experimental melting of biotite plagioclase quartz muscovite assemblages and implications for crustal melting abstract in order to understand the role of mica rich rocks as a source of granite magmas a series of melting experiments was performed on two different starting materials the first composition is a model biotite gneiss consisting of 30 wt biotite 30 wt plagioclase and 40 wt quartz the second composition is a model two mica pelites consisting of 15 wt biotite 15 wt muscovite 30 wt plagioclase and 40 wt quartz experiments were performed under vapor absent conditions at 1 0 gpa and between 750 o and 950oc with only biotite in the starting material the volume of melt is always less than 15 vol below 900oc and reaches 25 vol at 950oc in experiments that involve both biotite and muscovite in the starting material the melt proportion increases up to 28 vol at 825oc and reaches 60 vol at 950oc for the biotite plagioclase quartz bpq assemblage the solidus is located at 800oc at 1 0 gpa the melting reaction produces a metaluminous granitic liquid and leaves a residuum consisting of garnet biotite orthopyroxene plagioclase quartz in addition the experiments show that at 1 0 gpa biotite can be stable above 950oc with both micas in the starting material bpqm the solidus at 1 0 gpa is located at 750oc the melting reactions produce a peraluminous granitic liquid and leave a residuum of garnet sillimanite biotite quartz plagioclase kfeldspar in experiments below 900oc at 950oc the residuum consists of garnet orthopyroxene biotite plagioclase the melt fraction is determined by the proportions of the hydrous phases and of the amount of feldspar relative to quartz mineral modes of the source rocks particularly the amount of quartz are at least as important as the amount of available h20 in controlling the melt fraction generated during crustal anatexis

influence of pore pressure on the event size distribution of induced earthquakes 1 during an enhanced geothermal system egs experiment fluid is injected at high pressure into crystalline rock to enhance its permeability and thus create a reservoir from which geothermal heat can be extracted the fracturing of the basement caused by these high pore pressures is associated with microseismicity however the relationship between the magnitudes of these induced seismic events and the applied fluid injection rates and thus pore pressure is unknown here we show how pore pressure can be linked to the seismic frequency magnitude distribution described by its slope theb value we evaluate the dataset of an egs in basel switzerland and compare the observed event size distribution with the outcome of a minimalistic model of pore pressure evolution that relates event sizes to the differential stress d we observe that the decrease of b values with increasing distance of the injection point is likely caused by a decrease in pore pressure this leads to an increase of the probability of a large magnitude event with distance and time

development of a shear flow test apparatus and determination of coupled properties for a single rock joint abstract a new laboratory technique for coupled shear flow tests of rock joints was developed and used to investigate the coupled effect of joint shear deformation and dilatancy on hydraulic conductivity of rock joints this technique was used to carry out coupled shear flow tests with an artificial created granite joint sample under constant normal loads and up to residual shear deformations of 20 mm the hydraulic conductivity was estimated by using a finite difference method and an approximate equation assuming the cubic law the shear flow testing results revealed that the change of hydraulic conductivity is approximately similar to that of the dilatancy of a joint the hydraulic conductivity increases rapidly by about 1 2 1 6 orders of magnitude for the first 5 mm of shear displacement after passing the residual shear stress the hydraulic conductivity becomes gradually a constant value with increasing shear displacement on the other hand the hydraulic conductivity after shearing is about one order of magnitude larger than that prior to shearing shear flow coupling characteristics obtained from these tests have a consistent trend with barton s model prediction during the initial shear process however increasing deviation between measured and predicted hydraulic conductivity of rock joint samples has been observed with increasing shear displacement

tectonic evolution of nw iberia during the paleozoic inferred from the geochemical record of detrital rocks in the cantabrian zone the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges in age from ediacaran to early permian its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana ii an active continental margin or iii the margin of a drifting ribbon continent and iv a combination of the three aforementioned possibilities in this paper we present geochemical data from 21 samples taken in detrital rocks in the cantabrian zone from ediacaran to early permian age the results together with previously published detrital zircon ages from these strata allow a comprehensive analysis of changing tectonic scenarios and provenance through time collectively these data indicate that this portion of iberia was an active margin during the ediacaran that evolved to become part of the passive margin of gondwana at least from ordovician to late devonian times changes in geochemistry zircon age distribution and sm nd isotopes during the carboniferous are attributed to the far field effects of the onset of the collision between gondwana and laurussia and related processes such as recycling of older sedimentary sequences as well as the involvement of the rocks formed during the variscan orogeny and oroclinal buckling latest carboniferous and permian show a more juvenile sm nd and higher values of illite cristallinity that may support the hypothesis of lithospheric foundering after oroclinal buckling

emplacement of the cuera and picos de europa imbricate system at the core of the iberian armorican arc cantabrian zone north spain new precisions concerning the timing of arc closure the final stages of the variscan orogeny mostly carboniferous in the western european variscan belt involved the development of the iberian armorican arc which is cored by the cantabrian zone nw iberia the cantabrian zone is the foreland of the western european variscan belt and it is interpreted to record the waning stages of the closure of the rheic ocean the distalmost tectonic units within the cantabrian zone the cuera unit and the picos de europa province were the last tectonic units emplaced at the core of the western european variscan belt orocline together they form an imbricate system and associated wedge top basins that are key to understanding the development of the orocline the emplacement of the cuera unit and the picos de europa province occurred in the latest pennsylvanian between moscovian and gzhelian times new detailed mapping together with stratigraphic sedimentologic and biostratigraphic data analysis of syntectonic successions and structural information constrain the timing and evolution of this imbricate system our analysis indicates that the thrust sheets were emplaced roughly perpendicular to previous tectonic units of the cantabrian variscan foreland fold and thrust belt most probably during the oroclinal bending of the sw european variscan belt that formed the iberian armorican arc the n s directed imbricate system was characterized by a shallow dip of the topographic surface 150 15 km and complex structure of the orogenic wedge are thought to result from the progressive increase in the dip of the basal decollement during bending of the underlying gondwana lithosphere and may reflect the closure of the iberian armorican arc

scale invariant stress orientations and seismicity rates near the san andreas fault 1 we analyzed measurements of the direction of maximum horizontal compressive stress as a function of depth in two scientific research wells near the san andreas fault in central and southern california we found that the stress orientations exhibit scale invariant fluctuations over intervals from tens of cm to several km similarity between the scaling of the stress orientation fluctuations and the scaling of earthquake frequency with fault size suggests that these fluctuations are controlled by stress perturbations caused by slip on faults of various sizes in the critically stressed crust adjacent to the fault the apparent difference in stress scaling parameters between the two studies wells seem to correspond to differences in the earthquake magnitude frequency statistics for the creeping versus locked sections of the fault along which these two wells are located this suggests that stress heterogeneity adjacent to active faults like the san andreas may reflect variations in stresses and loading conditions along the fault

the importance of earthquake interactions for injection induced seismicity retrospective modeling of the basel enhanced geothermal system author s catalli f rinaldi ap gischig v nespoli m wiemer s abstract 2016 american geophysical union all rights reserved we explore the role of earthquake interactions during an injection induced seismic sequence we propose a model which considers both a transient pressure and static stress redistribution due to event interactions as triggering mechanisms by calibrating the model against observations at the enhanced geothermal system of basel switzerland we are able to reproduce the time behavior of the seismicity rate we observe that considering earthquake interactions in the modeling leads to a larger number of expected seismic events 24 more if compared to a pressure induced seismicity only the increase of the number of events is particularly evident after the end of the injection we conclude that implementing a model for estimating the static stress changes due to mutual event interactions increases significantly the understanding of the process and the behavior of induced seismicity

