shallow borehole tilt a reprise we describe results from nearly a decade of tilt measurements produced by two arrays of shallow borehole tiltmeters one in a semiarid environment at pinon flat observatory pfo california depth of burial 4 5 m and the other in a maritime arctic environment at adak alaska depth of burial 2 m although renovation and reinstallation of the instruments at the two sites reduced thermal noise it did not change the secular records significantly this implies that the large tilts observed reflect instability of the ground rather than the sensor so that deeper installations should give better results the pfo data show large rainfall related tilts caused by near surface weathering and also periodic temperature related tilts from several thermoelastic effects the adak data are dominated by tilts from the annual temperature cycle though by a smaller amount than at pfo but at other frequencies they show essentially the same power levels as at pfo both data sets confirm earlier results that burial at such shallow depths even in apparently stable material is inadequate for the measurement of tectonic tilts

chemical characteristics of migmatites accessory phase distribution and evidence for fast melt segregation rates equilibration between melt and solid is inhibited by rapid melt extraction and by restricted equilibration armouring slow dissolution when segregation occurs by channelised migration along high porosity pathways melt migration is more rapid than trace element diffusion rates in silicates and faster than accessory phase dissolution rates evidence for channelised flow and deformation enhanced melt segregation into boudin necks fractures and micro shears at low melt fractions is present in the moine kirtomy migmatitie suite kms in sutherland scotland melt migration distances are on a metre to tens of metres scale concordant leucosomes in stromatic migmatities in the kms have low zr contents low lree light rare earth element and h heavy ree contents and positive eu anomalies ree patterns of this type can be produced by removal of leucosome before complete equilibration with source due to the inhibited dissolution of lree and hree bearing accessory phases in water undersaturated melts melting in the kms however occurred at or near the wet granite solidus leaving biotite as a residual phase detailed back scattered electron imaging shows that ree bearing accessory phases remained as residual phases and were concentrated in the melanosome and at the melanosome leucosome boundary irregularly shaped patches of diatexite contain a small proportion of excess zr consistent with entrainment of melanosome schlieren enriched in zircon these data indicate that deformation enhanced melt extraction led to the rapid migration of small melt fractions from the melting site on a time scale less than that required to saturate the melt in zr leucosomes were thus prevented from equilibrating with accessory phases before extraction

tiltmeter measurements in long valley caldera california in 1983 the u s geological survey instrumented long valley caldera near mammoth lakes california with an array of borehole tiltmeters to detect the rapid deformation that might accompany incipient volcanic activity such as the intrusion of a dike at shallow depth the array consists of seven sites within and surrounding the zone of the 1983 earthquake swarm five of these sites have instruments installed at a depth of 6 m and consist of two closely spaced installations one containing a rockwell kinemetrics instrument and the other an instrument designed by j westphal of the california institute of technology the sixth site consists of two of westphal s instruments and one rockwell kinemetrics instrument and the seventh site has two of the rockwell kinemetrics instruments only installed at a depth of 2 m since july 1985 all but one site have been equipped with a telemetry system that uses the geostationary operational environmental satellite goes most sites are also equipped with a telemetry system based on telephone lines and radio links this tiltmeter array affords a unique opportunity to compare the relative merits of different instrument types and installation techniques under similar settings in general the results are similar for the two different instruments and installations but local geologic conditions substantially affect the performance at each site the records from the 6 m deep boreholes have better stability and lower noise than those from the 2 m deep boreholes at one site the tilt record appears to be dominated by perturbations related to local geothermal production performance at most other sites indicates that sudden deformation with time spans shorter than about 1 week and amplitudes of about 5 10 rad or greater should easily be detected by the instruments within the array

global geologic context for rock types and surface alteration on mars petrologic interpretations of thermal emission spectra from mars orbiting spacecraft indicate the widespread occurrence of surfaces having basaltic and either andesitic or partly altered basalt compositions global concentration of ice rich mantle deposits and near surface ice at middle to high latitudes and their spatial correlation with andesitic or partly altered basalt materials favor the alteration hypothesis we propose the formation of these units through limited chemical weathering from basalt interactions with icy mantles deposited during periods of high obliquity alteration of sediments in the northern lowlands depocenter may have been enhanced by temporary standing bodies of water and ice

disequilibrium partial melting model and its implications for trace element fractionations during mantle melting abstract a model to describe trace element evolutions during partial melting is proposed by assuming surface equilibrium instead of complete equilibrium between mineral grains and melt and by considering the following factors 1 chemical diffusion in minerals 2 melting and melt extraction for mantle melting rates of 10 7 yr 1 inferred from mid ocean ridge spreading rates elements with diffusivities d in minerals of 10 15 cm 2 s be at equilibrium between minerals of 0 5 cm in diameter and melt during mantle melting however the anomalous radioactive excess of 236 ra over 230 th up to 300 in young basalts which cannot be explained by equilibrium melting because of the highly incompatible nature of both elements can be accounted for by the present model only when melting rate is in the orders of 10 6 to 10 5 yr 1 the disequilibrium effect cannot be ignored for elements with d 13 cm 2 s if the melting rate is 10 5 yr 1 disequilibrium melting lowers the incompatibility of an incompatible element the high the incompatibility the more obvious this effect is fractionation of trace elements during disequilibrium melting depends both on their equilibrium partition coefficients k and their diffusivities this has two consequences 1 fractionation of two highly incompatible elements k 0 01 by more than 20 is still possible at melting degree as high as 15 if their diffusivities differ by a factor of 5 or more as compared to the requirement for less than 5 partial melting for the same extent of fractionation by equilibrium melting and 2 the incompatibility order of two incompatible elements can be reversed under certain circumstances i e k 1 k 2 but d 1 d 2 the pb paradox namely that the u pb ratio in mantle sources inferred from pb isotopes has increased during the differentiation of the earth even though pb is less incompatible than u is explainable if the diffusivity of pb is higher than that of u e g by a factor of 10 and if the melting rate is in the order of 10 5 yr 1

response of the u pb chronometer and trace elements in zircon to ultrahigh temperature metamorphism the kadavur anorthosite complex southern india abstract zircon from quartzites in the contact aureole and wider environs of the kadavur anorthosite complex se india was studied by laser ablation icp ms to assess the response of the u pb isotope system and trace element concentrations to ultrahigh temperature uht contact metamorphism 1000 c combined cathodoluminescence imaging and la icp ms analyses show that zircon grains contain detrital cores which yield ages between 3 4 ga and 1 8 ga and exhibit a large spread in ree concentrations these cores are associated with one or two rims that provide concordant age populations at 955 16 ma and 810 7 ma 2 and relatively uniform ree patterns the older ages of ca 955 ma record the imprints of regional early neoproterozoic metamorphism related to rodinia assembly the younger age is interpreted to date anorthosite emplacement and its associated contact metamorphism during a second regional metamorphic episode coeval with the intrusion of a type granites in the area zircon grains from the country rocks away from the anorthosite show additional rims yielding ages from 590 to 490 ma corresponding to regional pan african tectonometamorphism the zircon rims are either newly grown domains or represent recrystallized and re equilibrated parts of precursor zircon discordance is only observed in 25 of the detrital cores these cores have relatively high u concentrations 400 ppm which strongly suggests that pb loss occurred under u ht conditions by partial recrystallization annealing of zircon that had become metamict the preservation of old concordant ages and source ree characteristics by the low u i e weakly to non metamict detrital cores shows that pristine zircon is extremely robust to thermal disturbance and that its u pb systematics can remain unaffected despite multiple u ht metamorphic episodes these observations imply that u pb ages in zircon cannot be reset by volume diffusion under any crustal conditions disturbance of the u pb system in individual zircon crystals or parts thereof can only be achieved by recrystallization of radiation damaged zircon

