deformation of the long valley caldera eastern california from mid 1983 to mid 1988 measurements using a two color geodimeter deformation of long valley caldera in eastern california has been closely monitored since june 1983 with a two color geodimeter which has a precision between 0 12 and 0 20 ppm of the baseline length initial trilateration measurements were concentrated in the south moat of the caldera the source of the january 1983 earthquake swarm since then the network has been expanded from 13 to 42 baselines the additional baselines span the resurgent dome and the western half of the caldera with a combination of measurements made several times per week on 7 11 baselines and monthly or yearly measurements on the remaining baselines we have measured both the temporal and spatial deformation of the caldera although the m 6 4 chalfant valley earthquake was located 35 km from the nearest baseline in the network we observed coseismic strain changes of up to 1 5 ppm on several baselines using fault planes selected to coincide with the hypo centers of the main shock and the m 5 9 foreshock we have inferred a combined moment of 3 0 1025 dyn cm from the two color data this cumulative moment of both earthquakes is equivalent to a magnitude 6 3 shock over the period from mid 1983 to mid 1988 the long term trends determined from the length changes of the baselines within the caldera show a general decrease in the rate of extension during 1983 measurements show a maximum extension rate of 5 0 ppm yr after mid to late 1984 the extension rates decreased by a factor of 2 4 relative to those in the first year with the exception of the coseismic step the extension rates have been fairly constant during the interval between late 1984 and mid 1988 the long term changes in rate can be modeled by using two mogi point sources representing inflation beneath the resurgent dome of the caldera and a dislocation surface representing slip in the south moat modeling indicates that both the rates of slip and inflation have decreased since mid 1983 but these two processes are still occurring at a measureable and constant rate

an unusual high mg garnet spinel orthopyroxenite from southern india evidence for ultrahigh temperature metamorphism at high pressure conditions we report here a garnet spinel orthopyroxenite in close association with an ultrahigh temperature uht granulite from the central part of the madurai granulite block in southern india the garnet spinel orthopyroxenite is almost entirely composed of orthopyroxene spinel and rare garnet in a granular texture spinels in the rock are characterized by high mg x mg 0 69 0 71 with low cr and fe 3 consistent with compositions reported from spinels occurring within xenoliths in kimberlites and high pressure temperature p t alpine complexes the orthopyroxenes have high al content al 2 o 3 up to 4 85 wt typical of equilibration under high p t conditions the p t estimates derived for the garnet spinel orthopyroxenite indicate temperatures of around 1000 c and pressures exceeding 17 kbar the data indicate that uht metamorphism in this locality traversed from above 17 kbar to 11 kbar prior to the final stage of isothermal decompression our study reports the highest pressures obtained by far for extreme crustal metamorphism in southern india and elsewhere in gondwana the multi stage decompression observed in the uht rocks associated with the high p t garnet spinel orthopyroxenite could be correlated to extension of the crust and possibly of the lithospheric mantle and or its delamination with the asthenospheric mantle as the ultimate heat source during the final stage of amalgamation of the gondwana supercontinent

dynamic deformation of seguam island aleutian islands alaska 1993 2000 implications for magmatic and hydrothermal processes 1 the most recent volcanic eruption on seguam island was detected in december 1992 and lasted until july or august of 1993 a series of radar interferograms generated from imagery collected by the european space agency s ers 1 and ers 2 satellites are used here to map the spatial and temporal evolution of the deformation field from june of 1993 to september of 2000 this evolution consists of a coeruptive period of uplift surrounding the eruption two posteruptive subsidence signals centered within each of the two calderas on the island each of which lasted for 4 years after the end of the eruption and then uplift beneath the caldera on the eastern side of the island and the surrounding region from mid 1999 through the end of the time series in september 2000 the locations and magnitudes of the sources of the deformation are inferred from the interferometric synthetic aperture radar displacement data using an elastic half space earth model with multiple mogi point sources and a levenburg marquardt nonlinear inversion technique this inference reveals that the sources of subsidence are located within the upper 2 km of the earth s crust while the sources of uplift are located below 2 5 km the locations of dilatational sources are consistent with a model of the volcanic system where ascending magma accumulates below a density or structural boundary near 2 5 3 km depth the subsidence signals are most likely associated with cooling of shallow magmatic bodies emplaced during the 1992 1993 eruption however the possibility that they indicate a combination of magma cooling and depressurization of a hydrothermal system cannot be ruled out using the geodetic data alone implications for the long term evolution of the volcanic edifice are discussed

inherited smnd isotope components preserved in monazite inclusions within garnets in leucogneiss from east antarctica and implications for closure temperature studies abstract application of the sm nd dating technique using garnet can be hampered by the presence of minor microscopic to submicroscopic ree rich inclusions in particular monazite and to a lesser extent zircon an acid leaching method using a mixture of hno 3 hcl has been used to tackle the monazite contamination problem with this leaching technique we demonstrate that inherited monazite inclusions have been preserved in garnet formed under high grade metamorphic conditions even very low levels of monazite contamination 0 001 wt have a profound effect on the sm nd system in the garnet analysis of a garnet from a leucogneiss dyke in the northern prince charles mountains eastern antarctica reveals that unleached garnet concentrates and the monazite inclusions as represented by the leachates define an apparent isochron of 1150 30 ma mswd 1 88 which represents a mixing age of inclusions preserving their inherited sm nd isotopic systematics in contrast analyses of the leached garnet concentrates yield much younger ages of 630 to 555 ma model calculations show that the presence of as little as 0 01 wt of monazite inclusions would swamp the sm nd isotope signature of the garnet concentrates this very low level of contamination is beyond the reach of physical purification processes and without leaching contamination is inevitable in inclusion bearing garnets such contamination can lead to older apparent ages than those of garnet formation and thus has profound implications for sm nd as well as u pb garnet chronometry

study of volcanic sources at long valley caldera california using gravity data and a genetic algorithm inversion technique we model the source inflation of the long valley caldera california using a genetic algorithm technique and micro gravity data while there have been numerous attempts to model the magma injection at long valley caldera from deformation data this has proven difficult given the complicated spatial and temporal nature of the volcanic source recent work illustrates the effectiveness of considering micro gravity measurements in volcanic areas a genetic algorithm is a problem solving technique which combines genetic and prescribed random information exchange we perform two inversions one for a single spherical point source and another for two sources that might represent a more spatially distributed source the forward model we use to interpret the results is the elastic gravitational earth model which takes into account the source mass and its interaction with the gravity field the results demonstrate the need to incorporate more variations in the model including another source geometry and the faulting mechanism in order to provide better constraints on intrusion volumes future work should include the joint inversion of gravity and deformation data during the same epoch

geodetic methods for detecting volcanic unrest a theoretical approach in this paper we study the application of different geodetic techniques to volcanic activity monitoring using theoretical analysis this methodology is very useful for obtaining an idea of the most appropriate and efficient monitoring method mainly when there are no records of geodetic changes previous to volcanic activity the analysis takes into account the crustal structure of the area its geology and its known volcanic activity to estimate the deformation and gravity changes that might precede eruptions the deformation model used includes the existing gravity field and vertical changes in the crustal properties both factors can have a considerable effect on computed deformation and gravity changes topography should be considered when there is a steep slope greater than 10 the case study of teide stratovolcano tenerife canary islands for which no deformation or gravity changes are available is used as a test to avoid considering topography we worked at the lowest level of las canadas and examined a smaller area than the whole caldera or island therefore the results are only a first approach to the most adequate geodetic monitoring system the methodology can also be applied to active areas where volcanic risk is not associated with a stratovolcano but instead with monogenetic scattered centers especially when sites must be chosen in terms of detection efficiency or existing facilities the canary islands provide a good example of this type of active volcanic areas and we apply our model to the island of lanzarote to evaluate the efficiency of the monitoring system installed at the existing geodynamic station on this island topography is not important the results of our study show clearly that the most appropriate geodetic volcano monitoring system is not the same for all different volcanic zones and types and the particular properties of each volcano zone must be taken into account when designing each system

geophysical investigation of lithosphere structure beneath sri lanka using surface and internal load variations this paper describes a method for interpreting the lateral changes in the geological structure of the lithosphere by iteratively calculating the spatial variation of a bottom to top loading ratio we use forsyth s coherence method based on multi taper spectral analysis on overlapping window areas we discuss the geological findings and their comparison with existing geological interpretations for the sri lankan region the low thickness of the elastic plate in the sri lankan region suggests a high geothermal gradient the lithospheric geological structure beneath the sri lankan region is consistent with the main crustal units of sri lanka which belong to different ancient plates together with interpretations based on metamorphic rock units our findings support the existence of thrust contact in the central highlands of sri lanka

ultrahigh temperature metamorphism followed by two stage decompression of garnet orthopyroxene sillimanite granulites from ganguvarpatti madurai block southern india the mg al granulites from ganguvarpatti consist of orthopyroxene sillimanite garnet quartz as peak assemblage with a few porphyroblasts of cordierite and sapphirine these assemblages were strongly overprinted by late symplectites and coronas orthopyroxene inclusions in garnet and porphyroblast cores have the highest xmg 0 80 and al2o3 content 10 7 wt the estimated near peak metamorphic conditions 1 000 50 c and 11 kbar using garnet orthopyroxene geothermobarometry are consistent with those determined using a petrogenetic grid the proposed multi stage evolution process implies an initial decompression deduced from multi phase symplectites followed by cooling during biotite formation further late decompression is explained from the orthopyroxene rims on biotite this proposed p t path thus suggests a unique and complex evolution history for the uht granulites of southern india present results are comparable with similar adjacent terranes in the gondwana supercontinent but the lack of structural and geochronological data makes a link with any major metamorphic event uncertain