kinematics of orocline tightening in the core of an arc paleomagnetic analysis of the ponga unit cantabrian arc northern spain 1 paleomagnetic and structural analyses of the western european variscan belt wevb suggest that the most viable kinematic model for variscan deformation in northern iberia is oroclinal bending of an originally linear belt in a two stage tectonic history this history represents two regional compression phases e w in the late carboniferous and n s in the permian both in present day coordinates which resulted in the refolding about steeply plunging axes of initially north south trending thrusts and folds in the hinge zone and oroclinal tightening due to vertical axis rotation of the belt s limbs however the orocline model has yet to be critically tested in the wevb s core this study reports new paleomagnetic rock magnetic and structural data from the inner core of the wevb in order to test opposing kinematic models for the well documented fault and fold interference structures formed by late stage variscan deformation and to better understand the overall development of the wevb arc map scale structural features within the wevb core have a highly sinuous geometry characterized by transverse and thrust parallel fold systems formed by fault bend folding over footwall ramps the intersections of these two fold systems produce steeply plunging interference folds which are best exposed in the ponga thrust unit a total of 67 paleomagnetic sites were collected in the barcaliente formation of the ponga unit with emphasis placed on detailed spatial coverage of individual structural domains a multicomponent paleomagnetic remanence was measured which is interpreted to be composed of two components a low unblocking temperature recent viscous magnetization and a high unblocking temperature ancient magnetization that was acquired in the latest stephanian to early permian after initial d1 thrusting and folding bur prior to major secondary rotation rock magnetic experiments show that the characteristic remanence magnetization is carried by secondary authigenic pseudosingle domain magnetite these paleomagnetic results are used to determine the kinematics and geometry of postmagnetization deformation by restoring in situ magnetic vectors back to a defined reference direction on the basis of the restored ponga unit geometry the steeply plunging interference folds found in the wevb core are best described by a combination of secondary buckling of frontal ramp parallel hanging wall anticlines and modification of d1 lateral oblique ramps as frontal ramps during late stage north south shortening this combination of structural modification of the inner core of the wevb was necessary to accommodate oroclinal bending during the final amalgamation of pangea in the late paleozoic

phase relations in silicic systems at one atmosphere pressure an important control on magma rheology is the extent to which the magma crystallizes during ascent as a result of the effective undercooling created by volatile exsolution to assess this undercooling we need to know the final anhydrous one atmosphere phase relations of silicic magmas for this reason we have performed one atmosphere controlled fo2 crystallization experiments on dacitic to rhyolitic melt compositions 67 78 wt sio2 and determined equilibrium phase assemblages melt fractions and some phase compositions over a range of temperatures experiments were run at oxygen fugacities between nno 1 and nno 2 and temperatures of 1 000 to 1 250 c constant phase compositions and sample crystallinities in runs longer than 3 5 days suggest that these runs closely approached compositional equilibrium additionally melting experiments with similar compositions yielded results closely resembling those obtained in crystallization experiments all samples have liquidus temperatures between 1 250 and 1 200 c with plagioclase the liquidus phase for the two most mafic samples and quartz for the most silicic sample when associated glass compositions are projected into the qz ab or system they define a revised one atmosphere quartz feldspar cotectic 5 10 less quartz normative than previously estimated glass compositions from each sample plot along this cotectic between 1 100 and 1 000 c consistent with the plagioclase quartz co crystallization textures found in runs at these temperatures this cotectic constrains glass compositions to a maximum silica content of 76 1 wt sio2 reported glass compositions in excess of 77 wt sio2 in volcanic samples suggest non equilibrium crystallization perhaps a consequence of large melt undercoolings

styles of folding within thrust sheets examples from the appalachian and rocky mountains of the u s a and canada abstract folds in a single thrust sheet can be classified as trailing edge formed over footwall ramps intraplate due to shortening within the body of the sheet and leading edge leading edge folds are usually removed by erosion at the front of a thrust sheet but their unfaulted equivalents can be examined at the lateral termination of the thrusts most folds of these various classes possess a kink band geometry with sharp hinges and long planar limbs the orderly nature of kink band folds breaks down when thin incompetent units separate thicker competent members the resulting fold geometry ranging from simple disharmonic to hinge collapse is controlled by the thickness of interbedded incompetent materials the appalachian and rocky mountain thrust belts provide examples of these various classes and styles of folds

anorogenic magmatism chemical evolution of the mount el sibai a type complex egypt and implications for the origin of within plate felsic magmas the mount el sibai alkaline granitic complex eastern egypt forms an elongate body which was emplaced at the extension of a nw trending shear zone within voluminous calc alkaline pan african host rocks the complex is hypersolvus in nature and is composed of perthite quartz alkali amphibole fe rich biotite and accessory zircon apatite fluorite aenigmatite and ilmenite data on mineral chemistry show that the amphibole ranges in composition from hastingsite to pure end member arfvedsonite and the biotite is largely titaniferous annite geochemically the complex is highly evolved in composition with 72 78 wt sio 2 and di values of 85 98 is enriched in rb 48 291 ppm nb 28 237 ppm y 47 269 ppm zr 58 618 ppm ga 17 41 ppm and the ree 176 437 ppm and depleted in al mg ca sr and eu the complex exhibits a wide trace element compositional range the ree patterns are uniform parallel to sub parallel fractionated la yb n 4 7 lree enriched over hree and show prominent negative eu anomalies the albitized facies of this complex shows the highest concentrations of large ion lithophile lil and high field strength hfs elements the complex exhibits mineralogical and chemical traits typical of within plate a type granites mount el sibai is interpreted to have been developed during a phase of cooling relaxation crustal attenuation and fracturing of the newly formed pan african crust results of geochemical modelling indicate that the magma may have formed by a large degree of batch partial melting f 0 65 of pan african calc alkaline rocks which had been metasomatized metasomatism of source rocks may have been caused by a na f rich fluid phase compositionally similar to that which produced the albitized facies the volatile flux may have caused fenitization type reactions along fissures and re activated pan african fractures prior to anatexis and is considered to have played a role as an important agent of heat transfer temperature necessary for crustal anatexis is likely to have been produced as a result of shear heating caused by a rapid change in the direction of plate motions beneath eastern egypt during early palaeozoic times the confining pressure must have been released by fissuring of the crust magma ascent may have been facilitated by reactivation of pre existing faults and shear zones this model may have wider implications for the generation of within plate felsic magmas in other regions

late archaean felsic alkaline igneous rocks in the eastern goldfields yilgarn craton western australia a result of lower crustal delamination numerous felsic alkaline intrusions were emplaced in the granite greenstone terrain of the eastern goldfields yilgarn craton between c 2650 and 2630 ma intrusion coincided with the waning stages of voluminous regional granitic magmatism the alkaline rocks are localized along and adjacent to major crustal structures that separate lithostratigraphic greenstone terranes and are divided into four geographically and compositionally distinct suites viz the mount monger emu claypan and ninnis suites the rocks have features in common with a type granites including high total alkalis an anhydrous primary mineralogy an apparent anorogenic setting and enrichment in some hfse although they also have low concentrations of nb and y they show a range of features that together point to a crustal source e g the alkaline rocks are not associated with voluminous mafic magmatism they have high lile concentrations they carry zircon xenocrysts of similar age to whole rock sm nd model ages they show evidence for regional source heterogeneity suites have constant and distinct la y and nb y ratios and the rocks show many compositional similarities to the slightly older voluminous crust derived granites extreme enrichments in sr and ba in the mount monger suite however suggests that their source may have been metasomatized before or during melting intrusion of the alkaline rocks heralds a poorly recognized third tectonothermal event in the evolution of the eastern part of the yilgarn craton and was broadly synchronous with the intrusion of voluminous felsic fractionated granites crustal extension and widespread au mineralization the late archaean geology of the eastern goldfields shows many features in common with phanerozoic collisional belts thought to be profoundly influenced by processes of lithospheric delamination or convective thinning this newly recognized third tectonothermal event in the eastern goldfields may relate to the delamination or convective thinning of a dense garnet rich lower crust that resulted from extraction of earlier voluminous granite magma and it is possible that such processes have had a greater influence on late archaean crustal evolution than previously recognized