measurement and interpretation of tidal tilts in a small array strain induced tilts caused by variations in material properties can produce large perturbations in the tilt field near a material discontinuity this is evidenced in data from a closely spaced array of borehole tiltmeters that has been operating at piiion flat geophysical observatory in southern california since 1987 the array is composed of three borehole tiltmeters located at depths of 24 36 and 120 m the tiltmeters have a sensitivity of a few nanoradians tidal measurements show differences between the boreholes of up to 50 the largest percent difference occurs in the semidiurnal band where the tilt field measured in the 36 m borehole differs by 10 nrad at the m2 frequency this difference has been modeled as a strain induced tilt caused by the weathering layer

displacement of surface monuments vertical motion several measurements of vertical ground motion at pinon flat observatory california indicate the overall weakness and instability of the earth s weathered surface with respect to the underlying rock cumulative long period motions of order 0 5 mm per year dominate these records though smaller elastic deformations caused by precipitation loading atmospheric loading and tidal strains are evident at higher frequencies all of these help to characterize the near surface material the long period records suggest that near surface weathering is the dominant influence on monument motion at least at this site on crystalline rock in a semi arid environment rainfall loading gives an average vertical modulus of 2 6 gpa for the material in the uppermost 26 m of the ground compared with 88 gpa for granite under moderate confining stress atmospheric loading gives similar results but indicates the ground is permeable to airflow at periods longer than a few hours earth tide records show poisson s ratio to be 0 09 in contrast to the normative range of 0 2 0 25 establishing that horizontal strains couple only weakly into vertical ones so that vertical strains near the surface are a poor measure of areal strain the form of the ground surface displacement power spectrum indicates that analyses of geodetic surveys would be improved with the inclusion of a monument positioning error budget that increases with time because of ground instability and the generally small rate of crustal tectonic motions deeply emplaced monuments will be needed for observational programs designed to detect short term changes in crustal deformation over baselengths of order 10 km and less

point stability at shallow depths experience from tilt measurements in the lower rhine embayment germany and implications for high resolution gps and gravity recordings summary from 1996 to 1999 we have studied ground tilts at depths of between 2 m and 5 m at three sites in the lower rhine embayment lre western germany the lre is a tectonically active extensional sedimentary basin roughly 50 kmr100 km the purpose of the tilt measurements was a to provide insight into the magnitude nature and variability of background tilts and b to assess possible limitations of high resolution gps campaigns and microgravity surveys due to natural ground deformation the tilt readings sensed by biaxial borehole tiltmeters of baselength 0 85 m cover a frequency range from 10 x8 hz to 10 x2 hz periods from minutes to years assuming that the tilt signals represent ground displacements on a scale typically not larger than several times the tiltmeters baselength and that tilt signals at shallow depth could in a simple geometric way be related to changes in surface elevation and gravity we try to estimate the magnitude level of point movements and corresponding bouguer gravity effects that is generally not surpassed the largest tilt signals observed were some t50 mrad yr x1 i f they were observable over a ground section of extension e g 10 m the converted rates may correspond to about t0 5 mm per 10 m yr x1 in vertical ground displacement and t0 1 mgal yr x1 in bouguer gravity effect respectively large signals are mostly related to seasonal effects probably linked to thermomechanical strain other causes of ground deformation identified include seepage effects after rainfalls order of t10 mrad and diurnal strains due to thermal heating and or fluctuations in the water consumption of nearby trees order of t1 mrad episodic step like tilt anomalies with amplitudes up to 22 mrad at one of the observation sites might reflect creep events associated to a nearby active fault except for short term ground deformation caused by the passage of seismic waves from distant earthquakes amplitudes of non identified tilt signals in the studied frequency range seem not to exceed t2 mrad as the larger tilt signals are close to the precision achieved with modern gps systems and superconducting gravimeters when converted into height and gravity changes further enhancement in resolution of these techniques may require simultaneous recording of local ground deformation at the observation sites

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization crystal size distributions csds measured in metamorphic rocks yield quantitative information about crystal nucleation and growth rates growth times and the degree of overstepping t of reactions during metamorphism csds are described through use of a population density function n dn dl where n is the cumulative number of crystals per unit volume and l is a linear crystal size plots of ln n vs l for olivine pyroxene and magnetite in high temperature 1000 c basalt hornfelses from the isle of skye define linear arrays indicating continuous nucleation and growth of crystals during metamorphism using the slope and intercept of these linear plots in conjunction with growth rate estimates we infer minimum mineral growth times of less than 100 years at t 10 c and nucleation rates between 10 4 and 10 1 cm3 s garnet and magnetite in regionally metamorphosed pelitic schists from south central maine have csds which are bell shaped we interpret this form to be the result of two processes 1 initial continuous nucleation and growth of crystals and 2 later loss of small crystals due to annealing the large crystals in regional metamorphic rocks retain the original size frequency distribution and may be used to obtain quantitative information on the original conditions of crystal nucleation and growth the extent of annealing increases with increasing metamorphic grade and could be used to estimate the duration of annealing conditions if the value of a rate constant were known finally the different forms of crystal size distributions directly reflect differences in the thermal histories of regional vs contact metamorphosed rocks because contact metamorphism involves high temperatures for short durations resulting csds are linear and unaffected by annealing similar to those produced by crystallization from a melt because regional metamorphism involves prolonged cooling from high temperatures primary linear csds are later modified by annealing to bell shapes

ostwald ripening as a possible mechanism for zircon overgrowth formation during anatexis theoretical constraints a numerical model and its application to pelitic migmatites of the tickalara metamorphics northwestern australia a fundamental dichotomy exists between the low solubility of zircon in peraluminous melt predicted by experimental and geochemical studies and the large volume proportions of zircon overgrowths formed during high temperature metamorphism and anatexis that are revealed by cathodoluminescence imaging we investigate the potential of ostwald ripening as a possible mechanism for overgrowth formation by presenting a numerical solution to the continuity equation governing open system diffusion rate limited ostwald ripening in a zircon saturated melt application of the model to a typical log normal initial zircon crystal size distribution csd suggests that despite uncertainties associated with the interfacial free energy of zircon significant grain coarsening is possible via this mechanism under geological conditions and time scales relevant to high grade metamorphism primary influences on the rate at which ostwald ripening proceeds are i the temperature of the system ii the duration of the time interval for which the system is above its solidus and iii the nature of the initial premelting zircon csd to test the viability of the model we examine zircon csds from three high grade pelitic migmatites of the tickalara metamorphics northwestern australia assuming that zircon crystals hosted by melanosome biotite were permanently occluded from the melt and therefore approximate the premelting csd the model predicts that within 1 to 2 ma these biotite hosted zircon csds will evolve into the observed leucosome hosted zircon csds via melt present ostwald ripening under geological conditions applicable to peak metamorphism although we have not conclusively demonstrated that ostwald ripening contributed to changes in zircon csds during anatexis of the tickalara metapelites our results suggest that ostwald ripening is a viable mechanism for zircon volume transfer in a zircon saturated melt and capable of playing a significant role in overgrowth formation in rocks where the total volume of zircon overgrowths substantially exceeds the concentration of zircon dissolvable in the coexisting melt

