p t t evolution of ultrahigh temperature granulites from the saxon granulite massif germany part ii geochronology the granulites of the saxon granulite massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near isothermal decompression this raises the question of whether p t t data on the peak metamorphism may still be retrieved with confidence felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate p t estimates with isotopic ages presented in a companion paper the assemblage garnet clinopyroxene in mafic granulite records peak temperatures of 1010 1060 c consistent with minimum estimates of around 967 c and 22 3 kbar obtained from the assemblage garnet kyanite ternary feldspar quartz in felsic granulite multiple partial overprint of these assemblages reflects a clockwise p t evolution garnet and kyanite in the felsic granulite were successively overgrown by plagioclase spinel plagioclase sapphirine plagioclase and biotite plagioclase most of this overprinting occurred within the stability field of sillimanite garnet clinopyroxene in the mafic granulite were replaced by clinopyroxene amphibole plagioclase magnetite the high p t conditions and the absence of thermal relaxation features in these granulites require a short lived metamorphism with rapid exhumation the ages of peak metamorphism 342 ma and shallow level granitoid intrusions 333 ma constrain the time span for the exhumation of the saxon granulites to about 9 ma

modelling palaeostress magnitude and age in extensional basins a case study from the mesozoic bristol channel basin u k analysis of normal faults and extensional veins developed in the upper triassic and lower jurassic succession exposed along the bristol channel coast indicate a protracted period of rift related deformation from late triassic to early cretaceous during this period it is demonstrated that a consistently oriented stress system operated with 3 oriented ne sw but with stress ratios 2 1 3 varying from 0 9 0 1 two approaches are described to estimate rifting stress magnitudes the first involves data from synsedimentary faults and yields for late triassic rifting 1 0 714 mpa 2 0 169 mpa and 3 0 033 mpa the second is developed for faulting in the lower jurassic section where no direct evidence of age is available the method calculates stress magnitudes with the rifting stress ratio of 0 9 for varying increments of overburden load each increment represents the possible magnitude of tectonic stresses at the time indicated by the amount of burial by incrementally adding the estimated remaining lower jurassic lower cretaceous overburden load to the stress magnitudes of each case a plot of reducing stress ratio for each age of faulting is determined and that which best reproduces the range of stress ratios calculated represents the modelled estimate of stress magnitude and timing the results suggest an end early jurassic onset of faulting with principal stress magnitudes of 1 12 98 mpa 2 12 56 mpa 3 8 80 mpa several simplifying assumptions used in the analysis are discussed

relations between surface deformation fault geometry seismicity and rupture characteristics during the el asnam algeria earthquake of 10 october 1980 abstract the el asnam earthquake of october 10 1980 ms 7 3 produced surface faulting on a northeast trending thrust fault of 30 km length with displacements of up to 6 5 m though average displacements were about 3 m in addition widespread tensional features were formed some in clear association with folding above the thrust and others in an area beyond the exposure of the thrust at the surface which may be related to buried reverse faults the observed thrust fault is split into southern central and northern segments local and teleseismic data are examined to show that the main shock nucleated at the southwest end of the fault and propagated 12 km northeast where a second rupture of approximately equal moment occurred continuing the faulting a further 12 km northeast along the central segment both ruptures nucleated at about 8 10 km depth displacements were largest on the central segment where they were probably enlarged by aftershocks including one of mb 6 1 three hours after the main shock the northern segment was much shorter than the other two and showed smaller displacement the junctions between fault segments are marked by distinct geomorphological characteristics and a change in strike of the faulting as well as a sudden drop in the observed displacement it appears that the rupture development is influenced by the changes in fault geometry between segments and that such junctions or barriers have persisted through much of the late quaternary

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

metamorphic evolution during deep subduction and exhumation of continental crust insights from felsic granulites in south altyn tagh west china

control of the geometry of precursor brittle structures on the type of ductile shear zone in the adamello tonalites southern alps italy abstract amphibolite facies ductile shear zones developed during the immediate post intrusive cooling history of the adamello tonalites southern alps italy shear zones include i thin a few mm s thick fault like shear zones that accommodate shear strain values up to several 100 s the dominant type ii mylonitic horizons dm s thick in sharp contact with the undeformed wall rock iii continuous shear zones with sigmoidal shaped s or composite s c foliations a transition between the different types occurs along strike over short distances discontinuous shear zones are also localized on pegmatite dykes and quartz veins and both i and ii are discontinuously outlined by quartz layers detailed surface mapping reveals that most fault like shear zones are arranged en echelon mainly forming contractional steps markers crosscut by shear zones allow the displacement to be measured at several positions along shear zones and this reveals very steep displacement gradients close to the shear zone tips differential displacement is mainly accommodated at contractional steps by the development of foliated domains geochemical analyses of major and trace elements show that there is no compositional change along strain gradients the overall features are consistent with nucleation of shear zones on former sets of en echelon joints in many cases intruded by pegmatite dykes or filled with quartz reactivation of joints produced strongly localized shear zones whereas broader foliated zones evolved from the contractional jogs between adjacent stepped joint terminations during progressive shearing these jogs were progressively involved in the accommodation of shear displacement and overall shear zone development

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

prediction of fracture induced permeability and fluid flow in the crust using experimental stress data the results of a series of two dimensional photoelastic experiments on dilational jogs cut in perspex sheets have been used to determine the second order fracture patterns and fluid flow network associated with tectonically loaded dilational jogs the stress data derived from these experiments orientation of the principal stresses and the distributions and magnitudes of the differential stress and the mean stress are used in combination with the theory of brittle failure to predict the orientation approximate distribution and likelihood of formation of second order brittle structures associated with the modeled jog geometries the orientations of brittle structures predicted compare favorably with those found in natural large scale dilational jogs such as the vienna basin the dead sea basin and the brawley jog california as well as in mesoscopic examples and those formed in analog models from the experimental results it is possible to determine areas of high and low mean stress and by assuming that fluid migration occurs in response to mean stress gradients it is possible to predict the fluid migration associated with the development of a dilational jog in this way it is possible to show that the intrajog region of underlapping and neutral dilational jogs will not be favorable sites for fluid ingress when the jog has an overlapping geometry the intrajog region is a mean stress low and consequently fluid ingress into the jog is more likely in addition as the fault overlap increases the distribution of mean stress and thus the associated fluid flow in the intrajog region increases in complexity by combining the predicted second order fracture patterns and the mean stress distribution derived from the photoelastic analysis it is possible to construct a map showing sites of likely fluid accumulation and mineralization these sites correspond to regions of low and high fluid throughflow respectively predictions based on this model compare well with natural examples of fluid accumulation and mineralization in dilational jogs

diamond from the dabie shan metamorphic rocks and its implication for tectonic setting diamond occurs in ultrahigh pressure metamorphic rocks from dabie shan anhui province eastern china diamond bearing rocks include eclogite gamet pyroxenite and jadeitite diamond occurs in a mineral assemblage with coesite and jadeite the diamonds and diamondiferous rocks of dabie shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the yangtze continental plate during the early mesozoic at greater than 4 0 gigapascals and 900 c this interpretation is based on the distribution of rock units the stability field of diamond and isotopic data indicating a crustal origin for the rocks most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across although some are up to 700 micrometers across

exhumation mechanisms of melt bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates a series of 2d petrological thermomechanical numerical experiments was conducted to i characterize the variability of exhumation mechanisms of ultrahigh pressure metamorphic uhpm rocks during collision of spontaneously moving plates and ii study the possible geodynamic effects of melting at ultrahigh pressure conditions for the exhumation of high temperature ultrahigh pressure metamorphic ht uhpm rocks to this end the models include fluid and melt induced weakening of rocks five distinct modes of exhumation of u hpm rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows vertical crustal extrusion large scale crustal stacking shallow crustal delamination trans lithospheric diapirism and channel flow the variation in exhumation mechanisms for u hpm rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and or did not include fluid and melt induced weakening of rocks this difference in the range of exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid and melt related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments such as slab breakoff vertical crustal extrusion large scale stacking shallow crustal delamination and relamination and eduction of the continental plate consequently the significance of channel flow for the exhumation of uhpm rocks may have been overstated based on the results of the earlier numerical experiments in addition the results from this study extend over a larger proportion of the high temperature range of p t conditions documented from uhpm rocks including those retrieved from ht uhpm rocks than the results of experiments from previous numerical models in particular the highest peak metamorphic temperatures up to 1000 degrees c are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab nonetheless some extreme temperature conditions which have been suggested for the kokchetav and bohemian massifs perhaps up to 1100 1200 degrees c are still to be achieved in experiments using numerical models

variscan veins record of fluid circulation and variscan tectonothermal events in upper palaeozoic limestones of the moravian karst czech republic numerous variscan syntectonic calcite veins cross cut palaeozoic rocks in the moravian karst a structural petrographic and stable isotopic analysis of the calcite veins and a microthermometric study of fluid inclusions in these vein cements have been carried out to determine the origin of the variscan fluids and their migration during burial and deformation the isotopic parameters of white older more deformed and rose younger calcites are 87 sr 86 sr is between 0 7078 and 0 7082 white and 0 7086 rose 18 o is between 17 7 and 26 1 white and between 14 8 and 20 7 smow rose 13 c ranges from 0 1 to 2 5 white and from 0 3 to 1 6 v pdb rose the isotopic signatures point to precipitation in an older fluid system buffered by the host rock white calcites and to an open younger fluid dominated system rose calcites parent fluids h 2 o nacl system had salinities between 0 35 and 17 25 eq wt nacl the pressure corrected and confined homogenization temperatures suggest formation of the calcite veins from a fluid with a temperature between 120 and 170 c a pressure of 300 880 bar at a depth between 2 1 and 3 2 km the fluids were most likely confined to a particular sedimentary bed as a bed scale fluid migration white older calcite veins or later to a pile of palaeozoic sediments as a stratigraphically restricted fluid flow rose younger calcite veins the low temperatures and pressures during precipitation of calcites which took place close to a peak of burial deformation confirm the distal position of the moravian karst region within the variscan orogen

