mechanics of oblique convergence the distribution and magnitude of the strike parallel component of velocity in an obliquely converging thrust wedge or accretionary prism are determined by the geometry and mechanical properties of the wedge a mechanical analysis based on the assumption of a critical or stable geometry of the wedge for which the rate of cross strike deformation is zero leads to the following conclusions for different bulk rheologies 1 in a linear viscous wedge the strike parallel motion relative to the underthrust slab decreases exponentially away from the rear and is effectively concentrated in a shear zone with a width comparable to the thickness of the wedge at the rear the wedge also deforms by corner flow producing a circulation in the cross strike plane the strike parallel and corner flow velocities depend on the thickness and viscosity of the wedge and on the shear stresses applied to its lower and rear boundaries convergence at the wedge front is normal to strike 2 a critically tapered perfect plastic wedge moves coherently without internal deformation for low and moderate obliquities of the convergence vector the wedge moves at the same velocity as the backstop upper plate for high angles of obliquity the wedge moves laterally relative to the underthrust slab at a maximum velocity dependent on its dimensions and the stress conditions on its boundaries so that it is separated from the upper plate by a strike slip fault defining a forearc sliver no geometrical configuration exists that allows the strike parallel motion to be distributed through the wedge 3 a noncohesive coulomb wedge behaves in much the same way as a plastic wedge but the geometry and velocity depend only on its mechanical properties and the shear stresses on its boundaries and they are independent of scale

coastwise transport reconsidered lateral displacements in oblique subduction zones and tectonic consequences coastwise transport margin parallel displacement of arc and forearc slivers is a common feature of zones of oblique subduction today but at rates that rarely exceed a few tens of kilometers per million year paleomagnetic studies strongly indicate that coastwise transport in the north american cordillera went on during late cretaceous early tertiary time at rates several times higher than this other studies have shown that obliquity is commonly partitioned between oblique subduction and coastwise transport of a detached sliver of over riding crust the analysis developed here indicates that in the absence of a buttress a physical impediment to sliver motion at its leading edge sliver motion should obey the relationship tan p r tan 1 1 r2 sin2 12 where is the angle of obliquity is the angle of subduction and r rfrs is the ratio of resistance to slip on strike slip faults cutting the over riding plate to resistance to slip on the subduction zone in this equation p is a partitioning factor 0 p 1 where p 1 implies oblique subduction and no sliver motion and p 0 is the case in which the sliver moves at velocity v sin equivalent to the transverse component of relative motion rates of coastwise transport calculated from this relationship are systematically too high presumably indicating that most modern subduction zones are buttressed key factors in the buttress effect are 1 the greater strength of oceanic vs continental lithosphere and 2 the energy required to thicken continental crust large scale tectonic transport in the north american cordillera probably was the result of a combination of favorable factors long continued highly oblique convergence and a plate margin geometry that avoided the buttress problem

mobility and fractionation of rees during deep weathering of geochemically contrasting granites in a tropical setting malaysia in this study we describe the mobility and fractionation of rees in two deep up to 30 m tropical weathering profiles developed on two granites from the kuala lumpur pluton malaysia sampled at cheras and rawang on the basis of na2o and k2o both are s type granites but rawang has higher cao mgo and feo than cheras and lower sio2 with respect to al saturation rawang is i type and cheras is s type we compared the two profiles in terms of total rees magnitude and changes in ce and eu anomalies ree mobility and lree hree fractionation rawang profiles have higher ree contents display lower mobility for most except the heaviest rees and show higher lree hree fractionation than those from cheras these differences can be linked to differences in primary mineralogy and degree of weathering the latter controlling the type and volume of secondary minerals specifically bowl shaped parent rock normalised patterns in the cheras saprolites appear to be a result of apatite dissolution moreover moderate weathering evident in lower mineralogical indices of alteration mia at cheras has conserved parent rock ree patterns and fractionation factors in the saprolites by contrast more intense weathering observed in rawang profiles has produced abundant kaolinite group minerals that have preferentially retained lree which consequently display high lree hree fractionation this study provides important insights into the factors controlling ree mobility during tropical weathering and its potential as an indicator of weathering intensity

a new model for the indochina and south china collision during the late permian to the middle triassic article i nfo a comparative analysis of indochina and south china during the early paleozoic indicates the former may be extended to include north vietnam part of the qinzhou tectonic zone and southern hainan island these three regions were traditionally regarded as parts of south china separated from indochina by the song ma suture in central north vietnam a new suture called the dian qiong suture is proposed here approximately along the southern margin of the present nanpanjiang basin this suture is linked to its eastern counterpart in hainan island through a nne trending dextral transform fault zone along the eastern margin of the nanpanjiang basin the conventional song ma suture originally constitutes its western extension and was translated to its present location by sinistral displacement along the red river fault zone during the tertiary upper paleozoic deep water turbidites and associated mid oceanic ridge basalts along the suture in the nanpanjiang basin illustrate a coeval ocean between indochina and south china the ocean was consumed by south directed subduction beneath indochina during the late permian to the middle triassic this suture zone was finally re shaped by the indentation between the active margin of indochina and the irregular passive continental margin of south china during the late triassic as well as displacement along the red river fault zone during the cenozoic

intersecting intracontinental tertiary transform fault systems in the north american cordillera in central british columbia north trending dextral strike slip faults that cut late eocene granite also truncate northwest trending dextral strike slip faults the northwest trending strike slip faults bound the wolverine metamorphic complex wolverine complex which has been uplifted primarily by northwest southeast eocene crustal extension and somewhat by late eocene northerly extension the crustal extension is indicated by shallow dipping extensions faults dyke complexes and stretching lineations the wolverine complex and its bounding faults define a crustal pull apart in an en echelon dextral transform the northwest and north trending dextral strike slip faults in central british columbia are the continuations of faults that transect the interior of the north american cordillera and they represent at least two distinct plate boundaries intermittently active during the early to middle eocene and the late eocene to early oligocene each of these systems consists of en echelon strike slip fault

development of inverted metamorphic isograds in the western metamorphic belt juneau alaska an inverted metamorphic gradient is preserved in the western metamorphic belt near juneau alaska the western metamorphic belt is part of the coast plutonic metamorphic complex of western canada and southeastern alaska that developed as a result of tectonic overlap and or compressional thickening of crustal rocks during collision of the alexander and stikine terranes detailed mapping of pelitic single mineral isograds systematic changes in mineral assemblages and silicate geothermometry indicate that thermal peak metamorphic conditions increase structurally upward over a distance of about 8 km peak temperatures of metamorphism increase progressively from about 530 c for the garnet zone to about 705 c for the upper kyanite biotite zone silicate geobarometry suggests that the thermal peak metamorphism occurred under pressures of 9 11 kbar the metamorphic isograds are in general parallel to the tonalite sill that is regionally continuous along the east side of the western metamorphic belt although truncation of the isograds north of juneau indicates that the sill intrusion continued after the isograds were established our preferred interpretation of the cause of the inverted gradient is that it formed during compression of a thickened wedge of relatively wet and cool rocks in response to heat flow associated with the formation and emplacement of the tonalite sill magma garnet rim compositions and widespread growth of chlorite suggest partial re equilibration of the schists under pressures of 5 6 kbar during uplift in response to final emplacement and crystallization of the tonalite sill the combined results of this study with previous studies elsewhere in the western metamorphic belt indicate that high t high p metamorphism associated with the collision of the alexander and stikine terranes was a long lived event extending from about 98 ma to about 67 ma

