effect of fault rupture characteristics on near fault strong ground motions the effect of fault rupture characteristics on near fault strong ground motions is investigated using a kinematic modeling approach in an attempt to identify physical processes that lead to specific ground motion patterns the shear stress distribution on the causative fault plane of four well documented seismic events i e 1979 imperial valley 1985 michoacan 1989 loma prieta and 1999 izmit is calculated based on fault slip models available in the literature using the methodology proposed by bouchon 1997 for stress field computations in order to associate the fault rupture characteristics i e slip rupture velocity state of stress of the investigated earthquakes with near fault ground motions generated by the events forward ground motion simulations are performed using the discrete wavenumber representation method and the concept of the s wave isochrones is exploited the results indicate that the seismic energy radiated from the high isochrone velocity region of the fault arrives at the receiver within a time interval that coincides with the time window of the long period ground motion pulse recorded at the site furthermore the near fault ground motion pulses are strongly correlated with large slip on the fault plane locally driven by high stress drop in addition the local rupture velocity seems to be inversely correlated to the spatial distribution of the strength excess over the fault plane confirming findings of previous studies for various events the area of the fault that contributes to the formation of the near fault pulse encompasses more than one patch of significant moment release subevent e g 1979 imperial valley 1989 loma prieta this observation explains why a dislocation model with average properties i e slip rise time etc reproduces successfully near fault ground motions for strike slip faults and for dip slip faults with intermediate to large earthquake magnitudes aki 1979 however for very large earthquakes such as megathrust events on subduction zones e g 1985 michoacan the fault region that contributes to the pulse formation encompasses individual subevents and consequently cracklike slip functions rather than dislocation models may be more appropriate for the simulation of the near fault ground motions

three dimensional dynamic simulation of the 1992 landers earthquake the 1992 landers california earthquake magnitude 7 3 was modeled as the propagation of a spontaneous rupture controlled by a realistic prestress distribution with the use of a three dimensional finite difference method the dynamic rupture reproduces the general slip pattern used to compute the initial stress distribution and generates near fault ground motions at the surface similar to observations the simulated rupture propagates on the fault along a complex path with highly variable speed and rise time changing the magnitude and pattern of the stress significantly the method provides the framework to estimate earthquake rupture parameters from recorded seismic and geodetic data

model of earthquake recurrence role of elastic wave radiation relaxation of friction and inhomogeneity we have constructed a dynamical model of seismicity wherein a fault is embedded in an infinite continuous elastic medium therefore the influence of the energy dissipated in seismic wave radiation oil the sequential history of model earthquakes is fully taken into account in this model the drop in friction at rupture takes place gradually thus introducing a relaxation dimension as an example we consider a finite homogeneous fault that is terminated by infinitely strong barriers at the ends the features of the seismicity are dominated by the stresses that are reflected from the unbreakable barriers there is a strong dependence of the patterns of seismicity on the ratio of the relaxation dimension to the distance between the barriers at the ends for large values of this parameter we find that periodicity begins after a short transient interval and a dominance of the statistical distributions by large events that break completely through the fault from end to end for small values of this parameter smaller scale seismicity is interspersed with the large events and no periodicity is observed within the time spanned by the computations we conclude that major unquenched heterogeneities such as those found at barriers which we suppose arise in nature due to the nonuniform geometry of faults are vital ingredients for generating complex seismic histories as well as giving rise to the complex features of individual earthquake source time functions in models of seismicity as a dynamical process

weights of evidence and logistic regression modeling of magmatic nickel sulfide prospectivity in the yilgarn craton western australia abstract bayesian weight of evidence and logistic regression models are implemented in a gis environment for regional scale prospectivity modeling of greenstone belts in the yilgarn craton western australia for magmatic nickel sulfide deposits the input variables for the models consisted of derivative gis layers that were used as proxies for mappable exploration criteria for magmatic nickel sulfide deposits in the yilgarn about 70 of the 165 known deposits of the craton were used to train the models the remaining 30 was used to validate the models and therefore had to be treated as if they had not been discovered the weights of evidence and logistic regression models respectively classify 71 4 and 81 6 validation deposits in prospective zones that occupy about 9 of the total area occupied by the greenstone belts in the craton the superior performance of the logistic regression model is attributed to its capability to accommodate conditional dependencies amongst the input predictor maps and provide less biased estimates of prospectivity

dynamic faulting studied by a finite difference method we have developed a finite difference method that is especially adapted to the study of dynamic shear cracks we studied a number of simple earthquake source models in two and three dimensions with special emphasis on the modeling of the stress field we compared our numerical results for semi infinite and self similar shear cracks with the few exact solutions that are available in the literature we then studied spontaneous rupture propagation with the help of a maximum stress criterion from dimensional arguments and a few simple examples we showed that the maximum stress criterion depended on the physical dimensions of the fault for a given maximum stress intensity the finer the numerical mesh the higher the maximum stress that had to be adopted a study of in plane cracks showed that at high rupture velocities the numerical results did not resolve the stress concentration due to the rupture front from the stress peak associated with the shear wave propagating in front of the crack we suggest that this is the reason why transonic rupture velocities are found in the numerical solutions of in plane faulting when the rupture resistance is rather low finally we studied the spontaneous propagation of an initially circular rupture two distinct modes of nucleation of the rupture were studied in the first a plane circular shear crack was formed instantaneously in a uniformly prestressed medium after a while once stress concentrations had developed around the crack edge the rupture started to grow in the second type of nucleation a preexisting circular crack became unstable at time t 0 and started to grow the latter model appeared to us as a more realistic simulation of earthquake triggering in this case the initial stress was nonuniform and was the static field of the preexisting fault

supercontinents and superplume events distinguishing signals in the geologic record abstract simultaneous supercontinent and superplume events may reinforce or cancel signals preserved in the geologic record alternatively one signal may overwhelm the other for instance relatively low sea level during a 2 7 ga superplume supercontinent event may reflect direct hits of superplumes beneath the supercontinent although both supercontinent formation and superplume events occurred at 0 28 1 9 and 2 7 ga global warming at these times indicates the dominance of superplume events in controlling climate enhanced deposition of black shales correlates with superplume events and with supercontinent breakup carbon isotopes in seawater however show positive excursions during supercontinent breakup at 2 2 2 1 and 0 8 0 6 ga but show mixed signals or no signal during other supercontinent or superplume events probably due to negative feedbacks peaks in marine organism originations at 100 280 and 480 ma correlate with possible superplume events whereas an overall decrease in origination rate in the early paleozoic correlates with the growth of pangea and destruction of shallow marine environments increased production rates of juvenile crust correlate with formation of supercontinents and with superplume events there may be two types of superplume events catastrophic events which are short lived

3d simulation of near field strong ground motion based on dynamic modeling abstract we simulate the strong ground motion generated from the earthquake rupture process on a shallow strike slip fault using a 3d finite difference method the faulting process is modeled using a crack model with fixed rupture velocity the variability of peak ground velocity patterns correlated with fault location and source parameters such as stress drop or rupture velocity is investigated our findings suggest that these patterns are strongly affected by rupture directivity and the uppermost depth of the fault or that of the asperity when a fault breaks the ground surface the peak ground velocity and the peak ground acceleration show a narrow region of strong motion when a fault is buried under the ground the high peak ground velocity zone of the fault parallel component is apart from the fault trace by a distance comparable to the fault depth on the other hand the fault normal peak ground velocity is a maximum along the fault trace the fault length or asperity length is not so effective for peak ground velocities the effect of heterogeneity in stress drop and rupture velocity on strong ground motion is also investigated when stress drop is not uniform but increases linearly with depth from zero at the uppermost depth the peak ground velocity is reduced these results help better predict the strong ground motion generated from a potential fault

redistribution of ore elements during serpentinization and talc carbonate alteration of some archean dunites western australia serpentinization of barren and weakly mineralized archean dunites in the yilgarn block western australia occurred without any apparent loss of major components including the important ore elements ni and fe sulfur addition during initial serpentinization resulted in about 30 percent of original silicate ni being redistributed to newly formed sulfide by reactions similar to olivine water lizardite brucite and lizardite brucite s lizardite brucite magnetite heazlewoodite water h 2 serpentinization at a higher temperature or metamorphism of the lizardite bearing assemblage formed antigorite serpentinite in which up to 60 percent of bulk rock ni is incorporated in sulfide form lizardite brucite magnetite heazlewoodite co 2 s antigorite magnesite magnetite ni sulfide water h 2 complete co 2 metasomatism resulted in talc carbonate rocks with still more ni redistributed to sulfides and the formation of some fe sulfide antigorite magnesite magnetite ni sulfide co 2 s talc magnesite pentlanditc fe sulfide or magnetite water o 2 in areas where low s activity precluded sulfide formation ni was strongly enriched in magnetite in serpentinites or talc in talc carbonate rocks preexisting disseminated ni sulfides of magmatic origin were significantly upgraded by serpentinization up to 30 percent of sulfide ni in ores averaging about 0 7 percent ni is of metamorphic origin and this proportion may be higher in talc carbonate ores fe sulfides of metamorphic origin form up to 20 percent of the total sulfide fraction in talc carbonate altered disseminated ni ores so that about half the sulfide content of these ores may be metamorphically derived by redistribution of elements originally held in olivine

