ore genesis of pb zn deposits in the nappe zone of northern tunisia constraints from pb s c o isotopic systems abstract the jalta and jebel ghozlane ore deposits are located in the extreme north of tunisia within the nappe zone the mineralization of jalta hosted in triassic dolostones and the overlying mio pliocene conglomerates consists of abundant galena barite and cerussite with accessory sphalerite pyrite and jordanite at jebel ghozlane large pb zn concentrations occur in the triassic dolostones and eocene limestones the mineral association consists of galena sphalerite barite and celestite and their oxidation products cerussite smithsonite and anglesite lead isotope ratios in galena from both districts are relatively homogeneous 206 pb 204 pb 18 702 18 823 207 pb 204 pb 15 665 15 677 208 pb 204 pb 38 725 38 875 the 34 s values for sulfates from both areas 12 2 to 16 2 at jalta and 14 3 to 19 4 at jebel ghozlane are compatible with a derivation of sulfur from marine sulfates possibly sourced from the triassic evaporites the 34 s values of the sulfides have a range between 10 and 12 5 at jalta and between 9 1 and 22 1 at jebel ghozlane the large range of values suggests reduction of the sulfate by bacterial and or thermochemical reduction of sulfate to sulfur the high 34 s values of sulfides require closed system reduction processes the isotopically light carbon in late calcites 6 3 to 2 5 and authigenic dolomite 17 6 suggests an organic source of at least some of the carbon in these samples whereas the similarity of the 18 o values between calcite 24 8 and the authigenic dolomite 24 7 of jalta and their respective host rocks reflects oxygen isotope buffering of the mineralizing fluids by the host rock carbonates the secondary calcite isotope compositions of jalta are compatible with a hydrothermal fluid circulation at approximately 100 to 200 c but temperatures as low as 50 c may be indicated by the late calcite of jebel ghozlane 18 o of 35 9 given the geological events related to the alpine orogeny in the nappe zone nappe emplacement bimodal volcanism and reactivation of major faults such as ghardimaou cap serrat and the neogene age of the host rocks in several localities a late miocene age is proposed for the pb zn ore deposits considered in this study remobilization of deep seated primary deposits in the paleozoic sequence is the most probable source for metals in both localities considered in this study and probably in the nappe zone as a whole

ground motions in the fukushima hamadori japan normal faulting earthquake a crustal normal faulting earthquake graphic 1 m w 6 7 occurred in eastern tohoku japan on 11 april 2011 k net and kik net stations recorded 82 records from within 100 km of fault rupture these data and data from associated foreshocks and aftershocks will make a critical contribution to future improvements of ground motion prediction for normal faulting earthquakes peak ground accelerations pga and peak ground velocities pgv are compared with four ground motion prediction equations gmpes that include the style of faulting as a predictor parameter for distances under 100 km and using a network average value of v s 30 the average ratio of pga to the selected gmpes the event term is high by factors of 2 3 3 7 event terms for pgv are high by factors of 1 4 1 8 adjusting pga and pgv with customized site terms kawase and matsuo 2004a b the standard deviations of pga and pgv residuals are reduced from 0 59 to 0 43 and from 0 53 to 0 35 respectively the event terms decreased to relatively small factors of 1 1 1 8 for pga and increased slightly to 1 5 2 0 for pgv thus site terms are very important but positive event terms remain the remaining positive event terms are not explained by high stress drop which was typical of crustal events of all mechanisms globally or in japan two subparallel faults ruptured but source inversions which we reviewed revealed that they ruptured sequentially so simultaneous contributions from the two faults did not cause high motions although these observations may tend to suggest that ground motions in large normal faulting events are larger than predicted by the tested models we are not aware of any observations from this event that contradict the precarious rock evidence of brune 2000 that ground shaking is low on the footwall near the rupture 1 embed inline graphic 1 gif

tin metallogenesis associated with granitoids in the southwestern sanjiang tethyan domain nature deposit types and tectonic setting article i nfo tin mineralization in the southwestern sanjiang tethys sn metallogenic domain extends from the southeast asia tin belt in the south the eastern part of the domain does not contain significant sn deposits whereas the central part of the domain composed of the changning menglian sn belt and related to ca 239 178 ma s type granitoids generated by the collision between tengchong baoshan and simao blocks carries wide spread the hydrothermal vein type mineralization the western part of the domain composed of the tengchong baoshan sn metallogenic belt is related to both 136 113 ma and 89 52 ma s type granitoids the former granitoids resulted from the collision between western burma and tengchong baoshan blocks and generated skarn type tin deposits the younger groups correspond to the india asia continental collision and are associated with the xiaolonghe and lailishan greisen type sn deposits

sea99 a revised ground motion prediction relation for use in extensional tectonic regimes we present sea99 a revised predictive relation for geometric mean horizontal peak ground acceleration and 5 damped pseudovelocity response spec trum appropriate for estimating earthquake ground motions in extensional tectonic regimes which we demonstrate to have lower ground motions than other tectonic regimes sea99 replaces sea96 a relation originally derived by spudich et al 1996 1997 the data set used to develop sea99 is larger than that for sea96 and minor errors in the sea96 data set have been corrected in addition a one step regression method described by joyner and boore 1993 1994 was used rather than the two step method of joyner and boore 1981 sea99 has motions that are as much as 20 higher than those of sea96 at short distances 5 30 km and sea99 s motions are about 20 lower than sea96 at longer periods 1 0 2 0 sec and larger distance 40 100 km sea99 dispersions are significantly less than those of sea96 sea99 rock motions are on the average 20 lower than motions predicted by boore et al 1994 except for short distances at periods around 1 0 sec where sea99 motions exceed those predicted by boore et al 1994 by as much as 10 com parison of ground motions from normal faulting and strike slip events in our data set indicates that normal faulting horizontal ground motions are not significantly different from extensional regime strike slip ground motions

consistency of precariously balanced rocks with probabilistic seismic hazard estimates in southern california abstract the overturning fragility of a freestanding block such as a precariously balanced rock pbr has been parameterized as a function of a vector of ground motion intensity measures methodologies are outlined to estimate the failure probabilities of such objects given their residence times for deterministic seismic hazard analyses dshas a pbr is exposed to the scenario earthquakes that occur during its exposure time providing an estimate of the probability that the pbr survives the ensemble of events for probabilistic seismic hazard analyses pshas the pbr overturning fragility is multiplied by the ground motion occurrence rate from a vector valued probabilistic seismic hazard analysis vpsha yielding the marginal overturning rate for each ground motion bin summing the marginal rates over all ground motion bins produces the total overturning rate for time independent poisson based psha estimates the probability of block failure can be easily calculated as a function of exposure time this latter method is used to test vpsha estimates similar to the 2002 u s geological survey usgs national seismic hazard maps via pbr residence times pbr overturning fragilities are estimated at sites in southern california near the san andreas fault between the san jacinto and elsinore faults and near the white wolf fault the resulting failure probabilities for several of the pbrs are very high suggesting that they are inconsistent with the 2002 usgs ground motions an investigation of the hazard calculated with zero aleatory variability in the ground motion prediction equations gmpes suggests that the median ground motions or the earthquake rupture rates are too high at certain pbr sites

magmatic evolution and related w mo mineralization in the larong deposit eastern tibet evidence from zircon u pb ages geochemistry and sr nd hf isotopes abstract the late cretaceous ca 92 ma larong porphyry w mo deposit 39 44 mt wo3 0 1541 is the first discovered giant w mo deposit in the leiwuqi zogang metallogenic belt sanjiang region multi stage granitoids including the dacite porphyry dp granodiorite porphyry gp biotite granite porphyry bgp monzogranite porphyry mp biotite granite bg and granite aplite ga have been identified in the larong deposit however the magmatic evolution and related w mo mineralization in this deposit are still poorly understood la icp ms zircon u pb dating of the dp gp bgp mp bg and ga yield weighted mean 206pb 238u ages of 214 0 0 7 ma 213 8 1 3 ma 104 4 0 9 ma 93 9 1 3 ma 91 7 0 5 ma and 36 2 0 6 ma respectively the late triassic dp and gp should be classified as peraluminous a cnk values of 0 95 1 57 s type granitoids they are characterized by pronounced negative end t values 10 01 to 13 92 and ehf t values 8 5 to 16 6 with old tdm2 nd and tdm2 hf ages 1 81 2 12 ga and 1 79 2 30 ga respectively indicating that the dp and gp were mainly derived from partial melting of ancient crustal materials after break off of the paleo tethyan oceanic slab the late cretaceous bgp mp and bg and late eocene ga have low a cnk ratios 0 92 1 06 but high sio2 70 57 77 34 and differentiation index di 87 55 98 06 similar to the characteristics of highly fractionated i type granite detailed elemental e g depletion in nb ta ti sr ba eu and p and isotopic data 87sr 86sr i 0 71192 0 72932 end t 11 81 to 9 11 ehf t 10 4 to 1 4 demonstrate that they were the result of magma mixing between ancient crust derived melts and mantle derived melts the bgp was produced during the final stage of bangong nujiang tethyan ocean bnto subduction the mp and bg were formed under a post collisional tectonic setting related to slab break off of the bnto and the ga was likely associated with delamination of the thickened eclogitic lower crust together with lithospheric mantle the w mo mineralization in the larong deposit is genetically associated with the mp with four favorable factors that can account for the formation of this deposit 1 high w background 2 multi stage magmatism 3 highly evolved relatively oxidized and f rich magmatic system and 4 sufficient calcium obtained by intense fluid rock interaction

cenozoic evolution of neotethys and implications for the causes of plate motions africa north america eurasia plate circuit rotations combined with red sea rotations and new estimates of crustal shortening in iran define the cenozoic history of the neotethyan ocean between arabia and eurasia the new constraints indicate that arabia eurasia convergence has been fairly constant at 2 to 3 cm yr since 56 ma with slowing of africa eurasia motion to 1 cm yr near 25 ma coeval with the opening of the red sea ocean closure occurred no later than 10 ma and could have occurred prior to this time only if a large amount of continental lithosphere was subducted suggesting that slowing of africa significantly predated the arabia eurasia collision these kinematics imply that africa s disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern eurasia slowed its motion the slow steady rate of northward subduction since 56 ma contrasts with strongly variable rates of magma production in the urumieh dokhtar arc implying magma production rate in continental arcs is not linked to subduction rate

