identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

zircon xenocrysts from the kambalda volcanics age constraints and direct evidence for older continental crust below the kambalda norseman greenstones abstract the hangingwall basalt at kambalda western australia contains zircons that have been shown by ion microprobe analyses to have very high u and th contents and a wide variety of crystallization ages nearly all of these zircons certainly are xenocrysts a few might relate to intrusive veinlets the age of the youngest xenocrysts 2693 50ma 2 shows that the eruptive age of the basalt cannot exceed 2743 ma this confirms that the apparent sm nd isochron giving 3200 ma 1 2 for kambalda mafic and ultramafic rocks is a mixing line 2 between unrelated components enriched and depleted in light rare earth elements mixing probably occurred at depth by erosion of 3200 3500 ma old felsic crust from the walls of the hwb conduits the zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern yilgarn block the latter implies that the kalgoorlie norseman greenstone sequences were formed in a continental rather than an oceanic environment

2d kernel based imaging of coda q space variations in the gargano promontory southern italy abstract crustal seismic attenuation in the gargano promontory southern italy was inferred by analyzing coda waves of about one hundred small magnitude earthquakes recorded by the otrions local seismic network coda attenuation qc was computed using the sato 1977 model i e the assumption of single back scattering at different lapse times and at different frequencies average qc is comparable with the same quantity measured in the adjacent area of the umbria marche region central italy a recent kernel based imaging technique for qc was then applied and the results evidence a well confined zone located south west of the investigated area showing a sharp qc anomaly while qc cannot be directly related to the physical properties of the propagation medium its spatially dependent modeling allows an interpretation in the light of other geophysical and geological information our results were then interpreted following the two extreme hypotheses according to which qc could be representative of the effect of only intrinsic attenuation or of only scattering from the heterogeneities both the hypothesis correlate with the observation of anomalies in several geophysical parameters such as heat flow magnetic field gravity density seismic velocity in the southwestern gargano moreover the qc images support the results obtained in a recent study that shows clear variations in the rheological properties of the gargano the results obtained at local scale confirm and detail previous regional scale studies on attenuation and highlight for the first time the presence of a positive qc anomaly in the south western part of the gargano promontory

stress relaxation at the lower dislocation edge of great shallow earthquakes abstract the problem of post seismic stress evolution at the lower dislocation edge of great shallow earthquakes is investigated by a model reproducing a transcurrent boundary zone subject to a constant strain rate dislocation models with frictional stress threshold for slip arrest must be considered in order to study post seismic stress evolution in the proximity of dislocation edges it is found that the boundary zone is vertically divided in two regions an upper region where stress increases with time and a lower region where stress decreases dislocations which are nucleated in the seismogenic region can propagate downward into the aseismic region in this case earthquakes are followed by relaxation of the stress concentration at the lower dislocation edge this may have effects on the space time distribution of aftershocks

the gassmann burgers model to simulate seismic waves at the earth crust and mantle the upper part of the crust shows generally brittle behaviour while deeper zones including the mantle may present ductile behaviour depending on the pressure temperature conditions moreover some parts are melted seismic waves can be used to detect these conditions on the basis of reflection and transmission events basically from the elastic plastic point of view the seismic properties seismic velocity and density depend on effective pressure and temperature confining and pore pressures have opposite effects on these properties such that very small effective pressures the presence of overpressured fluids may substantially decrease the p and s wave velocities mainly the latter by opening of cracks and weakening of grain contacts similarly high temperatures induce the same effect by partial melting to model these effects we consider a poro viscoelastic model based on gassmann equations and burgers mechanical model to represent the properties of the rock frame and describe ductility in which deformation takes place by shear plastic flow the burgers elements allow us to model the effects of seismic attenuation velocity dispersion and steady state creep flow respectively the stiffness components of the brittle and ductile media depend on stress and temperature through the shear viscosity which is obtained by the arrhenius equation and the octahedral stress criterion effective pressure effects are taken into account in the dry rock moduli using exponential functions whose parameters are obtained by fitting experimental data as a function of confining pressure since fluid effects are important the density and bulk modulus of the saturating fluids water and steam are modeled using the equations provided by the nist website including supercritical behaviour the theory allows us to obtain the phase velocity and quality factor as a function of depth and geological pressure and temperature as well as time frequency we then obtain the ps and sh equations of motion recast in the velocity stress formulation including memory variables to avoid the computation of time convolutions the equations correspond to isotropic anelastic and inhomogeneous media and are solved by a direct grid method based on the runge kutta time stepping technique and the fourier pseudospectral method the algorithm is tested with success against known analytical solutions for different shear viscosities an example shows how anomalous conditions of pressure and temperature can in principle be detected with seismic waves

evidence for a fold nappe in the high grade basement of central sri lanka terrane assembly in the pan african lower crust abstract structural analysis in the kadugannawa complex kc in central sri lanka reveals the presence of an early steep gneissic layering s 1 which has been overprinted by a younger event d 2 in the structurally deepest levels d 2 folds become progressively tighter towards the contact with the underlying highland complex hc the distribution of lithologies in the kc the distribution of steep s 1 and subhorizontal s 2 and the strain gradient towards the kc hc contact suggest that the kc is a d 2 fold nappe overlying the hc vergence of asymmetric sheath folds in sections parallel to a strong nnw sse lineation is towards nnw indicating a top to the nnw displacement granulite facies assemblages were formed during and after the final stages of d 2 in the kc d 2 was the main fabric forming event in the hc where meta igneous and metasedimentary gneisses show l s fabrics defined by the peak granulite facies assemblages the sense of shear deduced from asymmetric foliation boudinage is consistent with the top to the nnw displacement in the kc both units have been refolded around large scale upright d 3 folds the nappe model and nd model ages suggest that the kc and the eastern part of the hc may represent distinct lower crustal terranes juxtaposed during pan african high grade high strain events d 2 the contact between them is interpreted as a terrane boundary in a lower crustal environment the top to the nnw movement sense suggests that the kc may have been originally located south of the eastern highland complex this and the similarity in isotope signatures indicate a pre assembly link of the kc the western highland complex and the vijayan complex of eastern sri lanka

nd isotopic mapping of the sri lanka basement update and additional constraints from sr isotopes abstract the high grade crystalline basement of sri lanka can be subdivided into three provinces distinguished by distinct isotopic model ages this confirms the conclusions of milisenda et al 1988 made on the basis of much more limited data high grade ortho and paragneisses from the highland complex in central sri lanka yield palaeoproterozoic to archaean nd model ages from 2 0 to 3 4 ga and reflect an extended premetamorphic crustal history the wanni complex in the western and northwestern part of the island is characterized by meso to neoproterozoic nd model ages ranging from 1 0 to 2 0 ga the vijayan complex in the southeast is occupied by neoproterozoic i type granitoids with nd model ages from 1 0 to 1 8 ga this distinct crustal province appears to fit best a detached continental margin plutonic terrane the isotopic data preclude a common premetamorphic history of the wanni and vijayan complexes with the ancient highland complex the combined nd sr and pb isotope data together with recent geochronological studies strongly suggest that the distinct crustal age provinces are separated by tectonic discontinuities and that they represent terranes that were accreted just prior to the high grade metamorphic event in neoproterozoic time

rb sr and sm nd geochronology of the eppawala metamorphic rocks and carbonatite wanni complex sri lanka abstract petrological studies on the surrounding metamorphic rocks of the eppawala carbonatite body wanni complex sri lanka revealed that these rocks had been metamorphosed under amphibolite to granulite facies conditions garnet sillimanite biotite gneiss shows lower range of metamorphic temperature 730 770 c than the migmatite gneiss 750 780 c and the pressure varies from 6 6 7 8 kbar to 5 6 6 4 kbar respectively the metamorphic age of the garnet sillimanite biotite gneiss and migmatite gneiss dated 607 23 ma and 626 16 ma respectively for mineral whole rock isochron in sm nd system these ages are compatible with the ages of regional high grade metamorphism occurred 610 550 ma in the three crustal units in sri lanka rb sr system for biotite apatite and whole rock fractions suggests 493 5 ma for the eppawala carbonatite body this age indicates the cooling age of the biotite the presence of non crystalline carbonatite matrix and large hexagonal apatite crystals suggests a slow cooling history further low closure temperature of biotite in rb sr system suggests that the intrusion age of carbonatite body should be more than 493 ma but non metamorphosed nature provides evidence that the intrusion age of the carbonatite body should be less than the period of regional metamorphism 610 550 ma therefore eppawala carbonatite body has a strong possibility to be a late to post magmatic intrusion the other late to post magmatic intrusions in the wanni complex and highland complex are dated between 580 550 ma therefore the most probable intrusion age of the eppawala carbonatite body is suggested to be around 550 ma