behaviour of rare earth elements and yttrium in ferromanganese concretions gibbsite spots and the surrounding terra rossa over dolomite during chemical weathering abstract the behaviour of rare earth elements rees and yttrium y during chemical weathering processes has been investigated on a 4 05 m thick terra rossa profile over dolomite on the yunnan guizhou plateau china in this profile ferromanganese concretions and gibbsite spots coexist in the terra rossa solum analyses of rees y zr f s and toc as well as mineralogical studies were conducted on a suite of ferromanganese concretions gibbsite spots terra rossas dolomites and insoluble residues from the underlying dolomite these analyses helped us to understand the mobilization redistribution and fractionation of rees and y during chemical weathering the rees and y are mobilized and redistributed during the terra rossa formation rees except for ce are removed from the upper and middle profiles transferred downwards and then precipitated in the base profile resulting in remarkable enrichment in the terra rossa near the weathering front the significant increase of ph near the weathering front was responsible for ree excluding ce and y enrichment in the base profile y is quite mobile during extreme chemical weathering and most of it was carried away from the profile because of very efficient oxidative fractionation of ce a significant positive ce anomaly in terra rossa was found in the middle of the profile whereas the terra rossa near the weathering front exhibited notably negative ce anomalies moreover the presence of cerianite and its content in the ferromanganese concretions can explain the markedly positive ce anomaly and the variation of ce ce values with depth in profile these ce anomalies are characteristic of the geochemical environment especially the redox condition studies of ree distribution in the ferromanganese concretions gibbsite spots and surrounding terra rossa revealed that significant ree fractionation occurred rees and y were preferentially concentrated in the ferromanganese concretions rather than in the gibbsite spots the water rock interaction resulted in m type tetrad effects in some of the ferromanganese concretions gibbsite spots and terra rossa samples in addition the tetrad effect in terrestrial weathering processes likely played an important role in the fractionation of rees and y such as y ho sm nd and the eu anomaly change

geometric test for late cretaceous paleogene intracontinental transform faulting in the canadian cordillera the tintina trench northern rocky mountain trench tt nrmt fault zone and the fraser river straight creek fr sc fault zone are separate en echelon concentric small circle fault segments of a composite intracontinental transform fault zone more than 2500 km long that cuts diagonally across the canadian cordillera from the outboard part of a tectonic collage of accreted foreign terranes in the south into the north american preaccretionary continental margin in the north most of the 450 km of right hand slip on the tt nrmt fault zone was transformed southward during the late cretaceous and paleocene into oblique convergence in the southern canadian rockies the remainder probably comprising less than 100 km was transformed southwestward during early and middle eocene time via a zone of distributed shear and east west crustal stretching into right hand slip on the en echelon fr sc fault zone these interpretations based on regional systematic mapping of geologic structures are in conflict with interpretations of paleomagnetic measurements that call for more than 1000 km of post mid cretaceous right hand displacement along the general locus of the tt nrmt fault zone involving foreign terranes and the parts of north america to which they had been accreted this paradox must be resolved

magma emplacement in a convergent tectonic orogen southern revillagigedo island southeastern alaska the western metamorphic belt of the british columbia alaska coastal orogen consists of lithostratigraphic units assigned to the taku terrane and the gravina sequence on revillagigedo island and cleveland peninsula in the ketchikan quadrangle of southeastern alaska greenschist and amphibolite grade schists of the western metamorphic belt are intruded by mid cretaceous tonalite and leucotonalite plutons and sills a number of these igneous bodies are spatially associated with northwest trending shear zones developed along thrust faults that formed during the main phase of metamorphism of the schists observations of field relations and structural fabric in the igneous rocks and the country rock schist show the following i along the southern shore of revillagigedo island the moth bay pluton was emplaced along a shear zone magmatic and submagmatic flow foliation in the southern border zone of the pluton parallels foliations in the sheared country rock rocks in the interior of the pluton are general

genesis of karst bauxite bearing sequences in baofeng henan china and the distribution of critical metals abstract bauxite bearing sequences are not only the main economic sources of aluminum but also of many critical metals the distribution of critical metals was determined from six bauxite profiles in baofeng and an evolution model was proposed for the formation of bauxite bearing sequences following a terrestrial deposition of the lower pisolitic bauxite horizon and the underlying clayey layer transportation and redeposition of bauxitic materials during submergence led to the formation of the upper bauxite horizon presenting finely clastic and oolitic textures as well as the intercalated clayey layers both bauxite ores and clay samples contain highly elevated contents of li b sc v ga zr nb w and total rees li contents are extremely low in high grade bauxite ores 10 92 44 77 ppm compared to highly elevated values in low grade bauxite ores 35 81 1601 ppm and clay samples 38 54 1965 ppm enlightening a new potential resource for li the bauxite bearing sequences resting on karstified carbonate the distribution pattern of ga is similar to that of li and the correlation analysis indicates that ga is primarily associated with silicates i e clay minerals the vertical distribution of li b v cr and ga shares the same pattern similar to that of k2o and sio2 being highly enriched in clayey layers relative to bauxite horizons both sc and total rees are also highly enriched in baofeng bauxite profiles highlighting a potential sc and rees resource

climatic and hydrologic controls on upper paleozoic bauxite deposits in south china abstract the late paleozoic era was an interval of major tectonic and climatic changes including formation of the supercontinent pangea and the 60 myr long late paleozoic ice age lpia although bauxite formation declined globally from the carboniferous to the permian in conjunction with global cooling it was an interval of widespread bauxite formation in china in south china 1 the jiujialu formation bauxite deposits in central and northern guizhou zunyi area are of early middle visean age 2 the dazhuyaun formation bauxite deposits in northern guizhou wuchuan zheng an daozheng area to southern chongqing nanchuan area and southeastern guizhou fuquan kaili area are of late pennsylvanian early permian age and 3 the heshan formation bauxite deposits in western guangxi to yunnan are of middle late permian age in north china the benxi formation bauxite deposits are of late mississippian middle pennsylvanian age the contrasting trends in bauxite metallogenesis between china and the rest of the world imply different climatic patterns in the eastern tethys high annual humidity with seasonal dryness and pangea aridification this hypothesis is further supported by differences in the chemical index of alteration or cia 80 in the eastern tethys vs 50 in western pangea and in paleotemperatures mean annual temperature or mat 20 c in the eastern tethys vs 4 c in western pangea determined from permo carboniferous siliciclastic deposits permo carboniferous bauxite deposits in south china formed in coastal plain and coastal karstic depression environments in which the position of the groundwater table was related to sea level changes during the late paleozoic high frequency eustatic fluctuations caused by waxing and waning of gondwana icesheets controlled sedimentation in these coastal depositional systems leading to cyclic accumulation of coal and bauxite deposits and cycles of vadose and phreatic type bauxite formation the results of the present study show that during interglacial stages lateritization resulted from high pco2 high sea level and groundwater table elevations low precipitation and limited vegetation cover whereas during glacial stages bauxitization of these ferralitic weathering products was promoted by low pco2 low sea level and groundwater table elevations high precipitation and more extensive vegetation cover thus a unique combination of geographic climatic and eustatic factors accounted for widespread formation of bauxite in south china during the lpia

mobilization and redistribution of rees and thorium in a syenitic lateritic profile a mass balance study abstract ree th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite at akongo in sw cameroon ree and th mass balance calculations for the host rock minerals show that at least 70 of the lree s and 50 of the hrees are contained in allanite apatite titanite and epidote and at least 50 of the th is controlled by the same accessory minerals which represent about 2 wt of the unaltered syenite these accessory phases are destroyed during the first stages of weathering causing most of the rees and th to be rapidly released into the soil comparison of the variation in the zr ti and th content as a function of the apparent density of the different zones of the saprolite shows that th is the least mobile element the presence of secondary thorianite tho2 the etched surface on zircon grains and the presence of ti in secondary cerianite support this geochemical interpretation the concentration of thorium was therefore chosen as invariant relative to the concentration of the other elements especially the rees in mass balance calculations most of the rees are leached in the iron rich upper horizons loose nodular horizon iron crust and top of mottled clay horizon where the groundwater table moves saprolite and bottom of the mottled clay horizon the rees are fractionated and redistributed there is a juxtaposition of leached and accumulation zones with precipitation of lree aluminous hydrated phosphates this study supports the existence of two different cycles for the redistribution of elements in the soil 1 1 as dissolved ions in the saprolite horizon and 2 2 as individual particles in the upper part of the profile