observations of strain accumulation across the san andreas fault near palmdale california with a two color geodimeter two color laser ranging measurements during a 15 month period over a geodetic network spanning the san andreas fault near palmdale california indicate that the crust expands and contracts aseismically in episodes as short as 2 weeks shear strain parallel to the fault has accumulated monotonically since november 1980 but at a variable rate improvements in measurement precision and temporal resolution over those of previous geodetic studies near palmdale have resulted in the definition of a time history of crustal deformation that is much more complex than formerly realized

u pb single zircon pb evaporation and sm nd isotopic study of a granulite domain in se madagascar madagascar is a poorly known precambrian domain at the eastern border of the mozambique belt strongly affected by the pan african orogeny this large island represents a key part of gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent in the southeastern zone of the island metamorphism has reached high temperature and low pressure conditions 850 c and 5 kbar with moderate retrogression a u pb conventional multigrain single zircon stepwise pb evaporation and sm nd geochronological study on meta felsic rocks has shown that basement of at least lower proterozoic age was strongly metamorphosed around 570 580 ma this tectometamorphic event nearly obliterated any pre pan african zircon isotopic memory the granulite facies metamorphism is associated with the syn metamorphic emplacement of the anosyan charnock ites and granites a strong channelized fluid contribution at 545 ma has produced the re crystallization of biotite apatite

stability of pargasite during ultrahigh temperature metamorphism a consequence of titanium and ree partitioning orthopyroxene clinopyroxene plagioclase needles and symplectite along the cleavage planes and grain boundaries of fluorine bearing titanian ferroan pargasite from the highland complex sri lanka are interpreted as evidence for dehydration melting at ultrahigh temperature conditions high ti up to 0 4 pfu and f x f up to 0 56 content in pargasite extends its stability to higher temperatures and the composition indicates the dehydration melting reaction may take place at ultrahigh temperatures 950 c at a pressure around 10 kbar close to peak metamorphic conditions the increase of ti content close to the grain boundaries and cleavage planes in pargasite indicates titanium partitioning from the melt during dehydration melting enhanced the stability of the mineral toward ultrahigh temperature conditions the ree content in the pargasite shows a similar behavior to that of titanium the cores with no breakdown assemblage consist of low and flat ree concentrations with respect to the high and eu depleted rim clinopyroxene in symplectite and needle shaped lamellae within the pargasite porphyroblasts have similar ree patterns with slightly low concentrations relative to that of pargasite in the breakdown assemblage lrees are partitioned mainly into plagioclase while the hrees are partitioned into orthopyroxene the ree enrichment in the pargasite rims signals their relative partitioning between pargasite rims and melt modeling of the partitioning of ti and rees associated with pargasite breakdown demonstrates that its stability is greatly enhanced at uht conditions this investigation implies that the stability of hydrous minerals such as amphibole can be extended to uht conditions and expands our knowledge of metamorphism in the lower crust

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

sm nd dating and cooling history of scourian granulites sutherland the scourian gneisses of the lewisian complex of north west scotland have been the subject of many geochronological investigations sm nd whole rock measurements suggest that the protoliths of the lewisian complex differentiated 2 92 0 05 gyr ago from an approximately chrondritic mantle1 rb sr and pb pb whole rock ages between 2 8 and 2 6 gyr reflect subsequent depletion in rb and u refs 2 3 which has generally been associated with granulite facies metamorphism zircons which are thought to have crystallized during this metamorphism have also given u pb ages of 2 66 gyr ref 4 this granulite facies metamorphism involving pressures in excess of 10 kbar and peak temperatures estimated between 820 50 c ref 5 and 1 250 c refs 6 7 affects a terrain including supracrustal rocks7 and thus implies a major tectonic event we report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in sm nd ratios garnet especially is strongly enriched in sm ref 8 data for three garnetiferous basic gneiss assemblages suggest that closure of the sm nd system occurred significantly later than the peak of the metamorphism

inflation of long valley caldera california basin and range strain and possible mono craters dike opening from 1990 94 gps surveys five years of annual global positioning system gps surveys of a network centered on long valley california constrain displacement rates for these stations relative to a central station in the network these observations are consistent with recent models of resurgent dome inflation in long valley langbein et al 1995 and have sufficient signal to detect the presence of basin and range strain in the long valley region the data also allow for the possibility of dike inflation beneath the mono craters dike intrusion is consistent with the mono craters recent geologic history of ash eruptions with seismic tomography leveling data and geologic studies of these volcanic domes and flows

nd and sr systematics in an eclogite xenolith from tanzania evidence for frozen mineral equilibria in the continental lithosphere combined isotopic nd sr and petrographic analyses are reported for an eclogitic xenolith from the vissury mine tanzania an exceedingly large difference in measured nd values in garnet and clinopyroxene 200 units suggest a major mineral equilibration at 1 75 0 014 ga nd isotopic heterogeneities are demonstrated in different generations of garnet and zonation in sr and nd isotope compositions is present within the clinopyroxene the variation in measured nd isotope compositions in different generations of garnet corresponds to 16 units exsolution of garnet from clinopyroxene occurred in an essentially closed system with respect to sr and nd petrographically different generations of garnet are present one garnet with rutile inclusions as well as the rims of clinopyroxene are clearly off the isochron features like this can originate either if a system is not closed e g affected by a migrating fluid or if a remanent isotopic heterogeneity survived the homogenisation at 1 75 ga the subsolidus partitioning of sm and nd between garnet and clinopyroxene results in higher fractionation of the smnd ratio in garnet than in the liquid crystal partitioning since the closure temperature for the sm nd exchange between garnet and clinopyroxene may be around 850 c in a dry system many of the lithospheric ultramafic rocks may exhibit frozen mineral equilibria the isotopic data suggest that this eclogite was emplaced at 1 75 ga into the lithosphere mantle at temperatures of about 1040 c and then cooled as a closed system at a rate of 6 50 c my down to 840 c where all the exchange reactions were efficiently frozen in at 1 75 ga ago the coincidence of the emplacement of granites 1 7 1 8 ga into the central part of the tanzanian craton with the age of the eclogite suggests that lower crustal melting was triggered by a thermal event in the upper lithosphere

analysis of two color geodimeter measurements of deformation within the long valley caldera june 1983 to october 1985 line length changes from several baselines in a trilateration network within the long valley caldera clearly define a decrease in strain rate from june 1983 through october 1985 the data consist of more than 1600 length measurements on 23 baselines using a two color geodimeter which has a precision of 0 2 ppm of the line length initial measurements made during the summer of 1983 show extension rates were as high as 6 ppm yr on some baselines the rates subsequently decreased steadily to less than 1 ppm yr during the summer of 1985 ih contrast the strain rates on a few other baselines were less than 0 2 ppm yr averaged over the 2 3 year interval a model is constructed using these observations as well as yearly observations of a trilateration network within and near the caldera the model contains two points of inflation located at 5 and 10 km beneath the resurgent dome of the caldera plus dextral slip on a fault plane within the south moat within an elastic half space to mimic the observed time dependence in the two color geodimeter data we conclude that the rates of both slip and inflation have steadily decreased over the survey interval because the two color geodimeter network is closer to all three sources than the single color geodimeter network these data are important for defining the factor of 2 3 rate decrease of slip and inflation comparing our estimates of slip and inflation with similar estimates for the year prior to july 1983 we conclude that the rate decrease is probably a result of postseismic and postinflation relaxation following the earthquake swarm in the south moat in january 1983

spherical and ellipsoidal volcanic sources at long valley caldera california using a genetic algorithm inversion technique abstract we model the second inflation period at long valley caldera california using a genetic algorithm technique and high quality geodetic measurements of elevation changes and baseline extensions we compare two source inversions for both spherical mogi point sources and the finite prolate ellipsoid of yang and davis a sensitivity analysis for the genetic algorithm is performed based upon synthetic data set inversions on similar sources in order to better constrain the areal location orientation and volume of the potential sources the spherical sources are well constrained the larger located at 9 9 km beneath the resurgent dome with a volume of 0 036 km 3 while the second at only 0 008 km 3 is located at a depth of 7 3 km beneath the south moat the depths to the ellipsoidal sources are switched with the larger source at a depth of 9 6 km and the smaller at 11 8 km with volumes of 0 037 and 0 002 km 3 respectively