thermal fronts generated by internal waves propagating obliquely along the continental slope abstract rapid temperature falls occurring at semidiurnal periods are observed close to the bottom above the continental slope in the bay of biscay simultaneous current measurements reveal that the abrupt temperature decrease o 0 5 k within one minute is associated with a brief downslope current contrary to previous observations it is suggested that the flow field associated with internal waves propagating obliquely downslope is responsible for advecting denser water higher on the slope than lighter fluid resulting in a gravitationally unstable stratification the collapse of this stratification is observed as a thermal front passing the moored instruments

correlation between seismic reflectivity and rheology in extended lithosphere southern tuscany inner northern apennines italy abstract in southern tuscany inner zone of the northern apennines the lithosphere has been affected by extensional deformation since the middle miocene and is characterized by high heat flow 120 mw m 2 both local shallow and regional deep seismic reflection profiles clearly show the occurrence of a sharp reflector k horizon within the upper crust which is probably located at the top of a shear zone acting as the decollement horizon for the latest pliocene quaternary normal faults its depth varies between 3 and 6 km in geothermal areas and 9 and 12 km elsewhere borehole control in the geothermal areas shows that it occurs at a temperature of approximately 450 c the hypothesis that the k horizon represents the top of the brittle ductile transition agrees with two lines of evidence a it approximately follows the same isotherm over the whole region and b its depth coincides with rheological predictions of the brittle ductile transition for the range of high pore fluid pressures and strain rates estimated for southern tuscany it appears therefore that at least under specific conditions very high pore fluid pressures strain concentration the brittle ductile transition may have a seismic signature

observations of internal wave reflection off sloping bottoms current and temperature spectra exhibit intensification and polarization near sloping bottoms over a band of frequencies centered at the local internal wave critical frequency examples are drawn from a variety of island seamount and continental slope observations waves propagating down from mid depth change energy density wave number and azimuth when they reflect off the bottom providing a natural perturbation to the deep ocean interior internal wave spectrum deviations from linear theory are interpreted as evidence for dissipation through subsequent shear instability and or nonlinear interaction topographic features appear likely to be energy sinks for internal wave energy

zircon ion microprobe dating of high grade rocks in sri lanka the high grade gneisses of sri lanka display spectacular in situ granulitization phenomena similar to those observed in southern india and of current interest for evolutionary models of the lower continental crust the absolute ages of these rocks are poorly constrained and so using the shrimp ion microprobe we have analyzed small spots on zircons from upper amphibolite to granulite grade quartzitic and pelitic metasediments detrital grains from a metaquartzite of the highland group preserve premetamorphic u pb ages of between 3 17 and 2 4 ga and indicate derivation of the sediment from an unidentified archean source terrain the pb loss patterns of these zircons and the other samples suggest severe disturbance at ca 1100 ma ago which we attribute to high grade regional metamorphism two pelitic gneisses contain detrital zircons with ages up to 2 04 ga and also record an approx 1100 ma event that is also apparent from metamorphic rims around old cores and new zircon growth a granite intrusive into

u pb isotopic systematics of zircons from prograde and retrograde transition zones in high grade orthogneisses sri lanka we present u pb zircon isotopic data from locally restricted prograde arrested in situ charnockitization and retrograde metamorphic transition zones which are well exposed in proterozoic orthogneisses tectonically interbanded with granulite facies supracrustal rocks of the highland group in sri lanka these granitoid rocks yield apparent ages of 1942 pm 22 ma 770 ma 660 ma and 560 ma all samples show severe pb loss some 550 560 ma ago the main phase of granulite formation could not be dated unambiguously but is bracketed between 660 ma and 550 ma the pervasive pb loss event around 550 560 ma reflects the end of this period of high grade metamorphism and was associated with widespread igneous activity and retrogression this is constrained by the 550 pm 3 ma intrusion age for a post tectonic granite we relate this late phase of thermal activity to crustal uplift of the sri lankan granulites our data unambiguously prove the high grade history of the sri lanka gneisses to be a late

distinct s wave reflector in the midcrust beneath nikko shirane volcano in the northeastern japan arc distinct s waves reflected from a midcrustal seismic velocity discontinuity are detected beneath nikko shirane volcano in the southernmost part of the northeastern japan arc a detailed travel time analysis of the reflected s waves by using data acquired through a dense seismic network temporarily set up in this region shows that this unusual s wave reflector is distributed over an area of 15 15 km2 at depths ranging from 8 to 15 km the reflector has a conical shape becoming shallow toward the summit of nikko shirane volcano observed amplitude spectral ratios of reflected s waves to direct s waves show that the reflector body has a strong velocity contrast to the surrounding medium and its thickness is of the order of 100 m at most the reflector body is approximated by two thin layers probably filled with partially molten materials cutoff depth for shallow seismicity in this area is 3 5 km above the reflector and becomes shallow toward nikko shirane volcano nearly parallel to the reflector the depth to brittle ductile transition zone seems to be prescribed by the existence of the reflector body which is perhaps a thin magma body

global prediction of abyssal hill root mean square heights from small scale altimetric gravity variability 1 abyssal hills which are pervasive landforms on the seafloor of the earth s oceans represent a potential tectonic record of the history of mid ocean ridge spreading however the most detailed global maps of the seafloor derived from the satellite altimetry based gravity field cannot be used to deterministically characterize such small scale 10 km morphology nevertheless the small scale variability of the gravity field can be related to the statistical properties of abyssal hill morphology using the upward continuation formulation in this paper i construct a global prediction of abyssal hill root mean square rms heights from the small scale variability of the altimetric gravity field the abyssal hill related component of the gravity field is derived by first masking distinct features such as seamounts mid ocean ridges and continental margins and then applying a newly designed adaptive directional filter algorithm to remove fracture zone discontinuity fabric a noise field is derived empirically by correlating the rms variability of the small scale gravity field to the altimetric noise field in regions of very low relief and the noise variance is subtracted from the small scale gravity variance suites of synthetically derived abyssal hill formed gravity fields are generated as a function of water depth basement rms heights and sediment thickness and used to predict abyssal hill seafloor rms heights from corrected small scale gravity rms height the resulting global prediction of abyssal hill rms heights is validated qualitatively by comparing against expected variations in abyssal hill morphology and quantitatively by comparing against actual measurements of rms heights although there is scatter the prediction appears unbiased

crustal q in southern italy determined from regional earthquakes abstract we estimated the quality factor q of s waves traveling through the crust in the southern region of italy for that purpose 65 regional earthquakes with magnitudes ml 2 0 5 3 and hypocentral distance ranging between 7 and 375 km were analyzed to estimate q we first calculated a set of empirical functions that describe the s wave spectral amplitude decay with distance for 15 frequencies between 1 and 25 hz within the distance range of the recordings the average q can be approximated as q 25 7 f 1 4 to separate the effect of upper and lower crust in the total q we used the first 50 km of the attenuation functions to estimate q of the upper crust and found that q 18 8 f 1 7 then using the 50 100 km segment of the attenuation functions we estimated that q 17 8 f 1 3 for the lower crust we also found that at high frequencies f 10 hz the upper crust q increases considerable relative to lower crust q being a factor of nine greater at 25 hz this higher attenuation in the lower crust may be the effect of deep structures as reported by menardi noguera and rea 2000 that were subjected to brittle ductile deformation during past tectonic events