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

kinematics and tectonic significance of transpressive structures within the coast plutonic complex british columbia abstract structural data from the coast plutonic complex near prince rupert british columbia are consistent with a deformational history dominated by dextral transpression from campanian to paleocene time penetrative east side up southwest directed ductile shearing produced moderately northeast plunging overturned kilometer scale isoclinal folds these folds are dextrally sheared and refolded into kilometer scale upright northwest plunging folds and steeply dipping transposed foliations with moderate to shallow northwest plunging lineations along their western side an east side up component to the transcurrent shearing is kinematically compatible with east side up shearing found within the great tonalite sill kinematic and geometric gradients and the spatial distribution of the finite stretching direction are interpreted to result from partitioning of transpression the location of these structures and overprinting relationships suggest the great tonalite sill intruded late kinematically into a crustal scale dextral transpressive shear zone these results indicate this shear zone could form part of the baja b c fault system that would have accommodated large northward displacements of the terranes making up western british columbia and southeast alaska this conclusion is based on 1 it is favorably located to accommodate the proposed displacements 2 deformation occurred during the time period of proposed large displacement 83 59 ma 3 the 15 km thickness of the shear zone indicates it records large displacements

discovery of the ree minerals and its geological significance in the quyang bauxite deposit west guangxi china most of the karstic bauxite deposits in china are enriched in ree and the ree has commonly been considered to be adsorbed on the surface of clay or diaspore minerals as ion state however occurrence of ree minerals in the bauxite deposits has not been reported by far in the quyang bauxite deposit from guangxi province we find ree minerals the minerals are parisite and churchite parisite is widely distributed in the ooids and matrix inducing positive ce anomaly of the ores whereas churchite is rare and occurred in the core of ooid the compositions of the ree minerals show the eu anomaly can change gently during the weathering and the eu anomaly analyses suggest that the ree in the ores are mostly derived from the underlying carbonates the parisite is formed in an alkaline condition while churchite in an acidic situation the co existence of the two minerals in the ores suggests the permian environment for bauxite formation is complex with great change of ph value moreover parisite is inferred to be formed earlier than the churchite

detrital zircon geochronology and regional correlation of metasedimentary rocks in the coast mountains southeastern alaska u pb ages have been determined for 55 detrital zircon grains from a metasedimentary sequence along the west flank of the coast mountains in southeastern alaska these rocks belong to the port houghton assemblage which consists of upper paleozoic pelitic and psammitic schist metaconglomerate metabasalt and marble the port houghton assemblage rests unconformably on metamorphosed and deformed mid paleozoic arc type volcanics endicott arm assemblage which gradationally overlie upper proterozoic lower paleozoic continental margin strata tracy arm assemblage three main clusters of ages are present 330 365 ma 19 grains 1710 2000 ma 27 grains and 2450 2680 ma 6 grains additional grains are approximately 2334 2364 and 3324 ma comparison of these ages with detrital zircon ages in other cordilleran assemblages supports previous interpretations that metasedimentary rocks in the coast mountains i form a southwestern continuation of the yukon tanana terrane of eastern alaska and yukon

strontium and oxygen isotopic variations in mesozoic and tertiary plutons of central idaho regional variations in initial 87sr 86sr ratios r i of mesozoic plutons in central idaho locate the edge of precambrian continental crust at the boundary between the late paleozoic mesozoic accreted terranes and precambrian sialic crust in western idaho the r i values increase abruptly but continuously from less than 0 704 in the accreted terranes to greater than 0 708 across a narrow 5 to 15 km zone characterized by elongate lens shaped highly deformed plutons and schistose metasedimentary and metavolcanic units the chemical and petrologic character of the plutons changes concomitantly from ocean arc type diorite tonalite trondhjemite units to a weakly peraluminous calcic to calcalkalic tonalite granodiorite granite suite the idaho batholith plutons in both suites yield late cretaceous ages but permian through early cretaceous bodies are confined to the accreted terranes and early tertiary intrusions are restricted to areas underlain by precambrian crust the two major terranes were juxtaposed between 75 and 130 m y ago probably between 80 and 95 m y oxygen and strontium isotopic ratios and rb and sr concentrations of the plutonic rocks document a significant upper crustal contribution to the magmas that intrude precambrian crust magmas intruding the arc terranes were derived from the upper mantle subducted oceanic lithosphere and may have been modified by anatexis of earlier island arc volcanic and sedimentary units plutons near the edge of precambrian sialic crust represent simple mixtures of the precambrian wall rocks with melts derived from the upper mantle or subducted oceanic lithosphere with r i of 0 7035 rb sr varies linearly with r i producing pseudoisochrons with apparent ages close to the age of the wall rocks measured 18o values of the wall rocks are less than those required for the assimilated end member by sr o covariation in the plutons however indicating that wall rock 18o was reduced significantly by exchange with circulating fluids metasedimentary rocks of the belt supergroup are similarly affected near the batholith documenting a systematic depletion in 18o as much as 50 km from the margin of the batholith plutons of the bitterroot lobe of the idaho batholith are remote from the accreted terranes and represent mixtures of precambrian wall rocks with melts dominated by continental lower crust r i 0 708 rather than mantle pseudoisochrons resulting from these data are actually mixing lines that yield apparent ages less than the true age of the wall rocks and meaningless ri assimilation fractional crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region

the dynamic evolution of the palaeozoic geography of eastern asia abstract new palaeogeographical reconstructions are presented for eleven time intervals through the palaeozoic of the eastern asia region from the middle cambrian at 510 ma to the end of the permian at 250 ma they centre on the continental blocks of north china south china and annamia indochina and their relationships with northeastern gondwana which was united to form part of pangea from the late carboniferous onwards also shown is the continent of tarim during the lower palaeozoic as well as the hutag uul songliao and khanka jiamasu bureya terranes both of which straddle the russian mongolian and chinese borders today from silurian times onwards we conclude that annamia and south china were united as a single continent throughout the lower palaeozoic and early devonian and were translocated by major strike slip faulting along the northeastern gondwana margin during that period from off afghanistan to outboard of the sibumasu and australian sectors of the superterrane they left the gondwana marginal area together during the lower devonian opening of the palaeotethys ocean but very shortly afterwards they themselves divided into the two separate continental blocks that we recognise today not to reunite until the triassic the various cambrian to permian rocks found in japan largely represent active volcanic arcs which originally lay to the southeast of south china although the carboniferous was more quiescent there the neotethys ocean opened during the permian dividing sibumasu and the tibetan terranes from gondwana and the palaeotethys ocean started to close progressively in the upper palaeozoic as most of the east asian continents and smaller terranes moved towards siberia the positions of the various continents and terranes have been deduced from a mixture of palaeomagnetic and faunal data the positions of large igneous provinces and kimberlites and the need to provide kinematic continuity between maps of successive ages however many uncertainties remain