zircon geochronology of late archean komatiitic sills and their felsic country rocks south central zimbabwe a revised age for the reliance komatiitic event and its implications abstract the 2 8 2 6 ga bulawayan supergroup of the zimbabwe craton is inferred to be a passive margin and plume related conformable sequence comprising in part the felsic volcanic koodoovale sedimentary manjeri komatiitic reliance basaltic zeederbergs units the reliance unit is spatially associated with subjacent komatiitic sills of the mashaba ultramafic suite the inferred subvolcanic intrusive phase of the reliance flow fields we report u pb zircon ages for several units the pangani sill in the filabusi belt yielded an age of 2912 ma indicating it may belong to the 3 0 2 9 ga belingwean supergroup the koodoovale unit appears to range in age from 2866 to 2788 ma inferred syn eruptive mass flow deposits within the manjeri basinal shallow water sub facies in the midlands se belt yield ages of 2877 and 2870 ma sills of the mashaba ultramafic suite together with a previous result of 2743 ma for the mashaba igneous complex yield a mean age of 2746 ma which we propose as a proxy age for the reliance unit and for the reliance komatiitic event as a whole the 2 80 ga age for the top of the koodoovale unit in the western filabusi and gweru nw belts confirms with greater stratigraphic control earlier precise geochronology and provides an age for the start of sedimentation in the overlying manjeri basinal deep water sub facies the 2 79 ga felsic sample from within the reliance unit in the shangani n belt is likely from a koodoovale derived post eruptive mass flow deposit in the midlands se belt the new felsic rock ages support the conformable relationship between the uppermost koodoovale unit and overlying manjeri unit but place the transition at c 2 87 ga significantly earlier than in belts further west the new ages for the koodoovale unit and the revised age for the reliance unit introduce new perspectives on the development of the bulawayan supergroup 1 the koodoovale manjeri transition is diachronous across the zimbabwe craton 2 the duration of manjeri sedimentation varied from 50 ma to 120 ma in the basinal deep water and shallow water sub facies respectively to 80 ma in the platformal facies in the east where sedimentation is estimated to have begun at 2 83 ga such prolonged sedimentation is controversial but can be explained by extended periods of non deposition in the basinal deep water sub facies 3 using all published precise ages reliance zeederbergs volcanism may have lasted 70 ma 2 75 2 68 ga much longer than phanerozoic plume systems and 4 the previously proposed correlation and late archean nearest neighbour relationship between the zimbabwe and slave cratons are provided further support

the importance of the dynamic source effects on strong ground motion during the 1999 chi chi taiwan earthquake brief interpretation of the damage distribution on buildings the 1999 chi chi taiwan earthquake that originated on a low angle reverse fault showed complexity and uncommon characteristics the records show that the hanging wall side is characterized by larger particle motions than the foot wall and the ground motion is stronger in the northern part than in the southern part of the causative fault although the strongest ground motion occurred near the north ern part of the trace structural damage was heavier in the southern part in order to get a better understanding of the complex damage distribution caused by this earth quake the dynamic rupture process was numerically simulated because of the dif ferences between the observed features of the rupture process in the northern and southern parts of the fault each part was modeled independently by using a 2d discrete element model dem the principal results of the simulation show that the velocity ground motions in the northern part in the frequency range of 0 5 2 hz natural frequency range of standard structures are small near the surface break thus light structural damage might be predicted near the surface rupture moreover in the northern part the fault rupture propagation reaches the surface with a very slow velocity about 1 2 km sec however in the southern part the rupture propa gation reaches the surface with higher velocity about 3 0 km sec these differences between the models could explain why the ground motion near the surface rupture in the northern part caused less damage in structures than the ground motion in the southern part

supershear mach waves expose the fault breakdown slip abstract mikumo et al 2003 showed that it is possible to estimate the breakdown slip dc as the slip at the time of the peak slip rate for rupture propagation with subshear speeds cruz atienza et al 2009 later attempted to extend this method to extract information about dc as the displacement at the time of the peak particle velocity from seismic strong motion records however a reasonably accurate estimate of dc was only possible in a narrow zone adjacent to the fault typically on the order of hundreds of meters due to the fast decay with distance from the fault of the seismic energy related to the stress breakdown process when the rupture propagates with supershear speeds on the other hand this energy is carried much farther away from the fault by mach waves in particular rayleigh mach waves when rupture reaches the earth s surface dunham and bhat 2008 here we present a new approach to estimate dc from strong motion records containing mach waves first we show that the method by mikumo et al is valid for supershear rupture propagation this method is then used to estimate dc via an asymptotic approximation of the slip and slip rate time histories from the mach waves using spontaneous rupture simulations we demonstrate that for a visco elastic half space model dc can be estimated within an error of 40 from mach waves that have propagated a distance of at least 3 km from the fault the method is applied to estimate dc for the 2002 mw7 9 denali alaska earthquake 1 5 m and for the 1999 mw7 6 izmit turkey earthquake 1 7 m

dynamic rupture processes on a dipping fault and estimates of stress drop and strength excess from the results of waveform inversion summary the dynamic rupture processes on a dipping fault in a horizontally layered medium are investigated on 3 d spontaneous shear crack models the wave equations for the 3 d space are solved numerically by a finite difference method under appropriate boundary conditions the displacement components right above and right below the dipping fault plane have been obtained by solving the boundary conditions across the fault it was found that the final fault slip on the hanging wall side is appreciably larger than that on the foot wall side and that their amount is quite sensitive to the fault depth and the elastic heterogeneities of the medium particularly of the existence of low velocity surface layers on the basis of the above model the dynamic rupture process of the 1961 kita mino earthquake in central japan has been investigated with constraints of the fault parameters derived from the results of waveform inversion analysis for this purpose a locking fracture criterion was introduced for rupture propagation from which a lower bound of the maximum shear stress before the rupture and hence of the strength excess has been estimated the dynamic stress drop has also been evaluated by repeated iterations to minimize the difference between the dynamic and kinematic fault slips the results revealed quite heterogeneous distribution of strength excess indicating large values near the sw section of the fault at shallow depths and in the central bottom section in contrast to small values at mid depths in the ne section around the rupture nucleus zone the distribution of stress drop was also found to be heterogeneous with the existence of high stress drop zones in the ne shallow and bottom sections and a low stress drop zone at most of mid depths except in the sw section these results suggest that the dynamic rupture of this earthquake initiates at a small nucleus zone with low strength excess and small stress drop then broke the shallower and deeper fault sections with moderate strength excess and high stress drop and finally ruptured barrier zones with high strength excess these zones may be interpreted as weak and strong asperities and barriers respectively

remnants of an archean oceanic plateau belingwe greenstone belt zimbabwe stratigraphic and structural data from the archean zimbabwe craton suggest that a major detachment surface exists within the belingwe greenstone belt the surface separates ultramafic and mafic magmatic rocks of the upper greenstones in the hanging wall from an ancient gneiss complex older volcanic sedimentary rocks and a shallow water sedimentary sequence in the footwall rocks dated at ca 2 7 ga above the detachment surface form the proposed mberengwa allochthon the regionally extensive upper greenstone succession represents tectonically emplaced allochthonous sheets not indigenous magmas erupted within autochthonous continental rifts magmatic rocks of the mberengwa allochthon resemble oceanic plateaus preserved in younger mountain belts comparison of the zimbabwe craton with the proterozoic birrimian terranes of west africa leads us to suggest that precambrian continental growth may have been characterized by intense structural imbrication related to the difficulty of subduction of buoyant oceanic lithosphere

breaking of a single asperity rupture process and seismic radiation the problem of spontaneous shear rupture of a single circular asperity on an infinite fault plane is studied initially the fault plane is broken everywhere except at a circular asperity an applied displacement at infinity results in a stress concentration along the bounding edge of the asperity the frictional stress on the broken part of the fault plane is taken to be a constant once a point on the asperity breaks the stress there drops to the same value as on the main fault surface the rupture is started by relaxing the shear stress at a point on the asperity edge and is then allowed to propagate spontaneously using a critical stress level fracture criterion the rupture process is calculated numerically it is found that for asperities of constant strength the rupture first propagates around the edge of the asperity and then inward a phenomenon best described by the well known term of classic military maneuver the double encircling pincer movement in the appendix the expressions for the far field seismic radiation due to the rupture of such an asperity are derived it is shown that the nth cartesian component of the far field displacement at x t for p sv and sh waves using the notation of aki and richards 1980 is given by un x t dni 4 c2r s0 i3 t r c ds thus the far field pulses can be directly found from the stress drops on the fault plane this formula is also true for crack or dislocation problems the directivity function dni for displacement for the asperity problem is found to be that for the double couple modified by some factor in particular the fault plane is a nodal plane for sv waves for the rupturing of asperities on a finite fault these directivity functions are applicable only to the initial part of observed pulses at a receiver provided the receiver is not located on the fault plane outside the broken part of the main crack edge in which case it is inapplicable for all time