earthquake faulting as a structural process abstract structural geology is concerned with the history of movement in the earth s crust and the processes by which displacements occur in the upper one third to one half of deforming continental crust displacement is accommodated largely by seismic slip increments on existing faults it follows that earthquakes and related processes are an integral part of structural geology traditionally structural geologists have been preoccupied with the complexity of the finite deformation within fault zones and with the stress states prevailing at the initiation of faults in intact crust future structural work should be directed more towards understanding the dynamic character of fault reactivation during incremental slip and related effects questions of interest include rheological and geometrical controls on the initiation perturbation and termination of ruptures directivity effects associated with rupture propagation the recognition of structures resulting from repeated stress cycling within seismogenic crust and identification of structural features diagnostic of shear stress levels during faulting structures arising from the inter relationships between slip episodes and induced fluid flow are of special importance because these dynamic fault processes appear influential in the development of much fault hosted mineralization mesothermal gold quartz lodes hosted in high angle reverse shear zones of mixed brittle ductile character form illustrative examples of structures that arguably can only be interpreted by seismo structural analysis embodying the concepts listed above

phase equilibria modelling and zircon age dating of pelitic granulites in zhaojiayao from the jining group of the khondalite belt north china craton the jining group occurs as the eastern segment of the khondalite belt north china craton and is dominated by a series of granulite facies rocks involving normal pelitic granulites recording peak temperatures of 850 c and ultrahigh temperature uht pelitic granulites recording peak temperatures of 950 1100 c the p t paths and ages of these two types of granulites are controversial three pelitic granulite samples in the jining group comprising two sillimanite garnet gneiss samples j1208 and j1210 and one spinel garnet gneiss sample j1303 were collected from zhaojiayao where normal pelitic granulites occur for determination of their metamorphic evolution and ages samples j1208 and j1210 are interpreted to record cooling paths from the tmax stages with p t conditions respectively of 870 890 c 7 8 kbar and 840 c 7 5 kbar constrained from the stability fields of the observed mineral assemblages and the isopleths of plagioclase garnet and biotite compositions in pseudosections sample j1303 is interpreted to record pre tmax decompression from the kyanite stability fields to the tmax stage of 950 1020 c 8 9 kbar and a post tmax cooling path revealed mainly from the stability field of the observed mineral assemblage the plagioclase zoning and the biotite composition isopleth in pseudosections the post tmax cooling stage can be divided into suprasolidus and subsolidus stages the suprasolidus cooling may not result in an equilibrium state at the solidus in a rock therefore different minerals may record different p t conditions along the cooling path the inferred maximum temperature is commonly higher than the solidus as well as different solidi being recorded for different samples from the same outcrop but experiencing different degrees of melt loss plagioclase compositions especially its zoning in plagioclase rich granulites are predicted to be useful for recording the higher temperature conditions of a granulite s thermal history the three samples studied seem to record the temperature range covering those of the normal and uht pelitic granulites in the jining group suggesting that uht conditions may be reached in normal granulites without diagnostic uht indicators la icp ms zircon u pb data provide a continuous trend of concordant 207pb 206pb ages from 1 89 to 1 79 ga for sample j1210 and from 1 94 to 1 80 ga for sample j1303 these continuous and long age spectrums are interpreted to represent a slow cooling and exhumation process corresponding to the post tmax cooling p t paths recorded by the pelitic granulites which may have followed the exhumation of deeply buried rocks in a thickened crust region resulted from a collision event at c 1 95 ga as suggested by the pre tmax decompression p t path

lateral fluid flow in a compacting sand shale sequence south caspian basin the article lateral fluid flow in a compacting sand shale sequence south caspian basin by john d bredehoeft rashid d djevanshir and kenneth r belitz aapg bulletin v 72 no 4 april 1988 p 416 424 needs the following corrections there are several typographical errors that were the author s error equation 2 should be equation 2 equation 3 should be equation 3 although the equations were incorrect in the article the associated discussion and equations 4 and 5 are based upon the correct expressions as shown above table 3 the calculated regional sand permeabilities is also incorrect the hydraulic conductivity which in our case has the units of meters per second was incorrectly converted to millidarcys values in meters per second were multiplied by the standard conversion 1 m sec equals 1 04 108 md in retrospect this conversion was a conceptual error table 3 should read table 3 calculated regional sand permeabilities acknowledgment we appreciate the comments of stan lohman who brought our attention to these errors end of article last page 1525

evidence of former stishovite in uhp eclogite from the south altyn tagh western china abstract we report here for the first time long prism needle shaped polycrystalline quartz aggregates and square parallelogram shaped single phase quartz inclusions in omphacite and garnet of ultrahigh pressure uhp eclogite from the yinggelisayi area in the south altyn uhp belt based on their morphology and mineral composition these quartz inclusions in omphacite and garnet are quartz paramorphs after stishovite the minimum peak pressure of the eclogite is estimated to be 8 9 gpa at 800 1000 c based on the stability field of stishovite this new evidence together with previous discovery of kyanite spineal exsolutions in former stishovite of the pelitic gneiss from the same region suggests a ultra deep subduction and exhumation of the south altyn continental rocks to from mantle depths in the stability field of stishovite evidence of ultra deep subduction of continental materials might be more common and diverse than previous thought

timescales of exhumation and cooling inferred by kinetic modeling an example using a lamellar garnet pyroxenite from the variscan granulitgebirge germany we present a numerical modeling approach to infer timescales of the exhumation and cooling history recorded in the chemical composition of minerals in a garnet pyroxenite from the granulitgebirge saxony germany the studied sample contains remarkable exsolution textures from former megacrysts that produced up to millimeter wide alternating lamellae of garnet grt and clinopyroxene cpx compositional profiles of major and minor elements measured with the electron microprobe perpendicular to the grt cpx interfaces reveal systematic zoning patterns for fe mg al si cr ti in clinopyroxene and ca fe mg mn in garnet respectively in addition to simple thermal modeling that is used to constrain the conditions of emplacement of the granulitgebirge massif at shallow crustal levels we combine thermodynamic data with a numerical finite difference scheme that simulates growth and simultaneous diffusive exchange between garnet and clinopyroxene along a virtual cooling path the latter model assumes local equilibrium at the interface diffusive fluxes are constrained by mass balance it is shown that zoning patterns such as fe mg exchange between garnet and clinopyroxene can be used to extract cooling rates and thus timescales of exhumation while the profiles for the minor elements are provisionally related to the growth history of the lamellae furthermore zoning profiles in the lamellae can only be reproduced with ultrahigh cooling rates similar to contact metamorphic conditions this in turn suggests that the massif was emplaced at temperatures above 900 c in agreement with the observed spatial extent of a contact aureole within low grade metasedimentary rocks surrounding the granulite massif as predicted by thermal modeling exhumation of the massif without cooling below 900 c requires an exhumation rate of several centimeters per year thus we propose an almost isothermal exhumation period of 1 ma followed by isobaric cooling from 900 to 600 c within less than 10 ka

bridge structures as sense of displacement criteria in brittle fault zones abstract fault zones often occur as arrays of en echelon fractures there are two main types of en echelon patterns involving right or left stepping fractures in such arrays first generation fractures define compressive or tensile bridges the structural features of such natural bridges at any scale can be used as criteria to determine the sense of displacement small compressive bridges provide criteria based on the increase in linear density of solution seams related to the degree of asymmetry of the fault trace and to the sequence of formation of the different types of en echelon fractures the criteria deduced from large compressive bridges involve upward displacement of the material by folds and hill or mountain ranges asymmetry of the fault profile and relative chronology of the segments tensile bridges yield criteria based upon vein fillings asymmetry and relative chronolog in large scale structures some pull apart basins are shown to be bounded by normal faults parallel to but outside the strike slip segments from a comparison of analytical models with well known large scale fault zones a new criterion can be proposed in fault zones formed through coalescence of en echelon fractures the bridging segments are wide if facing leeward of the bulk movement sense and narrow if forward facing

possible paleostress tensor configurations derived from fault slip data in eastern vermont and western new hampshire the configuration of six possible paleostress tensors have been derived from 152 faults measured in eastern vermont and western new hampshire populations of potentially genetically related faults were separated using two techniques tensor configurations for each population were derived using a linear least squares inversion method based in part on the work of reches 1987 and a grid search inversion method which tests over 100 000 possible tensors for compatibility with all or portions of the data faults belonging to the oldest population set r n 24 occur primarily in high grade rocks this set is composed of semiductile northeast trending reverse and west northwest trending left lateral faults fault fabrics of quartz rods and thin mylonite layers suggest that the host rocks were at crustal levels of 8 10 km during faulting the derived tensor indicates a roughly east west 1 and near vertical 3 set r faults are offset by normal faults sets n1 and n2 and are interpreted to be pre mesozoic in age perhaps related to late paleozoic alleghanian compression normal and normal oblique faults mineralized with chlorite calcite and strained quartz n 73 have been separated into two populations sets n1 n 49 and n2 n 24 even though these faults are likely of similar age host rocks were probably at moderate crustal depths of perhaps 5 km during faulting faults of both sets are most likely related to mesozoic rifting tensor configurations indicate that 1 plunges steeply northwest n1 and southeast n2 and 3 plunges gently roughly east west these faults cut presumably mesozoic age dikes and are themselves offset by normal oblique faults set t and right lateral faults set rl strongly deviatoric near vertical 1 suggests a thermally driven tectonic regime during the development of normal faulting in new england set t n 25 is composed of normal oblique slip faults mineralized similarly as sets n1 and n2 the plunge of 1 is 50 n and 3 plunges gently east field relations indicate set t faults document the transition from normal sets n1 and n2 to strike slip faulting regimes set rl a compatible sequence of mesozoic stress fields is suggested de boer et al 1988 north northeast trending right lateral faults of set rl n 7 are mineralized with quartz limonite calcite chlorite calcite twins are bent and fractured and quartz grains show minor internal strain suggesting that these faults developed while host rocks were at relatively shallow crustal levels the plunge of 1 is gently northeast and 3 plunges gently east southeast seven distinctly mineralized feldspar siderite quartzoalcite 1imonite chlorite gently northwest dipping thrust faults of set f define the sixth tensor these faults are cut by essentially vertical east side up faults and are assumed to be mesozoic in age by analogy with similar faults in western massachusetts