thin skinned tectonics in the ponga region cantabrian zone nw spain the variscan thrust and associated fold structures of the ponga region cantabrian zone nw spain are interpreted in terms of thin skinned tectonics with a dominant eastward transport direction

crustal magnetization and long wavelength aeromagnetic anomalies of the minto block quebec measured susceptibilities of 3500 samples from a 500 500 km area of the minto block superior province quebec have an average value of 0 03 si equivalent to induced magnetizations of 1 5 a m charnockitic lithologies igneous orthopyroxene bearing granodiorite granite diorite that constitute large parts of the minto region yield the highest susceptibilities the variations in susceptibility show a good correlation with the variations in the observed magnetic field including those at long wavelengths 100 km suggesting that charnockitic and related granitoid rocks could be responsible for the long wavelength anomalies modeling of a magnetic profile perpendicular to the strike of mapped lithotectonic subprovinces in the minto block supports this contention charnockitic lithologies are not usually considered a possible sources of long wavelength anomalies mafic rock types are generally favored the dominant minto lithologies are more felsic but are more efficient in the production of magnetite from the available iron

control of hazard due to seismicity induced by a hot fractured rock geothermal project in 2003 hydraulic stimulations were carried out in a geothermal field in eastern el salvador central america as part of a project to explore the feasibility of commercial hot fractured rock energy generation a key requisite for this environmentally friendly energy source is that the fracturing of the hot rocks at depths of 1 2 km must not produce levels of ground shaking at the surface that would present a serious disturbance or threat to the local population thresholds of tolerable ground motion were inferred from guidelines and regulations on tolerable levels of vibration and from correlations between instrumental strong motion parameters and intensity considering the vulnerability of the exposed housing stock the thresholds were defined in terms of peak ground velocity pgv and incorporated into a traffic light system that also took account of the frequency of occurrence of the induced earthquakes the system was implemented through a dedicated seismograph array and locally derived predictive equations for pgv the traffic light was used as a decision making tool regarding the duration and intensity of pumping levels during the hydraulic stimulations the system was supplemented by a small number of accelerographs and re calibrated using records obtained during the rock fracturing

microseismicity and permeability enhancement of hydrogeologic structures during massive fluid injections into granite at 3 km depth at the soultz hdr site summary a high rate injection of 20 000 m3 of water into granite between 2 8 and 3 4 km depth at the soultz hot dry rock hdr test site in france in 1993 september led to a 200 fold increase in borehole transmissivity and produced a subvertical cloud of microseismicity of dimensions 0 5 km wide 1 2 km long 1 5 km high and oriented 25 nw the resulting data set is unusually complete and well suited to studying permeability creation enhancement processes in crystalline rock and the utility of microseismic data for revealing them although the microseismic cloud defined using joint hypocentre determination jhd locations was diffuse and showed little structure application of the collapsing method showed it to be composed largely of discrete tubes and planes that propagated coherently one prominent structure that extended 350 m downwards from the vicinity of a flow inlet early in the injection and that appears to contain a major flow path was subjected to detailed investigation to establish its hydrogeologic nature and the mechanisms underpinning its inferred permeability enhancement high resolution microseismic mapping techniques i e multiplets and clustering showed it to be a subvertical nnw sse striking fracture zone of width 10 20 m the strike and scale of the structure identifies it as a member of a family of hydrothermally altered cataclastic shear structures that constitute the primary permeable paths for fluid migration within the rock mass both under ambient and forced fluid flow conditions the microseismicity occurred on subvertical small scale fractures within the cataclastic shear zone whose azimuths scatter within 22 of parallel to the parent structure although the structure is likely to have been naturally permeable to some degree its permeability appears to have been significantly enhanced as a consequence of the injection the most likely mechanism of permeability enhancement which is in accord with the strong preference for the microseismicity to grow downwards involves strike slip shearing which produced the opening of vertical tubes at along strike jogs in the fault the so called hill mesh seismic moment release averaged over the structure suggests shear displacements of at least 0 3 mm occurred which are sufficient to generate aperture changes that are hydraulically significant the preponderance of discrete structures within the microseismic cloud after collapsing suggests that significant flow and permeability enhancement i e stimulation within the rock mass is largely confined to the interiors of shear zones that appear to have a spacing of approximately 100 m

permeability creation and damage due to massive fluid injections into granite at 3 5 km at soultz 2 critical stress and fracture strength 1 the stress acting on fractures in a 3 5 km deep borehole in granite is examined to place constraints on fracture strength and to evaluate whether permeable fractures tended to be critically stressed two data sets consisting of some 500 fractures are analyzed the first considers the permeable impermeable fracture populations in their natural state the second considers the populations after the hole had been subject to a prolonged 9 mpa overpressure in two major injections which increased the number of permeable fractures from 18 to more than 95 for both data sets it is found that permeable fractures are critically stressed inasmuch as they support levels of shear stress that would be verging on failure if their strength were governed by a coulomb friction law with a coefficient of 0 6 1 0 however the vast majority of impermeable fractures are also critically stressed implying that critical stressing is a necessary but not a sufficient condition for permeability to develop most permeable fractures showed evidence of shear failure after the injections and tended to occur in hydrothermally altered cataclastic shear zones suggesting that they may have been relatively weak because of the presence of illite to prevent failure of the impermeable fractures during the injections requires a cohesion of at least 6 5 mpa to augment a maximum allowable friction coefficient of 1 0 these are probably early mode 1 fractures that are sealed primarily with calcite the strength of the shear zones appears to be significantly greater than the illite rich fractures from which they are composed this imposes constraints on their internal architecture

three dimensional geometry and interference of fault bend folds examples from the ponga unit variscan belt nw spain abstract examples of the three dimensional geometry and interference patterns between fault bend folds occur in the ponga unit of the variscan cantabrian foreland thrust and fold belt of nw spain the ponga unit consists of several e directed thrust sheets with a cumulative displacement of 93 km which were emplaced during upper westphalian stephanian time two major lateral ramps developed in the basal thrust of the ponga unit reaching 15 and 20 km in length respectively and some 3 km in height lateral structures are also common within the hangingwall thrust sheets the three dimensional staircase trajectories of thrust faults together with the tectonic superposition of thrust sheets has produced a complex interference pattern of fault related folds most map scale folds formed as fault bend folds related to interference between frontal and lateral ramps fold geometry and size varies depending on the geometry and size of the associated thrust ramp and flat the axial traces are grouped into lateral and frontal sets which are sub parallel or sub perpendicular to the tectonic transport direction respectively corner folds form with plunging axes at the intersections of frontal and lateral ramps and generally link pairs of frontal lateral folds they have relatively short axial traces that are oblique to the tectonic transport direction fold plunge varies due to the superposition of lateral and frontal structures of a thrust fault and also depends on the geometry of subsequent thrusts in the footwall the final geometry of frontal folds in the ponga unit are not only the result of the bending mechanism but have been modified by simple shear parallel to the emplacement direction major lateral structures have been reactivated as reverse faults and are locally overturned due to post emplacement north south compression