a layered basic intrusion deformed and metamorphosed in granulite facies of the sri lanka basement a layered basic intrusion has been found in the central granulite belt of the sri lanka continental basement it intruded parallel to bedding before all or early during deformation of neighbouring metasediments deformation affecting metasediments and the intrusion alike includes flattening to c 1 20 of the original thickness and nnw stretching to c 20 times the original length the intrusion is now 170 300 m thick most of the deformation was acquired under granulite facies metamorphism the intrusion was then folded still at high t by a large f4 synform with an axis parallel to str1 and a steep axial plane a steep axial plane cleavage and minor folds are related to this big fold stretching continued along its axis late during formation of this fold a granite intruded mainly following s4 cleavage planes the intrusion shows a homogeneous gabbroic series at the bottom followed upwards by a differentiated and layered series a thin sequence of ultramafic rocks occurs near the middle this indicates multiple melt injection more homogeneous partly biotite bearing amphibolites form the top of the succession magmatic layering is well preserved but no magmatic minerals or grain fabrics have escaped deformation or metamorphism static annealing under granulite facies conditions outlasted all deformation and was accompanied and followed by the beginning of cooling hornblende plagioclase coronas formed round garnets at this stage geochemical work carried out by stosch 1991 on our samples confirms the cumulate nature of the rocks

earthquakes in the ductile regime pseudotachylytes from a crustal scale shear zone in central australia have developed in a cyclical manner once developed an individual pseudotachylyte is deformed in a ductile manner only to be overprinted at a later stage by a new generation of pseudotachylytes such cyclic generation and deformation of pseudotachylyte has been interpreted in the past as representing conditions at the brittleductile transition a different interpretation however is presented here it is proposed that psuedotachylytes and associated ultramylonites can develop entirely within the ductile regime as ductile instabilities such instabilities are different in nature to those previously discussed at length in the geophysical literature but are identical in principle with the instabilities that develop for velocity weakening frictional behavior in spring slider systems at a given strain rate a critical temperature tc is defined at which the transient work hardening equals the product of stress relaxation due to a thermal fluctuation and the heat generated by shearing a necessary condition for ductile instability at a given strain rate is that the temperature is belowtc then the rate of change of stress with respect to strain is negative an additional requirement is that this rate of change exceeds in magnitude the effective elastic stiffness of the loading system ductile instabilities are marginally possible at geological strain rates in quartzites but are possible at mid crustal temperatures in other rock types on the basis of these observations a new interpretation is presented for the base of the seismogenic zone in crustal regions

the formation of a subsurface anticyclonic eddy in the peru chile undercurrent and its impact on the near coastal salinity oxygen and nutrient distributions the formation of a subsurface anticyclonic eddy in the peru chile undercurrent pcuc in january and february 2013 is investigated using a multi platform four dimensional observational approach research vessel multiple glider and mooring based measurements were conducted in the peruvian upwelling regime near 12 30 s the dataset consists of 10000 glider profiles and repeated vessel based hydrography and velocity transects it allows a detailed description of the eddy formation and its impact on the near coastal salinity oxygen and nutrient distributions in early january a strong pcuc with maximum poleward velocities of 0 25 m s at 100 to 200 m depth was observed starting on january 20 a subsurface anticyclonic eddy developed in the pcuc downstream of a topographic bend suggesting flow separation as the eddy formation mechanism the eddy core waters exhibited oxygen concentrations 1 mol kg an elevated nitrogen deficit of 17 mol l and potential vorticity close to zero which seemed to originate from the bottom boundary layer of the continental slope the eddy induced across shelf velocities resulted in an elevated exchange of water masses between the upper continental slope and the open ocean small scale salinity and oxygen structures were formed by along isopycnal stirring and indications of eddy driven oxygen ventilation of the upper oxygen minimum zone were observed it is concluded that mesoscale stirring of solutes and the offshore transport of eddy core properties could provide an important coastal open ocean exchange mechanism with potentially large implications for nutrient budgets and biogeochemical cycling in the oxygen minimum zone off peru

rubidium strontium whole rock ages of kataragama and pottuvil charnockites and east vijayan gneiss indication of a 2 ga metamorphism in the highlands of southeast sri lanka highland group granulite facies rocks of the kataragama klippe in southeast sri lanka yield a rb sr whole rock apparent age of 1930 pm 130 ma mswd 39 and a 87 sr 86 sr intercept of 0 715 pm 0 005 indicating a highlandian metamorphism about 2 0 ga ago a charnockitic gneiss at komari near pottuvil east sri lanka gives a rb sr whole rock isochron age of 820 pm 70 ma mswd 0 78 initial 87 sr 86 sr 0 725 pm 0 007 suggesting a metamorphic resetting at about 0 8 ga the rb sr whole rock data of an east vijayan biotite hornblende gneiss fit a reference isochron of 800 ma with a 87 sr 86 sr intercept of 0 705 the low 87 sr 86 sr intercept may be explained by a juvenile addition to the older crust a review of available data from various isotopic dating methods suggests that the highland group supracrustals were deposited 2 5 2 0 ga ago metamorphosed in the granulite facies about 2 0 ga ml ago and disturbed by resetting events about 1 1 ga m2 0 8 ga m3 and

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

energetics of seamount wakes part ii wave fluxes abstractseamounts are thought to facilitate ocean mixing through unsteady wake processes and through the generation of internal waves which propagate away from the seamount and later break the r

upper crustal structure of the larderello geothermal field as a feature of post collisional extensional tectonics southern tuscany italy abstract reflection seismic profiles crossing the larderello geothermal field permit the distinction of three different seismic facies units in the upper continental crust the upper 3 4 km of the crust is characterized by low reflectivity its base corresponding to a reflector referred to as the k horizon and recognized in many other areas of southern tuscany beneath this reflector a seismic facies of 0 8 1 0 twt thickness is characterized by strong convergent and divergent reflections forming a lozange shaped geometry the lowermost seismic facies is characterized by almost horizontal reflections based on the distribution of the hypocentral microearthquakes temperature data recorded in deep boreholes and the geometry of normal faults the k horizon is explained here as a kinematically active rheological boundary which separates a brittle upper part from a ductile lower part according to this hypothesis the k horizon and the lozange reflections could originate from a mylonite band located at the contact between two geological bodies characterized by different rheological behavior it is inferred that this horizon acts as a decoupling plane in the upper continental crust it developed as a consequence of the extensional tectonics and the high heat flow which characterize southern tuscany

rubidium strontium whole rock ages of kataragama and pottuvil charnockites and east vijayan gneiss indication of a 2 ga metamorphism in the highlands of southeast sri lanka a discussion

caledonian ages in variscan rocks rb sr and sm nd isotopic variations in dioritic intrusives from the northwestern bohemian massif west germany abstract dioritic intrusives and granites from the northwestern bohemian massif west germany were analyzed for the chemical and isotopic composition of rb sr and sm nd the marktredwitz intrusive suite and the tirschenreuth mahring sill intrusions yield apparent rb sr whole rock isochron ages of 468 9 ma and 470 33 ma respectively while the reuth erbendorf granodiorites display a scatter with an apparent age of 545 16 ma the rb sr whole rock ages conflict with geological evidence the diorites intruded metamorphic country rocks which experienced an early devonian metamorphism contact relationships between the diorites and porphyritic granites indicate nearly contemporary emplacement the intrusion ages of two porphyritic granites from this area have been dated at 319 ma and 311 ma the diorites display a variation in nd 320 ma ranging from 1 0 to 4 4 whereas the two granites yield homogeneous initial nd values of 3 7 and 6 6 the variations in elemental concentrations and initial isotopic ratios clearly show a mixing trend within the dioritic rock suites the mixing partners are a presumably mantle derived magma and crustal components of varying composition the isotope systematics of the diorites and the concordance of their rb sr whole rock age values with caledonian rock forming events in central europe point to a pre intrusive caledonian event inherited in these rocks we suggest that the diorites were formed by multistage anatectic processes

internal tide generation in nonuniformly stratified deep oceans we present numerical and experimental studies of the conversion of tidal motions of an exponentially stratified fluid over two dimensional knife edge gaussian and complex bottom topography to radiated internal waves in a model of the deep ocean we compare the radiated internal wave power for cases of strong stratification where the buoyancy frequency profile n z proportional to the square root of the density gradient is much larger than the tidal frequency to the power radiated for weak stratifications we consider particularly internal wave generation for topography below a turning depth ztd where n ztd for z ztd although the radiated power in these waves is much weaker than in cases without turning depths the radiated power is predicted well by prior analytical theory if the nonuniform stratification is averaged over depths spanning from the bottom boundary up to an effective height zeff in the absence of a turning depth we find zeff is approximately equal to the height of the topography indicating that only the stratification for depths spanned by the topography is relevant however in the presence of a turning depth the vertical scale of the internal tide becomes larger and zeff increases approximately linearly with the turning depth height toward values comparable to the total fluid depth

the role of fluids in granulite facies metamorphism as deduced from oxygen and carbon isotopic compositions abstract based on stable isotope data of whole rock samples the main fluid rock exchange reactions in the high grade rocks of the crystalline basement of sri lanka is deduced to have occurred prior to granulite facies metamorphism the influx of external fluids during and after granulite facies conditions was only channellized and did not give rise to the formation of interconnected fluid films in rock volumes larger than several cm3 fluid present conditions were thus only manifest for relatively short periods of time much less than 106 y so that the conditions of granulite facies metamorphism of the sri lankan basement rocks in general can be characterized as fluid absent