the armenian earthquake of 1988 december 7 faulting and folding neotectonics and palaeoseismicity summary the spitak earthquake of 1988 december 7 is the first well documented event directly associated with surface breaks in the transcaucasian regions of the ussr the earthquake was located within the ese wnw oriented pambak sevan thrust and fold zone corresponding to the southern front of the lesser caucasus the mechanism of the earthquake is consistent with the nearly ns compressive tectonics due to the active continental collision between the arabian block and the russian platform the rupture is composed by several branches two of which reach the surface the first branch oriented n140 begins near the village of alavar has a length of 11 km and disappears at about 4 km se of spitak it consists of a right lateral en echelon system of strike slip faults with a maximum offset of 50 cm the second branch is the main one and breaks for about 8 km between spitak and gekhasar with a general orientation n120 showing reverse faulting dipping to the north with a right lateral component surface ruptures between gekhasar and spitak show either a narrow band of pressure ridges in alluvial deposits and soil or a fault scarp in the bed rocks when soil is absent maximum displacements observed between spitak and gekhasar attain 160 cm of vertical motion and 90 cm of horizontal dextral offset the displacement also varies within this branch showing finer segmentation few secondary deformations are observed some normal faults near gekhasar correspond to the collapse of the uplifted block an anticline fold oriented parallel to the fault scarp and situated along its northwestern prolongation emphasizes the regional compressive tectonics landslides were activated on its flanks a 200 m long reverse fault break observed along the hinge of a secondary fold suggests the occurrence of a blind thrust at depth and that the hidden branches which continue the main central segment towards the nw are associated to surface folding therefore the total length of the fault and fold zone is about 40 km in agreement with the rupture length and seismic moment obtained on the basis of surface wave modelling well developed uplifted terraces in the northern block and subsiding valleys in the southern block indicate past quaternary activity in the fault region palaeoseismological evidence of ancient earthquakes has been recognized in trenches across the spitak fault one old event occurred between 17 000 years bp and the beginning of the formation of present day soil

ultra deep subduction of yematan eclogite in the north qaidam uhp belt nw china evidence from phengite exsolution in omphacite phengite exsolution in omphacite from the yematan eclogite north qaidam uhp belt nw china is described mineralogical investigations show that the precursor omphacite in the yematan eclogite contains up to 1 16 wt k2o and 10 000 ppm h2o experimental studies document this omphacite to be formed at pressures higher than 6 gpa at 900 c the pressure temperature conditions of 3 68 gpa and 892 c for phengite exsolution in omphacite associated with garnet during the exhumation were obtained by using grt omp ph geothermobarometer we conclude that omphacitic clinopyroxene in subducted eclogites may act as a robust medium to transport h2o and potassium deep into the interior of the earth this study suggests that the yematan eclogite in north qaidam uhp metamorphic belt nw china may have been subducted and exhumed from depths of more than 200 km

first finding of microdiamond coesite and other uhp phases in felsic granulites in the moldanubian zone implications for deep subduction and a revised geodynamic model for variscan orogeny in the bohemian massif abstract heavy mineral fractions and polished thin sections from felsic granulites from the moldanubian zone of the bohemian massif were thoroughly studied by means of raman microspectroscopy combined with optical microscopy and scanning electron microscopy the following phases were identified among others as inclusions in robust minerals such as garnet and zircon diamond characterized by an intense narrow peak at 1332 cm 1 was found in two inclusions in zircon they have a size of 5 m coesite identified by its very characteristic peak at 520 cm 1 was found in an inclusion in garnet together with quartz coesite has been almost completely transformed into quartz only minor coesite nano domains remain kumdykolite the orthorhombic polymorph of naalsi 3 o 8 characterized by strong peaks at 220 456 and 492 cm 1 occurs either as single crystals or as a part of multiphase inclusions in garnet and in zircon along with other mineral phases such as k feldspar phengite rutile moissanite sic exhibiting the characteristic raman bands at 767 788 and 969 cm 1 occurs as inclusions in garnet diamond and coesite are considered to have formed at the peak ultrahigh pressure metamorphic uhpm conditions kumdykolite has been proposed to be a metastable phase formed during rapid cooling from high temperature moissanite points to extremely reduced conditions during subduction to great depths the finding of uhp phases in felsic granulites in the moldanubian zone is clear evidence for subduction of crustal materials to mantle depths the garnet hosting the uhp phase inclusions usually preserves prograde compositional zoning this in combination with the uhpm mineral inclusions suggests that the felsic material should have passed uhp metamorphism at a low temperature gradient isothermal decompression the commonly accepted model at temperatures of 850 950 c would have substantially modified and homogenized the garnet composition eliminating any compositional zoning therefore the uhpm felsic rocks could have experienced only a short term granulite facies metamorphism after they were exhumed to crustal levels

from orientation to magnitudes in paleostress determinations using fault slip data abstract determinations of reduced stress tensors using fault slip data yield the orientation of principal stress axes and the ratio of the differences between principal stress magnitudes the use of rupture and friction laws allows determination of the two remaining unknowns that is the reconstruction of the complete stress tensor taking into account the depth of overburden brings an additional constraint the method is applied and discussed in the case of the hoover dam site western u s a where large data sets and rock mechanics information are available differences between intact sample and rock mass properties account for apparent disagreements between paleostress levels determined in similar tectonic environments pore pressure plays an important role where information about pore pressure is absent zero and hydrostatic pore pressure cases should be considered as limits

uht metamorphism peaking above 1100 c with slow cooling insights from pelitic granulites in the jining complex north china craton

strike slip relay ramps areas of reorientated bedding at contractional oversteps between strike slip faults are here called strike slip relay ramps metre scale examples are described from the jurassic sediments at east quantoxhead somerset u k larger strike slip relay ramps occur in the rio de peixe basin ne brazil along the newport inglewood trend california and in the bovey basin sw england although the geometry and development of strike slip relay ramps are similar to those of relay ramps in normal fault systems there are differences in the structures which accommodate the transfer of displacement between the overstepping faults whereas strike slip relay ramps are typically transpressional with pressure solution often occurring relay ramps in normal fault systems are dominated by extension or transtension care needs to be taken when interpreting areas of reorientated bedding between overstepping faults particularly when displacement directions are unknown for example when using seismic data this is because relay ramps can occur in both strike slip and normal fault zones

the dinevar transtensional pull apart basin nw zagros mountains iran a geological study and comparison to 2d finite element elastic models the dinevar transtensional pull apart basin is a nw se trending basin located in the northwest part of the zagros mountains sanandaj sirjan tectonic zone iran this basin was formed by oblique extension in a strike slip fault system which was controlled by the pre existing sahneh and the mianrahan faults in addition to a field study we constructed a 2d finite element elastic model based on the major fault segments according to the field study various geological structures are formed around the pre existing fault segments including pencil structures with 13 aspect ratio 19 tight to open asymmetric folds oblique normal faults synthetic and antithetic faults normal and reverse fault drags and negative and hybrid type flower structure fe model results show that around the fault step is a zone of displacement gradient that indicates extension transtension within the overlapping zone consequently subsidence is predicted over the releasing fault step the modelled stress distributions within the fault step and around the fault segment tips are asymmetric elastic modelling shows that the mean normal stress within the step is tensional which would impede plastic yield while the region outside the step is characterised by compressive mean normal stress the mean normal stress pattern is characterised by a regional minimum localised inside the extensional step transtension zone and two local maximum and minimum couples formed at the advancing and retreating sides respectively in the fault segment tips the results show that two main subsidence regions depocentres or sub basins form along and in the proximity of the fault segment tips the imposed relative motion is entirely accommodated by creep on the faults which causes plastic strain to accumulate outside the faults mostly localised within the fault step this localised strain between fault segments could initiate connecting faults or reactivate earlier faults generally at high angles relative to the master fault segments as suggested by the stress and strain patterns the rotation and magnitude variation in the stress and strain trajectories reflects heterogeneous deformation in general the dinevar transtensional pull apart basin is characterised by an elongate rhomboidal to trapezoidal shaped geometry the modelled stress distribution and strain localisations within the basin are in general agreement with the locations of fault segments within the dinevar transtensional pull apart basin

relationships among in situ stress fractures and faults and fluid flow monterey formation santa maria basin california we used borehole televiewer bhtv data from four wells within the onshore and offshore santa maria basin california to investigate the relationships among fracture distribution orientation and variation with depth and in situ stress our analysis of stress induced well bore breakouts shows a uniform northeast maximum horizontal stress sh max orientation in each well this direction is consistent with the sh max direction determined from well bore breakouts in other wells in this region the northwest trend of active fold axes and kinematic inversion of nearby earthquake focal plane mechanisms in contrast to the uniformity of the stress field fracture orientation dip and frequency vary considerably from well to well and within each well with depth fractures can be divided into distinct subsets on the basis of fracture frequency and orientation which correlate with changes of lithology and physical properties although factors such as tectonic history diagenesis and structural variations obviously have influenced fracture distribution integration of the in situ stress and fracture data sets indicates that many of the fractures faults and bedding planes are active small scale strike slip and reverse faults in the current northeast trending transpressive stress field in fact we observed local breakout rotations in the wells providing kinematic evidence for recent shear motion along fracture copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received april 15 1996 revised manuscript received december 27 1996 final acceptance july 15 1997 2department of geophysics stanford university stanford california 94305 2215 we wish to thank tom zalan of the chevron u s a production company for providing the offshore well data unocal corporation for providing the data on the onshore well and marcia mclaren from pacific gas and electric company for providing the earthquake focal mechanisms used in the stress inversion analysis the data used as background seismicity in figure 1 were extracted from the world wide web of the southern california seismic network scsn catalog operated jointly by the seismological laboratory at caltech and the u s geological survey both in pasadena california we appreciate the comments and helpful discussions from daniel moos steve graham and lev vernik