the 1999 izmit turkey earthquake nonplanar fault structure dynamic rupture process and strong ground motion we simulated dynamic rupture propagation along various nonplanar fault models of the 1999 izmit turkey earthquake using a boundary integral equation method these models were inferred from geological and geodetic observations based on these results we modeled seismic wave propagation around the fault system using a finite difference method we focused on the effect of different fault geometries on the rupture process and seismic wave propagation numerical simulation results imply a rapid and continuous rupture propagation from the izmit sapanca lake segment to the sapanca akyazi segment the rupture under sapanca lake appears to have propagated not on a disconnected fault segment but along a smooth fault structure with a bend of only a few degrees the observational complexity of the surface breaks however can be best simulated by a highly segmented fault model this infers that fault geometric characters observed in the field reflect near surface structure and that seismological and geodetic features are controlled by global fault structure at depth then we investigated the effect of frictional parameters and the initial stress field in order to explain near field seismograms at station skr located a distance of a few kilometers from the fault we had to force the rupture to propagate at shallow depth close to the station in order to obtain this we had to introduce a finite cohesive force in the friction law that allows stress accumulation and release in the shallow crust the external stress field had to be large enough for the rupture to propagate at very rapid speed our simulation results show that it is important to include detailed fault geometry in the numerical simulation and to constrain frictional parameters and the initial stress field for understanding of the full dynamic process of an earthquake

the age of the permian triassic boundary abstract the 5 cm boundary clay bed in the chinese stratotype section through the permian triassic boundary has been recognised as a bentonite shrimp ion microprobe dating of zircons in the bentonite indicates a magmatic age of 251 2 3 4 ma 2 this is the first direct constraint on the numerical age of the permian triassic boundary future refinements of ages at this important but poorly constrained level of the phanerozoic timescale may depend on re analysis of this uniquely placed volcanic horizon and other bentonites in the fossiliferous chinese upper permian and lower triassic the utility of defining the permian triassic boundary in the chinese stratotype section in the vicinity of known dateable horizons should be considered

tectonic setting and terrane accretion of the archean zimbabwe craton the archean zimbabwe craton is made of a number of distinct tectonostratigraphic terranes assembled by plate tectonic processes the central tokwe terrane consists of 3 5 2 95 ga gneissic rocks and structurally complex inliers of possibly older greenstone belts these are overlain unconformably by a 2 9 2 8 ga assemblage of mafic and felsic volcanic rocks and conglomerates and a separate 3 0 2 7 ga southeastward thickening platform sequence of sandstone shale and limestone 2 7 ga greenstone belts form two distinctly different domains flanking the central terrane northwest of the ancient gneissic terrane ca 2 7 ga greenstone belts comprise a series of calc alkaline lavas and intercalated sedimentary rocks intruded by syn volcanic plutons southeast of the ancient gneissic complex 2 7 ga greenstone belts consist of thick piles of tholeiitic basalts overlying ultramafic lavas resting allochthonously over the shallow water platform sequence and older gneissic terrane this division of the zimbabwe craton is interpreted to show that the central tokwe terrane had a continental magmatic arc built on its northwestern edge as its southeastern margin rifted from another fragment forming the sea of umtali a passive margin sedimentary wedge formed on the rifted southeastern edge of this ancient continent and prograded onto the craton during sedimentary and tectonic loading of the craton margin the southeastern greenstone belts formed as thick oceanic crust oceanic plateau in this back arc basin and were later obducted on to the rift and passive margin sequence as the sea of umtali closed ca 2 7 ga this was followed by intrusion of granitic plutons of the chilimanzi suite ca 2 6 ga in a tectonic regime of intracontinental strike slip faulting representing a response to the zimbabwe kaapvaal continent continent collision crustal and lithospheric thickening during intrusion of these late granites may have played a role in stabilizing the zimbabwe craton and forming the lithospheric root

3 d simulation of near field strong ground motion basin edge effect derived from rupture directivity the heavily damaged area in a narrow belt in kobe was a very characteristic feature of the 1995 hyogoken nambu kobe earthquake the processes responsible for the amplification of the strong ground motion there have not yet been clarified we model the basin edge effect derived from rupture directivity by simulating dynamic rupture for an event with m 7 on a strike slip fault buried near a basin ground velocity seismograms up to 1 hz are computed using a 3 d finite difference method patterns of the peak ground velocities show that they are amplified in a very narrow belt along the basin edge a few kilometers in width when the fault runs parallel to the edge this amplification effect also strongly depends on the fault location however the large amplitudes are not concentrated in a narrow belt when the whole fault ruptures at once this suggests that an intense ground shaking in a narrow belt along a basin edge occurs only when rupture propagates parallel to the basin edge with a finite velocity this appears to have been the case in the kobe area in which the basin edge effect derived from rupture directivity may have been the cause of the heavily damaged belt

a hydrothermal ni as pge geochemical halo around the miitel komatiite hosted nickel sulfide deposit yilgarn craton western australia the remobilization of metals during postdeposition hydrothermal alteration of magmatic sulfide ores has the potential to result in large haloes the recognition of which could potentially enlarge the detectable footprint of this ore type the miitel komatiite hosted nickel sulfide deposit in western australia was used as a case study to investigate the nature and 3 d geometry of the geochemical halo created by the remobilization of base metals gold and platinum group elements pge into the rocks surrounding the mineralization at miitel anomalous metal enrichment is found in the country rocks surrounding the massive sulfides up to 250 m away from the ore this enrichment detected using portable x ray fluorescence pxrf and backed up by laboratory analyses occurs in the mount edwards footwall basalt within decimeters of the contact with the overlying widgiemooltha komatiites it is associated with the presence of nickel arsenides gersdorffite and minor nickeline are concentrated within small quartz and carbonate veinlets and are interpreted to form during the circulation of arsenic rich hydrothermal fluids results of lead fire assay analyses and in situ laser ablation inductively coupled plasma mass spectrometer la icp ms analyses indicate high pge concentrations pd and pt and minor gold associated with the observed nickel and arsenic enrichment results from a larger regional scale study combined with this pge enrichment suggest that the massive nickel sulfides from the miitel ore are the source of the remobilized nickel in the country rocks the presence of pd and pt enriched trace arsenide phases in country rocks and shear zones may be a generally applicable proximity indicator for nickel sulfides in hydrothermally altered terranes

pt and pd mobility in hydrothermal fluids evidence from komatiites and from thermodynamic modelling abstract komatiite host rocks and ores provide a natural laboratory for the study of pt and pd during hydration and carbonation of ultramafic rocks the degree of correlation between pt and pd and of those elements with lithophile elements such as ti in komatiitic rocks and ores is strongly dependent on the magmatic sulfide content of the rock pt and pd are typically very strongly correlated within s poor komatiites and in disseminated ores containing 5 volume or less magmatic sulfides in contrast they are almost completely decoupled in sulfide rich ore samples thermodynamic modelling based on the existing body of published experimental data on pt and pd solubilities suggests that pd and pt can be readily transported as bisulfide complexes in acidic neutral solutions under reduced and moderate oxidation condition at 300 c and chloride complexes are only important under extremely acidic oxidised conditions pd is much more soluble than pt in the case of fluids in sulfide rich environments in equilibrium with the characteristic assemblages found within altered and metamorphosed komatiites solubilities of both elements are much lower in sulfide poor environments especially where ph values are constrained to neutral values by the near ubiquitous presence of carbonate minerals in the alteration assemblage this accounts for the evident near immobility of pt and pd in even the most altered silicate host rocks as compared with their wide dispersion in sulfide rich ores

a re appraisal of the epoch nickel sulphide deposit filabusi greenstone belt zimbabwe a hydrothermal nickel mineral system abstract the neoarchaean epoch nickel sulphide nis deposit discovered in 1970 and subsequently mined from 1972 to 1998 has been previously interpreted to be a magmatic ore system however on the basis of field petrographic and analytical data obtained during the initial exploration and evaluation stages integrated with limited new analytical data sem and ore microscopy we propose that the epoch nis may be of hydrothermal origin key features that have prompted a re appraisal of the genesis of the epoch mineralisation include a ore lenses are irregularly distributed along foliation fabrics of the host talc carbonate rocks and as such no relationship to basal ultramafic mafic units b the thickness of the ore lenses are highly variable from tens of centimetres to several metres c ore mineralogy is characterised by dominant millerite pyrite chalcopyrite assemblages and d high pd pt ratios although our hydrothermal model is somewhat speculative we suggest that on present evidence there is little doubt that the epoch nis lacks the features of typical magmatic sulphide deposits more geochemical and isotopic data are needed to test our model