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

role of the brittle ductile transition on fault activation abstract we model a fault cross cutting the brittle upper crust and the ductile lower crust in the brittle layer the fault is assumed to have stick slip behaviour whereas the lower ductile crust is inferred to deform in a steady state shear therefore the brittle ductile transition bdt separates two layers with different strain rates and structural styles this contrasting behaviour determines a stress gradient at the bdt that is eventually dissipated during the earthquake during the interseismic period along a normal fault it should form a dilated hinge at and above the bdt conversely an over compressed volume should rather develop above a thrust plane at the bdt on a normal fault the earthquake is associated with the coseismic closure of the dilated fractures generated in the stretched hangingwall during the interseismic period in addition to the shear stress overcoming the friction of the fault the brittle fault moves when the weight of the hangingwall exceeds the strength of the dilated band above the bdt on a thrust fault the seismic event is instead associated with the sudden dilation of the previously over compressed volume in the hangingwall above the bdt a mechanism requiring much more energy because it acts against gravity in both cases the deeper the bdt the larger the involved volume and the bigger the related magnitude we tested two scenarios with two examples from l aquila 2009 italy and chi chi 1999 taiwan events gps data energy dissipation and strain rate analysis support these contrasting evolutions our model also predicts consistently with data that the interseismic strain rate is lower along the fault segment more prone to seismic activation

seismic modeling study of the earth s deep crust we use seismic modeling methods to validate the interpretation of deep crust seismic exploration an approximation of the stacked section is obtained with the nonreflecting wave equation and the exploding reflector approach using this technique and ray tracing algorithms we obtain a geological model by comparing the synthetic section with the real stacked section an isotropic constitutive equation is assumed in this phase the exact synthetic stacked section is then obtained by applying the standard processing sequence to a set of synthetic common shot profiles computed with the variable density acoustic wave equation we introduce elliptical p wave anisotropy and the effects of small scale inhomogeneities by using a von karman autocovariance probability function that simulates scattering q effects verification of the geological model by poststack migration constitutes an additional test the methodology which is suitable for areas of complex geology is applied to a seismic line acquired in the nort

p wave seismic attenuation by slow wave diffusion effects of inhomogeneous rock properties recent research has established that the dominant p wave attenuation mechanism in reservoir rocks at seismic frequencies is because of wave induced fluid flow mesoscopic loss the p wave induces a fluid pressure difference at mesoscopic scale inhomogeneities larger than the pore size but smaller than the wavelength typically tens of centimeters and generates fluid flow and slow diffusion biot waves continuity of pore pressure is achieved by energy conversion to slow p waves which diffuse away from the interfaces in this context we consider a periodically stratified medium and investigate the amount of attenuation and velocity dispersion caused by different types of heterogeneities in the rock properties namely porosity grain and frame moduli permeability and fluid properties the most effective loss mechanisms result from porosity variations and partial saturation where one of the fluids is very stiff and the other is very compliant such as a highly permeable sandstone at shallow depths saturated with small amounts of gas around 10 saturation and water grain and frame moduli variations are the next cause of attenuation the relaxation peak moves towards low frequencies as the background permeability decreases and the viscosity and thickness of the layers increase the analysis indicates in which cases the seismic band is in the relaxed regime and therefore when the gassmann equation can yield a good approximation to the wave velocity

seismic rheological model and reflection coefficients of the brittle ductile transition it is well established that the upper cooler part of the crust is brittle while deeper zones present ductile behaviour in some cases this brittle ductile transition is a single seismic reflector with an associated reflection coefficient we first develop a stress strain relation including the effects of crust anisotropy seismic attenuation and ductility in which deformation takes place by shear plastic flow viscoelastic anisotropy is based on the eigenstrain model and the zener and burgers mechanical models are used to model the effects of seismic attenuation velocity dispersion and steady state creep flow respectively the stiffness components of the brittle and ductile media depend on stress and temperature through the shear viscosity which is obtained by the arrhenius equation and the octahedral stress criterion the p and s wave velocities decrease as depth and temperature increase due to the geothermal gradient an effect which is more pronounced for shear waves we then obtain the reflection and transmission coefficients of a single brittle ductile interface and of a ductile thin layer the pp scattering coefficient has a brewster angle a sign change in both cases and there is substantial ps conversion at intermediate angles the pp coefficient is sensitive to the layer thickness unlike the ss coefficient thick layers have a well defined brewster angle and show higher reflection amplitudes finally we compute synthetic seismograms in a homogeneous medium as a function of temperature

u pb zircon geochronology of the adirondack mountains and implications for their geologic evolution u pb zircon studies of metamorphosed igneous rocks in the adirondack mountains have yielded preliminary ages within the range 1420 990 ma several geochronologically and geochemically distinct episodes of igneous intrusion and at least one pre granulite facies dynamothermal metamorphic event are documented this information is consistent with recent field and geochronological studies throughout the grenville province and suggests that a complex sequence of events occurred in the adirondack mountains prior to the widespread deformation and metamorphism commonly attributed to the approximately 1100 1000 ma ottawan phase of the grenvillian orogenic cycle

upb geochronology of the sri lankan basement abstract u pb ages of zircons and monazites from ortho and paragneisses representing the major crystalline basement units of sri lanka establish a new geochronological framework which is consistent with previously published work on sm nd model ages rb sr whole rock ages and u pb mineral ages in the highland complex three newly dated orthogneisses one previously dated by baur et al 1991 and one dated by kroner and williams 1992 all intruded 1 85 to 1 9 ga ago this widespread intrusion event occurred soon after the end of sediment deposition which is estimated at 1 9 to 2 0 ga using upper intercept ages of zircons from metapelites in the highland complex within the boundaries of this complex established by sm nd model age mapping milisenda et al 1994 only one orthogneiss has been found which is substantially younger than 1 8 ga this is the metatonalite from polonnaruwa which intruded 670 ma ago baur et al 1991 three orthogneisses from the vijayan complex which lies to the east of the highland complex yield similarly consistent crystallization ages of 1023 8 1016 28 and 1033 121 ma in the wanni complex which lies to the west of the highland complex a sillimanite bearing gneiss which is apparently of metasedimentary origin but contains zircons of igneous appearance yields a primary crystallization age of 793 4 ma consistent with a previously dated orthogneiss from kurunegala of 770 ma baur et al 1991 in addition single zircon data by kroner et al 1994 indicate primary crystallization ages of 1 0 to 1 1 ga for orthogneisses from the vavuniya charnockite province which is considered to be part of the wanni complex these results confirm the age contrast between the highland complex on the one hand and the vijayan complex and wanni complex on the other as previously inferred from sm nd model ages milisenda et al 1988 1994 post tectonic granitoids and pegmatites intruded throughout all three complexes 550 ma ago the age of regional high grade metamorphism mostly granulite facies in the highland complex mostly amphibolite facies in the vijayan complex and both granulite and amphibolite facies in the wanni complex is constrained to be less than 670 ma the primary crystallization age of the polonnaruwa metatonalite it is more closely defined by lower intercept zircon ages which include one concordant age of 608 4 ma and one discordant but well defined age of 611 23 ma and by a nearly concordant monazite age of 592 2 ma this indicates that peak metamorphic conditions occurred 610 ma ago numerous other lower intercept ages between 590 and 550 ma and post tectonic intrusion ages of 550 ma set a lower age limit for high grade conditions and deformation the temperature history between 610 and 550 ma is not yet well resolved and it is not clear whether there was a separate thermal event 550 ma ago previously published whole rock rb sr and sm nd isochrons for gneisses from the highland complex yielding ages of 2 ga and greater and previously interpreted to date the time of granulite grade metamorphism must be reinterpreted in the light of the new data presented here as well as other recently published results e g burton and o nions 1990b baur et al 1991 and in view of the accumulated evidence for very limited element mobility during high grade metamorphism these whole rock ages are in fact identical or close to the ages of primary granitoid emplacement rather than the age of granulite facies metamorphism