density of morb eclogite in the upper mantle in order to study the role of eclogite subducted oceanic crust in geodynamics we carried out high pressure experiments on natural mid oceanic ridge basalt morb using a multi anvil apparatus we determined phase relations of morb as a function of pressure and temperature in the temperature range of 1200 1700 c and pressures from 5 to 14 gpa the present results differ from a previous high pressure study of morb earth planet sci lett 77 1986 245 conducted at 1200 c as follows 1 coesite coe or stishovite st appears throughout the studied pressure range and 2 the clinopyroxene garnet cpx gar ratio is smaller particularly in the low pressure range 12 gpa our results show that the calculated bulk density of subducted oceanic crust increases discontinuously at around 9 gpa due to the coesite stishovite phase transition the density is considerably higher than estimates based on the previous experiments at pressures less than 12 gpa thus the role of eclogite in driving subduction should be more important than previously thought on the other hand the density difference between peridotite and eclogite in ascending mantle plumes becomes smaller as temperature increases a result of the increase in clinopyroxene garnet sio2 ratio in eclogite with increasing temperature as eclogite is still denser than peridotite even under subsolidus conditions recycled oceanic crust entrained in ascending plumes must be sustained by the upward flow of ambient peridotite in mantle plumes

favoured states of palaeostress in the earth s crust evidence from fault slip data abstract in a major survey of published palaeostress estimates obtained by the use of fault slip data over 2000 stress tensors from shallow crustal levels have been compiled the results are derived from regions where deformation is dominated by brittle processes and consist of incomplete stress tensors in which the orientations of the principal stresses 1 2 3 are known together with the stress ratio 2 3 1 3 the orientations of the stress axes display a preferred orientation with a tendency for one of the axes to be vertical this accords with anderson s assumed standard state stress configuration near to the earth s free surface although the tendency is strong there are frequent deviations from the arrangement in 25 of cases the steepest stress axis deviates by more than 25 from the vertical in undertaking studies of palaeostress it would be therefore unwise to use methods that assume a priori one vertical stress axis unless previous local results indicate this to be the case normal strike slip and thrust stress arrangements with 1 2 and 3 as the steepest stress respectively occur in the database with frequencies approximately in the ratio 2 2 1 in the compiled results the raw stress ratios which have a theoretical range from 0 to 1 show a lack of very high values and to a lesser extent very low values this suggests that triaxial stress states are more common than axial compression deviatoric stress and axial tension deviatoric stress however this relative abundance of triaxial stress states is considered to be a natural feature of a uniform distribution of stress tensors in addition there is a marked bias in the collated ratios towards values less than 0 5 and this is expressed by an overall mean of 0 39 several possible explanations for the distribution of stress ratios are discussed these explanations are related to the tendency for tectonic deformation to be of plane strain type coupled with volume change and to the fact that gravitational loading makes an important contribution to the state of stress in the crust

geochemical evidence of preferential flow of water through fractures in unsaturated tuff apache leap arizona abstract the occurrence and significance of aqueous flow through fractures in unsaturated tuff was investigated at the apache leap research site near superior arizona water samples for geochemical and isotopic analysis were collected from water seeping from fractures in a mine haulage tunnel from the saturated zone in a vertical borehole usw uzp 4 and from both the unsaturated and saturated zones in an angled borehole dsb the geochemistry and14c activity of water samples from the dsb suggest that most of the recharge to the saturated zone has occurred through fractures especially beneath the ephemeral streams evidence of substantial recent recharge through fractures was found in saturated zone samples from the mine haulage tunnel using 3h 34s and so42 cl analyses evidence of partial imbibition of fracture flow into the rock matrix was found at multiple depths throughout the 147 m unsaturated zone at the dsb using geophysical measurements from the borehole water content analyses from core samples and 14c and 3h analyses from pore water extracted from preserved core samples post bomb 14c activity was measured in pore water near fractures just above the saturated zone

crack seal microstructure evolution in bi mineralic quartz chlorite veins in shales and siltstones from the rwth 1 well aachen germany abstract in core samples from the deep geothermal well rwth 1 we studied variscan quartz chlorite veins formed by crack seal processes in siliciclastics at the brittle to ductile transition these sheared veins are common in sections of the well which are interpreted as variscan thrust zones based on image logs and seismic data microstructures interpreted to reflect different stages in the evolution of such crack seal veins suggest the veins started in microcracks sealed by quartz and chlorite to veinlets crossing multiple grains and bundles of veinlets evolving by progressive localization into low angle extensional shear veins and high angle dilational jog veins in the sheared veins chlorite and quartz ribbons show evidence for crack seal and simultaneous ductile shearing during vein evolution forming peculiar fin shaped microstructures in quartz ribbons in high angle dilational jogs fibrous crystals of quartz and chlorite point to multiple crack seal events with simultaneous growth of two different mineral phases this is interpreted to be the basic microstructural process in the veins we extend earlier models of polycrystal growth in fractures and present a series of 2d simulations of the kinematics of crystal growth in these bi mineralic veins for both localized and non localized cracking results are compared with the observed microstructures we show that when the relative growth rates of the two mineral phases are different serrated grain boundaries evolve the similarities between observation and model suggest that the assumption of our model is valid although many second order processes require a more detailed study we propose that the principles observed here can be applied to other bi mineralic crack seal veins

faulting induced by forced fluid injection and fluid flow forced by faulting an interpretation of hydraulic fracture microseismicity carthage cotton valley gas field texas we analyzed precisely located microearthquake data detected during five hydraulic fracture treatments in the carthage gas field of east texas the treat ments were conducted in two adjacent boreholes within interbedded sands and shales of the upper cotton valley formation the microearthquakes were induced within narrow horizontal bands that correspond to the targeted sandstone layers events throughout all the treatments show strike slip faulting occurring uniformly along vertical fractures trending close to maximum horizontal stress direction these events are consistent with the reservoir s prevalent natural fractures known to be isolated within the sands and trending subparallel to the expected hydraulic fracture orien tation when this uniform fracture system was activated exclusively the detected shear deformation measured as the moment release per unit volume of fluid injected was constant independent of various fluid viscosities and flow rates used within the base of the upper cotton valley formation anomalous event counts and moment release occurred within dense clusters that delineate bends or jogs in the fracture zones the mechanisms are also strike slip but the fault planes are more favorably oriented for failure the dense clusters show location patterns diverging in time suggesting the expulsion of fluids from compressive fault jogs fluid flow forced by the adjacent slip induced loading appears to initially extend the treatments but the loading also tends to lock up and concentrate stress at the jogs as evident by fewer but larger events populating the structures as treatments progress as a result effec tive drainage lengths from the boreholes may be shorter than would be inferred from the seismicity extending past the jogs these high moment asperities are similar to dense patches of seismicity observed along creeping sections of the san andreas fault where they have been attributed to localized zones of strength or stress con centration surrounded by larger regions undergoing stable aseismic slip this sim ilarity plus large moment deficits in terms of volume injected suggests a large component of aseismic slip is induced by the cotton valley treatments

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

petrology and geochronology of eclogites from the western segment of the altyn tagh northwestern china abstract the eclogites from the western segment of the altyn tagh occur as lens or boundins within quartzofeldspathic gneisses or pelitic gneisses characterized by amphibolite facies parageneses the petrographic features and reaction textures testify to three main metamorphic stages 1 the peak eclogite facies stage p 15 kbar t 665 880 c 2 the granulitic facies stage p 11 14 kbar t 670 800 c and 3 the amphibolite facies stage p 6 3 9 5 kbar t 619 738 c during decompression the trace element rare earth element and sm nd isotopic data suggest that most of the eclogites have protolith features resembling transitional type t type mid ocean ridge basalt morb the well preserved eclogite was selected for sm nd and u pb isotopic dating the sm nd isotopic data yield a whole rock garnet omphacite isochron of 500 10 ma age the u pb isotopic measurement of zircons from the same eclogite shows that the four grain populations are near concordant and are well plotted on concordia giving a weighted mean age of 503 9 5 3 ma the two different geochronological methods yield a similar age which is interpreted as the peak metamorphic age of eclogites and reflects the existence of a mountain root related to continental subduction during early paleozoic moreover based on the comparison with similar rocks in the northern margin of the qaidam basin a displacement of 350 400 km for the altyn tagh fault have been estimated

relation between permeability and effective stress along a plate boundary fault barbados accretionary complex in situ bulk permeability was measured in a borehole that intersected the decollement zone a low angle detachment fault between the north american and caribbean plates permeability measurements were made at a variety of fluid pressure conditions defining a quantitative relation between bulk permeability and effective stress for this plate boundary fault zone the bulk permeability in this zone changed by several orders of magnitude as a consistent function of fluid pressure this relation may help to explain the dynamics of fluid fault interactions and the transient nature of hydrologic processes during deformation at convergent margins

p t t path of garnetites in south altyn tagh west china a complete record of the ultradeep subduction and exhumation of continental crust

quaternary rate of folding of the ventura avenue anticline western transverse ranges southern california upper quaternary terraces of the ventura river california are uplifted tilted and folded over the ventura avenue anticline rates of uplift and tilting have decreased since inception of the structure over the past 200 ka assuming that the chronology based on amino acid racemization 14c dates and soils correlation is approximately correct then the minimum possible average rate of uplift in the axial region of the fold has decreased from 14 mm yr to 2 mm yr during the past 200 ka interval rates of uplift for the periods 200 ka to 80 or 105 ka 80 or 105 ka to 30 ka and 30 ka to present are respectively about 20 mm yr 9 mm yr and 5 mm yr the rate of tilting shows a similar trend decreasing from 5 8 urad yr 2 5 urad yr and 1 2 urad yr for the same time intervals respectively based on the mechanics of flexural slip folds in stratified sedimentary rocks these data suggest that the rootless ventura avenue anticline is a fold that has been shortening at a relatively constant rate of about 9 mm yr since its inception

preservation exhumation of ultrahigh pressure subduction complexes abstract ultrahigh pressure uhp metamorphic terranes reflect subduction of continental crust to depths of 90 140 km in phanerozoic contractional orogens rocks are intensely overprinted by lower pressure mineral assemblages traces of relict uhp phases are preserved only under kinetically inhibiting circumstances most uhp complexes present in the upper crust are thin imbricate sheets consisting chiefly of felsic units serpentinites dense mafic and peridotitic rocks make up less than 10 of each exhumed subduction complex roundtrip prograde retrograde p t paths are completed in 10 20 myr and rates of ascent to mid crustal levels approximate descent velocities late stage domical uplifts typify many uhp complexes sialic crust may be deeply subducted reflecting profound underflow of an oceanic plate prior to collisional suturing exhumation involves decompression through the p t stability fields of lower pressure metamorphic facies scattered uhp relics are retained in strong refractory watertight host minerals e g zircon pyroxene garnet typified by low rates of intracrystalline diffusion isolation of such inclusions from the recrystallizing rock matrix impedes back reaction thin aspect ratio ductile deformed nappes are formed in the subduction zone heat is conducted away from uhp complexes as they rise along the subduction channel the low aggregate density of continental crust is much less than that of the mantle it displaces during underflow its rapid ascent to mid crustal levels is driven by buoyancy return to shallow levels does not require removal of the overlying mantle wedge late stage underplating structural contraction tectonic aneurysms and or plate shallowing convey mid crustal uhp decollements surfaceward in domical uplifts where they are exposed by erosion unless these situations are mutually satisfied uhp complexes are completely transformed to low pressure assemblages obliterating all evidence of profound subduction

role of fluids in faulting inferred from stress field signatures the stress orientation signature of weak faults containing high pressure fluids has been observed for segments of the san andreas fault system in southern california the inferred lithostatic fluid pressures extend into the surrounding relatively intact rock in a zone scaling with the width of the interseismic strain accumulation repeated strain related fracturing and crack sealing may have created low permeability barriers that seal fluids into the network of currently active fractures

