theoretical models of magnetic anisotropy to strain relationship revisited abstract mathematical modelling of the relationships between the low field magnetic anisotropy and strain are redeveloped and the calculated data are compared with the degree of anisotropy for various rock types it is found that the degree of anisotropy for realistic strain magnitudes is unrealistically high in the passive model the ductile model for which the degree of anisotropy depends on the magnetic grain the matrix viscosity ratio becomes realistic at high viscosity contrasts in the line plane model the theoretical degree of anisotropy corresponds well to the natural degree of anisotropy for rocks in which the carrier of magnetic anisotropy is either magnetite or phyllosilicate minerals by contrast in the case of haematite and pyrrhotite the modelled degree of anisotropy is much higher than the natural values in the viscous model the calculated degree of anisotropy corresponds well to that measured in sedimentary and volcanic rocks in the models imposing pure shear strain the natural logarithm of the degree of anisotropy to natural strain relationship can be approximately represented by a straight line at least for low to intermediate strains the proportionality constant varies according to the specific model and the specific carrier of the anisotropy of magnetic susceptibility

tectonothermal evolution of the mayuan assemblage in the cathaysia block implications for neoproterozoic collision related assembly of the south china craton the mayuan assemblage in the cathaysia block southeast china consists of felsic paragneiss pelitic schist greenschist amphibolite marble calcsilicate and quartzite that underwent three episodes of deformation d1 d3 and four episodes of metamorphism m1 m4 in the early neoproterozoic the m1 assemblage consists of mineral inclusions defining an early foliation s1 within porphyroblasts represented by chlorite muscovite biotite plagioclase quartz in pelitic schist and actinolite chlorite epidote albite quartz enclosed in amphibolite m2 coincides with the development of the regional schistosity s2 and represents the formation of the porphyroblasts and growth of matrix minerals resulting in development of prograde metamorphic zones chlorite biotite garnet staurolite and kyanite zones m3 is simultaneous with the third phase of deformation d3 and produced sillimanite bearing mineral assemblages in pelitic schist and hornblende bearing assemblages in amphibo lite the last metamorphic episode m4 gave rise to the retrogressive assemblage chlorite muscovite in pelitic rock and actinolite chlorite epidote in amphibolite the sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near isothermal decompression clockwise p t path for the mayuan pelitic schists using the tweequ software program the garnet biotite thermometer and garnet muscovite biotite plagioclase barometer yield p t conditions for m1 of 5 5 to 6 0 kb and 450 to 500 c and conditions for the garnet staurolite and kyanite zones of m2 of 6 0 to 7 0 kb and 550 to 600 c 6 0 to 7 5 kb and 600 c and 11 0 to 11 5 kb and 600 c the p t conditions of m3 were estimated at 570 to 625 c and 4 0 to 4 5 kb using the muscovite biotite thermometer and hornblende plagioclase geothermo barometer the garnet chlorite thermometer yields temperatures of 300 to 400 c for m4 but the pressures of m4 cannot be quantitatively estimated because of the lack of a suitable geobarometer these p t estimates also define an near isothermal decompression clockwise p t path which is involved in initial crustal thickening followed by rapid exhumation and final cooling and is related to amalgamation of the cathaysia and yangtze blocks to form the south china craton

fabric transitions in the saxony granulite terrain abstract quartz fabric transitions reported from the saxony granulite terrain and from the nearby ore mountains are interesting because of the changes in the pattern types and the variation in the opening angles of the fabric skeletons a short summary is presented of salient features in research documented in the east german literature between 1961 and 1978 the changes in fabric skeleton with decreasing metamorphic grade are proposed as mainly consequent to the deactivation of prism c glide systems as temperature decreases fabrics indicative of the appearance of the c burgers vector might be diagnostic for deformation in regimes of comparatively high temperatures or low strainrates or for deformation under conditions such that there is a relatively high kinetically effective hydroxyl content in the quartz structure

the 1999 mw 7 1 hector mine california earthquake sequence complex conjugate strike slip faulting the 1999 m w 7 1 hector mine mainshock showed right lateral strike slip faulting with an initial strike of n6 w and vertical dip the mainshock was preceded within 20 hours by 18 recorded foreshocks of 1 5 m 3 8 within a few kilometers distance of the mainshock hypocenter the aftershocks delineate how the hector mine earthquake ruptured with strike n6 w to the south for a distance of 15 km and possibly to the north for a distance of several kilometers the two largest aftershocks of m 5 9 and m 5 7 occurred near the north and south ends of the first mainshock rupture segment the second segment of rupture starting 15 km to the south away from the mainshock hypocenter delineated by strike slip and thrust faulting aftershocks extends 10 km farther away with a strike of s140 e along the bullion fault the aftershocks also outline an unusual third rupture segment extending from about 5 km south of the hypocenter with a strike of n30 w to n35 w for a distance of 20 km approximately 10 to 25 km farther to the north and west of the mainshock epicenter several clusters form a complex aftershock distribution three dimensional vp and vp vs models of the region exhibit only small regional changes as is typical for the mojave region nonetheless the mainshock rupture started within a region of rapidly varying vp and at least three regions of low vp vs are imaged within the aftershock zone the rate of decay for the hector mine earthquake sequence has been slightly above the mean for both p values and b values in southern california the focal mechanisms of the aftershocks and the state of stress are consistent with strike slip faulting including a component of normal faulting most prominent to the north the orientation of the regional maximum horizontal stress the variation in orientation of the mainshock fault segments by 30 and scattered distribution of aftershocks suggest that the mainshock and aftershock deformation field exhibit volumetric shear deformation accommodated by complex conjugate sets of strike slip faults

microstructures and quartz lattice preferred orientations in the eclogite bearing migmatitic gneisses of the d entrecasteaux islands papua new guinea 1 lattice preferred orientations lpo of quartz in gneiss domes of the d entrecasteaux islands woodlark rift shed insight into exhumation of the world s youngest 5 7 ma eclogite bearing terrane at cm yr rates we focus on deformation that affected the terrane as it transited between lower crustal depths and the surface including 1 grain scale deformation mechanisms and 2 style of flow and mode of emplacement of the domes electron backscatter diffraction was used to analyze microstructure and lpos of 37 quartzofeldspathic gneiss samples that enclose meter scale mafic blocks preserving original eclogite facies assemblages during exhumation of the ultrahigh pressure uhp terrane gneisses were retrogressed in the amphibolite facies at lower crustal depths the lpos change from dome cores to carapaces consistent with decreasing deformational temperatures in the relatively chilled outer carapaces of the domes the quartz lpos consist of mostly crossed girdle c axis patterns with some cleft girdle and small circle lpos and record dislocation creep accommodated by mixed slip in the cores of the migmatitic domes a chessboard pattern of subgrains is common and quartz lpos primarily record prism c slip probably at 630 c other microstructures indicate recovery by high temperature grain boundary migration grain boundary mobility was anisotropic leading to strong grain shape fabrics oblique to foliation but not obviously relatable to shear sense evidence for melt present deformation is abundant and microstructures including partially dissolved feldspar grains indicate some deformation by fluid assisted grain boundary diffusion creep lpos in carapace rocks are symmetrical recording flow that was dominantly coaxial we interpret the gneiss domes to have been emplaced into the rift as partially molten diapirs

moment tensor solutions for small and moderate earthquakes in the ibero maghreb region 1 we applied time domain moment tensor inversion of local and regional waveforms to small and moderate mw 3 5 5 7 shallow earthquakes from the iberian peninsula northern morocco and northern algeria for the 6 years period from november 1995 to march 2002 and the previous network of autonomously recording seismograms nars experiment moment tensor solutions were obtained for 58 events considerably increasing the total number of available solutions in the study area for each event we performed a moment tensor inversion and a double couple grid search for simple faulting events the grid search is valuable as a quality test for its ability to reveal potential ambiguities of the solutions and to assess confidence limits of fault plane parameters or principal axes orientation the computed mechanisms show regional consistency a large part of the iberian peninsula is characterized by normal faulting mechanisms with sw ne oriented t axes thrusting and se nw compression is dominant in algeria in the alboran sea the westernmost part of the mediterranean and the transition between both regimes strike slip mechanisms dominate with approximately n s oriented p axes this pattern suggests a regional anomaly characterized by clockwise rotation of the principal horizontal stress orientations

