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

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

the site response of two rock and soil station pairs to strong and weak ground motion abstract the site response to strong and weak ground motion depends largely on the subsurface conditions at the soil site for the two rock soil station pairs studied the first station pair consists of a soft soil site treasure island and a sandstone and shale site yerba buena island these stations recorded strong ground shaking from the loma prieta mainshock and weak ground motion from four aftershocks the range of peak ground acceleration is from approximately 0 00006 to 0 07 g at the rock site compared to the rock site the strong ground motion at the soft soil site is amplified by a factor of about 3 over a frequency range from 0 5 to 2 0 hz the amplification is much higher for weak motion and suggests a dependence on signal amplitude for example near 1 hz the site response shows an increasing amplification as magnitude and the peak velocity at the rock site decreases for events of local magnitude 7 0 4 3 4 1 3 5 and 3 3 the maximum soil site amplifications are 4 12 17 19 and 25 respectively a second station pair consisting of a stiff soil site gilroy 2 and a sandstone site gilroy 1 was also studied with contrasting results these two stations recorded strong ground shaking from the 1979 coyote lake 1984 morgan hill and 1989 loma prieta mainshocks weak ground motion was recorded at these stations after the loma prieta mainshock the range of peak ground acceleration is from 0 006 to 0 43 g at the rock site unlike the results for the soft soil study above the estimated stiff soil site responses are not significantly different for strong and weak motion from 0 5 to 2 0 hz near 0 7 hz the stiff soil site responses range from 2 5 to 4 5 for strong ground shaking from three mainshocks and from 1 5 to 4 0 for weak ground shaking from thirteen aftershocks

the relation between site amplification factor and surficial geology in central california abstract site amplification factors at frequencies of 1 5 3 6 and 12 hz were determined for 132 stations of the usgs seismic network in central california from coda waves of 185 local earthquakes in this area using a recursive stochastic inversion method we found that the site amplification at a station is systematically related to the geology underlying that station the site amplification is high for young quaternary sediments and decreases with increasing geologic age at all frequencies between 1 5 and 12 hz the rate of decrease varies with frequency where site amplification at low frequencies shows a faster rate of decrease with age than at higher frequencies to quantify the relation between site amplification factor and site geology the surface geology of station sites was classified into five groups namely 1 quaternary sediments 2 pliocene sediments 3 miocene through cretaceous sediments 4 franciscan formation and mesozoic granitic rocks and 5 pre cretaceous metamorphic rocks the station site amplification factors for each group were logarithmically averaged and the mean value was assigned to the median geologic age of that group a smooth power law relation was found between the mean site amplification and the median geologic age this relation provides a simple way of estimating site effect at a specific site with known surficial geology our site amplification factors were compared with magnitude station residuals determined by eaton 1990 for the same network a remarkable linear correlation was found between the logarithmic coda amplification factor and the magnitude station residual value confirming that the coda method provides an effective means of estimating site amplification factor comparison of our site amplification factor with strong motion results obtained from the loma prieta earthquake boore et al 1989 maley et al 1989 shakal et al 1989 chin and aki 1991 suggests that weak and strong motion site amplification factors correlate well in the region outside the epicentral source region beyond epicentral distances of about 50 km within the epicentral source region however the weak motion amplification factors estimated from coda waves do not agree with the observed for strong ground motion suggesting a nonlinear behavior at sediment sites these results together with studies on the correlation between weak motion site amplification and earthquake intensity borcherdt 1970 borcherdt and gibbs 1976 chavez garcia et al 1990 king et al 1990 demonstrate the importance of in situ determination of weak motion site amplification for seismic zonation

geochronology of the sequence hosting the broken hill pb zn ag orebody australia robust stratigraphic control and constraints on the timing of events have been established for the paleoproterozoic metasedimentary and metavolcanic succession that hosts the broken hill pb zn ag orebody in western new south wales geologically consistent depositional intrusive and metamorphic ages are elucidated by means of shrimp u pb analyses of zircon and integrated field studies comparisons of the new shrimp results with those of previous studies clarify some previously untenable age interpretations a range of zircon provenance ages is found in albitic metasedimentary rocks from one of the oldest recognized parts of the willyama supergroup thackaringa group the zircons indicate maximum depositional ages of 1720 to 1730 ma and a minimum stratigraphic age for the lower thackaringa group is constrained by the 1704 3 ma intrusive alma gneiss the latter is the same age as gneissic granitoid rocks in the redan zone southern broken hill domain 1703 3 1705 3 ma felsic magmatism of this age has long been known in the contiguous olary domain south australia but has not been recognized previously at broken hill depositional ages in the broken hill group the upper part of which hosts the pb zn ag orebody indicate that the entire package was deposited within a period of ca 10 m y an age of 1685 3 ma for felsic metavolcaniclastic rocks in the hores gneiss spatially and stratigraphically associated with the ore horizon corroborates previous work this age is within error of that determined for the hanging wall and footwall unit rasp ridge gneiss of the orebody 1683 3 ma it suggests that felsic magmatism in the rasp ridge gneiss was coeval with that of the hores gneiss previous interpretations which suggested that the hores gneiss is not a single stratigraphic entity but comprises younger intrusions having ages from 1690 to1640 ma are not supported by our data and conclusions the stratigraphic integrity of the broken hill group excluding some mafic intrusions is further supported by results from an older formation near its base where tuffaceous metasedimentary rocks from the ettlewood calc silicate member allendale metasediments define an age of 1693 4 ma this is in good agreement with the age of 1693 5 ma for the immediately overlying parnell formation provenance ages obtained from detrital zircons in broken hill group psammites indicate a remarkable uniformity of paleoproterozoic and late archean source s generally matching provenance ages are found in the overlying sundown group psammopelites suggesting a stable flux of sediment fill as the basin deepened and matured similar provenance is indicated for upper paragon group sedimentary rocks and inherited zircons in most of the younger granitoids have age spectra similar to those of the detrital zircons the perpetuation of these age patterns in both clastic detritus and magmatic xenocrysts thus broadly fingerprints exposed and buried older crustal elements that provided material to produce the willyama supergroup basin fill tuffaceous metasiltstones in the middle paragon group bijerkerno metasediments are interpreted to have been derived partly from air fall volcaniclastic material and give consistent depositional ages of 1655 4 and 1657 4 ma in the northern broken hill and euriowie blocks respectively younger tuffaceous metasiltstones in the upper paragon group dalnit bore metasediments provide a stratigraphically consistent maximum depositional age of 1642 5 ma zircon ages from granitoid intrusions that bracket the deformation events suggest that the d 2 and d 3 deformations are inseparable within the interval 1597 3 to 1591 5 ma low th u zircon overgrowths are ubiquitous and irrespective of stratigraphic position record metamorphic ages of 1600 ma metamorphic rutile has the same age confirming that high grade metamorphic event s and d 2 and d 3 structural events were approximately synchronous we find no isotopic support for any pre 1600 ma high grade metamorphic events using newly acquired local age control for the galena pb isotope growth curve the new data support a stratigraphic syngenetic or diagenetic model of primary ore formation for the broken hill orebody at ca 1690 ma although the present distribution of the lodes likely reflects displacement as a result of later structural remobilization

the global climate response to lowering surface orography of antarctica and the importance of atmosphere ocean coupling abstracta global climate model is used to study the effect of flattening the orography of the antarctic ice sheet on climate a general result is that the antarctic continent and the atmosphere aloft warm while there is modest cooling globally the large local warming over antarctica leads to increased outgoing longwave radiation which drives anomalous southward energy transport toward the continent and cooling elsewhere atmosphere and ocean both anomalously transport energy southward in the southern hemisphere near antarctica poleward energy and momentum transport by baroclinic eddies strengthens anomalous southward cross equatorial energy transport is associated with a northward shift in the intertropical convergence zone in the ocean anomalous southward energy transport arises from a slowdown of the upper cell of the oceanic meridional overturning circulation and a weakening of the horizontal ocean gyres causing sea ice in the northern hemisphere to expand and the arctic to cool comparison wi