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

timing tectonic implications and genesis of gold mineralization in the xincheng gold deposit china c h o isotopes pyrite rb sr and zircon fission track thermochronometry abstract the xincheng gold deposit located at the northwestern edge of the jiaodong peninsula in the north china craton has a reserve of 96 t au and is among the largest in the jiaodong gold province here disseminated and stockwork style ores are hosted in mesozoic granitoids the mineralization and alteration are largely controlled by the regional jiaojia fault the c h o isotope values in the ores indicate that the ore forming fluids were derived mainly from magmatic sources new rb sr geochronological data on hydrothermal pyrite from the xincheng deposit presented in this study indicate that the main gold mineralization occurred between 126 ma and 122 ma the zircon fission track of convergent ages and decomposed divergent ages from the ores and altered rocks show four groups of ages 135 ma 124 ma 96 75 ma and 66 52 ma the 135 ma is related to the exhumation and cooling of the linglong granitoids the 124 ma is considered to represent the principal stage of gold mineralization and is consistent with the pyrite rb sr ages the younger ages of 96 75 ma and 66 52 ma resulted from subsequent annealing caused by later tectonic activity the timing of gold mineralization in the xincheng gold deposit broadly coincides with the switch in stress field in eastern china from extension to transpression resulting from the change in subduction direction of the pacific plate from ne sw to nw se and marked by extension related voluminous magmatism associated with the cretaceous giant igneous event in eastern china

mayum an orogenic gold deposit in tibet china abstract located in western tibet china the mayum orogenic gold deposit was discovered in 2002 with estimated resources of 80 tonnes gold the gold mineralization is hosted by neoproterozoic cambrian schists and is controlled by nearly parallel e w trending bedding concordant fracture zones the au orebodies are composed of auriferous quartz veins and altered rocks with the au grades ranging from 2 23 g t to 69 56 g t and containing the 34s values of 18 separates of sulfides from auriferous quartz veins show a large variation from 0 2 to 16 8 the lead isotope compositions of sulfides from the gold ore are characterized by highly radiogenic values and very wide ranges of ratios 18 324 to 20 819 for 206pb 204pb 15 679 to 15 856 for 207pb 204pb and 38 401 to 41 204 for 208pb 204pb these data indicate that lead and sulfur in the quartz veins have multiple sources and were derived from the different wall rocks as the fluid flowed through them fluid inclusions indicate that the ore fluid was co2 rich with salinities mainly between 1 and 6 wt nacl equiv and homogenization temperatures predominantly ranging from 260 to 280 c the 18o values for quartz from auriferous veins range from 13 7 to 16 3 and the calculated 18oh2o values in equilibrium with quartz vary from 5 54 to 9 48 it suggests that the ore fluid may be derived from deep metamorphic deep crustal gold transporting fluids although a contribution from magmatic source cannot be ruled out 40ar 39ar age dating on the sericite from the alteration associated with the auriferous quartz veins in the mayum gold deposit gives a plateau like age of 59 34 0 62 ma later than the onset of the indo asian collision it is believed the gold mineralization was related to the indo asian collision and was formed during the early stage of orogenesis

relationship between crust mantle reaction and gold mineralization in ailaoshan metallogenic belt a case study of daping gold deposit in yuanyang the ailaoshan metallogenic belt is located in the middle south part of yunnan province where there are several large superlarge gold polymetallic deposits such as jinchang in mojiang laowangzhai in zhenyuan and daping in yuanyang regional geophysical characteristics show that the nw striking ailaoshan metallogenic belt has a complicated crust mantle structure the moho discontinu ity isobath features reveal that the crust in the northeast is thicker than that in the southwest and represents a mantle slope on the whole regional studies of the seismic wave velocity of the lower crust and the upper mantle indicate that there exists a large area low velocity anomalous belt in southwest yunnan province which perhaps has relationship with the thermal dynamical effect caused by partial melt of the lower crust materials resulting from the uplift of the upper mantle materials in the crust mantle boundary such a phenomenon also implies that there once occurred a strong crust mantle reaction in the ailaoshan metallogenic belt the high anomalous features of the continental geothermal gradient and the terrestrial heat flow demonstrate that the metallogenic belt has characteristics of high temperature and low pressure mantle slope which might be attributed to the heat flow uplift caused by intense crust mantle reaction at depth detailed isotope studies of the daping superlarge gold polymetallic deposit shows that the main source of the ore forming fluid was deep crust mantle mixed fluid rather than gas saturation water magmatic water or their mixed water in the upper crust the coherence between regional geophysical and isotope characteristics shows that the ore forming progress was accompanied by the strong deep magmatic activity in the ailaoshan metallogenic belt and that the crust mantle reaction made an important contribution to the mineralization in ailaoshan area

late cenozoic uplift along the northern dead sea transform in lebanon and syria an edited version of this paper was published in earth and planetary science letters by elsevier science elsevier science retains the copyright to this paper copyright 2006 elsevier science see also http dx doi org 10 1016 j epsl 2005 10 029 http atlas geo cornell edu deadsea publications gomez2006 epsl htm

displaced rocks strong motion and the mechanics of shallow faulting associated with the 1999 hector mine california earthquake the paucity of strong motion stations near the 1999 hector mine earthquake makes it impossible to make instrumental studies of key questions about near fault strong motion patterns associated with this event however observations of displaced rocks allow a qualitative investigation of these problems by observing the slope of the desert surface and the frictional coefficient between these rocks and the desert surface we estimate the minimum horizontal acceleration needed to displace the rocks combining this information with observations of how many rocks were displaced in different areas near the fault we infer the level of shaking given current empirical shaking attenuation relationships the number of rocks that moved is slightly lower than expected this implies that slightly lower than expected shaking occurred during the hector mine earthquake perhaps more importantly stretches of the fault with 4 m of total displacement at the surface displaced few nearby rocks on 15 slopes suggesting that the horizontal accelerations were below 0 2 g within meters of the fault scarp this low level of shaking suggests that the shallow parts of this rupture did not produce strong accelerations finally we did not observe an increased incidence of displaced rocks along the fault zone itself this suggests that despite observations of fault zone trapped waves generated by aftershocks of the hector mine earthquake such waves were not an important factor in controlling peak ground acceleration during the mainshock

nubia arabia eurasia plate motions and the dynamics of mediterranean and middle east tectonics summary we use geodetic and plate tectonic observations to constrain the tectonic evolution of the nubia arabia eurasia plate system two phases of slowing of nubia eurasia convergence each of which resulted in an 50 per cent decrease in the rate of convergence coincided with the initiation of nubia arabia continental rifting along the red sea and somalia arabia rifting along the gulf of aden at 24 4 ma and the initiation of oceanic rifting along the full extent of the gulf of aden at 11 2 ma in addition both the northern and southern red sea nubia arabia plate boundary underwent changes in the configuration of extension at 11 2 ma including the transfer of extension from the suez rift to the gulf of aqaba dead sea fault system in the north and from the central red sea basin bab al mandab to the afar volcanic zone in the south while nubia eurasia convergence slowed the rate of arabia eurasia convergence remained constant within the resolution of our observations and is indistinguishable from the present day global positioning system rate the timing of the initial slowing of nubia eurasia convergence 24 4 ma corresponds to the initiation of extensional tectonics in the mediterranean basin and the second phase of slowing to changes in the character of mediterranean extension reported at 11 ma these observations are consistent with the hypothesis that changes in nubia eurasia convergence and associated nubia arabia divergence are the fundamental cause of both mediterranean and middle east post late oligocene tectonics we speculate about the implications of these kinematic relationships for the dynamics of nubia arabia eurasia plate interactions and favour the interpretation that slowing of nubia eurasia convergence and the resulting tectonic changes in the mediterranean basin and middle east resulted from a decrease in slab pull from the arabia subducted lithosphere across the nubia arabia evolving plate boundary

dynamic geometrical effects on strong ground motion in a normal fault model we report results of measurement of particle motion in a foam rubber model of normal faulting and compare the results with similar results for a strike slip geometry standard computer modeling of strong ground motion from normal fault earthquakes has used dislocation theory in which slip along the shallow part of the fault is prescribed by assuming particular time functions for fault slip unfortunately in the case of normal faults there are essentially no near fault data from large earthquakes to constrain the modeling in an extensional faulting regime the static normal and shear stresses along the fault must approach zero at the surface and thus the upper few kilometers of the fault have inherently less stored strain energy than the maximum possible for strike slip faults in addition there are dynamic effects from geometry and drop in fault normal stress which affect the fault motion physical models of faulting such as foam rubber modeling are guaranteed to obey static and dynamic mechanical laws and thus can be used to gain insight into the physical processes involved in this study we compare surface accelerations from normal fault and strike slip geometries the data show surface accelerations near the normal fault trace that are systematically lower by anmore average factor of about 0 1 compared to the accelerations at the side sensors which represent strike slip motion these results suggest that kinematic modeling of ground motion using classical dislocation techniques is inappropriate on the shallow part of the fault the results of this study are qualitatively similar to those obtained for numerical models such as a dynamic lattice model ital shi ital et al 1997 and a finite element model ital oglesby ital et al 1998 lending support to the probability that similar effects take place in the real earth copyright 1999 american geophysical union less