hydraulically drained flows in rotating basins part i method abstract an asymptotic method for coupling circulations in basins to hydraulically controlled overflows is introduced the method is applicable when the forcing dissipation and coupling with the overflow are weak in which case the lowest order solution for the homogeneous or 1 layer model consists of the natural basin modes including gravity inertia gravity potential vorticity helmholtz and steady geostrophic modes at the next order of approximation the mode amplitudes are found to vary slowly with time as the result of forcing dissipation interior nonlinear mode interactions and most importantly coupling with the overflow even when the latter are absent the overflow dynamics generally introduce nonlinearity although the basin dynamics are assumed linear to lowest order the overflow is intrinsically nonlinear to couple the two systems the overflow model must be adapted to serve as a nonlinear boundary condition on the basin flow to do so a rotating channel model introduced by whiteh

genesis of ree minerals in the karstic bauxite in western guangxi china and its constraints on the deposit formation conditions abstract karstic bauxites in western guangxi china comprise two subtypes permian bauxite and quaternary bauxite the quaternary bauxite originated from the breaking up rolling and accumulating of permian bauxite in karstic depressions in quaternary various types of rare earth element ree minerals were discovered during the formation of the permian and quaternary bauxites from the xinxu longhe and tianyang bauxite deposits in this study five types of ree minerals including bastnasite parisite cerianite rhabdophane and churchite were identified bastnasite and parisite are the most abundant and they are widely developed in the permian ore and also present in the quaternary ore obvious variations in bastnasite and parisite ree compositions were observed which is ascribed to distinctions in the source materials in the primary weathering profile from different areas the mode of occurrence of bastnasite and parisite suggests they were mainly precipitated under alkaline and reducing conditions during the permian bauxite forming stage and underwent intensive corrosion in the quaternary churchite was formed during the permian weathering stage under acidic condition both cerianite and rhabdophane occur in fractures within the permian bauxite ore indicating that both formed during the quaternary weathering stage it is considered that the rhabdophane enriched in ce have formed locally in the process of that the ce3 released from bastnasite rapidly entered the rhabdophane lattice before being oxidized to ce4 cerianite was mainly found in association with mn al hydroxides suggesting that the released ce3 was oxidized into ce4 and precipitated cerianite in fractures within the permian bauxite ore mass balance equations reveal a depletion in nearly all rees during the transformation from the permian to the quaternary bauxite ore mainly caused by the dissolution of bastnasite and parisite the genesis of the ree minerals together with the occurrence of other minerals indicates that intensively acidic and oxidizing conditions developed before the formation of the permian bauxite ore towards the end of the permian the conditions became reducing and alkaline favorable for the large scale bauxitization the quaternary bauxite forming stage was characterized by variable ph and eh conditions with acidic ph 4 6 and oxidizing eh 2 conditions at the surface of the exposed permian bauxite ore

age and nature of the basement in northeastern washington and northern idaho isotopic evidence from mesozoic and cenozoic granitoids k feldspar pb and whole rock nd isotopic analyses from 25 mesozoic and cenozoic plutonic rocks and two gneisses from ne washington and northern idaho are used to elucidate the age and nature of the concealed cratonic basement the plutons form two highly distinct isotopie groups group i hornblende biotite and two mica plutons of all ages have pb isotopie compositions suggesting derivation from rocks of the belt supergroup or their metamorphosed equivalents although nd isotopie data can only support this model if there was a significant input of juvenile mantle derived or ancient light ree depleted material group ii hornblende biotite of eocene age have highly retarded pb isotopie compositions relative to the present day crustal average and require a source region with long term u and other lil depletion characteristic of cratonic lower crust a u pb zircon upper intercept age of ca 2600 ma obtained from one of the group ii samples together with sm nd data from the gneisses indicates possible

direction and shear sense during suturing of the seven devils wallowa terrane against north america in western idaho a northeast dipping 1 5 km thick mylonite near dworshak dam marks the suture zone between precambrian north america and the seven devils wallowa terrane in western idaho the mylonite formed under amphibolite facies conditions from quartz diorite containing apparently synplutonic mafic and synkinematic pegmatite dikes of the kamiah plutonic complex mylonitic lineations and fold axes have a mean plunge of 48 toward 056 nearly down the dip of the mylonitic foliation shear sense given by offset of late stage crosscutting pegmatites is consistently top to the southwest reverse slip parallel to the mylonitic lineation folds that formed by progressive folding of the mylonitic foliation approach sheath fold geometry axial planes and fold limbs are nearly parallel to the mylonitic foliation mafic dikes that are apparently synplutonic in the undeformed quartz diorite immediately south of the mylonite zone and north of kamiah have variable dips and azimuths in the shear zone however these dikes lie nearly in the mylonitic foliation transposition of the dikes into near concordance with the foliation by simple shear requires high values of shear strain and suggests that cumulative top to the southwest reverse slip displacement across the mylonite zone is at least 27 km and likely more than 80 km this displacement involves underthrusting of the kamiah plutonic complex emplaced within the seven devils wallowa terrane beneath north america during late cretaceous docking with continental north america

genesis of the guangou karstic bauxite deposit in western henan china abstract the guangou bauxite deposit in western henan china is located in the north china block and to the north of the north qinling orogenic belt the orebody is hosted within the lower member of the carboniferous benxi formation which overlies unconformably upon the ordovician majiagou formation in the lower member of the benxi formation the bauxite orebody with a diaspore illite anatase mineral assemblage is sandwiched between underlying ferric clay illite hematite goethite and overlying top clay kaolinite quartz goethite according to field observations and geochemical evidence including trace and rare earth element ree compositions especially zr hf ratios the ferric clays were weathered from the underlying argillaceous limestones in the majiagou formation during this weathering process trace elements s zn ni cr v sc f ba and be are depleted li rb and b are enriched and zr hf bi ta ga nb and th are relatively immobile rees exhibit evident differentiation with enrichment of la ce pr and nd and depletion of gd tb dy ho er tm yb and lu the bauxite formed in weak reducing slightly basic water conditions according to the geneses of coexisting diaspore and anatase and it was altered during the epigenetic and second exposure periods forming kaolinite and goethite respectively interelemental relationship analyses of the ores suggest that the elemental behaviors of trace elements and rees in the bauxitization process are mainly controlled by the mineral compositions in bauxite ore and chemical properties of the elements for instance zircon rutile and anatase contain zr hf nb ta w and u based on geochemical evidence together with age data and chemical compositions of the detrital minerals both distant igneous rocks in the plate margin and paleozoic carbonates and precambrian rocks inside north china block provide the miscellaneous material for the bauxite ore and the top clay