critical role of water in the formation of continental crust continental arcs are the sites of production of continental crust but the origin of these magmatic systems is not well understood although a number of processes have been suggested to be important the role of water migrating from slab to surface during subduction has been underappreciated directly below the moho hot approximately 1 100 c hydrous basaltic magmas fractionate as they cool to the regional geotherm at 750 to 800 c ultimately solidifying as mafic underplates cooling and fractionation cause water to exsolve and ascend triggering fluid fluxed melting of overlying mafic underplates and other crust melting of prior mafic underplates buffers temperatures and generates the voluminous juvenile low k magmas of cordilleran batholiths these granitoid magmas comprise a low temperature slurry of melt and residue and recrystallize into silicic mush during adiabatic ascent such hydrous mushes are intermittently infused by hotter more mafic magmas which hybridize and facilitate ascent and potentially eruption fluid fluxed melting overcomes many of the general petrological and geochemical problems associated with models dominated by fractional crystallization the role of water during repeated episodes of mafic underplating is critical to generate the juvenile granitoid infrastructure of the continents migration of water from the slab to the surface during subduction is highlighted as a key process in the formation of continental crust

analysis of kilometric scale superposed folding in the central coal basin cantabrian zone nw spain abstract the carboniferous paralic sequence of the central coal basin cantabrian zone hercynian nw spain contains an outstanding example of kilometric scale superposed folding that allows comparison with previous experimental models first generation folds have a n s trend and mainly constitute a fold train of fault propagation folds in which an increase of interlimb angle and an increase in the dip of the axial plane is observed from west to east a second folding phase gave rise to two types of upright roughly e w oriented superposed folds the superposed folds generated in general by buckling but their trend and situation are controlled by lateral ramps of the previous thrusts in many cases superposed folds of the first type are the most common and have developed on the limbs of previous folds without folding their axial surfaces they occur in zones and usually have kilometre scale hinge lines with syngenetic curved traces these folds present wavelengths distinctly smaller than those of the previous folds the second type of superposed folds affect both the limbs and axial planes of the previous folds these second type folds are in some cases smaller than previous folds and have developed on closed early folds but in other cases they are large wavelength folds related to the tightening of lateral ramps of the thrusts

fold interference structures in thrust systems abstract the accommodation problems associated with the generation of interference folds in thrust belts are examined there are several ways in which interference folds may grow in thrust systems such as moulding on to non planar ramps or via the interaction of non coaxial structures the geometries of interference folds from thrust belts in spain and pakistan are described and form the basis of an analog model the model is used to quantify the volume of rock which must be accommodated during the growth of these structures mechanisms which might solve the inherent space problem are discussed the simplest geometrical solution would involve a mechanism which caused thickening in the fold hinges but this was not observed in the field field evidence is used to construct a linked fold and fault model which reconciles the absence of thickening in the interference fold hinge with the volume of material which must be accounted for

a model for monazite melt equilibrium and application to the generation of granitic magmas abstract in order to model the behaviour of zr hf ti p u th y and the rare earth elements ree in the continental crust the parameters which control the equilibria between accessory minerals and granitic melts must be determined this paper proposes equations for the equilibrium between monazite and ca poor felsic melts in a first step all the light ree are considered as a single component and an equation describing monazite solubility is deduced from the available experimental data in a second step the fractionation of ree by monazite is investigated on the basis of a natural example of coexisting monazite and obsidian from macusani peru the solubility of monazite can be used as a thermometer based on the total ree content of natural magmatic rocks application of ree thermometry to various peraluminous rock series suggests that it is a reliable tool for determining temperatures of magmatic processes the equation governing ree fractionation by monazite is used to discuss genetic relationships between a melt and a presumed parental rock using the composition of the monazite in the parental rocks and the ree pattern of the daughter rock this method has been applied successfully to high himalayan manaslu and tibetan slab granites it is suggested that detailed petrographical work on monazite including electron microprobe analyses and sem imaging of its internal structures combined with the model presented here can be a useful tool for deciphering the genesis and the differentiation of peraluminous granitic magmas in the continental crust

petrogenesis of the linxi granitoids northern inner mongolia of china constraints on basaltic underplating five representative granitoid plutons in the linxi area northern inner mongolia of china divided into two suites are mainly granodiorite and syenogranite and correspond to i and a type granites respectively zircon u pb dating yields an age of 230 ma but most ages range from 149 ma to 111 ma for intrusion of the five plutons suite 1 granitoids show lower sio2 66 1 72 51 and k2o higher al2o3 cao mgo feo and tio2 higher sr ba and eu and lower sm gd nb y and yb than suite 2 granitoids 71 73 78 14 sio2 modeling of major and trace elements shows that fractional crystallization of hornblende and lesser amounts of feldspars for suite 1 and plagioclase and alkali feldspar for suite 2 with overprint of magmatic hydrothermal fluids can explain the geochemical variations the linxi granitoids have initial end t values varying between 5 60 and 2 89 and initial 87 sr 86 sr t ratios ranging from 0 701764 to 0 707654 geochronology of inherited zircons coupled with the nd sr isotopic comparison between granitoids late jurassic basalts and proterozoic gneisses indicates that the linxi granitoids derive from a mixed source of dominantly underplated material or mafic magma with various amounts of the proterozoic gneiss basaltic underplating provides both material and heat to produce the late jurassic to early cretaceous granitoids variation of end t and 87 sr 86 sr t with age suggests involvement in the melting of increasingly greater amounts of the old lower crustal rock our refined equation yields less scattered nd isotope depleted mantle model ages the least fractionated granitoids from each suite have low ca sr rb sr rb ba and k ba ratios and hence represent parental magmas suite 1 parent magma is produced by dehydration melting or derives principally from the fluid metasomatized mantle melting of the underplated tholeiites and their differentiated products generates suite 2 parent magma d 2005 elsevier b v all rights reserved

influence of detachment layer thickness on style of thin skinned shortening abstract there are several parameters which may determine whether detachment folds or thrust faults develop in thin skinned compressional systems a fundamental but little publicised parameter is apparent from consideration of detachment fold growth as a detachment fold amplifies its core must be filled by redistribution of ductile material if the mobile material were insufficient to supply the volume required by the fold core through redistribution fold growth would be inhibited and eventually faulting would occur the volume of a detachment fold core is compared here with the volume of available underlying mobile material for various amounts of shortening and sizes of fold the occurrence of detachment folds versus thrusts in several compressional belts is successfully predicted by this relationship

variations in earthquake size distribution across different stress regimes the earthquake size distribution follows in most instances a power law1 2 with the slope of this power law the b value commonly used to describe the relative occurrence of large and small events a high b value indicates a larger proportion of small earthquakes and vice versa statistically significant variations of b values have been measured in laboratory experiments mines and various tectonic regimes such as subducting slabs near magma chambers along fault zones and in aftershock zones3 however it has remained uncertain whether these differences are due to differing stress regimes as it was questionable that samples in small volumes such as in laboratory specimens mines and the shallow earth s crust are representative of earthquakes in general given the lack of physical understanding of these differences the observation that b values approach the constant 1 if large volumes are sampled4 was interpreted to indicate that b 1 is a universal constant for earthquakes in general5 here we show that the b value varies systematically for different styles of faulting we find that normal faulting events have the highest b values thrust events the lowest and strike slip events intermediate values given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress

generic dependence of the frequency size distribution of earthquakes on depth and its relation to the strength profile of the crust 1 we explore the idea that the relative size distribution of earthquakes quantified using the so called b value is negatively correlated with differential stress because the maximum possible differential stress increases linearly in the brittle upper crust we expect to find a decrease of b with depth we test this expectation for seven continental areas around the world each of which is described by a regional earthquake catalog we find a monotonic decrease in b value between 5 and 15 km depth the decrease stops near the brittle ductile transition we specifically focus on the high quality catalogs of earthquakes in california to perform a sensitivity test with respect to depth uncertainty we also estimate the probability depth gradient for the occurrence of a target magnitude event and study the behavior of b with depth in near and off fault zones we also translate the observed b depth gradients into b differential stress gradients our findings suggest that b values are negatively correlated with differential stress and hus have the potential to act as stress meters in the earth s crust