rapidity of orogenesis in the paleoproterozoic halls creek orogen northern australia evidence from shrimp zircon data cl zircon images and mixture modeling studies combining u pb shrimp zircon geochronology with cathodoluminescence imaging enables the resolution of temporally closely spaced geological events important for understanding tectonothermal processes in the paleoproterozoic halls creek orogen of northern australia the youngest detrital zircon grains from a low grade quartz muscovite psammite of the tickalara metamorphics have a 207 pb 206 pb shrimp age of 1864 or 4 ma defining a maximum depositional age for the unit zircon crystals from a high grade garnet biotite metapelite approximately 5 10 volume percent leucosome from the same sequence are considerably more complex and shrimp analyses form a single large concordant group in the range approximately 1885 to 1830 ma the zircon crystals contain three distinct cl zoning patterns and individual shrimp spots show a corresponding variation in 207 pb 206 pb ages concentric oscillatory zoned zircon dominates pre 1850 ma ages and is interpreted as detritus from igneous source rocks narrow structureless zircon rims infrequently overgrow and truncate the oscillatory zoning these rims comprise most of the post 1850 ma analyses and are inferred to be the product of uppermost amphibolite facies metamorphism reflecting the interaction of a robust pre existing detrital zircon suite with a very limited melt volume in addition some zircon cores of uncertain geological affinity contain large areas devoid of oscillatory zoning with individual analyses clustering around 1850 ma dividing the data into detrital and metamorphic suites solely on the basis of cl imaging yields an older group of 18 analyses 207 pb 206 pb age 1867 or 4 ma and a younger group of 10 analyses 207 pb 206 pb age 1843 or 4 ma not including five shrimp spots within areas of unzoned zircon mixture modeling of all 33 analyses in the post 1900 ma data set resulted in a best fit solution composed of two distinct components 1 an older group of 19 analyses with an age of 1867 or 4 ma and 2 a younger group of 14 analyses with an age of 1845 or 4 ma these results suggest that the unzoned patches of zircon might be related to metamorphism rather than being detrital cores importantly the ages and proportions of populations predicted by mixture modeling are otherwise very similar to those derived from analysis of cl zoning patterns these data imply that high temperature metamorphism occurred in the metasedimentary rocks less than 25 my after the crystallization of the igneous detrital source such rapid rates of erosion deposition and burial have rarely been proposed for proterozoic rocks despite evidence for analogous orogenic processes in the mesozoic and cainozoic on a comparable timescale careful evaluation of geological and geochronological data in proterozoic provinces elsewhere may reveal similar patterns with potential implications for the possible rates of proterozoic orogenesis and crustal evolution

water table effects on the measurement of earth strain abstract forced changes in the water head within a granite penetrating borehole were found to induce anomalously large free surface strains and tilts in the vicinity of the hole this deformation is shown to be due to fluid pressure induced changes in the aperture of a compliant hydraulically conductive fracture at a depth of 100 m and a quantitative analysis of the deformation data is performed to recover fracture characteristics the importance of the effect for secular earth strain measurements is discussed in the light of the ubiquitous nature of both water table fluctuations and fractures in the shallow crust

s type granitic magmas petrogenetic issues models and evidence abstract despite a perception that it represents a perverse divergence it is perfectly possible to believe in the existence of s and i type granites and the implications for the nature of their protoliths and to disbelieve in the applicability of the restite unmixing model for chemical variation in granitic magmas white and chappell erected the s i classification with impeccable validity the isotopic evidence demands contrasting source reservoirs for s and i type granitic magmas however the major advance was not the classification but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas the restite unmixing model is commonly seen as a companion to the s i classification but it is really a separate issue this model implies that the compositions of granites image those of their source rocks in a simple way however there are other equally valid models that can explain the data and none of them represents a unique solution the most cogent explanation for the high grade metasedimentary enclaves in most s type granites is that they represent mid crustal xenoliths restitic enclaves are either rare or absent inherited zircons in s type rocks are certainly restitic however the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock zircon and zirconium behaviours are controlled by disequilibrium and kinetics and zr contents of granitic rocks can rarely be used to infer magma temperatures since the dominant ages among inherited zircons in lachlan fold belt lfb s type granites are ordovician and proterozoic it seems likely that crust of this age but geochemically different from the exposed rocks not only underlies much of the lfb but also forms a component in the granite magma sources the evidence is overwhelming that the dark microgranular enclaves that occur in both s and i type granites are igneous in origin they represent globules of quenched more mafic magma mingled and modified by exchange with the host granitic magma however magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas likewise the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and in some cases violate constraints from isotopic studies s and i type magmas commonly retain their distinct identities this suggests limited source mixing limited magma mixing and limited wall rock assimilation though intermediate types certainly exist they are probably relatively minor in volume crystal fractionation probably plays the major role in the differentiation of very many granitic magmas including most s types especially those emplaced at high crustal levels or in the volcanic environment minor mechanisms include magma mixing wall rock assimilation and restite unmixing isotopic variations within plutons and in granite suites could be caused by source heterogeneities magma mixing assimilation and even by isotopic disequilibrium however source heterogeneity coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust and crustal thickening likewise fails to provide sufficient heat generally the mantle must be the major heat source this might be provided through mantle upwelling and crustal thinning and possibly through the intra and underplating of mafic magmas upper crustal extension seems to have been common in regions undergoing granitic magmatism migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid present reactions granitic magmas are mostly formed by fluid absent processes where we do see rare evidence for arrested fluid absent partial melting the melt fraction is invariably concentrated into small shear zones veinlets and small dykes thus it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels however one major missing link in the chain is the mechanism by which melt fractions in small scale segregations occurring over a wide area can be gathered and focused to efficiently feed much wider spaced major magma conduits answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones self organization almost certainly plays a role