age nd hf isotopes and geochemistry of the vijayan complex of eastern and southern sri lanka a grenville age magmatic arc of unknown derivation abstract the ca 1 0 1 1 ga vijayan complex vc of eastern and southeastern sri lanka is one of three high grade metamorphic terranes making up the basement of the island and is in tectonic contact with the adjacent older highland complex it consists predominantly of granitoid gneisses ranging in composition from diorite to leucogranite with a distinct calc alkaline geochemical signature and is interpreted as a magmatic arc strong ductile deformation has obliterated almost all original intrusive relationships high grade metamorphism during the pan african event at ca 610 520 ma has produced widespread granulite facies assemblages that are now largely retrogressed and were affected by extensive late metamorphic k metasomatism in southern sri lanka the vijayan gneisses are tectonically interlayered with rocks of the highland complex in a so called mixed zone we report zircon ages whole rock nd and hf in zircon isotopic systematics and geochemical data for a large selection of vijayan gneissses in order to better characterize this complex and its tectonic setting the zircon ages are predominantly in the range 1100 1000 ma with a few early neoproterozoic intrusions and identify the vc as a grenville age magmatic arc many zircons experienced minor to significant lead loss at about 580 ma the nd and hf isotopic data confirm a generally primitive origin for most vijayan gneisses but significant variations in both isotopic systems argue for source heterogeneities and the possible involvement of minor amounts of older continental material in their genesis immobile trace and rare earth element distributions favour an origin of the gneiss protoliths through melting at relatively shallow depth and the chemical variation from diorite and tonalite via granodiorite to granite in the vc can be explained by increasing fractionation of plagioclase and biotite accompanied by amphibole and accessory phases the vc may be comparable in some respect to the composition and evolution of the kohistan arc in pakistan the unique composition age and isotopic characteristics of the vc find no convincing counterpart in other fragments of the gondwana supercontinent and its origin therefore remains exotic and enigmatic

uplift across long valley caldera california the results of levelings along u s highway 395 between lee vining and toms place california in 1957 1975 1980 1982 and 1983 define the development of a dome shaped uplift across the long valley caldera these levelings indicate that toms place remained virtually invariant with respect to lee vining during the full interval 1957 1983 implying that the end points of the survey route lie beyond the range of the developing dome no significant uplift occurred before 1975 but by october 1980 the uplift near casa diablo hot springs reached 0 25 m and the half width of the dome was of the order of 25 km the uplift of casa diablo hot springs increased to 0 35 m by august 1982 and to about 0 40 m by august 1983 without any appreciable increase in the lateral extent of the dome the august 1982 to august 1983 uplift apparently occurred before february 1983 with little or no uplift during the period february august 1983 the 1975 1983 uplift is attributed to the injection of 0 19 km3 of magma into a spherical chamber at a depth of 10 km beneath the southern quadrant of the ancient resurgent dome within the caldera examination of the full vertical control record through the long valley area indicates that the 1975 1983 uplift is unique within the period 1905 1983 although comparisons of the results of a 1932 leveling against a 1914 datum disclose a 1914 1932 down to the northwest tilt of about 10 rad over the reach between toms place and lee vining this tilt if real is more reasonably attributed to regional tectonic deformation than it is to the movement of magma at depth

ultra high temperature metamorphism and multistage decompressional evolution of sapphirine granulites from the palni hill ranges southern india sapphirine granulites from a new locality in the palni hill ranges southern india occur in a small enclave of migmatitic highly magnesian metapelites mg 85 72 within massive enderbitic orthogneiss they show a variety of multiphase reaction textures that partially overprint a coarse grained high pressure assemblage of bt opx ky grt pl qtz the sequence of reactions as deduced from the corona and symplectite assemblages together with petrogenetic grid considerations records a clockwise p t evolution with four distinct stages 1 equilibration of the initial high p assemblage in deep overthickened crust 12 kbar 800 900 c was followed by a stage of near isobaric heating presumably as a consequence of input of extra heat provided by the voluminous enderbitic intrusives during heating kyanite was converted to sillimanite and biotite was involved in a series of vapour phase absent melting reactions which resulted in the ultra high temperature assemblage opx crd kfs spr sil grt qtz bt coexisting with melt equilibration at c 950 1000 c 11 10 kbar 2 subsequently as a result of decompression of the order of 4 kbar at ultra high temperature a sequence of symplectite assemblages opx sil spr spr crdopx spr crdopx crdopx crd spl crd spl developed at the expense of garnet orthopyroxene and sillimanite this stage of near isothermal decompression implies rapid ascent of the granulites into mid crustal levels possibly due to extensional collapse and erosion of the overthickened crust 3 development of late biotite through back reaction of melt with residual garnet indicates a stage of near isobaric cooling to c 875 c at 7 8 kbar i e relaxation of the rapidly ascended crust to the stable geotherm 4 a second period of near isothermal exhumation up to c 6 5 kbar 850 c is indicated by the partial breakdown of late biotite through volatile phase absent melting reactions available isotope data suggest that the early part of the evolutionary history stages 1 3 is presumably coeval with the early proterozoic metamorphism in the extended granulite terrane of the nilgiri biligirirangan and shevaroy hills to the north while the exhumation of the granulites from mid crustal levels stage 4 occurred only during the pan african thermotectonic event which led to the accretion of the kerala khondalite belt to the south

constraints on continued episodic inflation at long valley caldera based on seismic and geodetic observations 1 long valley caldera a large and potentially explosive silicic system has experienced highly anomalous continued inflation since late 1970s we characterize an episode of rapid episodic uplift occurring between 2002 and 2003 following similar episodes of 1979 1980 1983 1989 1990 and 1997 1998 this most recent episode was the first to be observed by a dense array of 13 continuous global positioning system gps stations similar to previously observed episodes of deformation uplift is quasi radially symmetric and is mostly explained by a compact pressure source located 3 km west of the resurgent dome the maximum uplift during the 2002 2003 episode is 35 8 mm about 1 3 the magnitude but with a similar time dependent behavior as the 1997 1998 episode the horizontal source location is well constrained at 118 930 37 678 for a small spherical source and indistinguishable from the location of a vertically dipping prolate spheroidal source a trade off between depth and volume change is observed for both spherical and prolate models with depth between 7 5 and 13 5 km and a volume change of 0 01 0 03 km3 at 95 confidence for prolate spheroidal models depth and volume change are additionally affected by the source axis ratio b a which is greater than 0 55 though the background seismicity remained low during the 2002 2003 episode we identified a significant spike in activity during the maximum rate of uplift similar to observations in both the much larger episodes in 1989 1990 and 1997 1998 more interestingly we additionally find that all three episodes begin immediately after a short period of seismic quiescence with background seismicity falling to levels below background levels following the prior uplift event with the dense gps coverage we also identify increased opening of the mono inyo volcanic chain after the 2002 2003 episode suggesting potential interaction of magmatic fluids between the two systems

recent plate motions and crustal deformation reports by u s workers on geodetic measurements of recent plate motions or crustal deformation published in 1987 1990 are reviewed the review begins with global plate motions proceeds through plate boundaries in california alaska and the pacific northwest and finishes with volcanic phenomena monument stability and longevity and gps relative position measurements 184 refs

thermal and baric evolution of garnet granulites from sri lanka abstract garnet granulites from sri lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700 750 c and pressures in the vicinity of 6 8 kbar associated strain patterns suggest prograde metamorphism occurred during and immediately following an episode of crustal thickening with the prograde p t conditions probably reflecting a combination of the conductive and advective transport of heat at the mid levels of tectonically thickened crust the occurrence of prograde wollastonite provides evidence for internally buffered fluid compositions or fluid absent conditions during peak metamorphism and precludes pervasive advection of a co2 rich fluid the advective heat component is therefore likely to have been provided by the transport of silicate melt intricate symplectitic textures record partial re equilibration of the garnet granulites to lower pressures 4 6 kbar at high temperatures 600 750 c and testify either to the erosional denudation of the overthick crust prior to significant cooling i e quasi isothermal decompression or to a subsequent static heating possibly of early palaeozoic age pan african the metamorphic history of the sri lankan granulites is compared with high grade terrains in the neighbouring fragments of gondwana with the emphasis on similarities with proterozoic granulites of the east antarctic craton

partial melting and p t evolution of the kodaikanal metapelite belt southern india abstract the kodaikanal region of the madurai block in southern india exposes a segment of high grade metamorphic rocks dominated by an aluminous garnet cordierite spinel sillimanite quartz migmatite suite designated herein as the kodaikanal metapelite belt kmb these rocks were subjected to extreme crustal metamorphism during the late neoproterozoic despite the lack of diagnostic ultrahigh temperature assemblages the rocks preserve microstructural evidence demonstrating initial heating dehydration melting to generate the peak metamorphic assemblage and later retrogression of the residual assemblages with remaining melt the peak metamorphic assemblage is interpreted to be garnet sillimanite k feldspar spinel fe ti oxide quartz melt which indicates pressure temperature p t conditions around 950 1000 c and 7 8 kbar based on calculated phase diagrams a clockwise p t path is proposed by integrating microstructural information with pseudosections we show that evidence for extreme crustal metamorphism at ultrahigh temperature conditions can be extracted even in the cases where the rocks lack diagnostic ultrahigh temperature mineral assemblages our approach confirms the widespread regional occurrence of uht metamorphism in the madurai block during gondwana assembly and point out the need for similar studies on adjacent continental fragments