kinematics of the western africa eurasia plate boundary from focal mechanisms and gps data summary the western mediterranean displays a complex pattern of crustal deformation distributed along tectonically active belts developed in the framework of slow oblique plate convergence we used earthquake and global positioning system gps data to study the present day kinematics and tectonics of the africa eurasia plate boundary in this region crustal seismicity and focal mechanisms analysed in terms of seismic moment release and seismic deformation outline the geometry of major seismic belts and characterize their tectonics and kinematics continuous gps data have been analysed to determine euler vectors for the nubian and eurasian plates and to provide the global frame for a new mediterranean gps velocity field obtained by merging continuous and campaign observations collected in the 1991 2005 time span gps velocities and displacements predicted by the nubia eurasia rotation pole provide estimates of the deformation accommodated across the tectonically active belts the rather simple deformation occurring in the atlantic region characterized by extension about perpendicular to the middle atlantic and terceira ridges and right lateral motion along the gloria transform fault turns into a complex pattern of deformation occurring along broader seismic belts where continental lithosphere is involved our analysis reveals a more complex fragmentation of the plate boundary than previously proposed the roughly e w trending mainly compressive segments i e southwestern iberia northern algeria and southern tyrrhenian where plate convergence is largely accomodated across rather localized deformation zones and partially transferred northward to the adjacent domains i e the algero balearic and tyrrhenian basins are interrupted by regions of more distributed deformation i e the rif alboran betics tunisia libya and eastern sicily or limited seismicity i e the strait of sicily which are characterized by less homogeneous tectonics regimes mainly transcurrent to extensional in correspondence of the observed breaks tectonic structures with different orientation interfere and we find belts with only limited deformation i e the high and middle atlas the tunisian atlas and the offshore tunisia libya belt that extends from the plate boundary into the nubian plate along pre existing tectonic lineaments our analysis suggest that the sicilian pelagian domain is moving independently from nubia according to the presence of a right lateral and extensional decoupling zone corresponding to the tunisia libya and strait of sicily deformation zone despite the space variability of active tectonic regimes plate convergence still governs most of the seismotectonic and kinematic setting up to the central aeolian region in general local complexities derive from pre existing structural features inherited from the tectonic evolution of the mediterranean region on the contrary along calabria and the apennines the contribution of the subducted ionian oceanic lithosphere and the occurrence of microplates i e adria appear to substantially modify both tectonics and kinematics finally gps data across the gibraltar arc and the tyrrhenian calabria domain support the hypothesis that slab rollback in these regions is mostly slowed down or stopped

in situ la mc icp ms and id tims u pb geochronology of cassiterite in the giant furong tin deposit hunan province south china new constraints on the timing of tin polymetallic mineralization abstract the furong deposit located in southern hunan province china is a newly discovered giant tin deposit spatially associated with the a type qitianling granite the petrogenetic link between the giant furong tin deposit and the qitianling granite batholith remains controversial because of the lack of precise dating for the tin polymetallic mineralization here we report for the first time in situ u pb data on cassiterite obtained by la mc icp ms and the results are compared to id tims data the in situ la mc icp ms analyses of the cassiterites provide a reliable age constraint for tin polymetallic mineralization in the furong deposit yielding an isochron age of 159 9 1 9 ma at 95 confidence level mswd 18 which is indistinguishable from the id tims 206 pb 238 u weighted mean age of 158 2 0 4 ma the in situ u pb ages corroborate the ar ar dates published for the furong tin deposit which indicate that the mineralization is coeval with the emplacement of the qitianling granite batholith these results provide further evidence of a close temporal link between the pluton s emplacement and tin mineralization in the furong area the data obtained clearly demonstrate that in situ la mc icp ms u pb dating of cassiterite is a robust geochronometer for direct dating of tin polymetallic mineralization

trace and rare earth element geochemistry in tourmaline and cassiterite from the yunlong tin deposit yunnan china implication for migmatitic hydrothermal fluid evolution and ore genesis abstract yunlong tin deposit is a medial sized hydrothermal deposit in western yunnan province of china this deposit is unusual because the tin mineralization occurs exclusively within the migmatite bodies the genesis of the ore deposit has been hotly debated and various models have been proposed including granite related magmatic hydrothermal and migmatitic hydrothermal origins this paper reports a systematic trace and rare earth elements ree geochemical study in tourmaline and cassiterite from the orebodies and associated migmatites from the yunlong tin deposit tourmaline and cassiterite from this deposit show light ree lree enriched patterns with variable eu and ce anomalies consistent with hydrothermal fluids being derived from local high grade metamorphism and migmatization the data suggest that tourmaline and cassiterite were likely precipitated from migmatitic hydrothermal fluids and indicate that the yunlong tin deposit was probably formed during migmatitic hydrothermal process the high concentrations of transition metals e g sc ti v cr mn and zn in tourmalines are also consistent with this model the high concentrations of sn sr and especially sc v and cr in tourmalines can be used to discriminate tourmaline from barren deposits relative to those associated with economic tin mineralization

seismotectonic constraints at the western edge of the pyrenees aftershock series monitoring of the 2002 february 21 4 1 lg earthquake this work was carried out with the support of the marconi ren2001 1734 project from the spanish mcyt mr and cl benefited from a phd grant from the spanish mcyt we would like to thank to the responsible of the ign renass omp and cea databases for sharing the seismological data used in this study

monitoring velocity variations in the crust using earthquake doublets an application to the calaveras fault california we present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates we compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal a moving window analysis of the entire seismograms including the coda gives t the difference in arrival times versus running time along the seismogram the time resolution of the method is an order of magnitude better than the digitization interval the t technique is illustrated with a pair of microearthquakes m 1 7 and 2 0 that occurred before and after the coyote lake california earthquake m 5 9 of august 6 1979 and on the same segment of the calaveras fault that ruptured during the earthquake the coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet so that its seismogram appears as a stretched version of the earlier event we interpret this systematic variation in t along the coda as a change in the average s velocity in the upper crust in the time interval between the two doublets s wave velocities appear to have decreased by 0 2 in an oblong region 5 10 km in radius at the south end of the aftershock zone

amplitude spectra moment tensor inversion of shallow earthquakes in spain summary the study of focal mechanisms for shallow earthquakes of low to moderate magnitude at regional distances requires a proper inversion technique one of the major problems is the correct separation of path effects from source effects the method used should take into account problems arising from errors in the definition of crustal structure and inclusion of the high frequency content of the waveforms in this study a method of frequency domain moment tensor inversion to determine deviatoric moment tensor solutions from body wave amplitude spectra is described the method has been tested with synthetic data to define instabilities that are due to mismodelling of several parameters the inversion technique is applied to broadband seismic data of shallow 10 km depth earthquakes in spain solutions are obtained for a range of depths and compared with solutions obtained with different methods such as first motion polarities and moment tensor inversion in the time domain a combined fit of amplitude spectra first motion polarities and double couple component is introduced as a method which resolves better the non uniqueness of solution the proposed method can be implemented in fast moment tensor inversion when a green functions database has been created in advance for the studied area

quartz exsolution topotaxy in clinopyroxene from the uhp eclogite of weihai china abundant oriented silica precipitates of quartz 4 0 1 0 vol in part coexisting with calcic amphiboles 0 2 vol have been identified in clinopyroxenes of eclogites from the weihai area sulu ultrahigh pressure terrane eastern china electron backscatter diffraction ebsd analyses demonstrate that the majority 97 of quartz precipitates have topotactic relationships with their host clinopyroxenes three types of crystallographic topotactic relationship have been identified between quartz and host clinopyroxene 1 52 quartz precipitates share the same orientation for the c axes with 0001 qz 001 cpx 2 34 quartz precipitates share the same orientation for the a axes with 112 0 qz 001 cpx and 3 11 quartz precipitates share the same orientation for the s planes with 112 1 qz 100 cpx other quartz axes and planes disperse in large or small girdles around the shared axes or planes many quartz rods needles are elongated parallel to the 001 cpx with the long axes of quartz being either 0001 qz or 112 0 qz amphibole precipitates have also a strong crystallographic relationship with host clinopyroxene i e 100 amp 100 cpx 010 amp 010 cpx and 001 amp 001 cpx these results provide quantitative microstructural evidence supporting an exsolution origin for oriented quartz needles rods in clinopyroxene and demonstrate that the exsolution of quartz from clinopyroxene occurred within the stability field of quartz rather than coesite the oriented precipitates of quartz in part coexisting with calcic amphiboles in host clinopyroxene are probably promoted by supercritical fluid or partial melting during the early exhumation of eclogites our results suggest that oriented quartz precipitates in clinopyroxene cannot be used as an indisputable uhp indicator