frictional processes in straits friction and mixing can be dynamically significant in straits as in many oceanographic situations for weak time dependent homogeneous flows geostrophy friction and acceleration can all play roles in limiting the flow through a strait and the ratio of the geostrophic elevation difference across the strait to the frictional difference along the strait may be of importance in more general situations a specific question for quasi steady exchange flows through straits concerns the influences of friction and entrainment on hydraulic control within the context of layered models these effects tend to drive a flow towards hydraulic criticality in the downstream direction and to shift control points downstream however friction and entrainment also induce vertical gradients in velocity and density rendering layered models dubious in such situations the conditions for hydraulic control have been uncertain even for a homogeneous fluid there is an apparent contradiction between control conditions based on a similarity assumption for the velocity profile on the one hand and on the speed of long waves on the other the resolution of this contradiction is discussed it seems that if the frictional forces only involve local flow properties then inviscid long waves are arrested at the control section though this is shifted from its location for inviscid flow but that if flow derivatives along the channel are involved the condition changes for a homogeneous flow with shear internal friction and bottom friction the control section is shifted downstream partway to where it would be for a slab flow with bottom friction while the parameterization of bottom friction actually seems to be reasonably well established the nature and quantification of internal and lateral frictional processes are uncertain in a rotating system secondary cross strait flows are expected to be driven by the vertical gradient of the vertical reynolds stress and can provide estimates of the magnitude of bottom and internal friction

paleogene history of the kula plate offshore evidence and onshore implications paleocene to middle eocene magnetic anomalies were mapped over oceanic crust that accreted at the kula pacific spreading center and is now obliquely entering the western aleutian trench between 179 e and 168 e the strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56 55 ma when north south spreading veered to northwest southeast 310 130 kula pacific spreading ceased in 43 ma a 75 km long section of the fossil kula rift axis has avoided subduction although it now intersects the trench axis almost orthogonally near 171 5 e a narrow remnant of the former kula plate northwest of this fossil spreading center is bounded by a fossil kula pacific transform with a high transverse ridge alongside a sediment filled transform valley anomalies on this remnant show that eocene kula pacific spreading was highly asymmetric 2 1 the 56 55 ma change in kula plate rotation inferred from the change in spreading direction coincided with birth of the aleutian subduction zone and was probably a consequence of the resulting change in slab pull stresses on the oceanic lithosphere the change in direction of kula north american motion is a plausible explanation for the detachment of continental terranes from the pacific northwest and their migration around the gulf of alaska and for the early eocene demise of alaska range are volcanism the cessation of kula pacific spreading coincides with a major change in pacific plate rotation and the subsequent direction of convergence of the pacific plate with the aleutian arc was similar to the 55 43 ma direction of kula plate convergence

late triassic arc related potassic igneous rocks in the north american cordillera igneous rocks of late triassic age are widespread in the cordillera of western north america and except in wrangellia consist of subduction related plutonic and volcanic suites many of these including those in the stikinia quesnellia rattlesnake creek and jackson terrenes and in southern california are clinopyroxene rich and belong to high potassium and shoshonitic rock series features that are generally absent from older and younger igneous rocks in the same terranes the late triassic subduction related rocks are exposed in two discontinuous belts that lie east and west of the cache creek terrane in canada and correlative melange terranes farther south stratigraphic and structural data suggest that these belts were spatially separate magmatic arcs in late triassic time tectonic implications of this analysis include an explanation of middle jurassic cordilleran deformation as the result of collision of the western with the eastern belt absence of late triassic links between stikinia and quesnellia disassociation of stikinia with terranes in northwestern nevada and tentative correlation of the wallowa seven devils terrane with stikinia rather than wrangellia

solid liquid ree fractionation in the lateritic system of goyoum east cameroon the implication for the present dynamics of the soil covers of the humid tropical regions the ree th weathering geochemistry and mineralogy has been investigated in a lateritic soil cover in relation to the close hydrographical system at goyoum east cameroon the stutied area composed of gneissic hills covered by humid tropical forest belongs to the sanaga river basin and corresponds to a transition vegetation zone between rainforest and savannah a representative soil catena was taken as a reference zone close to the sanaga river the gneissic parent rock soil and groundwater samples have been cut off in pits in the lower part of the catena waters were also sampled both in brooks rich in organic matter and in the sanaga river all the waters were sampled seasonally during the rainy and dry periods the water rock interactions and the exportation of ree and th have been discussed as a function of the evolution of specific ratios considering th as the less mobile element during laterization variation of parent rock pr normalized ratios ln th pr vs depth show that the upper ferruginous horizons are depleted in ree while the basal saprolite presents accumulation zones which are differently located for ce and the other lrees the hrees are also depleted the la accumulations are located near the saprolite gneiss weathering front associated with negative ce anomalies ce ce 0 4 the ce accumulations inducing strong positive ce anomalies ce ce 4 are located in the upper saprolite beneath the ferruginous horizons all along the soil catena the secondary lree bearing minerals which reflect lree fractionation during incipient weathering of the gneiss are hydrous phosphates rhabdophane lreepo4 nh2o and cerianite ceo2 whose precipitation depends on the eh ph changes in the soil profiles on the other hand hree depletion is controlled by the dissolution of xenotime the major hree bearer in the parent gneiss in the 0 45 m filtered acidic groundwater ph 5 5 which have flooded the saprolite for a long residence time th contents are extremely low 0 th 10 ppt while lree contents are high 90 la 450 ppt in these filtered waters th seems to be strongly controlled by minerals i e thorianite which lead to its immobilization in the soil while lree remain in the soluble and or colloidal pools moreover the groundwaters show significant negative ce anomalies throughout the year 0 1 ce ce 0 4 the mobilization of lree except redox active ce results from the dissolution and or the mobilization in colloidal form of the secondary phosphates which may present major negative ce anomalies as indicated by the bulk soil analyses at the saprolite gneiss weathering front therefore the groundwaters have the signature of saprolitic materials we propose that the strong positive ce anomalies of the upper saprolite result from ancient weathering processes these processes would have led to much stronger ce iii ce iv dissolution reprecipitation conditions than those occurring at the present time in the saprolite in the 0 45 m filtered brook acidic blackwaters lree and th contents are high 100 th 400 ppt 400 la 1700 ppt in this organic matter rich environment lree and th are highly mobilized the colloial pool controls half of the whole lree th load the geochemical signature of these waters is independent of the gneissic parent rock and soil lree contents there is no ce anomaly this signature is similar to those of other organic rich rivers draining humid tropical ecosystems in the world in the 0 45 m filtered sanaga waters lree and th contents are lower than in the brooks 15 th 40 ppt 40 la 300 ppt these concentrations are dependent on the time and on the colloidal composition however 90 of these elements are mainly controlled by the suspended pool

magmatism and deformation southern revillagigedo island southeastern alaska the mid cretaceous cale alkaline lree enriched plutons on southern revillagigedo island southeastern alaska form two distinct groups 1 biotite epidote hornblende tonalite of the moth bay pluton and 2 biotite bearing hornblende absent leucotonalite higher total ree and lower na 2 o al 2 o 3 and sr distinguish the moth bay tonalite from the leucotonalite the plutons are surrounded by narrow contact metamorphic aureoles containing kyanite and staurolite pressure estimates for emplacement based on metamorphic conditions of contact rocks and from igneous amphibole compositions are 8 9 kbar detailed analyses of pluton host rock relationships indicate that pluton emplacement was synchronous with orogen deformation pre emplacement thrusting placed the metavolcanic and metasedimentary rocks of the taku terrane structurally above the younger metasedimentary rocks of the gravina belt the contact between these units was subsequently cut by the moth bay pluton the southern margin of the moth bay pluton has been transformed to blastomylonite contact metamorphic textures and undeformed dikes which cut the blastomylonite indicate that shearing did not outlast crystallization of the pluton a still later phase of deformation transposed the pluton related structures and generated a northeast dipping ductile thrust fault southern revillagigedo shear zone along the southern shore of revillagigedo island the shear zone and earlier structures all trend approximately northwest fission track age determinations and track length measurements in apatite were used to determine the cooling histories of the plutonic units fission track ages from the western part of the study area are 51 3 4 0 ma apatite 57 8 4 4 ma zircon and 69 8 4 2 ma sphene eastward toward the coast batholith fission track ages are 10 m y younger these younger ages may result from a tectonic thermal event associated with batholith emplacement this same event may have generated north trending structures that overprint older northwest trending structures on the east side of the study area