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

first application of titanium in zircon thermometry to ultrahigh temperature metamorphism titanium in zircon thermometry has been applied to evaluate the temperatures of metamorphism for two ultrahigh temperature uht granulite localities the neoproterozoic anapolis itaucu complex in central brazil and lower crustal xenoliths from the neo archean kaapvaal craton in south africa uht metamorphism in the anapolis itaucu complex occurred ca 640 ma ti in zircon thermometry on metamorphic zircon yields temperatures of 965 811 c a population of late zircon that formed from ilmenite and rutile breakdown records temperatures of 780 c maximum temperatures are less than those inferred from quantitative phase equilibria modeling based on textural setting inclusions in peak phases and as matrix grains these zircons likely record growth during the pro grade and or postpeak evolution in contrast the kaapvaal xenoliths record two phases of zircon growth during uht metamorphism at 2720 2715 ma followed by growth during isobaric cooling in kyanite bearing leucosomes ca 2690 ma temperatures recorded by the two phases of growth are 1024 878 c and 936 839 c respectively suggesting that zircon growth occurred during uht conditions and continued throughout the postpeak evolution over a 200 c interval these examples demonstrate that the ti in zircon thermometer is a powerful new tool with the potential to determine crystallization temperatures of zircon during granulite facies metamorphism throughout the prograde to postpeak evolution of such terranes

generation and maintenance of abnormal fluid pressures beneath a ramping thrust sheet isotropic permeability experiments abstract we have investigated the mechanisms responsible for the evolution of excess pore pressures within and beneath a ramping thrust sheet i e fluid flow porosity compression and thermal expansion of water and the sensitivity of pore pressure to a variety of physical parameters e g permeability thrust sheet velocity heat flux coupled pore pressure and temperature equations were solved numerically in two dimensions using a generalized hydrostratigraphy of north american thrust belts because of the lack of either symmetry or a steady state in this problem both deposition and thrust loading stages were simulated the dominant mechanisms controlling pore pressure evolution were fluid flow and compression of pore space by vertical loading thermal expansion of the fluids was found to be insignificant in generating excess pore pressures at common thrust loading rates the results indicate that it is possible to generate high pore pressure to lithostatic pressure ratios within thrust sheets by depositional loading prior to thrusting high values of are generated and maintained during thrust loading for reasonable assumptions about the conditions thought to have existed in thrust belts values of were not constant throughout the model the highest values tended to concentrate near the surface of the model and within and below the toe of the thrust sheet the magnitude and distribution of excess pore pressures and values were found to be especially sensitive to variations in permeability excess pore pressure generation by compression exceeded pore pressure dissipation by fluid flow for permeabilities less than approximately 10 16 m 2 permeabilities greater than approximately 10 16 m 2 produced hydrostatic pore pressure gradients the models demonstrate that permeability inhomogeneity due to lithologic variations may exert a strong control on the magnitude and spatial distribution of excess pore pressures within thrust sheets in addition these models indicate that it is unlikely that fluid pressure is high everywhere in a moving thrust sheet

numerical modeling of fault controlled fluid flow in the genesis of tin deposits of the malage ore field gejiu mining district china a distinct element finite difference model based on geologic data from the tin polymetallic hydrothermal deposits of malage is constructed to simulate the geologic processes of fault controlled fluid migration during hydrothermal ore genesis the history of hydrothermal ore genesis is strongly affected by the interaction between fault deformation and fluid flow an injected fluid induces fault slip by lowering the effective stress across a fault the subsequent movement on the fault enhances the fault aperture which provides a channelway that both facilitates and focuses fluid flow the variation of fluid injection under certain stress conditions causes cyclic movement on faults this cyclic failure gives rise to pulsating fluid flow along the faults which may be responsible in part for the precipitation zoning of mineral deposits as an important structural condition for ore formation in any fault related mineralized region like malage the linking up of fracture networks not only allows appropriate long distance channels for fluid migration but also provides the appropriate conditions for ore deposition at particular structural sites this study indicates that different fluid flow patterns i e changes in fluid flow rate or velocity with time and space can reflect the variation of structural environment for instance the flow velocity is increased gradually over time on the relatively connected faults such as the northwest southeast trending fault and the western half of the east west trending yuanlao fault in the center of the malage ore field these are the main pathways for the ore forming fluids in the smaller secondary faults the flow velocity fluctuates more rapidly with time and space in some instances causing conditions that are favorable to mineral deposition the main ore deposition sites occur on the smaller secondary faults the detailed variation of fluid and aperture conditions modeled in this complex fault array can be related to the formation and location of mineral deposits both predictively and conceptually the modeling also demonstrates that the fault deformation and fluid flow are sensitive to the principal stress orientation if the orientation of sigma 1 is changed by more than 15 degrees there are very different results for the calculated fault motion pattern hydraulic aperture and fluid flow

earthquakes as a coupled shear stress high pore pressure dynamical system the migration coalescence and localization of slip seismicity and zones of high pore pressure are modeled using a porosity reduction mechanism to drive pore pressure within a fault zone in excess of hydrostatic increased pore pressure in discrete cells creates zones of low effective stress which induces slip that may propagate to surrounding cells depending on the local state of stress at slip stress is transferred using the solution for a rectangular dislocation in an elastic half space and pore pressures are redistributed by conserving fluid mass using simple assumptions about fault rheology and permeability it is shown that the interaction between shear stress and effective stress evolves to a state of earthquake clustering with repeated events locked zones and large variations in fault strength the model evolves from a uniform shear stress state on a strong fault to a heterogeneous shear stress state on a weak fault

response of fractured rock subject to fluid injection part i development of a numerical model abstract the selection and development of a numerical model used to investigate the response of fractured rock to injection are described the distinct element method was chosen on the basis of its ability to represent explicitly the fractures and to couple the mechanical deformation to the fluid flow the numerical model was used to simulate fluid injection from a single well into each of four basic model reservoirs the results of these simulations form the basis of the interpretation given in parts ii and iii

subduction of continental crust and terrigenous and pelagic sediments an experimental study abstract an important role for the recycling of terrigenous sediments in the mantle was proposed by armstrong 1 2 aspects of this hypothesis have been tested via an experimental study of phase equilibria in a composition similar to that of average upper continental crust over the pressure interval 6 24 gpa this composition is sufficiently close to those of the principal argillaceous and siliceous classes of pelagic sediments to use the results to interpret the behaviour of these latter sediments in addition to terrigenous sediments during subduction the subsolidus phase relationships displayed by lithologies derived from these materials are complex with major roles being played by k hollandite garnet na clinopyroxene and stishovite continentally derived lithologies achieve densities similar to or greater than surrounding mantle at depths greater than 200 km the geodynamical implications of these buoyancy relationships are discussed a reconnaissance study has also been made to assess the melting and element partitioning behaviours in the subducted continental crust lithologies during partial melting at relatively low pressures 5 10 gpa orthoclase wadeite and k hollandite are eliminated near the solidus whereas the stability field of na clinopyroxene extends to temperatures well above the solidus liquids formed by small to moderate degrees of partial melting of subducted continentally derived lithologies in this pressure interval have high k na ratios and high sio2 contents at higher pressures 16 24 gpa the stability fields of k hollandite and stishovite extend towards the liquidus whereas sodium bearing phases are eliminated much closer to the solidus resultant partial melts consequently possess relatively low k na ratios and sio2 contents lead possesses a high crystal liquid partition coefficient in k hollandite whereas uranium is excluded the broad stability field of k hollandite during partial melting of these lithologies at pressures above 15 gpa therefore has the capacity to cause fractionation of lead from uranium the petrological and geochemical implications of these results are discussed in the context of the partial melting of terrigenous materials during subduction and the interactions of the partial melts with the surrounding mantle

strike slip fault bridge fluid pumping mechanism insights from field based palaeostress analysis and numerical modelling abstract we present a finite element study of stress perturbation in evolving compressive and extensional strike slip fault bridges the results are compared with a fracture study of a compressive bridge at st donats uk horizontally interbedded calcareous mudstone and bioclastic calcilutite at st donats have a distinct vertical permeability anisotropy this sedimentary sequence behaves as a set of horizontal aquifers the fluid flow in these aquifers is sensitive to mean stress gradients paleostress analysis of field fracture data verified by finite element modelling indicates a rotation of 1 towards parallelism with boundary faults inside the growing compressive bridge boundary faults and bridge faults recorded numerous fluid flow events the modelled mean stress pattern shows a regional maximum within the bridge and local maxima minima pairs at boundary fault tips finite element modelling of an extensional bridge indicates that 3 rotates towards parallelism with boundary faults the mean stress pattern is similar to the pattern in compressive bridge but with maxima and minima locations interchanged the stress patterns are reestablished by each stress build up preceding the rupturation of the boundary faults throughout the development stages of strike slip fault bridges mean stress gradients developed pre failure control the fluid flow in fractures of the strike slip fault system at and after the end of each stress build up and the fluid flow in boundary faults post failure fracture reactivation and new fracture generation within an evolving bridge is a process consisting of multiple successive events that retain the storage capacity of the bridge rupture and sealing of the main bounding faults is a step wise process that opens and closes fluid conduits between areas with different pressures