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

the dilemma of asymmetric porphyroclast systems and sense of shear abstract asymmetric porphyroclast systems have long been used as independent shear sense indicators based on their clast tail asymmetry and or stair stepping tail geometry over the last two decades advances in exploring the mechanical behavior of rigid clasts in a viscous flow has shed new light on understanding the formation of porphyroclast systems embedded in noncoaxial viscous flow and subsequently quantifying many aspects of flow kinematics using examples from naturally deformed rocks combined with results from recent advances our study indicates that 1 in general only porphyroclast systems composed of equant shaped clasts can be treated as independent shear sense indicators 2 special attention should be given to type porphyroclast systems composed of high angle intermediate aspect ratio clasts which may contain meaningful information on bulk flow kinematics 3 bookshelf structures cannot serve as independent shear sense indicators multi criteria and thorough three dimensional shape analyses are suggested for a proper interpretation of flow kinematics in ductile shear zones

c slip in quartz from subsolidus deformed granite abstract the slip behaviour of quartz from a granite and granitic veins during subsolidus high temperature conditions 700 800 c of deformation is studied by lattice preferred orientation lpo and microstructural techniques lpos with a point maximum of c axes parallel to the shear direction which is close to the stretching lineation are presented and interpreted with the aid of light and electron microscopy in terms of c as the dominant slip direction during plastic deformation microscopy of such samples reveals the presence of 0001 tilt boundaries and free 0001 burgers vector dislocations in edge orientations a change in lpo in a decreasing temperature environment from c axes parallel to the shear direction to c axes normal to the shear direction documents a fabric transition from the rare c slip to the more common a slip fabric it is concluded that conditions of high temperature and partial pressure of water are required to initiate dominant c slip during deformation at geological strain rates

cassiterite u pb and muscovite40ar 39ar age constraints on the timing of mineralization in the xuebaoding sn w be deposit western china abstract the xuebaoding deposit in the songpan garze orogenic belt western china spatially associated with the xuebaoding granitic complex is a unique pegmatite type sn w be deposit within the giant songpan garze polymetallic belt a lack of precise geochronological constraints on the timing of mineralization for this deposit has precluded definitive statements relating to a genetic link between ore formation and granitoid intrusion up to now to address the temporal relationship between the emplacement of granite and the spatially associated sn w be polymetallic mineralization in the xuebaoding deposit systematic cassiterite u pb and muscovite ar ar dating were undertaken u pb data of cassiterite analyzed by in situ la mc icp ms provided an isochron age of 193 6 6 ma at 95 confidence level mswd 4 the 40 ar 39 ar dating on the muscovite intergrown with the cassiterite sample yielded a plateau age of 194 50 1 02 ma mswd 1 26 and an inverse isochron age of 194 53 1 05 ma mswd 1 34 both the ar ar dates overlap the u pb date within uncertainties placing the timing of tin mineralization at 193 194 ma this is consistent with field geological relationships that show veins cutting marble and granite and with previous sm nd dating within experimental error for scheelite and indicates that sn w polymetallic mineralization at the xuebaoding deposit occurred during the early jurassic combined with previous published trace element and ree data from the xuebaoding granites and other ore minerals as well as the timing of granite emplacement the temporal and genetic associations between granite intrusion events and sn w mineralization of this deposit have been discussed in this paper the results suggest that the pankou granite is genetically related to the polymetallic mineralization in xuebaoding sn w be deposit and further demonstrate that the latest phase of granitic intrusion is commonly favorable for sn and or w mineralization although the pukou granite also exhibits close relationships with the ore formation in its spatial distribution and element characteristics the possibility as a parent for this deposit is excluded duo to the time span from its emplacement to mineralization event that exceed the estimated life of modern geothermal systems

the relationship between ductile shear zone and mineralization in the jiufeng sn deposit northern guangxi south china evidence from structural analysis and cassiterite u pb dating abstract the jiufeng sn deposit is located in the outer contact zone southwest of the neoproterozoic motianling granitic pluton in northern guangxi south china the deposit is characterized by a nne striking 30 200 m wide ductile shear zone named the jiufeng ductile shear zone majority of the sn orebodies including four large ones are hosted by the shear zone field and microstructural observations electron backscatter diffraction measurements and magnetic fabric analyses show that the shear zone is characterized by sinistral thrust shearing the shear zone has also slightly modified the sn orebodies quartz in the shear zone exhibits core mantle structure and undulose extinction quartz has experienced significant sub grain rotation and grain boundary migration recrystallization lattice preferred orientation analysis reveals primarily basal rhomb and prism slips cassiterite grains in the shear zone however demonstrate brittle deformation therefore the deformation temperature during the ductile shearing is estimated lower than 650 c and below u pb closure temperature of cassiterite as such the u pb isotopic composition of the cassiterite was not disturbed by the shearing and the u pb age of the cassiterite could be used for the age of the sn mineralization la icp ms u pb dating of cassiterite grains selected from a cassiterite bearing tourmalinite sample performed in this study yielded a tera wasserburg u pb lower intercept age of 832 3 5 3 ma this age is consistent within error with that of the neighboring neoproterozoic motianling granitic pluton which suggests a close genetic relationship between the sn mineralization and the granitic magmatism studies show that the jiufeng ductile shear zone was produced by a widespread caledonian or kwangsian tectonic event in south china indicating that the shearing was much later than the sn mineralization and has no generic relationship to the sn deposit

the single slip hypothesis revisited crystal preferred orientations of sheared quartz aggregates with increasing strain in nature and numerical simulation this study discusses the fabric development in naturally sheared quartz aggregates in comparison to results from texture modeling according to the polycrystalline plasticity theory with particular emphasis on the formation of a single c axis maximum the investigated natural shear zone samples were deformed at about 650 50 c with increasing strain up to 14 and show dynamic recrystallization microstructures of grain boundary migration recrystallization neutron diffraction texture analysis results in c axis pole figures with a single maximum at the periphery of the pole figure this maximum does not align with the shear plane normal towards higher strain but rotates towards an inclined orientation in accordance with the sense of shear such a rotation is inconsistent with the single slip hypothesis and suggests that the formation of this c axis pattern is controlled by multi slip on several slip systems based on the polycrystalline plasticity theory this quartz fabric can develop if combined view the mathml source r view the mathml source z and view the mathml source prism slip dominates and must not be related to the commonly proposed view the mathml source basal slip the multi slip texture development is in agreement with the shear sense interpretation from the asymmetry between well defined quartz fabrics and the foliation for dominant view the mathml source basal slip in quartz and 2 numerical simulations predict a single peripheral maximum perpendicular to the shear plane and two a maxima with a 30 inward position parallel to the shear plane this simulation corresponds to naturally observed cpo patterns of quartz formed at different deformation conditions and it is in agreement with the single slip hypothesis hence our combined natural and numerical data suggest that the single slip hypothesis is a possible explanation for a single c 0001 maximum but not universally true

spatial temporal analysis of a seismic series using the principal components method the antequera series spain 1989 a method for the characterization of a seismically active zone from a distribution of hypocenters is presented this method is based on principal components analysis a powerful multivariant statistical technique that is used to find the rupture local ellipsoid rle the ellipsoid is a planar structure with which two variations of the method are developed the spatial principal components analysis and the spatial temporal principal components analysis using these methods it is possible to find the dominating tendencies in the fracturing of the seismically active volume as well as the temporal evolution of the process the methodology developed has been applied to a series of earthquakes that occurred near antequera spain with the result that the main fracture series trends n70 80 e moreover the temporal evolution of the system from the most relevant rle has been characterized

spatial analysis of the la paca se spain 2005 seismic series through the relative location of multiplets and principal component analysis abstract on 29 january 2005 a m w 4 8 i max vi vii earthquake occurred near the village of la paca se spain the aftershock sequence shows distinct heterogeneity manifested in a non uniform temporal decay of activity and different source mechanisms for the main shock and the largest aftershocks we analysed 262 earthquakes of the seismic series in order to characterize the active seismic structures moving window cross correlation of the p and s waveforms recorded at local broad band stations and the application of a selection algorithm have allowed to identify events with similar waveforms multiplets and to classify them into 16 clusters of 3 25 earthquakes each in each multiplet cluster events have been relocated with respect to a master event by the linear inversion of cross correlation time lags for p and s waves clusters show epicentral alignments with a predominant direction n145 e and two secondary directions n6 e and n40 e spatio temporal patterns of the seismicity have been analysed using principal component analysis pca upon the relocated epicentre distribution and we have obtained three trends n42 e n14 e and n145 e present during all the sequence these directions inferred from relative location and pca are coincident with nodal planes of moment tensor solutions for the larger events within the series they can be interpreted as the simultaneous activity of conjugate strike slip faults under a n160 170 e orientation of the principal compressive stress 1