evolution of the archaean granite greenstone terranes of the eastern goldfields western australia shrimp upb zircon constraints abstract thirty five high precision shrimp u pb zircon dates with an average 95 confidence error of 6 ma have been obtained from the granite greenstone terranes in the southern part of the eastern goldfields of western australia these dates establish the timing of major deformation metamorphism and granite intrusion events and test inferred stratigraphic relationships within the greenstones these new data reveal that felsic volcanic rocks were erupted throughout the southern part of the eastern goldfields between c 2713 and c 2672 ma analytically indistinguishable dates of c 2705 ma were obtained for felsic volcanic rocks interbedded with komatiitic lavas at three sites dates on felsic volcanic rocks within tectonostratigraphic packages that include komatiites are also consistent with an inferred eruption age for these komatiites of c 2705 ma komatiitic lavas in the southern part of the eastern goldfields were probably emplaced during a single voluminous eruption that resulted in the flooding of a vast area of the late archaean landscape filling the topographic lows around older felsic volcanic vents the stratigraphic complexity within tectonostratigraphic domains is attributed to the existence of an undulating topography during the eruption of the mafic and ultramafic lavas and to the rapid and localized deposition of felsic volcanic rocks close to a number of isolated volcanic centres prior to and after the komatiite lava eruption event between c 2685 and c 2675 ma early pre d2 intrusive equivalents of the felsic volcanic rocks were emplaced as thick granite sheets into the base of the greenstone sequences regional east west compression between c 2675 and c 2657 ma reactivated early extensional structures and resulted in the stacking of the greenstones this was accompanied by further intrusion of granite within developing antiforms burial of greenstone slices by stacking and the effect of hydrothermal fluids emanating from both the granites and buried greenstones resulted in regional metamorphism to greenschist facies prior to c 2640 ma monzogranite and granodiorite emplaced near the western southern and eastern margins of the greenstones were deformed and metamorphosed to upper amphibolite facies and converted into gneisses during d2 and d3 compression events movement along large scale transcurrent d3 shear zones had ceased by c 2635 ma remnants of a c 2958 to c 2930 ma greenstone sequence preserved near norseman and ravensthorpe in the southeastern yilgarn craton and xenocryst zircons ranging in age from c 2910 2730 ma in the c 2713 2672 ma felsic volcanic rocks indicate that the exposed greenstone sequences were deposited on continental basement the eastern goldfields granite greenstone terranes were formed by the closure of a series of narrow back arc rift basins that developed along a continental margin and above an active subduction zone located to the east of the rift basins

three dimensional spontaneous rupture propagation and implications for the earthquake source mechanism summary we consider spontaneous cracks spreading out areally over a fault plane in an infinite medium as a realistic earthquake source model the boundary integral equation technique described in das is used to determine the displacements and stresses everywhere on the crack plane for faults spreading out symmetrically in all four quadrants over infinite planes of constant yield strength we find that the terminal rupture velocity viiicr in the direction of purely anti plane rupture is given by viiicr in the purely in plane direction of rupture propagation we find the terminal rupture velocity viicr to be given by viicr or viicr 0 5 depending on the yield strength we also find that crack propagation in the purely antiplane direction does not influence crack propagation in the purely in plane direction and vice versa for infinitely long shear cracks of finite width the slip at a point is found to grow more slowly after the arrival of a shear wave diffracted from the nearer crack edge the slip virtually ceases after the shear arrival from the further crack edge this implies that for such faults the slip is controlled by the fault width for a rectangular fault in an infinite medium our final dynamic solution is in closer agreement with the static elliptical crack solution of eshelby than with the static rectangular dislocation model solution of chinnery we find the relationship between the average slip and average dynamic stress drop e on such a rectangular fault of half width w to be e c w where c 0 7 the slip at an interior point is again controlled by the fault width for faults that are much longer than wide so that the slip may stop in the region where the fault initiated before the completion of the rupture process this implies that two rectangular faults of varying lengths but of the same width have the same slip for the same average stress drop a fact clearly contradicted by observations that fault slip increases with earthquake size for great earthquakes occurring along the same fault this contradiction is resolved if stress drops are also bigger for bigger earthquakes

dynamic source modeling of the 1978 and 2005 miyagi oki earthquakes interpretation of fracture energy 1 we constructed spontaneous dynamic rupture models of the 1978 and 2005 miyagi oki japan earthquakes which occurred on the same plate boundary repeatedly with different magnitudes to reproduce the slip and the rupture velocity predicted by kinematic source models comparison of the dynamic source parameters of these events enables us to consider the behavior of rupture dynamics during different source processes on the same fault in particular we compared the stress drop and the fracture energy which can be evaluated stably between two events the maximum and average values of the stress drop over the whole fault plane and on an asperity that ruptured during both events are almost identical in the two models on the other hand the 1978 event whose magnitude was larger than the 2005 event has a fracture energy value larger than that of the 2005 event not only over the whole fault but also on the asperity that ruptured repeatedly these results are consistent with previous studies of the scaling of fracture energy with the seismic moment we also compared the fracture energy values with those of other inland events estimated for previous studies our values for the miyagi oki earthquakes are smaller differences in tectonic setting between shallow inland events and deep subduction zone events might cause this discrepancy

a multielement geochronologic study of the great dyke zimbabwe significance of the robust and reset ages abstract new sm nd u pb and pb pb age determinations indicate that the great dyke of zimbabwe an elongate intrusion of mafic and ultramafic rocks some 550 km long and between 3 and 10 km wide is over 100 ma older than previously believed based on rb sr ages the intrusion was emplaced as a series of subchambers with similar stratigraphy comprising a lower ultramafic sequence with cyclic layering of dunite or harzburgite grading upwards into bronzitite the top sections of which include pt enriched sulfide zones and an upper mafic sequence of pyroxenites capped by olivine gabbro and gabbronorite the sm nd method has yielded a combined mineral whole rock isochron of 2586 16 ma and e nd t of 1 1 for samples from the darwendale sebakwe and wedza subchambers as well as the satellite east dyke this isochron age is in excellent agreement with the u pb age for three concordant rutile fractions extracted from a feldspathic pyroxenite of the selukwe subchamber with an error weighted mean at 2587 8 ma two zircon fractions from the same feldspathic pyroxenite sample as the rutile are discordant and although not well constrained suggest pb loss from the zircons at ca 830 ma this may be related to the onset of the widespread and diachronous pan african tectonothermal event in southern africa whole rock samples and clinopyroxene and plagioclase separates from a darwendale subchamber drill core yielded a 207 pb 204 pb vs 206 pb 204 pb isochron age of 2596 14 ma which is in agreement with the sm nd isochron and the rutile u pb crystallization age this new age information shows that emplacement of the great dyke and its satellite dikes closely followed the amalgamation of the kaapvaal and zimbabwe cratons and was contemporaneous with emplacement of the youngest of the trondhjemite tonalite granodiorite granitoid suite in the zimbabwe craton assuming that amalgamation of the kaapvaal and zimbabwe cratons was largely by nnw directed convergence it follows that the source of the great dyke was asthenospheric mantle hydrated and enriched in incompatible elements by subduction processes isochrons of 206 pb 204 pb vs 238 u 204 pb and 207 pb 204 pb vs 235 u 204 pb yield ages with large errors but well constrained initial pb ratios 206 pb 204 pb 14 15 0 30 and 207 pb 204 pb 15 04 0 06 assuming a two stage model for common lead evolution this result yields a value of 9 5 along with the calculated initial sr and nd isotopic compositions these data are consistent with derivation of the great dyke magmas by large volume melting of a mantle that has been hydrated and enriched by subduction while a small amount of crustal contamination of magma derived from depleted mantle could produce the composition of the great dyke the uniformity of initial ratios between subchambers supports the notion of enrichment in incompatible elements being an intrinsic characteristic of the mantle source