the ca 1740 1710 ma leichhardt event inversion of a continental rift and revision of the tectonic evolution of the north australian craton abstract interpretation and modelling of high resolution regional geophysical data of the central leichhardt river fault trough in the mount isa inlier are used to determine the timing of a major basin inversion event following the development of the ca 1780 1740 ma leichhardt superbasin inversion of the leichhardt superbasin formed the regional north south trending leichhardt anticline during east west shortening the limbs of the anticline are overprinted by several east west trending wedge shaped non magnetic sub basins filled with ca 1710 ma calvert and isa superbasin successions these relationships suggest inversion of the leichhardt superbasin occurred between ca 1740 and 1710 ma the event is also known to have affected the northern and eastern north australian craton the scale of the inversion suggests it was a significant event that we have defined as the leichhardt event this event requires a major tectonic driver to the east of the north australian craton possibly the accretion of a micro continental ribbon to the east of the mount isa inlier the results of this study have implication for paleogeographic reconstruction of the australian continent during the formation of nuna because eastern north australian craton faced an ocean at ca 1740 1720 ma the results also challenge the significance and intensity of crustal shortening associated with the ca 1600 1500 ma isan orogeny throughout the western mount isa inlier

spatial variation of ground motion determined from accelerograms recorded on a highway bridge a set of time synchronized strong motion accelerograms obtained on the san juan bautista 156 101 separation bridge in california during the 6 august 1979 coyote lake earthquake m l 5 9 are used to study the spatial variation of ground motion at the bridge site including traveling wave effects and the influence of multiple support excitation analysis of the ground motion recorded at the base of two of the bridge supports 32 6 m apart revealed the presence of a differential support excitation having a period of 3 sec much longer than any structural periods of the bridge this signal also appeared as a noticeable long period component in the superstructure displacements analysis of the vertical and radial components of the 3 sec ground motion indicated that ground displacements were retrograde for the duration of strong shaking with several cycles exhibiting elliptical particle motions these findings suggest that long period differential support motions were induced by phase delays in a rayleigh wave traveling across the bridge site further support to this premise is given by the location of the bridge site near a maxima of the rayleigh wave radiation pattern for the coyote lake earthquake based on published focal mechanism data traveling wave effects were also detected for compressional body waves by a correlation analysis which indicated a time delay of 7 msec between p wave arrivals at two of the bridge supports

reinterpretation of the tectonic context of high temperature metamorphism in the broken hill block nsw and implications on the palaeo to meso proterozoic evolution abstract the origin of high temperatures during regional low pressure granulite facies metamorphism within the proterozoic broken hill block australia has been reinterpreted to be the result of burial of anomalously hot rock packages for which the lithospheric geothermal gradient was initially elevated during early rifting ca 1 71 1 67 ga and then maintained during a ca 1 62 ga short lived mid crustal extensional event mid crustal extension at ca 1 62 ga was associated with amphibolite facies metamorphism and elevated lithospheric geothermal gradients and occurred prior to activity along d2 high temperature shear zones peak low pressure granulite facies metamorphism ca 1 60 ga and crustal shortening during the olarian orogeny ca 1 60 1 59 ga this reinterpretation places the broken hill block within an environment in which multiple episodes of transient extension ca 1 71 1 62 ga were followed by a switch to shortening at ca 1 60 ga the interpreted tectonic environment is a continental back arc setting located in the over riding plate of a subduction zone along the southern margin of palaeoproterozoic australia

site amplifications for generic rock sites seismic shear wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities and these velocities have been used to compute frequency dependent amplifications for zero attenuation for use in simulations of strong ground motion we define a genetic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data most of the boreholes are in populated areas for that reason the rock sites sampled are of particular engineering significance we consider two genetic rock sites rock corresponding to the bulk of the borehole data and very hard rock such as is found in glaciated regions in large areas of eastern north america or in portions of western north america the amplifications on rock sites can be in excess of 3 5 at high frequencies in contrast to the amplifications of less than 1 2 on very hard rock sites the consideration of unattenuated amplification alone is computationally convenient but what matters for ground motion estimation is the combined effect of amplification and attenuation for reasonable values of the attenuation parameter k 0 the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 hz with a maximum level of less than 1 8 the combined effect is about a factor of 1 5 at 1 hz and is less than unity for frequencies in the range of 10 to 20 hz depending on t using these amplifications we find provisional values of about ao 70 bars and k 0 0 035 sec for rock sites in western north america by fitting our empirically determined response spectra for an m 6 5 event to simulated values the borehole data yield shear velocities 033o of 618 and 306 rrdsec for rock and soil sites respectively when averaged over the upper 30 m from this we recommend that v30 equals 620 and 310 m sec for applications requiring the average velocity for rock and soil sites in western north america by combining the amplifications for rock sites and the site factors obtained from our analysis of strong motion data we derive amplifications for sites with 17 3o 520 m sec nehrp class c corresponding to a mix of rock and soil sites and v3o 310 and 255 m sec average soil and nehrp class d sites respectively for the average soil site the combined effect of amplification and attenuation exceeds a factor of 2 0 for frequencies between 0 4 and about 4 hz with a peak site effect of 2 4 the peak of the nehrp class d site effect is 2 7

textural evidence for extensive melting of the broken hill orebody galena pyrrhotite and chalcopyrite in samples of zn rich ore from the broken hill orebody show textures similar to those in partially melted metamorphic rocks including strings of low melting phases separating lithons of minerals likely to be restite low dihedral angles between low melting temperature minerals and restitic minerals and enlargement of these dihedral angles due to partial annealing analysis of existing experimental work from the system fe pb zn s indicates that at 1 bar galena and sphalerite coexisting with pyrrhotite of the compositions found at broken hill would have been partially melted at temperatures of about 750 c at 5 kbars this melting would occur at 780 c this temperature is well within the range of metamorphic temperatures reported from the area from these data we conclude that the broken hill orebody was partially molten at peak metamorphism with much of the pb cu ag sb and as in the melt and fe and zn residing mostly in the restite differentiation during cooling enriched the residual melt in sb as ag and other low melting temperature chalcophile elements the main sulfide melt froze after the last penetrative deformation to affect the orebody at temperatures below 720 c and possibly as low as 650 c sulfosalts crystallized out of the remaining melt at temperatures of 600 to 450 c and the final melt crystallized native bismuth at ca 250 c

site response shallow shear wave velocity and wave propagation at the san jose california dense seismic array ground motion records from a 52 element dense seismic array near san jose california are analyzed to obtain site response shallow shear wave velocity and plane wave propagation characteristics the array located on the eastern side of the santa clara valley south of the san francisco bay is sited over the evergreen basin a 7 km deep depression with miocene and younger deposits site response values below 4 hz are up to a factor of 2 greater when larger regional records are included in the analysis due to strong surface wave development within the santa clara valley the pattern of site amplification is the same however with local or regional events site amplification increases away from the eastern edge of the santa clara valley reaching a maximum over the western edge of the evergreen basin where the pre cenozoic basement shallows rapidly amplification then decreases further to the west this pattern may be caused by lower shallow shear wave velocities and thicker quaternary deposits further from the edge of the santa clara valley and generation trapping of surface waves above the shallowing basement of the western evergreen basin shear wave velocities from the inversion of site response spectra based on smaller local earthquakes compare well with those obtained independently from our seismic reflection refraction measurements velocities from the inversion of site spectra that include larger regional records do not compare well with these measurements a mix of local and regional events however is appropriate for determination of site response to be used in seismic hazard evaluation since large damaging events would excite both body and surface waves with a wide range in ray parameters frequency wavenumber plane wave analysis is used to determine the backazimuth and apparent velocity of coherent phases at the array conventional high resolution and multiple signal characterization f k power spectra and stacked slowness power spectra are compared these spectra show surface waves generated scattered at the edges of the santa clara valley and possibly within the valley at the western edge of the evergreen basin

electron microprobe monazite dating of ca 1 71 1 63 ga and ca 1 45 1 38 ga deformation in the homestake shear zone colorado origin and early evolution of a persistent intracontinental tectonic zone the homestake shear zone one of the principal precambrian structures within the colorado mineral belt has a history of tectonism that extends from the proterozoic to the tertiary new field mapping microstructural analysis and electron microprobe u th pb monazite dates define the style kinematics and timing of multistage proterozoic ductile deformation early subhorizontal shearing d1 progressed to northwest southeast crustal shortening d2 during a protracted ca 1710 1630 ma orogeny involving high temperature metamorphism partial melting and emplacement of the ca 1 7 ga cross creek batholith the subvertical northeast trend of the shear zone was established by d2 shortening a major episode of simple shear displacement occurred ca 1 4 ga during emplacement of the st kevin batholith complex kinematics are recorded by 1380 ma southeast side down dextral mylonite zones d3 and post 1380 ma southeast side up dextral ultramylonite pseudotachylyte zones d4 that were both localized in d2 foliation domains the succession of ductile tectonites in the homestake records 300 m y of episodic deformation at progressively shallower crustal levels as colorado evolved from an active accretionary margin 1 8 1 7 ga through crustal assembly 1 7 1 6 ga to a tectonically active continental interior post 1 45 ga the homestake shear zone constitutes a classic example of ductile shear zone reactivation and the development of persistent tectonic zones in the continental lithosphere