structural and geochronologic relations along the western flank of the coast mountains batholith stikine river to cape fanshaw central southeastern alaska abstract geologic and u pb geochronologic studies in central southeastern alaska provide constraints on the mid cretaceous to mid tertiary deformation west of the coast mountains batholith the ne dipping sumdum and fanshaw faults record the w directed emplacement of the ruth assemblage yukon tanana terrane and taku terrane over the subjacent gravina belt and alexander terrane ductile fabrics of the sumdum fanshaw fault system truncate early formed foliation and thrust faults in albian and older gravina belt strata u pb age data from syn and post tectonic plutons suggest that deformation was ongoing at 92 9 3 0 ma but had mostly ceased by 90 ma the sumdum fanshaw fault system marks the tectonic boundary between the alexander wrangellia terrane and inboard yukon tanana and stikine terranes and lies within a thrust belt that extends from southern alaska to northern washington mid cretaceous structures are truncated to the east by ne dipping ductile fabrics of the coast shear zone sheet like paleocene tonalites were emplaced into and deformed within the shear zone undeformed mid eocene dikes cross cut the tonalites kinematic relations in the coast shear zone suggest a complex displacement history that includes both east side up reverse and west side up normal shear the shear zone probably accommodated the collapse of overthickened crust developed during mid cretaceous shortening

terrane motion by strike slip faulting of forearc slivers forearc slivers bounded by a trench and an active strike slip fault occur in about 50 of modern subduction zones analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1 2 cm yr tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction active arc parallel strike slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique strike slip rate increases with greater convergence obliquity

tilt and northward offset of cordilleran batholiths resolved using igneous barometry considerable controversy surrounds the suggestion based on palaeomagnetic evidence that large segments of the north american cordillera travelled long distances parallel to the coast during the latest cretaceous and early tertiary periods well after the amalgamation of exotic terranes discordant palaeomagnetic data from mid cretaceous plutonic rocks of the peninsular ranges batholith of coastal southern california and baja and the mount stuart batholith of the cascade range in washington play a pivotal role in the controversy the discordant data were originally interpreted to reflect northward transport of 1 000 km relative to cratonal north america after the batholiths cooled through their magnetic blocking temperatures1 5 more recently it has been argued that the discordances arise from local tilting of batholiths rather than northward offset6 7 here we present and implement new methods based on hornblende barometry8 for determining the palaeohorizontal in granitic batholiths and correcting palaeomagnetic data for tilting our results indicate that the peninsular ranges and mount stuart batholiths have undergone northward offsets of 1 000 450 and 2 900 700 km respectively and also significant tilting

mineralogy and geochemistry of claystones from the guadalupian lopingian boundary at penglaitan south china insights into the pre lopingian geological events abstract the guadalupian lopingian g l boundary at a stratigraphically well documented outcrop in penglaitan guangxi autonomous region south china has been approved as the global stratotype section and point gssp several volcanic ashes or tuffs occur at this boundary but their mineralogy and geochemistry are not available yet and no reliable age for this boundary has been obtained a combined study of mineralogy geochemistry and geochronology has been carried out in this study on six layers of claystones collected below group 1 and above group 2 the g l boundary at the penglaitan section both group 1 and group 2 claystones are likely clastic in origin rather than volcanic ashes as previously thought thus the penglaitan claystones are not suitable for age determination of the g l boundary they are significantly different in terms of mineralogy and geochemistry specifically group 1 claystones are likely derived from a mafic source which is genetically related to the emeishan large igneous province therefore providing additional evidence for the synchroneity between the g l boundary mass extinction and the emeishan volcanism group 2 samples were derived from a felsic source of which zircons yield an age spectrum peaked at 262 3 ma undistinguishable within the uncertainty from the currently accepted g l boundary age 260 4 0 4 ma nevertheless group 2 samples are not related to emeishan volcanism because their negative zircon ehf t values differ significantly from those of emeishan magmas and trace element compositions of zircons are indicative of an arc source rather than a within plate source in consideration of paleogeographic reconstruction we propose that the group 2 claystones may have been derived from continental arcs during the palaeo tethys evolution this is the first sedimentary evidence for permian continental arc in the northern margin of palaeo tethys

genetic relationship between the emeishan plume and the bauxite deposits in western guangxi china constraints from u pb and lu hf isotopes of the detrital zircons in bauxite ores abstract the formation processes and alumina sources of the large and super large bauxite deposits that have developed in west guangxi in the yangtze block have been debated for a long time shrimp zircon u pb dates combined with in situ hf isotopic data of the detrital zircons from permian bauxite ores in the western guangxi province china provide new constraints on the genesis of the bauxites in west guangxi u pb dating ages of the detrital zircons cluster around 256 ma and 261 ma this is consistent with the emplacement age of the emeishan plume thus it has been determined that the detrital zircons come from magmatic rocks related to the emeishan plume and that the emeishan plume has significant control on the formation of bauxites in addition this work presents new evidence for the evolution of the emeishan plume most of the e hf t values of the zircons around 256 ma and 261 ma are negative varying widely from 1 3 to 16 1 this data indicates the melting of yangtze block basement rocks and the mixing of magmas generated both from the mantle and from the crust occurred at the periphery of the plume in the main phase of emeishan lip magmatism this study verified that the plume induced uplift exposed carbonates as well as the mafic and felsic rocks generated by the emeishan plume to long periods of intense weathering under humid tropical conditions the weathering remnants of the high alumina magmatic rocks and carbonates were deposited on or transported to the paleokarst surface and converted to bauxites

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

ce anomalies in the textural components of upper cretaceous karst bauxites from the apulian carbonate platform southern italy the chemical and mineralogical composition of upper cretaceous apulian karst bauxites southern italy and their textural components i e ooids and matrix has been studied the bulk samples are composed of boehmite hematite anatase and kaolinite the samples collected along a vertical profile show a downward enrichment for the elements rb sr ba ni and cr a similar distribution is observed in deposits bauxitized in situ at the expense of matrix like material collected in the karst zone the ooids consist mainly of hematite with minor boehmite and anatase whereas in the matrix boehmite prevails on hematite kaolinite and anatase in the void fillings in the matrix there is a ca fluorocarbonate having a ce ce of 5 8 the ooids with the exception of ce are enriched in ree and show a higher la yb ch ratio relative to the matrix the matrix exhibits a large positive ce anomaly whereas the ooids have negative ce anomaly the ce fractionation between the textural components can be explained assuming 1 ce oxidation and cerianite precipitation in the uppermost part of the deposits 2 scavenging of ree from ce depleted percolating solutions by the iron oxide inducing both ree enrichment and ce negative anomalies in ooids 3 remobilization of cerium as fluoride complex as a consequence of more acidic conditions in the uppermost part of the deposit and precipitation of ce3 as fluorocarbonate mineral toward the carbonate bedrock barrier at alkaline ph alternatively the cerium remobilization possibly as a carbonate fluoride complex could be due to an eh decrease favoured by a rise of the groundwater level