correlated errors in geodetic time series implications for time dependent deformation analysis of frequent trilateration observations from the two color electronic distance measuring networks in california demonstrate that the noise power spectra are dominated by white noise at higher frequencies and power law behavior at lower frequencies in contrast earth scientists typically have assumed that only white noise is present in a geodetic time series since a combination of infrequent measurements and low precision usually preclude identifying the time correlated signature in such data after removing a linear trend from the two color data it becomes evident that there are primarily two recognizable types of time correlated noise present in the residuals the first type is a seasonal variation in displacement which is probably a result of measuring to shallow surface monuments installed in clayey soil which responds to seasonally occurring rainfall this noise is significant only for a small fraction of the sites analyzed the second type of correlated noise becomes evident only after spectral analysis of line length changes and shows a functional relation at long periods between power and frequency of 1 where is frequency and 2 with 2 this type of correlated noise is termed random walk noise and its source is mainly thought to be small random motions of geodetic monuments with respect to the earth s crust though other sources are possible because the line length changes in the two color networks are measured at irregular intervals power spectral techniques cannot reliably estimate the level of 1 noise rather we also use here a maximum likelihood estimation technique which assumes that there are only two sources of noise in the residual time series white noise and random walk noise and estimates the amount of each from this analysis we find that the random walk noise level averages about 1 3 mm yr and that our estimates of the white noise component confirm theoretical limitations of the measurement technique in addition the seasonal noise can be as large as 3 mm in amplitude but typically is less than 0 5 mm because of the presence of random walk noise in these time series modeling and interpretation of the geodetic data must account for this source of error by way of example we show that estimating the time varying strain tensor a form of spatial averaging from geodetic data having both random walk and white noise error components results in seemingly significant variations in the rate of strain accumulation spatial averaging does reduce the size of both noise components but not their relative influence on the resulting strain accumulation model

metamorphic evolution of high pressure and ultrahigh temperature granulites from the highland complex sri lanka abstract sapphirine quartz bearing pelitic granulites garnet clinopyroxene quartz bearing mafic granulites and quartzo feldspathic granulites with corundum garnet quartz constitute rare but important members of the highland complex in sri lanka peak metamorphic conditions from the highland complex generally have been considered to be up to 850 900 c and 8 10 kbar however this study on the above mentioned rocks indicates that ultrahigh temperature and high pressure conditions 1100 c and 12 kbar were attained during peak conditions a metamorphic evolution of the ultrahigh temperature metamorphic rocks was determined from careful analyses of shifts in divariant assemblages and reaction textures this shows a clockwise p t path from more high pressure conditions 1000 c and 17 kbar stage 0 as part of the prograde metamorphic path to lower pressure and temperature conditions 950 c and 9 kbar stage 4 as part of the retrograde path through the peak metamorphic conditions stage 1 widespread lower pressure and temperature granulite facies metamorphic rocks surround the ultrahigh temperature granulites and are interpreted to have formed by the strong effect of the retrograde metamorphism and deformation the widely reported pan african metamorphic ages derived from similar granulite facies metamorphic rocks in the gondwana fragments highland complex in sri lanka lutzow holm complex in east antarctica etc may also be the result of retrograde metamorphism of ultrahigh temperature metamorphic rocks there remains a possibility that this early ultrahigh temperature high pressure granulite facies metamorphism in the highland complex as well as that in the lutzow holm complex might pre date pan african metamorphism

slip rate increase at parkfield in 1993 detected by high precision edm and borehole tensor strainmeters on two of the instrument networks at parkfield california the two color electronic distance meter edm network and borehole tensor strainmeter btsm network we have detected a rate change starting in 1993 that has persisted at least 5 years these and other instruments capable of measuring crustal deformation were installed at parkfield in anticipation of a moderate m6 earthquake on the san andreas fault many of these instruments have been in operation since the mid 1980s and have established an excellent baseline to judge changes in rate of deformation and the coherence of such changes between instruments the onset of the observed rate change corresponds in time to two other changes at parkfield from late 1992 through late 1994 the parkfield region had an increase in number of m4 to m5 earthquakes relative to the preceding 6 years the deformation rate change also coincides with the end of a 7 year period of sub normal rainfall both the spatial coherence of the rate change and hydrological modeling suggest a tectonic explanation for the rate change from these observations we infer that the rate of slip increased over the period 1993 1998

the 1989 earthquake swarm beneath mammoth mountain california an initial look at the 4 may through 30 september activity abstract mammoth mountain is a 50 000 to 200 000 year old cumulovolcano standing on the southwestern rim of long valley in eastern california on 4 may 1989 two m 1 earthquakes beneath the south flank of the mountain marked the onset of a swarm that has continued for more than 6 months in addition to its longevity noteworthy aspects of this persistent swarm include 1 an exponential like increase in the rate of activity through the first month 2 a vertically oriented planar distribution of hypocenters at depths between 6 and 9 km with a north northeast strike roughly perpendicular to the average t axis orientation for the swarm earthquakes 3 recurring spasmodic bursts rapid fire sequences of similar sized earthquakes with overlapping coda and occasional earthquakes with enhanced low frequency energy 4 a uniform temporal distribution of the four largest m 3 events over the first 4 months of the swarm with a cumulative seismic moment for the entire sequence through 30 september corresponding to a single m 4 earthquake 5 a b value of 1 2 and 6 submicrostrain perturbations on the nearby borehole dilatometer the first of which led the onset of swarm activity by more than 2 weeks these aspects of the swarm together with its location along the southern extension of the youthful mono inyo volcanic chain which last erupted 500 to 600 years ago point to a magmatic source for the modest but persistent influx of strain energy into the crust beneath mammoth mountain

decrease in deformation rate observed by two color laser ranging in long valley caldera after the january 1983 earthquake swarm the last period of notable seismicity the rapid rate of deformation of the south moat and resurgent dome of the long valley caldera diminished frequently repeated two color laser ranging measurements made within a geodetic network in the caldera during the interval june 1983 to november 1984 reveal that although the deformation accumulated smoothly in time the rate of extension of many of the baselines decreased by factors of 2 to 3 from mid 1983 to mid 1984 areal dilatation was the dominant signal during this period with rates of extension of several baselines reaching as high as 5 parts per million per annum during the summer of 1983 within the south moat shear deformation also was apparent the cumulative deformation can be modeled as the result of injection of material into two points located beneath the resurgent dome in addition to shallow right lateral slip on a vertical fault in the south moat

u pb and sm nd dating of high pressure granulite and upper amphibolite facies rocks from sw sweden abstract geochronological data from different radiometric systems on high grade metamorphic rocks in the southwest swedish granulite region southwestern sweden are generally analytically precise but sometimes in conflict with each other age data from seven samples on which a variety of radiometric methods have been applied are presented the age of peak metamorphism is determined by metamorphic zircons from two garnet amphibolites at c 975 ma using the conventional u pb isotope and pb pb evaporation methods titanites from the same type of rocks give ages of c 923 3 ma and 945 2 ma the sm nd data on mafic granulites and garnet amphibolites yield surprisingly young ages between 945 and 900 ma similar to previous sm nd ages these ages are comparable to or even younger than the 40 ar 39 ar hornblende ages that range between 1030 and 930 ma assuming that the sm nd systematics are entirely controlled by volume diffusion the closure temperature of the sm nd system in minerals from these rocks would be comparable to or lower than that of ar diffusion in hornblende i e around 500 c

ca 820 640 ma sims u pb age signal in the peripheral vijayan complex sri lanka identifying magmatic pulses in the assembly of gondwana abstract sri lanka comprises three roughly north south trending amphibolite to granulite facies lithotectonic complexes from west to east the highland complex the wanni complex and the vijayan complex these terranes were correlated with other east gondwana continental terranes with similar lithologies forming at similar ages the wanni complex and the vijayan complex have been interpreted as volcanic arc terranes brought together by a double sided subduction the highland complex represents the metamorphosed accretionary prism within the suture when the wanni and vijayan complexes were juxtaposing against each other in contrast to the wanni and highland complexes the vijayan complex has yielded only a few geochronological data with satisfactory precision previous studies suggested that the vijayan complex comprises 1100 924 ma granitic gneisses which were metamorphosed during 590 456 ma more recently 772 617 ma mafic intrusions have been identified this study divides the vijayan granitic gneisses and the associated melt products geochemically into a low nb series and a more primitive high nb series our sims u pb zircon data suggested that both series have protolith magmatic ages of 1062 935 ma and metamorphic ages of 580 521 ma which is consistent with previous work however some of the vijayan granitic gneisses and granitic anatectic melt products at the highland vijayan tectonic mixed zone preserve an additional tonian cryogenian 820 630 ma age signal this age signal suggested that felsic magmatism also occurred when mafic granulites were emplaced along the highland vijayan boundary which is broadly coeval with to the bimodal magmatism occurring along the highland wanni boundary this study also suggests that charnockitisation in the vijayan complex occurred at 562 6 ma during the neoproterozoic regional metamorphism the tonian cryogenian signal preserved in the highland vijayan tectonic mixed zone can also be found in the alkaline intrusion hosted by the namuno terrane and the lurio belt in mozambique this indicates a relationship between the vijayan granitic gneisses and the lurio foreland metagranitic basement while the namuno terrane and the lurio belt are correlated with the highland vijayan tectonic mixed zone the ages and the isotope signatures of these granitic bodies further suggest a genetic relationship of these granitic bodies with various magmatic intrusions in east antarctica