nickel sulfide deposits in australia characteristics resources and potential abstract australia s nickel sulfide industry has had a fluctuating history since the discovery in 1966 of massive sulfides at kambalda in the eastern goldfields of western australia periods of buoyant nickel prices and high demand speculative exploration and frenetic investment the nickel boom years have been interspersed by protracted periods of relatively depressed metal prices exploration inactivity and low discovery rates despite this unpredictable evolution the industry has had a significant impact on the world nickel scene with australia having a global resource of nickel metal from sulfide ores of 12 9 mt five world class deposits 1 mt contained ni and a production status of number three after russia and canada more than 90 of the nation s known global resources of nickel metal from sulfide sources were discovered during the relative short period of 1966 to 1973 australia s nickel sulfide deposits are associated with ultramafic and or mafic igneous rocks in three major geotectonic settings 1 archean komatiites emplaced in rift zones of granite greenstone belts 2 precambrian tholeiitic mafic ultramafic intrusions emplaced in rift zones of archean cratons and proterozoic orogens and 3 hydrothermal remobilized deposits of various ages and settings the komatiitic association is economically by far the most important accounting for more than 95 of the nation s identified nickel sulfide resources the ages of australian komatiitic and tholeiitic hosted deposits generally correlate with three major global scale nickel metallogenic events at 3000 ma 2700 ma and 1900 ma these events are interpreted to correspond to periods of juvenile crustal growth and the development of large volumes of primitive komatiitic and tholeiitic magmas caused by large scale mantle overturn and mantle plume activities there is considerable potential for the further discovery of komatiite hosted deposits in archean granite greenstone terranes including both large and smaller high grade 5 to 9 ni deposits that may be enriched in pges 2 to 5 g t especially where the host ultramafic sequences are poorly exposed analysis of the major komatiite provinces of the world reveals that fertile komatiitic sequences are generally of late archean 2700 ma or paleoproterozoic 1900 ma age have dominantly al undepleted al 2 o 3 tio 2 15 to 25 chemical affinities and often occur with sulfur bearing country rocks in dynamic high magma flux environments such as compound sheet flows with internal pathways facies kambalda type or dunitic compound sheet flow facies mt keith type most precambrian provinces in australia particularly the proterozoic orogenic belts contain an abundance of sulfur saturated tholeiitic mafic ultramafic intrusions that have not been fully investigated for their potential to host basal ni cu sulfides voisey s bay type mineralization the major exploration challenges for finding these deposits are to determine the pre deformational geometries and younging directions of the intrusions and to locate structural depressions in the basal contacts and feeder conduits under cover stratabound pge ni cu cr deposits hosted by large archean proterozoic layered mafic ultramafic intrusions munni munni panton of tholeiitic affinity have comparable global nickel resources to many komatiite deposits but low grades

shear zone controlled post magmatic ore formation in the huangshandong ni cu sulfide deposit nw china abstract the huangshandong ni cu sulfide deposit located in the kangguer jing erquan ductile shear zone belongs to the jueluotage island arc belt in eastern tianshan of the central asian orogenic belt the huangshandong mafic ultramafic pluton which has a fusiform shape is believed to have intruded strongly foliated late carboniferous volcanic rocks during the development of the regional ductile shear zone some orebodies in the deposit show intense ductile deformation among them the no 17 orebody is entirely controlled and hosted by the ductile shear zone host rocks and ores from this orebody show intense deformation and alteration and could be called ore mylonites furthermore numerous anastomosing veinlets can be observed in copper rich carbonate sulfide veins in the ductile shear zone geological field investigation and ore petrography studies indicate that sulfide ores at huangshandong are composed of three types undeformed magmatic ores deformed magmatic ores and hydrothermal overprinted ores magmatic ores with net fabrics remain unaltered and undeformed and sulfide minerals in them show only brittle fractures silicate and sulfide minerals in deformed magmatic ores are characterized by intense deformation fabrics examples for such fabrics are kink bands wavy extinction of uralite domino structure of phlogopite and elongated grains of pyrrhotite and pentlandite hydrothermal overprinted ores are characterized by strong deformation flattened grains and recrystallization of pyrrhotite moreover annealing of pyrrhotite is frequently observed pyrrhotite pentlandite and chalcopyrite in hydrothermal overprinted ores have high ni and co concentrations in pyrrhotite a higher ni concentration in pentlandite and high ag and cd concentrations respectively we attribute ni and co in fluid overprinted ores mainly to the alteration of olivine and the dissolution of primary chalcopyrite is the likely source of copper in chalcopyrite rich hydrothermal overprinted ores during the intruding and cooling of the huangshandong pluton intense regional ductile shearing caused the ductile deformation of sulfide ores in the deposit and subsequently ore mylonites were formed gangue minerals in the deformed orebodies were crushed to fine grained minerals and altered to serpentine chlorite and uralite during these processes fluid and ore elements were released and reacted with deformed rocks and ores forming anastomosing veinlets on hydrothermal overprinted ores

estimation of the breakdown slip from strong motion seismograms insights from numerical experiments fukuyama and mikumo 2007 presented a method to extract informa tion about the friction controlling earthquake rupture directly from near field seismo grams recorded out to several kilometers from a fault this method computes a parameter d 00 approximating the breakdown slip dc from the rake parallel dis placement at the time of the peak particle velocity the validation of the method was based on simple 2d steady state green s functions an approach lacking sufficient physics of the rupture process to support their conclusions here we use 3d simula tions of subshear strike slip spontaneous rupture propagation to demonstrate that d 00 is almost always controlled by rupture and wave propagation effects rather than the fault friction process only if rupture reaches the earth s surface and within a short distance from the fault rc is it possible to extract information about dc from strong motion data due to the fast decay with distance from the fault of the seismic energy related to the stress breakdown process beyond rc d 00 is controlled by the dynamic stress drop from the earthquake rupture without information about dc rc is comparable to the length of the fault cohesive zone where the breakdown process takes place during rupture and approximately equal to 80 of the wavelength associated with the break down frequency defined as the reciprocal of the time span required by the stress to drop to the dynamic level our findings suggest that the dc estimate by fukuyama and mikumo 2007 for the 2000 mw 6 6 tottori earthquake is unrelated to the breakdown slip moreover the very narrow widths for rc obtained from our simulations are in agreement with previous studies challenging the resolution of bandlimited strong motion data to estimate dc from kinematic rupture models guatteri and spudich 2000 spudich and guatteri 2004

field relationships and geochemical constraints on the emplacement of the jinchuan intrusion and its ni cu pge sulfide deposit gansu china field mapping and petrological geochemical investigation of the jinchuan intrusion in north central china clarifies how the intrusion was emplaced and provides a new model that explains how its large and rich ni cuplatinoid deposits may have formed the intrusion was emplaced into high grade gneisses and marbles along a disconformity at the base of an overlying cover sequence indicating that it was emplaced as a sill not a nearvertical dike as previously proposed after emplacement the intrusion was rotated to its present orientation and deformed and metamorphosed under greenschist facies conditions relative enrichment of incompatible trace elements coupled with negative u th and nb ta anomalies in all samples from the intrusion provide evidence that the parental magma assimilated granitoid rocks in the lower crust the presence of abundant marble xenoliths now decarbonatized to diopside rich skarns and chemical indices such as high cao sio2 indicate that the magma assimilated carbonate on reaching its present site this contamination may be linked to the formation of the ni cu platinoid ores we propose that the assimilation of carbonate rich fluids increased the oxygen fugacity of the magma and led to the segregation of metal rich sulfides

the nickeliferous late archean reliance komatiitic event in the zimbabwe craton magmatic architecture physical volcanology and ore genesis the widespread komatiitic reliance lithostratigraphic unit of the late archean bulawayan supergroup consists of many submarine flow fields directly or indirectly overlying the sedimentary manjeri unit which varies from a generally carbonaceous sulfide rich deep water basinal facies in the north and west parts of the zimbabwe craton to a generally sulfide poor shallow water platformal facies in the south and east the flow fields are associated with high level subvolcanic feeder sills intruded into pre manjeri basement made up of the conformable sulfide bearing felsic koodoovale unit and older barren granitoid greenstone crust in the north and west and south and east respectively the sills and flow fields together constitute largely discrete sill flow complexes very approximately 100 km in diameter of which five are well preserved in the north and west and three in the south and east parts of the craton the reliance sills up to 40 km in diameter and 2 000 m thick some composite and locally overlapping to slightly discordant mostly comprise a thick lower dunite peridotite and a relatively thin to intermittent upper pyroxenite gabbro other larger probably related sills intruded at greater structural depths link adjacent complexes and form midcrustal magma chambers the reliance sills are interpreted as an interconnected system of flow through feeders at different crustal levels the reliance flow fields are 33 to 85 km in diameter and up to 2 500 m thick dominantly of komatiitic basalt composition of very low height breadth aspect ratio locally linked to the sills via structures interpreted as fissure vents and partly extrusive cryptodomes and thin out toward their lateral margins most display a simple coherent volcanological growth sequence from sheet and or channelized sheet flows and lava channel complexes in lower and proximal positions to marginal lava lobes in upper and distal positions comparable in general organization and probable emplacement processes to those of hawaiian and flood basalt flow fields flow field development was influenced by episodic eruption preexisting topographies and penecontemporaneous sedimentation and felsic and mafic volcanism and some flow fields in the south and east were partly emergent sills in the north and west are weakly mineralized but those in the south and east are barren the principal reliance nickel sulfide deposits are confined to flow fields in the north and west occur within lava channel complexes trojan and shangani mines damba silwane and hunters road prospects and a fissure vent epoch mine and include type 1 basal sequences of massive to matrix to disseminated sulfides lowermost flows trojan shangani and type 2 central accumulations of weakly disseminated sulfides hunters road upper flows trojan shangani epoch and shangani occupy central flow field positions whereas hunters road and damba silwane are proximal trojan may be either distal or proximal field evidence does not directly support the origin of the reliance deposits solely by magmatic erosion of sulfidic substrate during flow emplacement as proposed for komatiite hosted nickel deposits elsewhere instead the deposits origin and asymmetric distribution across the craton may be mainly due to assimilation of sulfidic felsic wall rocks during magma passage through the subvolcanic sills in the north and west possibly combined with sulfur degassing due to very shallow water lava emplacement in the south and east