evolution of crystal sizes in the series of dissolution and precipitation events in open magma systems article i nfo we propose a model that describes the evolution of crystal sizes and crystal size distributions csd of igneous phenocrysts in a sequence of dissolution and crystallization events this model is based on the assumption that crystal dissolution is rate limited by diffusion in melt while crystal growth is controlled by the slower kinetic of new nucleation and growth as a result the dissolution rate is inversely proportional to crystal size coming into effect through the curvature of the crystal s surface but the growth rate does not depend on the crystal size closed form analytical solution of equation for csd is obtained we apply results of modeling to quartz and zircon two prime minerals in silicic igneous systems that are widely used in geochemical and isotopic investigations the time series of multiple solution reprecipitation episodes generate concave downward csds and this result fits well with experimental and natural observations on the abundant concave down csd in silicic igneous rocks we suggest that maturation of crystal populations with sizes above several micrometers can not be caused by a size effect on the solubility of the crystals ostwald ripening but is rather driven by thermal oscillations in experiments and in nature the model predicts that mean crystal size increases with time proportionally to t 0 20 which is very close to the published experimental results for quartz maturation with the exponent of 0 19 0 22 our proposed model gives an opportunity to use natural csds for interpretation of pre eruptive magma history when solubilities and diffusion data are available for constituent elements of the dissolving mineral in particular we present time estimates for maturing zircon populations in large volume ignimbrites and estimate that it takes 100 1000 yrs to mature an initially exponential csd to a lognormal csd

melt segregation in the continental crust before melt will segregate from its restite and source there must be both a driving force and space into which the melt can move the continental crust is anisotropic and in most cases is subject to deviatoric stresses during melting under these conditions melt segregation begins when an interconnected network of melt forms and the source becomes permeable melt is driven by pressure gradients from its restite toward dilatant sites set up in the heterogeneously deforming source the source is continually drained of melt and so need never contain more melt than that required to maintain permeability 26 where the solid matrix breaks down and diapirism can occur

ion probe u th pb zircon dating of polymetamorphic orthogneisses from northern labrador canada rounded zircons included within igneous zircons from the uivak i gneisses northern labrador have ages of up to 3863 12 ma 2 showing the former existence of rocks that predate the uivak gneisses by more than 100 ma the igneous zircons themselves have experienced varying amounts of early pb loss but age estimates based on maximum 207pb 206pb from three separate gneiss samples agree to within error at 3732 6 ma 2 which is taken as the age of emplacement of the igneous precursor to the dominant component of the uivak gneisses a new generation of zircons was formed during migmatization at ca 3620 ma the volumetrically less important lister gneiss was emplaced at 3235 8 ma 2 in the late archaean different parts of the gneiss complex were subjected to different grades of metamorphic overprinting the zircon chronology of gneisses affected by granulite facies metamorphism suggests a rapid sequence of events with simultaneous recrystallization of old zircons and growth of new u poor zi

estimating source parameters from deformation data with an application to the march 1997 earthquake swarm off the izu peninsula japan we have applied two monte carlo optimization techniques simulated annealing and random cost to the inversion of deformation data for fault and magma chamber geometry these techniques involve an element of randomness that permits them to escape local minima and ultimately converge to the global minimum of misfit space we have tested the monte carlo algorithms on two synthetic data sets we have also compared them to one another in terms of their efficiency and reliability we have applied the bootstrap method to estimate confidence intervals for the source parameters including the correlations inherent in the data additionally we present methods that use the information from the bootstrapping procedure to visualize the correlations between the different model parameters we have applied these techniques to gps tilt and leveling data from the march 1997 earthquake swarm off of the izu peninsula japan using the two monte carlo algorithms we have inferred two sources a dike and a fault that fit the deformation data and the patterns of seismicity and that are consistent with the regional stress field

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

displacement of surface monuments horizontal motion measurements of tectonic deformation depend on both accurate instrumentation and adequate coupling of the apparatus to the earth s crust existing techniques capable of resolving the signals of interest strain rates of 10 n yr are mainly observatory based the limitation in the baselength of these instruments 1 km results in a requirement that the reference monuments be exceptionally stable 10 tzm yr however records from pition flat observatory california show that the actual horizontal displacements for massive near surface monuments eraplaced in competent weathered granite are of the order of 50 tzm yr the low noise level of the strain measurements at this site indicates that this magnitude of monument displacement is abnormally small until high accuracy geodetic techniques are developed sophisticated monuments or monument monitoring devices will be necessary to record faithfully continuous crustal deformation

a seafloor long baseline tiltmeter long term monitoring of seismicity and deformation has provided constraints on the eruptive behavior and internal structure and dynamics of subaerial volcanoes but until recently such monitoring of submarine volcanoes has not been feasible little is known about the formation of oceanic crust or seamounts and we have therefore developed a stand alone long baseline tiltmeter to record deformation on active seafloor volcanoes the instrument is a differential pressure two fluid sensor adapted for use on the seafloor combined with an autonomous data logger and acoustic navigation release system the tiltmeter can be installed without use of remotely operated vehicles or manned submersibles and to first order is insensitive to noise driven by temperature or pressure gradients we recorded 65 days of continuous data from one of these tiltmeters on axial seamount on the juan de fuca ridge during a multidisciplinary experiment that included ocean bottom seismographs magnetotelluric instruments and short baseline tiltmeters after instrument equilibration the 100 m iono tiltmeter provided a record with long term drift rates of 0 5 5 rad day and higher frequency variations of the order of 5 10 rad comparison with records of subaerial volcanic tilt shows that this instrument can discriminate volcanic deflation events though none occurred during our deployment a conclusion supported by nearby short baseline tilt and bottom pressure recordings the short and long baseline data constrain volcanic inflation of axial seamount to be below 0 5 1 rad day 1 during mid 1994 analysis of the long baseline tilt data in conjunction with electric field temperature and short baseline tiltmeter data shows that high frequency signals are largely driven by ocean currents improved coupling between the tiltmeter and seafloor should reduce this noise improve stability and drift and further enhance our ability to record tilt related to active submarine volcanism

zr lree rich minerals in residual peraluminous granulites another factor in the origin of low zr lree granitic melts there is a significant enrichment in some trace elements in the major residual minerals of peraluminous granulite xenoliths from the lower crust those trace elements are released from the breakdown of accessory phases at high t granulite facies conditions n850 c around 10 35 of zr is hosted in granulite rutile and garnet whereas the entire lree eu budget is controlled by feldspar the zr and ree compatible behaviour of the major granulite phases combined with the scarcity of accessory phases which are mostly included in major granulite minerals leads to a disequilibrium in accessory dissolution in the peraluminous partial melts thus the melt extracts less zr and lree and consequently generates the false impression of having lower t when applying current accessory phase dissolution models