distribution of precarious rocks at the nevada test site comparison with ground motion predictions from nuclear tests 1 precarious rocks have been proposed as ground motion seismoscopes that have been in place thousands of years thereby providing important constraints on seismic hazard estimates numerous precarious rocks exist in the region of large nuclear explosions at nevada test site nts ground motion recordings from nuclear tests have provided regression equations for ground acceleration and ground velocity as a function of distance and explosion yield for comparison with constraints from precarious rocks very near large nts explosions there are no precarious rocks or semiprecarious rocks cliff faces are shattered and numerous recent rockfalls have fresh white surfaces covered by caliche proof that the rockfalls have been caused by the explosions as the distance from the explosions increases and the contours for predicted ground motion decrease fewer and fewer rockfalls are observed some semiprecarious rocks begin to appear and finally numerous precarious rocks are found with no evidence of recent rockfalls to quantify the calibration of precarious rocks and ground motion we 1 do field tests on the rocks to determine the quasi static and dynamic toppling accelerations 2 carry out computer tests using estimates of waveforms from nts explosions and 3 compare the results with ground motions predicted by empirical regressions for nuclear explosions we conclude that these comparisons provide an important validation of the precarious rock methodology and an approximate validation of preliminary estimates of toppling accelerations

h o s and pb isotope geochemistry of the awanda gold deposit in southern tianshan central asian orogenic belt implications for fluid regime and metallogeny abstract the awanda gold deposit is located within the southern tianshan accretionary complex a late paleozoic orogenic belt in the northern margin of tarim block in nw china the gold mineralization here occurs in primarily within shear zones and secondary s faults with quartz mica schist as the principal host rock the sulfide mineral assemblage is dominated by arsenopyrite and pyrrhotite with minor pyrite and gudmundite alteration includes silicification sericitization and rare carbonatization the measured 18 o values for the gold bearing quartz range from 15 9 to 20 3 and the estimated 18 o fluid values range from 7 8 to 12 2 the d values for fluid inclusions from quartz range from 116 3 to 87 6 the 34 s values of gold associated sulfide minerals range from 3 6 to 6 2 and the calculated 34 s h2s values of h 2 s for the hydrothermal fluids range from 4 8 to 5 8 with an average of 0 4 lead isotopic compositions of sulfides from gold bearing ores show 206 pb 204 pb ratios ranging from 17 4625 to 18 5356 207 pb 204 pb ratios from 15 4943 to 15 6468 and 208 pb 204 pb ratios from 37 8825 to 38 8238 respectively collectively the isotope h o s pb characteristics suggest that the ore forming fluids were of metamorphic origin derived through dehydration of carbonaceous black shale with the sulfur sourced from crustal components and the lead from a mixture of host rocks and ore forming metamorphic fluids this deposit can be classified as orogenic gold deposit and share many major characteristics with those of muruntau type gold deposits during late paleozoic the southern tianshan accretionary complex was intensely deformed into an orogenic belt by folding and thrusting generating regional thrust faults at different scales the ore controlling subsidiary faults also formed subsequently with the ore forming fluids derived from dehydration of deeply buried carbonaceous black shale and the ore fluids channeled into shear zones leading to gold precipitation

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

origin of the daping gold deposit in the ailaoshan metallogenic belt sw china insights from geology isotope geochemistry and geochronology abstract the daping gold deposit the most representative gold deposit in the ailaoshan metallogenic belt is located between the southeast sanjiang region three rivers jinshajiang lancangjiang and nujiang and the western yangtze block ore bodies in the daping gold deposit are mainly vein types hosted in the late proterozoic diorite and silurian limestone and controlled by nw trending faults la icp ms zircon u pb dating of granodiorite yielded a late eocene age of 36 76 0 38 ma constraining the upper limit of the main mineralization quartz has d values ranging from 80 to 50 4 average 63 1 and 18 o fluid values ranging from 7 29 to 8 29 in the early stage of mineralization in the late stage quartz has d values ranging from 87 to 67 average 77 4 and 18 o fluid values ranging from 0 24 to 3 86 carbonate minerals have 13 c pdb and 18 o smow values ranging from 2 3 to 7 6 average 5 1 and 9 to 14 9 mean 12 6 the sulfides have a narrow range of 34 s values 0 2 to 6 7 average 3 51 in the main stages of mineralization the 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb values of the sulfides range from 18 685 to 18 765 15 653 to 15 728 and 39 014 to 39 280 in general according to the h o c s pb isotopic compositions the ore forming fluids are dominated by metamorphic fluids and the ore forming fluids and materials might have been mainly derived from the mantle and lower crust with a small input of wall rock material they were affected by meteoric water in the late stage of mineralization given the regional geology ore geology ore forming fluids material features and ore forming age the daping gold deposit can be classified as a typical orogenic gold deposit

rocking and overturning of precariously balanced rocks by earthquakes abstract precariously balanced rocks in seismically active regions may be used to provide constraints on probabilistic estimates of strong ground motion a computer program is developed to study the rocking response of models of precariously balanced rocks when subjected to various levels of ground accelerations the computer code validated by comparing the numerical and experimental results for simple shaped rigid blocks is used to study the rocking response of rectangular blocks with various aspect ratios to the el centro strong ground motion recording of the 18 may 1940 imperial valley earthquake results show that during earthquakes the horizontal components of ground motion play a dominant role in overturning rocking objects the rederived and corrected housner9s equation 1963 could be used to estimate the minimum overturning acceleration of precarious rocks at different frequencies by measuring the quasi static toppling acceleration in the field

late archean collision and delamination in the southwest yilgarn craton the driving force for archean orogenic lode gold mineralization orogenic lode gold deposits formed within a range of crustal environments over most of the yilgarn craton from 2640 to 2630 ma but there is little evidence for a causative widespread tectonomagmatic event at this time in the most highly mineralized greenstone terranes in the exposed deeper crustal levels of the southwest yilgarn craton however field geochemical pb isotope and shrimp u pb zircon studies define a continent continent collisional event with a widespread thermal anomaly probably due to lithospheric delamination at this time it is proposed that this event was of sufficient scale to drive the giant upper crustal fluid circulation systems with lateral and vertical flow at the scale of hundreds and tens of kilometers respectively that produced the widespread gold mineralization

methodology for using precarious rocks in nevada to test seismic hazard models abstract fields of precariously balanced rocks indicate that strong earthquake motions have not occurred at that site since the precarious rocks developed these fields can be characterized with an estimate of the peak acceleration that would be sufficient to topple the rocks and an estimate of how long the rocks have been precarious this article uses this information to test the input to probabilistic seismic hazard analysis the fundamental assumption is that the probability of exceeding a ground motion capable of toppling a precarious rock during a time period equal to the age of the rock is equal to the confidence level at which the inputs to the probabilistic seismic hazard analysis can be rejected we performed a probabilistic seismic hazard analysis for 26 sites of observed precarious rocks in nevada using preliminary estimates of the toppling acceleration and the age of the features two standard models are rejected with over 95 confidence by most of the precarious rock observations we consider several possible explanations for the inconsistency a probabilistic seismic hazard analysis that eliminates the area sources and only includes faults is consistent with the precarious rock observations at most of the sites spatial clustering of small magnitude events could maintain this effect without contradicting the precarious rock observations however it may not be necessary to completely reject the area sources from our probabilistic seismic hazard analysis the physics of rock stability may allow increasing the minimum magnitude to 6 0 in the area sources because the short duration of high frequency accelerations in smaller events may not topple all precarious rocks the precarious rocks could generally have small site effects attenuation models may overestimate ground motions on the foot wall of normal faults and a different attenuation model in general might decrease the ground motion predictions the ergodic assumption that is made in estimating the uncertainty in attenuation models may be contributing to overestimate the hazard individually each of these effects might allow more of the precarious rock sites to be consistent with the area source zones

precariously balanced rocks and ground motion maps for southern california groups of precariously balanced rocks are effectively low resolution strong motion seismoscopes that have been operating on solid rock outcrops for thousands of years and once the methodology has been developed can provide important information about seismic risk in one zone near victorville only 30 km from the nearest point on the san andreas fault more than 50 precarious rocks have been documented widespread rock varnish suggests that many of these rocks have been in their current unstable positions for thousands of years we have established the mechanical basis for rough estimates of the horizontal accelerations necessary to topple these rocks using field observations and numerical and physical modeling to verify that zones of precarious rocks do not occur near historic earthquakes searches using binoculars were made along roads with occasional foot surveys near large earthquakes based on these reconnaissance searches we conclude that no precarious rock zones are found within 15 km of zones of high energy release of historic large earthquakes to document the occurrence of precarious rocks in southern california road surveys were carried out along major roads four zones of precarious rocks and seven other zones of somewhat less precarious rocks have been documented published probabilistic ground motion maps for southern california are compared with the occurrence of zones of precarious and semi precarious rocks the results are encouraging and suggest that eventually studies of precarious rocks will provide important constraints on the assumptions on which the maps are based results from studies of precarious rocks may eventually provide important information for siting and design of sensitive structures such as hospitals and power plants precarious rocks give a direct indication of past ground shaking in contrast to the indirect inference provided by fault trenching studies which may be subject to uncertainties in the actual time history of slip due to the fault e g fault creep slow earthquakes or unknown dynamic stress drop it is concluded that precarious rocks warrant further study and quantitative analysis

in situ evidence for dextral active motion at the arabia india plate boundary the arabia india plate boundary also called theowen fracture zone is perhaps the least known boundary among large tectonic plates1 6 although it was identified early on as an example of a transform fault converting the divergent motion along the carlsberg ridge to convergent motion in the himalayas7 its structure and rate of motion remains poorly constrained here we present the first direct evidence for active dextral strike slip motion along this fault based on seafloor multibeam mapping of the arabia india somalia triple junction in the northwest indian ocean there is evidence for 12km of apparent strike slip motion along the mapped segment of the owen fracture zone which is terminated to the south by a 50 km wide pull apart basin bounded by active faults by evaluating these new constraints within the context of geodetic models of global plate motions we determine a robust angular velocity for the arabian plate relative to the indian plate that predicts 2 4mmyr 1 dextral motion along the owen fracture zone this transformfault was probably initiated around 8 million years ago in response to a regional reorganization of plate velocities and directions8 11 which induced a change in configuration of the triple junction infrequent earthquakes of magnitude 7 and greater may occur along the arabia india plate boundary unless deformation is in the formof aseismic creep