near surface attenuation and site effects from comparison of surface and deep borehole recordings abstract near surface attenuation and site effects are investigated using the seismograms of 17 local earthquakes recorded at depths of 0 0 3 1 5 2 5 and 2 9 km in the cajon pass borehole southern california the borehole penetrates 500 m of miocene sandstone and then crystalline granitic basement rock previous estimates of site response have been limited to shallower holes where the surface reflection interferes with the upgoing direct wave and the deepest sensor is not below the severe near surface effects in bedrock spectral ratios of the direct p and s waves for each earthquake between the different recording levels are well modeled with frequency independent q and amplification at the borehole q p 27 8 and q s 21 7 in the upper 2 9 km increasing from q p 26 and q s 15 in the upper 300 m to q p 133 and q s 47 between 1 5 and 3 km one event was also recorded at a surface granite site less than 1 5 km from the wellhead comparison of the 2 9 km recording with that at this granite site gives q p 50 and q s 23 the similarity of these values with those of previous studies at a wide range of sites suggests that q is very low in the near surface independent of rock type near surface amplification appears considerably more site dependent at the wellhead the amplifications at 1 hz of the direct p and s waves are 12 7 and 13 7 respectively these values include the free surface effect and are in reasonable agreement with the impedance contrast from the borehole logs at the granite site amplification of both p and s waves is less than four direct wave amplification at the wellhead is therefore at least three times that of the granite site the spectra of the direct and coda waves of the one earthquake recorded at both the wellhead and granite sites are compared with the corresponding 2 9 km recording coda wave amplification is in good agreement with the direct wave at the rock site but at the borehole the coda wave amplification factor overestimates the direct wave amplification by a factor of 3

baroclinic waves propagating from a high latitude source a time dependent primitive equation model of the southern hemisphere winter troposphere with an idealized dome shaped continent centred at 80 s has been used to assess the potential of antarctica to force stationary waves and to determine the location of the mid latitude baroclinic storm track several integrations of the model for two hundred days were made with the zonal mean fields fixed at the climatological values in the time mean of each run an anticyclonic wave response formed over the mountain and a planetary wave propagated into the mid latitudes with substantial amplitude this suggests that much of the observed stationary asymmetry in the southern mid and high latitudes may be due to antarctica the response to the antarctic forcing was different in a steady linear version of the model however interaction between the steady waves appears to be of minor importance to the time mean pattern rather e vector fields point to a substantial effect of the transients on the pattern in particular the low frequency transients appear to be important in the balance of the time mean equations near the mountain

a model for low pressure facies metamorphism during crustal thickening low pressure metamorphic facies i e high t p ratios are widespread in a wide range of tectonic settings explanations offered for the occurrence of these facies include extensional and or magmatic models however these fail to explain that the low p facies metamorphism is commonly coeval with a phase of pervasive crustal thickening with t p ratios increasing during or slightly lagging behind the thickening we propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion thinning of the mantle lithosphere

the willyama supergroup in the broken hill and euriowie blocks new south wales abstract the willyama supergroup consists of highly deformed metasedimentary schists and gneisses with abundant quartzofeldspathic gneisses lesser basic gneisses and minor lode rocks prograde metamorphism ranged from andalusite through sillimanite to granulite grade radiometric dating in the broken hill block indicates separation of the source of the quartzo feldspathic gneisses from the mantle at 2300 2100 ma a rb sr model source age of 1820 60 ma has been interpreted as the maximum age of deposition deformation d 1 and or d 2 was coincident with high grade metamorphism at 1660 10 ma minor mundi mundi type granitoids and some pegmatites were intruded at 1490 20 ma minor pyroxenites at 561 7 ma dolerite dykes soon after and zoned pegmatites at 500 ma in association with the delamerian orogeny which reactivated retrograde schist zones at 520 40 ma subsequent history mostly comprises faulting weathering uplift and erosion the willyama supergroup has an estimated total thickness of 7 9 km with neither top nor basement exposed no unconformities have been recognized in the sequence the supergroup was deposited in a deepening environment passing upward from sandy facies to more shaly and fine sandy facies and from interpreted bimodal acid basic volcanics to non volcanic facies the metasediments have been interpreted as shelf sediments overlain by deeper water turbidites and minor contourites recently they have been reinterpreted as shallow marine with the quartzo feldspathic gneisses interpreted as fluvio deltaic arkoses in this paper the sequence to the top of thackaringa group is interpreted as mainly fluvio deltaic and lacustrine the broken hill group and sundown group are interpreted as shallow marine the paragon group may have comprised shelf muds overlain by delta front and lacustrine sediments in turn overlain by deeper water fine grained turbidites the broken hill orebody near the top of broken hill group consists of a series of stacked lenses much recent work has concluded that the deposit is volcanic exhalative in origin probably formed from seawater convected through the sequence and driven by heat from high level magma chambers however some features of the deposit are more appropriate to a sedimentary exhalative origin and models have been proposed involving seismic pumping of connate water with ore deposition on or below the seafloor devolatilization of the mantle has been proposed as a source of ore constituents deposition may have occurred in a narrow ne sw trending seafloor depression the willyama supergroup was probably deposited in a rift zone with thin crust the basic gneisses are interpreted as fe rich tholeiites similar to basalts from some oceanic spreading ridges the intense folding post folding leucogranitic intrusions and extensive granulite amphibolite facies metamorphism could have resulted from a collision following subduction however there is no direct evidence of oceanic crust or subduction and the low pressure metamorphic path does not suit a collision model the rocks were probably deposited in a continental rift with some similarities to those interpreted for proterozoic fold belts in northern australia

influence of the antarctic ice sheet on southern high latitude climate during the cenozoic albedo vs topography effect sensitivity of high latitude temperature distribution to the variation of the antarctic ice sheet is investigated using an atmospheric general circulation model coupled with a mixed layer ocean results suggest that cooling effect of the ice sheet is not negligible compared to the effect of atmospheric co2 decrease such as during the cenozoic cooling effect of high surface albedo of the ice sheet is enhanced over the ice sheet and slightly suppressed over the circumpolar ocean by the high topography of the ice sheet high surface albedo and high topography of the ice sheet each causes increase and decrease of southward atmospheric heat transport respectively whose contribution to maintaining the heat balance anomaly of atmosphere ocean system is comparable to that of local adjustment of radiation balance

local site effects on weak and strong ground motion abstract this is a review of the current state of the art in characterizing effects of local geology on ground motion a new horizon is clear in this aspect of strong motion studies non linear amplification at sediment sites appears to be more pervasive than seismologists used to think several recent observations about the weak motion and the strong motion suggest that the non linear amplification at sediment sites may be very common first on average the amplification is always greater at the younger sediment sites for all frequencies up to 12 hz in the case of weak motion while the relation is reversed for frequencies higher than 5 hz in the case of strong motion secondly the application of the amplification factor determined from weak motion overestimates significantly the strong motion at sediment sites observed during the loma prieta earthquake within the epicentral distance of about 50 km thirdly the variance of peak ground acceleration around the mean curve decreases with the increasing earthquake magnitude finally the above non linear effects are expected from geotechnical studies both in the magnitude of departure from the linear prediction and in the threshold acceleration level beyond which the non linearity begins