the terminology of structures in thrust belts abstract a review of structures and geometric relationships recognized in thrust belts is presented a thrust is defined as any contractional fault a corollary being that thrusts must cut up section in their transport direction flats are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ramps are those portions of thrusts which cut across datum surfaces ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike slip movement an array of thrusts which diverge in their transport direction may form by either of two propagation models these are termed piggy back propagation which is foreland directed and overstep propagation which is opposed to the thrust transport direction an array of thrust surfaces is termed an imbricate stack and should these surfaces anastamose upwards a duplex will result the fault bounded blocks are termed horses a duplex is bounded by a higher roof thrust and a lower floor thrust the intersection of any two thrust planes is termed a branch line thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut a further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one the movement of thrust sheets over corrugated surfaces or the local development of thrust structures beneath will fold higher thrust sheets these folds are termed culminations and their limbs are termed culmination walls accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust these accommodation surfaces are termed hangingwall detachments and they need not root down into the active thrust this category of detachment includes dip slip hangingwall drop faults which are developed by differential uplift of duplex roofs and out of the syncline thrusts which develop from overtightened fold hinges back thrusts as well as forming as hangingwall detachments may also form due to layer parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp

probability of inducing given magnitude earthquakes by perturbing finite volumes of rocks 1 fluid induced seismicity results from an activation of finite rock volumes the finiteness of perturbed volumes influences frequency magnitude statistics previously we observed that induced large magnitude events at geothermal and hydrocarbon reservoirs are frequently underrepresented in comparison with the gutenberg richter law this is an indication that the events are more probable on rupture surfaces contained within the stimulated volume here we theoretically and numerically analyze this effect we consider different possible scenarios of event triggering rupture surfaces located completely within or intersecting only the stimulated volume we approximate the stimulated volume by an ellipsoid or cuboid and derive the statistics of induced events from the statistics of random thin flat discs modeling rupture surfaces we derive lower and upper bounds of the probability to induce a given magnitude event the bounds depend strongly on the minimum principal axis of the stimulated volume we compare the bounds with data on seismicity induced by fluid injections in boreholes fitting the bounds to the frequency magnitude distribution provides estimates of a largest expected induced magnitude and a characteristic stress drop in addition to improved estimates of the gutenberg richter a and b parameters the observed frequency magnitude curves seem to follow mainly the lower bound however in some case studies there are individual large magnitude events clearly deviating from this statistic we propose that such events can be interpreted as triggered ones in contrast to the absolute majority of the induced events following the lower bound

experimental constraints on the origin of the 1991 pinatubo dacite crystallisation and mantle interaction experiments have been performed on the 1991 pinatubo dacite to constrain its petrogenesis in the dacite h2o system at 1000 mpa magnetite and either clinopyroxene or amphibole are the liquidus phases no garnet is observed at this pressure dacite mantle interaction at 920 mpa produces massive orthopyroxene crystallisation in addition to amphibole phologopite amphibole crystallising in dacite at 1000 mpa faithfully reproduces aluminium rich hornblende preserved in the cores of amphibole phenocrysts of the 1991 dacite suggesting a high pressure stage of dacite crystallisation under high melt h2o contents 10 wt and low temperatures 950c altogether plagioclase amphibole and melt inclusion compositions suggest that the pinatubo dacite was water rich oxidized and not much hotter than 900c when emplaced in the shallow reservoir in which most phenocrysts precipitated until the onset of the 1991 eruptions the strongly fractionated ree pattern demands garnet somewhere during dacite genesis which in turn requires high pressure derivation partial melting of subducted oceanic crust gives melts unlike the pinatubo dacite interaction of these slab melts with sub arc peridotite is unable to produce such type of dacite nor is a direct mantle origin conceivable on the basis of mantle dacite interaction experimental results dehydration melting of underplated basalts requires temperatures too high to match with petrological constraints and does not allow production of dacite melts having the low feo mgo high h2o ni and cr contents typical of pinatubo like magmas on the face of chemical and phase equilibrium constraints the most plausible origin of the pinatubo dacite is via high pressure fractionation in the upper mantle of an hydrous oxidised primitive basalt that crystallises amphibole and garnet upon cooling as shown by recent phase equilibrium work dacite melts so produced are directly expelled from the uppermost mantle or lower crust to shallow level storage reservoirs from which they erupt occasionally following injection of h2o poor mantle derived basalts that can ascent without massive crystallisation unlike their hydrous counterparts parental to dacite magmas that rest at mantle depths dacites such as the pinatubo one may thus witness particular h2o rich conditions of the sub arc mantle rather than the melting of a young and hot subducting oceanic plate

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

investigating the kinematics of local thrust sheet rotation in the limb of an orocline a paleomagnetic and structural analysis of the esla tectonic unit cantabrian asturian arc nw iberia the esla tectonic unit lies along the southern boundary of the cantabrian asturian arc a highly curved foreland fold thrust belt that was deformed during the final amalgamation of the pangea supercontinent previous structural and paleomagnetic analyses of the cantabrian asturian arc suggest a two stage tectonic history in which an originally linear belt was bent into its present configuration creating an orocline the esla tectonic unit is a particularly complex region due to the interaction of rotating thrust sheets from the southern limb of the arc and the southward directed thrusts of the picos de europa tectonic domain during late stage north south shortening and oroclinal bending these structural interactions resulted in intense modification of early phase thin skinned tectonic structures that were previously affected by a deeper out of sequence antiformal stack that passively deformed the early thrust stack a total of 75 paleomagnetic sites were collected from the portilla and santa lucia formations two carbonate passive margin reef platform units from the middle devonian similar to other regions of the cantabrian asturian arc esla unit samples carry a secondary remanent magnetization that was acquired after initial thrusting and folding of variscan deformation in the late carboniferous protracted deformation during late stage oroclinal bending caused reactivation of existing thrust sheets that include the esla and younger corniero and valbuena thrusts when combined with existing structural data and interpretations these data indicate that the present day sinuosity of the esla unit is the consequence of both secondary rotation of originally linear features in the western esla exposures e g frontal thrusts and secondary modification and tightening of originally curvilinear features in the eastern esla exposures e g hanging wall lateral oblique ramps differences in structural style between the esla and other tectonic units of the arc highlight the complex kinematics of oroclinal bending which at the orogen scale buckled an originally linear north south in present day coordinates trending cantabrian asturian thrust belt during the final stages of pangea amalgamation

conical folding in the core of an orocline a geometric analysis from the cantabrian arc variscan belt of nw iberia abstract the cantabrian arc of the variscan belt has been recently defined as a true orocline constraining kinematics and deformation timing this curved sector of the orogenic belt is characterized by two different fold sets 1 one runs parallel to the outcrops of the main thrusts and describes a horseshoe shape concave towards the east and 2 another is radial to the arc a detailed geometric study of the fold interference patterns in the cantabrian arc revealed the conical nature of the folds belonging to the radial set these conical folds developed with different geometrical characteristics semi apical angles and axis attitudes depending on the initial orientation and geometry of the folded surfaces they are interpreted to result from a vertical axis rotation during oroclinal buckling of the variscan belt in nw iberia this study of conical folds in the cantabrian arc highlights that conical folds in curved orogenic arcs are a powerful tool for establishing the sequence of tectonic events because interference patterns due to vertical axis secondary differential rotations provide unique geometrical characteristics observed in the cantabrian arc that can be extrapolated to other oroclines