zircon response to high grade metamorphism as revealed by u pb and cathodoluminescence studies correct interpretation of zircon ages from high grade metamorphic terrains poses a major challenge because of the differential response of the u pb system to metamorphism and many aspects like pressure temperature conditions metamorphic mineral transformations and textural properties of the zircon crystals have to be explored a large c 450 km2 coherent migmatite complex was recently discovered in the bohemian massif central european variscides rocks from this complex are characterized by granulite and amphibolite facies mineral assemblages and based on compositional and isotopic trends are identified as the remnants of a magma body derived from mixing between tonalite and supracrustal rocks zircon crystals from the migmatites are exclusively large 200 400 m and yield 207pb 206pb evaporation ages between 342 328 ma and single grain zircon fractions analysed by u pb id tims method plot along the concordia curve between 342 and 325 ma high resolution u pb shrimp analyses substantiate the existence of a resolvable age variability and yield older 206pb 238u ages 342 330 ma weighted mean age 333 6 3 1 ma for inner zone domains without relict cores and younger 206pb 238u ages 333 320 ma weighted mean age 326 0 2 8 ma for rim domains pre metamorphic cores were identified only in one sample 206pb 238u ages at 375 0 3 9 420 3 4 4 and 426 2 4 4 ma most zircon ages bracket the time span between granulite facies metamorphism in the bohemian massif 345 ma and the late variscan anatectic overprint bavarian phase 325 ma it is argued that pre existing zircon was variously affected by these metamorphic events and that primary magmatic growth zones were replaced by secondary textures as a result of diffusion reaction processes and replacement of zircon by dissolution and recrystallization followed by new zircon rim growth collectively the results show that the zircons equilibrated during high grade metamorphism and record partial loss of radiogenic pb during post peak granulite events and new growth under subsequent anatectic conditions

an investigation into shallow borehole tiltmeters shallow borehole tiltmeters are frequently used for ground deformation monitoring in volcanic areas where they are usually installed within a few meters of the surface a major problem concerns the noise affecting the installation at shallow depth where large tilt and strain are caused by temperature effects the long term stability of these instruments and the best shallow depth of installation in terms of best cost benefit are the crucial points we conducted an experiment in order to compare the signals of three shallow borehole electronic tiltmeters installed in the same place at different depths we verified the signal reliability in revealing the long term slow deformation and looked into the reduction of the temperature effects with depth we present three years of data and discuss the limits and the advantages of the different installation depths

eruption prediction aided by electronic tiltmeter data at mount st helens telemetry from electronic tiltmeters in the crater at mount st helens contributed to accurate predictions of all six effusive eruptions from june 1981 to august 1982 tilting of the crater floor began several weeks before each eruption accelerated sharply for several days and then abruptly changed direction a few minutes to days before extrusion began each episode of uplift was caused by the intrusion of magma into the lava dome from a shallow source causing the dome to inflate and eventually rupture release of magma pressure and increased surface loading by magma added to the dome combined to cause subsidence just prior to extrusion

revealing hidden structures the application of cathodoluminescence and back scattered electron imaging to dating zircons from lower crustal xenoliths abstract the use of cathodoluminescence cl and or back scattered electron bse imaging techniques with in situ ion probe analyses of zircons can help unravel complex crustal histories of metamorphic rocks that otherwise might remain elusive using these techniques we have imaged zircons from three lower crustal xenolith suites that have previously been dated by shrimp sensitive high resolution ion microprobe in all three cases the zircons are featureless in transmitted light but cl and bse reveal internal structures that correlate with distinct growth events generally cl and bse images reveal similar structures with cl showing finer detail neither imaging technique is capable of delineating all growth features in every sample and the best results are obtained using a combination of the two techniques igneous cores in zircons commonly emit a different color cl emission in many cases zoning in the cores is truncated indicating that the zircons either spent time in the supracrustal environment after their initial crystallization and prior to the granulite facies event s or that part of the core zircon was resorbed during the subsequent metamorphic event metamorphic rims when present are commonly 10 to 30 m thick and are nearly always unzoned and featureless igneous cores and metamorphic overgrowths commonly have distinctive cl emission spectra and trace element concentrations however the cl spectra can only be used to qualify chemical differences as a linear relationship has not been shown to exist between cl intensity and trace element concentration in natural zircons in many cases the hf y p u and the heavy rare earth elements hrees concentrations can be correlated to igneous and metamorphic growth using a combination of cl and bse imaging techniques and in situ trace element analyses with either the electron microprobe or pixe particle induced x ray emission

contrasting cw and ccw tectono metamorphic belts in the eastern himalayan syntaxis quantification of p t t paths and tectonic interpretation abstract in the eastern himalayan syntaxis the indus tsangpo suture zone separates the himalaya to the south from the southern lhasa terrane to the north in this study we combine petrology zircon u pb geochronology and phase equilibrium modeling to quantify the p t t paths of metapelites from the namche barwa complex part of the himalaya and the nyingchi complex part of the southern lhasa terrane the namche barwa complex is interpreted to record slow cooling across a hp granulite facies p t field at 12 kbar from 790 c to the solidus at 750 c between c 50 ma and c 17 ma followed by decompression with slight heating to a p t field at 5 5 8 kbar and 790 825 c at c 3 ma consistent with the post peak segment of a cw p t t path by contrast the nyingchi complex records a ccw p t t path characterized by compression through peak granulite facies p t conditions of 7 5 9 kbar at 760 780 c before re crossing the solidus at p of 11 kbar at c 35 31 ma based on the youngest published spot date from a detrital core in zircon from metapelite the maximum depositional age for the protoliths of the nyingchi complex is c 53 ma we propose a tectonic scenario in which the nyingchi complex ccw p t t path was buried in the interval c 53 35 ma synchronous with heating then compression during ongoing convergence related to the india asia collision by contrast the namche barwa complex cw p t t path which represents the subducted indian continental lithosphere records relatively fast exhumation from c 17 to c 3 ma that was initiated by crustal scale folding linked to orogen scale gravitational spreading and thrusting enhanced by surface processes

a tilt and seismicity episode in the new hebrides vanuatu island arc tilt and seismicity have been monitored in the central new hebrides island arc since 1978 using bench mark arrays long tube water tiltmeters borehole tiltmeters and a local seismometer network releveling of the bench mark array on efate island in late november 1986 revealed a 10 rad tilt up to the nnw since the previous leveling in april 1986 the tilt event was preceded by a magnitude 5 9 thrust event that occurred on october 25 1986 at a depth of 48 km and about 11 km nw of the tiltmeter instruments six days later a shallow 20 km swarm of earthquakes occurred 12 km nnw of the tiltmeter instruments and 5 km north of the epicenter of the magnitude 5 9 earthquake closely coincident in time with the swarm a 5 rad tilt up to the nnw that occurred over a period of 5 days was recorded on both the 100 m baseline water tube tiltmeter and the borehole bubble level tiltmeter a composite focal mechanism of 191 earthquakes selected from the swarm indicates a thrust mechanism with some component of strike slip calculations show that the seismic slip associated with a swarm of this magnitude is apparently inadequate to cause the observed surface deformation two similar shallow swarms in november 1987 and july 1988 have occurred within 15 km of the 1986 swarm but with no apparent surface deformation the most likely explanation supported by simple modeling is that the swarm and tilt are the result of a magmatic intrusion from island arc volcanism an alternate hypothesis is that both the seismicity and the tilt are due to an episode of largely aseismic creep in the upper crust

twenty five years of continuous borehole tilt and vertical displacement data at mount etna insights on long term volcanic dynamics we report a 25 year long data set 1990 2014 of combined continuous tilt and gps vertical displacement series recorded at etna volcano to our knowledge this is the first time that such a data set on an active volcano has been presented we show the coherence of the two series which help draw new insights on how the volcano characterized by frequent flank eruptions works in the long term tens of years this data set provides evidence that after the 1992 1993 flank eruption the biggest in the last three centuries and the following major recharging phase 1994 2001 all the ensuing eruptions fall within a single long term reequilibrium phase 2001 2014