a comparison of the evolution of diamondiferous quartz rich rocks from the saxonian erzgebirge and the kokchetav massif are so called diamondiferous gneisses magmatic rocks abstract the petrography and chemical composition of minerals of quartz rich diamondiferous rocks from the kokchetav massif especially the zonation of garnet were studied and compared with diamondiferous quartzofeldspathic rocks from the saxonian erzgebirge many compositional and textural features were found to be similar for instance microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks on the basis of experimental data a magmatic scenario was constructed to check if the quartz rich diamondiferous rocks are of magmatic origin by this the p t paths derived here for the kokchetav rocks and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts these melts originated by anatexis of deeply submerged metasediments erzgebirge at t as high as 1200 c kokchetav massif at 50 100 c lower t and ascended from at least 200 km depth relics of the pre anatectic evolution are still present for instance as garnet cores after ascent and emplacement of the magma in deep portions of thickened continental crust kokchetav massif 45 50 km close to 800 c erzgebirge 55 60 km at 30 50 c lower t considerable quantities of white and or dark micas formed by peritectic reactions from melt for instance garnets could be resorbed at this stage and biotite grew instead after the magmatic stage retrogression took place much stronger in the kokchetav massif this was accompanied by deformation transforming broadly the magmatic texture of quartz rich diamondiferous rocks from the kokchetav massif to a gneissic texture

linking thermal hydrological and mechanical processes in fractured rocks abstract in this paper an overview is presented of coupled processes linking thermo hydro mechanical thm effects in fractured rocks a formulation is first presented to show the linkage mathematically which can be used as a basis for numerical solutions and for further developments two simple examples of hydromechanical hm and thermo hydro mechanical thm coupled processes are discussed to convey physical insight into such couplings finally three large scale long term experiments currently under way are described these are being conducted specifically to study coupled processes in situ

recycling of crustal materials through study of ultrahigh pressure minerals in collisional orogens ophiolites and mantle xenoliths a review abstract newly recognized occurrences of ultrahigh pressure uh p minerals including diamonds in ultrahigh temperature uh t felsic granulites of orogenic belts in chromitites associated with ophiolitic complexes and in mantle xenoliths suggest the recycling of crustal materials through deep subduction mantle upwelling and return to the earth s surface this circulation process is supported by crust derived mineral inclusions in deep seated zircons chromites and diamonds from collision type orogens from eclogitic xenoliths in kimberlites and from chromitities of several alpine himalayan and polar ural ophiolites some of these minerals contain low atomic number elements typified by crustal isotopic signatures ophiolite type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone in addition to ringwoodite and inferred stishovite a number of nanometric minerals have been identified as inclusions employing state of the art analytical tools reconstitution of now exsolved precursor uh p phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier uh p conditions for example tibetan chromites containing exsolution lamellae of coesite diopside suggest that the original chromitites formed at p 9 10 gpa at depths of 250 300 km the precursor phase most likely had a ca ferrite or a ca titanite structure both are polymorphs of chromite and at 2000 c would have formed at minimum pressures of p 12 5 or 20 gpa respectively some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin including zircon feldspars garnet kyanite andalusite quartz and rutile the zircons possess much older u pb ages than the time of ophiolite formation these uh p mineral bearing chromitite hosts evidently had a deep seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes these new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere recycling of surface organic carbon into the lower mantle and ascent to the earth s surface through mantle upwelling intensified study of uh p granulite facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling

pore fluid and seismogenic characteristics of fault rock at depth on the wasatch fault utah fluid inclusions and structural fabric in fault rock in the exhumed footwall of the wasatch fault allow measurement of fluid pressure temperature and composition and estimation of rheological characteristics of fault behavior hydrothermally altered and deformed oligocene quartz monzonite of the little cottonwood stock forms a partially preserved footwall carapace at the southern end of the salt lake segment of the wasatch normal fault zone cataclasite and phyllonite contain two syndeformational alteration mineral assemblages that formed during progressive faulting and displacement of the footwall an early epidote chlorite sericite magnetite alteration assemblage developed in phyllonite and cataclasite and a later laumontite prehnite hematite alteration assemblage developed during cataclasis fluid inclusions associated with the formation of alteration minerals are preserved in healed fractures in quartz grains fluid inclusions that record the transition temperature from quasi plastic flow to frictional deformation are associated with hydrothermal alteration to chlorite epidote and sericite and were trapped at minimum temperatures of 223 c 353 c and minimum fluid pressures of 1150 2800 bars the fluids average 13 mol co2 and 8 2 wt nacl a radiometric age of hydrothermal sericite suggests that the age of the alteration is 17 6 0 7 m y if maximum fluid pressure is near lithostatic the minimum depth of formation of the alteration was 11 km at 337 c consistent with a thermal gradient of 30 c km the ratio of pore fluid pressure to lithostatic pressure varied from 0 52 to nearly 1 0 and indicates that effective normal stress on the fault at these depths may have approached zero structural fabrics and fluid inclusion characteristics are used to estimate seismogenic characteristics of the fault cleavage lineation and s c bands in the phyllonite indicate that deformation at temperatures above 350 c and depths greater than 11 km was dominated by aseismic creep unstable fracture propagation in cataclasite developed during seismic slip at shallower levels the downdip seismogenic length of the salt lake segment is estimated as 16 km using a fault zone dip of 45 and a minimum depth of 11 km for the quasi plastic frictional transition the segment strike length is 35 km and the seismogenic area is about 600 km2 compatible with generation of m 7 0 earthquakes

thermobarometry and geotectonic significance of high pressure granulites examples from the moldanubian zone of the bohemian massif in lower austria

flexural slip folding along the eastern elmore ranch fault in the superstition hills earthquake sequence of november 1987 co seismic flexural slip folding of a small anticline along the eastern elmore ranch fault associated with the superstition hills earthquake sequence demonstrates that variations in left lateral slip can be explained locally by folding surface rupture on the eastern branch of the elmore ranch fault was greatest to the northeast of the fold but decreased to zero across the southern limb a kilometer long surface slip gap exists just southwest of the fold being bounded on the northeast with 75 mm of slip and to the southwest with about 60 mm of slip lateral slip continues southwesterly to the faults intersection with the superstition hills fault the gentle to openly folded moderately plunging upright asymmetric anticline experienced left lateral slip on the northern limb and right oblique slip on the southern limb with maximum documented lateral slip along bedding planes of 10 mm slip values were greatest on the limbs and decreased to zero at the fold hinge which is consistent with the flexural slip folding process secondary left lateral faults ruptured both limbs of the anticline and bedding plane slip was not observed hingeward of these faults these data suggest that left lateral strain associated with seismogenic fault rupture was continuous in the subsurface but was accommodated locally by folding at the surface

diamond former coesite and supersilicic garnet in metasedimentary rocks from the greek rhodope a new ultrahigh pressure metamorphic province established abstract we report here the first discovery of ultrahigh pressure uhp indicator minerals and textures from the rhodope metamorphic province rmp northern greece in particular we document the exsolution of quartz rutile and apatite in sodic garnet from metapelites garnet biotite kyanite gneisses which attests to the presence of a si ti na p rich precursor garnet phase similar textures in garnet have been reported in the literature only once before for eclogites from the su lu uhp metamorphic province china we also document the presence of microdiamonds and multicrystalline quartz pseudomorphs after coesite included in garnet from both eclogites and metapelites we then argue that these rocks had once been transported to depths exceeding 220 km well into the upper mantle thus establishing the rmp as another important uhp metamorphic belt in the world

characterization of stress perturbations near major fault zones insights from 2 d distinct element numerical modelling and field studies jura mountains abstract we consider and discuss the presence of discontinuities in the crust as a major source of stress perturbations based on 2 d distinct element modelling we reconstruct the local stress field around a vertical discontinuity in various geological contexts the resulting stress distribution reveals that major directional stress changes occur near the tips of the discontinuity so that stress deviations can reach values as large as 50 we establish simple relationships controlling stress changes around a pre existing fault zone as a function of 1 the remote differential stress magnitude 1 3 2 the friction coefficient on the discontinuity and 3 the strike of the discontinuity relative to the far field stress as a geological example we present the morez fault zone in the internal jura paleostress reconstruction in forty two sites indicates that the trends of the mio pliocene compression are n110 on average near the fault whereas they are n130 in the surrounding areas a comparison between the results of the tectonic study and those of theoretical modelling suggests that the 20 counterclockwise deviation is directly related to the reactivation of this large weak zone we thus evaluate the role of mechanical decoupling along pre existing zones of weakness especially with consideration to the accommodation of the alpine deformation in the jura belt

diamond and coesite discovered in saxony type granulite solution to the variscan garnet peridotite enigma diamond and coesite were discovered in situ as inclusions in garnet kyanite and zircon in high pressure granulites from northern bohemian massif these continental crustal rocks were therefore subducted to depths of c 140 km which also explains their common association with mantle garnet bearing peridotites models involving crustal thickening for these high pressure granulites need to be significantly modified whole variscan belt with numerous hp granulite occurrences can represent a large ultrahigh pressure terrain