phase relationships of i type granite with h2o to 35 kilobars the dinkey lakes biotite granite from the sierra nevada batholith the dinkey lakes biotite granite from the sierra nevada batholith was reacted with varying percentages of h 2o in sealed platinum capsules in a piston cylinder apparatus between 10 and 35 kbar the results were combined with the results from previously published experiments to provide comprehensive phase relationships for an i type granite a p t diagram with excess h 2o isobaric t x h 2o diagrams at 25 30 and 35 kbar showing h 2o undersaturated relations the h 2o undersaturated liquidus surface mapped with contours for constant h 2o contents and fields for near liquidus minerals and the solubility of h 2o in granite liquids to 35 kbar results and their implications show 1 the solidus temperature decreases from 680 c at 2 kbar to 620 c at 10 kbar then increases to 700 c at 35 kbar because of changes from less dense to more dense subsolidus mineral assemblages 2 the melting interval with excess h 2o which is only 35 c at 2 kbar increases to 105 c at 10 kbar and 150 c at 35 kbar because the liquidus minimum in the complex rock system departs from granite composition with increasing pressure 3 the solubility of h 2o in granite liquid is 27 2 5 weight percent at 35 kbar and 850 c indicating that a miscibility gap persists between h 2o saturated silicate magmas and aqueous vapor phase at least to pressures corresponding to 120 km depth in the mantle dissolution of alkali feldspar 20 of rock in the subsolidus aqueous vapor phase indicates that deep seated aqueous fluids are concentrated solutions 4 quartz and coesite are the liquidus minerals at mantle pressures for all h 2o contents indicating that granites and rhyolites cannot be primary magmas from mantle peridotite or subducted oceanic gabbroic crust 5 the liquidus surface at crustal pressures with plagioclase and quartz as primary minerals indicates that primary liquids of granite composition with moderate h 2o contents can be generated in the crust at reasonable temperatures these liquids could rise to near surface levels without vesiculation granite liquid together with residual crustal minerals could constitute plutonic magmas of intermediate composition

high fluid pressure and triggered earthquakes in the enhanced geothermal system in basel switzerland 1 we analyzed 118 well constrained focal mechanisms to estimate the pore fluid pressure field of the stimulated region during the fluid injection experiment in basel switzerland this technique termed focal mechanism tomography fmt uses the orientations of slip planes within the prevailing regional stress field as an indicator of the fluid pressure along the plane at the time of slip the maximum value and temporal change of excess pore fluid pressures are consistent with the known history of the wellhead pressure applied at the borehole elevated pore fluid pressures were concentrated within 500 m of the open hole section which are consistent with the spatiotemporal evolution of the induced microseismicity our results demonstrate that fmt is a robust approach being validated at the meso scale of the basel stimulation experiment we found average earthquake triggering excess pore fluid pressures of about 10 mpa above hydrostatic overpressured fluids induced many small events m 3 along faults unfavorably oriented relative to the tectonic stress pattern while the larger events tended to occur along optimally oriented faults this suggests that small scale hydraulic networks developed from the high pressure stimulation interact to load hydraulically isolated high strength bridges that produce the larger events the triggering pore fluid pressures are substantially higher than that predicted from a linear pressure diffusion process from the source boundary and shows that the system is highly permeable along flow paths that allow fast pressure diffusion to the boundaries of the stimulated region

water saturated and undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kb evidence for the origin of silicic magmas in the taupo volcanic zone new zealand and other occurrences

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

meta igneous granulite xenoliths from mount ruapehu new zealand fragments of altered oceanic crust meta igneous granulite mig xenoliths in lavas from mount ruapehu taupo volcanic zone new zealand have variable but relatively high 18o 8 2 to 11 7 and 87sr 86sr 0 70506 to 0 70872 and end ranging from 1 5 to 6 2 they show a strong positive correlation between 87sr 86sr and 18o both of which are also broadly correlated with mg number but lack any correlation between 87sr 86sr and end the xenoliths have been mineralogically re equilibrated at lower crustal temperatures 800 930 c and pressures 7 10 kbar geochemical and isotopic evidence suggests they are fragments of oceanic crust that have been altered previously in an ocean floor hydrothermal system alternatively they may be igneous rocks of unknown origin hydrothermally altered in the lower crust irrespective of this uncertainty the xenoliths provide rare samples of the lower crustal basement beneath taupo volcanic zone and represent a potential source rock for the voluminous rhyolites and ignimbrites that dominate the zone

balancing reservoir creation and seismic hazard in enhanced geothermal systems summary fractureshear dilatancyisanessentialprocessforenhancingthepermeabilityofdeepgeothermal reservoirs and is usually accompanied by the radiation of seismic waves however the hazard and risk perspective of induced seismicity research typically focuses only on the question of how to reduce the occurrence of induced earthquakes here we present a quantitative analysisofseismichazardasafunctionofthetwokeyfactorsdefininganenhancedgeothermal system the permeability enhancement and the size of the stimulated reservoir our model has two coupled components 1 a pressure diffusion model and 2 a stochastic seismicity model permeability is increased in the source area of each induced earthquake depending on the amount of slip which is determined by the magnitude we show that the few largest earthquakes i e 5 10 events with m 1 5 contribute more than half of the total reservoir stimulation the results further indicate that planning and controlling of reservoir engineering operations may be compromised by the considerable variability of maximum observed magnitude reservoir size the gutenberg richter b value and shapiro s seismogenic index i e a measure of seismic reactivity of a reservoir that arises from the intrinsic stochastic nature of induced seismicity we also find that injection volume has a large impact on both reservoir size and seismic hazard injection rate and injection scheme have a negligible effect the impact of site specific parameters on seismicity and reservoir properties is greater than that of the injected volume in particular conditions that lead to high b values possibly a low differential stress level have a high impact on seismic hazard but also reduce the efficiency of the stimulation in terms of permeability enhancement under such conditions target reservoir permeability can still be achieved without reaching an unacceptable level of seismic hazard if either the initial reservoir permeability is high or if several fractures are stimulated the proposed methodology is a first step towards including induced seismic hazard analysis into the design of reservoir stimulation in a quantitative and robust manner

nucleation and arrest of dynamic slip on a pressurized fault 1 elevated pore pressure can lead to reactivation of slip on pre existing fractures and faults when the static coulomb failure is reached locally as the pressurized region spreads diffusively slip can accumulate quasi statically paced by the pore fluid diffusion or dynamically in this work we consider a prestressed fault with a locally peaked diffusively spreading pore pressure field to study 1 conditions leading to the escalation of slip and nucleation of dynamic rupture and 2 rupture run out distance before it is arrested nucleation appears in this model when the fault friction decreases from its peak value with slip while arrest of dynamic propagation is imminent on aseismic faults i e such that prestress b is less than the residual fault strength rat ambient conditions when fluid overpressure is a small to moderate fraction of the ambient value of normal effective stress and prestress is large enough for fault slip to be activated by overpressure dynamic rupture always nucleates and the nucleation length increases with decreasing prestress practically independently of the overpressure value transition from the ultimately unstable b r to the ultimately stable b r fault loading is marked by a strong increase of the nucleation length 1 b r 2 as b approaches r from above for aseismic faults b r no dynamic rupture is nucleated at large fluid overpressures for all but the smallest values of prestress the largest run out distances of dynamic slip on aseismic faults correspond to overpressure prestress just sufficient for slip activation in such cases the dynamically accumulated slip can lead to enhanced dynamic fault weakening resulting in a sustained dynamic rupture and generating a large earthquake this is consistent with field observations when the largest injection induced seismicity occurred after fluid injection ended