buoyancy frequency profiles and internal semidiurnal tide turning depths in the oceans 1 we examine the possible existence of internal gravity wave turning depths depths below which the local buoyancy frequency n z becomes smaller than the wave frequency at a turning depth incident gravity waves reflect rather than reaching the ocean bottom as is generally assumed here we consider internal gravity waves at the lunar semidiurnal m2 tidal frequency m2 profiles of n2 z the quantity in the equations of motion are computed using conductivity temperature and depth data obtained in the world ocean circulation experiment woce values for n2 z computed using gibbs seawater routines in two thermodynamically equivalent expressions for n2 z are found to yield values that are in excellent accord but differ significantly from n2 z computed from often used but inexact expressions that involve potential density uncertainties in n2 z are estimated using a monte carlo method where the data are averaged over a range in depth 80 200 m which is determined by minimizing a cost function our principal result reached from an analysis of all 18 000 woce casts is that turning depths are common for zonal east west propagating internal tides in the deep oceans inclusion of the full coriolis effect i e not making the so called traditional approximation leads to the conclusion that turning depths cannot occur for meridional and near meridional internal tides but the non traditional component has little impact on turning depths for internal tides that are near zonal i e propagating within about 30 of the east west direction at low and midlatitudes

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

creep of rocksalt abstract a review is presented of the fundamental flow properties and processes in experimentally deformed natural and synthetic halite single crystals and polycrystalline aggregates included in the summary are discussions of a microstructures induced during steady state creep b creep rupture of rocksalt c experiments associated with project salt vault and more recent field studies and d brine migration a representative steady state flow law determined for natural aggregates and maximum natural deviatoric stresses deduced from subgrain sizes are applied briefly to considerations of creep in waste repositories and of salt dome dynamics while the mechanical behavior of rocksalt is probably better understood than for all other rock types further investigations especially on load path stress history and creep rupture are clearly mandated furthermore additional investigations of brine migration and of bench and field scale deformations are needed the latter incorporating realistic rocksalt flow properties into numerical simulations of natural rock mass response

age of metamorphism in the high grade rocks of sri lanka controversy concerning the age of regional granulite grade metamorphism in sri lanka stems from the fact that different isotopic systems apparently record different events published isotopic data show that zircons from igneous and sedimentary protoliths experienced considerable pb loss at 550 ma and that new growth of zircon monazite rutile and garnet between 539 pm 6 and 608 pm 3 ma probably dates near peak metamorphic conditions upper concordia intercept ages between 670 and 1950 ma for magmatic zircons from high grade orthogneisses and charnockites date igneous emplacement and also support the occurrence of granulite metamorphism after 670 ma a granulite of igneous origin from the kataragama klippe in southern sri lanka yielded discordant ion microprobe data that suggest emplacement of the protolith at 1770 1880 ma thus a rb sr whole rock age of 1930 pm 130 ma from the same suite is unlikely to reflect isotopic homogenization during granulite formation and we interpret this and oth

the precambrian of sri lanka a historical review early studies of the geology of sri lanka lasted from the beginning of the century until 1935 the modern phase began in the forties whereas intensive studies of the precambrian rocks took place in the last ten years the term vijayan gneiss first used for the gneisses around colombo was changed in 1947 to vijayan series for all rocks outside the central khondalite charnockite belt the latter was named the highland series in 1961 and the vijayan series was subdivided into an eastern and a western component and the name was altered to vijayan complex in 1978 in 1959 a southwestern group was recognized and in 1935 the kadugannawa gneiss was named at present there is a general agreement that the following nomenclature should be used by all workers on the precambrian of sri lanka highland complex hc formerly highland series and southwestern group an intimate association of interbanded ortho and paragneisses including pelitic gneiss metaquartzite marble and charnockitic gneiss which forms the broad belt of rocks extending from northeast to southwest through the central highlands of the island outliers of hc rocks tectonic klippen are found in the vijayan complex at kataragama and maligawila vijayan complex vc formerly eastern vijayan complex consisting of migmatites granitic gneisses granitoids and scattered metasediments and occurring in the east and south of the hc wanni complex wc formerly western vijayan complex consisting of migmatites granitic gneisses charnockitic gneisses minor metasediments and granitoids lying west of the hc kadugannawa complex kc partly the former kadugannawa gneiss for the hornblende and biotite hornblende gneisses within the elongate synformal basins arenas around kandy in the early sixties it was noted that the highland series rocks had been subjected to granulite facies metamorphism and the vijayan series rocks to amphibolite facies metamorphism charnockitic rocks in the highland series showed the characteristics of metamorphic rocks recently near peak metamorphic conditions in the hc were established at 770 900 c and 5 kbar in the west wc to 9 kbar in the east hc with a pronounced regional gradient in palaeopressure relict inclusions of staurolite and kyanite in garnet of hc pelitic gneisses have been shown to document a late stage of a prograde metamorphic p t path a retrograde p t path with coronitic assemblages of retrograde mineral phases like andalusite has also been recorded outliers of hc rocks in the vc are klippen thrust over the vc parts of kc have been interpreted as a metamorphosed layered basic complex in the late seventies three phases of deformation were recognized in the hc presently four phases are known to occur an early view was that the highland series was younger than the vijayan series the former sediments having been laid down on a vijayan floor the hc is now known to be older than the vc the boundary between them being a thrust contact the exact position and nature of the hc wc boundary however is still to be resolved recent geochronological data suggest that 1 hc sedimentation as well as igneous activity in the form of basic lava flows basic sills and dykes took place 2 0 ga ago 2 intrusion of the earliest granitoids into the hc took place 1 8 to 1 94 ga ago 3 orthogneisses of the wc were intruded between 0 67 and 1 1 ga ago 4 orthogneisses of the vc were intruded 1030 to 1040 ma ago and 5 high grade regional metamorphism of the hc vc and wc took place between 455 and 610 ma ago

pacific abyssal transport and mixing through the samoan passage versus around the manihiki plateau abstractthe main source feeding the abyssal circulation of the north pacific is the deep northward flow of 5 6 sverdrups sv 1 sv 106 m3 s 1 through the samoan passage a recent field campaign

internal wave generation by tidal flow over periodically and randomly distributed seamounts we examine numerically the conversion of barotropic tidal energy into internal waves by flow over an isolated seamount and over systems of periodically and randomly distributed 1100 m tall seamounts with gaussian profiles the simulations use the massachusetts institute of technology general circulation model mitgcm to calculate for an infinitely deep ocean the dependence of the energy conversion on seamount slope seamount separation tidal direction and the size and aspect ratio of the simulation domain for neighboring seamounts with a slope greater than the internal wave beam slope wave interference reduces the conversion relative to that calculated for an isolated seamount and relative to that predicted by linear theory for a seamount of slope less than the beam slope the conversion by an individual seamount in a system of random seamounts separated by an average distance of 18 km is found to be suppressed by 16 relative to the conversion by an isolated seamount this study provides insight into tidal conversion by ocean seamounts modeled as gaussian mountains with slopes both smaller and larger than the beam slope we conclude that the total energy conversion by all seamounts peak height 1000 m and knolls peak height 500 1000 m taking into account interference affects is of the order of 1 of the total barotropic to baroclinic energy conversion in the oceans which is about twice as large as previous estimates