modeling dynamic rupture in a 3d earthquake fault model we propose a fourth order staggered grid finite difference method to study dynamic faulting in three dimensions the method uses an implementation of the boundary conditions on the fault that allows the use of general friction models including slip weakening and rate dependence because the staggered grid method defines stresses and particle velocities at different grid points we preserve symmetry by implementing a two grid row thick fault zone slip is computed between points located at the borders of the fault zone while the two components of shear traction on the fault are forced to be symmetric inside the fault zone we study the properties of the numerical method comparing our simulations with well known properties of seismic ruptures in 3d among the properties that are well modeled by our method are full elastic wave interactions frictional instability rupture initiation from a finite initial patch spontaneous rupture growth at subsonic and supersonic speeds as well as healing by either stopping phases or rate dependent friction we use this method for simulating spontaneous rupture propagation along an arbitrarily loaded planar fault starting from a localized asperity on circular and rectangular faults the shape of the rupture front is close to elliptical and is systematically elongated in the in plane direction of traction drop this elongation is due to the presence of a strong shear stress peak that moves ahead of the rupture in the in plane direction at high initial stresses the rupture front becomes unstable and jumps to super shear speeds in the direction of in plane shear another interesting effect is the development of relatively narrow rupture fronts due to the presence of rate weakening friction the solutions for the thick fault boundary conditions scale with the slip weakening distance do and are stable and reproducible for do greater than about 4 in terms of 2t t ax finally a comparison of scalar and vector boundary conditions for the friction shows that slip is dominant along the direction of the prestress with the largest deviations in slip rate direction occurring near the rupture front and the edges of the fault

simulation of long period 3 d elastic responses for whole earth models summary improvements in gdsn seismic data quality and advances in numerical algorithms for solution of the wave equation now make it useful and feasible to simulate long period elastic responses for 3 d whole earth models the algorithm used is eighth order finite differencing on a staggered grid this allows stable computation at boundaries with large contrasts in poisson s ratio such as the core mantle boundary as the solution automatically generates all wave types it may lead to prediction of previously unobserved or misinterpreted phases and allow more detailed analysis of all phases simulations of large e g nuclear explosions and double couple earthquake sources reveal the effects on relative amplitudes of their respective radiation patterns in both wavefield snapshots and seismogram profiles a large number of phases are affected even by a single smooth bump on the core mantle boundary a 3 d whole earth model implicitly contains the travaltimes for all source receiver positions and for any source configuration the corresponding amplitudes of all phases thus 3 d numerical modelling of whole earth responses is a potentially valuable diagnostic and interpretive tool and a basis for globalization in contrast to regionalization of traveltime tables

magmatic nickel sulfide mineralization in zimbabwe review of deposits and development of exploration criteria for prospectivity analysis significant magmatic nickel sulfide deposits in zimbabwe occur in archean greenstone belts and archean to proterozoic mafic ultramafic intrusions such as madziwa and the great dyke here we review exploration criteria for magmatic nickel sulfide deposits taking into account nickel sulfide mineral systems in general zimbabwe s cratonic evolution and the geological setting of the deposits we propose that the essential ingredients for nickel sulfide mineral systems in zimbabwe are i proximity to greenstone belts craton margins and orogenic belts ii large mafic ultramafic intrusions and their feeders iii crustal contamination of basic ultrabasic magma with sulfidic crustal lithologies and iv the vicinity to major fault zones that allowed fertile magma to ascend into the upper crust we suggest that there is potential for new nickel sulfide discoveries both within and outside the greenstone belts including the karoo flood basalt province the mashonaland dolerites and the limpopo magondi and zambezi orogenic belts

rupture process of an earthquake with kilometric size fault inferred from the modeling of near source records abstract we use the exceptional set of strong ground motion records obtained during one of the largest aftershocks m l 5 0 of the 1979 imperial valley earthquake to study the time history of rupture on a kilometric size fault we apply single crack composite cracks and dislocation earthquake models to simulate the source and calculate the ground velocity and acceleration at the recording sites the best fit to the data is obtained for a crack like source where rupture is initiated at one edge and expands circularly over the fault the stress drop inferred is 720 bars and the fractured area has a diameter of about 1 km the fault is vertical and strikes in the n320 e direction this simple model explains surprisingly well the amplitudes and waveforms of the ground velocities and accelerations recorded in the near source region during the event

dynamic modeling of the 1992 landers earthquake we have used observed band pass filtered accelerograms and a previously determined slip distribution to invert for the dynamic rupture propagation of the 1992 landers earthquake in our simulations dynamic rupture grows under the simultaneous control of initial stress and rupture resistance by friction which we modeled using a simple slip weakening law we used a simplified landers fault model where the fault segments were combined into a single vertical planar fault by trial and error we modified an initial stress field inferred from the kinematic slip distribution proposed by wald and heaton 1994 until dynamic rupture generated a rupture history and final slip distribution that approximately matched those determined by the kinematic inversion we found that rupture propagation was extremely sensitive to small changes in the distribution of prestress and that a delicate balance with energy release rate controls the average rupture speed for the inversion we generated synthetic 0 5 hz ground displacements using an efficient green s function propagator method axitra this method enables us to propagate the radiation generated by the dynamic rupture to distances greater than those feasible using the finite difference method the dynamic model built by trial and error inversion provides a very satisfactory fit between synthetics and strong motion data we validated this model using records from stations used in the slip inversion as well as some which were not included we also inverted for a complementary model that fits the data just as well but in which the initial stress was perfectly uniform while rupture resistance was heterogeneous this demonstrates that inversion of ground motion is nonunique

the bulawayan supergroup a late archaean passive margin related large igneous province in the zimbabwe craton the four principal lithostratigraphic units koodoovale manjeri reliance and zeederbergs of the widespread late archaean bulawayan supergroup in the zimbabwe craton record the early to middle stage development of a major c 250 000 km 2 volcanosedimentary basin the craton wide c 2 70 ga manjeri unit varies systematically from a tens of metres thick dominantly chemogenic basinal sediment in gradational conformable contact with the underlying several kilometres thick diachronous 2 83 2 71 ga felsic koodoovale unit in the north and west to an up to hundreds of metres thick dominantly siliciclastic retrogradational platformal sediment unconformably overlying protocratonic basement in the south and east the intermittent c 2 70 ga reliance unit consists of the flow fields of several discrete komatiitic sill flow complexes scattered throughout the craton and fed by an interlinked system of mid to upper crustal flow through feeder sills the c 2 70 ga zeederbergs unit comprises a submarine flood basalt sequence of several kilometres thickness that erupted from ene trending parallel mafic dykes preserved in the south and east the basalts form a craton wide blanket to the reliance flow fields and intervening manjeri sediments certain structural stratigraphic features rule out primary construction of the bulawayan supergroup by early lateral accretion as proposed by several previous workers instead evolution of the bulawayan basin can best be explained by vertical accretion involving 1 abundant episodic and bimodal plume related volcanism and 2 extension and rapid subsidence of continental lithosphere many aspects of the bulawayan supergroup provide important insights into archaean tectonics mantle plume activity magma generation and lithospheric plumbing

the age of the kambalda greenstones resolved by ion microprobe implications for archaean dating methods abstract a chert horizon within the archaean basalt pile at kambalda western australia contains zircons from air fall tuffs erupted as the sediment was laid down ion microprobe analyses of these magmatic zircons constrain the long sought age of the kambalda lavas to 2692 4ma 2 and show that regional metamorphism and deformation of the greenstone pile occurred within 30 ma of deposition rare zircon xenocrysts are evidence of pre greenstone crust as old as 3450 ma exposed remnants of which are yet to be identified subsets of both the magmatic and xenocryst zircons show evidence of hitherto unsuspected low temperature u and or pb mobility in the proterozoic long after stabilisation of the craton the ion microprobe data unravel complexities of isotopic inheritance and alteration illustrating the hazards of using bulk or whole rock samples in archaean isotopic investigations identification of interflow cherts as a precisely dateable rock type opens the way to direct ion probe dating of mafic ultramafic piles and close numerical age control of the complex but ill exposed stratigraphic and structural relations in this richly mineralised greenstone belt