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

first evidence of ultrahigh temperature decompression from the granulite province of southern india sapphirine bearing orthopyroxene sillimanite garnet granulites of the palni hill ranges record evidence of ultrahigh temperature metamorphism during which primary mineral assemblages were partially overprinted by a succession of reaction coronas and symplectites that together suggest decompression model system petrogenetic grids allow constraints to be placed on the evolution of p t conditions as recorded by reaction relationships of successively developed mineral assemblages thermobarometry on garnet bearing assemblages allows minimum constraints to be placed on p t conditions for the primary mineral assemblages primary mineral assemblages suggest p t conditions of c 12 kbar and c 900 1000 c secondary mineral assemblages in reaction textures show a two stage decompression to probable p t conditions of c 5 kbar and c 850 c the inferred p t path is clockwise

structural controls on the emission of magmatic carbon dioxide gas long valley caldera usa we present a degassing study of long valley caldera that explores the structural controls upon emissions of magmatic carbon dioxide gas a total of 223 soil gas samples were collected and analyzed for stable carbon isotopes using a field portable cavity ring down spectrometer this novel technique is flexible accurate and provides sampling feedback on a daily basis sampling sites included major and minor volcanic centers regional throughgoing faults caldera related structures zones of elevated seismicity and zones of past and present hydrothermal activity the classification of soil gases based on their 13c and co2 values reveals a mixing relationship among three end members atmospheric biogenic and magmatic signatures dominated by biogenic contributions 4 vol 24 are found on the caldera floor the interior of the resurgent dome and areas associated with the hilton creek and hartley springs fault systems with the introduction of the magmatic component 100 vol 4 5 samples acquire mixing and hydrothermal signatures and are spatially associated with the central caldera and mammoth mountain in particular they are concentrated along the southern margin of the resurgent dome where the interplay between resurgence related reverse faulting and a bend in the regional fault system has created a highly permeable fracture network suitable for the formation of shallow hydrothermal systems this contrasts with the south moat where despite elevated seismicity a thick sedimentary cover has formed an impermeable cap inhibiting the ascent of fluids and gases to the surface

zoning patterns in orthopyroxene and garnet in granulites implications for geothermometry compositional maps of orthopyroxene and garnet of contrasting grain size and in contact with different minerals were made from two paragneiss granulites from the minto terrane of northern quebec the compositional maps provide clear evidence of late exchange of fe fe mg after ca in garnet and al in orthopyroxene had been quenched in the extent of late fe mg exchange was controlled by neighbouring minerals with negligible fe mg gradients against plagioclase and quartz and substantial gradients against exchangeable fe mg minerals cores of grains in contact with exchangeable fe mg neighbours are progressively more reset in fe fe mg as grain size decreases whereas cores of even small grains surrounded by only plagioclase and quartz are not significantly different in fe fe mg than cores of the largest grains gradients of ca in garnet and of al in orthopyroxene in grains of uniform fe fe mg preserve a high temperature retrograde history during which intergranular exchange effected compositional uniformity of mineral rims and intragranular fe mg diffusion in garnet and orthopyroxene was rapid enough to homogenize fe fe mg the transition from efficient intergranular exchange at relatively high temperatures to local fe mg exchange at lower temperatures may have been controlled by loss of an intergranular exchange medium in the rock possibly an internally generated dehydration melt phase implications for geothermometry of granulites include the following numerical values are particular to this study 1 core compositions of garnet and orthopyroxene grains in contact with exchangeable neighbours may be reset in fe fe mg relative to the most refractory compositions by an amount equivalent to 120d c 2 fe mg exchange thermometry using even the most refractory fe fe mg compositions may not record peak granulite conditions possibly recording instead the temperature at which an intergranular exchange medium was lost from the rock and 3 temperature sensitive net transfer equilibria involving al solubility in orthopyroxene yield temperatures up to 150d c higher than maximum fe mg exchange temperatures even in grains with flat fe fe mg compositional profiles making them a better means of estimating peak granulite temperatures than fe mg exchange thermometry

crustal growth in the mesoarchean plutonic belt from the memve ele area ntem complex southwestern cameroon evidence of early earth transpressional tectonics abstract the mesoarchean crust of the plutonic belt of the memve ele area lies in the southwestern part of the ntem complex ntc cameroon emplacement of these rocks resulted from both gravity and tectonic driven vertical flow perpendicular to the direction of shortening at the time of partial melting and magmatic accretion such deformation patterns suggest the occurrence of a long lived hot orogen and may explain a nearly isobaric retrograde pressure temperature p t path recorded within high t low to medium p metamorphic rocks however in the memve ele shear zone msz a metamorphic gradient runs from granulite to green schist facies structural analysis reveals four continuous ductile deformations the first stage is characterized by the emplacement of dome and basin structures the second stage is marked by coeval diapiric movements and a transpressional shear zone the third stage is marked by both transpressional and lateral flow regimes and the last stage is marked by horizontal constrictional strain these features indicate that the mesoarchean crust is affected by a long lived horizontal shortening deformation d1 d3 coeval with vertical movements in which concentric cleavages occur around domes thus the studied area can be considered as exemplifying an early earth style tectonics the structural patterns in the msz contain zone of high strain rocks belonging to the deep and shallow mid crusts that have recorded transpressional tectonics the compressional strain regime in which results coeval steep to shallow dipping structural features and dome and basin geometries associated to msz activities contributed to maintaining a stable crustal thickness in the memve ele area during mesoarchean orogeny

sm_nd garnet ages from the uluguru granulite complex of eastern tanzania further evidence for post metamorphic slow cooling in the mozambique belt abstract two samples from the uluguru granulite complex yield garnet sm nd ages of 633 7 and 618 16 ma similar to previously published hornblende 40ar 39ar and k ar ages the similarity of the sm nd to the k ar age suggests that the closure temperature of garnet to nd diffusion is similar to that of hornblende to ar diffusion assuming that published zircon u pb ages of about 700 ma date peak granulite facies metamorphism a mean post metamorphic cooling rate of 2 3 c ma can be calculated for the time interval 700 to 630 ma such slow cooling rates imply thermal relaxation with a thickness length scale greater than the thickness of average continental crust this in turn implies that the thermal perturbation responsible for metamorphism was preceded by regional crustal thickening probably in a collisional orogen

deformation near the casa diablo geothermal well field and related processes long valley caldera eastern california 1993 2000 abstract regional first order leveling lines which extend from lee vining ca to tom s place ca have been surveyed periodically since 1957 by the u s geological survey usgs the national geodetic survey ngs and caltrans two of the regional survey lines or leveling networks intersect at the casa diablo geothermal well field these leveling networks referenced to a distant bench mark c916 near lee vining provide time series vertical control data of land surface deformation that began around 1980 these data are also useful for delineating localized subsidence at casa diablo related to reservoir pressure and temperature changes owing to geothermal development that began in 1985 a comparison of differences in bench mark elevations for five time periods between 1983 and 1997 shows the development and expansion of a subsidence bowl at casa diablo the subsidence coincides spatially with the geothermal well field and temporally with the increased production rates and the deepening of injection wells in 1991 which resulted in an increase in the rate of pressure decline the subsidence superimposed on a broad area of uplift totaled about 310 mm by 1997 the usgs established orthogonal tilt arrays in 1983 to better monitor deformation across the caldera one tilt array dbr was established near what would later become the casa diablo geothermal well field this array responded to magmatic intrusions prior to geothermal development tilting away from the well field with the start of geothermal fluid extraction in 1985 tilt at the dbr array reversed direction and began tilting into the well field in 1991 geothermal power production was increased by a factor of four and reservoir pressures began a period of steep decline these changes caused a temporary three fold increase in the tilt rate the tilt rate became stable in 1993 and was about 40 lower than that measured in 1991 1992 but still greater than the rates measured during 1985 1990 data from the local leveling networks spanning the well field and the bounding graben were analyzed for several 2 year periods 1993 1995 1995 1997 and 1997 1999 annual rates of change across the normal faults bounding the graben have steadily decreased for each 2 year period between 1993 and 1999 reflecting the slowing decline in geothermal reservoir pressure horizontal control data from a two color electronic distance meter edm defined the lateral extent of subsidence at casa diablo the edm and leveling data elucidate the localized effect of the shallow source of subsidence and the broader effect of the deeper magmatic inflation source data from bench marks common to both the vertical and the horizontal control networks were used to assess the effect of subsidence on the edm base station casa modeling of geodetic data collected during periods of little or no magmatic inflation indicated that the casa two color edm station is being drawn toward the well field at a rate of 3 5 mm yr

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

genesis of super cyclone pam 2015 modulation of low frequency large scale circulations and the madden julian oscillation by sea surface temperature anomalies abstractsuper cyclone pam 2015 formed in the central tropical pacific under conditions that included el nino modoki and the passage of a convectively enhanced phase of the madden julian oscillation mjo in the western pacific this study examines the influence that sea surface temperature anomalies sstas have on the mjo and low frequency large scale circulation and establishes how they modulated the genesis of pam two series of numerical experiments were conducted by using a nonhydrostatic global atmospheric model with observed obssst and climatological clmsst ssts the results suggested that low frequency westerly winds at 850 hpa u850 were intensified in the central tropical pacific due to the observed ssta the amplitude of the mjo simulated in obssst was larger than in clmsst in addition the experiments initialized 26 february 2 march exhibited that the phase of the mjo in obssst was ahead of that in clmsst and that the genesis location in obssst was 10 to the east of that in clmsst