a precise u pb age on cassiterite from the xianghualing tin polymetallic deposit hunan south china we report the first precise u pb isotope data on cassiterite from the large xianghualing tin polymetallic deposit in the central nanling district south china the results show that four separates from sample xf 51 have a relatively narrow range of 206pb 238u apparent ages varying from 152 to 157 ma and the three 206pb 238u apparent ages yield a weighted average value of 156 4 ma mswd 0 32 separates from two other cassiterite samples do not have sufficient radiogenic pb to generate a reliable 206pb 238u age seven separates from the above three cassiterite samples define a well constrained 238u 206pb isochron corresponding to an age of 157 6 ma mswd 34 a comparison of the u pb cassiterite ages with published ar ar dates on muscovite from this deposit and k ar age data on biotite from the pluton genetically related to the tin mineralization in this area demonstrates that the u pb isotope system of cassiterite is a potential geochronometer combined with the ar ar dates of muscovite from this deposit we can constrain the absolute age of tin polymetallic mineralization in xianghualing at 154 157 ma the dates obtained in this study consistent with the published geochronological results from other important deposits in this region reveal that the large scale tungsten tin mineralization in the central nanling region was predominantly emplaced during 150 161 ma

the 1998 papua new guinea earthquake and its fault plane estimated from relocated aftershocks the 1998 papua new guinea earthquake of m w 7 0 occurred near the wewak trench where the north bismarck plate is subducting beneath the australian plate its mechanism is thrust type and one of the nodal planes is almost parallel to the plate interface to determine which of the two nodal planes of the main shock is the fault plane we relocated the main shock and aftershocks using a method of modified joint hypocenter determination we combined and employed two types of data in this study firstly we used data reported by the national earthquake information center neic of the u s geological survey usgs which includes three stations at the northeastern edge of irian jaya and one station in northern papua new guinea from which the epicentral distances are less than 2 degrees secondly in addition to the above permanent station data we used data from temporary aftershock observations near the epicentral area around the sissano lagoon carried out by tsuji et al 1998 using three component seismometers they carried out observations from august 2 to october 2 1998 at three sites although the network did not record the main shock and immediate aftershocks the data obtained by temporary observation sites can clearly assist in identifying their absolute locations since it is possible to apply the joint hypocenter determination jhd method hypocenters were relocated between the coastline and the wewak trench distributed along a nodal plane dipping shallowly to the southwest therefore we can conclude that this nodal plane is the main shock fault and that the 1998 papua new guinea earthquake was an interplate earthquake between the north bismarck and australian plates

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

dating multiply overprinted sn mineralized granites examples from the erzgebirge germany granites and primary tin mineralization in the erzgebirge were dated using 1 conventional u pb dating of uraninite inclusions in mica 2 rb sr dating of inclusions in quartz that represent highly evolved melts 3 re os dating of magmatic hydrothermal molybdenite and 4 chemical th u pb dating of uraninite conventional isotope dilution and thermal ion mass spectrometry and chemical th u pb dating of uraninite in granites from the ehrenfriedersdorf mining district provide ages of 323 9 3 5 ma 2 greifenstein granite and 320 6 1 9 and 319 7 3 4 ma 2 both sauberg mine in agreement with u pb apatite ages of 323 9 2 9 and 317 3 1 6 ms 2 both sauberg mine rb sr analysis of melt inclusions from zinnwald gives highly radiogenic sr isotopic compositions that with an assumed initial sr isotopic composition permit calculation of precise ages from single inclusions the scatter of the data indicates that some quartz hosted melt inclusions have been affected by partial loss of fluid exsolved from the melt inclusion re os dating of two molybdenite samples from altenberg provides ages of 323 9 2 5 and 317 9 2 4 ma 2 together with age data from the literature our new ages demonstrate that primary tin mineralization and the emplacement of the large sn specialized granites in the erzgebirge fall in a narrow range between 318 and 323 ma primary sn mineralization occurred within a short interval during post collisional collapse of the variscan orogen and was essentially synchronous over the entire erzgebirge in contrast to earlier claims no systematic age difference between granites of the eastern and western erzgebirge was established furthermore our data do not support a large age range for late variscan granites of the erzgebirge 330 290 ma as has been previously suggested

trace and rare earth elements in cassiterite sources of components for the tin deposits of the mole granite australia trace and rare earth elements have been determined for cassiterite from deposits associated with the mole granite and hosted by granite metasediments and metavolcanics the ree of cassiterite is controlled by the ree of the the ore fluid and the rocks through which this fluid circulated the ree distribution factor and lree hree value of cassiterite is strongly influenced by the associated mineral assemblage the fluid chemistry and the crystal chemical characteristics of the host mineral cassiterite from deposits hosted by granite have trace and rare earth element characteristics similar to those determined for the mole granite cassiterite from deposits hosted by metasediments or acid volcanics have most trace and rare earth element characteristics similar to those of the enclosing rocks and some characteristics similar to the mole granite the ore fluid had chemical components derived from the parental granite and components acquired by passage through the metamorphosed aureole

role of microfabrics and magnetic fabrics in the tectonic evolution of the achankovil shear zone south india abstract achankovil shear zone a major crustal discontinuity in south india which extends for about 120 km in a nw se direction separates the khondalite belt from the terrain of massive charnockites in the north progressive mylonitisation and shear fabrics corresponding to a progression in strain as displayed by grain size reduction flattening elongation and dimensional preferred orientation of constituent minerals characterize the shear zone the present work incorporates the results of petrofabric and magnetic susceptibility analyses with a view to interpret the kinematic and microstructural evolution of the shear zone ebsd derived lpo suggests plastic deformation through the activation of intracrystalline slip systems and modifications in the fabric due to the reactivation magnetic fabric data show significant correlation with the field and lpo fabrics reflecting the regional strain kinematics and fabric development the fabrics and kinematic indicators on meso and microscopic scales reveal dominant dextral shear sense with a minor component of sinistral shearing in micro domains due to strain partitioning and or reactivation of the shear zone

seismic sources on the iberia african plate boundary and their tectonic implications the plate boundary between iberia and africa has been studied using data on seismicity and focal mechanisms the region has been divided into three areas a the gulf of cadiz b the betics alboran sea and northern morocco and c algeria seismicity shows a complex behavior large shallow earthquakes h 30 km occur in areas a and c and moderate shocks in area b intermediate depth activity 30 h 150 km is located in area b the depth earthquakes h 650 km are located to the south of granada moment rate slip velocity andhvalues have been estimated for shallow shocks and show similar characteristics for the gulf of cadiz and algeria and quite different ones for the central region focal mechanisms of 80 selected shallow earthquakes 8 mb 4 show thrust faulting in the gulf of cadiz and algeria with horizontal nnw sse compression and normal faulting in the alboran sea with e w extension focal mechanisms of 26 intermediate depth earthquakes in the alboran sea display vertical motions with a predominant plane trending e w solutions for very deep shocks correspond to vertical dip slip along n s trends frohlich diagrams and seismic moment tensors show different behavior in the gulf of cadiz betic alboran sea and northern morocco and northern algeria for shallow events the stress pattern of intermediate depth and very deep earthquakes has different directions vertical extension in the nw se direction for intermediate depth earthquakes and tension and pressure axes dipping about 45 o for very deep earthquakes regional stress pattern may result from the collision between the african plate and iberia with extension and subduction of lithospheric material in the alboran sea at intermediate depth the very deep seismicity may be correlated with older subduction processes

spatial characterization of agron southern spain 1988 1989 seismic series abstract during november 1988 to april 1989 a seismic swarm of about 400 earthquakes of magnitude between 0 4 and 4 0 occurred near the village of agron andalusia southern spain the pattern of the energy release versus time clearly shows that the dataset is representative of a seismic swarm where it is not observable a typical mainshock aftershock sequence basing on the gutenberg richter relationship the catalogue is considered complete down to a magnitude of 1 we revise the dataset by selecting those digital data which were recorded by at least four stations the revised dataset is relocated using a probabilistic approach based on a bayesian formalism the epicenters are barely aligned along two main structures trending nne ssw nw se and wnw ese respectively the focii depth is confined in the 5 20 km interval application of a clustering technique based on the principal component analysis pca of the local rupture ellipsoids depicts a spatial clustering of the seismicity according to five main planes of which only one coincides with the horizontal alignment of epicenters cross correlation among waveforms allows to individuate five different groups of similar events multiplets high resolution estimate of time differences among events belonging to the same multiplet and least squares adjustment of arrival times allow for precise relative locations from which we derive the corresponding rupture planes focal mechanisms are calculated for three of the five multiplets for these cases the planes obtained from the relative locations are coincident with one of the fault plane solutions the results are consistent with those previously inferred from pca and indicate a high variability in the spatial setting of fault planes over a short distance range these observations are consistent with a fragmentation of the upper crust into small 2 4 km tectonic blocks and indicate an heterogenous stress regime with different stresses acting simultaneously in adjacent areas