horizontal accretion and stabilization of the archean zimbabwe craton structural metamorphic data and mineral ages from the northern parts of the zimbabwe craton indicate that archean crustal formation and stabilization evolved in two stages in the shamva bindura greenstone belt early layer parallel shear zones began to form at 2670 ma and accommodated imbricate stacking of oceanic and volcanic arc material between large nappe structures with felsic gneiss cores the resultant crustal pile of anomalously hot felsic and mafic crustal slices reached isostatic and mechanical equilibrium at a thickness of 35 km further shortening of this pile caused strain partitioning into vertical strike slip zones the subsequent establishment of an equilibrium geotherm resulted in large scale crustal melting and diapirism between 2620 and 2600 ma the rise of the melt and diapirs caused a second transient metamorphic imprint and much of the strain pattern regarded as typical for the zimbabwe craton this diapiric stage led to cooling and stabilization of the craton

a stochastic model for fracture growth on a heterogeneous seismic fault summary we present a model for the propagation of seismic fracture on a heterogeneous fault on which the strength and applied stress vary locally along the fault plane in a random fashion the propagation of fracture is controlled by three macroscopic parameters the average of the difference between the applied stress and fault strength a measure of fault heterogeneity and the average coupling between elementary faults evolution of fracture can then be represented by an epidemics model whose domains of slip are obtained from percolation theory the geometry of these domains contains the principal features of both barrier and asperity models

geology geochemistry and genesis of the longhua low temperature hydrothermal ni co arsenide deposit in sedimentary rocks guangxi south china abstract the longhua ni co deposit hosted in cambrian carbonaceous sandstone and siltstone in south china consists of disseminated niccolite mineralization and high grade ni co arsenides quartz carbonate vein mineralization the disseminated mineralization is characterized by niccolite hosted in the center of quartz fragments and in argillaceous concretions in cambrian banded pisolitic carbonaceous silty mudstone and by parallel niccolite grains in cambrian pisolitic pelite these characteristics indicate that the disseminated niccolite originated through redeposition the ni co vein mineralization forms high grade ores and is composed mainly of ni co arsenides the homogenization temperatures of fluid inclusions in the mineralized vein quartz range from 118 to 219 c with corresponding salinities of 1 2 to 8 8 wt nacl equiv the ore forming fluids yield dv smow and 18oh2o values of 54 to 66 and 3 0 to 7 7 respectively indicating that the source was circulating meteoric water the longhua ni co vein mineralization gives a niccolite re os isochron age of 462 6 8 5 ma mswd 1 03 the geological and geochemical characteristics of the longhua ni co deposit indicate that it formed as follows 1 precambrian quartz niccolite mineralization was eroded and transported to the longhua sediment trap forming low grade disseminated niccolite mineralization or ni co rich source beds during the cambrian 2 high grade ni co arsenide vein mineralization was formed by further enrichment of ni co from the ni rich source beds by circulated meteoric water during the ordovician caledonian orogeny the formation of the longhua low temperature high grade ni co vein ore indicates that ni can be transported in low temperature fluids that are rich in arsenic and hco3 or co32 the longhua ni co deposit might represent a new type of sedimentary hosted low temperature hydrothermal ni deposit

high frequency seismic wave radiation from antiplane cohesive zone model and fmax as source effect while a high frequency cutoff fmax is widely observed in strongmotion seismic data there is no consensus on whether it is due to source processes or to attenuation and scattering of high frequency radiation in the crust or near the surface to investigate the ability of source processes to control fmax we use a standard antiplane crack propagation formalism to numerically model the effect of rupture nucleation and arrest on high frequency radiation based on a simple physical hypothesis a slip weakening model we model rupture arrest due to three types of inhomogeneity 1 a strong portion of rupture medium barrier 2 a drop in the pre existing stress distribution of rupture medium and 3 a finite length of unruptured medium asperity lying between previous ruptures for cases 2 and 3 high frequencies fall off more steeply than 2 and fmax cannot be properly defined for case 1 we find that fmax vflfllj 2 where vf is the final crack velocity lf is the final rupture length and l i is the initial crack size we extrapolate this result to the rupture for a three dimensional model and try to explain observed fm x if we assume that an earthquake is a single crack li is large for a large earthquake however if we assume that an earthquake is made up of set of cracks and asperities fmax will be determined by the interaction of small cracks and barriers if the distribution of these cracks and asperities is independent of source size then fmax will be nearly constant for all earthquakes

effects of hydrous alteration on the distribution of base metals and platinum group elements within the kevitsa magmatic nickel sulphide deposit abstract platinum and nickel are commonly assumed to be immobile in most conditions especially during low temperature hydrothermal alteration however only a small number of studies have rigorously tested this assumption the ni cu pge sulphide ore body hosted by the kevitsa intrusion northern finland provides a natural laboratory to study the behaviour of base metals and platinum group elements pge during low temperature alteration this ca 2060 ma mafic ultramafic intrusion located in the central lapland greenstone belt hosts disseminated ni cu pge sulphide mineralisation in the middle part of the main ultramafic body the mineralisation which contains a range of ni cu and pge grades is affected by three main alterations serpentinisation amphibolitisation and epidotisation and is cross cut by various types of veins the effect of the circulation of hydrothermal fluids on the distribution of base metals and pge was studied at two different scales interrogation of an extensive deposit wide assay database provided information on the deposit scale kilometre scale effect of these different alteration styles and a detailed study involving laboratory x ray fluorescence xrf portable xrf and micro xrf mapping of drill core samples containing cm scale cross cutting veins provided information on the small scale centimetre to decimetre scale remobilisation of base metals and pges results show that the hydration and carbonation of the kevitsa mineralised mafic ultramafic intrusion did not significantly affect the distribution of ni and pge at scales larger than a few mm and that cu and au are the only metals that are affected by small to large scale remobilisation from centimetre to kilometre scale

organic geochemistry and mineralogy i characterisation of organic matter associated with metal deposits abstract as geochemical appraisals of mineral regions of commercial prospectivity evolve the organic matter associated with metal rich ores has attracted greater attention petroleum basin and modern seafloor hydrothermal vent studies have suggested that organic matter can have a significant influence on the behaviour of mineralising fluids there have been many isolated reports of certain organic compositional or morphological e g pyrobitumen features showing an apparent relationships with hydrothermal fluids or minerals raising expectations that organic based parameters might be useful to mineral exploration however the understanding of organic inorganic relationships in earth systems is far from complete for example the detailed mechanics of the interaction of organics with hydrothermal fluids over geological time remain largely undefined organic geochemistry studies have traditionally involved the measurement and interpretation of the hydrocarbon composition of sedimentary rocks here we review the types of aliphatic hydrocarbons aromatic hydrocarbons and metalloporphyrins often detected from organic geochemical investigations in mineral rich regions such molecular data can be particularly diagnostic of biochemical sources and the palaeo environments at the time mineral associated organic matter was deposited sub surface trends of hydrocarbon alteration may also reflect major biogeochemical processes such as thermal maturity and biodegradation organic geochemistry data can also occasionally provide information about the nature e g origin composition temperatures and migration pathways of hydrothermal fluids and can make a contribution to holistic ore genesis models the well preserved organic matter associated with the economic here s your chance pb zn ag mine paleoproterozoic barney creek formation mcarthur basin australia and the transition metal rich early permian kupferschiefer formation germany poland have attracted significant attention a more detailed summary of the organic character of these deposits is provided to highlight the contribution organic geochemistry can make to understanding mineralisation processes most organic geochemical studies of highly mineralised regions however have not adequately addressed the significance of organic matter to mineralisation a slightly different analytical focus than traditionally used for exploration appraisal of petroleum hydrocarbons may be required to properly evaluate the significance of organic species to the mobilisation transport and deposition of ore metals the characterisation and subsequent thermodynamic modeling of organic substances and complexes within metalliferous hydrothermal systems will contribute to a better understanding of the nature and role of organic inorganic fluids or other affiliated organics in ore systems

hydrothermal nickel deposits secular variation and diversity abstract hydrothermal nickel systems are one of the most puzzling mineral systems that involve trace element mobility in the earth s crust nickel is an abundant element in the crust however the processes that are involved in its mobility and accumulation at ore scale are challenging a new empirical stochastic model is required to gain a deeper insight into these systems this special issue presents several studies on hydrothermal nickel accumulations world wide aiming to stimulate the debate on this intriguing deposit type

lizardite versus antigorite serpentinite magnetite hydrogen and life the serpentinization of peridotite operates according to one or the other or a combination of two end member mechanisms in low temperature environments 50 300 c where lizardite is the predominant serpentine mineral olivine is consumed by reaction with h2o but its composition mg remains unchanged mg rich lizardite magnetite and dihydrogen gas brucite are products of the reaction at higher temperatures 400 600 c rates of mgfe diffusion in olivine are orders of magnitude faster with the result that the growth of mg rich antigorite can be accommodated by a compositional adjustment of olivine eliminating the need to precipitate magnetite and evolve hydrogen this latter end member mechanism probably best reflects the situation in the forearc mantle wedge