textural development of monazite during high grade metamorphism hydrothermal growth kinetics with implications for u th pb geochronology monazite has become an important tool for geochronology but it commonly exhibits complex internal zoning of composition and age experiments were conducted to characterize the textural development and the rate and mechanism of growth of finely powdered 2 o at 1 0 gpa and 1000 degrees c coarsely crushed quartz crystals 500 mu m in diameter grew rapidly and progressively engulfed monazite crystals to form arrays of monazite inclusions the mean diameter of all monazite crystals decreased in the first 24 h then increased at a constant rate consistent with growth by grain boundary diffusion controlled ostwald ripening with a minimum rate constant k1 4 4 41x10 super 2 mu m s 4 using small quartz crystals of uniform diameter approximately 0 5 mu m in the starting material reduced quartz grain boundary mobility and limited the development of inclusions monazite grew by matrix volume diffusion controlled ostwald ripening with k1 3 1 02x10 super 2 mu m s 3 in all run products matrix coarsening produced linear crystal size distributions that reflect continuous recrystallization and nucleation textural evidence suggests that matrix coarsening induced coalescence was also an important growth mechanism during annealing of fluid filled rock growing host crystals may occlude small monazite crystals preserving their isotopic composition large monazite crystals may pin grain boundaries while smaller crystals may move with grain boundaries by recrystallizing a process that resets isotopic systems monazite crystals on grain boundaries may grow by ostwald ripening to form rims and by coalescence accurate interpretations of monazite ages therefore require knowledge of the texture growth history of the rock and its dated grains

comparing tiltmeters for crustal deformation measurement a preliminary report a collection of high precision tiltmeters is being operated at pinon flat observatory southern california both to compare instruments and to measure tectonic deformation we report on 1 2 years of data from four of these two michelson gale long fluid tiltmeters one long center pressure tiltmeter and a shallow borehole tiltmeter the three long base instruments are all located on the same baseline with a precise leveling line running between their end monuments at nontidal frequencies only the two michelson gale instruments show some coherence 3 for periods of 2 to 4 days while the center pressure instrument is correlated with air temperature at periods from a few days to a few weeks the most stable tilt record shows a secular rate of 0 28 rad a which may be real over much longer times leveling to specially stabilized benchmarks should confirm this comparing instruments has identified more and less successful measurement techniques it appears that low noise data will most probably be produced only by relatively complex and expensive instruments though even for these the operating costs over any reasonable lifetime will exceed the capital cost even the best existing sensors must be improved to measure continuous tectonic motions

measurements and interpretations of tilt strain gauges in seismically active areas abstract an overview of different aspects of deformation measurements is given an introduction to the general theory of point crustal deformations with an application to the stress strain relations at the earth surface is made the instrumentation used in crustal deformation measurements is briefly explained emphasizing different constructions of tilt and strainmeters the problems connected with the housing of the instruments i e the cavity and topography effects and with the ambient noise factors as the hydrological and pressure effects are discussed the last part of the paper is concerned with the relation between deformation measurements and seismic events both from a theoretical and observational standpoint a number of examples are shown that report on pre co and post seismic and the secular term deformation in different areas of the world

accessory phase controls on the geochemistry of crustal melts and restites produced during water undersaturated partial melting the profound geochemical conseqences of accessory phase behaviour during partial melting of highgrade metapelites are demonstrated with reference to two geochemically distinct crustal melts produced by biotite dehydration melting reactions under granulite facies kbar 860 cc conditions these two leucogneiss suites from the brattstrand bluffs coastline eastern antarctica have similar field relations transport distances 10 100 s of metres and major element chemistry type 1 leucogneisses have low zr th and lree positive eu anomalies and zr zr and lreet lreet values less than 1 0 i e less than required to saturate the melt mass balance constraints suggest that these melts have been extracted before equilibration with host melanosomes the dry peraluminous nature of vapour undersaturated melts inhibits monazite and zircon solubility and results in concentration of these phases in the residue melts are consequently depleted in lree and hree melanosomes show complementary enrichment in lree while hree patterns are dominated by residual garnet type 2 leucogneisses in contrast have strongly enriched zr th and lree abundances negative eu anomalies and zr zr and lreet lreet 1 resulting from accessory phase entrainment vapour absent partial melting under moderate 6 8 kbar pressure granulite facies conditions of a pelitic source containing monazite is likely to give disequilibrium melts depleted in lree and hree as monazite and garnet are concentrated in the residue if temperatures are high enough 850 870 c to permit relatively large degrees of partial melting then the feldspar component of the source will be removed almost completely giving melts with large positive eu anomalies melts formed under vapour present conditions are unlikely to show such extreme lree and hree depletion or positive eu anomalies even at high degrees of partial melting disequilibrium melting coupled with source entrainment could fortuitously produce ree and trace element signatures similar to those typical of s type granites and usually ascribed to equilibrium melting conditions

a simple pendulum borehole tiltmeter based on a triaxial optical fibre displacement sensor sensitive instruments like strainmeters and tiltmeters are necessary for measuring slowly varying low amplitude earth deformations nonetheless laser and fibre interferometers are particularly suitable for interrogating such instruments due to their extreme precision and accuracy in this paper a practical design of a simple pendulum borehole tiltmeter based on laser fibre interferometric displacement sensors is presented a prototype instrument has been constructed using welded borosilicate with a pendulum length of 0 85 m resulting in a main resonance frequency of 0 6 hz by implementing three coplanar extrinsic fibre fabry perot in terferometric probes and appropriate signal filtering our instrument provides tilt measurements that are insensitive to parasitic deformations caused by temperature and pressure variations this prototype has been installed in an underground facility rustrel france where results show accurate measurements of earth strains derived from earth and ocean tides local hydro logic effects as well as local and remote earthquakes the large dynamic range and the high sensitivity of this tiltmeter render it an invaluable tool for numerous geophysical applications such as transient fault motion volcanic strain and reservoir monitoring