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

deflation acting on pantelleria island sicily channel inferred through geodetic data abstract pantelleria is a volcanic island situated at the centre of the sicily channel continental rift since 1980 the island has been monitored by geodetic techniques electro optical distance measurements edm and levelling and gravimetry after more than 10 years the edm network was re measured in 1995 and successively it has been improved and measured also with global position system gps techniques it has therefore been possible to carry out a long term analysis 1980 1996 of the deformation pattern the levelling in the 1980 1996 period revealed a marked lowering of the central southern part of the island with respect to the northern sector the horizontal edm measurements all prove to be in contraction and suggest that a deflation is ongoing in the central southern part the effects of areal contraction and lowering are interpreted as being due to the volcanic field which appears to predominate over possible stretching induced by the sicily channel dynamics we modelled the horizontal and vertical changes accumulated over 16 years using a 3d ellipsoidal depressuring model which indicates a ca 5 km deep source located under the area within the most recent caldera this model could explain a short term tens of years acting deflation we also propose an interpretive framework in the case where the deflation is acting over a long period thousands of years in this case the deflation is interpreted as readjustment following the loss of mass occurring with recent volcanism 18 3 kyr

possible tectonomagnetic effect observed from mid 1989 to mid 1990 in long valley caldera california precise measurements of local magnetic fields have been obtained with a differentially connected array of three proton magnetometers in the long valley caldera region since 1984 two magnetometers are located inside the caldera with a third reference magnetometer located 26km southeast of the caldera after correction for secular variation it is apparent that an anomalous 2 nt decrease in the magnetic field occurred from mid 1989 to mid 1990 at the magnetometer located closest to the center of the resurgent dome inside the caldera during this period a significant increase in geodetic strain rate of 8 5 ppm a was observed on the two color geodimeter network within the caldera from october 1989 to mid 1990 and a dramatic increase in seismic activity occurred from december 1989 to july 1990 a simple dilatational point source model with pressure increasing by 52 mpa from october 1989 to august 1990 at a depth of about 7km beneath the center of the resurgent dome can be fit to the strain data if this same model is used to calculate piezomagnetic fields in the caldera the results obtained agree with the observed local magnetic field data provided the curie point isotherm is at a depth of 5km taken together these magnetic seismic and geodetic data suggest that an episode of active magmatic intrusion occurred from late 1989 to mid 1990 at a depth of about 7 8km beneath the resurgent dome within the long valley caldera other indications of this intrusion should be evident in measurements of leveling local gravity and seismic imaging data

shallow and peripheral volcanic sources of inflation revealed by modeling two color geodimeter and leveling data from long valley caldera california 1988 1992 we refined the model for inflation of the long valley caldera near mammoth lakes california by combining both geodetic measurements of baseline length and elevation changes baseline length changes measured using a two color geodimeter with submillimeter precision revealed that the resurgent dome started to reinflate in late 1989 measurements between late 1989 and mid 1992 revealed nearly 13 cm of extension across the resurgent dome geodetic leveling surveys with approximately 2 mm precision made in late 1988 and in mid 1992 revealed a maximum of about 8 cm of uplift of the resurgent dome two ellipsoidal sources satisfy both the leveling and two color measurements whereas spherical point sources could not the model s primary inflation source is located 5 5 km beneath the resurgent dome with the two horizontal axes being nearly equal in size and the vertical axis being 4 times the length of the horizontal axes a second ellipsoidal source was added to improve the fit to the two color measurements this secondary source is located at a depth between 10 and 20 km beneath the south moat of the caldera and has the geometry of an elongated ellipsoid or pipe that dips down to the northeast in addition the leveling data suggest dike intrusion beneath mammoth mountain during the 1988 1992 interval which is likely associated with an intense swarm of small earthquakes during the summer of 1989 at that location our analysis shows the dike intrusion to be the shallowest of the three sources with a depth range of 1 3 km below the surface to the top of the intrusion

petrology of granulite facies metapelites of the highland complex sri lanka implications for the metamorphic zonation and the p t path in the pan african granulite facies highland complex of sri lanka the metamorphic grade increases from the central northwestern part towards boththe east and the southeast in metapelites the following prograde changes in metamorphic assemblage are recognized biotite ilmenite garnet or co cordierite sillimanite alkali feldspar quartz in the central northwestern part give way to garnet rutile sillimanite ilmenite or biotite alkali feldspar quartz in the e and se and are replaced by gar garnet cordierite biotite magnetite ilmenite alkali feldspar quartz bearing assemblages in the southwest cordierite coexisting with garnet is restricted to the western highland complex where metamorphic pressures of 5 7 kbar havebeen determined in the eastern highland complex where garnet clinopyroxene quartz is stable in metabasic rocks metamorphic pressures of 8 10 kbar have been obtained from barometers on metapelites phase relations of metapelites as well as geobarometry point to a continuous metamorphic transition between the eastern and the western part of the highland complex a clockwise p t path for the whole highland complex is indicated by the succession kyanite sillimanite andalusite an even earlier stage of the prograde metamorphism documented by sillimanite needles older than kyanite inclusions in garnet hhave been found in two samples of the eastern highland complex this points to a strong pressure increase at amphibolite facies conditions probably due to tectonic thickening of the crust during prograde metamorphism other inclusion assemblages in garnet and reaction textures indicate the breakdown of staurolite to kyanite spinel garnet and subsequent continued garnet growth in the sillimanite field at the expense of biotite sillimanite and quartz during further heating ternary feldspars in metapelites bear evidence of extreme metamorphic temperatures of about 900 c previously only indicated by coexisting pyroxenes in metabasic and charnockitic rocks within a few garnets inclusions of staurolite which are ti rich 1 34 wt tio2 and richer in fe xfe 0 745 than the enclosing garnet have survived the very high grade metamorphism the high grade stage of the prograde metamorphism was accompanied by strong flattening and a general n s stretching which is many metapelites ceased after peak temperatures but proceeded in others tectonic uplift of the whole highland complex occurred only after a phase of near isobaric cooling in the lower crust zircon 610 ma and biotite cooling ages 460 ma holzl et al 1991 as well as retrograde p t paths which passed the andalusite stability field are similar in the eastern and western part of the highland complex this suggests that the lower crustal cross section exposed in the highland complex was already tilted during cooling

temperatures of granulite facies metamorphism constraints from experimental phase equilibria and thermobarometry corrected for retrograde exchange this study assesses temperatures of formation of common granulites by combining experimental constraints on the p t stability of granulite facies mineral associations with a garnet orthopyroxene grt opx thermobarometry scheme based on al solubility in opx corrected for late fe mg exchange we applied this scheme to 414 granulites of mafic intermediate and aluminous bulk compositions our findings suggest that granulites are much hotter than traditionally assumed and that the p t conditions of the amphibolite granulite transition portrayed in current petrology textbooks are significant underestimates by over 100 c for aluminous and intermediate granulites mean corrected temperatures based on our method are 890 17 and 841 11 c respectively uncertainties reported as 95 confidence limits on the mean consistent with minimum temperatures for orthopyroxene production by fluid absent partial melting in these bulk compositions in contrast mean temperatures based on grt opx fe mg exchange equilibria using the same thermodynamic data are 732 22 and 723 11 c respectively well below the minimum temperatures for opx stability for mafic granulites the mean corrected temperature using our method is 816 12 c similar to the mean temperature of 793 13 c from fe mg exchange reasons for the differences between the mafic granulites and aluminous intermediate granulites are unclear but may be due to the lower al concentrations in opx in the mafic rocks and possible deficiencies in the thermodynamic modelling of these low concentrations we discuss a number of well known granulite terrains in the context of our findings including the adirondacks the acadian granulites of new england the incipient charnockites of southern india and sri lanka and the kerala khondalite belt our findings carry implications for thermotectonic models of granulite formation a computer program to perform our thermobarometry calculations rclc is available from the journal of petrology website at http www petrology oupjournals org or from the authors at http www geo ucalgary ca pattison drm pattison rclc htm

continuous monitoring of surface deformation at long valley caldera california with gps continuous global positioning system gps measurements at long valley caldera an active volcanic region in east central california have been made on the south side of the resurgent dome since early 1993 a site on the north side of the dome was added in late 1994 special adaptations for autonomous operation in remote regions and enhanced vertical precision were made the data record ongoing volcanic deformation consistent with uplift and expansion of the surface above a shallow magma chamber measurement precisions 1 standard error for absolute position coordinates i e relative to a global reference frame are 3 4 mm north 5 6 mm east and 10 12 mm vertical using 24 hour solutions corresponding velocity uncertainties for a 12 month period are about 2 mm yr in the horizontal components and 3 4 mm yr in the vertical component high precision can also be achieved for relative position coordinates on short 10 km baselines using broadcast ephemerides and observing times as short as 3 hours even when data are processed rapidly on site comparison of baseline length changes across the resurgent dome between the two gps sites and corresponding two color electronic distance measurements indicates similar extension rates within error 2 mm yr once we account for a random walk noise component in both systems that may reflect spurious monument motion both data sets suggest a pause in deformation for a 3 5 month period in mid 1995 when the extension rate across the dome decreased essentially to zero three dimensional positioning data from the two gps stations suggest a depth 5 8 1 6 km and location west side of the resurgent dome of a major inflation center in agreement with other geodetic techniques near the top of a magma chamber inferred from seismic data gps systems similar to those installed at long valley can provide a practical method for near real time monitoring and hazard assessment on many active volcanoes

timing and rate of isothermal decompression in pan african granulites from rundvagshetta east antarctica geochronological data combined with field and petrological evidence constrain the timing and rate of near isothermal decompression at granulite facies temperatures in rocks from the lutzow holm complex of east antarctica granulite facies gneisses from rundvagshetta in lutzow holm bay experienced a peak metamorphic temperature of over 900 c at c 11 kbar as evidenced by primary orthopyroxene sillimanite bearing assemblages and secondary cordierite sapphirine bearing assemblages in metapelites peak metamorphic assemblages show strong preferred mineral orientation interpreted to have developed synchronously with pervasive ductile deformation zircon from a syndeformational leucosome has a u pb age of 517 9 ma which is interpreted as a melt crystallization age this age provides the best estimate of the time of peak metamorphic conditions the post peak metamorphic history is characterized by near isothermal decompression recorded by mineral textures in a variety of rock compositions field and textural relations indicate that decompression post dated pervasive ductile deformation k ar and 40ar 39ar ages from hornblende and biotite represent closure ages during cooling subsequent to decompression and indicate cooling to temperatures between c 350 and 300 c by c 500 ma thus placing a lower time limit on the duration of the high temperature isothermal decompression episode the combination of the zircon age from a syndeformational melt with k ar and 40ar 39ar closure ages indicates that near isothermal decompression from c 11 to c 4 kbar at granulite facies temperatures followed by cooling to c 300 c took place within a time interval of 20 10 myr simple one dimensional models for exhumation controlled cooling indicate that these data require exhumation rates of the order of c 3 km myr 1 for several million years then cessation of exhumation followed by relatively isobaric cooling during thermal re equilibration