mineralogical characteristics of the superlarge quaternary bauxite deposits in jingxi and debao counties western guangxi china abstract in recent decades more than 0 5 billion tons of ores scattered in the quaternary laterite in western guangxi china have been explored the ores were derived from a bauxite horizon in permian via physical break up and re sediment process utilizing various test methods i e xrd dta tg dtg sem eds and epma the mineralogical characteristics of the quaternary bauxite ores in jingxi and debao counties were investigated xrd was used together with tg dtg to obtain relatively accurate ore mineral abundance diaspore is the major phase whereas hematite kaolinite anatase chamosite gibbsite goethite illite and rutile are minor diaspore is characterized by a small particle size low degrees of crystallinity and complex chemical composition both gibbsite and goethite have a varied particle size and goethite crystals contain high al substitution and si it is clarified that diaspore chamosite and anatase were formed in a mildly reduced and alkaline depositional environment in permian while gibbsite hematite goethite and part kaolinite were precipitated from al 3 si 4 and fe 3 enriched solutions within an quaternary oxidized environment the ions al 3 si 4 and fe 3 are mostly released from chamosite in its dissolution process the different physicochemical conditions between the permian depositional and the quaternary weathering periods resulted in a complex mineral assemblage in the quaternary bauxite

wide angle seismic imaging across accreted terranes southeastern alaska and western british columbia abstract this study addresses the question of crustal moho and uppermost mantle structure across an accreted terrane continental arc and fold and thrust belt in southeastern alaska and western british columbia the 186 km long portland canal line of the accrete wide angle seismic dataset across the coast mountains is analyzed using a combination of travel time analysis techniques including delay time tomography turning ray tomography reciprocal time analysis intercept time inversion and forward ray tracing additional constraints on the model are obtained from a coincident vertical incidence seismic section and from correlation with the mapping of geologic structures plunging into the seismic section the study reveals moderately high crustal velocities low velocity gradient in the middle crust and decreasing average crustal velocity and a north deepening moho termination of crustal reflectivity across a vertical zone the coast shear zone csz indicates that the csz is most likely a strike slip fault associated with a transpressive tectonic regime a mid cretaceous thrust system mapped on the surface to the west of the csz is imaged by several groups of mid to lower crustal reflectors extending close to the moho indicating it was a thick skinned thrust system ne dipping fabric imaged within the coast mountains batholith cmb is associated with a ductile deformation during early eocene crustal extension the crustal section under the cmb which has an average velocity of 6 55 km s and shallower than average crustal thickness of 31 km can be considered as corresponding to the lower two thirds of an average crustal section which has been inflated by intrusions of high velocity tonalite to gabbro sills

87 sr 86 sr and 18 o 16 o isotopic systematics and geochemistry of granitoid plutons across a steeply dipping boundary between contrasting lithospheric blocks in western idaho a spatially abrupt geochemical boundary is preserved within four plutonic complexes along the western margin of the cretaceous idaho batholith near mccall idaho these intrusives ranging in composition from tonalite to granite were emplaced across a regional boundary between accreted oceanic arc terranes and the continental margin and their isotopic major element and trace element geochemistry provide detailed information about this change in crustal characteristics at depth indicating that the boundary is nearly vertical and extends deep into the lithosphere the hazard creek complex emplaced west of the transition in wall rock lithology has initial 87sr 86sr ri less than 0 7045 and 18o greater than 7 5 indicating little or no continental crust in its source region however elevated 18o requires some incorporation of rocks formed or altered at the earth s surface a large shift in ri and 18o is observed across the 5 8 km wide little goose creek complex which was emplaced across the wall rock boundary this is interpreted as mixing between 1 a basaltic or andesitic magma with low k2o and high na2o al2o3 and sr similar to that forming the hazard creek complex and 2 materials similar to precambrian sedimentary sedimentary rocks with low sr high 18o 15 and high ri 0 83 at 100 ma the payette river complex emplaced east of the wall rock boundary exhibits at least one additional component with low 18o 6 moderate ri 0 708 and mafic composition this component is inferred to be old basaltic material in the lower crust or upper mantle similar to that inferred to be a minor part of the peninsular ranges batholith in se california silver et al 1979 hill et al 1986 the easternmost complex in the idaho transect is made up of granites that may contain a component of granitic cratonal basement the entire west to east geochemical transition from oceanic arc magmas to cratonal magmas takes place over a lateral distance of less than 20 km although the zone of transitional protolith dominated by metasedimentary rocks is unusually narrow and may have been in part tectonically removed the striking geochemical similarities between this traverse and several other transects across much broader areas of nevada and california suggest that the craton itself was not rifted apart but that juxtaposition of the accreted oceanic arc terranes occurred along the preexisting craton margin the data confirm that the isotopic geochemistry of granitoid plutons can be used as a probe of deep lithospheric character and that major lateral variations in the lithosphere on the order of one to two kilometers in width can be recognized in favorable circumstances

transformation from permian to quaternary bauxite in southwestern south china block driven by superimposed orogeny a case study from sanhe ore deposit abstract permian karstic bauxite and its quaternary derivative in western guangxi southwestern south china block possess a total tonnage greater than 0 5 billion tons the primary late permian karstic bauxite formed in reduced environment in the background of tethyan accretionary orogenesis and as one consequence of cenozoic convergence of the indian and eurasia continents the primary orebody was uplifted eroded and re sedimented within quaternary laterite the geochemical variation and its controls during the ore transformation from permian to quaternary remain poorly understood quaternary ore blocks comprise an inner zone of fresh ore and then it gradually transited through a middle zone to a margin with extensive weathering one such bauxite block was selected and further subdivided into twenty three samples for geochemical and mineralogical analysis the inner and middle zones contain similar mineralogical compositions dominated by diaspore and amesite with minor illite anatase goethite pyrite zircon and rutile the margin is composed of diaspore with small amounts of amesite boehmite illite goethite anatase kaolinite zircon rutile and barite bauxite in all three zones is composed of mainly al si fe and ti and high contents of zr cr li f s zn v sr nb ba and ree variations in fe2 and fe3 between the three zones were observed the elements si al fe2 mg ba cr f li ni zn and ree decrease from the core of the ore block outwards corresponding to an increase in s and fe3 depletions in si al fe2 mg ba and cr were caused by the dissolution of amesite most of the al and si in amesite were lost during the weathering and minor retained to form kaolinite depletions in li ni and zn resulted from changes in the depositional environment between the late permian and quaternary dissolution of ree bearing fluorocarbonates resulted in depletions of ree and f the enrichment of fe3 and s was related to the precipitation of goethite hematite and barite in an oxidizing environment while local enrichment of ce resulted from the redox change of ce3 ce4 under the same condition this shows that the chemical composition of laterite enwrapping the bauxite also took part in quaternary bauxite transformation this study shows that the elements migrations during bauxite transformation were influenced by multiple independent factors except for the elemental attributes