the late cretaceous donlin creek gold deposit southwestern alaska controls on epizonal ore formation the donlin creek gold deposit southwestern alaska has an indicated and inferred resource of approximately 25 million ounces moz au at a cutoff grade of 1 5 g t the ca 70 ma deposit is hosted in the late cretaceous kuskokwim flysch basin which developed in the back part of the arc region of an active continental margin on previously accreted oceanic terranes and continental fragments a hypabyssal mainly rhyolitic to rhyodacitic and commonly porphyritic 8 3 km dike complex part of a regional ca 77 to 58 ma magmatic arc formed a structurally competent host for the mineralization this deposit is subdivided into about one dozen distinct prospects most of which consist of dense quartz carbonate veinlet networks that fill north northeast striking extensional fractures in the northeast trending igneous rocks the sulfide mineral assemblage is dominated by arsenopyrite pyrite and typically younger stibnite gold is refractory within the arsenopyrite sericitization carbonatization and sulfidation were the main alteration processes fluid inclusion studies of the quartz that hosts the resource indicate dominantly aqueous ore fluids with also about 3 to 7 mol percent co2 ch4 and a few tenths to a few mole percent nacl kcl the gold bearing fluids were mainly homogeneously trapped at approximately 275 to 300 c and at depths of 1 to 2 km some of the younger stibnite may have been deposited by late stage aqueous fluids at lower temperature measured 18o values for the gold bearing quartz range between 11 and 25 per mil the estimated 18o fluid values range from 7 to12 per mil suggesting a mainly crustally derived fluid a broad range of measured d values for hydrothermal micas between 150 and 80 per mil is suggestive of a contribution from devolatilization of organic matter and or minor amounts of mixing with meteoric fluids gold associated hydrothermal sulfide minerals are characterized by 34s values mainly between 16 and 10 per mil with the sulfur derived from diagenetic pyrite and organic matter within the flysch basin a smaller group of 34s measurements which shows values as depleted as 27 per mil suggests a different local sulfur reservoir in the basin for the later hydrothermal episode dominated by stibnite initial nd of 8 7 to 3 1 and 87sr 86sr measurements of 0 706 to 0 709 for the ore hosting dikes also indicate a crustal reservoir for some of the late cretaceous magmatism overlapping lead isotope data for these intrusive rocks and for sulfide minerals suggest a crustal contribution for the lead in both copper and gold bearing stockwork veinlets in hornfels occur at dome a prospect located at the northern end of the donlin creek deposit these stockworks are cut by the younger auriferous gold veins that define the main donlin creek gold mineralization highly saline gas rich heterogeneously trapped fluids deposited the stockworks at temperatures approximately 100 c hotter than those of the main gold forming event at donlin creek the genetic relationship of the dome prospect to the main donlin creek gold resource is equivocal the epizonal donlin creek deposit shows affinities to the gold systems interpreted by various workers as orogenic or intrusion related it shows important differences from typical epithermal and carlin like deposits the ore forming fluids were derived by either broad scale metamorphic devolatilization above rising mantle melts or exsolution from a magma that was dominated by a significant flysch melt component

palaeomagnetic constraints on the geodynamic evolution of the gibraltar arc subduction zone roll back was recently put forward as a convincing model to explain the geometry and evolution of the gibraltar arc for other subduction related arc systems of the mediterranean such as the calabrian arc and the hellenic arc palaeomagnetic rotation data from neogene extensional basins provided important constraints on geodynamic evolution models here we present the results of a palaeomagnetic study of 13 continuous sections that are located in e w transects across the neogene sedimentary basins of morocco and spain they provide evidence that no significant rotation about vertical axes has occurred in the gibraltar arc since the late tortonian comparison with other mediterranean arc systems shows strong similarities as regards geodynamic evolution the timing of rotation in the gibraltar arc is markedly older than in the calabrian and hellenic arcs and suggests that it is related to the first neogene extensional phase of the western mediterra

cenozoic plate reconstruction of the south china sea region abstract reconstructions of the south china sea region at 60 ma 40 ma 30 ma 20 ma 10 ma and 5 ma are presented we have attempted to place the south china sea basin in a regional tectonic framework the tectonic evolution of the major blocks surrounding the south china sea were analyzed as well as the relative motions of the indian and australian plates we have tried to correct the tectonic models available in this region a 3 d graphics terminal was used to derive rotation poles for the different tectonic blocks and our model was then tested to determine its self consistency when the model conflicted with previous interpretations the input data were evaluated for alternative explanations at least two and possibly three stages of extension can be recognized in this region the earliest one active in the late cretaceous to eocene involved nw se extension the second one active from the late eocene to early miocene involved north south extension the third stage of extension which probably trended nw se can be dated as post oligocene the first extensional event produced the ne sw trending proto south china sea and a series of sedimentary basins along the south china margin following the southeastward extrusion of indochina the proto south china sea was mostly consumed at the palawan trough renewed north south extension in the south china continental margin started the present day south china sea spreading in the oligocene the southeastward extrusion of indochina blocked by sundaland resulted in the nw se trending opening of the south china sea basin in the early miocene collision of the north palawan microcontinental block with the west philippines block stopped the opening of the south china sea at the end of early miocene spreading activity switched to the sulu sea basin in the middle miocene but collision between the sulu ridge and the west philippines at mindanao halted the opening of the sulu sea at the end of the middle miocene in the late miocene greater india continued its northward path and seems to have ripped open the andaman sea in the pliocene subduction along the northern manila trench placed the north luzon arc on a collision path with the east asia continental margin at taiwan our reconstructions along with detailed geological and geophysical information may be used as a predictive tool for basin evolution models and block interactions in this region the development of the south china sea basin the gulf of thailand the malay basin and the central thailand basins are the result of collision induced extensional forces the sulu celebes and sumatra basins were formed as a consequence of prolonged subduction the opening of the pearl river mouth west natuna south china sea sulu and possibly celebes basins were terminated by various plate collisions during the course of plate reorganizations major boundary faults have changed their slip senses during different stages of evolution

the identification of the guqiong orogenic ag au polymetallic deposit in southern tibet evidences from mineralogy geochronology and fluid evolution abstract guqiong is an ag au polymetallic deposit located in the eastern part of the au sb ore belt along the indus yarlung zangbo suture in the north himalaya in this study we have identified three major alteration mineralization stages for guqiong based on the mineral assemblages and their paragenetic relationships namely the i pre ore alteration forming mainly ore barren thick quartz veins and vuggy quartz ii main ore stage that comprises quartz veins with sulfide mineralization including ag au bearing minerals in this stage sericite occurs closely associated with sulfides iii the post ore stage characterized by the formation of calcite and quartz with trace sulfides fluid inclusion assemblage fia microthermometry was conducted on barren and mineralized quartz and sphalerite three types of inclusions are identified based on their petrographic features and laser raman spectra i e the pure carbonic pc aqueous carbonic three phase c1 and two phase c2 and aqueous w types the fias in stage i ore barren quartz are mainly pc type and c1 type which are characterized by being high temperatures 260 to 300 c low salinities 0 8 to 3 5 wt nacl eqv and with abundant co2 sphalerite of main ore stage contains mainly w type with limited c2 type of low temperatures 158 to 180 c and medium salinities 5 6 to 6 7 wt nacl eqv meanwhile the fia in mineralized quartz is featured by the general coexistence of c2 type and w type with local pc type and c1 type inclusions the fluid system may have been a mixture of the two aforementioned fluids in barren quartz and sphalerite as indicated by two significantly different fias c type and w type with contrasting homogenization temperatures 260 to 290 c and 180 to 220 c respectively but comparable salinities 0 2 to 8 2 and 0 2 to 8 1 wt nacl eqv respectively the fluid d 18o and 13c values of the main ore stage quartz are 172 to 114 8 8 to 13 1 and 13 4 to 7 7 respectively the negative 13c values are interpreted as devolatilization products of biogenic carbon with a metamorphic origin while the d and 18o values likely reflect meteoric water involvement into metamorphic fluids we propose that fluid mixing had occurred and caused sulfide precipitation at the main ore stage additionally the estimated fluid trapping depth of 5 to 6 km obtained using 3 d phase diagram is consistent with the formation depth of many orogenic type deposits worldwide 40ar 39ar dating on sericite yielded a well defined plateau age of 42 03 0 42 ma with normal and inverse isochron ages of 42 8 1 6 ma and 42 9 1 9 ma respectively these ages bracket the ag au mineralization at guqiong place it in the main stage india asia collision and further support its orogenic type origin

terrane boundary and spatio temporal distribution of ore deposits in the sanjiang tethyan orogen insights from zircon hf isotopic mapping abstract the sanjiang tethyan orogen sjto of the eastern tethyan tectonic belt in china records a transition from late paleozoic tethys realm to cenozoic continental collision associated with the formation of large metallic mineral deposits this contribution combines geological and hf isotopic data for igneous rocks from the sjto together with the salient features associated with mineralization to gain insights into the crustal architecture and metallogeny integrating the available information from tectonic events sedimentary successions paleobiogeography and paleolatitude based on paleomagnetic data the hf isotopic maps that we construct highlight the changning menglian suture as an important tectonic boundary between the gondwana and cathaysian continents the changning menglian suture separates two distinct hf isotopic domains with an old reworked crustal block to the southwestern part of the sjto and a juvenile crustal block with significant mantle derived components to the northeastern part of the orogen the hf isotopic mapping of the sjto also provides important indications on the location of different deposits the jinshajiang ailaoshan suture appears to have exerted a first order control on the localisation of cenozoic magmatism and intrusion related mineral systems the magmatic source for intrusions in the eastern high ehf low tdmc domain is dominated by mantle components and gave rise to porphyry and porphyry skarn cu mo mineralization in contrast the magma sources for intrusions in the low ehf high tdmc domains contain an old and reworked crustal component and gave rise to granite related sn w mineralization those for the intrusions characterized by variable ehf and tdmc are dominantly reworked older crustal components together with subordinate juvenile material which generated the granite related pb zn cu ag mineralization