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

co seismic displacements of the 1992 landers earthquake sequence we present co seismic displacement vectors derived from global positioning system gps measurements of 92 stations in southern california these gps results are combined with five well determined gps displacement vectors from continuously tracking stations of the permanent gps geodetic array as well as line length changes from usgs geodolite and two color laser trilateration observations to determine a self consistent set of geodetic data for the earthquake these combined displacements are modeled by an elastic dislocation representation of the primary fault rupture planes on average the model residuals are about twice the estimated measurement errors

effects of geothermal development on deformation in the long valley caldera eastern california 1985 1994 long valley caldera in east central california has been the site of crustal unrest in the form of seismicity and ground deformation in response to magmatic inflation since 1980 uplift of the resurgent dome has totaled 0 6 m over the 1975 1992 period within this region of uplift and near the southwestern edge of the resurgent dome is the casa diablo area which experienced relative subsidence of 0 17 m between 1985 and 1992 geothermal fluid production began at casa diablo in 1985 to supply a binary electric power plant currently three such plants generate a total of about 40 mw of electricity the plants are supplied by 170 c water pumped from depths near 150m all the produced water is reinjected at cooler temperatures at depths near 600 m analyses of data from 1 regional leveling lines and a network of bench marks in the geothermal well field area 2 tilt observations from an l shaped array of bench marks and 3 calculations relating pressure and temperature changes to subsidence indicate that relative subsidence at casa diablo results mainly from reductions in pressure in the shallow production reservoir and overlying formations and reductions in temperature in the underlying injection reservoir secondary effects on deformation are caused by offsets along the taylor bryant fault bounding the west side of the well field

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

sensitivity test of the geodetic network in las canadas caldera tenerife for volcano monitoring abstract a 17 benchmark geodetic network in the volcanic area of the teide caldera canary islands has been utilised several times since 1982 to detect crustal movements associated with volcanic activity within the network as well as a procedure for solving configuration problems the network is located on the mid western side of the caldera where there are two different morphological zones that both have benchmarks the authors performed a sensitivity test of this geodetic network for volcano monitoring purposes to do so we use a deformation model to calculate surface displacement caused by a dike intrusion in a homogenous half space the depth and location of dike are changed to study the variation of the effects produced displacements the size and location of the intruded dike are found to play a major role in determining both the displacement pattern and magnitude when the dike is close to the surface there is an inversion of the surface displacement pattern and very large surface displacement at certain benchmarks such phenomena can serve as precursors of such dike eruptions our study show a clear need to extend the existing geodetic network to cover the full island for volcano monitoring purposes

variations in fault slip and strain accumulation at parkfield california initial results using two color geodimeter measurements 1984 1988 repeated length measurements of several geodetic baselines near parkfield california have revealed significant variations in the local rates of shallow fault slip and strain this network of baselines surveyed several times each week straddles the san andreas fault in the transition zone between the creeping section to the northwest and the locked section to the southeast the length measurements characterized by a precision approaching 0 1 ppm reveal large fluctuations in the rates of baseline extension principal mode analysis of the length change data indicates that the two largest components of the signal are 1 secular extensions and contractions consistent with surface slip on the main strand of the san andreas fault and 2 a large seasonal oscillation with no obvious spatial coherence on most of these baselines the second component appears to be in phase with seasonal rainfall when data from the baselines with the largest amplitude of the seasonal signal are excluded the remaining data can be modeled in terms of both spatial and temporal variations in surface slip variations in the components of the changes in uniform strain and the possible displacement of the central monument in this radial network in parameterizing this model the spatial variation of slip beneath the near surface is reflected by changes in shear strain although the computed secular shear is highly dependent upon the specified parameterization of surface slip the data are consistent with the hypothesis that slip at intermediate depths lags behind the surface slip rate however the range in models that fit the data does not necessarily imply that there is a deficit in slip at depth relative to the surface comparison of the inferred values of surface slip from the model with the observed fault slip measured by very short baseline creep meters indicates close agreement in secular rates but the short term variations observed with the creep meters are either highly attenuated or nonexistent in the modeled slip since the modeled slip is a spatial average which smooths out possible short wavelength variations in the surface slip for which the creep instruments are most sensitive an interesting conclusion from the two color data is that surface slip on the san andreas fault appears to be spread over a 2 km wide zone on the south flank of middle mountain but is confined to a very narrow zone to the south as the fault passes through the center of the network this conclusion is dependent upon the assumption that a few critical monuments are stable and track tectonic displacements in the long term finally the largest observed strain change is an extensional strain coincident with the kettleman hills earthquake m5 5 in august 1985

the mechanics of unrest at long valley caldera california 1 modeling the geometry of the source using gps leveling and two color edm data we surveyed 44 existing leveling monuments in long valley caldera in july 1999 using dual frequency global positioning system gps receivers we have been able to tie gps and leveling to a common reference frame in the long valley area and computed the vertical deformation by differencing gps based and leveled orthometric heights the resurgent dome uplifted 74 7 cm from 1975 to 1999 to define the inflation source we invert two color edm and uplift data from the 1985 1999 unrest period using spherical or ellipsoidal sources we find that the ellipsoidal source satisfies both the vertical and horizontal deformation data whereas the spherical point source cannot according to our analysis of the 1985 1999 data the main source of deformation is a prolate ellipsoid located beneath the resurgent dome at a depth of 5 9 km 95 bounds of 4 9 7 5 km this body is vertically elongated has an aspect ratio of 0 475 95 bounds are 0 25 0 65 and a volume change of 0 086 km3 95 bounds are 0 06 0 13 km3 failure to account for the ellipsoidal nature of the source biases the estimated source depth by 2 1 km 35 and the source volume by 0 038 km3 44

high pressure granulites formation recovery of peak conditions and implications for tectonics high pressure granulites are characterised by the key associations garnet clinopyroxene plagioclase quartz in basic rocks and kyanite k feldspar metapelites and felsic rocks and are typically orthopyroxene free in both basic and felsic bulk compositions in regional metamorphic areas two essential varieties exist a high to ultrahigh temperature group and a group representing overprinted eclogites the high to ultrahigh temperature type formerly contained high temperature ternary feldspar now mesoperthite coexisting with kyanite is associated with garnet peridotites and formed at conditions above 900 c and 1 5 gpa clinopyroxene in subordinate basic rocks is al rich and textural evidence points to a high pressure high temperature melting history the second variety contains symplectite like or poikilitic clinopyroxene plagioclase intergrowths indicating former plagioclase free i e eclogite facies assemblages this type of rock formed at conditions straddling the high pressure amphibolite high pressure granulite field at around 700 850 c 1 0 1 4 gpa importantly in the majority of high pressure granulites orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure in contrast to low and medium pressure granulites which form at conditions attainable in the mid to lower levels of normal continental crust high pressure granulites of nonxenolith origin mostly represent rocks formed as a result of short lived tectonic events that led to crustal thickening or subduction of the crust into the mantle short times at high temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene high pressure granulites of both regional types although rare are known from both old and young metamorphic terranes e g c 45 ma namche barwa e himalaya 400 340 ma european variscides 1 8 ga hengshan china 1 9 ga snowbird saskatchewan and 2 5 ga jianping china this spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the archean