delamerian aged metamorphism in the southern curnamona province australia implications for the evolution of the mesoproterozoic olarian orogeny monazite electron microprobe u th pb and garnet sm nd isotopic data from metapelitic assemblages in the willyama supergroup in the southern curnamona province south central australia indicate that the terrain underwent regional greenschist to amphibolite grade metamorphism during the c 500 ma delamerian orogeny the delamerian aged mineral assemblages include prograde garnet staurolite and kyanite bearing associations that overprint andalusite and sillimanite bearing assemblages that are interpreted to have developed during the c 1600 ma olarian orogeny importantly the development of secondary kyanite bearing assemblages in the southern curnamona province has been used previously to suggest that the olarian orogeny followed an anticlockwise p t evolution if such assemblages are the product of c 500 ma metamorphism then the anticlockwise p t path is an apparent path due to the overprint of a distinct metamorphic cycle c 1100 ma later making such distinctions is therefore extremely important when using the textural and metamorphic evolution of polycyclic terrains to model the thermal behaviour of the crust during orogeny this study highlights the utility of in situ geochronology linking age data to petrologically important phases and assemblages

monazite paragenesis and u pb systematics in rocks of the eastern mojave desert california u s a implications for thermochronometry abstract studies of the paragenesis and u pb systematics of monazite in rocks from the eastern mojave desert california corroborate its potential usefulness as a prograde thermochronometer and in dating granite inheritance unmetamorphosed latham shale and its equivalents at grades ranging from greenschist to upper amphibolite facies are virtually identical in composition monazite is absent in the shale and low grade schists but it is abundant in schists at staurolite and higher grades lower grade schists instead include minute th and ce oxides and unidentified ce poor lree phosphates that apparently are lower temperature precursors to monazite thus monazite originates when the pelite passes through lower amphibolite facies conditions monazites from three upper cretaceous granites yield ages that are strongly discordant upper intercepts of 1 6 1 7 ga are similar to those defined by u pb data for coexisting zircons and coincide with a period of copious magmatism in the mojave crust as the host upper cretaceous granitic magmas were all above 700 c effective closure of the restitic monazites to pb loss must be well in excess of this temperature u pb compositions of monazite from proterozoic granitoids and schist also indicate high pb retentivity taken together these studies support the suggestion that monazite can be an effective prograde thermochronometer at least in pelites it is not usually retained as a detrital mineral but rather forms during moderate temperature metamorphism its u pb system should not be reset by subsequent higher grade metamorphism

caustics and focusing produced by sedimentary basins applications of catastrophe theory to earthquake seismology summary as high frequency elastic waves propagate through real media it is common for caustics and focusing to occur typically rays may envelop a caustic surface in space or exceptionally they may all coalesce at a focal point in strong motion seismology the observed large fluctuations in peak acceleration and intensity of ground shaking may just be a consequence of focusing and caustics created by waves propagating through irregularly shaped sedimentary basins these basins acting as deformed optical lenses are capable of producing a complex network of patches and seemingly isolated pockets of intensified damage or high intensity shaking where the caustic intersects the ground surface we adapt methods from optics and catastrophe theory to study the properties of caustics induced by typical sedimentary basins several hypothetical examples are shown that reflect the fact that these properties are useful to assess quantitatively the degree of wavefield amplification to be expected a good correlation is found between actual damage patterns and caustic locations computed for the caracas venezuela earthquake of 1967

zonation of monazite in metamorphic rocks and its implications for high temperature thermochronology a case study from the lewisian terrain abstract monazite is a valuable chronometer for many geological processes and a proper interpretation for the age derived from monazite requires an adequate understanding of the behaviour of this phase and its chemical and isotopic systems during geological processes this contribution reports the results of a detailed study on chemical and isotopic zonation of monazite in lewisian granulites which is of general significance for monazite geochronology in high grade metamorphic rocks detailed investigations on two lewisian granulite samples using sem epma and sims have revealed that lewisian monazites frequently show petrographic chemical and age zonation the petrographical zonation which reflects the chemical compositional variation can be characterised as follows on the basis of the bse images 1 concentric zoning 2 patchy zoning and 3 intergrowth like zoning the chondrite normalised ree distribution patterns are remarkably similar between different zones of single monazite grains this rules out the possibility that any portion of a zoned monazite in these two samples is detrital in origin some possible mechanisms are proposed for the formation of monazite zonation these include 1 intergrowth of monazite crystals with different composition 2 episodic growth or regrowth of monazite in response to the change of environment conditions during thermal event s which may involve recrystallisation or replacement of the original crystals in short the diversity of zoning patterns in monazites observed in this study cannot be explained assuming a single mechanism accordingly interpretation of the chemical and age information from any zoned monazite requires an adequate understanding of its formation mechanism line scans of th u pb la ce nd sm ca and si across monazite zones have been performed on two monazite grains which experienced granulite facies metamorphism the very sharp boundaries in these line scans indicate that the diffusive transport of these elements in monazite is low even at the temperature of granulite facies metamorphism the effective closure of monazite grains may also depend upon other factors such as the armouring effects of the host phase and recrystallisation of the mineral itself thus great care needs to be taken for geological application of the closure temperature concept

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

age mapping and dating of monazite on the electron microprobe deconvoluting multistage tectonic histories high resolution x ray mapping and dating of monazite on the electron microprobe are powerful geochronological tools for structural metamorphic and tectonic analysis x ray maps commonly show complex th u and pb zoning that reflects monazite growth and overgrowth events age maps constructed from the x ray maps simplify the zoning and highlight age domains microprobe dating offers a rapid in situ method for estimating ages of mapped domains application of these techniques has placed new constraints on the tectonic history of three areas in western canada age mapping has revealed multiphase monazite with older cores and younger rims included in syntectonic garnet microprobe ages show that tectonism occurred ca 1 9 ga 700 m y later than mylonitization in the adjacent snowbird tectonic zone in new mexico age mapping and dating show that the dominant fabric and triple point metamorphism occurred during a 1 4 ga reactivation not during the 1 7 ga yavapai mazatzal orogeny in norway monazite inclusions in garnet constrain high pressure metamorphism to ca 405 ma and older cores indicate a previously unrecognized component of ca 1 0 ga monazite in all three areas microprobe dating and age mapping have provided a critical textural context for geochronologic data and a better understanding of the complex age spectra of these multistage orogenic belts

earthquake site response in santa cruz california abstract aftershocks of the 1989 loma prieta california earthquake are used to estimate site response in a 12 km 2 area centered on downtown santa cruz a total of 258 s wave records from 36 aftershocks recorded at 33 sites are used in a linear inversion for site response spectra the inversion scheme takes advantage of the redundancy of the large data set for which several aftershocks are recorded at each site the scheme decomposes the observed spectra into source path and site terms the path term is specified before the inversion the undetermined degree of freedom in the decomposition into source and site spectra is removed by specifying the site response factor to be approximately 1 0 at two sites on crystalline bedrock the s wave site responses correlate well with the surficial geology and observed damage pattern of the mainshock the site response spectra of the floodplain sites which include the heavily damaged downtown area exhibit significant peaks the largest peaks are between 1 and 4 hz five floodplain sites have amplification factors of 10 or greater most of the floodplain site response spectra also have a smaller secondary peak between 6 and 8 hz residential areas built on marine terraces above the floodplain experienced much less severe damage site response spectra for these areas also have their largest peaks between 1 and 4 hz but the amplification is generally below 6 several of these sites also have a secondary peak between 6 and 8 hz the response peaks seen at nearly all sites between 1 and 4 hz are probably caused by the natural resonance of the sedimentary rock column the higher amplifications at floodplain sites may be caused by surface waves generated at the basin margins the secondary peak between 6 and 8 hz at many sites may be a harmonic of the 1 to 4 hz peaks we used waveforms from a seven station approximately linear array located on the floodplain to calculate the apparent velocity and azimuth of propagation of coherent arrivals within moving windows of the s wave codas the initial windows give results that are consistent with direct s wave arrivals the apparent velocities are high greater than 4 0 km sec and azimuths are from the source waves arriving later than 2 sec after the direct s waves have apparent velocities of less than 1 km sec indicating that they are surface waves and arrive from divergent azimuths this analysis indicates that after the direct s wave arrival long duration shaking comes from surface waves that are generated at the basin margin and reverberate in the floodplain sediments