internal waves and mixing near the kerguelen plateau abstractin the stratified ocean turbulent mixing is primarily attributed to the breaking of internal waves as such internal waves provide a link between large scale forcing and small scale mixing the internal wave field north of the kerguelen plateau is characterized using 914 high resolution hydrographic profiles from novel electromagnetic autonomous profiling explorer em apex floats altogether 46 coherent features are identified in the em apex velocity profiles and interpreted in terms of internal wave kinematics the large number of internal waves analyzed provides a quantitative framework for characterizing spatial variations in the internal wave field and for resolving generation versus propagation dynamics internal waves observed near the kerguelen plateau have a mean vertical wavelength of 200 m a mean horizontal wavelength of 15 km a mean period of 16 h and a mean horizontal group velocity of 3 cm s 1 the internal wave characteristics are dependent on regional dynamics suggesting tha

behavior of isotopic systematics during deformation and metamorphism a hf nd and sr isotopic study of mylonitized granite a combined compositional and hf nd and sr isotopic study was performed on a suite of samples of progressively deformed granite from a mylonite zone in the harquahala mountains western arizona to evaluate the effects of deformation and metamorphism on the isotopic systematics of typical continental crustal rocks the 1 4 ga harquahala granite was deformed during mesozoic thrusting along the harquahala thrust granite in the resulting 60 m wide shear zone ranges from protomylonite to ultramylonite in most of these mylonites the protolith is not megascopically recognizable and can be discerned only by the progressive transition to undeformed granite isotopic analyses of hf nd and sr from the shear zone document the immobility of the hf and nd isotopic systems relative to that of the sr isotopic system during deformation the rb sr isotopic data show considerable scatter on an isochron plot exhibiting both gains and losses of rb and sr from the whole rock systems in contrast the sm nd and lu hf isotopic systematics are mostly well behaved on isochron diagrams plotting mostly in tight clusters or along 1 4 ga isochrons these results show that while the sr isotopic system in crustal rocks is quite susceptible to later tectonic disturbance both hf and nd isotopic systems can provide reliable model age information in continental crustal terranes even when the rocks have been subjected to low to medium grades of deformation and metamorphism

evidence for isotopic equilibration of smnd whole rock systems in early archaean crust of enderby land antarctica smnd isotopic data indicate that differential ree mobility occurred on a whole rock scale during transitional amphibolite to granulite facies regional metamorphism 700 c 7 kbar in early archaean rocks 3930 ma of the napier complex of enderby land antarctica the degree of mobility is independent of metamorphic grade but correlates directly with development of tectonic fabric whole rock samples with d3 m3 internal fabrics lie along an array corresponding to an age of 2410 100ma whereas samples preserving only earlier fabrics preserve an older albeit imprecisely defined isochron age in contrast to a widely held belief such changes did not require the presence of a large hydrous fluid flux if the mechanism responsible for smnd resetting at this locality where tchur ages range from 1990 ma to 6090 ma is more widespread than is currently recognised isolated smnd model ages particularly in complex terrains should be treated with caution

focal mechanisms in anisotropic media summary focal mechanisms of seismic sources in anisotropic media are more complicated than in isotropic media planar shear faulting produces pure double couple dc mechanism in isotropy but generally non double couple non dc mechanism in anisotropy the non dc mechanism can comprise both the isotropic iso and compensated linear vector dipole clvd components the amount of the iso and clvd components depends on strength and symmetry of anisotropy and on the orientation of faulting shear faulting in anisotropy generates a pure dc mechanism provided that faulting is situated in symmetry planes of orthorhombic or of higher anisotropy symmetries the fault plane solution i e the orientation of the fault normal and the slip direction can be retrieved from the seismic moment tensor and from elastic parameters describing anisotropy at the source numerical modelling shows that shear faulting in anisotropic rocks present in the earth crust the earth mantle or in the subduction zones can produce mostly mechanisms with the clvd up to 30 per cent and with the iso up to 15 per cent the fault plane solutions calculated under the assumption of isotropy typically deviate from the true solutions by an angle of 5 to 10

deformation of a high grade gondwana fragment sri lanka abstract the sri lankan gondwana fragment is a high grade gneiss terrain consisting of at least three crustal units namely the wanni complex wc the highland complex hc and the vijayan complex vc which were juxtaposed during the pan african collision at least six phases of ductile deformation were recognized in the smallest gondwana fragment sri lanka there is microscopic and mesoscopic evidence for a first deformation d 1 which occurred during prograde metamorphism but before the main granulite facies metamorphism this first deformation is represented by straight to curved and crenulated inclusion trails in garnet porphyroblasts microfolds in some metapelites and quartzo feldspathic gneisses and by a s 1 foliation preserved in crenulation folds and some rootless intrafolial folds d 2 has two main stages of which the second phase is the most intense and dominant deformation which produced most part of the major composite and transposed compositional layering as well as the prominent stretching lineation l 2 the first stage of d 2 may have been responsible for the formation of a crenulation cleavage of s 1 as seen in garnet porphyroblasts in some metapelites the compositional layering s 2 in metapelites and metasemipelites appears to have formed through the development of a crenulation cleavage f 2 folds are mesoscopic isoclinal rootless and occur as intrafolial folds and their limbs are highly attenuated major part of the deformation during the latter stage of d 2 was non coaxial many minor and large scale recumbent isoclinal folds f 3 were produced during d 3 both d 2 and d 3 were coeval with the peak conditions of granulite facies metamorphism d 4 produced very large gentle nearly e w upright folds probably during the beginning of uplift of the entire high grade assemblage from deeper crustal levels still under granulite facies conditions d 5 is the second strongest deformation and was responsible for the large scale upright folds f 5 which control the present map pattern of sri lanka the superimposition of d 5 folds on d 4 folds gave rise to a large scale type i fold interference pattern of ramsay 1967 d 6 deformation caused local refolding of the f 5 folds and was accompanied by amphibolite facies conditions during uplift the deformation represented by the majority of vein type structures such as patchy charnockite veins cordierite bearing veins and graphite veins clearly post date d 5 it is argued that the wc the hc and the vc in sri lanka may be three distinctly different proterozoic crustal units brought into contact by tectonism in late proterozoic at two different stages

depth of the brittle ductile transition in a transcurrent boundary zone a model is proposed which describes the boundary zone between two transcurrent plates as a viscoelastic body with rheological properties changing with depth in this model the brittle ductile transition is defined as the depth at which the time derivative of shear stress changes from positive to negative values variations of this depth are studied as functions of geothermal gradient rheological parameters and strain rate using a power law rheology with exponent ranging from 1 to 4 stress relaxation in the ductile zone is controlled by a local characteristic time which depends on petrology temperature and in the case of non newtonian rheology on strain rate the composition and the hydration degree of crustal rocks may also sensibly influence the depth of the brittle ductile transition the model predictions are compared with observations regarding the san andreas imperial valley and north anatolian faults it is found that values ofn from 1 to 3 are more appropriate to reproduce the transition depth inferred by the seismicity distribution

extensional shear zones as imaged by reflection seismic lines the larderello geothermal field central italy the larderello geothermal field is located in the inner northern apennines in an area which has been subject to extension since the early miocene the latest extensional episode pliocene present has resulted in the formation of nw trending nedipping listric normal faults whose geometry is controlled down to f3 km by borehole data in this paper we integrate a new interpretation of seismic reflection lines with existing seismic field and borehole data to analyse the relations among listric normal faults the top of the brittle ductile transition and the migration of geothermal fluids in accordance with previous interpretations we consider the strong reflector k horizon marking the top of the reflective mid lower crust and located at a depth of 3 5 km in the geothermal area to represent the top of the brittle ductile transition its reflectivity most probably derives from the presence of overpressured fluids we identify three main nw trending nedipping extensional brittle shear zones showing listric geometry and soling out in the vicinity of the k horizon the latter appears to be dislocated in correspondence of the soling out of the shear zones these shear zones because of the associated intense fracturing represent the most natural channels of upward migration of geothermal fluids from the magmatic sources located below the k horizon we suggest that these two conclusions that listric normal faults root at or near the brittle ductile transition and that they act as preferential upward migration paths for magmatic fluids may be of general validity for geothermal fields located in extensional settings d 2002 elsevier science b v all rights reserved