high precision relative locations of two microearthquake clusters in southeastern sicily italy in november 1999 and january 2000 two microearthquake swarms occurred in southeastern sicily italy they were analytically located in the depth range 17 25 km some kilometers northward from the buried front of a regional foredeep below the active thrust zone of the sicily mountain chain their hypocentral distribution showed two distinct clusters and comparison of the waveforms revealed clearly that the two swarms formed two distinct families of multiplet events this led us to 1 carry out a precise relocation relative to two chosen master events of the families and 2 better define the geometrical structure of the two clusters the cross spectral method was applied to obtain precise readings of the wave onsets sh wave onsets were used instead of p waves as they showed clearer onsets and a good signal to noise ratio residuals of the relative locations showed small values no more than several meters on average the vertical extent of the two relocated clusters was 500 and 250 m respectively while the horizontal extent was 250 m hypocenters of the first cluster clearly delineate a north northwest trending plane with almost vertical dip matching one nodal plane of the focal mechanism obtained as a composite solution of all events of the cluster given the considerable gap angles because of unfavorable network geometry with respect to the events the stability of our results was tested carrying out a monte carlo experiment varying the onset times randomly in the range of 5 msec a dispersion of the locations less than 10 m in longitude and less than 50 m both in latitude and depth was found similar results were obtained when comparing relocations carried out with different master events thus the overall geometrical characteristics of the clusters were not affected seriously by random errors considering the geostructural framework of the region together with the location and time evolution of the two clusters fluids of plutonic origin are suggested as the trigger mechanism manuscript received 26 june 2002

high precision locations of multiplets on south eastern flank of mt etna italy reconstruction of fault plane geometry abstract on 9 january 2001 a seismic swarm located on the south eastern flank of mt etna and with nearly identical waveforms caused some damage to zafferana etnea village 3 km from the epicentral area an analysis of the seismicity occurring in the last 8 years in this area has revealed other earthquakes with the same characteristics some pre empted and followed up to a few months the 2001 january swarm others were recorded more than five years beforehand using similarity of waveforms these earthquakes were classified into three families the use of a multiplet technique has allowed to obtain the spatial distribution of the events with higher precision mean error of 10 20 m with respect to traditional localization techniques mt etna earthquakes relocation clearly describes the geometry of the seismogenic tectonic structure the hypocenters lie on a ne sw oriented plane that is coincident with one of the focal planes obtained by first arrival polarities this alignment is also coherent with one of the main regional tectonic trends cutting the mt etna area and can be interpreted as a right lateral strike seismic source on the south eastern flank of mt etna distant from eruptive centres which repeats from time to time and is able to produce strong energy releases

neoproterozoic tin mineralization in south china geology and cassiterite u pb age of the baotan tin deposit in northern guangxi the baotan tin deposit 23 mt 0 43 sn is located in the jiuwandashan yuanbaoshan area south china it is hosted in neoproterozoic mafic metasedimentary rocks and apical portions of the pinying granite pluton six alteration and mineralization stages have been identified pre ore alteration cassiterite greisen cassiterite tourmaline quartz vein cassiterite quartz vein cassiterite sulfide vein and post ore quartz calcite quartz vein stages tin mineralization is mainly in the cassiterite greisen cassiterite tourmaline quartz and cassiterite quartz vein stages the deposit is characterized by widespread tourmalinization both pre ore and ore stage tourmaline is schorl tourmaline from pre ore tourmaline quartz nodules has elevated al2o3 and f contents and fe fe mg and na na ca ratios which are probably controlled by the initial magmatic fluid ore stage tourmaline shows low al2o3 and f contents and fe fe mg and na na ca ratios which are likely influenced by the surrounding mafic rocks la icp ms u pb dating on two cassiterite samples from disseminated cassiterite tourmaline quartz ore and cassiterite quartz vein yields 206pb 238u weighted mean ages of 832 5 ma and 834 4 ma 2 respectively these two dates are consistent with the previously reported zircon u pb ages of 834 835 ma for the pingying granite which indicates that tin mineralization is related to the granite the granite has low magnetic susceptibility and zircon ce4 ce3 ratios which are similar to those of sn bearing ilmenite series granites our study confirms the neoproterozoic tin mineralization event in south china and indicates that the neoproterozoic highly fractionated s type granites in the southeastern margin of yangtze block have a great potential for tin mineralization

comment on the usage of 238u 207pb vs 206pb 207pb linear regressions for the la icpms u pb dating of cassiterite

early paleozoic tectonic reactivation of the shaoxing jiangshan fault zone structural and geochronological constraints from the chencai domain south china abstract the shaoxing jiangshan fault zone sjfz as a fundamental neoproterozoic block boundary that separates the yangtze block from the cathaysia block is the key to understanding the evolution of south china from neoproterozoic block amalgamation to early paleozoic crustal reworking new structural observations coupled with geochronological ages from the chencai domain indicate that intense ductile deformation and metamorphism along the sjfz occurred at 460 420 ma in response to the early paleozoic orogeny in south china to the east of the sjfz the deformation involves widespread generations of ne striking foliation intrafolial folds and local development of sinistral oblique shear zones the shearing deformation occurred under amphibolite facies conditions at temperatures of 550 c locally even 650 c to the west of the sjfz the deformation corresponds to sinistral oblique shearing along ne striking steep dipping zones under greenschist facies conditions at temperatures of 400 500 c these deformation styles as typical mid crustal expressions of continental reworking reflect tectonic reactivation of the pre existing deeply rooted neoproterozoic block boundary in the early paleozoic we infer that the tectonic reactivation possibly induced by oblique underthrusting of north cathaysia facilitated ductile shearing and burial metamorphic reactions giving rise to the high strain zones and high grade metamorphic rocks with respect to pre existing mechanical weakness our work highlights the role of tectonic reactivation of early structures in localizing later deformation before it propagates into yet undeformed domains

a new estimate of the conditions for transition from basal a to prism c slip in naturally deformed quartz abstract quartz c axis fabrics of naturally deformed quartz in metacherts from the ryoke metamorphic belt in the yanai district southwestern japan suggest that the prism c prism a and basal a slip systems were activated during the plastic deformation under high temperature metamorphic conditions with increasing temperature quartz c axis fabric patterns change from girdle type through x maximum with a faint crossed girdle to x maximum the dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature closely correlated with the change in quartz c axis fabric the fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size moreover observations with transmission electron microscopy tem clearly identify the dominant activation of c slip through the presence of basal subgrain boundaries and free dislocations with the 0001 burgers vector in the quartz from the metachert samples exhibiting x maximum c axis fabric based on these observations it is concluded that the mechanism switch from basal a to prism c slip systems occurred with increasing temperature in the metacherts the transition temperature is petrologically estimated to be ca 550 600 c at the natural strain rate

the link between strength of lattice preferred orientation second phase content and grain boundary migration a case study from the alpine fault zone new zealand abstract we analyse the microstructure and quartz lpos of 36 layers of varying composition from a several meter thick sequence of amphibolite facies metacherts and related mica garnet plagioclase quartz bearing schists from the central southern alps in the mylonite zone related to the alpine fault quartz contents vary from 10 to 100 and all of the lpo fabric skeletons are similar featuring an asymmetric single girdle of c axes inclined 30 away from the zy plane lpo strength is typically low at quartz contents