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

tethys tectonic evolution and its bearing on the distribution of important mineral deposits in the sanjiang region sw china abstract the sanjiang region in se tibet plateau and nw yunnan is known to have formed by amalgamation of gondwana derived continental blocks and arc terranes as a result of oceanic subduction followed by continental collision from paleozoic to mesozoic in this paper we provide a synthesis of tectonic evolution magmatism and metallogeny in the region based on data from literatures early paleozoic ophiolites 473 439 ma in the changning menglian belt indicate the existence of a proto tethys ocean in this region two episodes of subduction related magmatism in the early paleozoic one occurred in the baoshan and tengchong blocks at 502 455 ma and the other occurred in the simao block at 421 401 ma are regarded as evidence for two different events of subduction of the proto tethys ocean at different locations the proto tethys was succeeded in early devonian by the paleo tethys which comprised the main ocean and three branches ailaoshan jinshajiang and garze litang the changning menglian main ocean existed from middle devonian to middle triassic the remnants of the oceanic crust are preserved in a few places in the longmu tso shuanghu suture as well as in the changning menglian ophiolite belt the eastward subduction of the main oceanic plate from early permian to early triassic formed a prominent arc terrane stretching 1500 km from yunnan to eastern tibet from the waning stage of subduction to post subduction numerous s type granite plutons with ages varying between 230 and 219 ma such as the lincang batholith in yunnan were emplaced at or close to the suture this event produced several hydrothermal w sn deposits in the region the tectonic evolution and associated magmatism of the jinshajiang and ailaoshan branch oceans are generally comparable to those of the main ocean however the branch oceans were subducted westward instead the garze litang branch ocean also underwent westward subduction from middle devonian to late triassic arc related high sr y porphyry intrusions and associated porphyry skarn cu mo au deposits are common in the jinshajiang ailaoshan region especially in the yidun arc which formed prior to jurassic the vms deposits in the sanjiang region formed in diverse tectonic settings including middle silurian back arc basins carboniferous oceanic islands paleozoic subduction zones and triassic post subduction rifting environments the mesozoic and early cenozoic evolution of the baoshan and tengchong blocks was largely influenced by eastward oceanic subduction of the meso and neo tethys from late permian to middle cretaceous and from late cretaceous to 50 ma respectively abundant early cretaceous granitoids and associated skarn type pb zn and sn fe deposits in the baoshan and tengchong blocks were produced in the background of the shan boundary oceanic slab subduction to the west and the break off of the nujiang bitu oceanic slab to the north the subduction of the neo tethys oceanic plate beneath the tengchong block from late cretaceous to paleogene formed abundant s type granitoids and many skarn type and greisen type sn w deposits granitoids formed at 105 to 81 ma and contemporaneous hydrothermal w mo ag and au deposits which temporally coincided with the subduction of the neo tethys are common in the yidun arc terrane

the jurassic danba hypozonal orogenic gold deposit western china indirect derivation from fertile mantle lithosphere metasomatized during neoproterozoic subduction the genesis of hypozonal orogenic gold deposits is highly controversial with both crustal metamorphic fluids and variously derived sub crustal fluids suggested as fluid and metal sources the lower jurassic danba gold deposit a rare phanerozoic hypozonal orogenic gold deposit on the western margin of the yangtze craton china provides an opportunity to resolve these controversies danba is characterized by biotite amphibole alteration and pyrrhotite dominant ore assemblages formed at 500 600 c the 34s values of ore related pyrrhotite range from 3 1 to 9 9 in sharp contrast to those of syn sedimentary pyrrhotite from devonian host rocks which are between 6 8 and 9 5 the pb isotope compositions of ore related pyrrhotite with 206pb 204pb 17 85 18 25 and 207pb 204pb 15 48 15 67 are less radiogenic and more variable than those of unaltered host rocks these differences combined with the hypozonal feature of the deposit indicate that ore metals and sulfur were derived from a sub crustal rather than a local crustal source the pge patterns of ore related pyrrhotites are similar to those of sulfides in baltic mantle lithosphere that was metasomatized by aqueous fluids in terms of marked enrichment of pd and ru but significantly differ from those of magmatic intrusions and related magmatic hydrothermal deposits this suggests that the metals that formed the danba deposit were transported via aqueous fluid from metasomatized mantle lithosphere rather than from a magmatic source the 18o of mantle fluid calculated from hydrothermal quartz and biotite ranges from 10 to 12 the anomalously high s and o isotope ratios and variable pb isotope ratios together with pge data support a model in which the inferred mantle lithosphere source was significantly hydrated metasomatized and fertilized by fluid derived from subducted oceanic sediments with elevated 34s and 18o values geological evidence suggests that this subduction event was neoproterozoic in age this study defines a model in which hydrated and metasomatized mantle lithosphere was formed during early subduction but it was over 500 million years later during later lower jurassic asthenosphere upwelling that metal and sulfur rich fluid was released into the crust to form the danba gold deposit

metallogenic model of the jinchang au ni deposit in the ailaoshan belt sw china determined on the basis of pyrite trace element contents in situ sulfur isotope composition and pge geochemistry abstract the ailaoshan metallogenic belt in southwestern china contains numerous shear zone related au deposits with au reserves greater than 200 tonnes however the source of ore forming fluids and metals is controversial hampering the construction of genetic models in this study we selected the jinchang au ni deposit in the northern segment of the ailaoshan belt for detailed investigation of pyrite chemistry and in situ sulfur isotope compositions using laser ablation inductively coupled plasma mass spectrometry la icp ms the petrographic and pyrite chemistry analyses revealed four types of hydrothermal pyrite py1 pyrite is euhedral to subhedral and has in situ sulfur isotope 34s values of 5 30 to 6 26 median 2 93 and low concentrations of au ni as sb cu zn pb and ag py1 is interpreted to have formed during the early hydrothermal stage py2 pyrite is subhedral to anhedral and porous with variable size although py2 and py3 show different textural relationships we conclude that both of them formed during the intermediate hydrothermal stage as evidenced by similar sulfur isotope compositions and trace element contents and common intergrowth textures py2 and py3 pyrite have in situ 34s values ranging from 11 6 to 1 29 median 4 45 and relatively elevated concentrations of au ni as sb cu zn pb and ag in py2 and py3 pyrite ni concentrations correlate with co concentration whereas au exhibits moderate positive correlations with ni co and as indicating that au ni and other metals were predominantly sourced from the jinchang ultramafic rocks py4 pyrite is euhedral in shape and has relatively low in situ 34s values of 23 5 to 4 34 median 8 33 the metal concentrations in py4 pyrite are lower than those in py2 and py3 pyrite in general the metal concentrations in the different pyrite populations display systematic changes with low levels in py1 higher levels in py2 andpy3 and low levels again in py4 the average total platinum group element pge concentration of the jinchang ni bearing ultramafic rocks is 26 3 ppb similar to the average value of the primitive mantle of 22 0 ppb the au ni ores from the jinchang deposit have an average pd pt ratio of 5 98 similar to that of hydrothermal ni ores but higher than that of typical magmatic ni ores in contrast the jinchang ultramafic rocks have low pd pt ratios between 0 57 and 1 13 suggesting that the pges in the jinchang intrusion are of magmatic origin similarly the rh ir ratios of au ni ores are consistent with those of hydrothermal ni ores but differ from those of magmatic ni ores the pyrite chemistry in situ sulfur isotope compositions and pge geochemistry suggest that the ore forming fluids and metals associated with the jinchang au ni deposit were dominantly derived from the jinchang ultramafic intrusion mesozoic felsic magmatism or cenozoic regional strike slip activity may have triggered migration of the ore forming fluids leaching of metals from the jinchang intrusion and precipitation of these metals at the boundary between the intrusions and carbonaceous rocks

a physical model of the effect of a shallow weak layer on strong ground motion for strike slip ruptures we report results of foam rubber modeling of the effect of a shallow weak layer on ground motion from strike slip ruptures computer modeling of strong ground motion from strike slip earthquakes has involved somewhat arbitrary assumptions about the nature of slip along the shallow part of the fault e g fixing the slip to be zero along the upper 2 kilometers of the fault plane in order to match certain strong motion accelerograms most modeling studies of earthquake strong ground motion have used what is termed kinematic dislocation modeling in kinematic modeling the time function for slip on the fault is prescribed and the response of the layered medium is calculated unfortunately there is no guarantee that the model and the prescribed slip are physically reasonable unless the true nature of the medium and its motions are known ahead of time there is good reason to believe that in many cases faults are weak along the upper few kilometers of the fault zone and may not be able to maintain high levels of shear strain required for high dynamic energy release during earthquakes physical models of faulting as distinct from numerical or mathematical models are guaranteed to obey static and dynamic mechanical laws foam rubber modeling studies have been reported in a number of publications the object of this paper is to present results of physical modeling using a shallow weak layer in order to verify the physical basis for assuming a long rise time and a reduced high frequency pulse for the slip on the shallow part of faults it appears a 2 kilometer deep weak zone along strike slip faults could indeed reduce the high frequency energy radiated from shallow slip and that this effect can best be represented by superimposing a small amplitude short rise time pulse at the onset of a much longer rise time slip a weak zone was modeled by inserting weak plastic layers of a few inches in thickness into the foam rubber model for the 15 cm weak zone the average pulse is reduced by a factor of 0 46 the factor for the 20 cm case reduction is 0 11 for the 30 cm case it is 0 045 from these results we can see that the thicker the weak layer the more difficult it is for a short rise time acceleration pulse to push its way through the weak layer to the surface this is thus an approximate justification for reducing the high frequency radiation from shallower parts of strike slip faults if it is known that the shallow part of the fault is weak or has not stored up shear stress