constraints on long lived mesoproterozoic and palaeozoic deformational events and crustal architecture in the northern mount painter province australia abstract meta sedimentary and igneous rocks preserved in the hanging wall of the paralana fault northern mount painter province record a protracted ca 1600 myr multi staged metamorphic and poly deformational evolution related to mesoproterozoic and palaeozoic orogenic events the mount painter province was located near the margin of both the south australian craton and the north australian craton which allows comparisons of the tectonic evolution of these margins and provides context for the evolution of the eastern proterozoic australia for both the columbia and gondwana supercontinents within the mount painter province meta sedimentary rocks form part of a basin system developed adjacent to the margins of the gawler craton and curnamona province following the olarian wartakan orogenic system sedimentation was followed by ca 1590 1552 ma ductile deformation d 1 d 3 at mid to upper crustal levels preserving a rapid ca 6 myr burial and exhumation cycle initial deformation is characterised by the development of a layer parallel fabric s 1 s 1 is overprinted by recumbent north verging folds f 2 formed during mid to upper amphibolite facies peak metamorphism m 2 which are overprinted by tight upright shallow ne sw plunging folds f 3 d 3 folds developed contemporaneous with sinistral oblique shearing and the development of proto mylonitic to ultra mylonitic fabrics along the paralana fault zone the timing of d 3 deformation is constrained by the development of a penetrative steeply dipping foliation s 3 in the ca 1585 1569 ma shallowly emplaced mount neill granite and relatively undeformed granites dated at ca 1552 ma further constraint on the absolute timing of mesoproterozoic tectonism is provided through the dating of zircon growth from samples of the meta sedimentary radium creek metamorphics this dating reveals a 1555 16 ma metamorphic event interpreted to be the consequence of m 3 amphibolite facies regional metamorphism and accompanying metasomatism the transition between d 2 and d 3 is interpreted to reflect a shift from ca 1591 1585 ma n s shortening to ca 1585 1552 ma nw se shortening this episode of orogenesis is correlated with the ca 1585 1540 ma kararan orogeny in the northern gawler craton which distinctly postdates the ca 1600 1590 ma olarian orogeny as previously interpreted the mesoproterozoic structural architecture was reactivated during neoproterozoic extension d 4 and the ca 500 ma delamerian orogeny delamerian structures are formed in a predominantly brittle regime early thin skinned thrusting was followed by thick skinned deformation d 5 d 6 during which open f 5 folds developed above oblique lateral ramps and produced type 0 overprinting patterns with f 3 mesoproterozoic folds steeply plunging f 6 buckle folds with se trending axial traces are the last phase of deformation recorded both mesoproterozoic ductile deformation phases and the palaeozoic ductile brittle to brittle overprints were focussed along the paralana fault emphasising the repeated long lived evolution of this crustal scale fault system

regional correlations of vs30 and velocities averaged over depths less than and greater than 30 meters using velocity profiles from sites in japan california turkey and europe we find that the time averaged shear wave velocity to 30 m vs30 used as a proxy for site amplification in recent ground motion prediction equations gmpes and building codes is strongly correlated with average velocities to depths less than 30 m vsz with z being the averaging depth the correlations for sites in japan corresponding to the kik net network show that vs30 is systematically larger for ag ivenvsz than for profiles from the other regions the difference largely results from the placement of the kik net station locations on rock and rocklike sites whereas stations in the other regions are generally placed in urban areas underlain by sediments using the kik net velocity profiles we provide equations relating vs30 to vsz for z ranging from 5 to 29 m in 1 m increments these equations and those for california velocity profiles given in boore 2004b can be used to estimate vs30 from vsz for sites in which velocity profiles do not extend to 30 m the scatter of the residuals decreases with depth but even for an averaging depth of 5 m a variation in logvs30 of 1 standard deviation maps into less than a 20 uncertainty in ground motions given by recent gmpes at short periods the sensitivity of the ground motions to vs30 uncertainty is considerably larger at long periods but is less than a factor of 1 2 for averaging depths greater than about 20 m we also find that vs30 is correlated with vsz for z as great as 400 m for sites of the kik net network providing some justification for using vs30 as a site response variable for predicting ground motions at periods for which the wavelengths far exceed 30 m online material estimates of vs30 at k net stations

interpretation of monazite ages obtained via in situ analysis abstract monazite grains from nepal and vietnam were compositionally analyzed with an electron microprobe and dated th pb with an ion microprobe five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population 1 pb loss due to diffusion 2 dissolution reprecipitation reactions along a retrograde path 3 analytical uncertainties 4 analyses of overlapping age domains and 5 episodic monazite growth the influence of these factors is sample dependent but can be evaluated 1 using peak metamorphic conditions and x ray element maps to assess potential polymetamorphism or retrogression 2 obtaining other geochronologic data including previous work or dating other minerals in the sample 3 evaluating any method related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses and 4 ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element ree oxide or allanite chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms instead of relying on monazite chemical composition major mn fe mg ca and minor y element garnet zoning patterns and peak p t conditions should be used to facilitate age interpretation this thermobarometric data records the sample s thermal history changes in garnet growth rate and mechanisms and accessory mineral breakdown

late paleo mesoproterozoic plate margin deformation in the southern gawler craton insights from structural and aeromagnetic analysis abstract the integration of structural analysis of outcrop and aeromagnetic interpretation has revealed a complex tectonic evolution for the southern gawler craton that reflects the regional tectonic signal during the late paleo to early mesoproterozoic results suggest that exposures at yarlbrinda hill comprise the margin of a highly deformed st peter suite pluton emplaced within the southwestern gawler craton at ca 1611 ma d 1 at outcrop records east to northeast directed ductile thrusting regionally this deformation overprints north south trending dextral yarlbrinda shear zone fabrics within ca 1622 ma st peter suite granite and ca 1690 1670 ma tunkillia suite granite progressive east to northeast directed thrust movements led to exhumation of the deformed ca 1611 ma granite d 2 deformation is recorded by the brittle reactivation of first generation mylonitic fabrics accompanied by cataclasis alteration and pseudotachylite development d 3 records a switch to northwest southeast shortening which led to the folding and tear faulting of the d 1 2 shear zone a switch to extension and subsequent rotations of the principle stress axes led to hiltaba suite granite emplacement and contemporaneous faulting under east west extension during d 4 and west northwest east southeast extension during d 5 deformation of ca 1622 ma st peter suite granite is consistent with movement on the yarlbrinda shear zone and requires the st peter suite to have been emplaced into or adjacent to the ca 1690 1670 ma tunkillia suite prior to shear zone activity and on the overriding plate during subduction d 1 d 3 deformation is constrained by the emplacement of ca 1580 ma hiltaba suite granite and correlates with the wartakan orogeny in the southeastern gawler craton and the olarian orogeny in the curnamona province a 52 disparity between the shortening directions in these respective areas suggests different degrees of mesoproterozoic rotation which likely aided the formation of an intervening basin province the remnants of which are presently preserved in the mount painter inlier the record of convergent deformation is geochronologically constrained to a 30 myr period and places this region of the central gawler craton near a late paleoproterozoic convergent plate margin prior to a period of transient extension beginning at ca 1580 ma the continental scale signal of tectonic events can be recognized at the outcrop scale highlighting the importance of detailed outcrop studies in sparsely exposed areas

low damage anomaly of the 1976 tangshan earthquake an analysis based on the explosion ground motions abstract during the 1976 tangshan earthquake m 7 8 a low earthquake damage anomaly was reported in yutian area this low damage area has not been explained satisfactorily because of insufficient ground motion records in this area the japan china joint research project conducted during 1987 to 1988 provides us with a unique opportunity to investigate this problem in this study the ground motion data generated by explosions are used to explain the origin of the low earthquake damage anomaly four recording profiles are arranged for this purpose and each profile contains about 20 stations the arrival times and waveform characteristics are different along profile b b wave trains with small amplitudes in the north are related to a thin sedimentary layer and an up dip underground structure in the south the wave trains have longer coda waves that can be linked with a thicker sedimentary layer site effects on ground motions can be identified in the west of yutian based on the small amplitudes observed during three explosions the distributions of small amplitude also depend on the locations of explosions this reveals that the path effects also contribute to the seismic ground motion the regions with small peak velocities are very similar to the low earthquake damage area from the 1976 tangshan earthquake the cause of low damage anomaly during the tangshan earthquake can be attributed to both the site and path effects