buoyancy driven rapid exhumation of ultrahigh pressure metamorphosed continental crust abstract preservation of ultrahigh pressure uhp minerals formed at depths of 90 125 km require unusual conditions our subduction model involves underflow of a salient 250 150 km wide 90 125 km long of continental crust embedded in cold largely oceanic crust capped lithosphere loss of leading portions of the high density oceanic lithosphere by slab break off as increasing volumes of microcontinental material enter the subduction zone buoyancy driven return toward midcrustal levels of a thin 2 15 km thick low density slice finally uplift backfolding normal faulting and exposure of the uhp terrane sustained over 20 million years rapid 5 mm year exhumation of the thin aspect ratio uhp sialic sheet caught between cooler hanging wall plate and refrigerating downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces the intracratonal position of most uhp complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone uhp metamorphic terranes consist chiefly of transformed yet relatively low density continental crust compared with displaced mantle material otherwise such complexes could not return to shallow depths relatively rare metabasaltic metagabbroic and metacherty lithologies retain traces of phases characteristic of uhp conditions because they are massive virtually impervious to fluids and nearly anhydrous in contrast h2o rich quartzofeldspathic gneissose schistose more permeable metasedimentary and metagranitic units have backreacted thoroughly so coesite and other uhp silicates are exceedingly rare because of the initial presence of biogenic carbon and its especially sluggish transformation rate uhp paragneisses contain the most abundantly preserved crustal diamonds

strain hardening and strain softening in fault zones from foreland thrusts the movement of thrust sheets is accomplished in part by deforming the rocks adjacent to thrust faults in foreland fold and thrust belts emplacement generates an array of mesoscopic faults in a thick layer along the base of a sheet ductile deformation of rocks in the layer occurs by displacing fault bounded blocks relative to one another in a kind of mesoscopic grain boundary sliding analyses of data from three southern appalachian thrust sheets indicate that this block sliding is first accompanied by strain hardening then by strain softening and it is accommodated by intra block fractures small faults and veins the density of accommodation features increases near thrusts and where these features become sufficiently numerous a layer of cataclastic rocks forms along the thrust we describe foliated cataclasites from two thrusts the microstructures of which indicate that the cataclasites exhibited extreme ductility interrupted occasionally by brittle processes deformation of the cataclasites was accompanied by strain softening strain softening deformation in fault arrays near thrusts is genetically related to strain softening deformation in cataclasites during the formation of the cataclasite rocks in the sheet are impelled to cross a strain energy hump after which they deform at either higher rates or lower stresses

diapiric exhumation of earth s youngest uhp eclogites in the gneiss domes of the d entrecasteaux islands papua new guinea abstract the woodlark rift in papua new guinea hosts the world s youngest 2 8 ma eclogite facies rocks and extensional deformation has played a key role in exhuming these u hp rocks at rates of 20 mm yr during the eocene papuan arc continent collision australian plate derived continental rocks were subducted to u hp depths there they remained for up to 30 m y until the pliocene when asthenospheric circulation ahead of the west propagating woodlark spreading ridge introduced heat and fluids this caused rocks to break away from the paleosubduction channel recrystallize in the eclogite facies and rise as rayleigh taylor instabilities the diapirs ascended adiabatically undergoing partial melting which lowered their viscosity and increased buoyancy u hp crust ponded near the moho at 2 4 ma thickening the crust to 40 km 11 kb domal uplifts emerged above sea level and these are still underlain by an unusually thick crust 26 km for a rift that has stretched by factor of 3 since 6 ma after ponding they acquired a flat lying foliation during amphibolite facies retrogression vertical shortening accompanied the gravitationally driven outflow of ponded lower crust the weak material was extended parallel to the rift margin thinning ductilely by w k 0 2 and was mechanically decoupled from and orthogonal to plate motion in the rift top e shear fabrics suggest that this flow was westward perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the woodlark spreading ridge at 100 c per m y and were mechanically incorporated into the rift s upper crust final exposure was by normal faulting and minor erosion such exhumation may also apply to other u hp terranes where less evidence for moho ponding is preserved

pore pressure generation in sedimentary basins overloading versus aquathermal the principal mechanism for abnormally high pore pressure generation has been an important matter of debate for more than a decade among the various mechanisms proposed mechanical overloading and aquathermal pressuring have probably received the greatest attention most previous discussions however have been based upon qualitative conceptual models here we propose a quantitative analysis by first examining the basic principles of porous flow and the fundamental properties of sediments and second by comparing the relative importance of the overloading versus the aquathermal mechanisms the coupling between pore pressure and compaction as well as the nonlinear and path dependent material properties are found to be critical in the present analysis we find that although changes in stress and temperature both contribute to pressure generation mechanical overloading is the main mechanism under normal geological conditions and is capable of producing the observed pore pressure profiles

fluid pressure transients on seismogenic normal faults abstract fluid inclusions in hydrothermally altered footwall rocks of the dixie valley fault nevada and the wasatch fault utah indicate that pore fluid pressure fluctuated minimum entrapment pressures for fluid inclusions consisting of h2o co2 nacl ranged from 295 mpa to 60 mpa in the temperature range 350 to 170 c on the wasatch fault and from 158 to 35 mpa in the temperature range 350 to 200 c on the dixie valley fault scatter in the pressure estimates at constant temperature is interpreted as paleo fluid pressure transients at depths of up to 11 km on the wasatch fault and 3 to 5 km on the dixie valley fault observed pressure transients range from 5 mpa within the limits of error in pressure determination to 120 mpa on the wasatch fault and 7 to 126 mpa on the dixie valley fault the pressure transients are greatest on both faults in the temperature range 270 to 310 c the fluids represented by fluid inclusions play a key role in nucleation and propagation of earthquake ruptures high fluid pressures may initiate rupture then dilatancy pore pressure reduction and dilatant hardening may arrest the rupture however decompression of the fluids and phase separation produces a decrease in fluid bulk modulus of 41 to 90 which reduces the dilatant hardening effect and may permit ruptures to propagate

behaviour of zircon in high grade metamorphic rocks evidence from hf isotopes trace elements and textural studies hf isotopic data of minerals in a mafic pyroxene granulite from the southern bohemian massif together with their major and trace element composition and petrological observations were used to decipher the metamorphic history and behaviour of zircon in the granulite the hf isotopic composition in the minerals was used to estimate whether the decompression reaction namely the consumption of garnet and rutile could have provided zr for the formation of newly grown metamorphic zircon the age of the decompression reaction indicated by the evolution of hf isotopes in garnet and orthopyroxene is between 333 and 331 ma i e ca 7 ma younger than the available u pb zircon ages from the moldanubian granulites and than the newly obtained 343 2 ma laser ablation icp ms u pb age of zircons the combination of bulk and in situ hf isotopic data major and trace element composition and petrological modeling of p t evolution revealed that the formation of zircons can not be related to the decompression phase of the evolution of the mafic granulites

anatexis during high pressure crustal metamorphism evidence from garnet whole rock ree relationships and zircon rutile ti zr thermometry in leucogranulites from the bohemian massif in situ mineral garnet zircon trace element and garnet rock ree distribution data obtained on leucogranulites from the bohemian massif are consistent with an origin of these now strongly deformed rocks as migmatitic leucogranites formed through dehydration melting of muscovite bearing protoliths at high pressures during the variscan orogeny partial melting at p t conditions of 900 9408c and 1 6 1 8 gpa consumed mica and feldspar to produce peritectic kyanite and high ca 20 24 mol grossular high eu garnet along with leucogranitic melt dree grt rock values calculated from this garnet and its host rock are consistent with garnet melt equilibrium at these p t conditions ternary feldspar and rutile 2000 3200 ppm zr crystallized from the leucogranite melts in the migmatites at minimum temperatures of 4880 9208c these formed along with apatite prior to and accompanying the crystallization of garnet rims and the majority of the garnet that occurs in the leucogranulites this garnet is typified by strong depletion in eu and dree grt rock values consistent with garnet melt equilibrium only after the fractionation of apatite 50 3 wt and ternary feldspar 5 15 wt zircons in one leucogranulite are dominated by oscillatory zoned grains that yield pre variscan u pb ages 495 and 433 ma and crystallized from ti in zircon thermometry at 700 8108c in their original host magmas these xenocrystic grains are potentially preserved because of the short timespan of the high p t variscan event at 340 ma and the moderate amounts of leucogranitic partial melt present in the rocks estimated as at least 30 wt melt from zr mass balance the production of new zircon associated with the variscan event in this leucogranulite is limited to weakly zoned zircon rims characterized by flat heavy ree patterns that approach ree equilibrium with garnet rims this post peak zircon was formed at minimum p t conditions of 8408c and 0 8 1 0 gpa along the decompression cooling path defined by previous work for the bohemian massif granulites the garnet whole rock ree relationships observation of eu depletion between early and later garnet growth phases and zr ti thermometric estimates indicating temperatures of48408c for this evolution cannot be explained by the alternative interpretation of the leucogranulites as reworked and recrystallized pre variscan low p granitoids but support a model in which the bohemian leucogranulites were high p variscan age migmatites containing significant amounts of melt

transform margins development controls and petroleum systems an introduction this paper provides an overview of the existing knowledge of transform margins including their dynamic development kinematic development structural architecture and thermal regime together with the factors controlling these this systematic knowledge is used for describing predictive models of various petroleum system concept elements such as source rock seal rock and reservoir rock distribution expulsion timing trapping style and timing and migration patterns the paper then introduces individual contributions to this volume and their focus supplementary material location table and map of specific transform examples structural elements of the romanche transform margin and glossary of terms used in this article is available at https doi org 10 6084 m9 figshare c 3276407 gold open access this article is published under the terms of the cc by 3 0 license this volume shows dynamic linkage between new transform margin research and increasing transform margin exploration it offers a critical set of predictive tools via an understanding of the mechanisms involved in the development of play concept elements at transform margins and their controlling factors our introduction to this volume reviews the current knowledge of the transform margins focusing on their dynamic development kinematic development structural architecture thermal regimes and petroleum systems the last part of this paper introduces the contributions to this volume

coesite exsolution from supersilicic titanite in uhp marble from the kokchetav massif northern kazakhstan coesite exsolved from supersilicic titanite was discovered in an impure calcite marble at kumdy kol in the kokchetav uhp ultrahigh pressure metamorphic terrane northern kazakhstan this impure marble consists mainly of calcite k feldspar diopside and symplectites of diopside zoisite with minor amount of titanite phengite and garnet no diamond was found in the marble coesite and quartz which have needle or platy shapes measuring about 20 60 m in length occur as major exsolved phases in the cores and mantles of titanite crystals with minor calcite and apatite the strongest raman band for the coesite needles and plates was confirmed at about 524 cm 1 with a weak band at about 271 cm 1 to estimate the initial composition of the titanite before coesite exsolution exsolved phases were reintegrated by measuring their area fractions on digital images the highest excess si in titanite was thus determined to be 0 145 atoms per formula unit apfu this composition requires a pressure higher than 6 gpa on the basis of phase relations in the system catisio5 casi2o5 this pressure is consistent with other evidence of high pressure in the same marble such as 1 4 1 8 wt k2o and over 1000 ppm h2o in diopside supersilicic titanite and coesite exsolution also indicate that sio2 exsolution occurred in the coesite stability field during exhumation of the uhp metamorphic unit