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

mathematical model relationship between the paramagnetic anisotropy and strain in slates abstract a simple mathematical model is used to investigate the relationship between the paramagnetic anisotropy and strain in slates the relationship found is similar to the relationship of whole rock magnetic anisotropy to strain in weakly magnetic slates the anisotropy is probably mostly due to paramagnetic phyllosilicates and the whole rock relationship reflects the paramagnetic fraction

precise and accurate in situ pb pb dating of apatite monazite and sphene by laser ablation multiple collector icp ms to evaluate in situ pb dating by laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icp ms we analysed apatite sphene and monazite from paleoproterozoic metamorphic rocks from west greenland pb isotope ratios were also determined in the national institute of standards and technology nist 610 glass standard and were corrected for mass fractionation by reference to the measured thallium isotope ratio the nist 610 glass was used to monitor pb isotope mass fractionation in the low tl pb accessory minerals replicate analyses of the glass 1 to 2 min yielded ratios with an external reproducibility comparable to conventional analyses of standard reference material 981 by thermal ionisation mass spectrometry tims mineral grains were generally analysed with a 100 m laser beam although some monazite crystals were analysed at smaller spot sizes 10 and 25 m the common pb isotope ratios required for age calculations were either measured on coexisting plagioclase by la mc icp ms or could be ignored as individual crystals exhibit sufficient pb isotopic heterogeneity to perform isochron calculations on replicate analyses of single crystals mean mineral ages with the 204pb ion beam measured in the multiplier were as follows apatite 1715 23 m y sphene 1789 11 m y and monazite 1783 to 1888 m y with relative uncertainties on individual monazite ages of 0 2 but highly reproducible age determinations on single monazite crystals 1 isochron ages calculated from several mineral analyses without assumption of common pb also yield precise age determinations apatite and monazite pb ages determined by in situ pb isotope analysis are identical to those determined by conventional tims analysis of bulk mineral separates and the analytical uncertainties of these short laser analyses with no prior mechanical or chemical separation are comparable to those obtained by tims detailed examination of the sphene in situ age data does however show a small discrepancy between the la mc icp ms and tims ages 1 younger high resolution mass scans of the sphene during ablation clearly showed several small and as yet unidentified isobaric interferences that overlap with the 207pb peak at the resolution conditions for measurement of isotope ratios these might account for the age discrepancy between the la mc icp ms and tims sphene ages la mc icp ms is a rapid accurate and precise method for in situ determination of pb isotope ratios that can be used for geochronological studies in a manner similar to an ion microprobe albeit currently at a somewhat degraded spatial resolution further modifications to the la mc icp ms system such as improved sensitivity ion transmission and la methodology may lead to this type of instrument becoming the method of choice for many types of in situ pb isotope dating

controversial pb pb and sm nd isotope results in the early archean isua west greenland oxide iron formation preservation of primary signatures versus secondary disturbances pb stepwise leaching pbsl determinations on two magnetite enriched fractions of a bif sample from the northeastern part of the isua supracrustal belt west greenland yield an isochron of 3691 22 ma mswd 0 4 in combination with previously published geochronological constraints for a minimum deposition age of 3 71 ga for volcanic sequences nutman et al 1997 comprising the studied bifs and supported by microtextural observations this demonstrates an early archean amphibolite facies metamorphic event in the supracrustal pbsl data on magnetite from slightly discordant veins within the same iron stone together with bulk pb isotope values of paragenetically late pyrite yield a 3 63 0 07 ga mswd 8 6 errorchron with significantly different pb isotopic compositions from those obtained from the main magnetite layers and this suggests that the discordant layers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later the fluids controlling this redistribution of magnetite can be tentatively correlated with metasomatic alteration features produced during the widespread intrusion of 3 6 ga granitic into 3 7 ga tonalitic amitsoq gneisses enclosing the belt tremolite rich layers in the bif are characterized by sm nd ratios close to chondritic end 3800 of 1 7 and 2 1 within error of published results from adjacent basic units in the supracrustal suite in contrast the magnetite dominated layers yield unrealistically high end 3800 of 14 8 and 14 4 indicative of sm nd ratios resembling ree fractionated continental sources these high end 3800 together with radiogenic sr leached from the magnetite enriched separates is ascribed to secondary hydroxyapatite which predominantly forms as crystal overgrowths in the magnetite rich bands the timing of the hydrothermal event during which apatite was deposited within the bif remains uncertain but a tchur model age of 1 85 ga from the apatite dominated hcl leachate may point to a close genetic relationship with local proterozoic metamorphism and granite formation by crustal remelting in the isukasia area step leaching of magnetite from a similar silicate oxide facies bif from the western part of the isua supracrustal belt yield a pb pb isochron age of 2 84 0 05 ga mswd 1 43 the agreement between the pbsl age and previously published field and isotopic evidence for a major late archean metamorphic event affecting the western area suggests there was widespread equilibration accompanying the intrusion of the 2 75 2 83 ga granodioritic ikkattoq gneisses west and southwest of the western limb of the isua supracrustal belt we argue that the pbsl isochron ages date the peak of amphibolite facies metamorphic events which respectively affected the eastern and western sections of the isua supracrustal belt during the early and late archean our results give additional support to the suggestion that the scatter on published end t values from the isua supracrustal belt and adjoining gneisses can be assigned to post formational hydrothermal processes and underline the need for care in the interpretation of sm nd bulk data from polymetamorphic rocks to constrain isotopic models of early earth s evolution

strain indicators and magnetic fabric in intraplate fault zones case study of daroca thrust iberian chain spain abstract anisotropy of magnetic susceptibility ams has been applied to the study of shallow fault zones although interpretation of the results requires establishing clear relationships between petrofabric and magnetic features magnetic behaviour of fault rocks and an extensive knowledge of p t conditions in this work we demonstrate that magnetic methods can be applied to the study of heterogeneous fault zones provided that a series of requisites are met a major fault zone within the iberian plate daroca thrust showing transpressional movements during cenozoic time was chosen for this purpose because of the exceptional outcrops of fault gouge and microbreccia and its relevance within the context of the northeastern iberian plate magnetic fabrics were analysed and the results were compared with foliation and s c structures measured within the fault zone clay mineral assemblages suggest maximum burial depths shallower than 2 km

rupture geometry from high precision relative hypocentre locations of microearthquake clusters summary in 1987 two microearthquake sequences with a duration of about a week each and consisting of 37 and 46 events respectively occurred in the upper crust below the jura mountains of northern switzerland the seismograms within each sequence exhibit a high degree of similarity indicating tight clustering of hypocentres and similar focal mechanisms using a cross correlation technique applied to the seismograms in the time domain together with a new least squares adjustment procedure it was possible to determine relative hypocentre locations of the earthquakes in each cluster with a precision of a few tens of metres the results show that hypocentres of events with the same focal mechanism lie on a plane which coincides exactly with one of the nodal planes of the fault plane solution and that consequently swarm like sequences of similar earthquakes are due to repeated slip on the same fault in one case slip occurred as right lateral motion on a steeply dipping plane striking in wnw ese direction whereas the other case corresponds to left lateral slip on two almost vertical planes striking roughly n s in 1988 an additional earthquake triplet occurred on a wnw ese oriented normal fault nearby all three mechanisms are consistent with a general nnw sse oriented direction of maximum crustal shortening and a corresponding wsw ene oriented extension repeated slip on the same fault is indicative of a large degree of heterogeneity and of short term temporal variability of both frictional resistance and stress distribution on the fault an explanation for the occurrence of such swarm like seismic activity in terms of barriers or asperities would require that shear stress on the unbroken patches increase from one event to the next since however the overall shear stress would be expected to decrease as a consequence of the stress released in each event a more plausible mechanism involves pore pressure fluctuations caused by fluids under suprahydrostatic pressures migrating upward through pre existing zones of weakness in the crust

neoproterozoic mineralization in a hydrothermal cassiterite sulfide deposit at jiumao northern guangxi south china mineral scale constraints on metal origins and ore forming processes abstract most tin deposits worldwide are inferred to have formed from fluids derived from evolved granitic magma systems neoproterozoic silicate and sulfide stage sn deposits at jiumao of south china in contrast have been thought to be derived from metasedimentary and mafic ultramafic host rocks the sn deposit may thus have formed by components and processes distinct from those that commonly contribute to sn deposits although spatially associated greisen hosted sn deposits also occur to characterize in detail the formation of the jiumao ore deposits which contain 28 000 t of sn we present new field and petrographic observations major and trace element compositions for silicate and ore minerals and u pb age data of cassiterites and zircons for the deposits and granites our research shows that 1 the granite greisen system formed at 830 ma which is the earliest sn mineralization event in south china 2 the silicate and sulfide stage ores in the country rocks formed at an fo2 of nno with temperatures of 350 c and 170 c respectively 3 all vein type sn ores at jiumao were formed by sn f b rich fluids that were derived from the yuanbaoshan granite magma system rather than ultramafic rocks 4 the fe rich character of the ores hosted by the metasedimentary rocks and the mg rich character of the ores hosted by the ultramafic rocks largely controlled the reactions and efficacy of cassiterite precipitation and thus ore grade we highlight that differences in mineral assemblages largely relate to differences of the protolith compositions and fluid rock reactions e g b and tourmaline deposition in the ores hosted by the metasedimentary rocks but not in the ores hosted by the ultramafic rocks cassiterite and titanite trace element compositions reflect fluid composition protolith composition and intensive parameters of crystallization