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

deformation near the long valley caldera eastern california 1982 1986 annual surveys of trilateration and leveling networks in and around long valley caldera in the 1982 1985 interval indicate that the principal sources of deformation are inflation of a magma chamber beneath the resurgent dome and right lateral strike slip on a vertical fault in the south moat of the caldera the rate of inflation of the magma chamber seems to have been roughly constant 0 02 km3 yr in the 1982 1985 interval but the slip rate on the south moat fault has decreased substantially in addition there is evidence for a shallow source of dilatation possibly dike intrusion beneath the south moat of the caldera in 1983 and less certain evidence for a deep source possibly magma chamber inflation beneath mammoth mountain in the western caldera in 1983 1985 deformation in the 1985 1986 interval as inferred from trilateration alone seems to be similar to that observed in 1984 1985

higher order interrelations between seismogenic structures and earthquake processes abstract aki k 1992 higher order interrelations between seismogenic structures and earthquake processes in t mikumo k aki m ohnaka l j ruff and p k p spudich guest editors earthquake source physics and earthquake precursors tectonophysics 211 1 12 to advance our understanding of higher order smaller scale interrelations between the fault zone structure and the earthquake process we start with the observation that the source parameters of major california earthquakes deviate systematically from the self similarity this departure from the self similarity is interpreted in terms of both the specific barrier model and the asperity model the asperity model however is found to be inconsistent with the following observations 1 sharp impulsive displacements waveform in the direction perpendicular to the fault trace 2 the complementary relationship between fault slip and aftershock and 3 existence of barriers in the creeping segment of the san andreas fault the application of the specific barrier model on the other hand leads to a predominant subevent crack size of a few hundred meters in the creeping segment this result is consistent with the negative correlation between coda q 1 and b value observed in the stone canyon area for 1 5 3 temporal increase in coda q 1 is sometimes accompanied by an increase in the rate of surface deformation but not always so this is attributed to the erratic coupling between the creeping brittle region and the overlying brittle region the applicability of the conclusions obtained from california earthquakes to other regions is investigated using the observed scaling laws of seismic source spectra and it is suggested that the crack size of the creeping brittle zone may vary regionally from a few hundred meters to a few kilometers as an alternative to the widely held view that the aseismic creep in the plate boundary may occur as a giant slow earthquake our result suggests that it may be occurring as small fractures with scale lengths from a few hundred meters to a few kilometers

evidence for prograde metamorphic evolution of sri lankan pelitic granulites and implications for the development of continental crust petrological evidence for prograde metamorphic evolution is obtained for pelitic granulites of the newly defined highland complex in sri lanka it includes the occurrences of relict kyanite quartz staurolite corundum kyanite hercynite kyanite and sapphirine kyanite spinel within garnet in the sillimanite quartz rich rocks of special interest is the mineralogy of khondalites graphitic garnet sillimanite k feldspar quartz gneisses the matrix of which is free of hydrous minerals indicating that dehydration reactions have been completed in this kind of rocks however relict hydrous minerals such as al ti rich more than 58 wt al2o3 and more than 1 wt tio2 staurolite and extremely ti rich up to 9 8 wt tio2 biotine occur as inclusions within garnet the local relict mineral assemblages within garnet in khondalites indicate the following continuous staurolite breakdown reactions which may have progressed as temperature increased and pressure decreased staurolite quartz kyanite garnet h2o staurolite corundum kyanite garnet h2o staurolite kyanite hercynite garnet h2o staurolite sillimanite hercynite garnet h2o some of these dehydration reactions are inferred to have also taken place in other widespread pelitic granulites of the highland complex moreover garnet in the highland complex pelitic granulites occasionally preserves growth zoning though modified slightly to extensively by post growth volume diffusion based on the evidence a clockwise prograde p t path of evolution is inferredfor meta sedimentary granulites belonging to the highland complex on the other hand no evidence for prograde metamorphism has so far been obtained in lower grade rocks of the wanni complex to the northwest of the highland complex such a difference between these complexes may be the first petrological feature which supports the new geotectonic division of the sri lankan basement rocks based virtually on the distinctly different neodymium model ages it suggests that these two complexes were amalgamated during the very last part of prograde metamorphism of the highland complex rocks that is during decompression and heating probably by the lower crustal lateral flow after tectonic overthickening and delamination the clockwise prograde p t t path followed by the meta sedimentary granulites of the highland complex is in marked contrast with the near isobaric cooling path for meta igneous granulites suggested by some authors it is proposed that both types of p t t paths albeit for different rock types are possible and resulted from diverse deep crustal processes such as underplating of continental crust by sediments and magmas or near convergent plate boundaries

sensitive high resolution ion microprobe u pb dating of prograde and retrograde ultrahigh temperature metamorphism as exemplified by sri lankan granulites ultrahigh temperature uht granulites of the central highland complex sri lanka underwent some of the highest known peak temperatures of crustal metamorphism zircon and monazite u pb systems in granulites near kandy the highest grade region 1050 c 0 9 gpa preserve both a record of the timing of prograde and retrograde phases of uht metamorphism and evidence for the ages of older protolith components zircon grains from a quartz saturated granulite containing relics of the peak uht assemblage have remnant detrital cores with dates of ca 2 5 0 83 ga date clusters of ca 1 7 and 1 04 0 83 ga record episodes of zircon growth in the source region of the protolith sediment two generations of overgrowths with contrasting th u record metamorphic zircon growth at 569 5 and 551 7 ma probably in the absence and presence of monazite respectively the age of coexisting metamorphic monazite 547 7 ma is indistinguishable from that of the younger low th u zircon overgrowths zircon from a quartz undersaturated monazite absent uht granulite with a mainly retrograde assemblage is mostly metamorphic 551 5 ma the ca 570 ma zircon overgrowths in the quartz saturated granulite probably record partial melting just before or at the metamorphic peak the ca 550 ma zircon in both rocks and the ca 550 ma monazite in the quartz saturated sample record post peak isothermal decompression a possible model for this pressure temperature time evolution is ultrahot collisional orogeny during the assembly of gondwana locally superheated by basaltic underplating followed by fast extensional exhumation

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

post seismic slip on the san andreas fault at the northwestern end of the 1989 loma prieta earthquake rupture zone a small geodetic network spanning the san andreas fault was measured 7 77 157 and 200 days following the october 17 1989 loma prieta m7 1 earthquake this network is located at the northwestern end of the rupture plane defined by the locations of numerous aftershocks in the initial 70 day interval the measured line length changes revealed that 5 4 0 4 mm of right lateral slip occurred within the network however during the later 4 month interval only a marginally significant rate 3 2 1 4 mm yr of right lateral slip could be detected thus it appears that the measured slip is a typical response of the fault following a major shock in that the rate of slip decreases rapidly with time however the magnitude of the post seismic slip is less than 0 5 of the inferred co seismic slip at depth

extreme crustal metamorphism during a neoproterozoic event in sri lanka a study of dry mafic granulites abstract garnet clinopyroxene quartz granulites of the central highland complex of sri lanka preserve textural and compositional features indicative of high pressure ultrahigh temperature hp uht crustal metamorphism and multistage retrogression grains of the peak metamorphic assemblage garnet clinopyroxene quartz are commonly separated and embayed by late orthopyroxene plagioclase symplectites however in some domains rare grain to grain associations of the peak assemblages are still preserved thermodynamic modeling in the cao na2o k2o feo mgo al2o3 sio2 system indicates peak metamorphic conditions of 12 5 kbar at 925 c the temperature estimates using garnet and clinopyroxene core compositions are in the range 844 982 c in agreement with the thermodynamic modeling in conclusion the textural geochemical and thermodynamic modeling and thermobarometric data indicate a multistage decompression after hp uht metamorphism u pb zircon laser ablation inductively coupled plasma mass spectrometry

inflation of long valley caldera from one year of continuous gps observations a permanent global positioning system receiver at casa diablo hot springs long valley caldera california was installed in january 1993 and has operated almost continuously since then the data have been transmitted daily to the jet propulsion laboratory for routine analysis with data from the fiducial laboratories for an international natural sciences network flinn by the jpl flinn analysis center results from these analyses have been used to interpret the on going deformation at long valley with data excluded from periods when the antenna was covered under 2 5 meters of snow and from some periods when anti spoofing was enforced on the gps signal the remaining time series suggests that uplift of the resurgent dome of long valley caldera during 1993 has been 2 5 1 1 cm yr and horizontal motion has been 3 0 0 7 cm yr at s53w in a no net rotation global reference frame or 1 5 0 7 cm yr at s14w relative to the sierra nevada block these rates are consistent with uplift predicted from frequent horizontal strain measurements spectral analysis of the observations suggests that tidal forcing of the magma chamber is not a source of the variability in the 3 dimensional station location these results suggest that remotely operated continuously recording gps receivers could prove to be a reliable tool for volcano monitoring throughout the world

two stage decompression in orthopyroxene sillimanite granulites from forefinger point enderby land antarctica implications for the evolution of the archaean napier complex magnesian metapelites of probable archaean age from forefinger point sw enderby land east antarctica contain very high temperature granulite facies mineral assemblages which include orthopyroxene 8 9 5 wt al2o3 sillimanite garnet quartz k feldspar that formed at 10 1 5 kbar and 950 50 c these assemblages are overprinted by symplectite and corona reaction textures involving sapphirine orthopyroxene 6 7 wt al2o3 cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene sillimanite these textures mainly pre date the development of coarse biotite at the expense of initial mesoperthite and the subsequent formation of orthopyroxene 4 6 wt al2o3 cordierite plagioclase rinds on late biotite the early reaction textures indicate a period of near isothermal decompression at temperatures above 900 c decompression from 10 1 5 kbar to 7 8 kbar was succeeded by biotite formation at significantly lower temperatures 800 850 c and further decompression to 4 5 1 kbar at 700 800 c the later parts of this p t evolution can be ascribed to the overprinting and reworking of the forefinger point granulites by the late proterozoic c 1000 ma rayner complex metamorphism but the age and timing of the early high temperature decompression is not known it is speculated that this initial decompression is of archaean age and therefore records thinning of the crust of the napier complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post deformational near isobaric cooling history of this terrain