regional and local factors in attenuation modelling hong kong case study seismic attenuation behaviour is controlled by a large number of wave modification mechanisms the characteristics of some of these mechanisms are specific to a local area whilst the remainder can be generalised to the entire seismic region factors representing these mechanisms are often not resolved a new attenuation modelling approach is demonstrated in this paper using hong kong as a case study to evaluate individual regional and local wave modification factors shear wave velocity swv information for the four prevalent geological formations found in hong kong was first obtained a at shallow depths from instrumented boreholes b at depths of up to 100 200 m from measurements using the microtremor spatial auto correlation spac technique c at depths of up to 1 5 km from the monitoring of quarry blasts and d at depths from 1 5 to 8 km in the hard basement rock layers from results of seismological refraction surveys the upper crust amplification factor calculated from the four modelled rock swv profiles was then combined with predicted attenuation parameters to determine the upper crust modification factor filter function incorporating the local wave modification characteristics associated with hong kong geological formations such functions may then be combined with the regional attenuation characteristics in that part of the south china region a seismic attenuation model was developed by combining the upper crust modification factor with the regional source function of intra plate earthquakes based on stochastic simulations the ground shaking model developed from the presented methodology is supported by the comparison with macro seismic data of seven historical earthquake events affecting hong kong

cu au mineralisation in the curnamona province south australia a hybrid stratiform genetic model for mesoproterozoic iocg systems in australia abstract the proterozoic curnamona province lies between two of the most endowed cu bearing mineral belts on the australian continent the gawler craton which hosts the giant olympic dam iron oxide copper gold iocg system and the mount isa inlier which hosts the giant mount isa cu system and iocg belt of the eastern succession this study focuses on cu au deposits within the proterozoic rocks of the benagerie ridge magnetic complex in the central curnamona province the benagerie ridge is buried beneath neoproterozoic and phanerozoic cover sequences we use drillcore and geophysical data to determine the structural architecture and structural controls of cu au mineralisation in the benagerie ridge the portia and north portia deposits are the most significant cu au mineral deposits in the benagerie ridge and lie adjacent to an extensive regional redox boundary that separates lower magnetic units from upper non magnetic units the redox boundary is extensive throughout the curnamona province and is linked with mineralisation in the region significant chalcopyrite bornite and molybdenite occur along bedding parallel layers and shear structures these factors support a strong lithological control on cu au mineralisation at all scales deformation in the benagerie ridge involved the development of a d1 layer parallel foliation s1 which is overprinted by d2 recumbent folds and d3 upright folds the deformation history is associated with the c 1610 1580 ma olarian orogeny overprinting of d2 structures during d3 has resulted in a type 2 interference pattern which we have successfully mapped using aeromagnetic data several later phases of post olarian faults overprint f2 and f3 fold interference patterns and have resulted in the remobilisation of some of the cu au mineralisation we propose that the stratiform cu au mineralisation occurred prior to the onset of the olarian orogeny mineralisation is preserved within the thickened hinge zone of a similar type f2 fold proximal to the hinge of an overprinting f3 fold which we propose has enhanced mineralisation within the north portia deposit consequently the identification of the redox boundary within f2 folds elsewhere in the curnamona province particularly when located proximal to f3 fold hinges which place the mineralised zones into structurally favourable locations provide good targets for future mineral exploration

geologically constraining india in columbia the age isotopic provenance and geochemistry of the protoliths of the ongole domain southern eastern ghats india abstract the ongole domain in the southern eastern ghats belt of india formed during the final stages of columbia amalgamation at ca 1600 ma yet very little is known about the protolith ages tectonic evolution or geographic affinity of the region we present new detrital and igneous u pb hf zircon data and in situ monazite data to further understand the tectonic evolution of this columbia forming orogen detrital zircon patterns from the metasedimentary rocks are dominated by major populations of palaeoproterozoic grains ca 2460 2320 2260 2200 2100 2080 2010 1980 1920 1850 and 1750 ma and minor archaean grains ca 2850 2740 2600 and 2550 ma combined u pb ages and lu hf zircon isotopic data suggest that the sedimentary protoliths were not sourced from the adjacent dharwar craton instead they were likely derived from east antarctica possibly the same source as parts of proterozoic australia magmatism occurred episodically between 1 64 and 1 57 ga in the ongole domain forming felsic orthopyroxene bearing granitoids isotopically the granitoids are evolved producing ehf values between 2 and 12 the magmatism is interpreted to have been derived from the reworking of archaean crust with only a minor juvenile input metamorphism between 1 68 and 1 60 ga resulted in the partial to complete resetting of detrital zircon grains as well as the growth of new metamorphic zircon at 1 67 and 1 63 ga in situ monazite geochronology indicates metamorphism occurred between 1 68 and 1 59 ga the ongole domain is interpreted to represent part of an exotic terrane which was transferred to proto india in the late palaeoproterozoic as part of a linear accretionary orogenic belt that may also have included south west baltica and south eastern laurentia given the isotopic geological and geochemical similarities the proposed exotic terrane is interpreted to be an extension of the napier complex antarctica and may also have been connected to proterozoic australia north australian craton and gawler craton

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

dating prograde amphibolite and granulite facies metamorphism using in situ monazite u pb shrimp analysis abstract in situ shrimp u pb analysis of monazite grains from pelites within an early formed high temperature shear zone in the southern broken hill block australia has been used to constrain the timing of prograde amphibolite facies and peak granulite facies metamorphism within the terrain geochronological analyses from grains inhabiting texturally distinct localities revealed two distinct age populations ca 1619 and 1600 ma the older age was obtained from grains that occur as inclusions completely enclosed within coarse grained k feldspar and garnet grains which evidently armored the monazite inclusions against resetting during younger deformation and metamorphic events the ca 1619 ma monazite population occurs as part of an amphibolite facies inclusion assemblage hosted within the peak granulite facies mineral assemblage and constrains the timing of prograde amphibolite facies metamorphism within the broken hill block the younger 1600 ma monazite age population was from grains within the pe

felsic metavolcanic rocks related to the broken hill pb zn ag orebody australia geology depositional age and timing of high grade metamorphism two key issues related to the origin of the massive broken hill pb zn ag orebody are the genesis of its host rocks and their depositional age these granulite facies layered gneisses have been ascribed alternately to volcanic and nonvolcanic regimes determination of the depositional age of the sequence has never been seriously addressed north of broken hill amphibolite facies precursors to the granulite facies gneisses show compelling evidence of a volcanic origin in their texture chemistry and zircon morphology these felsic metavolcanic rocks are in the upper part of the broken hill group stratigraphically equivalent to the broken hill orebody the present work traces textural zircon morphological and isotopic changes across the amphibolite to granulite facies transition in order to determine the primary depositional age of this supracrustal sequence and the timing of highgrade regional metamorphism ion microprobe shrimp u pb data on zircons from the lower grade amphibolite facies felsic metavolcanic rocks define a major magmatic population at 1690 or 5 ma this is considered to be the age of zircon crystallization and eruption of felsic volcanics in this part of the broken hill group these shrimp data clarify inheritance and discordance problems that had plagued conventional multigrain u pb analyses zircon results from granulite facies stratigraphic equivalents of these rocks near southern cross and at broken hill have patterns similar to those of the amphibolite facies rocks including igneous relics that provide the same primary age additionally zircon in the highest grade granulite facies rocks approximately 800 degrees c forms metamorphic overgrowths surrounding the magmatic relics these younger zircon selvedges have a pooled age of 1600 or 8 ma which determines the timing of this granulite facies event consistent u pb monazite results yield this same age 1599 or 9 ma suggesting that the monazite blocking temperature is close to that of zircon or that cooling to 725 degrees c was achieved relatively rapidly this is some 60 m y younger than the previously accepted age for high grade metamorphism at broken hill irrespective of metamorphic grade in the gneisses all biotite rb sr ages have been reset to approximately 480 ma a reflection of early paleozoic delamerian orogenesis inherited zircon grains interpreted as xenocrysts and in some cases mantied by 1690 ma igneous rims have older proterozoic to late archcan ages 1780 2700 ma the presence of xenocrystic zircon components points to an ensialic tectonic setting for the broken hill block and reflects accidental incorporation in the felsic volcanic protoliths of lithic fragments and or complex igneous sources such as found in crust exposed in the gawler craton to the west the 1690 or 5 ma depositional age determined here provides a firm chronologic tie between the early proterozoic sequences enclosing major stratiform pb zn ag orebodies at broken hill mount isa 1670 or 19 ma and mcarthur river 1690 or 27 ma in northern australia in light of the uncertainties over timing and mode of origin of these deposits the similar ages of their host sequences have important implications for early proterozoic tectonics and metallogeny

on baroclinic instability over continental shelves testing the utility of eady type models abstractthis study examines the utility of eady type theories as applied to understanding baroclinic instability in coastal flows where depth variations and bottom drag are important the focus is