two phase evolution of accretionary margins examples from the north american cordillera abstract the coast mountains orogen southeastern alaska and the salmon river suture zone western idaho record the accretion of oceanic crustal fragments to the north american cordilleran margin structural elements associated with these boundaries include a western accretion related thrust belt and steeply dipping post accretionary shear zone the shear zones the coast shear zone southeastern alaska and western idaho shear zone idaho are steeply dipping crustal scale structures characterized by steep isotopic and geophysical gradients the presence of syndeformational tabular calc alkaline tonalitic plutons concordant with shear zone fabrics locally preserved dextral strike slip indicators and pervasive down dip mineral and elongation lineations are consistent with an intra arc transpressional origin for both structures in addition to strike slip translation the structures accommodated exhumation of high pressure moderate temperature metamorphic rocks within the thrust belt to the west and moderate pressure high temperature batholithic rocks to the east similarities between the structures in alaska and idaho suggest that the observed two stage history of underthrusting of accreting crustal fragments followed by modification by intra arc transpressional shear zones is a common tectonic process our analysis implies that recognition of processes active during the accretion of crustal fragments to continental or cratonal margins is commonly inhibited by post accretion deformation and provides insight into lower crustal processes presently operative under modern strike slip accretionary boundaries in addition the intra arc transpressional shear zones e g the coast and western idaho shear zones are structures that could have accommodated paleomagnetically determined large magnitude transcurrent displacements

tectonic framework of the upper paleozoic and lower mesozoic alava sequence a revised view of the polygenetic taku terrane in southern southeast alaska fragments of upper paleozoic and lower mesozoic metavolcanic and metasedimentary sequences of the taku terrane are exposed discontinuously along a narrow belt in southeast alaska and form a distinct lithostratigraphic package in the ketchikan area called the alava sequence crinoidal and argillaceous marble carbonaceous phyllite argillite mafic flows pillow breccia pyroclastic tuff and quartzite characterize the sequence these strata are unconformably overlain by upper jurassic to lower cretaceous fine to coarse grained epiclastic rocks of the gravina sequence the upper paleozoic part of the alava sequence may be correlative with the yukon tanana terrane whereas the middle and upper triassic portion of the alava sequence may represent a metamorphic vestige of the stikine terrane both parts are now exposed on the western flank of the coast plutonic complex in contrast with their correlatives to the east these relations suggest that the stikine and alexander terranes were juxtaposed prior to deposition of the gravina sequence the western boundary between rocks of north american affinity and allochthonous ensimatic crustal fragments of the alexander and wrangellian terranes lies west of the coast plutonic complex

orebody vertical structure and implications for ore forming processes in the xinxu bauxite deposit western guangxi china abstract the xinxu bauxite orebodies which are contained in quaternary depressions overlying strata of permian to devonian age were formed through the break up oxidation and re sedimentation of a pre existing permian bauxite horizon with progressive karsitification the underlying strata of the quaternary orebodies were eroded out downwards in sequence gradually transiting from permian to devonian thus an older underlying stratum generally infers a higher degree of weathering a total of 47 exploration wells sampled each meter were chosen from several orebodies to evaluate the vertical distribution of four major components al 2 o 3 fe 2 o 3 sio 2 and loss on ignition in the ores the vertical evenness is quantified via a box gliding multifractal model the small width of the multifractal spectrum suggests that the vertical ore quality is even overall indicating an intertwining break up and mixing up processes during evolution of the quaternary orebody although the four components show complex spatial distributions topographic elevation displays a positive relationship with the mean fe 2 o 3 content and a negative correlation with the vertical unevenness of al 2 o 3 and fe 2 o 3 in the wells this is taken to reflect the more uniform oxidizing environment in the higher depressions which facilitates a homogeneous and greater increase of fe 2 o 3 content throughout the orebody in contrast the redox conditions in lower depressions are less oxidizing and varied in space inducing great variation in degree of fe 2 o 3 increase in the ores this study also demonstrates that in the xinxu deposit the ore forming redox environment has more impact on the ore quality than the degree of weathering as the contents show irregular vertical changes in the lower depressions it follows that it is the ancient redox environment rather than the modern conditions which affect the orebody vertical structure this clarifies the fact that the vertical structure of the orebody is jointly constrained by the physical melange accumulation of ores and the differences between the ancient redox environments in the higher and lower locations these factors should be the primary factors affecting the vertical structure of karst bauxite deposits

40ar 39ar constraints on the emplacement uplift and cooling of the coast plutonic complex sill southeastern alaska a 700 km long and 5 to 15 km wide aggregation of sill like tertiary plutons separates low to medium grade greenschist to amphibolite facies metamorphic rocks to the west from medium to high grade upper amphibolite to granulite facies gneisses and granitic plutons to the east along the western boundary of the coast plutonic complex in southeastern alaska and british columbia 40ar 39ar analyses of hornblende biotite and plagioclase samples from a sea level traverse and a 1 to 2 km elevation traverse across one of the sills near holkham bay southeastern alaska yield dates that vary systematically with position samples collected on the eastern margin of the sill are 3 to 6 m y younger than samples of the same mineral collected at the same elevation from the western margin of the sill indicating that the western margin cooled earlier samples collected at sea level yield 40ar 39ar dates on hornblende biotite and plagioclase that are consistently younger than those collected at 1 and 2 km elevation dates from the center and eastern margin of the sill define isotherm migration rates km m y of 0 8 1 3 at 60 56 ma hornblende 0 8 0 6 at 56 53 ma biotite and 0 4 at 54 50 ma plagioclase 40ar 39ar dates from a single sea level sample collected 15 km farther south indicate no significant variation in dates parallel to the southeasterly strike of the sill for at least 30 km the younger dates calculated for the east side relative to the west side and metamorphic temperatures suggest that the country rock was hotter to the east than to the west when the pluton cooled through mineral blocking temperatures the rapid isotherm migration rates calculated from the vertically distributed samples and the suggestion of hotter country rock to the east during cooling is consistent with an interpretation that the pluton intruded during uplift of the rocks to the east because the relative precision of the 40ar 39ar technique is sufficient to detect subtle difference in the cooling ages its application to spatially distributed samples within a homogeneous plutonic body as in this study may yield new insights into the timing of metamorphism magmatism and tectonism of young mountain belts

redistribution and mobilization of titanium zirconium and thorium in an intensely weathered lateritic profile in western australia abstract the mobility of titanium zirconium and thorium elements commonly considered insoluble during supergene weathering is still not well understood especially in intensely weathered regolith thus an intensely weathered lateritic profile jg developed on meta granitoids in jarrahdale western australia was investigated the mobility of ti zr and th has been assessed at both mineral assemblage and profile scales and the mode of occurrence has been investigated through the combined use of geochemical data from bulk regolith particle size fractions and sequential extractions with in situ data determined by electron probe micro analyzer and synchrotron x ray powder diffraction neoformed poorly crystalline phases containing trace to minor amounts of zr ce and th unassociated with silicates or phosphates were identified on the walls of al fe rich pores in the ferruginous duricrust this implies that some mobilization and redistribution of zr and th occurs within a sample scale breakdown of primary thorite and rare earth element rich fluorocarbonates is thought to be the source for zr and th in the neoformed phases rather than zircon thus the mineral hosts of zr ti and th in the parent rock and their relative susceptibility to weathering are the fundamental controls on subsequent mobility during initial weathering trace amounts of th in secondary phases such as rhabdophane and florencite show translocation of th at the mineral scale while strong partitioning of th into gravel rather than matrix reflects redistribution of th at the profile scale the absence of primary sphene from the regolith and the presence of partially dissolved ilmenite and rutile grains in the ferruginous mottled zone suggest mobilization and translocation of ti at a mineral assemblage scale furthermore the fluctuation of ti zr in the ferruginous zone is in contrast to the consistency of zr hf throughout the profile in general within the range of parent meta granitoid this suggests that ti and zr fractionate from each other and partition between gravel and matrix during extreme weathering and advanced lateritization this study demonstrates that ti zr and th are mobile at a variety of scales an important consideration that is often overlooked when calculating element mass flux in intensely weathered regolith