improvement in the fault boundary conditions for a staggered grid finite difference method the staggered grid finite difference method is a powerful tool in seismology and is commonly used to study earthquake source dynamics in the staggered grid finite difference method stress and particle velocity components are calculated at different grid points and a faulting problem is a mixed boundary problem therefore different implementations of fault boundary conditions have been proposed 1982 chose the shear stress grid as the fault surface however this method has several problems 1 fault slip leakage outside the fault and 2 the stress bump beyond the crack tip caused by s waves is not well resolved 1998 solved the latter problem via thick fault implementation but the former problem remains and causes a new issue displacement discontinuity across the slip is not well modeled because of the artificial thickness of the fault in the present study we improve the implementation of the fault boundary conditions in the staggered grid finite difference method by using a fictious surface to satisfy the fault boundary conditions in our implementation velocity or displacement grids are set on the fault plane stress grids are shifted half grid spacing from the fault and stress on the fictitious surface in the rupture zone is given such that the interpolated stress on the fault is equal to the frictional stress within the area which does not rupture stress on the fictitious surface is given a condition of no discontinuity of the velocity or displacement fault normal displacement or velocity is given such that the normal stress on the fault is continuous across the fault artificial viscous damping is introduced on the fault to avoid vibration caused by onset of the slip our implementation has five advantages over previous versions 1 no leakage of the slip prior to rupture and 2 a zero thickness fault 3 stress on the fault is reliably calculated 4 our implementation is suitable for the study of fault constitutive laws as slip is defined as the difference between displacement on the plane of z 0 and that of z 0 and 5 cessation of slip is achieved correctly

woolly mammoth mammuthus primigenius blum and its environment in northern europe during the last glaciation abstract woolly mammoths were large herbivorous cold adapted mammals of the late pleistocene the diet and habitat requirements of the species set certain constraints on the palaeoenvironments it could occupy the relationship between the mammoth s shifting range and changing environments can be explored using independent data on ice sheet configuration temperature and vegetation provided the locality and age of the fossil remains can be validated here we present a comprehensive record of occurrence of the woolly mammoth in the circum baltic region of northern europe during the last glaciation based on a compilation of radiocarbon dated remains the record shows that the mammoth was widespread in northern and north eastern europe during marine isotope stage 3 mis 3 at 50 000 30 000 calibrated years ago 50 30 ka the presence of the species up to 65 n latitude supports the restriction of the scandinavian ice sheet sis during mis 3 the widest distribution range round 30 ka was followed by a decline that led to the disappearance of mammoths from the area during the maximum extent of the sis from 22 to 18 ka the woolly mammoth re colonized the baltic region and southern scandinavia after the onset of the late glacial deglaciation at 17 ka the late glacial record suggests a markedly fluctuating population changing its range in tune with the rapid environmental changes the last appearance of mammoth in our study region was in estonia during the younger dryas greenland stadial 1 gs1 at about 12 ka the two major periods of occurrence during mis 3 and the late glacial stadial suggest that mammoth had a wide tolerance of open to semi open tundra and steppe tundra habitats with intermediately cold climate whereas the 22 18 ka disappearance suggests a major southward and or eastward retreat in response to extremely cold glacial conditions near the sis margin the final regional extinction correlates with the re forestation during the rapid warming at the younger dryas holocene boundary

slip patterns and earthquake populations along different classes of faults in elastic solids numerical simulations of slip instabilities on a vertical strike slip fault in an elastic half space are performed for various models belonging to two different categories the first category consists of inherently discrete cellular fault models such are used to represent fault systems made of segments modeled by numerical cells that can fail independently of one another their quasi independence is assumed to provide an approximate representation of strong fault heterogeneity due to geometric or material property disorder that can arrest ruptures at segment boundaries the second category consists of models having a well defined continuum limit these involve a fault governed by rate and state dependent friction and are used to evaluate what types of property heterogeneity could lead to the quasi independent behavior of neighboring fault segments assumed in the first category the cases examined include models of a cellular fault subjected to various complex spatial distributions of static to kinetic strength drops and models incorporating rate and state dependent friction subjected to various spatial distributions of effective stress normal stress minus pore pressure the results indicate that gradual effective stress variations do not provide a sufficient mechanism for the generation of observed seismic response strong and abrupt fault heterogeneity as envisioned in the inherently discrete category is required for the generation of complex slip patterns and a wide spectrum of event sizes strong fault heterogeneity also facilitates the generation of rough rupture fronts capable of radiating high frequency seismic waves the large earthquakes in both categories of models occur on a quasi periodic basis the degree of periodicity increases with event size and decreases with model complexity however in all discrete segmented cases the models generate nonrepeating sequences of earthquakes and the nature of the large quasi periodic events is highly variable the results indicate that expectations for regular sequences of earthquakes and or simple repetitive precursory slip patterns are unrealistic the frequency size fs statistics of the small failure episodes simulated by the cellular fault models are approximately self similar with b 1 2 and ba 1 where b and ba are b values based on magnitude and rupture area respectively for failure episodes larger than a critical size however the simulated statistics are strongly enhanced with respect to self similar distributions defined by the small events this is due to the fact that the stress concentrated at the edge of a rupture expanding in an elastic solid grows with the rupture size when the fault properties e g geometric irregularities are characterized by a narrow range of size scales the scaling of stress concentrations with the size of the failure zone creates a critical rupture area terminating the self similar earthquake statistics in such systems events reaching the critical size become on the average unstoppable and they continue to grow to a size limited by a characteristic model dimension when however the system is characterized by a broad spectrum of size scales the above phenomena are suppressed and the range of apparent self similar fs statistics is broad and characterized by average b and ba values of about 1 the simulations indicate that power law extrapolations of low magnitude seismicity will often underestimate the rate of occurrence of moderate and large earthquakes the models establish connections between features of fs statistics of earthquakes range of self similar regimes local maxima and structural properties of faults dominant size scales of heterogeneities dimensions of coherent brittle zones the results suggest that observed fs statistics can be used to obtain information on crustal thickness and fault zone structure

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

3d eight order elastic finite difference modeling of refraction and strong motion data from the coyote lake region california abstract 3d elastic modeling of vertical component explosive source refraction data is performed to constrain the shallow part of a velocity model followed by 3d modeling of three component strong motion data from a spatially and temporally extended earthquake source in that model modeling is by eighth order finite differencing on a staggered grid which automatically generates all body and surface waves including multiples and conversions application is to the 1981 usgs refraction survey and the 6 august 1979 coyote lake event in central california 3d modeling was able to reproduce the main observed features in both the refraction and earthquake data for the refraction data the synthetics explain local variations in first arrival travel times reverberations in the near surface sediments and surface waves for the strong motion data the initial arrival times and amplitudes are determined by the position and orientation of the initial rupture the shapes of the trailing edges of the pulses are modified by the actual source distribution changes of source position of 1 to 2 km are easily resolvable through changes in the position of nodal planes discontinuities in material properties e g layering produce a coda of multiples and converted waves lateral variations in velocities especially in the upper few kilometers change the energy distribution among the trace components by 3d refraction and so shift the apparent source directivity and position as well as contributing to the coda remaining differences between the observed and synthetic data are attributed to local seismometer coupling effects very local site structures and phenomena such as anelasticity and anisotropy which are not explicitly modeled

spontaneous growth and autonomous contraction of a two dimensional earthquake fault abstract we numerically study a model of an earthquake as a spontaneous rupture on a fault we assume that a two dimensional antiplane shear crack initiates at a point in an infinite homogeneous and isotropic elastic medium and subsequently propagates with variable velocity under the influence of nonuniform stress drop on the crack and nonuniform cohesive resistance at the crack edges to begin with we analyze the dynamical rupture process immediately after nucleation we determine the subsequent extension of each edge by solving an ordinary differential equation of the first order as well as the dynamical stress in the regions between each edge and the nearest wave front application of this procedure to both edges of the crack in an alternating manner yields the complete history of extension of the crack we use the critical stress intensity fracture criterion to determine the conditions on propagation and arrest of the crack to verify the accuracy and the stability of our numerical technique we compare it with some special cases in which analytical solutions are available the comparison indicates that the numerical results are in good agreement with the analytical ones obtained previously by knopoff and chatterjee 1982 and chatterjee and knopoff 1983 we apply this technique to the computation of the dynamical extension of the crack for several representative cases among them are 1 1 uniform stress drop but nonuniform cohesion a barrier model 2 2 nonuniform stress drop but uniform cohesion an asperity model and 3 3 nonuniform stress drop and nonuniform cohesion a combination of both the barrier and the asperity models the numerical results indicate that the heterogeneities of both the stress drop and the cohesion are the main factors which control the growth cessation and healing of the crack and that the complexities in the seismic radiation are caused by the complex healing process as well as by the complex rupture propagation in contrast to the asperity model the barrier model is characterized by abrupt changes in the slope of the source time function and the theoretical seismogram the collision of the stress pulses from each pair of barriers is the potential source of the crack fission in the general model the fission process occurs in a complex sequence and the effects of this process have to be taken into account in the interpretation of observations