constraints on the viscosity of the continental crust and mantle from gps measurements and postseismic deformation models in western mongolia 1 we use gps measurements and models of postseismic deformation caused by seven m6 8 to 8 4 earthquakes that occurred in the past 100 years in mongolia to assess the viscosity of the lower crust and upper mantle we find an upper mantle viscosity between 1 1018 and 4 1018 pa s the presence of such a weak mantle is consistent with results from independent seismological and petrological studies that show an abnormally hot upper mantle beneath mongolia the viscosity of the lower crust is less well constrained but a weak lower crust 3 1016 to 2 1017 pa s is preferred by the data using our best fit upper mantle and lower crust viscosities we find that the postseismic effects of viscoelastic relaxation on present day horizontal gps velocities are small 2 mm yr 1 but still persist 100 years after the 1905 m8 4 bolnay earthquake this study shows that the gps velocity field in the baikal mongolia area can be modeled as the sum of 1 a rigid translation and rotation of the whole network 2 a 3 5 mm yr 1 simple shear velocity gradient between the siberian platform to the north and northern china to the south and 3 the contribution of postseismic deformation mostly caused by the 1905 bolnay tsetserleg sequence and by the smaller but more recent 1957 bogd earthquake

isotopic mapping of age provinces in precambrian high grade terrains sri lanka nd model ages of amphibolite and granulite grade rocks in sri lanka form a simple regional pattern that broadly correlates with mappable geological units and is in effect an isotopic map of the island s basement the granulite grade units of the highland group and southwest group have model ages of 2 2 3 0 ga indicating derivation mainly from late archean sources they are bounded to the east and west by late proterozoic gneisses of the vijayan complex with model ages of 1 1 2 0 ga the isotopic data identify three distinct crustal provinces and are not consistent with earlier suggestions that the vijayan gneisses are retrograde equivalents of the highland granulites sri lanka is not a direct continuation of the archean dharwar craton of southern india identification of vijayan type juvenile crustal terrains in other gondwana fragments may play a key role in determining the precise attachment of southern india sri lanka in eastern gondwana

pb loss patterns in zircons from a high grade metamorphic terrain as revealed by different dating methods upb and pbpb ages for igneous and metamorphic zircons from northern sri lanka abstract we report u pb and 207 pb 206 pb ages for multigrain fractions single grains and grain domains in zircons from highgrade igneous and sedimentary rocks of the vavuniya charnockite province vcp in northern sri lanka using the ionmicroprobe shrimp a new vapour digestion technique vdt and evaporation charnockitic enderbitic and mangeritic gneisses are derived from i type tonalitic syenitic granodioritic and granitic precursors for which the u pb systematics of zircons as determined by shrimp suggest crystallization ages between 1000 and 1100 ma many translucent round and multifacetted zircons in these rocks are not of metamorphic but of igneous origin and acquired their shape through partial zircon dissolution and recrystallization the large majority of zircons exhibit severe and variable pb loss some 540 580 ma ago which we ascribe to granulite grade metamorphism almost immediately followed by local retrogression most of the zircons lost pb from the entire grain with no domains remaining that reflect the original pb isotopic composition it was therefore impossible to obtain meaningful 207 pb 206 pb evaporation ages for such grains and this cautions against use of this method in zircons that experienced severe pb loss however zircons formed during high grade metamorphism apparently retain their primary pb isotopic composition and were dated successfully by the evaporation technique yielding ages between 553 26 and 560 30 ma conventional multigrain analysis and small fractions following vdt for two samples although much more precise than the shrimp data yielded only strongly discordant data points with patterns reflecting variable pb loss these data alone would not have made it possible to obtain reliable primary zircon crystallization ages 207 pb 206 pb evaporation ages for detrital zircons from two garnetiferous gneiss samples of sedimentary origin in the centre of the vcp revealed a maximum age of 1329 18 ma whereas a metaquartzite near the southern margin of the vcp contained zircons with 207 pb 206 pb ages between 1857 14 and 2543 14 ma we interpret the latter rock as belonging to the highland complex which occurs adjacent to the vcp our zircon data in combination with nd model ages milisenda et al 1994 suggest the vcp to be part of the wanni complex a distinct crustal domain in northern sri lanka characterized by relatively young ages in comparison with much older ages in the adjacent highland complex

three dimensional tomography and rock properties of the larderello travale geothermal area italy in a geothermal area a detailed knowledge of the three dimensional velocity structures aids the management of the field and the further development of the geothermal source here we present a high resolution study of the three dimensional s wave velocity structures from microearthquake travel times for the larderello travale geothermal field italy we have also deduced the vp vs and vp vs parameters for this area to emphasize the deep variations in the physical rock properties due to fluid content and porosity furthermore effective porous medium modelling has been performed for site relevant lithologies to improve our interpretation of the results in terms of rock physics signatures this has allowed us to estimate the variation range of the seismological parameters investigated as well as their sensitivity for suitable rock under specific physical conditions low vp vs anomalies arising from a lower vp compared to vs dominate the geothermal field of larderello travale these have been interpreted as due to steam bearing formations on the contrary analysis of vp vs images provides information on the relative changes in rock porosity at depth comparison of tomographic section images with previously interpreted seismic lines suggests that the reflective k horizon delineates a transition between zones that have different porosities or crack gatherings the k horizon also lies on low vp vs anomalies which suggests a steam saturation zone despite the reduced porosity at this depth

isotopic constraints on emplacement age of anorthositic rocks of the marcy masiff adirondack mts new york the igneous emplacement age of anorthositic rocks comprising the marcy massif have been constrained by fixing minimum and maximum ages for their intrusion minimum ages have been fixed by a u pb dating of cores of air abraded zircons from the anorthosite 1113 ma b u pb dating of baddeleyite within the anorthosite 1087 ma c u pb dating of baddeleyite from an olivine metagabbro crosscutting marcy massif anorthosite 1109 ma d u pb zircon dating of jotunitic rocks crosscutting the marcy anorthosite massif 1143 pm 31 ma maximum emplacement ages are based upon a sheets of anorthositic rock crosscutting mangeritic charnockitic gneiss dated by u pb zircon at 1125 pm 10 ma 1134 pm 4 ma and 1135 pm 43 ma and b anorthosite related pyroxenite dikes crosscutting mangerite that contains xenoliths of identical pyroxenite these results constrain emplacement of the marcy anorthosite to 1113 1138 ma with the most probable age in the interval 1125 1135 ma this time of in

high temperature experiments on the elastic and anelastic behaviour of magmatic rocks measurements of the complex shear modulus have been performed on various ultrabasic and basic rocks dunite peridotite gabbro and basalts and a synthetic forsterite at temperatures between 600 and 1500 c depending on the sample material and in the frequency range from 0 003 to 30 hz under forced torsional oscillations the maximum shear stress was 0 3 mpa and the strains were of the order 5 10 6 to 5 10 5 measurements of the electrical resistivity as a function of temperature have been carried out together with the mechanical experiments to indicate the beginning of partial melting the absolute values of the shear modulus below tm are comparable with the results of other authors the onset of melting shows up in a rapid decrease of both shear modulus and electrical resistivity at high temperatures t 0 8 tm the absorption factor q 1 follows a power law q 1 exp a rt with 0 15 0 30 and a between 100 and 200 kj mol 1 towards lower temperatures this high temperature background is superimposed by a flat absorption peak which shifts from high to low frequencies with falling temperature this peak can often be described by a lognormal distribution of simple relaxation processes with a standard deviation of about three and a mean relaxation time exp a rt with a 400 600 kj mol 1 above tm a third contribution to q 1 is observed that can be regarded as independent of frequency between 1 and 30 hz this last component which contributes 50 to the total absorption is attributed to the melt phase in the samples an explanation for the high temperature background and the peak is proposed which is based on the relaxation of pinned dislocation lines by some coupling mechanism via the reorganisation of the stress field with increasing deformation this process may be responsible for both features at very high temperatures and low frequencies a small influence of steady state flow on q 1 is observed and below t 0 6 tm thermal cracking or thermoelastic losses may contribute to q 1 the influence of lithostatic pressure on q 1 is estimated and the extrapolation of the experimental data to the p t conditions of the uppermost mantle are in good agreement with published earth models an attempt is made to establish a general q t relationship from the experimental data