cassiterite potential for direct dating of mineral deposits and a precise age for the bushveld complex granites u pb and pb isotopic measurements of cassiterite from the 200 ma tin deposits of belitung indonesia and 2100 ma zaaiplaats deposit in south africa demonstrate that cassiterite can be used to directly date mineral deposits rather than associated minerals such as zircon and rutile a precise u pb age of 2099 3 ma has been determined for cassiterites hosted in granites from the bushveld complex on conventional 207 pb 204 pb 206 pb 204 pb plots the data for the indonesian deposits give an age apparently older than that obtained from u pb plots perhaps indicative of inheritance of pb from an older source

faulting parameters of the 1999 mula earthquake southeastern spain abstract the seismicity of southern spain is characterized by low to moderate earthquake activity magnitudes

study of the damaging earthquakes of 1911 1999 and 2002 in the murcia southeastern spain region seismotectonic and seismic risk implications a detailed study of four earthquakes that occurred in the murcia region southeastern spain in 1911 1999 and 2002 has been carried out new intensity maps have been plotted for the march and april 1911 shocks these show maximum values of vii viii ems we have found values of vi and v respectively for the 1999 and 2002 earthquakes surface wave magnitudes range from 5 2 for the bullas 2002 event to 5 7 for the march 1911 event focal mechanisms for the mula 1999 and bullas 2002 events indicate reverse and strike slip motions with scalar seismic moments of 5 9 10 16 n m and 8 6 10 15 n m respectively and focal dimensions of 4 0 and 1 5 km both earthquakes were recorded at epicentral distances greater than 21 km showing low values of peak ground acceleration pga 0 020 g these records involve a ground motion less than the ones expected in the region according to different hazard studies the seismic hazard map that we obtained for the murcia region for a return period of 475 years shows higher values than those given in the spanish building code ncse 02 2002 e g 0 24 g for pga in the city of murcia compared with 0 16 g in the code

shearing of magma along a high grade shear zone evolution of microstructures during the transition from magmatic to solid state flow abstract syntectonic plutons may record short lived geological events related to crustal melting and deformation of the continental crust therefore interpretation of microstructure and orientation of fabrics is critical in order to constrain space time temperature deformation relationships during pluton crystallization here we describe the transition from magmatic to solid state deformation in the late variscan diorite granite suite dgs emplaced along the santa lucia shear zone the systematic collection of meso microstructural and quartz axis data allow us to examine the spatial distribution and the mode of superposition of different fabrics we identify three magmatic microfabric types thought to reflect the microstructural evolution at decreasing melt content during pluton crystallization our data suggest that diffusion creep dislocation creep and grain scale fracturing cooperated in accommodating the shearing of the partially molten quartzofeldspathic aggregate we suggest that the switch from upward to horizontal magmatic flow occurred at melt fractions gt 0 40 and that most of the hypersolidus fabrics formed during horizontal flow reflecting the stress field imposed by the shear zone and preserving no memory of the ascent stage

evolution of c axis pole figures and grain size during dynamic recrystallization results from experimentally sheared quartzite 1 quartzite samples experimentally sheared at conditions where subgrain rotation and grain boundary migration recrystallization are active show a striking change in the c axis crystallographic preferred orientation cpo with increasing shear strain up to 8 and degree of recrystallization up to 100 we used optical methods to determine the c axis cpo for the bulk samples and for porphyroclasts and recrystallized grains separately and to track the strengths of the cpos in five different orientation domains with increasing the c axis pole figure evolves from a broad peripheral maximum indicative of basal a slip to an inclined single girdle with two maxima indicative of rhomb a slip and finally an elongate single maximum at the girdle center indicative of prism a slip throughout this sequence the fabric skeleton rotates with the sense of shear the bulk preferred c axis orientation results from growth and shrinkage of differently oriented crystallographic domains grain boundary density analysis indicates that the size of prism a recrystallized grains is 1 5 times that of average and 2 times that of rhomb grains indicating that prism a slip is significantly easier at our experimental conditions this inference is supported by autocorrelation function analysis of the various orientation domains and how they evolve with strain similar progressive changes in c axis cpo are observed in quartzites naturally sheared at conditions where grain boundary migration recrystallization operates these results indicate that achievement of microstructural and mechanical steady state requires high strain and complete recrystallization

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

constraints of in situ zircon and cassiterite u pb molybdenite re os and muscovite 40ar 39ar ages on multiple generations of granitic magmatism and related w sn mineralization in the wangxianling area nanling range south china abstract the nanling range in south china hosts numerous w sn rare metal deposits associated with several cycles of polygenetic granitic intrusions in the wangxianling area of the middle nanling range igneous intrusions include the wangxianling and hehuaping granite plutons and more than 20 granite porphyry dykes the wangxianling pluton has a central phase of medium grained tourmaline two mica granite and a marginal phase of medium grained tourmaline muscovite granite the pluton hosts several tungsten deposits including shuiyuanshan and yejiwo mineralization type is dominantly scheelite greisen with subordinate scheelite skarn and wolframite quartz veins the hehuaping pluton composed of coarse grained biotite granite and granite porphyry dykes to the east of the wangxianling pluton have an associated hehuaping tin deposit with tin skarn and subordinate porphyry and greisen mineralization in situ zircon and cassiterite u pb dating molybdenite re os and muscovite 40ar 39ar dating techniques are used to refine the age spectra of granitic rocks and w sn mineralization in the wangxianling area granites of the wangxianling pluton have zircon u pb ages of 224 ma tungsten bearing quartz veins from both the shuiyuanshan and yejiwo tungsten deposits have an average molybdenite re os isochron age of 224 ma and muscovite of scheelite greisen from the shuiyuanshan tungsten deposit yields a 40ar 39ar plateau age of 214 ma the two mineralization ages are consistent with the zircon u pb age of the wangxianling pluton indicating a close association between pluton emplacement and w mineralization in the late triassic both the hehuaping pluton and granite porphyry dykes have zircon u pb ages of 156 ma in situ la icp ms u pb dating of cassiterite in both the tin skarn and tin greisen yields an average age of 156 ma showing that the sn mineralization occurred at a similar time to the late jurassic granitic magmatism thus two mineralization events are identified in the wangxianling area and include an earlier w dominant mineralization event and a later sn dominant mineralization event the earlier event is related both spatially and temporally to intrusion of the late triassic s type wangxianling granite pluton whereas the later mineralization is associated both spatially and temporally with intrusion of the late jurassic hehuaping a2 type granite pluton and a2 type granite porphyry dykes

cassiterite u pb geochronology constrains magmatic hydrothermal evolution in complex evolved granite systems the classic erzgebirge tin province saxony and bohemia

recent and present day stresses in the granada basin betic cordilleras example of a late miocene present day extensional basin in a convergent plate boundary the diffuse convergent boundary between the eurasian and african plates in the western mediterranean is associated with a seismicity zone more than 300 km wide although the two plates are converging nw se the betic and rif cordilleras contain extensional structures that have been active since the miocene the extensional tectonics in the region which occurred simultaneously with the uplift of the cordillera have been analyzed in the southeastern sector of the late miocene to recent granada basin using earthquake focal mechanisms the determination of paleostresses from the study of the orientation and kinematics of microfaults and the study of the major structures both the geological surface data and the focal mechanisms indicate present day regional conditions of ne sw extension with triaxial to prolate stress ellipsoids however the stress field is heterogeneous with local variations in stress over time with different stresses sometimes even acting simultaneously in adjacent areas the most frequent changes consist of pluridirectional or ne sw extension favored by the prolate character of the stress ellipsoids and nw se subhorizontal compression favored by the regional tectonic setting strike slip faults are scarce even though they are the most likely structure to be expected in a region with sw ne extension and nw se compression seismicity is concentrated in the upper crust and may correspond to the activity of low to high angle normal faults similar to the surface faults although they can not be correlated with them the lower cutoff of this seismicity probably coincides with the 300 c isotherm and suggests a low thermal gradient for the region present day regional stresses have 1 vertical at the surface but in depth plunge toward the sw