tectonic evolution of proterozoic australia proterozoic australia has long been interpreted as a single intact continent in which all tectonic and magmatic activity was intracratonic this paper proposes an alternative hypothesis in which numerous fragments of continental crust were assembled by plate tectonic processes the assembly was completed between 1300 and 1100 ma when the crustal fragments were combined as an early component of the rodinian supercontinent rifting and fragmentation of archaean continents began in the late archaean and continued into the proterozoic passive margin deposits such as those of the hamersley basin accumulated on isolated fragments of archaean crust these numerous fragments were subsequently assembled into three cratons by 1830 ma a west australian craton was established by collision of the archaean pilbara and yilgarn cratons which were joined along the capricorn orogen similarly a south australian craton developed by amalgamation of the proto gawler and proto curnamona cratons along the kimban orogen a north australian craton appears to have formed by accretion of numerous crustal fragments including the kimberley pine creek lucas and altjawarra cratons with sutures marked by the king leopold halls creek tennant creek and proto isan orogens the southern margin of the north australian craton was the site of repeated terrane accretion and orogenic activity between 1880 ma and 1400 ma this included an orogenic event at 1880 1850 ma the strangways 1780 1730 ma argilke 1680 1650 ma and chewings 1620 1580 ma orogenies and the intracratonic anmatjira uplift 1500 1400 ma intracratonic rifting at 1750 to 1710 ma and 1640 to 1600 ma produced the mcarthur basin and related minor basins parts of which were deformed by the isan orogeny at 1600 and 1530 ma rifting along the line of the capricorn orogen led to deposition in the overlying intracratonic bangemall basin between 1630 and 1300 ma along the eastern margin of the south australian craton the 1670 to 1600 ma olarian orogeny marks interaction with now obscured continental crust to the east tectonic activity between 1300 and 1100 ma led to the assembly of proterozoic australia as an early component of the supercontinent of rodinia this first involved the amalgamation of the west australian and north australian cratons followed by collision with the south australian craton the centralian superbasin developed over the junction of the north south and west australian cratons between 830 and 750 ma rifting to the east formed the adelaide geosyncline at 830 ma this was followed by the breakup of rodinia with the rifting apart of laurentia and gondwanaland along the eastern margin of proterozoic australia at 750 ma and the subsequent formation of the palaeo pacific ocean after the breakup of rodinia a series of northeast southwest compressional events followed by periods of relaxation reflect the assembly of a new supercontinent old lines of weakness were reactivated culminating in the intracratonic king leopold paterson petermann ranges and pinjarra orogenies between 620 and 540 ma subsequent reactivation continued into the phanerozoic with the widespread eruption of continental flood basalts and the formation of intracratonic basins 540 530 ma

inclusion host relations involving accessory minerals in high grade metamorphic and anatectic rocks in general accessory minerals are expected to participate in partial fusion of their host rocks to a degree determined simply by their solubilities in the melt the possibility must be recognized however that a given accessory grain may be physically isolated from the melt by inclusion within a residual major mineral because of the importance of accessory minerals to crustal rock trace element and isotope geochemistry and because of their common existence as inclusions in major phases we undertook an evaluation of the factors that affect inclusion formation during ultrametamorphism three approaches are taken 1 a review of interfacial energy considerations is used to show that the free energy of the system is lowered by location of accessory minerals at major phase grain perimeters and that the magnitude of this effect is proportional to the square of the accessory grain radius 2 annealing and partial melting experiments 1000 c 10 and 15 kbar on rock analogs are described and the results are shown to confirm the predicted tendency of accessory minerals to occupy grain boundaries and 3 the results of a study of accessory phase rutile and zircon distribution in a migmatite from the tibetan slab are reported again in confirmation of the prediction that accessories tend to be situated at majorphase grain perimeters the latter two aspects of the study reveal that although included accessory grains are common their generally small size results in only a minor contribution to the bulk rock budget of accessory mineral components most of the mass of these components is contained within populations of generally larger accessory grains located at major phase grain boundaries accordingly the assumption that accessory minerals are involved in crustal melting is generally valid

prograde and near peak zircon growth in a migmatitic pelitic schist of the southeastern canadian cordillera abstract u pb dates and trace elements acquired simultaneously on zircon from a migmatitic pelitic schist from the southeastern canadian cordillera are linked with results of phase equilibria modeling the first of three phases of zircon growth occurred between 77 and 72 ma along a prograde path in the kyanite melt field between the wet melting and muscovite dehydration melting reactions 675 775 c this protracted phase of prograde growth is characterized by a shallow positive slope in heavy rare earth elements hree the second phase occurred at c 69 ma after significant garnet growth by biotite dehydration melting at 775 875 c which produced zircon depleted in hree the last phase occurred at c 63 ma on the retrograde path as garnet and rutile were consumed at 780 c as indicated by enrichments in hree and nb but prior to feldspar crystallization in the melt as suggested by the moderate eu anomaly this path is consistent with rutile and monazite petrogenesis documented herein and in a previous study respectively in contrast with the high temperatures at which zircon grew 675 875 c ti in zircon thermometry yields unrealistically low estimates 470 739 c with 80 below the wet solidus there are no uncertainties in the buffering assemblage as rutile and quartz were present throughout and no effect of pressure because a pressure dependent calibration was used this study demonstrates that zircon growth along a prograde path is possible and warrants further investigation on factors controlling the ti content of zircon thermometry in metamorphic rocks

relationships between zircon u pb shrimp ages and leucosome type in migmatites of the halls creek orogen western australia field relations and zircon zoning and shrimp u pb patterns introduction from an outcrop of deformed pelitic migmatites reveal relationships both the amount and isotopic variability of accessory between leucosome type and the amount of zircon growth during minerals incorporated into granitic magmas and their migmatization zircons from the oldest stromatic migmatites show constituent trace elements are a complex function of several pb pb intragrain analyses with 1850 1840 ma source and crystallization processes bea et al 1994 anatectic ages and distinctive cathodoluminescence cl patterns brown 1994 ayres et al 1995 barbey et al 1995 bea although the sample is dominated by inferred pre 1855 ma detrital 1996 it is recognized that initial bulk compositions x zircons subsequent thick contact sheath leucosomes and backand pressure p temperature t conditions of anatexis veins developed at 1845 1830 ma in response to intrusion of are major controls on the chemistry and subsequent mafic dykes into the stromatic migmatites zircons show cl evidence behaviour of granitic magmas e g clemens 1990 for substantial anatectic growth and overgrowths with post 1850 brown 1994 also there is increasing awareness that ma ages and a single population age 20 my younger than the deformation in the source region can also control the early other migmatites zircons from the structurally youngest shearand physical pathways and chemical evolution of magmas e g vein hosted leucosomes show few overgrowths and retain a large sawyer 1991 hand dirks 1992 brown et al 1995 detrital population correlation between shear zones and minimal rushmer 1995 watt et al 1996 oliver barr 1997 anatectic zircon growth is inferred to reflect rapid deformationwhich in turn may have a profound impact on subsequent induced melt extraction from the protolith and or rapid quenching granitic magma evolution for a given bulk source comthe earlier contact sheath and stromatic leucosomes probably had position magma extracted at low melt fractions from a combination of slow melt segregation larger interacting melt an actively deforming partially molten zone may differ volume and longer residence times at high temperatures results substantially in composition from one extracted at high suggest that rapid melt extraction by deformation will increase the melt volumes from rocks undergoing little deformation tendency for preservation of source rock isotopic patterns in some rushmer 1995 the link between frozen migmatites granites and complete granite bodies may not be direct clemens