the origin of the pink granites of sri lanka another view abstract pink granites are common throughout the precambrian metasedimentary terrain of sri lanka and play a significant role in reconstructing the geologic evolution of this country essentially microcline bearing rocks they form conformable layers with the associated metasedimentary rocks and are always interlayered and associated with rocks possessing amphibolite facies mineral assemblages this association is seen throughout the terrain at all scales further they always tend to fold around the major structures of the terrain and contain conformable layers and enclaves of rare metasediments and have a gneissic appearance in places their field relations associations mineralogy petrography petrofabrics chemistry and also age suggest that they are metasediments probably arkoses which have undergone the same set of metamorphic and deformational events as other precambrian metamorphic rocks of sri lanka their present granite like character does not necessarily imply that they are the products of crystallization of magmatic or anatectic melts or that they have had a replacement origin rather chemical composition has determined their present granite like character due to granitization by simple isochemical recrystallization treptomorphism during highgrade metamorphism at a ph2o which was sufficient to retard their grade to amphibolite facies under granulite facies metamorphic conditions at which the other metasediments of sri lanka have metamorphosed

secondary siderite oxide sulphide and carbonate andalusite assemblages in cordierite granulites from sri lanka post granulite facies fluid evolution during uplift we report here that some of the pelitic rocks from the wanni and highland complexes of sri lanka reacted with co2 rich fluids to produce a wide range of unusual secondary carbonate silicate oxide sulphide assemblages these enable the depth temperature and fluid compositions of co2 reactions to be calculated more rigorously than is generally possible for the patches of arrested charnockite that have been described from sri lanka magnesite andalusite quartz has partially replaced primary cordierite and siderite rutile replaced ilmenite paragenetic sequences involving primary pyrrhotite ilmenite and magnetite and secondary pyrite siderite rutile magnetite hematite demonstrate the control which carbonate equilibria have upon evolving fluid compositions during cooling direct evidence for the role of graphite as a source of co2 is found in the highland complex where primary graphite partially reacted with silicates to form secondary siderite assemblages it is proposed that following peak metamorphism continued uplift along a clockwise p t t path was accompanied by a series of devolatilization reactions involving breakdown of graphite and the continuous production of secondary co2 rich fluids the limited extent of disseminated secondary carbonate reflects the small amount of graphite inferred to have been present in the source rocks these rocks demonstrate that co2 rich fluids as found in disseminated fluid inclusions need not form during peak granulite metamorphism but may be an inevitable consequence of continued uplift along a clockwise p t t path the arrested charnockite which overprinted some of the hornblende bearing felsic intermediate composition rocks in sri lanka most likely formed by the same process

summary and discussion of p t estimates from garnet pyroxene plagioclase quartz bearing granulite facies rocks from sri lanka abstract in granulite facies rocks of granitic to basic composition from the highland complex hc of sri lanka maximum p t estimates for about 115 locations have been obtained from core compositions of garnet orthopyroxene and plagioclase their distribution shows a decrease in pressure from the east and southeast where pressures exceed 9 kbar towards the west and southwest where pressures are about 5 kbar the accompanying temperature decrease in the order of about 200 c is less pronounced the regional distribution pattern of garnet quartz clinopyroxene reaction textures around orthopyroxene and plagioclase indicates that the lower crustal rocks of the eastern and southeastern parts of the hc have experienced an episode of near isobaric cooling subsequent uplift resulted in the breakdown of garnet and clinopyroxene to orthopyroxene plagioclase this reaction texture is found in all parts of the hc with the exception of the west and southwest where the higher crustal levels are exposed comparisons of the array of calculated maximum p t data with published continental geotherms for normal to elevated mantle heat flows 30 50 70 and 90 10 3 w m 2 at the base of a 36 km crust show that the depth versus temperature gradient is not a simple geothermal one the p t array in which the gradient is steeper than the geotherms would be consistent with magmatic heating this model would suggest a scenario in which intrusions that were generated in the lower crust were emplaced at higher crustal levels and heated up the surrounding rocks however since the number of known slightly pre to synmetamorphic intrusions in sri lanka is very limited other mechanisms of the heat supply seem to be required prior to uplift and exposure of this partial cross section through the lower to upper crust

transient and steady state deformation of synthetic rocksalt abstract synthetic rocksalt with a porosity less than 2 5 and an average grain size of about 0 35 mm was made by warm pressing at 100 c 150 mpa and for 15 min cylinders of this material 25 mm in diameter by 50 mm long were deformed at strain rates of 0 1 ksec 1 at confining pressures of 20 50 100 and 200 mpa and at temperatures of 100 200 and 300 c the resulting stress time data show transient stress behavior before steady state stress occurs very little variation in the stress time data occurs for the above confining pressures at a constant temperature many of the tests reach steady state at 10 strain where all the experiments were terminated the differential stress at 10 varies from about 22 mpa at 100 c to about 6 mpa at 300 c these strengths are slightly less than those measured by heard 1972 also on synthetic polycrystalline rocksalt and are similar to those measured by wawersik and hannum 1980 and by hansen and mellegard 1980 in modified creep tests on coarse grained natural rocksalt under similar pressure and temperature conditions activation energies computed from these steady state stresses vary between 7 5 and 25 kcal mole and are consistent with those obtained by heard 1972 and by parrish and gangi 1977 1981 the stress time data were fit using one and two mechanism transient stress functions which assume independent mechanisms or processes the rms errors decrease from about 1 mpa for the one mechanism fits to about 0 2 mpa for the two mechanism fits indicating at least two mechanisms are operative in these tests similar one and two mechanism fits were made to creep tests performed by w wawersik et al on new mexico bedded rocksalt similar improvements in the fits were obtained for those tests that lasted long enough so that the effect of a second mechanism could be noted it was found that the steady state strain rate found in creep tests could be interpreted as the beginning of another mechanism this raises the question of whether the steady state phenomena exist at all or whether it is just an approximation to a mechanism with a time constant that is long compared to the length of the test

theory of borehole stability when drilling through salt formations we present a mathematical analysis of borehole stability when drilling through rock salt first we consider an elastic transversely isotropic medium and find the optimal mud weight as a function of the vertical overburden and horizontal tectonic stresses then the zener and maxwell mechanical models are used to model the effects of transient and steady state creep flow respectively in isotropic media under certain conditions such as the absence of dilatational anelasticity the burger model can be used to describe the steady state flow including transient creep effects the type of creep is regulated by critical octahedral stress values that depend on temperature and pressure a typical drilling results in conditions of plane strain whose solution is given by kirsch s equations in this case the borehole is subject to minimum and maximum horizontal stresses which differ from the vertical stress the analysis provides expressions for the shape of the borehole cross section the boreholewall closure time and the optimal mud weight to avoid wall collapse or expansion it is shown that an anisotropic state of tectonic stress may require mud pressures exceeding the overburden stress and that the calculation should consider the joint optimization of the shape and area of the borehole cross section

fe ti oxide chronometry with application to granulite formation abstract ilmenites occurring in amphibolite and granulite facies orthogneisses in sri lanka yield precise u pb ages coexisting ilmenite magnetite in the granulite facies rocks indicate temperature conditions of 680 60 c and log o 2 17 0 1 2 the u pb age of 498 2 3 3 ma obtained for ilmenite is indistinguishable from the pb pb age of 501 5 ma derived for coexisting silicate minerals and suggests that oxide silicate equilibration has been preserved in contrast fe ti oxides in adjacent amphibolitefacies rocks indicate a temperature of 830 60 c and log o 2 12 7 0 8 and record u pb ages of 1093 7 7 6 ma and 1095 4 4 2 ma these ages are some 600 ma older than those obtained for coexisting silicate phases in the same sample or for both oxide and silicate minerals in the nearby charnockite and relate either to the time of emplacement of the granitic precursor or to some earlier metamorphic episode these results suggest that temperature o2 and fluid conditions estimated from fe ti oxides should be interpreted with extreme caution