detrital zircon geochronology of the northeastern tibetan plateau u pb geochronologic analyses have been conducted on 413 detrital zircon grains collected from 16 samples in the altun shan nan shan and qilian shan the samples come primarily from quartz arenites and metaturbidites of middle to late proterozoic age and from feldspathic and volcanic clast rich sandstones of early paleozoic age zircon grains in proterozoic strata resting on tarim basement yielded mainly 2 0 1 9 ga ages whereas proterozoic strata of the qaidam and qilian terranes yielded mainly ca 930 820 ma and ca 1 9 1 1 ga ages the younger grains were apparently shed from local igneous rocks whereas the grains older than 1 1 ga were shed from an undetermined continental source grains in the lower paleozoic strata are mainly ca 500 430 ma and were shed from nearby plutonic and possibly volcanic rocks that formed in a magmatic arc setting our detrital zircon ages are consistent with a model first proposed by e r sobel and n arnaud in which early paleozoic magmatism occurred within a single northeast facing magmatic arc that was constructed across an assemblage of middle to late proterozoic accretionary complexes remnants of magmatic arcs and shallow marine strata this arc system was accreted to the tarim and sino korean cratons during silurian devonian time the resulting suture has been reactivated as tertiary thrust faults that currently define the structural and topographic margin of the tibetan plateau our data also provide two new estimates for the offset along the eastern altyn tagh fault a belt of middle proterozoic shallow marine strata is offset by 400 km whereas a belt of 490 480 ma magmatic arc rocks is offset by 370 km these values are generally similar to the 350 400 km offset reported in most previous studies

challenges in high pressure granulite metamorphism in the era of pseudosections reaction textures compositional zoning and tectonic interpretation with examples from the bohemian massif high pressure granulites are an important record of geodynamic processes in overthickened or subducted continental crust orthopyroxene free assemblages in granitic ternary feldspar s quartz garnet kyanite rutile intermediate ternary feldspar s quartz garnet clinopyroxene kyanite rutile titanite and basic garnet clinopyroxene plagioclase quartz rutile compositions indicate formation conditions at mantle depths clinopyroxene compositions in variscan high pressure granulites are unusual in that they include omphacite in plagioclase bearing rocks thus not eclogite and al rich diopside i e indicating high ca tschermak content with both yielding temperatures above 900 c problems such as compositional zoning multiple generations of key phases in reaction domains and unmixing of high temperature solid solution phases during cooling ternary feldspars omphacite clearly indicate disequilibrium and require very careful interpretation as to which phases and compositions possibly represent a former equilibrium association pressure temperature p t determination by the pseudosection method although allowing prediction of mineral assemblages compositions and molar proportions for a fixed bulk composition for modelled p t conditions still requires reliable activity composition information for the key phases feldspar and clinopyroxene as well as an interpretation of former equilibrium compositions in the investigated samples i e the same restrictions applying to conventional thermobarometry the interpretations of some recently determined pseudosections for the composition of variscan clinopyroxene bearing high pressure granulites contradict numerous published p t paths however quantitative information from thermobarometry or pseudosections must be integrated with key petrographic observations in the case of the variscan example it is argued that petrographic observations and published p t paths are consistent with mineral assemblages predicted in pseudosections and support existing tectonometamorphic models

pore pressure response during failure in soils three experiments were performed on natural slopes to investigate variations of soil pore water pressure during induced slope failure two sites in the wasatch range utah and one site in the san dimas experimental forest of southern california were forced to fail by artificial subsurface irrigation the sites were instrumented with electronic piezometers and displacement meters to record induced pore pressures and movements of the slopes during failure piezometer records show a consistent trend of increasing pressure during the early stages of infiltration and abrupt decreases in pressure from 5 to 50 minutes before failure displacement meters failed to register the amount of movement due to location and ineffectual coupling of meter pins to soil observations during the experiments indicate that fractures and macropores controlled the flow of water through the slope and that both water flow paths and permeability within the slopes were not constant in space or time but changed continually during the course of the experiments

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

age petrogenesis and tectonic implications of early devonian bimodal volcanic rocks in the south altyn nw china abstract in this paper we report zircon u pb dating hf isotopes geochemical and sr nd isotopic data with the aim the petrogenesis and regional tectonic evolution of early devonian bimodal volcanic rock in the south altyn nw china new la icpms zircon u pb isotopic data constrained them at ca 406 ma the mafic samples are characterized by high fe cr and ni contents low ti and mg contents slightly enriched lree patterns and low la yb n la nb and la ta ratios and positive e nd t values 3 3 to 3 4 indicating that they were likely derived from strong batch melting of the asthenosphere in the spinel facies field the felsic rocks show an a type affinity with high alkalis fe ga zr nb ce and y contents low mg sr content high rb sr and ga al ratios enrichment in lile e g rb k th u and lree and depletion in ba sr nb ta p and ti and fractionated ree patterns with very strong negative eu anomalies these features along with distinct e nd t values 0 5 to 2 3 and mostly positive e hf t 0 29 to 5 18 indicate that the felsic rocks were mainly generated by partial melting of the crust in low pressure and high temperature conditions and simultaneously underwent slight magma mixing of such melts with mantle magma according to the petrogenetic schemes and geological background of the early devonian bimodal volcanic rocks tholeiite and a type dacite rhyolite they should have formed in a post collisional extensional setting moreover on the basis of spatial and temporal distribution and formation mechanism the tectonic magmatic evolution of the early paleozoic south altyn tagh could be divided into three stages 505 472 ma continental collision the magmatite formed under high pressure conditions due to the deep subduction and initial tearing of continental slab 467 450 ma continental slab break off the magmatite formed at high temperature and low pressure in virtue of felsic upper crust uplifting and mantle magma underplating and 432 385 ma post collisional extension the magmatite consists of a type granites and bimodal volcanic rocks which are the products of the interaction between mantle and crust and asthenosphere upwelling

combined rutile zircon thermometry and u pb geochronology new constraints on early paleozoic hp uht granulite in the south altyn tagh north tibet china abstract hp granulite and garnet peridotite constitute a hp metamorphic unit in the south altyn tagh northern tibet previous data suggest that hp granulites experienced high pressure ultrahigh temperature metamorphism 870 1050 c 19 25 kbar followed by a medium pressure granulite facies overprint 780 820 c 9 5 12 kbar previous zircon u pb geochronology of hp granulites gave an age of ca 500 ma but it is unclear if this age reflects the metamorphic peak or the post peak overprint in this contribution new shrimp analyses indicate that the zircons associated with the peak metamorphic assemblage in hp felsic granulite are characterized by flat heavy ree patterns and 30 62 ppm ti yielding a temperature of 892 1002 c by ti in zircon thermometry suggesting that the growth of the zircons occurred under or close to peak ultrahigh temperature conditions rather than under post peak conditions these zircons gave a shrimp u pb age of 495 4 ma representing the age of peak metamorphism in contrast oscillatory zoned zircon cores yield magmatic ree patterns ti in zircon temperatures of 720 820 c and neoproterozoic u pb ages 936 22 ma this indicates that the protoliths of the felsic granulites were formed in the early neoproterozoic and were taken to extreme temperatures and pressures during the early paleozoic orogenic event in situ la icpms and emp analyses indicate that rutiles are characterized by broad scatter and bimodal distribution in both zr concentration and u pb age which are interpreted as a result of recrystallization or growth rather than volume diffusion the first rutile u pb age cluster is at ca 485 ma with a zr concentration of ca 3000 ppm yielding ca 950 c at 20 kbar implying that they grew at or near the peak metamorphic conditions the second rutile u pb age cluster is at ca 450 ma with a zr concentration of ca 1000 ppm corresponding to a temperature around 780 c at 10 kbar which is related to post peak reheating under medium pressure medium temperature granulite facies conditions these results demonstrate that hp granulites in the south altyn tagh experienced a peak ultrahigh temperature and high pressure metamorphism at ca 495 ma followed by medium pressure medium temperature granulite facies overprint before ca 450 ma

surface rupture of the cariaco july 09 1997 earthquake on the el pilar fault northeastern venezuela abstract this paper discusses the surface rupture of the cariaco july 09 1997 ms 6 8 earthquake in northeastern venezuela located at 10 545 n and 63 515 w and about 10 km deep the field reconnaissance of the ground breaks confirms that this event took place on the ene wsw trending onshore portion of the dextral el pilar fault between the gulfs of cariaco and paria which is part of the major wrenching system within the caribbean south america plate boundary zone dextral slip along this fault was further supported by the structural style of this rupture en echelon right lateral r shears connected by mole tracks at restraining stepovers and by larger geometric complexities pop ups at las manoas and guarapiche as well as by the focal mechanism solutions determined for the event by several authors this 1997 surface ruptre comprised two distinct sections from west to east a a main very conspicuous continuous 30 km long rather straight 075 n trending alignment of en echelon surface breaks with a rather constant purely dextral coseismic slip of about 25 cm but reaching a maximum value of 40 cm slightly northwest of pantono and b a secondary discontinuous 10 km long boomerang shaped rupture with a maximum coseismic slip of 20 cm at guarapiche the onshore extent of the surface rupture totalled 36 km but may continue westward underwater as suggested by the very shallow aftershock seismicity this aftershock activity also clearly defined the steep north dip of the fault plane along the western rupture suggesting tectonic inheritance on this major fault from many locals accounts the rupture seems to have propagated from pantono to the west highly asymmetric bidirectionality this suggests that earthquake nucleation happened at or near the casanay guarapiche restraining bend and rupture quickly propagated westward allowing only a small fraction to progress eastwards beyond the bend additionally the large fraction of after slip or creep released is to be related to such restraining bend which seems to have partly locked slip during rupture