precambrian geology of china preface abstract the north china south china and tarim cratons compose the tectonic framework of china in the last decade extensive structural metamorphic geochemical geochronological and geophysical investigations have been carried out on these cratonic blocks producing an abundant amount of new data and competing interpretations which have resulted in refinement of and in some cases significant reinterpretations of the precambrian crustal formation and evolution of china this forms a justification for organizing this special issue in which we invited representative experts working in the north china south china and tarim cratons to present overviews or detailed studies on the formation and tectonic evolution of major precambrian blocks in china twenty four contributions collected in this special issue provide current knowledge of the components and tectonic evolution of major precambrian blocks in china and discuss some key issues based on new data and interpretations which will provide important insights into understanding the precambrian geology of china

dynamics of earthquake normal faulting two dimensional lattice particle model a 2d lattice particle model is used to simulate the dynamic rupture process of a normal fault the system equations for the particle motions are solved numerically by the finite difference method under a given block boundary condition the flexibility of the implementation of a 2d lattice particle model to simulate an earthquake dynamic process was demonstrated in previous modeling of a shallow angle thrust fault shi et al 1998 numerical results indicate that the particle motions displacement velocity and acceleration along the fault are discontinuous both in the fault parallel and fault normal directions with a localized slip rupture and localized fault separation in the vicinity of the fault outcrop the position at which the fault intersects with the free surface the particle velocity and acceleration increase rapidly both on the hanging wall and the footwall the particle motions on the hanging wall are larger than those on the footwall these motions are amplified as the fault scarp develops rupture breaks out at the surface with strong asymmetry between the hanging wall and the footwall along the free surface as the distance from the fault outcrop increases the particle velocity and acceleration decrease rapidly on the footwall and less rapidly on the hanging wall the asymmetrical particle motion results from the geometrical effect of the dip of the fault the free surface and the dynamic source rupture the combination of all of these effects causes a strong asymmetry in stress when the rupture pulse approaches the free surface the dynamically propagating rupture is characterized by a ramp slip time function accompanied with fault opening the slip pulse becomes sharper when the rupture approaches the free surface consequentially the hanging wall in the vicinity of the fault exhibits a large vibration which generates a strong surface wave propagating along the free surface away from the fault scarp on the hanging wall side this result is similar but significantly different from the numerical simulation of a normal fault with a moving double couple dislocation source benz and smith 1988 in addition the numerical result is qualitatively in agreement with recent foam rubber experiments brune and anooshehpoor 1999 and similar to results from a finite element simulation oglesby et al 1998 oglesby 1999 comparing with a 2d strike slip fault and thrust fault the particle motions in the vicinity of the normal fault on the free surface are smaller for a same size rupture pulse

distinct asymmetry in rupture induced inelastic strain across dipping faults an off fault yielding model 1 off fault stresses associated with rupture propagation cause material yielding under a mohr coulomb condition leaving irrecoverable deformation in the fault zone 2d dynamic rupture simulations for a 30 reverse fault and a 60 normal fault in a depth dependent stress environment show that the yielding inelastic zone off the fault broadens as it nears the surface with decreasing confining pressure forming a skewed flower like structure bounded at the top by the free surface the inelastic strain in the hanging wall is significantly larger and broader than the footwall for both reverse and normal faults the occurrence of inelastic strain however significantly reduces ground motion especially on the hanging wall and gives rise to a reduced asymmetry in ground motion on the hanging wall and footwall compared to elastic solutions these results provide theoretical predictions for fault zone structure of dipping faults that can be tested by future observational experiments

constraints on arabian plate motion and extensional history of the red sea analysis of the geologic and geophysical data from the western and northern margin of the arabian plate combined with data from the red sea gulf of aden leads to an interpretation of arabian plate motion that supports an episodic extensional history in the red sea and gulf of aden phase 1 extension began in medial to late eocene and was caused by the nw propagation of the reoriented central indian ridge spreading center it stretched and thinned continental crust along numerous wrench and normal faults that reactivated preexisting structures first phase extension stopped when the arabian plate terminally sutured to eurasia in the medial miocene and halted northward movement of arabia relative to africa the combined african arabian plates moved north more slowly constricting and thickening continental crust in the bitlis zagros sutures while the red sea remained technically quiescent and accumulated salt by the early pliocene continued northward movement was accommodated by lateral extrusion of large continental wedges north of the bitlis zagros sutures this allowed arabia to move northward faster than africa and reopen the red sea where phase 2 extension was expressed by seafloor spreading extension within the red sea gulf of aden rift system has been controlled by the interactions and effects of two factors 1 convergent processes between the north margin of the arabian plate and the eurasian plate to the north and 2 northwest propagation of extension associated with the medial eocene reorganization of the central indian spreading center

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

rupture mechanism and interface separation in foam rubber models of earthquakes a possible solution to the heat flow paradox and the paradox of large overthrusts abstract spontaneous stick slip along the interface between stressed foam rubber blocks is a simple analog of earthquake rupture and stick slip results from this model are used to elucidate the role of normal stress variations along the interface in the stick slip process observations indicate significant normal interface vibrations and separation during slip suggesting that dynamic changes in normal stress rather than a drop in the coefficient of friction may control stick slip as suggested for example by tolstoi oden and martins and brune and co workers observations of particle trajectories indicate that stick slip shear motion is associated with various degrees of fault separation for an asymmetric model the motion is consistent with slipping motion of the type suggested by schallamach and price for a symmetric model the motion is similar to that suggested by comninou and dundurs if interface waves of this type involving separation during slip occur in earthquakes they may be a solution to the heat flow paradox since a major part of the slip occurs during separation and during low normal stress thus frictional heat generation is reduced normal interface vibrations during stick slip may explain the high corner frequency of p wave spectra and the generally high levels of p wave spectra beyond the corner frequency schallamach comninou type waves are consistent with the partial stress drop abrupt locking self healing models of brune and heaton

geodetic constraints on present day motion of the arabian plate implications for red sea and gulf of aden rifting 1 five years of continuously recording gps observations in the kingdom of saudi arabia together with new continuous and survey mode gps observations broadly distributed across the arabian peninsula provide the basis for substantially improved estimates of present day motion and internal deformation of the arabian plate we derive the following relative geodetic euler vectors latitude n longitude e rate myr counterclockwise for arabia nubia 31 7 0 2 24 6 0 3 0 37 0 01 arabia somalia 22 0 0 5 26 2 0 5 0 40 0 01 arabia india 18 0 3 8 87 6 3 3 0 07 0 01 arabia sinai 35 7 0 8 17 1 5 0 0 15 0 04 and arabia eurasia 27 5 0 1 17 6 0 3 0 404 0 004 we use these euler vectors to estimate present day stability of the arabian plate the rate and direction of extension across the red sea and gulf of aden and slip rates along the southern dead sea fault south of the lebanon restraining bend 4 5 4 7 0 2 mm yr left lateral 0 8 1 1 0 3 mm yr extension and the owens fracture zone 3 2 2 5 0 5 mm yr right lateral increasing from north to south 1 2 mm yr extension on a broad scale the arabian plate has no resolvable internal deformation weighted root mean square of residual motions for arabia equals 0 6 mm yr although there is marginally significant evidence for n s shortening in the palmyride mountains syria at 1 5 mm yr we show that present day arabia plate motion with respect to eurasia is consistent within uncertainties i e 10 with plate tectonic estimates since the early miocene when arabia separated from nubia we estimate the time of red sea and gulf of aden rifting from present day arabia motion plate tectonic evidence for a 70 increase in arabia nubia relative motion at 13 ma and the width of the red sea and gulf of aden and find that rifting initiated roughly simultaneously 2 2 myr along the strike of the red sea from the gulf of suez to the afar triple junction as well as along the west gulf of aden at 24 2 2 ma based on the present kinematics we hypothesize that the negative buoyancy of the subducted ocean lithosphere beneath the makran and the zagros fold thrust belt is the principle driver of arabia eurasia convergence and that resisting forces associated with arabia eurasia continental collision have had little impact on plate motion

the rotation of arabia and the levant fault system abstract we establish the kinematics of the rotation of arabia with respect to africa and show that the rotation rate of africa increased by a factor of about four in late serravallian time about 12 13 ma ago we discuss evidence that oceanic accretion started simultaneously in the gulf of aden and in the red sea at this time it is also at this time that the levant and anatolian fault systems were initiated our initial reconstruction of the red sea rift in late oligocene early miocene time approximately superposes the present shore lines which coincide with the mid point of the zones of rapid crustal thinning the future continental slopes the amount of extension during the early phase of continental rifting is obtained by assuming that the total volume of continental crust is conserved during extension and by checking these estimates through closure around the sinai triple junction in the late miocene the southern evant fault zone was a zone of distributed shear deformation through counterclockwise block rotation extending soul h ward within the northernmost red sea since the early pliocene the shear motion has been localized within the gulf of aqaba changing the tectonic pattern in the northern red sea