new time constraints on lithospheric scale oroclinal bending of the ibero armorican arc a palaeomagnetic study of earliest permian rocks from iberia abstract the palaeozoic variscan orogeny was a large scale collisional event that involved amalgamation of multiple continents and microcontinents previous palaeomagnetic and structural analyses of the western variscan orogen notably the ibero armorican arc suggest that this region underwent oroclinal bending of an originally near linear orogen during the latest stages of variscan deformation in the late palaeozoic these analyses necessitate a two stage tectonic history with east west compression forming a linear belt in the carboniferous followed by a change in regional compression directions resulting in crustal block rotation and formation of the present day arc this study presents new palaeomagnetic analyses of early permian samples from the sotres and cabranes formations 41 3 n 222 5 w 95 13 1 in the northern inner limb and the viar basin 51 8 n 199 2 w 95 6 7 in the southern outer limb of the arc both locations yield expected early permian palaeomagnetic pole positions for stable iberia implying that measured magnetizations have undergone no significant vertical axis rotation since acquisition and thus were acquired subsequent to oroclinal bending this new result places a time constraint of about 10 ma for oroclinal bending of the ibero armorican arc which agrees well with recent geodynamical models that relate oroclinal bending to lithospheric delamination in this region

sequences of thrusts and displacement transfer in the superposed duplexes of the esla nappe region cantabrian zone nw spain abstract in a cross section through the southern arm of the cantabrian zone several duplexes have been identified below the esla nappe which is the uppermost and main thrust sheet of the area the folds deforming the esla nappe are culmination walls linked to frontal and lateral ramps belonging to the lower thrust sheets the thrust sequence can be established on the basis of quantitative analysis of displacement transfer and out of sequence thrusting the primitive footwall ramps of the esla nappe region were often subsequently broken by decollements developed in successively lower stratigraphic levels of these footwalls the kinematics of the lowest duplex are more complicated than those of typical duplexes described elsewhere some thrusts transfer only part of their displacement to the roof thrust while the remaining part is accommodated along the higher thrusts of previously emplaced duplexes cutting out of sequence one or more floor or roof thrusts cumulative displacement of the thrusts in this region is about 90 km giving a present thickness 3 times that of the original pre orogenic sequence together with a translation of at least 60 km for the synorogenic basin

fluid flux melting generated postcollisional high sr y copper ore forming water rich magmas in tibet miocene postcollisional porphyry cu deposits in southern tibet are genetically associated with dacitic rhyolitic intrusions with unusually high sr y ratios 40 which have been attributed to dehydration melting of garnet amphibolite in a thickened lower crust to test this hypothesis and examine the hydration state of copper ore forming high sr y magmas we utilize a geohygrometer for granitoid rocks entailing zircon saturation thermometry and h 2 o dependent phase equilibria the results show that these tibetan high sr y magmas had dissolved h 2 o contents 10 wt which considerably exceeds the water supply by dehydration melting of basaltic amphibolites maximum of 6 7 1 4 wt our results indicate that high sr y dacitic rhyolitic magmas cannot be produced by dehydration melting of basaltic amphibolites while h 2 o added melting of basaltic amphibolites can produce high sr y dacitic rhyolitic melts it does not yield high enough mg 50 to match the tibetan ore forming porphyries we propose an alternative model for the genesis of copper ore forming high sr y magmas in tibet and suggest that the high sr y dacitic rhyolitic porphyries in southern tibet are residually h 2 o enriched high pressure differentiation products of hydrous mafic partial melts of tibetan mantle this hypothesis is based on the previous investigation of miocene mafic microgranular enclaves mantle derived melts which define a fractionation trend with and have sr nd hf isotopic compositions similar to the host tibetan ore forming porphyries

stress drop variations of induced earthquakes at the basel geothermal site 1 we determine stress drops from p wave spectra of about 1000 earthquakes induced by hydraulic stimulation in crystalline rock for a deep heat mining project in basel switzerland we observe an increase in stress drop by about a factor of five with radial distance from 10 m to 300 m which suggests that stress drop correlates with pore pressure perturbations due to the injection we test this hypothesis by calculating the injection related pore pressure perturbation based on a simple linear pore pressure diffusion model and find a good correlation of the expected pore pressure perturbation with the estimated stress drops

the origin of lavas and ignimbrites of the taupo volcanic zone new zealand in the light of oxygen isotope data abstract the taupo volcanic zone consists predominantly of andesites at each end and about a 20times larger volume of rhyolites incl ignimbrites and pumice in the center in a 18o vs sio2 diagram the rhyolites do not continue the increasingly 18o contaminated andesite basalt to dacite trend but have lower 18o values than the most differentiated andesites comparison with 18o of greywackes and conjectural granitoids seems to rule out a partial fusion origin in retrospect the sr isotope data of ewart and stipp 1968 are seen to confirm this conclusion isotopic adjustment of source rocks before partial fusion by exchange with circulating subsurface waters is highly unlikely because of the combined demands for o and sr isotope shifts and the constraint offered by the normal igneous to somewhat enriched 18o values of the rhyolites oxygen isotope results confirm separate origins or at least different modes of ascent of basalt and andesites on one hand and rhyolites on the other both magma types appear to have originated below crustal formations and both have become contaminated by greywacke through which they passed to reach the surface

investigation of injection induced seismicity using a coupled fluid flow and rate state friction model we describe a numerical investigation of seismicity induced by injection into a single isolated fracture injection into a single isolated fracture is a simple analog for shear stimulation in enhanced geothermal systems egs during which water is injected into fractured low permeability rock triggering slip on preexisting large scale fracture zones a model was developed and used that couples 1 fluid flow 2 rate and state friction and 3 mechanical stress interaction between fracture elements based on the results of this model we propose a mechanism to describe the process by which the stimulated region grows during shear stimulation which we refer to as the sequential stimulation ss mechanism if the ss mechanism is realistic it would undermine assumptions that are made for the estimation of the minimum principal stress and unstimulated hydraulic diffusivity we investigated the effect of injection pressure on induced seismicity for injection at constant pressure there was not a signifi

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

comment on blundy and holland s 1990 calcic amphibole equilibria and a new amphibole plagioclase geothermometer

origin of xenocrystic garnet and kyanite in clinopyroxene hornblende bearing adakitic meta tonalites from cape hinode prince olav coast east antarctica

paleomagnetic analysis of fold kinematics and implications for geological models of the cantabrian asturian arc north spain the ponga unit lies in the core of the cantabrian asturian arc a thrust system which was emplaced and then noncoaxially refolded during the variscan orogeny alternative models for thrust system geometry prior to refolding of the ponga unit have been proposed although the ponga unit becomes populated with steeply plunging folds under each model the alternative models imply differing kinematic histories for these folds paleomagnetic data were acquired from 22 sites in grey and red carboniferous limestones around these folds in order to determine the rotational histories of the structures and hence to identify the appropriate geological model for initial thrust system geometry low anisotropy of magnetic susceptibility indicated that the samples were suitable for rigid body rotation studies four remanence components were identified on stepwise demagnetization a very low temperature randomly orientated component v a low temperature component l present between 50 c and 200 c 64 349 95 4 4 clustered around the recent field in geographic coordinates an intermediate temperature component i present between 200 c and 400 600 c of reverse polarity and shallow inclination and showing significant between site dispersion and a high temperature component h 30 342 95 21 lying close to the eocene reference direction in geographic coordinates the i component fails a fold test based upon a first deformation phase d1 hanging wall anticline and is therefore a remagnetization the i component consistently gives positive fold tests of the steeply plunging fold structures when the structures are unfolded about their plunging hinges this demonstrates that these folds result from north south shortening of essentially homoclinal westerly dipping d1 hanging wall ramps and that alternative models involving steeply dipping interference folds arising from complex hanging wall deformation during d1 are invalid a partial restoration of the thrust system based on the former model with the paleomagnetic data pinned to a calculated stephanian reference direction shows that the thrust front was approximately north south in trend in the ponga unit as it was farther south in the esla unit where anticlockwise rotations and dextral wrench faulting have subsequently occurred the data also record a tilt to the north of up to 30 which has occurred incrementally since the end of variscan deformation