isoseismal map of the 2015 nepal earthquake and its relationships with ground motion parameters distance and magnitude abstract a devastating earthquake of mw 7 8 struck central nepal on 25th april 2015 6 11 25 ut which resulted in more than 9000 deaths and destroyed millions of houses standing buildings roads and electrical installations worth 25 30 billions of dollars are reduced to rubbles the earthquake was widely felt in the northern parts of india and moderate damage have been observed in the northern part of up and bihar region of india maximum intensity ix according to the usgs report was observed in the meizoseismal zone surrounding the kathmandu region in the present study we have compiled available information from the print electronic media and various reports of damages and other effects caused by the event and interpreted them to obtain modified mercalli intensities mmi at over 175 locations spread over nepal and surrounding indian and tibet region we have also obtained a number of strong motion recordings from india and nepal seismic network and developed an empirical relationship between the mmi and peak ground acceleration pga peak ground velocity pgv we have used least square regression technique to derive the empirical relation between the mmi and ground motion parameters and compared them with the empirical relationships available for other regions of the world further seismic intensity information available for historical earthquakes which have occurred in the nepal himalaya along with the present intensity data has been utilized for developing an attenuation relationship for the studied region using two step regression analyses the derived attenuation relationship is useful for assessing damage of a potential future large earthquake earthquake scenario based planning purposes in the region

antarctic ice sheet elevation impacts on water isotope records during the last interglacial knowledge of how the antarctic ice sheet ais has varied in response to past climates can inform the prediction of future ais behaviour water stable isotope records from antarctic ice cores tradi

euro seistest strong motion array near thessaloniki northern greece a study of site effects abstract the effects of local geology on the characteristics of strong ground motion have been and continue to be a field of active research despite the considerable efforts made so far there are still several unresolved and controversial issues remaining in particular debates still continue over the limits of applicability of one dimensional 1d wave propagation models there are also unresolved questions related to the implementation and reliability of site response estimation techniques such as the standard spectral ratio ssr and the horizontal to vertical spectral ratio hvsr this study addresses these issues on the basis of data from the euro seistest strong motion array at volvi near thessaloniki greece the data set used consists of accelerograms of 32 earthquakes almost exclusively local covering a magnitude range from m l 2 0 to 6 1 the range of recorded accelerations is 0 001 0 042 g we implement two 1d wave propagation models the haskell thompson matrix method and kennett9s reflectivity coefficient method and the hvsr site response assessment technique we test the applicability of the 1d models in a basin environment we use both fourier amplitude and response spectra to compute hvsr and investigate how hvsr is influenced by the choice of the time window and smoothing procedure we found the hvsr technique and 1d sh wave modeling to perform reasonably well in a sediment valley environment the modeling was performed for a site in the center of the valley

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

topographic influences on the general circulation of the southern hemisphere a numerical experiment abstract a july integration of a gfdl spectral general circulation model is repeated after eliminating from the model the topographic elevations zonally averaged mean fields regions of frequent cyclogenesis and cyclone tracks and the standing waves in geopotential at 500 mb simulated for the southern hemisphere in both integrations are compared it is argued that the differences in the results support the interpretation advanced by mechoso 1980 concerning the influence of antarctica an the general circulation of the southern hemisphere

quasi stationary waves in the southern hemisphere part ii generation mechanisms abstract in this part ii the authors investigate the role that antarctic elevations the rest of the world orography thermal forcing from lower latitudes and the transient eddy component of the flow play on the generation of the quasi stationary wave field in the southern hemisphere an approach based on the ucla gcm is followed results from a control simulation with full orography and from experiments without the antarctic elevations and without the rest of the world orography suggest that the quasi stationary wave with zonal wavenumber 1 qs wave 1 around antarctica is primarily generated by mechanisms other than the antarctic elevations comparison of a three dimensional eliassen palm flux vector in the control simulation and those where the antarctic elevation and the rest of the world orography are removed suggests that wave activity propagates both from the subtropics and from polar latitudes although in qualitative agreement with results of part i the horizontal and vertical structure of t

timing and nature of fluid flow and alteration during mesoproterozoic shear zone formation olary domain south australia the development of shear zones at mid crustal levels in the proterozoic willyama supergroup was synchronous with widespread fluid flow resulting in albitization and calcsilicate alteration monazite dating of shear zone fabrics reveal that they formed at 1582 22 ma at the end of the olarian d3 deformational event and immediately prior to the emplacement of regional s type granites two stages of fluid flow are identified in the area first an albitizing event which involved the addition of na and loss of si k and fe and a second phase of calcsilicate alteration with additions of ca fe mg and si and removal of na fluid fluxes calculated for albitization and calcsilicate alteration were 5 56 109 to 1 02 1010 mol m 2 and 2 57 108 5 20 109 mol m 2 respectively these fluxes are consistent with estimates for fluid flow through mid crustal shear zones in other terranes the fluids associated with shearing and alteration are calculated to have 18o and d values ranging between 8 and 11 and 33 and 42 respectively and nd values between 2 24 and 8 11 our results indicate that fluids were derived from metamorphic dehydration of the willyama supergroup metasediments fluid generation occurred during prograde metamorphism of deeper crustal rocks at or near peak pressure conditions shear zones acted as conduits for major crustal fluid flow to shallow levels where peak metamorphic conditions had been attained earlier leading to the apparent retrograde fluid flow event thus the peak metamorphism conditions at upper and lower crustal levels were achieved at differing times prior to regional granite formation during the same orogenic cycle leading to the formation of retrograde mineral assemblages during shearing

cooling and exhumation history of the northeastern gawler craton south australia abstract we present biotite 40ar 39ar ages and from the palaeo to mesoproterozoic mount woods inlier in the northeastern gawler craton south australia in combination with garnet biotite fe mg exchange modelling using the thermal history program these data are used to constrain a post peak metamorphic p t path cooling rates and a temperature time path for the inlier consistently well defined plateaus at 1 53 ga of biotite 40ar 39ar age spectra indicate that the rocks cooled through 300 c at this time cooling rates within the inlier are 3 9 c ma from peak high temperature low pressure metamorphism at 1 59 ga to 1 53 ga cooling rates subsequently decreased to 4 c ma cooling is suggested to be a function of cessation of voluminous magmatism and mantle heat input in conjunction with exhumation initiated along the southwestern margin of the inlier ca 1 592 1 582 ga the temperature time path of the inlier has a concave shape the peak metamorphic and ensuing cooling history of the mount woods inlier are akin to the nearby curnamona province coober pedy ridge and mabel creek ridge within the south australian craton deformation during high grade metamorphism within these terranes was accommodated at mid crustal levels and large scale exhumation did not occur until the terranes cooled and were able to be exhumed as coherent rigid blocks along discrete shear zones and faults

partial resetting of the upb isotope system in monazite through hydrothermal experiments an sem and upb isotope study abstract natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their u pb isotope systematics measured before and after the runs runs were carried out with monazite grain size fractions ranging from 40 to 125 m in diameter and on 207 pb 206 pb ages close to 380 ma u concentrations remained always unchanged all recrystallized monazites from the powdered starting material define a discordia with intercept ages of 379 3 2 3 and 7 18 ma the latter indicating the hydrothermally induced pb loss at the time of the experiment sem observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces run products from monazite powders showed recrystallization and grain growth induced by a dissolution precipitation mechanism increasing temperature resulted in a distinct enlargement of grain sizes at 650 and 750 c a dissolution precipitation process in presence of a fluid phase rather than pb volume diffusion acts as a very efficient mechanism for the resetting of the u pb isotope system of monazites even at low temperatures this process is most important in shear zones where channelized fluid flow may occur