cyclic ground tilt associated with the 2004 2008 eruption of mount st helens 1 the long term behavior of the 2004 2008 effusive eruption of mount st helens was characterized by a gradual decline in the rates of seismicity dome growth and broad scale ground deformation but shallow near periodic drumbeat earthquakes over timescales of minutes indicated episodic short term behavior in part to better characterize this behavior and any associated ground deformation a network of tiltmeters was installed and recorded thousands of cyclic tilt events within the crater the duration of these events from minutes to hours was too long for them to be directly caused by the drumbeat seismicity tilt events were characterized by a recoverable asymmetric pattern of rapid tilt away from the vent followed by a more gradual reversal were highly correlated between different tiltmeters in the crater sometimes occurred in association with volcanic seismicity and ceased at the end of the eruption tilt vectors converged on a point just south of the center of the preexisting 1980s lava dome and the absence of detectable tilt outside the crater suggests a shallow source 1 km we examine several models including cycles of conduit pressurization and plug slip or gas loss and stick slip behavior on the interface between the ascending plug and the 1980s lava dome the small number of stations within the crater prevents a unique determination of source type or geometry but results are consistent with a mechanism involving extrusion of the semisolid dacite plug and or cycles of conduit pressurization

zirconium abundance in granulite facies minerals with implications for zircon geochronology in high grade rocks zircon growth in high grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconium zr we have measured the concentration of zr in the major minerals in granulite facies rocks of differing bulk composition both garnet and hornblende contain tens of parts per million zr and no other major phase contains significant zr simple calculations show that reaction of either garnet or hornblende to form non zr bearing phases will release sufficient zr to account for at least some new zircon growth u pb ages from new zircon grown as a result of either hornblende or garnet breakdown are not expected to record the time of peak metamorphism but rather will record the time of particular metamorphic reactions allowing direct correlation of zircon ages with petrologically derived pressure temperature time paths this approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible

thermomechanical forcing of deep rock slope deformation 2 the randa rock slope instability 1 deformation monitoring between 2004 and 2011 at the rock slope instability above randa switzerland has revealed an intriguing seasonal trend relative dislocation rates across active fractures increase when near surface rock temperatures drop in the fall and decrease after snowmelt as temperatures rise this temporal pattern was observed with different monitoring systems at the ground surface and at depths up to 68 m and represents the behavior of the entire instability in this paper the second of two companion pieces we interpret this seasonal deformation trend as being controlled by thermomechanical tm effects driven by near surface temperature cycles while part 1 of this work demonstrated in a conceptual manner how tm effects can drive deep rock slope deformation and progressive failure we present here in part 2 a case study where temperature controlled deformation trends were observed in a natural setting a 2d discrete element numerical model is employed which allows failure along discontinuities and successfully reproduces the observed kinematics of the randa instability by implementing simplified ground surface temperature forcing model results were able to reproduce the observed deformation pattern and tm induced displacement rates and seasonal amplitudes in the model are of the same order of magnitude as measured values model results however exhibit spatial variation in displacement onset times while field measurements show more synchronous change additional heat transfer mechanisms such as fracture ventilation likely create deviations from the purely transient conductive temperature field modeled we suggest that tm effects are especially important at randa due to the absence of significant groundwater within the unstable rock mass

a comparison of tiltmeters at tidal frequencies different techniques for measuring crustal deformation are now being tested at pinon flat geophysical observatory in southern california as a part of this comparison we have analyzed five months of data from four different tiltmeters three borehole instruments two 4 5 m and one 26 m deep and a long base 535 m instrument at the surface for periods from 1 hour to 10 days the signals from the deep borehole and long base instruments show comparable noise levels the shallow instruments are much noisier tidal analyses give results for the different sensors which agree to within the errors which are several percent

zircon growth during progressive recrystallization of gabbro to garnet amphibolite eastern segment sveconorwegian orogen the interpretation of whether a dated metamorphic zircon generation grew during the prograde peak or retrograde stage of a metamorphic cycle is critical to geological interpretation this study documents a case at herrestad in the eastern part of the 1 0 ga sveconorwegian province involving progressive metamorphic recrystallization of gabbro to garnet amphibolite and associated behaviour of zr bearing minerals in this case textures show that baddeleyite is by far the main source of zr for metamorphic zircon growth the amount of metamorphic zircon formed was primarily controlled by the degree of metamorphic recrystallization which in turn was controlled by deformation and the presence of a fluid as a transport medium zircon in the herrestad rocks shows a range of morphologies and internal textures at different degrees of metamorphic recrystallization igneous zircon occurs together with baddeleyite in coarse grained olivine free facies of the gabbro metamorphic polycrystalline zircon rims on baddeleyite and minute 5 m bead like zircon grains at fe ti oxide boundaries characterize the transition to coronitic metagabbro with increasing metamorphic recrystallization polycrystalline zircon rims grow at the expense of baddeleyite and the amount of minute bead like zircon increases forming strings of zircon beads with increasing distance from fe ti oxide grains the progressive breakdown of baddeleyite results in polycrystalline zircon aggregates that become denser and finally form single grains in completely recrystallized garnet amphibolite late magmatic zircon crystallized at 156765 ma whereas metamorphic zircon dates amphibolite facies metamorphic recrystallization at 97067 ma the herrestad case illustrates a general rule that the bulk zr budget in originally baddeleyite bearing rocks will rapidly become locked into metamorphic zircon during the first event of metamorphic recrystallization when silica and zr are released from the igneous minerals incomplete metamorphic recrystallization and partial preservation of baddeleyite however also allows later stages of zircon formation thus in incompletely reacted rocks the final result may be highly complex with microscale zircon of several age generations less

poroelasticity observations of anomalous near surface tilt induced by ground water pumping abstract ground tilt signals observed at three different test sites in germany confirm earlier observations of surface deformation occurences caused by the withdrawal of ground water from nearby wells the strength of the signals presented here ranges from a few rad to 230 rad whereby ground tilt was measured in shallow boreholes at depths between 2 and 4 m analysis of the tilt signals suggests that local inhomogeneities in the subsoil can have significant impact on several signal parameters namely its maximum amplitude its variation in time and its strike with respect to the direction of the productive well the observed data emphasize the likelihood of various causes for deviations from the normal tilt response that holds for an isotropic poroelastic half space in particular at one site a reverse well level change also known as noordbergum effect in hydrogeology is seen together with a respective response in the tilt signal the study concludes that near surface deformation in the vicinity of pumped wells bears a wealth of information that may be useful to constrain the conditions of fluid flow at depth and that there is a need for model calculations to fully understand the involved phenomena

rates of surface processes on slopes slope retreat and denudation results taken from 270 publications on rates are summarized and collated with those from 149 publications reviewed previously young 1969 1974 the data are classified by major climatic zone normal or steep relief and consolidated or unconsolidated rocks representative rates and their ranges are given for soil creep solifluction surface wash solution chemical denudation rock weathering slope retreat cliff free face retreat marine cliff retreat and denudation the last being compared with representative rates of uplift solifluction is of the order of 10 times faster than soil creep but both cause only very slow ground loss solution is an important cause of ground loss for siliceous rocks on which it may be half as rapid as on limestones total denudation brought about mainly by surface wash reaches a maximum in the semi arid and probably also the tropical savanna zones acceleration of natural erosion rates by human activities ranges from 2 3 times with moderately intense land use to about 10 times with intensive land use and considerably higher still where there is recognized accelerated soil erosion where there is active uplift typical rates are of the order of 10 times faster than denudation although in some high steep mountain ranges these may approach equality

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