combined zircon and cassiterite u pb dating of the piaotang granite related tungsten tin deposit southern jiangxi tungsten district china abstract the large low grade piaotang w sn deposit in the southern jiangxi tungsten district of the eastern nanling range south china is related to a hidden granite pluton of jurassic age the magmatic hydrothermal system displays a zonation from an inner greisen zone to quartz veins and to peripheral veinlets stringers five floor zonation model most mineralization is in quartz veins with wolframite cassiterite the hidden granite pluton in underground exposures comprises three intrusive units i e biotite granite two mica granite and muscovite granite the latter unit is spatially associated with the w sn deposit combined la mc icp ms u pb dating of igneous zircon and la icp ms u pb dating of hydrothermal cassiterite are used to constrain the timing of granitic magmatism and hydrothermal mineralization zircon from the three granite units has a weighted average 206pb 238u age of 159 8 0 3 ma 2 mswd 0 3 the cathodoluminescence cl textures indicate that some of the cassiterite crystals from the wolframite cassiterite quartz vein system have growth zonations i e zone i in the core and zone ii in the rim dating on cassiterite zone ii yields a weighted average 206pb 238u age of 159 5 1 5 ma 2 mswd 0 4 i e the magmatic and hydrothermal systems are synchronous this confirms the classical model of granite related tin tungsten mineralization and is against the view of a broader time gap of 6 myr between granite magmatism and w sn mineralization which has been previously proposed for the southern jiangxi tungsten district the elevated trace element concentrations of zr u nb ta w and ti suggest that cassiterite zone ii formed in a high temperature quartz vein system related to the piaotang granite pluton

relocation of early and late aftershocks of the 2001 bhuj earthquake using joint hypocentral determination jhd technique implication toward the continued aftershock activity for more than four years we employed layered model joint hypocentral determination jhd with station corrections to improve location identification for the 26 january 2001 mw 7 7 bhuj early and late aftershock sequence we relocated 999 early aftershocks using the data from a close combined network national geophysical research institute india and center for earthquake research institute usa of 8 18 digital seismographs during 12 28 february 2001 additionally 350 late aftershocks were also relocated using the data from 4 10 digital seismographs accelerographs during august 2002 to december 2004 these precisely relocated aftershocks error in the epicentral location 30 meter error in the focal depth estimation 50 meter delineate an east west trending blind thrust north wagad fault nwf dipping 45 southward about 25 km north of kachchh main land fault kmf as the causative fault for the 2001 bhuj earthquake the aftershock zone is confined to a 60 km long and 40 km wide region lying between the kmf to the south and nwf to the north extending from 2 to 45 km depth estimated focal depths suggest that the aftershock zone became deeper with the passage of time the p and s wave station corrections determined from the jhd technique indicate that the larger values both ve and ve characterize the central aftershock zone which is surrounded by the zones of smaller values the station corrections vary from 0 9 to 1 1 sec for the p waves and from 0 7 to 1 4 sec for the s waves the b value and p value of the whole aftershock 2001 2004 sequences of mw 3 are estimated to be 0 77 0 02 and 0 99 0 02 respectively the p value indicates a smaller value than the global median of 1 1 suggesting a relatively slow decay of aftershocks whereas the relatively lower b value less than the average b value of 1 0 for stable continental region earthquakes of india suggests a relatively higher probability for larger earthquakes in kachchh in comparison to other stable continental regions of the indian peninsula further based on the b value mainshock magnitude and maximum aftershock magnitude the bhuj aftershock sequence is categorized as the mogi s type ii sequence indicating the region to be of intermediate level of stresses and heterogeneous rocks it is inferred that the decrease in p value and increase in aftershock zone both spatially as well as depth over the passage of time suggests that the decay of aftershocks perhaps could be controlled by visco elastic creep in the lower crust

kinematics of the iberia maghreb plate contact from seismic moment tensors and gps observations the iberian peninsula and the maghreb experience moderate earthquake activity and oblique nw se convergence between africa and eurasia at a rate of 5 mm yr coeval extension in the alboran basin and a n35 e trending band of active left lateral shear deformation in the alboran betic region are not straightforward to understand in the context of regional shortening and evidence complexity of deformation at the plate contact we estimate 86 seismic moment tensors mw 3 3 to 6 9 from time domain inversion of near regional waveforms in an intermediate period band those and previous moment tensors are used to describe regional faulting style and calculate average stress tensors the solutions associated to the trans alboran shear zone show predominantly strike slip faulting and indicate a clockwise rotation of the largest principal stress orientation compared to the regional convergence direction 1 at n350 e at the n algerian and sw iberian margins reverse faulting solutions dominate corresponding to n350 e and n310 e compression respectively over most of the betic range and intraplate iberia we observe predominately normal faulting and wsw ene extension 3 at n240 e from gps observations we estimate that more than 3 mm yr of african nubian eurasian plate convergence are currently accommodated at the n algerian margin 2 mm yr in the moroccan atlas and 2 mm yr at the sw iberian margin 2 mm yr is a reasonable estimate for convergence within the alboran region while alboran extension can be quantified as 2 5 mm yr along the stretching direction n240 e superposition of both motions explains the observed left lateral transtensional regime in the trans alboran shear zone two potential driving mechanisms of differential motion of the alboran betic gibraltar domain may coexist in the region a secondary stress source other than plate convergence related to regional scale dynamic processes in the upper mantle of the alboran region as well as drag from the continental scale motion of the nubian plate along the southern limit of the region in the atlantic ocean the 3 5 mm yr westward motion of the gibraltar arc relative to intraplate iberia can be accommodated at the transpressive sw iberian margin while available gps observations do not support an active subduction process in this area

high precision relocation and focal mechanism of the 2002 rain triggered earthquake swarms at mt hochstaufen se germany summary the 2002 seismicity of mt hochstaufen se germany is characterized by several swarm type earthquake sequences that follow above average rainfall events first investigations have recently provided indication that the rain events triggered this swarm type activity we present a detailed relocation of the 2002 seismicity which is composed of 1 cluster analysis 2 multievent semi automatic phase repicking 3 probabilistic location in a pseudo 3 d velocity model with topography and 4 master event relocation furthermore focal mechanisms of selected events are derived by taking into account the first motion polarities of p sv and sh waves we introduce a simple methodology to test the stability and estimate the variance of the focal mechanisms solutions location uncertainties of less than 100 m were achieved the hypocentres are mainly concentrated between 1000 m above and 4000 m below sea level and aligned on two planes which however cannot be associated with geological structures with reasonable certainty swarm type activity follows above average rain events and shows a depth migration of hypocentres over time evidence that the rain events triggered the earthquake swarms is reported by hainzl et al on basis of our relocation the derived focal mechanisms seem to indicate an influence of the saalachtal fault zone on the stress regime of the study area

the relative locations of multiplets in the vicinity of the western almeria southern spain earthquake series of 1993 1994 summary we have analysed 721 earthquakes 1 5jmbj5 0 of the 1993 1994 western almero a southern spain series and the following seismicity in the area until 1998 among the data there are several multiplets events characterized by very similar seismograms at the short period stations of the local network we detected similar seismograms using crosscorrelation analysis of the p and s arrivals and classified similar events into families or clusters we found 39 multiplet clusters of 3 33 events within each cluster relocations relative to a master event have been calculated by using the interpolated cross correlation maxima for the precise relative timing of p and s phases at each station relative arrival times have been compared for all the possible selections of the master event and adjusted by forming the mean value after removing the outliers the distribution of the stations does not permit a satisfactory resolution of focal depths but relative epicentres have been determined with an accuracy of a few tens of metres typically they draw well defined lineaments and show two dominant strike directions n120u 130ue and n60u 70ue these directions are coincident with known fault systems in the area and with the source parameters of three of the largest events mw 4 8 3 6 and 4 9 which were estimated from waveform modelling of near field acceleration records at a single station consistent with previous studies distances within multiplets typically several tens of metres are for the most part clearly smaller than the fracture radii of these events this indicates repeated slip on the same fault segment it was possible to obtain precise relative locations between several nearby clusters thereby imaging a very heterogeneous seismotectonic fine structure of the source area i e the positions of adjacent active fault segments and the fragmentation of the crust into small approximately 1 km tectonic blocks

lg wave cross correlation and double difference location application to the 1999 xiuyan china sequence a surprising discovery has been made that in some cases the complex highly scattered lg wave is found to be similar for clusters of events we analyze in detail a subset of 28 out of 90 events from the 1999 xiuyan sequence cross correlations provide highly accurate differential travel time measurements their error estimated from the internal consistency is about 7 msec these travel time differences are then inverted by the double difference technique to obtain epicenter estimates that have location precision on the order of 150 m the locations are computed with waveform data from four to five regional stations 500 to 1000 km away the epicenter estimates are not substantially affected by the sparseness of stations or large azimuthal gaps comparison with a surface trace a few kilometers away and location estimates based on much more dense networks led us to conclude that the absolute positions are accurate to the 5 km level regional event locations must often be based on a small number of phases and stations due to weak signal to noise ratios and sparse station coverage this is especially true for monitoring work that seeks to locate smaller magnitude seismic events with a handful of regional stations two primary advantages of using lg for detection and location are that it is commonly the largest amplitude regional wave enabling detection of smaller events and it propagates more slowly than p waves or sn resulting in smaller uncertainty in distance for a given uncertainty in travel time