the effect of quaternary alluvium on strong ground motion in the coyote lake california earthquake of 1979 abstract the effect of alluvium on strong ground motion can be seen by comparing two strong motion records of the coyote lake california earthquake of 6 august 1979 m l 5 9 one record at a site on franciscan bedrock had a peak horizontal acceleration of 0 13 g and a peak horizontal velocity of 10 cm sec the other at a site 2 km distant on 180 meters of quaternary alluvium overlying franciscan had values of 0 26 g and 32 cm sec amplifications by factors of 2 and 3 horizontal motions computed at the alluvial site for a linear plane layered model based on measured p and s velocities show reasonably good agreement in shape with the observed motions but the observed peak amplitudes are greater by a factor of about 1 25 in acceleration and 1 8 in velocity about 15 per cent of the discrepancy in acceleration and 20 per cent in velocity can be attributed to the difference in source distance the remainder may represent focusing by refraction at a bedrock surface concave upward there is no clear evidence of nonlinear soil response fourier spectral ratios between motions observed on bedrock and alluvium show good agreement with ratios predicted from the linear model in particular the observed frequency of the fundamental peak in the amplification spectrum agrees with the computed value indicating that no significant nonlinearity occurs in the secant shear modulus computations show that nonlinear models are compatible with the data if values of the coefficient of dynamic shear strength in terms of vertical effective stress are in the range of 0 5 to 1 0 or greater the data illustrate that site amplification may be less a matter of resonance involving reinforcing multiple reflections and more the simple effect of the low near surface velocity application of traditional seismological theory leads to the conclusion that the site amplification for peak horizontal velocity is approximately proportional to the reciprocal of the square root of the product of density and shear wave velocity

geophysical reconstruction of buried geological features and site effects estimation of the middle valle umbra basin central italy abstract the middle valle umbra central italy is a nw se 20 km long and 10 km wide quaternary extensional basin located in the internal sector of the apennine chain this area historically experienced strong earthquakes that caused significant damages to the outstanding historical heritage the same area has been recently hit by the 2016 seismic sequence of amatrice visso norcia with the aim to reconstruct the buried geological structures of the basin a multi technique geophysical approach was performed an extended campaign of ambient noise measurements was carried out to investigate the subsurface setting and to identify the main geological units we performed three 2d passive arrays to analyze two different sites within the basin their aperture was between 150 and 752 m for one site and of 48 m for the other site to characterize the geological units in terms of sediment thickness and shear wave velocity profile data collected were processed with f k and mspac analysis to extract dispersion curves with good resolution in a frequency range of 0 5 10 hz and 4 5 18 hz for the two sites respectively spectral ratios were computed for every single station ambient noise measurement performed and for all the stations of the bigger array our final target is to extend these results to the whole valley in order to retrieve the attitude of the main geological units and propose a reliable reconstruction of the subsurface geometry of the basin another point of this work is to evaluate the site response in the middle of the valley through the analysis of the earthquakes recorded by the accelerometric station it csa belonging to the italian civil protection and the corresponding recordings of the nearby rock station it ass

reference rock site condition for central and eastern north america abstract the reference rock site condition has two important applications for ground motion prediction in the stable continental region of central and eastern north america cena 1 it represents the site condition for which ground motions are computed using semiempirical ground motion prediction equations in addition 2 it represents the site condition to which site amplification factors which are used to modify ground motion intensity measures for softer site condition are referenced i e site amplification is unity for reference rock we define reference rock by its shear s and compression p wave velocities as well as a site attenuation parameter 0 which is used in stochastic ground motion simulation methods prior definitions of reference rock conditions in cena were based mostly on indirect large scale crustal velocity inversions and judgment we compile and interpret a unique database of direct velocity measurements to develop criteria for assessing the presence of reference rock site condition based on measured seismic velocities and their gradient with respect to depth we apply the criteria to available profiles and perform rigorous statistical analysis from which we recommend s and p wave velocities of 3000 and 5500 m s respectively for the reference rock condition we recommend that for practical applications use ranges of reference s and p wave velocities of 2700 3300 m s and 5000 6100 m s respectively the ranges are based on a 5 change in amplification using quarter wavelength theory we do not find evidence for regional dependence of the reference velocities which are derived principally from three general geographic regions 1 atlantic coast 2 continental interior and 3 appalachian mountains our data do not provide reference velocities for the gulf coast region the recommended velocity compatible reference rock site kappa is 0 006 s

observed variations of earthquake motion across a sediment filled valley the response to earthquake motion of a site in one sediment filled valley was observed to depend strongly on frequency and the position of the site within the valley and weakly on the input signal s azimuth and incidence angles the observed input motion was weak varying between 10 5 and 10 3 g measurements of ground velocity were made on profiles across and along the valley and simultaneously on two nearby rock sites the valley was 400 m wide and 700 m long with a maximum sediment thickness of 60 m the average seismic impedance contrast between the sediments and underlying basement rock was about 6 1 ratios of fourier spectra from soil sites to spectra from nearby rock sites showed apparent site amplifications of as much as a factor of ten depending strongly on frequency and the distance of the site from the valley edge the scatter in spectral ratios for earthquakes with different azimuths and angles of incidence was about a factor of two thus local site effects that changed the amplitude of incident motion by a factor of two or more could be identified in the records of most earthquakes ground motion at valley edge and mid valley sites separated by less than 100 m differed by as much as a factor of five this difference could cause large differential motion in structures spanning these sites a theoretical flat layer model of sediment response predicts the average behavior of the middle of the valley but not that of the valley edge the lowest frequency responance apparently involves the sediments across the width of the valley not just the vertical column of soil beneath each site

seismic ground response studies in olympia washington and vicinity relative seismic ground response characteristics in the cities of olympia lacey and tumwater washington were determined from analysis of instrumentally recorded ground motion induced by blasts at an open pit coal mine near centralia washington a ground response function grf defined as the ratio of fourier spectral amplitudes at an alluvium site to spectral amplitudes on hard rock is a measure of amplification of seismic waves by localized site conditions grf values in three frequency bands 0 5 to 1 0 hz 1 0 to 2 0 hz and 2 0 to 4 0 hz were compared with observed modified mercalli mm intensities from the 29 april 1965 puget sound earthquake and with mapped surficial geologic units typically the grf values relate well with the surficial geological units in addition mm intensities within the v to vii range appear to be directly related to the frequencies within the 0 5 to 4 0 hz bandwidth such that mm v intensity sites had a lower grf value in the 2 0 to 4 0 hz bandwidth as compared to the 0 5 to 2 0 hz bandwidth and the mm vii intensity sites had higher grf values in the 2 0 to 4 0 hz bandwidth as compared to the 0 5 to 2 0 hz bandwidth the set of grf values determined for the city of olympia and its vicinity should be useful in formulating a theoretical relative ground response model for the southern puget sound area

evidence and timing of crustal extension versus shortening in the early tectonothermal evolution of a proterozoic continental rift sequence at broken hill australia 1 low pressure p high temperature t metamorphism previously attributed to crustal thickening accompanying nappe emplacement in the paleoproterozoic willyama supergroup at broken hill is reinterpreted as a response to crustal extension d1 and bimodal magmatism commencing circa 1700 ma d1 deformation occurred under amphibolite granulite facies conditions producing layer parallel fabrics s1 and metapelitic mineral assemblages in which andalusite dominated over sillimanite crustal extension ceased soon after 1640 ma to be followed by crustal thickening higher pressure metamorphism and northeast vergent recumbent folding d2 attendant on inversion of the original d1 extensional structures d2 recumbent folding has previously gone unrecognized but probably peaked around 1600 ma before further amphibolite facies metamorphism between 1600 and 1590 ma accompanying upright folding and northwest directed thrust faulting d3 in the light of this revised tectonothermal history geodynamic models erected to explain elevated thermal gradients and low p high t granulite facies metamorphism in tectonically thickened crust may not be as relevant to broken hill as once thought above average crustal temperatures and low p high t metamorphism in the willyama supergroup more likely occurred in response to voluminous magmatic intrusion accompanying crustal thinning mantle upwelling and emplacement and unroofing of midcrustal rocks along a major detachment surface a second major structural and or stratigraphic break in the willyama supergroup is indicated by the absence of d1 fabrics from the uppermost part of the sequence

synchronous development of type 2 and type 3 fold interference patterns evidence for recumbent sheath folds in the allendale area broken hill nsw australia abstract synchronous development of type 2 and type 3 fold interference patterns occur in the poly deformed broken hill inlier the interference patterns have resulted from the superposition of recumbent f2 folds and n s oriented upright f3 folds the synchronous development of type 2 and type 3 fold interference patterns is attributed to variation in the hinge of f2 folds by as much as 90 suggesting the development of a regional scale sheath like fold geometry during d2 overprinting relationships along the eastern limb of the pap synform suggests that it formed part of a recumbent fold hinge that was flattened during horizontal crustal shortening this has resulted in the development of a modified type 3 fold interference pattern the results of this study show that the type 2 and type 3 fold interference patterns can develop during the same deformation event in an evolving orogen