the genesis of the liancheng cu mo deposit in the lanping basin of sw china constraints from geology fluid inclusions and cu s h o isotopes abstract liancheng is an important cu mo deposit hosted by red beds in the mesozoic to cenozoic lanping basin located in the sanjiang tethyan orogen of sw china the deposit is fault controlled where it has been affected and enriched by four hydrothermal events indicated by four distinct sets of veins and their cross cutting relationships a detailed cu s h o isotopic study is reported here on the mineralization focusing on the source of the metals and fluid and on the mineralizing processes the first two hydrothermal events are characterised by co 2 and fluid rich inclusions with homogenisation temperatures of 124 446 c and salinities ranging from 0 4 to 24 6 wt nacl equiv the next two hydrothermal events are characterised by liquid rich fluid inclusions that homogenise at 145 256 c with salinities ranging from 1 4 to 22 9 wt nacl equiv the h o isotope values obtain from the fluid inclusions indicate that the first three hydrothermal events relate to mineralizing fluids derived from a metamorphic source but the last event includes a mixed metamorphic and meteoric source chalcopyrite tennantite and bornite from the deposit have 34 s values between 11 5 and 3 the 65 cu values for quartz cu sulfide mineralization associated with the second hydrothermal event range from 0 31 to 0 2 but carbonate cu sulfide mineralization associated with the third hydrothermal event ranges from 1 01 to 1 08 the combination of the cu and s isotopic values and field observations indicate that the controlling factors include structures the temperature of regional metamorphism and the ph of the mineralizing fluid the zation is therefore characteristic of an orogenic type deposit

earthquakes on dipping faults the effects of broken symmetry dynamic simulations of earthquakes on dipping faults show asymmetric near source ground motion caused by the asymmetric geometry of such faults the ground motion from a thrust or reverse fault is larger than that of a normal fault by a factor of 2 or more given identical initial stress magnitudes the motion of the hanging wall is larger than that of the footwall in both thrust reverse and normal earthquakes the asymmetry between normal and thrust reverse faults results from time dependent normal stress caused by the interaction of the earthquake generated stress field with earth s free surface the asymmetry between hanging wall and footwall results from the asymmetric mass and geometry on the two sides of the fault

precarious rock evidence for low ground shaking on the footwall of major normal faults preliminary interpretations of precarious rock observations suggest relatively low near fault 3 km footwall ground motions for normal faults relative to strike slip faults as reported for a dynamic foam rubber normal fault model by brune and anooshehpoor 1999 since for large normal fault earthquakes there are no instrumental data at such close distances from the fault trace precarious rocks may provide important data for estimating the ground motion for these earthquakes use of empirical curves in the literature based mainly on strike slip data may lead to overestimates of seismic hazard for the near fault footwall of normal faults these preliminary results may warrant more comprehensive and more quantitative followup studies

age and origin of the bulangshan and mengsong granitoids and their significance for post collisional tectonics in the changning menglian paleo tethys orogen the subduction and collision tectonics in the sanjiang region provide important insights into the evolution of the paleo tethyan orogen here we present la icp ms zircon u pb ages sm nd and lu hf isotopic data whole rock major and trace element compositions of the late triassic magmatic rocks from the mengsong and bulangshan granitoids in this region zircon u pb dating of the bulangshan and mengsong granitoids yields crystallization age of 216 228 ma with zircon ehf t values ranging from 15 2 to 1 5 these granitoids are high k calc alkaline and muscovite bearing the rocks show peraluminous nature and are enriched in large ion lithophile elements rb k u and th and depleted in high field strength elements nb and ti they also display enrichment in light rare earth elements with strong negative eu anomalies eu 0 16 the geochemical and isotopic features suggest that the primary magma of the bulangshan and mengsong granitoids most likely originated from partial melting of the underlying proterozoic metasedimentary rocks the granitoid magmatism is correlated with post collisional tectonics and provides important constraints on the timing of closure of the paleo tethyan ocean

geological and isotopic evidence for magmatic hydrothermal origin of the ag pb zn deposits in the lengshuikeng district east central china the lengshuikeng ore district in east central china has an ore reserve of 43 mt with an average grade of 204 53 g t ag and 4 63 pb zn based on contrasting geological characteristics the mineralization in the lengshuikeng ore district can be divided into porphyry hosted and stratabound types the porphyry hosted mineralization is distributed in and around the lengshuikeng granite porphyry and shows a distinct alteration zoning including minor chloritization and sericitization in the proximal zone sericitization silicification and carbonatization in the peripheral zone and sericitization and carbonatization in the distal zone the stratabound mineralization occurs in volcano sedimentary rocks at 100 400 m depth without obvious zoning of alterations and ore minerals porphyry hosted and stratabound mineralization are both characterized by early stage pyrite chalcopyrite sphalerite middle stage acanthite native silver galena sphalerite and late stage pyrite quartz calcite the 34s values of pyrite sphalerite and galena in the ores range from 3 8 to 6 9 with an average of 2 0 the c o isotope values of siderite calcite and dolomite range from 7 2 to 1 5 with an average of 4 4 v pdb and from 10 9 to 19 5 with an average of 14 8 v smow respectively hydrogen oxygen and carbon isotopes indicate that the hydrothermal fluids were derived mainly from meteoric water with addition of minor amounts of magmatic water geochronology employing la icp ms analyses of zircons from a quartz syenite porphyry yielded a weighted mean 206pb 238u age of 136 3 0 8 ma considered as the emplacement age of the porphyry rb sr dating of sphalerite from the main ore stage yielded an age of 126 9 7 1 ma marking the time of mineralization the lengshuikeng mineralization classifies as an epithermal ag pb zn deposit

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

mantle crustal and atmospheric noble gases in ailaoshan gold deposits yunnan province china abstract helium and argon isotopic and elemental compositions of fluids released by crushing pyrite grains are reported from the ailaoshan gold province china the province is a series of gold deposits hosted on a 200 km segment of a major normal fault formed during eocene extension the deposits have been well characterised using conventional geochemical and microthermometric techniques the results of which are consistent with a predominantly high temperature magmatic fluid present in the ore minerals in contrast analyses of noble gases from three gold deposits demonstrate the palaeofluids were a mixture between a mantle derived magmatic fluid and two different low temperature fluids similar to modern groundwaters the specific isotopic and abundance characteristics of helium and argon allow unequivocal identification of both a mantle derived component and a surface derived component to these fluids the magmatic fluid dominates deposits from the northern end of the fault the second groundwater is present only in deposits in the south of the province by assuming fluid inclusion homogenisation temperatures in coexisting quartz are representative of fluid temperature 3he heat ratios can be estimated the 3he heat ratios vary systematically along the fault from 30 10 12 cm3 stp j 1 in the northern deposits to

rupture dynamics on a bimaterial interface for dipping faults dip slip faulting may juxtapose different geologic materials with differ ent properties such that a strong material contrast will naturally tend to form both the material contrast across a fault and dip slip motion on a nonvertical fault lead to normal stress variations during earthquake rupture this normal stress variation will significantly affect dynamic rupture propagation to demonstrate this we model dynamic rupture propagation on two dimensional 2d reverse and normal faults 30 45 and 60 dipping with 20 material contrasts for predominantly up dip rupture propagation we find that normal stress variations due to the free surface and material contrast can either reinforce or counteract each other depending on the configuration for reverse faults we find a larger strength drop for a more com pliant hanging wall and a lower strength drop for a more compliant footwall for normal faults we find a larger strength drop for a more compliant footwall and a lower strength drop for a more compliant hanging wall for both reverse and normal faults ground motion will be more symmetric between the hanging wall and footwall with compliant material on the footwall and more asymmetric if more compliant materials are on the hanging wall our results have important implications for the dynamics of crustal and perhaps subduction zone earthquake faulting where strong bimaterial contrasts across dipping faults are possible in continental settings reverse faulting will tend to advect rigid materials from greater depth onto the hanging wall such that the effects of fault dip and material contrast will counteract one another in subduction zones the hanging wall is likely to be more compliant and hence the material and geometric effects may reinforce one another

precarious rock evidence for low near source accelerations for trans tensional strike slip earthquakes abstract this paper describes precarious rock evidence for low ground motions associated with extensional sections of strike slip faults recent evidence from physical and numerical models and data regressions has indicated that ground motion for extensional strike slip regions may be lower than for strike slip faults with a large fault normal tectonic stress component and for thrust faults in general data from compressional strike slip and thrust earthquakes dominates the database used in most regression curves for ground acceleration and in the calculation of current probabilistic seismic hazard maps therefore estimates of ground accelerations on these seismic hazard maps may be too high for sites near extensional sections of strike slip faults this paper discusses precariously balanced rock data from three areas near extensional sections of strike slip faulting 1 the region of the honey fault california with an active holocene fault 2 the red rock canyon region of the garlock fault near a dilatational step over and 3 the region just south of beaumont california near the hemet dilatational step over in the san jacinto fault these are all active strike slip faults with at least a few large earthquakes in the holocene and in the case of the san jacinto example historic large earthquakes m 7 thus the precarious rocks at these sites are evidence of relatively low ground motions associated with extensional strike slip faulting the results of this study could be very important in developing more detailed seismic hazard maps in the future

evidence for a post 3 16 ma change in nubia eurasia north america plate motions we combine updated gps velocities from the nubian nu eurasian eu and north american na plates with 500 new 3 16 myr average seafloor spreading rates and nine transform fault azimuths from the northern atlantic and arctic basin seafloor spreading centers to estimate and test for changes in the relative motion between these plates the numerous new seafloor spreading rates and gps velocities improve our ability to detect recent changes in the relative motions of these plates the angular velocity vector that best fits the eu na gps velocities lies significantly north of the 3 ma average pole in accord with previously published geologic evidence that the eu na pole has migrated northward since v3 ma although we also find evidence for a significant post 3 ma change in nu na motion it is less compelling because the nubian plate gps velocity field is sparse and nu na seafloor spreading rates appear to have remained steady within the 1 mm yr 31 uncertainties if we systematically decrease the seafloor spreading rates to correct for outward displacement of seafloor spreading magnetic lineations the nu eu pole derived from gps site velocities lies more than 30 angular degrees south of the tightly constrained 3 ma average estimate and predicts significantly slower and more oblique present day nu eu convergence in the mediterranean both models for nu eu motion pass a key test for their accuracy namely they correctly predict strike slip motion along the well mapped gloria fault east of the azores the change to more oblique nu eu motion may reflect increasing difficulty in maintaining margin normal convergence within this continent continent collision zone 8 2003 published by elsevier b v

global positioning system constraints on plate kinematics and dynamics in the eastern mediterranean and caucasus we present and interpret global positioning system gps measurements of crustal motions for the period 1988 1997 at 189 sites extending east west from the caucasus mountains to the adriatic sea and north south from the southern edge of the eurasian plate to the northern edge of the african plate sites on the northern arabian platform move 18 2 mm yr at n25 5 w relative to eurasia less than the nuvel 1a circuit closure rate 25 1 mm yr at n21 7 w preliminary motion estimates 1994 1997 for stations located in egypt on the northeastern part of africa show northward motion at 5 6 2 mm yr also slower than nuvel ia estimates 10 1 mm yr at n2 4 e eastern turkey is characterized by distributed deformation while central turkey is characterized by coherent plate motion internal deformation of 2 mm yr involving westward displacement and counterclockwise rotation of the anatolian plate the anatolian plate is de coupled from eurasia along the right lateral strike slip north anatolian fault naf we derive a best fitting euler vector for anatolia eurasia motion of 30 7 0 8 n 32 6 0 4 e 1 2 0 1 myr the euler vector gives an upper bound for naf slip rate of 24 1 mm yr we determine a preliminary gps arabia anatolia euler vector of 32 9 1 2 n 40 3 1 1 e 0 8 0 2 myr and an upper bound on left lateral slip on the east anatolian fault eaf of 9 1 mm yr the central and southern aegean is characterized by coherent motion internal deformation of 2 mm yr toward the sw at 30 1 mm yr relative to eurasia stations in the se aegean deviate significantly from the overall motion of the southern aegean showing increasing velocities toward the trench and reaching 10 1 mm yr relative to the southern aegean as a whole

low footwall accelerations and variable surface rupture behavior on the fort sage mountains fault northeast california the fort sage mountains fault zone is a normal fault in the walker lane of the western basin and range that produced a small surface rupture 20 cm during an m l 5 6 earthquake in 1950 we investigate the paleoseismic history of the fort sage fault and find evidence for two paleoearthquakes with surface displacements much larger than those observed in 1950 rupture of the fort sage fault 5 6 ka resulted in surface displacements of at least 0 8 1 5 m implying earthquake moment magnitudes m w of 6 7 7 1 an older rupture at 20 5 ka displaced the ground at least 1 5 m implying an earthquake of m w 6 8 7 1 a field of precariously balanced rocks pbrs is located less than 1 km from the surface rupture trace of this holocene active normal fault ground motion prediction equations gmpes predict peak ground accelerations pgas of 0 2 0 3 g for the 1950 rupture and 0 3 0 5 g for the 5 6 ka paleoearthquake one kilometer from the fault surface trace yet field tests indicate that the fort sage pbrs will be toppled by pgas between 0 1 0 3 g we discuss the paleoseismic history of the fort sage fault in the context of the nearby pbrs gmpes and probabilistic seismic hazard maps for extensional regimes if the fort sage pbrs are older than the mid holocene rupture on the fort sage fault zone this implies that current gmpes may overestimate near fault footwall ground motions at this site

dating precariously balanced rocks in seismically active parts of california and nevada precariously balanced boulders that could be knocked down by strong earthquake ground motion are found in some seismically active areas of southern california and nevada in this study we used two independent surface exposure dating techniques rock varnish microlamination and cosmogenic 36 cl dating methodologies to estimate minimum and maximum limiting ages respectively of the precarious boulders and by inference the elapsed time since the sites were shaken down the results of the exposure dating indicate that all of the precarious rocks are 10 5 ka and that some may be significantly older at victorville and jacumba california these results show that the precarious rocks have not been knocked down for at least 10 5 k y a conclusion in apparent conflict with some commonly used probabilistic seismic hazard maps at yucca mountain nevada the ages of the precarious rocks are 10 5 to 27 0 ka providing an independent measure of the minimum time elapsed since faulting occurred on the solitario canyon fault

gps constraints on continental deformation in the africa arabia eurasia continental collision zone and implications for the dynamics of plate interactions 1 the gps derived velocity field 1988 2005 for the zone of interaction of the arabian african nubian somalian and eurasian plates indicates counterclockwise rotation of a broad area of the earth s surface including the arabian plate adjacent parts of the zagros and central iran turkey and the aegean peloponnesus relative to eurasia at rates in the range of 20 30 mm yr this relatively rapid motion occurs within the framework of the slow moving 5 mm yr relative motions eurasian nubian and somalian plates the circulatory pattern of motion increases in rate toward the hellenic trench system we develop an elastic block model to constrain present day plate motions relative euler vectors regional deformation within the interplate zone and slip rates for major faults substantial areas of continental lithosphere within the region of plate interaction show coherent motion with internal deformations below 1 2 mm yr including central and eastern anatolia turkey the southwestern aegean peloponnesus the lesser caucasus and central iran geodetic slip rates for major block bounding structures are mostly comparable to geologic rates estimated for the most recent geological period 3 5 myr we find that the convergence of arabia with eurasia is accommodated in large part by lateral transport within the interior part of the collision zone and lithospheric shortening along the caucasus and zagros mountain belts around the periphery of the collision zone in addition we find that the principal boundary between the westerly moving anatolian plate and arabia east anatolian fault is presently characterized by pure left lateral strike slip with no fault normal convergence this implies that extrusion is not presently inducing westward motion of anatolia on the basis of the observed kinematics we hypothesize that deformation in the africa arabia eurasia collision zone is driven in large part by rollback of the subducting african lithosphere beneath the hellenic and cyprus trenches aided by slab pull on the southeastern side of the subducting arabian plate along the makran subduction zone we further suggest that the separation of arabia from africa is a response to plate motions induced by active subduction

stable isotope studies and processes of carbonate formation in hungarian alkali basalts and lamprophyres evolution of magmatic fluids and magma sediment interactions processes of carbonate formation have been related to c and o isotopic compositions in the mesozoic alkali basalt mecsek mts and lamprophyre transdanubian range suites of hungary in the studied magrnatic rocks carbonates are present as ocelli amygdales xenoliths veins and groundmass carbonate c and o isotope studies of these types of carbonate have yielded information on the origin of the carbonates and indicated the following processes of formation that determined the 13c and 18o values of the carbonates 1 crystallization of magmatic carbonate textural characteristics and 13c values suggest formation of magmatic carbonate in alkali basalt and lamprophyre dikes whereas the 18o compositions of these carbonates indicate low temperature oxygen isotope exchange with magmatic fluids 2 assimilation of sedimentary carbonate by silicate magmas even completely recrystallized amygdales and ocelli of basalts and lamprophyres have preserved their sedimentary 13c values in contrast variations in the extent of mobilization and isotope exchange with magmatic fluids are reflected in differences in the ranges of the 18o values of amygdales ocelli and veins and can be attributed to different amounts of fluids involved in the magmatic events 3 low temperature alteration of magmatic rocks caused only 18o enrichment in the carbonate amygdales of basalts and the groundmass carbonates of lamprophyres indicating that no externally derived co2 was present in the alteration fluids 4 degassing of magma and magmatic fluid correlations between 13c and 18o data magma crystallization depths and amygdale sizes in the alkali basalts suggest that co2 degassing has been responsible for the negative 13c and positive 18o shifts observed a similar trend was found in the lamprophyres but the extent of the 18o shift indicates that in these rocks h2o degassing also played an important role

in situ u pb zircon dating using laser ablation multi ion counting icp ms high resolution in situ u pb zircon geochronology on zoned grains can obtain isotope signatures from multiple growth or thermal events we present a method using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms to overcome complications associated with intricately zoned zircon crystals through in situ sampling of zircon volumes as small as 12 m 25 m and 40 m in diameter by about 10 m in depth high precision u pb age of a series of zircon standard covering a wide age range of 30 to 1 065 ma was acquired using la mc icp ms the precision of measured pb u ratios in homogeneous zircon is about 2 2 resulting in routinely achieved precision of u pb ages obtained by external calibration of 1 2 or better all masses of interest can be simultaneously recorded with a multi ion counting system mic operating in static mode and the short ablation required to achieve such precision results in spatial resolution that is superior to comparable u pb zircon analyses by single collector icp ms the resulting present u pb age for five zircon reference samples and two geological samples show an excellent agreement with the previously reported id tims or shrimp data

insights into ore genesis of the jinding zn pb deposit yunnan province china evidence from zn and in situ s isotopes abstract the jinding zn pb deposit located in the mesozoic cenozoic lanping basin of southwest china has ore reserves of 220 mt with an average grade of 6 1 zn and 1 3 pb the mineralization is hosted by sandstone in the early cretaceous jingxing formation and limestone breccia in the paleocene yunlong formation mineralization in both types of host rocks is characterized by a paragenetic sequence beginning with marcasite sphalerite stage 1 followed by pyrite marcasite sphalerite galena stage 2 and then galena sphalerite pyrite sulfate carbonate stage 3 pyrite from these stages have different 33 s compositions with pyrite from stage 1 averaging 9 6 stage 2 averaging 8 9 and stage 3 averaging 0 3 sphalerite hosted by the sandstone has similar 66 zn values ranging from 0 10 to 0 30 in all stages of the mineralization but sphalerite samples from the limestone breccia hosted ore show variable 66 zn values between 0 03 and 0 20 our data on sphalerite precipitated during the earlier stages of mineralization has a constant 66 zn value and cogenetic pyrite displays a very light sulfur isotope signature which we believe to reflect a sulfur source that formed during bacterial sulfate reduction bsr the stage 3 sphalerite and pyrite precipitated from a late influx of metal rich basinal brine which had a relatively constant variable 66 zn isotopic composition due to open system isotope fractionation and a near zero 33 s composition due to the influence of abiotic thermochemical sulfate reduction from observed sulfates in the host rock