provenance analysis of the paleozoic sequences of the northern gondwana margin in nw iberia passive margin to variscan collision and orocline development abstract the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges from neoproterozoic to early permian in age its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana or ii an active continental margin or iii a drifting ribbon continent in this paper we present detrital zircon u pb laser ablation age data from 13 samples taken in detrital rocks from the cantabrian zone sequence ranging from early silurian to early permian in depositional age the obtained results together with previously published detrital zircon ages from ediacaran ordovician strata allow a comprehensive analysis of changing provenance through time collectively these data indicate that this portion of iberia was part of the passive margin of gondwana at least from ordovician to late devonian times zircon populations in all samples show strong similarities with the sahara craton and with zircons found in libya suggesting that nw iberia occupied a paleoposition close to those regions of present day northern africa during this time interval changes in provenance in the late devonian are attributed to the onset of the collision between gondwana and laurussia additionally the middle carboniferous to permian samples record populations consistent with the recycling of older sedimentary sequences and exhumation of the igneous rocks formed before and during the variscan orogeny late devonian to permian samples yield zircon populations that reflect topographic changes produced during the variscan orogeny and development of the lithospheric scale oroclinal buckling

timing and structural evolution in the limb of an orocline the pisuerga carrion unit southern limb of the cantabrian orocline nw spain abstract oroclines are the largest scale folds on earth and the process of oroclinal formation is a key topic in tectonics however most studies of oroclines have focused on the hinge areas where the changes in strike and therefore the orocline shape are most obvious in this paper we investigate the deformation mechanisms the timing and the structural and tectonic evolution of the pisuerga carrion unit situated on the southern limb of the cantabrian orocline at the nw of the iberian peninsula the cantabrian orocline located in the variscan belt of western europe has been recently defined as a secondary orocline constraining kinematics and deformation timing our study in the pisuerga carrion unit reveals that an out of sequence thrust system developed and reactivated existing structures by a flexural slip mechanism that was diachronous with respect to oroclinal formation joint analysis of unconformity bounded rock sequences provide a late moscovian age for oroclinal initiation ca 308 ma at least locally additionally comparing those joint sets found in different series we quantify a minimum of 40 counterclockwise vertical axis rotation for the pisuerga carrion unit during the late pennsylvanian

permo triassic anatexis continental rifting and the disassembly of western pangaea crustal anatectites are frequently observed along ocean continent active margins although their origins are disputed with interpretations varying between rift related and collisional we report geochemical isotopic and geochronological data that define an 1500 km long belt of s type meta granites along the andes of colombia and ecuador which formed during 275 223 ma these are accompanied by amphibolitized tholeiitic basaltic dykes that yield concordant zircon u pb dates ranging between 240 and 223 ma a model is presented which places these rocks within a compressive permian arc setting that existed during the amalgamation of westernmost pangaea anatexis and mafic intrusion during 240 223 ma are interpreted to have occurred during continental rifting which culminated in the formation of oceanic crust and initiated the break up of western pangaea compression during 275 240 ma generated small volumes of crustal melting rifting during 240 225 ma was characterized by basaltic underplating the intrusion of tholeiitic basalts and a peak in crustal melting tholeiitic intrusions during 225 216 ma isotopically resemble depleted mantle and yield no evidence for contamination by continental crust and we assign this period to the onset of continental drift dissected ophiolitic sequences in northern colombia yield zircon u pb dates of 216 ma the permo triassic margin of ecuador and colombia exhibits close temporal faunal and geochemical similarities with various crustal blocks that form the basement to parts of mexico and thus these may represent the relict conjugate margin to nw gondwana the magmatic record of the early disassembly of pangaea spans 20 ma 240 216 ma and the duration of rifting and rift drift transition is similar to that documented in cretaceous tertiary rift settings such as the west iberia newfoundland conjugate margins and the taupo lau havre system where rifting and continental disassembly also occurred over periods lasting 20 ma

orocline timing through joint analysis insights from the ibero armorican arc abstract the timing and kinematics of oroclinal bending in the core of the ibero armorican arc iaa has recently been constrained using paleomagnetic data from the cantabrian zone in northern iberia this study analyzes the joint patterns present in rock units deposited pre syn and post oroclinal bending systematic changes in the orientations of tensional joint sets in superimposed stratigraphic units are interpreted to record the progressive stages of oroclinal bending in the core of the iaa time constrains for joint set development are constrained by the known ages of the bounding unconformities that limit the studied stratigraphic units joint azimuth variability in the pre orocline rocks neoproterozoic to pre upper carboniferous is comparable to the present arc curvature of the orocline about 180 the joints in the syn orocline rocks upper pennsylvanian or stephanian 304 to 299 ma show a lower azimuthal variability that is comparable to about 50 70 of the total curvature seen in pre orocline rocks finally post orocline rocks permian contain joints that have uniform azimuths for each set across the entirety of the present day arc together these spatially and temporally distinct joint sets suggest that rotations in the cantabrian zone took place in the upper pennsylvanian during a ca 10 ma time period which agrees well with previous paleomagnetic arguments the data also provides supporting evidence for oroclinal bending by rotation around vertical axes of an initially linear or nearly linear orogenic belt and finally these data highlight the potential power in using tectonic joint sets for constraining thrust belt kinematics in curved orogenic systems when unconformity bounded stratigraphic sequences are present that are coeval with orocline development

geometry of thin skinned tectonic systems in relation to detachment layer thickness in sedimentary basins characterizing features of both extensional and compressional thin skinned systems may be controlled by the thickness of the underlying mobile material within extensional fault arrays detachment layer thickness in conjunction with detachment dip controls variations in fault spacing and polarity because rigid fault blocks are only free to rotate as long as there is enough underlying mobile material to redistribute between the fault block keels different styles of extensional fault array range from single polarity arrays with closely spaced faults thin detachment to arrays where the fault blocks rotate until the fault blocks separate from one another thick detachment departure from this relationship depends on the amount of internal deformation that can be sustained by the domino fault blocks within contractional zones detachment folds occur where the amount of mobile material can fill the evolving fold core via redistribution thrusts occur where there is insufficient mobile material to sustain detachment fold growth the variations in extensional and contractional style that are described here in relation to mobile layer thickness can be combined to give an idealized model of regional variation in structural style in relation to regional variation in stratigraphy applying this model to a salt basin an allochthon in the area of thickest salt may be characterized by updip raft tectonics and downdip detachment fold belts this style would be laterally transitional toward areas of thinner or lower net salt to updip domino fault arrays and downdip thrust systems

a classification of thrust fronts variations in structural style at thrust fronts are examined to determine whether they represent different stages of typical thrust sheet development or unique events as orogenic stresses declined thrust fronts are classified as buried or emergent in which either the sole thrust died out in the subsurface or emerged to the synorogenic surface buried thrust fronts are subdivided into two types type 1 fronts end in broad zones of low strain type 2 fronts end in zones of high strain emergent thrust fronts may display significant horizontal displacement strongly emergent or limited displacements of about 1 km weakly emergent tip stick thrust fronts exhibit high strains and out of sequence thrusting different types of thrust fronts are linked by transitional tip li e anticlines and strike slip zones most thrust front types represent different stages of typical thrust sheet development but those in type 1 display atypical development unique to thrust fronts

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex