erosional troughs in deep sea carbonates and their relationship to basement structure abstract a detailed survey of a small area in the eastern equatorial pacific with the marine physical laboratory s deep tow instrument package reveals erosionally excavated troughs in an area of relatively rapid accumulation of carbonate the troughs lie on top of a basement high implying a relationship between the troughs and the basement feature differential accumulation and compaction may explain the origin of the troughs sediment deposited around the basement high preferentially accumulates on gentle slopes and in troughs this results in a thick sequence of sediment on the flanks of the basement high and a thin sequence above on top of the hill unstable sediment masses and differential compaction generate a tensile stress field above the peak of the basement high which causes slumping and faulting the steep slopes generated by faulting and slumping may then serve to intensify tidal currents or act as spillways and locally generate increased turbulence this increased turbulence results in local erosion by dissolution a trough will form above a basement high however only when a combination of 1 basement steepness 2 high sedimentation rate 3 sediment thickness and 4 sediment erodibility are present

deformation of ductile inclusions in a multiple inclusion system in pure shear this paper analyzes the deformational behavior of mutually interacting spherical inclusions in a multiple inclusion system considering two physical factors viscosity ratio between inclusion and matrix m and the ratio of inclusion diameter to mean inter inclusion distance a b for a given value of m the strain partitioning between a stiff inclusion and the bulk system i e ratio of their natural extension rates increases non linearly with increasing a b ratios and the gradient of increase becomes steeper when the inter inclusion distance is less than about twice their diameter i e a b about 0 5 the strain distribution within a deformed inclusion is homogeneous when the a b ratio is less than about 0 6 for larger values of a b the internal deformation becomes heterogeneous with the strain increasing or decreasing towards the core in the case of stiff m 1 and soft m 1 inclusions respectively the deformed shape of inclusions in section also shows departure from an ideal ellipse with an increase in the a b ratio stiff inclusions develop shapes similar to that of a super ellipse in contrast to soft inclusions that resemble a sub ellipse the heterogeneity of internal deformation is also reflected in the distortion of passive foliations initially at right angles to the bulk extension direction which become curved with convexity outward and inward respectively within stiff and soft inclusions

heat flow variations correlated with buried basement topography on the juan de fuca ridge flank systematic heat flow variations are commonly observed on sedimented flanks of mid ocean ridges these variations have been attributed to the convective circulation of pore fluids within the oceanic crust and have been used to estimate the scale and pattern of this circulation here we report similar systematic variations observed in a detailed heat flow and seismic reflection survey over sediment covered 3 myr old sea floor on the eastern flank of the juan de fuca ridge we find that heat flow peaks are located above buried basement ridges and adjacent to two minor basement outcrops heat flow lows are found where thick sediments fill basement valleys the simple inverse relationship between heat flow and sediment thickness defined by the data indicates that hydrothermal circulation sealed within the permeable portion of the oceanic crust is vigorous enough to maintain approximately isothermal 10 k conditions at the sediment crust interface as no systematic variations in estimated basement temperatures are observed it seems that at this location the heat flow variations result only from sediment thickness variations and do not reflect a particular convective pattern

new precise zircon u pb and muscovite 40ar 39ar geochronology of the late cretaceous w sn mineralization in the shanhu orefield south china abstract the shanhu orefield located in the northeastern guangxi province is an important part of w sn polymetallic metallogeny in the nanling region the shanhu w sn deposit is spatially associated with the yantianling granite la icpms zircon u pb dating of the yantianling granite yields late cretaceous ages of 100 9 2 2 ma and 100 8 2 3 ma hydrothermal muscovite 40ar 39ar dating yields plateau ages of 102 7 1 7 ma and 100 8 0 7 ma this date coincides well with the la icpms zircon u pb age of the yantianling granite indicating a spatial temporal link between the w sn mineralization and the granitic magmatism in combination with regional geological data it is suggested that a late cretaceous mineralization event related to granitic magmatism occurred in the nanling region which might have resulted from regional lithospheric extension

non ellipsoidal inclusions as geological strain markers and competence indicators abstract geological objects that do not deform homogeneously with their matrix can be considered as inclusions with viscosity contrast such inclusions are generally treated as initially spherical or ellipsoidal theory shows that ellipsoidal inclusions deform homogeneously so they maintain an ellipsoidal shape regardless of the viscosity difference however non ellipsoidal inclusions deform inhomogeneously so will become irregular in shape geological objects such as porphyroblasts porphyroclasts and sedimentary clasts are likely to be of this kind with initially rectilinear prismatic or superelliptical section shapes we present two dimensional finite element models of deformed square inclusions in pure shear parallel or diagonal to the square as a preliminary investigation of the deformation of non ellipsoidal inclusions with viscosity contrast competent inclusions develop marked barrel shapes with horn like corners as described for natural ductile boudins or slightly wavy rhombs incompetent inclusions develop dumb bell or bone shapes with a surprising degree of bulging of the shortened edges or rhomb to sheath shapes the results lead to speculation for inclusions in the circle to square shape range and for asymmetric orientations anticipated shapes range from asymmetric barrels lemons or flags for competent inclusions to ribbon or fish shapes for incompetent inclusions we conclude that shapes of inclusions and clasts provide an important new type of strain marker and competence criterion

gold mineralization of the chencai suichang uplift and tectonic evolution of zhejiang province southeast china abstract a metallogenic model is proposed in which gold and base metal mineralization in se china is related to plate interactions along the pacific margin with southeast asia from about 900 to 70 ma all depositional orogenic and metallogenic processes can be explained by subduction events along northeast trending zones with progressive accretion and cratonization along the continental margin these subduction zones moved southeast for a distance of less than 150 km as a result the area is one of crustal reworking in which the rocks and mineral deposits of earlier events have been repeatedly modified the chencai suichang uplift is an uplifted crustal block in central zhejiang province of mainland china in which the proterozoic basement is exposed as tectonic windows in overlying mesozoic volcanics several gold and base metal deposits are located in these proterozoic windows as well as in the younger volcanics most economic deposits were formed during the last major tectono magmatic event affecting the area the yanshanian 160 170 ma event although there is evidence for earlier mineralization that has been remobilized during the yanshanian it is suggested that crustal reworking was particularly important in remobilising and upgrading gold mineralization the model presented is supported not only by local geology and isotopic data but also by the regional distribution of gold deposits which are arranged in a 50 km wide belt parallel to the yanshanian subduction zone three major episodes of mineralization occurred during 1 a subduction related period of metamorphism partial melting and mesothermal fluid generation and mineralization in the lower crust during the caledonian 2 subduction related volcano plutonic activity with epithermal mineralization during the early yanshanian and 3 hydrothermal mineralization during late yanshanian tectono magmatic event in the third two main categories of metallogenic control are recognized namely 1 hydrothermal fluid generation in the lower crust and 2 upper crust structural and lithological controls on mineralization

the nicoya convergent margin a region of exceptionally low heat flow over 100 measurements of seafloor heat flow reveal that the accretionary complex adjacent to the nicoya peninsula is characterized by remarkably low heat flow values over the accretionary prism average 28 mw m and values in the trench and the ocean crust seaward of the trench average 14 mw m we attribute the low heat flow to effective hydrothermal cooling of the upper crust on the subducting plate and suggest that extensional faults created by flexure of the lithosphere enhance hydrothermal circulation thermal models show that subduction of low temperature crust combined with significant frictional heating at the decollement can explain the low and uniform heat flow disparity between heat flow values observed on the lower trench slope with model results suggests upward advection of heat by porewater flux through broadly distributed conduits

modelling the bulk viscosity of two phase mixtures in terms of clast shape abstract the rheological behaviour of many rock types will depend on how mixtures of different minerals and rock clasts behave together this paper develops two dimensional modelling of the bulk viscosity of two phase viscous mixtures with different shape fabrics in pure shearing based on self consistent mechanics and behaviour of inclusion matrix systems an equation is presented for the bulk viscosity in terms of the phase viscosity contrast phase fractions and clast shape results are given for idealised two dimensional mixtures of circles squares or ellipses with a variety of phase viscosity contrasts in pure shearing parallel perpendicular to the shape fabric the normalised bulk viscosities of all these mixtures fall between the theoretical upper and lower viscous bounds given by homogeneous strain rate and homogeneous stress respectively but nearer to the upper bound with increasing shape fabric mixtures with a particular geometry are shown to have a constant relative horizontal distance between these two bounds as proposed by p d bons using graphs for the normalised bulk viscosities of mixtures with variably elliptical aligned shape fabrics the results for fabrics orthogonal to pure shearing can be extrapolated to a diagonal fabrics in pure shearing and b orthogonal or diagonal fabrics in simple shearing these reveal the bulk viscous anisotropy of mixtures with shape fabric in two dimensions it is speculated that during a progressive finite deformation two phase mixtures might undergo changes in bulk viscosity over time with steady stiffening during pure shear and perhaps cyclic stiffening and softening during simple shear the two dimensional models provide an approximation to the three dimensional properties of mixtures with shape fabrics with potential applications to rocks including conglomerates

role of weak flaws in nucleation of shear zones an experimental and theoretical study this paper investigates the role of inherent weak flaws in the formation of plastic zones in deforming solids to understand the development of geological shear zones physical experiments were carried out on polymethylmethaacrylate pmma models containing single and multiple circular cylindrical flaws under compression maintaining plane strain condition models with single flaws show development of shear zones against a flaw in the form of conjugate sets with an average dihedral angle of 84 and oriented at an angle of 42 to the bulk compression direction the shear zones are generally tapered with increasing width away from the flaw in models with multiple flaws shear zones nucleated against individual flaws which propagated and coalesced with one another forming through going band like shear zones with inclination varying from 35 to 53 with the bulk compression direction with an increase in flaw concentration the through going shear zones defined persistent conjugate sets we applied the plane theory of elasticity for numerical simulations of ductile shear zones under the influence of a single circular weak flaw the pattern of shear zones yielded from numerical runs grossly matches with those observed in physical model experiments theoretical analysis demonstrates that the presence of a flaw promotes nucleation of shear zones at a bulk stress below the yield strength of matrix this critical stress is a non linear function of the flaw matrix competence contrast and decreases asymptotically with increasing competence contrast

viscous inclusions in anisotropic materials theoretical development and perspective applications abstract theories and numerical solutions for a viscous ellipsoid in an infinite anisotropic viscous medium subjected to far field homogeneous deformation lie at the heart of self consistent homogenization models and multiscale simulations of texture and fabric development in earth s lithosphere there is considerable literature on ellipsoid inclusions focused on anisotropic elastic materials published in multi disciplinary fields to make this body of work more accessible as well as to advance viscous inclusion studies an effort is made here to summarize recent advances and to further develop formally more explicit and where possible analytic solutions for incompressible viscous materials the point force concept and equivalent inclusion method of eshelby are used together with the green function approach this leads to generalized equations for ellipsoid inclusion behaviors in anisotropic materials in the particular case of isotropic materials the new mathematical development here enables the use of existing methods for elastic materials to get solutions for corresponding viscous inclusion problems efficiently and accurately a 2d formulation is also presented for elliptic cylinders in plane straining flows of anisotropic materials using the same green function method that is adopted for 3d inclusions the 2d formulation is benchmarked with existing analytic solutions a reconnaissance investigation to compare the behaviors of 2d elliptic inclusions and triaxial ellipsoids in a matrix of planar anisotropy undergoing far field plane straining flows suggests that conclusions drawn from 2d cannot be applied to 3d in anisotropic cases the application of the viscous inclusion theory to the rheologically heterogeneous and non linear lithosphere is discussed by regarding a heterogeneous element as an ellipsoidal inclusion embedded in a hypothetical homogeneous equivalent matrix whose effective rheology is obtained through micromechanical homogenization and assuming that the conditions of scale separation are satisfied the viscous inclusion theory forms the backbone of multiscale models for natural deformation and the accompanying texture and fabric development

zircon u pb geochronology and geochemistry of two episodes of granitoids from the northwestern zhejiang province se china implication for magmatic evolution and tectonic transition abstract granitoids 175 80 ma representing a prominent yanshanian jurassic to cretaceous magmatic event in south china widely intrude the precambrian crystalline basement and paleozoic strata here we report zircon u pb age data geochemical characteristics and sr nd isotopes of the late jurassic and early cretaceous granitoids from the northwestern zhejiang province zxb of southeastern china our results reveal two distinct episodes for the yanshanian magmatism the jiemeng and datongkeng granodiorites formed at 148 6 1 1 ma whereas the huangshitan jiuligang and ruhong aluminous a type granites were generated between 129 0 0 6 ma and 126 1 1 1 ma the two magmatic phases represent a tectonic transition from an active continental margin to post orogenic setting during the late jurassic ca 150 ma to early cretaceous ca 128 ma geochemically these intrusions are granodioritic to granitic in composition and show an affinity of s type and a type granitoids respectively the s type granodiorites of jiemeng and datongkeng are characterized by moderate sio 2 65 0 69 6 wt high k 2 o na 2 o 5 0 7 6 wt k 2 o na 2 o 1 2 1 5 zr 31 109 ppm sr 71 190 ppm and high field strength elements low to intermediate mg and moderate nb depletion the a type granites of huangshitan jiuligang and ruhong are characterized by high sio 2 72 7 77 2 wt k 2 o na 2 o 6 9 8 8 wt k 2 o na 2 o 1 3 2 1 fe t fe t mg ga 17 29 ppm 20 ppm commonly zr 96 197 ppm and sr 8 45 ppm with slight nb depletion the s type granodiorites have higher mg a nk sr sr ba sr y la yb n and lree hree and lower sio 2 k 2 o na 2 o ga and zr with weak negative eu anomalies compared to those of the a type granites with negative eu anomalies all these rocks show y nb ratios 1 2 high initial 87 sr 86 sr i sr ratios and low e nd t and are depleted in nb ti and sr indicating crustal origin with subduction zone signatures we suggested that the zxb s type granitic bodies might have been derived from the mesoproterozoic metamorphic basement rocks through partial melting induced by mantle derived magma followed by limited fractional crystallization the zxb aluminous a type granites were also derived from a similar magma source but underwent fractional crystallization at higher crustal levels the a type granites in the zxb correlate with a post orogenic tectonic setting a geological comparison between zxb and adjacent areas indicates that the geochemical features of the zxb a type granites are comparable with the baijuhuajian a type granites and also the adjacent areas a type granites but are distinct from the late cretaceous a type granites 105 90 ma distributed along the southeastern coastal area of south china we correlate the formation of the zxb s type granodiorites to inland compression associated with the subduction and collision of the paleo pacific plate in the late jurassic 170 145 ma in contrast the a type granites formed under a post orogenic setting during the early cretaceous period 145 120 ma resulting from lithospheric thinning and continent extension accompanied by slab roll back of the paleo pacific plate following the subduction collision event

mesozoic overthrust tectonics in south china whether south china was a platform or a para platform after the mid paleozoic deformation or whether the mountains of huanan south china form a mesozoic collision type orogen depends upon whether the ophiolite melange in the yangtze region is precambrian basement or was overthrust onto a foreland thrust belt during the mesozoic this paper presents evidence of tectonic windows under the melange and basement klippes above the melange in the yangtze region

numerical modelling and comparison of the temporal evolution of mantle and tails surrounding rigid elliptical objects in simple shear regime under stick and slip boundary conditions abstract structures associated with rigid inclusions are a rich source of evidence to understand the local deformation regime the behaviour of rigid objects in modelled here as being immersed in a linear newtonian fluid with either i a stick boundary condition continuity of stress and velocity across the boundary or ii a slip boundary condition continuity of boundary normal stress and velocity across the boundary with zero shear stress at the boundary of particular interest are the types of structures developed in a concentric region adjacent to the object termed the mantle a model of the displacement of points around the inclusion comprises a set of ordinary differential equations which are solved numerically a comprehensive set of simulations for a variety of mantle sizes object aspect ratios initial orientations as well as different boundary conditions has been performed a comparison between natural examples and model output indicates a level of consistency the resulting structures differ in detail and in a broader sense in general type structures only develop when stick boundary conditions are in operation in contrast type structures at high strain are restricted to slip boundary conditions slip conditions also tend to be the source of complex mantle types involving more than one generation of mantle structures or wings furthermore our model indicates that using asymmetry of orientation of objects relative to the shear direction may be problematic when used alone particularly if stick boundary conditions prevail but that together with mantle structures there is less chance of confusion

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

high strain deformation of conglomerates numerical modelling strain analysis and an example from the wutai mountains north china craton abstract conglomerates have been widely used to investigate deformation history and rheology strain vorticity and viscosity previous studies reveal that several factors such as pebble shapes and concentrations as well as material properties affect conglomerate deformation however how pebble concentration and interaction between pebbles affect deformation is not understood very well we use the 2d numerical modelling platform elle coupled to the full field crystal visco plasticity code vpfft to simulate the deformation of conglomerates with various viscosity contrasts between pebbles and matrix and different pebble concentrations with both linear stress exponent n 1 and power law n 3 viscous rheologies under simple shear conditions up to a shear strain of ten pebbles can behave as effectively passive deformable or effectively rigid an increase in pebble concentrations viscosity contrasts enhances pebble deformation but reduces their rotation a mean aspect ratio rf orientation plot is proposed to gain an estimate of pebble deformation behaviour and the amount of bulk strain closely spaced rigid or deformable pebbles can form clusters that mechanically act as single inclusions rigid clusters rotate and survive for only short strain increments whereas the more stable deformable ones keep on elongating with minor rotation we provide a natural example of deformed conglomerates from the wutai mountains north china craton these consist of banded iron formation bif pebbles embedded in a schistose matrix using the mean rf plot a finite strain of 6 under simple shear could be determined the viscosity of the pebbles is estimated at about 5 8 times that of the matrix for a linear rheology n 1 or 2 to 5 times if a power law rheology with n 3 is assumed

petrology and geochemistry of siliciclastic rocks of mixed feldspathic and ophiolitic provenance in the northern apennines italy provenance in clastic sedimentary rocks in the northern apennines is determined by integrated petrographic and geochemical criteria thus mafic poor arkoses in the gottero formation senonian paleocene deposited prior to the alpine orogeny were derived from granitic provenance in the sardinian corsican massif in the gottero formation chemical criteria distinguish between very low input of detrital mafic material and co c and ni added entirely by adsorption on clay minerals post alpine orogeny feldspathic sandstones in the macigno formation mid oligocene lower miocene and cervarola formation lower miocene reflect a more diverse provenance of granitic metamorphic ophiolitic and sedimentary rocks in the apennines and alps the macigno and cervarola rocks are distinguished by serpentine gabbro and diabase fragments and higher chlorite contents than in the gottero rocks chemically the macigno and cervarola rocks contain distinctive signatures expressed in higher mgo co cr and ni contents of the mafic ultramafic detritus a wider range and more scattered cr and ni variation and higher niggli ti for a given niggli al alk content the mgo co cr and ni abundances indicate up to 10 ultramafic ophiolitic detritus in the macigno and cervarola sandstones and up to 17 ophiolitic detritus in associated shales estimated by comparison of mixtures of feldspathic sands with ligurian peridotites up to 10 basaltic ophiolitic detritus in these sediments is suggested by cao and fe2o3 total abundances petrographic data indicate that a major part of the detritus in the macigno and cervarola formations is from alpine granitic basement of calc alkaline granite to tonalite plutons and gneisses but apart from higher ba in macigno and cervarola sandstones than in the gottero sandstones this is difficult to identify chemically the trace elements cr ni ba rb ce la y co cu ga pb and zn generally have positive correlations with niggli al alk a chemical surrogate measure of sheet mineral clays micas and chlorites content whereas sr which is associated with carbonate and feldspar shows negative correlation with al alk an olistostrome unit contains the most weathered detritus but otherwise shows similar chemical variation to the other formations methods to 1 identify separately mafic and ultramafic inputs 2 recognize in shales co cr and ni derived from ophiolitic sources and non ophiolitic sources 3 estimate the minimum mafic ultramafic proportion in a sediment may have general application elsewhere in sedimentary and metasedimentary rocks

estimation of viscosity contrast and finite strain from deformed elliptical inclusions a method of strain analysis is described which takes into account a possible competence difference between the strain markers and the rest of the rock using data consisting of the sectional shapes and orientations of groups of inclusions from conglomerate like rocks limits are placed on the possible effective viscosity contrast between the inclusions and the rock as a whole as well as on the bulk finite strain suffered by the rock these results are calculated using a computer program based on the theory for the deformation of elliptical cylinders the method can be considered an extension of the rf method but provides more information and involves less restrictive assumptions

gold and silver metallogeny of the south china fold belt a consequence of multiple mineralizing events abstract the south china fold belt is part of the south china block that is interpreted to be the result of multiple tectonic and magmatic events that formed a collage of accreted proterozoic and phanerozoic terranes the jurassic to early cretaceous yanshanian period 180 90 ma a time of major tectono thermal events that affected much of eastern and southeastern china is of great metallogenic importance in the fold belt this period is linked to subduction of the pacific plate beneath the eurasian continent and is manifested by voluminous volcano plutonic activity of predominantly calc alkaline affinity the distribution of gold and silver deposits in the south china fold belt indicates the presence of two distinct metallogenic provinces a region of basement uplifts which are controlled by shear zones and form neoproterozoic inliers of metamorphosed iron rich rock types defines the first province in this province orogenic lodes and volcanic related epithermal deposits represent the more significant precious metal mineralization the second province is essentially confined to a belt of yanshanian felsic intermediate volcanic and subvolcanic rocks that extends along most of the southeastern china coast in an area known as the coastal volcanic belt deposits in the coastal volcanic belt are silver and or copper rich volcanic hosted and epithermal in character the precious metal metallogeny of the south china fold belt is interpreted to have developed in at least three stages one as a result of collision events during the caledonian orogeny ca 400 ma the second during the indosinian orogeny ca 200 ma and the third during or soon after the formation of the yanshanian magmatic belt yanshanian orogeny 180 90 ma the latter was responsible for a hydrothermal event that affected large sections of the belt and its proterozoic substrate this may have resulted in the redistribution and enrichment of precious metals from preexisting orogenic gold lodes in neoproterozoic basement rocks which are now exposed as windows in zones of tectonic uplift the yanshanian hydrothermal activity was particularly widespread in the coastal volcanic belt and resulted in the formation of both low and high sulfidation epithermal gold and silver and locally copper and other base metal mineralization it is suggested that the coastal volcanic belt has greater potential for world class epithermal and porphyry deposits than previously realised

collision between the north and south china blocks a crustal detachment model for suturing in the region east of the tanlu fault a crustal detachment model based on the interpretation of linear aeromagnetic anomalies surface geological observations and deep seismic profiles is proposed for the continent continent collision between the north and south china blocks east of the tanlu fault the model suggests that during the mid mesozoic collision between the two continental blocks the upper crust of the south china block in the subei yellow sea region was detached from the lower crust and thrust over the north china block for 400 km whereas the lower part of the lithosphere was subducted under the north china block along a subsurface suture running east from nanjing the sinistral offset of the qinling suture by the tanlu fault is only 110 120 km in the deep crust much less than previously suggested

types and distribution of silver ore deposits in china silver is generally recovered as a by and co product along with au pb zn cu sb bi hg sn and w from polymetallic and gold mines in china chinese silver deposits can be classified into five principal types according to host rocks these types and their host rocks are 1 marine volcanic sedimentary rocks hosting la massive sulfide pb zn cu ag ores and 1b stratiform ag au ores 2 continental volcanic sedimentary rocks containing 2a various polymetallic silver veins and or stockworks and 2b ag au veins in pyroclastic rocks 3 rocks affected by metasomatic processes including 3a complex sn polymetallic ag ores 3b pb zn cu ag skarns 3c associated w ag quartz veins and or skarns and 3d ag au zones and veins in altered carbonate rocks 4 metamorphic intrusive rocks principally ag au veins in sheared and brecciated metamorphic rocks and 5 sedimentary rocks including 5a stratiform pb zn ag ores in carbonate rocks 5b ag v mantos in black shales and 5c cu ag layers in red sandstones a sixth grouping includes gossans the tectonic settings geological features and temporal and spatial distribution of these different types of silver occurrences indicate that silver mineralization reflects to a great extent the evolution of tectonic environments in china throughout geological time type 1 is generally developed in association with several fold belts from the caledonian to yanshanian orogens while types 2 3 and 4 correlate with rejuvenation of the eastern china continent during the mesozoic period the three subgroups of type 5 are recognized in different stages and various sedimentry and diagenetic environments during the development of paraplatforms and fold belts type 6 results from recent weathering of existing sulfide ores or protores

the galapagos spreading center at 86 w a detailed geothermal field study appendix is available with entire article on microfiche orderfrom american geophysical union 2000 florida avenue n w washington d c 20009 document j80 013 01 00 payment mustaccompany order we report here measurements of the heat flow field of the galapagos spreading center on crust of age less than 1 0 m y the 443 measurements in an area of about 570 km2 reveal the general planform of the geothermal flux and permit the first truly areal estimate of the near axis conductive heat flux the intrusion process and associated hydrothermal circulation dominate the surface heat flow pattern with circulation apparently continuing beyond the limits of our survey the areal average of the conductive heat flux is 7 1 0 8 hfu 295 33 mw m2 about one third the heat flux predicted by plate models the remaining heat is apparently removed by venting of hydrothermal waters at the spreading axis and through basalt outcrops and hydrothermal mounds off axis the pattern of surface heat flux is lineated parallel to the axis and the strongly lineated topography sharp lateral gradients in the heat flow greater than 10 hfu km near escarpments and commonly expressed as high heat flow at the tops of the scarps and lower heat flow in the valleys may indicate a local concentration of the circulation by surface fault systems and or variable sediment thickness

chert in the palaeoproterozoic bartle member killara formation yerrida basin western australia a rift related playa lake and thermal spring environment abstract palaeoproterozoic chert in the bartle member of the killara formation yerrida group yerrida basin of western australia contains an anomalous association of crystal structures and rock fabrics that are difficult to interpret the association appears to indicate an evaporitic pyroclastic thermal spring environment associated with rifting at about 2 2 ga the chert member contains silica pseudomorphs after evaporite minerals that in places enclose relict isolated crystals and aggregates of crystals of gypsum and anhydrite the evaporite minerals are associated with minerals such as barite and analcime the association of these minerals together with palaeoenvironmental evidence invites comparison with rocks of the afar region and lake magadi in the east african rift system the bartle member locally contains anomalous gold up to twelve times the average crustal values of 4 ppb and barium it also contains finely disseminated kerogen together with numerous spheroids classified here as microdubiofossils and some curious structures showing organisation into complex petal like structures classified here as putative microfossils the bartle member chert has many characteristics of playa lake and thermal spring deposits and may host epithermal precious metal mineralisation

spatiotemporal reconstruction of late mesozoic silicic large igneous province and related epithermal mineralization in south china insights from the zhilingtou volcanic intrusive complex silicic large igneous provinces slips generally reflect large scale melting of lower crustal materials and represent significant metal reservoirs the south china block coastal region scb cr slip hosts several large epithermal deposits to better understand these deposits we document the spatio temporal framework of the host slip across the scb cr using zircon u pb dating and geochemical and isotopic analysis we identify four stages of emplacement stage 1 felsophyre ca 149 ma shows a chemical affinity to highly fractionated i type granites stages 2 and 3 of low mg felsic volcanics ca 128 to 111 ma and stage 4 felsite ca 100 ma have higher ehf t and end t values than stage 1 felsophyre suggesting a significant contribution of newly underplated juvenile crust to the magma sources stage 4 diabase ca 101 ma was likely produced by melting of subduction metasomatized asthenospheric mantle together with reliable published data we build a new spatio temporal framework of volcanics and infer that the majority of the scb cr slip was related to the gradual northwestward subduction of the izanagi plate beneath south china in a continental arc setting during ca 170 to 110 ma and minor contribution was from the eastward retreat of the subducting slab in a back arc setting during ca 110 to 90 ma we conclude that the large scale epithermal mineralization was generated by melting of the metal rich thin 30 40 km newly underplated hydrous juvenile crust during the tectonic transition from arc to back arc settings

numerical modelling of deformation and fluid flow in the shuikoushan district hunan province south china the shuikoushan district in hunan province south china contains major pb zn au ag polymetallic mineralisation two groups of numerical models have been constructed to study the interactions between deformation and fluid flow in the district during the yanshanian compression event 180 to 90 ma the first group includes district scale conceptual models of coupled deformation and fluid flow during folding the models show that fluid flow patterns are controlled by deformation within the fold system inferred for the district during regional shortening and folding fluids are generally focused towards the fold hinge core areas along higher permeability layers in particular permian limestone units in preference to flowing across the low permeability seal units permian and jurassic terrestrial sequences the efficiency of this fluid focusing can only be significantly increased if these folded seal units are allowed to undergo permeability increase as a result of tensile failure the modelling results show that permeability enhancement localises mostly at fold hinges dominantly within the silicified zone on the top of the permian limestone unit this process results in increased flow velocities and facilitates fluid focusing towards fold hinge core locations at this silicified rock horizon the second group includes deposit scale models for the kangjiawan deposit which is one of the two major deposits in the shuikoushan district the models show patterns of tensile failure permeability creation fluid focusing and mixing and fracture development along a selected exploration cross section through the deposit these results are consistent with the observed brecciation and mineralisation features regions of maximum brecciation in the district are associated with 1 a combination of fold hinge and fault intersection locations structural and 2 the silicified zone and permian limestone unit lithological such brecciation zones are associated with extensive fluid focusing and mixing and therefore represent the most favourable locations for mineralisation in the district on the basis of this work ideas for future research work and mineral exploration in the district are proposed

thermal models of the middle america trench at the nicoya peninsula costa rica 1 we present thermal models of the 24 ma cocos plate subducting under the caribbean plate at the nicoya peninsula costa rica our modeling incorporates the effect of hydrothermal cooling of the seafloor and the absence of an accretionary prism initial geotherms of the incoming plate representing different effective cooling depths of hydrothermal circulation affect the thermal structure and temperatures along the main subduction thrust excluding the deforming prism results in a higher heat flow immediately landward of the trench than previous models and therefore significant frictional heating is not required conductive plate cooling over predicts heat flow across the frontal forearc with no or very low frictional heating models with hydrothermal cooling to depths of 2 km under predicts the heat flow comparisons with recently determined hypocenter locations indicate that the updip limit of seismicity is consistent with temperatures between 100 150 c but there are large along strike variations in both the seismicity and thermal regime

hydrothermal pits in the biogenic sediments of the equatorial pacific ocean 1 a recent expedition to the equatorial pacific surveyed over 60 000 km2 of seafloor using swath map imaging and high resolution seismic profiling this survey revealed the common occurrence of large pits in the biogenic sediments that are usually associated with basement highs the pits range in size up to a few kilometers across and are on the order of 50 m 100 m deep they were found on seafloor ranging in age from 15 ma to 55 ma with overlying sediment cover up to 500 m thick we link their origin with hydrothermal discharge through conduits created by differential compaction and fracturing over basement highs and by basement faulting that penetrates the overlying sediments although carbonate dissolution is likely to be associated with the formation of the pits their occurrence shows no consistent relationship with the present calcite compensation depth ccd

the behaviour of deformable ellipsoidal particles in three dimensional slow flows implications for geological strain analysis abstract eshelby s equations governing the motion of a viscous inclusion embedded in a matrix of contrasting viscosity are solved numerically for arbitrarily shaped and arbitrarily oriented ellipsoidal particles the theory has important implications for geological strains and strain analysis specifically it predicts that particles more competent than their matrix will have deformed shapes with larger k values than equivalently shaped and oriented passive markers the effect is significant even for viscosity ratios as low as r 1 5 and increases with increase in r for general applications numerical solutions must be sought nevertheless there are useful exceptions 1 1 the two dimensional deformed shape of viscous particles can be computed graphically provided the matrix strain is irrotational pure shear and the section ellipse is a principal plane of the particle ellipsoid for sections through a three dimensional system with arbitrarily oriented particles this method and those based on sections through an elliptic cylinder is inaccurate the reliability is further diminished for simple shear 2 2 irrotational deformations in three dimensions can be computed on transformed flinn plots if the particle or average particle axial orientations are parallel with the applied flow similar diagrams may be used to make independent estimates of viscosity contrasts in polymict conglomerates

space time patterns of magmatism along the tethysides a preliminary study five maps showing spatio temporal evolution of magmatism along the tethysides for the following time intervals late carboniferous and permian 320 248 ma triassic and early jurassic 247 188 ma

epithermal deposits in south china geology geochemistry geochronology and tectonic setting abstract south china block scb is the broad area including the yangtze craton in the northwest and huanan orogen in the southeast it is an important epithermal metallogenic province in china containing at least 1 high sulfidation hs and 42 low sulfidation ls au ag cu pb zn sb epithermal deposits porphyry type mineralization was recognized in four of the ls deposits and thus they were regarded as ls p type these 43 deposits are mainly located in 1 the lower yangtze river belt and 2 the northeastern jiangnan orogenic belt in the yangtze craton 3 the wuyi yunkai orogenic belt and 4 the southeast coastal volcanic belt in the huanan orogen they are mostly located in mesozoic volcanic basins especially where the regional faults and their subsidiaries occurred the host rocks include jurassic cretaceous volcanic sedimentary rocks coeval or slightly older subvolcanic granitoids and breccias and metamorphic basement rocks the alteration of the hs epithermal deposit zijinshan cu au zoned from silicic vuggy quartz through alunite to dickite and phyllic alteration zones from the ore veins outwards the alteration of the ls deposits is zoned from adularia chalcedony bladed calcite or quartz pseudomorphs after bladed calcite in ore veins to distal illite sericite chlorite kaolinite assemblages for those ls p systems besides the dominated ls alteration assemblages phyllic and potassium silicate alteration related to porphyry mineralization were identified acid leaching textures and vein stockwork and breccia structures are common in hs deposit while the ls epithermal deposits are characterized by open space filling crustifications colloform banding and comb structures the ore forming fluids are low temperature low salinity meteoric water dominated in most epithermal deposits in scb with variable input of magmatic water the ore components were derived from both the deep magma and host rocks and transported upwards or laterally and precipitated in the fracture systems by fluid boiling mixing and cooling most of the epithermal deposits are formed at depth of

strain localization during high temperature creep of marble the effect of inclusions abstract the deformation of rocks in the earth s middle and lower crust is often localized in ductile shear zones to better understand the initiation and propagation of high temperature shear zones induced by the presence of structural and material heterogeneities we performed deformation experiments in the dislocation creep regime on carrara marble samples containing weak limestone or strong novaculite second phase inclusions the samples were mostly deformed in torsion at a bulk shear strain rate of 1 9 10 4 s 1 to bulk shear strains between 0 02 and 2 9 using a paterson type gas deformation apparatus at 900 c temperature and 400 mpa confining pressure at low strain twisted specimens with weak inclusions show minor strain hardening that is replaced by strain weakening at 0 1 0 2 peak shear stress at the imposed conditions is about 20 mpa which is 8 lower than the strength of intact samples strain progressively localized within the matrix with increasing bulk strain but decayed rapidly with increasing distance from the inclusion tip microstructural analysis shows twinning and recrystallization within this process zone with a strong crystallographic preferred orientation dominated by r and c slip in recrystallization induced weakening starts at local shear strain of about 1 in the process zone corresponding to a bulk shear strain of about 0 1 in contrast torsion of a sample containing strong inclusions deformed at similar stress as inclusion free samples but do not show localization the experiments demonstrate that the presence of weak heterogeneities initiates localized creep at local stress concentrations around the inclusion tips recrystallization induced grain size reduction may only locally promote grain boundary diffusion creep accordingly the bulk strength of the twisted aggregate is close to or slightly below the lower isostress strength bound determined from the flow strength and volume fraction of matrix and inclusions

chemical plumes from low temperature hydrothermal venting on the eastern flank of the juan de fuca ridge we report evidence for chemical anomalies in the water column from low temperature ridge flank hydrothermal venting during cruises in 1992 and 1994 samples were taken from the water column for trace metals nutrients dissolved gases and particles near each of three basaltic outcrops overlying 3 5 m y old crust on the eastern flank of the juan de fuca ridge in cascadia basin the water column above one of these outcrops baby bare which rises about 70 m above a flat turbidite plain was the most thoroughly sampled thermal chemical mn fe 3he ch4 and o2 and particulate anomalies in the water column confirm the existence of 1 early diagenesis of organic matter in seafloor sediment which produces a flux of dissolved metals and nutrients to bottom seawater 2 hydrothermal emissions which are both focused spring like and diffuse and 3 resuspension of sediment by turbulent flow of tidal currents about a topographical high on the basis of data from the water column and thermal and chemical pore water data from 46 piston and gravity sediment cores near and on baby bare flankflux 90 and 92 we constrain the composition of seawater in basement and thus the composition of spring like water given this composition no measurable dissolved silica or phosphate hydrothermal anomalies are expected in the water column

influence of sedimentation local and regional hydrothermal circulation and thermal rebound on measurements of seafloor heat flux 1 we quantify the influence of environmental processes on measurements of seafloor heat flux with a new one dimensional thermal model that includes time varying sedimentation and boundary conditions represents several scales of hydrothermal and transient conductive processes within basement and allows fluid seepage through accumulating sediments variations in basement thermal conductivity the extent of hydrothermal mixing in upper basement and fluid seepage through sediments each influence seafloor heat flux by 2 8 conductive thermal rebound following the cessation of advective heat loss from the crust may lower seafloor heat flux values by 5 10 even after several to several tens of million years the new models indicate that thermal rebound takes much longer than suggested by earlier analytical calculations mainly because earlier models did not account for the heat capacitance of the conductive lithosphere application of the new model to 3 5 3 6 ma seafloor on the eastern flank of the juan de fuca ridge suggests that anomalously low heat flux in this area is best explained by incomplete conductive thermal rebound in the last 100 200 ka following the burial of numerous basement outcrops application of the new model to the eastern flank of the east pacific rise and extrapolation out to the age of some of the oldest remaining seafloor indicate that sedimentation corrections may be important even where accumulation rates are typical of global values model results also suggest that conductive thermal rebound through thick sediments may bias measurements made on moderate to old seafloor even where there is little evidence at present for ridge flank hydrothermal circulation

origin of elevated sediment permeability in a hydrothermal seepage zone eastern flank of the juan de fuca ridge and implications for transport of fluid and heat elevated sediment porosity and permeability may help to focus upward fluid seepage observed over a buried basement high on the eastern flank of the juan de fuca ridge consolidation and permeability tests of fine grained hemipelagic and turbidite sediments from the seepage site and from nearby sites that are not experiencing seepage indicate that sediments at the seepage site primarily hemipelagic have 10 15 higher porosity and 10 higher permeability at a given depth and 10 100 lower hydraulic impedance for the 40 m thick sediment column the correlation of consolidation and permeability properties with sediment type rather than location indicates that the relatively high porosity and permeability do not result from fluid flow but are intrinsic properties of the hemipelagic sediment on the basis of measured sediment properties fluid pressure at the top of basement is 5 kpa greater than hydrostatic a simple circulation model that incorporates this estimate and proximity to the nearest basement outcrops 4 20 km suggests that local basement permeability is between 3 10 13 and 3 10 11 m2 within the range measured in nearby boreholes the measured sediment properties combined with other published data indicate that a few tens of meters of fine grained terrigenous hemipelagic or calcareous marine sediment effectively seal the basement aquifer whereas much thicker siliceous or pelagic sediment may support thermally and chemically significant fluid flow the large contrast in hydraulic impedance among sediment columns of different types could cause regional variation in the evolution of ridge flank hydrothermal systems and in the contribution of seepage to ridge flank fluxes

quartz vein morphology and implications for formation depth and classification of archaean gold vein deposits abstract a simplified scheme to categorise quartz vein morphology from gold deposits is based on the growth direction of quartz or pseudomorphed chalcedony in the veins the categories are scale independent and are 1 face control 2 displacement control 3 parallel control 4 radiating 5 non directional control 6 replacement and 7 modified quartz vein textures do not indicate any particular type of mineralisation for the depth continuum of archaean lode gold deposits of the yilgarn craton all seven categories of texture are variously present however crustal depth is a major control parallel and radiating textures are characteristic of gold deposits formed at the near surface displacement and non directional control are typical of deposits at midcrustal greenschist and amphibolite conditions and face control shows a range of conditions from the near surface to midcrustal replacement and modified textures embrace a wide variety of alteration and deformation conditions and can occur in gold deposits at all crustal levels

invisible gold distribution on pyrite and ore forming fluid process of the huangshan orogenic type gold deposit of zhejiang se china implications from mineralogy trace elements impurity and fluid inclusion studies the huangshan orogenic type gold deposit in zhejiang of se china occurred in quartz pyrite veins it is hosted by phyllonite that underwent greenschist facies metamorphism along a large jiangshan shaoxing tectonic belt with a ne sw direction trace elemental characteristics ore forming process and invisible gold on different forms of pyrite and quartz are studied the au associated pyrite can be classified into two categories recrystallized pyrite and euhedral pyrite the precipitation of invisible au on pyrite is mainly derived by co and ni with auhs2 complex in the mineralizing fluids in different events the xps results revealed that valence states of au3 replaced 2fe2 in the pyrite and au0 replaced si4 in the quartz structure the electron paramagnetic resonance and trace elemental results suggested that the element pairs of ge li al in quartz and mn co ni in pyrite have distinct impurities as identified a fluid inclusion study showed that the auriferous quartz is characterized by low saline and co2 rich fluids coexistence of the type i type iii inclusions and same range of homogenization temperature with different mode are evidences of immiscible fluid process the temperature pressure values of ca 250 c 1250 bar and ca 220 c 780 bar for gold precipitation have been calculated by intersection of coexisting fluids during the entrapment the huangshan orogenic type gold deposit may be associated with the wuyi yunkai orogeny during the early paleozoic including an upper mid greenschist facies metamorphism 450 420 ma all the features suggest that the huangshan gold deposit is probably a product linking with the early paleozoic orogeny in south china

near axis heat flow measurements on the northern juan de fuca ridge implications for fluid circulation in oceanic crust present models of the cooling of oceanic crust suggest that convection of hydrothermal fluid is a major component of the process in axial regions abundant faults and open fissures are associated with the venting of high temperature hydrothermal fluid in older crust where the insulating sediment cover is thick previous studies have shown that basement topography is the dominant forcing factor for within crust fluid circulation in the intermediate region where young crust is lightly sedimented heat flow data are difficult to obtain with traditional techniques to determine whether topography or permeability is the dominant process controlling fluid circulation in the near axis region we conducted a profile of heat flow measurements using the submersible alvin on the endeavour segment of the juan de fuca ridge our data indicate that topographic forcing is responsible for the long wavelength variations with high heat flow at the ridge summits and low values in the inter ridge valleys the locations of the extreme values of heat flow taken within the context of subsurface faulting are consistent with a model where a ridge valley topographic pair comprises a single circulation cell this model predicts that the source area for the high temperature axial vents may be in the flanking inter ridge valleys

non steady homogeneous deformations computational techniques using lie theory and application to ellipsoidal markers in naturally deformed rocks abstract the dynamic theory of deformable ellipsoidal inclusions in slow viscous flows was worked out by j d eshelby in the 1950s and further developed and applied by various authors we describe three approaches to computing eshelby s ellipsoid dynamics and other homogeneous deformations the most sophisticated of our methods uses differential geometric techniques on lie groups this lie group method is faster and more precise than earlier methods and perfectly preserves certain geometric properties of the ellipsoids including volume we apply our method to the analysis of naturally deformed clasts from the gem lake shear zone in the sierra nevada mountains of california usa this application demonstrates how given three dimensional strain data we can solve simultaneously for best fit bulk kinematics of the shear zone as well as relative viscosities of clasts and matrix rocks

hydrothermal circulation juan de fuca ridge eastern flank factors controlling basement water composition pore water has been analyzed from sediment cores taken from three areas on the eastern flank of the juan de fuca ridge as part of flankflux 90 a study of hydrothermal circulation through mid ocean ridge flanks seismic reflection and heat flow surveys davis et al 1992a indicate that the three areas differ in sediment thickness basement topography abundance of outcrops basement temperature and fraction of heat lost by advection versus conduction area 1 is on 0 6 ma crust with nearly continuous basement outcrop area 2 is on 1 3 ma crust over the first buried ridge parallel to the present ridge axis and area 3 is on 3 5 3 8 ma crust over two axis parallel buried ridges that penetrate the sediment cover in three locations each area includes a hydrothermal system in which seawater flows into basement reacts with crustal basalt and then exits basement either through the sediment or directly into the overlying water column as constrained by concentrations of sulfate and lithium in the pore waters at least some seawater enters basement in all three areas without reacting fully with the overlying sediment even where no outcrops are known nearby speeds of up welling of pore water through the sediment have been estimated by fitting profiles of dissolved magnesium and chlorinity which behave conservatively in these areas to numerical time dependent transport models the estimated velocities range from 0 1 to 7 4 cm yr faster flows probably occur but were not sampled upwelling speed correlates positively with heat flow and basement highs and negatively with sediment thickness the correlation with heat flow differs from area 2 to area 3 along with differences in physical properties of the turbidite sediment we have documented pore water upwelling through sediment up to 100 m thick we estimate that upwelling continues at decreasing speeds through sediment up to 160 m thick corresponding to a heat flow of 0 44 w m2 in area 2 and 0 3 w m2 in area 3 concentrations of magnesium and chlorinity in the altered seawater upwelling from basement are uniform within each area but differ from one area to the next both species remain at the bottom seawater concentration in area 1 where basement is cooled to 10 c at the base of the sediments mainly by advection the concentration of magnesium decreases with increasing basement temperature in areas 2 and 3 to a minimum of 2 5 mmol kg at about 90 c in area 3 the transition from largely advective to largely conductive heat loss occurs over only 20 km between areas 1 and 2 and corresponds to a dramatic change in the composition of fluid circulating through basement as the uppermost basement is heated from 10 to 40 50 c chlorinity of the basement fluid increases above the present day bottom seawater concentration in areas 2 and 3 and in nearly all other mid ocean ridge flanks studied to date as a result of rock hydration and the higher chlorinity of bottom seawater during the last glacial period while chlorinity generally correlates positively with uppermost basement temperature in various ridge flank hydrothermal systems it reaches a maximum in area 2 at only 40 c probably because alteration there occurs at a lower water rock ratio than elsewhere for all mid ocean ridge flanks studied to date the temperature at the basement interface correlates better with the fraction of heat lost by advection versus conduction and with the average thickness of the sediment cover than with crustal age

geological strain analysis a manual for the rf φ technique by richard j lisle pergamon press oxford 1985 viii 99 pp 15 hard cover 8 95 soft cover

rotation behaviour of rigid inclusions in multiple association insights from experimental and theoretical models many rocks are representative of inclusion matrix systems where rigid inclusions float on a ductile matrix the geological analogues of such inclusion matrix systems are theoretically modelled with the help of jeffery s 1922 theory in recent years there have been a number of reinvestigations on the rotational motion of rigid inclusions to account for some geological observations that are not yet predicted in models based on jeffery s theory adding to this effort this paper investigates the effects of 1 inclusion concentration and 2 the degree of coherence at the inclusion matrix interface on the rotation behaviour of rigid inclusions in a multiple inclusion system during simple shear deformation shear box experiments were run on models containing multiple identical elliptic cylinders of wax mimicking rigid inclusion embedded within putty representing ductile matrix two sets of experiments were performed with and without lubrication at the inclusion matrix interfaces each model had an isolated inclusion to reveal the variation of rotations of single and multiple inclusions increase in inclusion concentration results in contrasting rotation behaviour of isolated and multiple inclusions initially oriented parallel to the shear direction when the interfaces were not lubricated inclusions in multiple association rotated antithetically whereas the isolated one almost remained stationary in models wherein the inclusion matrix interfaces were lubricated with liquid soap both isolated and multiple inclusions rotated antithetically but the isolated inclusion rotated at a lower rate with increasing inclusion concentration the difference in the rates of antithetic rotation between isolated and multiple inclusions tended to be larger in both lubricated and non lubricated conditions the sense of rotation of isolated as well as multiple inclusions initially oriented perpendicular to the shear direction was always synthetic irrespective of the nature of the interface lubricated or non lubricated however the instantaneous rotation rates of inclusions in multiple association were higher than that of the isolated inclusions disposed in the same orientation rotation of inclusion in an inclusion matrix system is basically induced by the traction exerted by the flowing matrix on the surface of the inclusion during deformation employing a hydrodynamic model it is shown that mutual mechanical interaction among inclusions which is a function of inclusion concentration modifies the stresses at the inclusion matrix interface to a large extent moment calculations reveal that inclusions oriented parallel to the shear direction experience an antithetic moment in response to the normal stress components the magnitude of which increases with increasing inclusion concentration this implies that rotation of shear parallel inclusions in antithetic sense is favoured by higher inclusion concentration on the other hand inclusions oriented perpendicular to shear direction experience a moment that induces synthetic rotation the magnitude of the synthetic moment is larger for larger inclusion concentration leading to increase in the rate of synthetic rotation using the theoretical model the moments are calculated as a function of the aspect ratio of inclusions and the inferences based on this moment calculation are complemented with experimental findings

regional heat flow variations across the sedimented juan de fuca ridge eastern flank constraints on lithospheric cooling and lateral hydrothermal heat transport seafloor heat flow has been estimated continuously along a transect on the sedimented eastern flank of the juan de fuca ridge to provide constraints on the scale and rate of lateral heat and fluid transport in the upper igneous crust and on the total flux from the young lithosphere beneath the profile extends from 20 km east of the ridge axis where turbidite sediments lap onto the oceanic crust at an age of less than 1 ma and ends 110 km east of the axis over 3 6 ma crust estimated heat flow is derived from the combination of the depth to igneous basement determined from seismic reflection profiles and basement temperatures measured in nine ocean drilling program boreholes along the transect the strategy is based on the knowledge that in this young area upper basement temperatures are locally homogenized by vigorous hydrothermal circulation and hence vary smoothly between drilling sites at the young end of the transect near the region of extensive outcrop heat flow is only 15 of that predicted for lithosphere of this age heat flow increases systematically to the east reaching the full predicted level about 20 km from the point of sediment basement onlap the anomaly can be accounted for by mixing and or eastward flow of cool water in the upper igneous crust beneath the sediment at a darcian rate of the order of 1 m yr a geochemical signature of fresh seawater in basement is observed many tens of kilometers farther to the east although the thermal signature is no longer present along the remainder of the transect the heat flow conforms very well to that predicted by simple boundary layer cooling theory once the thermal effects of rapid sedimentation on this ridge flank are accounted for

early mesozoic thrust tectonics of the northwest zhejiang region southeast china the nw zhejiang region of south china occupies a key tectonic position near the suture zone of the yangtze and cathaysian blocks and is of critical importance for the assembly of east asia sedimentological and tectonic analyses indicate that the region had a se dipping paleoslope in the late paleozoic to early triassic a transitional sedimentary environment from deep sea to continental molasse is documented in the early triassic late triassic interval associated structures are nw vergent folds and thrusts that root southeastward beneath the high grade chencai metamorphic complex the structural styles of this foreland fold and thrust belt are characterized by multifold duplexes and individual folds that are together zoned from se to nw as follows 1 core zone characterized by shear folds and ductile thrusts 2 se belt with out of sequence thrusting of multifold duplexes and an average shortening of 50 3 central belt with duplexes imbricate fans and an average shortening of 40 and 4 nw belt with jura type folds and a shortening of 10 a tectonic model for the foreland fold and thrust belt is discussed in relation to the early mesozoic archipelago paleogeography of south china

analysis of superposed strain a case study from barr conglomerate in the south delhi fold belt rajasthan india abstract the structural geometry in the barr conglomerate and the neighboring rocks in the western flank of the meso to neoproterozoic south delhi fold belt indicate superposed deformation with structures developed by horizontal dextral simple shear deformation superimposed on earlier structures formed during approximately ese wnw compression and subvertical maximum elongation the first deformation produced nne ssw trending subvertical schistosity and associated steeply plunging isoclinal folds mineral lineation and pebble elongation lineation having almost downdip alignment on the schistosity surface the second deformation produced dextral folds both on bedding and schistosity surfaces and modified the shape of the pebbles already deformed by the first deformation on vertical sections perpendicular to the schistosity trace the pebbles show sub ellipsoidal shape with their mean elongation direction parallel to the schistosity trace on the horizontal section the pebbles often show asymmetrical shape and asymmetrical deflection of the schistosity surface around the pebbles the mean elongation direction makes a small angle 2 8 in the counter clockwise sense with the schistosity surface this obliquity is due to modification of post first deformation sub ellipsoidal shape by later horizontal simple shear using the schistosity surface as the movement plane analytical expressions are derived for the modification of an original ellipse by simple shear parallel to the long axis of the ellipse a family of curves has been generated to depict the change in axial ratio and orientation of long axis for different initial axial ratios and for different values of shear strain using these curves it has been possible to factorize the total strain into two components representing the earlier compression and the later simple shear it is noted that the computed earlier shortening strain ellipsoids are consistently in the flattening field close to the line of pure oblate ellipsoids in the flinn plot there is no systematic spatial control on the variation in the values of shear strain strain ellipsoid shape parameter k and intensity of distortion d as one proceeds south to north along the strike of the conglomerate

geochronology and magmatic oxygen fugacity of the tongcun molybdenum deposit northwest zhejiang se china the tongcun mo porphyry deposit in northwest zhejiang is hosted in three porphyry units huangbaikeng songjiazhuang and tongcun from southwest to northeast u pb zircon ages of 162 3 0 ma for the huangbaikeng porphyry 159 9 3 0 ma for the songjiazhuang porphyry and 167 6 155 6 ma for the tongcun porphyry indicate that these intrusions formed during the jurassic and are most likely associated with the northwestward subduction of the izanagi plate trace element compositions of zircons from the tongcun deposit constrain the oxygen fugacity fo2 of the magma using zircon ce anomalies and ti in zircon temperatures the average magmatic fo2 for the porphyries in the tongcun deposit is fayalite magnetite quartz fmq 2 7 which is similar to the shapinggou fmq 3 2 and dabaoshan fmq 3 5 mo porphyry deposits but much higher than that of the reduced cretaceous ore barren shangjieshou porphyry fmq 1 1 around 8 km away from the tongcun deposit the distinct difference in magmatic oxygen fugacity between the jurassic and cretaceous porphyries may help to explain the absence of mo porphyry mineralization in northwest zhejiang during the cretaceous

the effect of non passive clast behaviour in the estimation of finite strain in sedimentary rocks abstract existing methodologies that use ellipsoidal objects for the analysis of geological strain typically assume that these objects acted passively during deformation this assumption when not valid can lead to significant underestimates of strain in rocks deformed under low grade conditions in orogenic forelands this is especially true when clastic sedimentary rocks are utilized to measure strain the competency contrast between clasts and matrix possibly leading to marked non passive behaviour the systematic nature of this finite strain underestimation may allow for a correction of strain estimates when this type of behaviour is evident

extensional detachment at the inclusion matrix interface in a multiple inclusion system abstract this paper deals with a theoretical analysis of extensional fissures around spherical rigid inclusions hosted in a ductile matrix undergoing bulk shear the analysis shows that the mechanical interaction of inclusions is a function of the inclusion concentration a b 2 a and 2 b are the inclusion diameter and mean inter inclusion distance respectively and intensifies the tensile stress imparted by the flowing matrix on the inclusion matrix interface inclusion concentration a b thus appears to be a crucial parameter in perturbing the inclusion matrix coherence and leading to formation of extensional fissures around an inclusion we performed numerical simulations to delineate the fields for coherent and incoherent interfaces in the t o a b space where t o is the tensile strength of the interface normalized to where and are the viscosity of matrix and bulk shear rate respectively for a given a b the interface cannot remain coherent unless the tensile strength is greater than a critical value the critical t o increases almost linearly at a gentle gradient with increasing a b however for a b 0 7 this increases nonlinearly assuming a large value when the concentration is high a b 0 8 it is also revealed that under the same deformation conditions the instantaneous opening in fissures may vary depending upon the a b values

influence of continental roots and asthenosphere on plate mantle coupling the shear tractions that mantle flow exerts on the base of earth s lithosphere contribute to plate driving forces and lithospheric stresses we investigate the sensitivity of these tractions to sub lithospheric viscosity variations by comparing shear tractions computed from a mantle flow model featuring laterally varying lithosphere and asthenosphere viscosity with those from a model with layered viscosity lateral viscosity variations generally do not change the direction of shear tractions but deeply penetrating continental roots increase traction magnitudes by a factor of 2 5 compared to 100 km thick lithosphere a low viscosity asthenosphere decreases traction magnitudes by a smaller amount and is important only if 100 km thick increased plate mantle coupling beneath thick continental lithosphere may increase plate driving forces surface deformation and mantle derived lithospheric stresses in these regions by contrast a low viscosity asthenosphere does not decouple the lithosphere from mantle flow highlighting the geological importance of mantle tractions on the lithosphere

a tale of two viscosities abstract we describe a series of 12 large strain two dimensional simple shear numerical experiments on two phase linear and non linear viscous materials the modal percentage of the two phases their relative viscosities and the stress exponent of the dependence of viscosity on stress were systematically varied allowing us to observe the effects of these parameters on the flow behaviour of the system all experiments exhibited localisation but the extent and type of localisation varied considerably and the range of parameters allows both classical sc as well as sc structures to form the inter granular finite strain geometries also varied systematically although there is a tradeoff between non linearity of viscosity and the viscosity contrast that results in similar geometries for quite different mechanical behaviour the finite rotation of the harder phases was similar within any one experiment originally parallel passive marker lines thus remain sub parallel after deformation and rotation which may be mistaken for non rotation the internal deformability and interaction between grains allows meta stable grain elongation orientations to form without any additional consideration of external slip surfaces the slight variability of initial grain geometry and subsequent grain interactions leads to a wide variability in final grain shapes for the same initial viscosities

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

the effects of sedimentation and compaction on oceanic heat flow summary the estimation of environmental effects forms an important part of the interpretation of oceanic heat flow measurements in particular the perturbations associated with sedimentation and surface temperature changes must be taken into account analytical solutions can be obtained only for individual simplified versions of these problems whereas any real example is complicated by the process of sediment compaction which changes the bulk thermal properties with depth a physical model is developed which uses sediment porosity trends to predict the thermal parameters and material advection rates for an evolving sediment basement system these values are then used in a numerical solution to the heat flow equation to give estimates of the perturbed surface heat flux through time in addition to variations in sedimentation rate sediment type radioactive heat production and surface temperature changes are considered heat flow corrections may vary by up to a factor of 2 according to sediment type while radioactive heat production can offset the effects of sedimentation by as much as 40 per cent the results also indicate that alterations determined from simple analytical models tend to over estimate the true perturbation to the flux

effects of object ellipticity on strain and implications for clast matrix rocks we examine the influence of object shape on strain taking the example of an elliptical object in a matrix of different viscosity the eshelby bilby equation for the cross sectional strain of circular cylindrical objects in a matrix of different viscosity is differentiated to express the relationship of developing sectional ellipticity on incremental strain this leads to an expression for the strain of initial elliptical objects whose axes are parallel or perpendicular to pure shearing graphs show that competent elliptical objects with axial ratios of ri 3 or more will strain significantly more than circular objects of the same viscosity less if the objects are incompetent while the effect is likely to be insignificant for competent objects with initial ellipticity of 2 which is indicated from statistics for undeformed pebbles in conglomerates any clast with greater ellipticity such as ri 5 to 10 could deform significantly more than an equant clast and thus appear to be materially less competent these principles have implications for geological strain studies and for competence contrasts in rocks

numerical modeling of the motion of rigid ellipsoidal objects in slow viscous flows a new approach abstract a simple algorithm for modeling the rotation of rigid ellipsoidal objects in viscous flows based on jeffery s 1922 proceedings of the royal society of london a102 161 179 theory is presented and is implemented in a fully graphic mathematics application mathcad http www mathsoft com the orientation of ellipsoidal objects is specified in terms of polar coordinate angles that can be easily converted to the trend and plunge angles of the three principal axes rather than the euler angles with the mathcad worksheets presented in the supplementary data associated with this paper modeling the rotation paths of individual rigid objects the development of inclusion trail geometry within syn kinematic porphyroblasts and the development of preferred orientation and shape fabrics for a population of rigid objects becomes as easy a task as using a spreadsheet the shape and preferred orientation fabrics for a population of rigid objects can be presented in both a three dimensional form and a two dimensional form allowing easy comparison between field data and model predictions the modeler can customize the type and format of the output to best fit the purpose of the investigation and to facilitate the comparison of model predictions with geological observations application examples are presented for various types of modeling involving rigid objects

pure shear deformation of square objects and applications to geological strain analysis abstract geological objects that deform differently from the rock matrix such as pebbles or other clasts are unlikely to have been originally circular or elliptical in section and must therefore be expected to deform heterogeneously and change shape irregularly we investigate this process with finite element models of pure shear deformation of square objects in three orientations square skew and diagonal in a matrix with different viscosity modelling shows that squares deform irregularly with competent objects becoming barrel shaped and fish mouthed cf boudins whereas incompetent objects become bone shaped or elongate lobes the object aspect ratios r o are less different from the bulk strain ratio r s than for equivalent circular objects in contrast diagonal squares deform almost homogeneously into rhombs with aspect ratios closer to those for circles asymmetrically oriented skew squares behave intermediately developing skew flag and hooked shapes according to competence contrasts that might be misdiagnosed as shear criteria all these square objects and circles in theory show almost linear strain paths of object versus bulk r 1 with slope related to viscosity ratio object shape and orientation linear relationships are also found for concavity convexity shape factors for squares the results have implications for strain analysis and competence contrasts in rocks

factors affecting finite strain estimation in low grade low strain clastic rocks the computer strain analysis methods sape mrl and dtnnm have permitted the characterization of finite strain in two different regions with contrasting geodynamic scenarios 1 the talas ala tau tien shan kyrgyzs republic and 2 the somiedo nappe and narcea antiform cantabrian to west asturian leonese zone boundary variscan belt nw of iberia the performed analyses have revealed low strain values and the regional strain trend in both studied areas this study also investigates the relationship between lithology grain size and percentage of matrix and strain estimates the two methodologies used the results show that these methods are comparable and the absence of significant finite strain lithological control in rocks deformed under low metamorphic and low strain conditions

cretaceous magmatism and mo mineralization in the south china mo province u pb and re os geochronology constraints from the sanzhishu porphyry mo deposit abstract compared with the mesozoic w sn cu au pb zn ag mineralization in the south china mo province scmp detailed studies on mo deposits especially cretaceous mo deposits are rare due to limited exposures the sanzhishu porphyry mo deposit located in the scmp is characterized by disseminated and veinlet type molybdenite quartz veins within monzogranite zircon sims u pb analysis of the monzogranite yields a concordia age of 112 3 ma indicating that the emplacement of the sanzhishu monzogranite was in the early cretaceous six molybdenite samples yield re os ages between 111 and 112 ma with an isochron age of 111 6 4 ma which is consistent with the zircon age of the ore bearing monzogranite a new compilation of reliable re os isotope age data indicate that cretaceous mo mineralization in the scmp occurred during two major episodes at 130 145 ma and 90 120 ma with most deposits forming in the later episode during the cretaceous a back arc extensional setting in se china is evidenced by the occurrence of numerous high f mo deposits widespread a type granites and the development of pull apart basins the increasing trend in re contents of molybdenite from the early mo mineralization period to the late mo mineralization period in the scmp indicates that the area experienced stronger crust mantle interaction in the later mineralization episode based on geological evidence we propose that the early episode of mo mineralization formed in a tectonic setting associated with a metamorphosed subducted slab that experienced an increase in density and an increase in the gravitational pull on the slab itself which induced rollback of the subducting paleo pacific slab the late episode of mo mineralization developed in an extensional tectonic setting caused by the continued rollback and the eventual slab break off

mapping the fluid flow of the mariana mounds ridge flank hydrothermal system pore water chemical tracers we present a conceptual model of fluid circulation in a ridge flank hydrothermal system the mariana mounds the model is based on chemical data from pore waters extracted from piston cores and from push cores collected by deep sea research vessel alvin in small meter sized mounds situated on a local topographic high these mounds are located within a region of heat flow exceeding that calculated from a conductive model and are zones of strong pore water upflow we have interpreted the chemical data with time dependent transport reaction models to estimate pore water velocities in the mounds themselves pore water velocities reach several meters per year to kilometers per year within about 100 m from these zones of focused upflow velocities decrease to several centimeters per year up to tens of centimeters per year a larger area of low heat flow surrounds these heat flow and topographic highs with upwelling pore water velocities less than 2 cm yr in some nearby cores downwelling of bottom seawater is evident but at speeds less than 2 cm yr downwelling through the sediments appears to be a minor source of seawater recharge to the basaltic basement we conclude that the principal source of seawater recharge to basement is where basement outcrops exist most likely a scarp about 2 4 km to the east and southeast of the study area

influence of object concentration on finite strain and effective viscosity contrast insights from naturally deformed packstones deformed conglomerates and ooidal oncoidal packstones are commonly used to evaluate finite strain in deformed sedimentary successions in order to obtain a correct estimate of finite strain it is necessary to consider not only the different behaviour of matrix and objects but also object concentration the analysis of two component rocks characterised by high values of packing commonly results in a substantial underestimate of bulk strain and of viscosity contrast between objects and matrix in this study the effects of the volumetric fraction of competent inclusions on both object and bulk measured finite strain as well as on apparent viscosity contrast have been investigated in naturally deformed packstones characterised by variable object concentration on the scale of the hand specimen and hence for homogenous viscosity contrast object finite strain has been obtained by rf f analysis whereas the fry method provides a measure of whole rock strain that is also a function of inclusion concentration therefore the finite strain measured by the fry method is better termed effective bulk strain in order to investigate the role of object concentration this parameter has been plotted against object and effective bulk strain and also against viscosity contrast these diagrams show that i for high values of packing measured object and effective bulk strain show values that are significantly lower with respect to the calculated maximum value that would result in the ideal case of no particle interaction and represents therefore the real bulk strain of the samples ii the viscosity contrast shows lower values with respect to the calculated maximum one that is equal for the three principal sections of the finite strain ellipsoid and as packing reaches the maximum value the viscosity contrast approaches a unit value empirical equations have also been found that link object concentration with both object and effective bulk finite strain q 2005 elsevier ltd all rights reserved

a combined fluid inclusion and isotopic geochemistry study of the zhilingtou mo deposit south china implications for ore genesis and metallogenic setting abstract the zhilingtou mo deposit is recently discovered and occurs in the qin hang metallogenic belt south china vein stockworks are hosted in the granite porphyry and the surrounding paleoproterozoic badu group basement three stages of mineralization and wallrock alteration were developed the early stage quartz k feldspar veins with potassic alteration the ore stage quartz molybdenite veins with strong phyllic alteration overprinting early potassic alteration and the late stage quartz pyrite calcite veins with propylitization two phase liquid rich aqueous type i inclusions in the early stage veins show homogenization temperatures th of 366 442 c with salinities of 5 3 14 1 wt nacl equivalent two phase gas rich aqueous type ii and coexisting halite bearing type iii inclusions in the ore stage veins display th of 335 433 c and 329 396 c and salinities of 0 5 6 2 and 38 6 44 8 wt nacl equivalent respectively type i inclusions in the late stage veins yield th of 187 282 c with salinities 3 5 7 7 wt nacl equivalent fluid boiling occurred during the ore stage and probably promoted a rapid precipitation of molybdenite laser raman analysis implies that ore fluids are co 2 poor hydrogen and oxygen isotopes indicate that mineralizing fluids are primarily magmatic in origin with apparently involvement of external meteoric water in the late stage lead isotopes suggest that the metals are sourced from the ore bearing granite porphyry trace elements of the ore bearing granite porphyry display obvious nb and ta depletion indicating arc magma geochemical affinities vein stockworks intensive phyllic alteration co 2 poor ore fluids and continental arc setting suggest that the zhilingtou mo deposit is likely to be an endako type porphyry mo deposit it is distinctly different from porphyry mo deposits in the east qinling dabie metallogenic belt in china which were formed in a post collisional setting and characterized by intensive potassic alteration and co 2 rich ore fluids

hydrothermal circulation through mid ocean ridge flanks fluxes of heat and magnesium thermally driven convection of seawater occurs through oceanic crust of all ages at the seafloor spreading axis on mid ocean ridge flanks and in the ocean basins at the ridge axis and on the flanks circulating seawater produces a major chemical exchange between the oceans and the crust based on heat budget constraints and the composition of hot springs between 10 and 40 of the river flux of mg can be taken up during high temperature alteration of the basaltic crust along the ridge axis most of the hydrothermal heat loss however occurs on the mid ocean ridge flanks where the temperatures are lower and the seawater flux correspondingly larger the estimated heat loss on the flanks is so large that upwelling must occur over a large fraction 5 30 of the seafloor less than 65 ma in age if temperatures are 20 c and seepage velocities are on the order of 10 to 100 cm y the circulating seawater needs to lose on average less than 1 2 of its mg content in order to solve the mg mass balance for the oceans chemical fluxes through mid ocean ridge flanks are poorly known because of the wide range of crustal conditions that prevail there and the paucity of study to date the most critical parameter for characterizing crustal conditions is temperature in basement which is a function of crustal age basement topography and sediment thickness and permeability basement temperature which can usually be inferred from heat flow and seismic reflection surveys largely determines the change in the composition of seawater circulating through basement this change can be inferred in turn from profiles of sediment porewater chemistry all sites studied to date with temperatures at the sediment basement interface 25 c have a large component of advective heat loss and show only a small 80 mg loss both types of sites may be important for chemical fluxes whether the cooler sites are important depends on how much the seawater changes in composition as it circulates through the crust only small changes are needed whether the warmer sites are important depends on how much heat is lost by advection in this type of setting the warmer sites could produce significant chemical fluxes even if they are scarce they could account for the entire river input of mg to the oceans even if they represent only 8 20 of the total advective heat loss on ridge flanks

evolution of meso cenozoic subduction zones in the ocean continent connection zone of the eastern south china block insights from gravity and magnetic anomalies abstract the evolution of unified east asian continental margin was accompanied by large scale magmatic and tectonic events here we investigate the tectonic evolution history of the ocean continent connection zone in the eastern south china block scb during the mesozoic cenozoic subduction of the paleo pacific plates based on gravity and magnetic methods that provide a window to the deep crustal structure the decomposed multi scale gravity wavelets reveal subduction related intra crustal structures and the deep structure of the mesozoic greater south china block gscb the depth of moho discontinuity was inverted by gravity data constrained by the latest global crustal model of crust 1 0 and the inverted depth is closely consistent with and slightly larger than the depth on seismic profiles the undulation of the moho discontinuity beneath the south china block and the deep supra crustal faults along the continental margin are considered to be associated with the subduction of the paleo pacific plates in general the moho depth in the eastern scb gets shallower from west to east the oblique subduction related strike slip faults can be traced on the 7 4 km upward continued field of the magnetic anomaly the geologically indistinct jurassic magmatic arc along the scb continental margin is verified by residual crustal field of the bouguer gravity anomaly the southern ryukyu taroko line is speculated as the inland segment of the early jurassic paleo subduction zone and represents the southeastern paleo plate boundary of the gscb in the jurassic as the backarc arc forearc paleo system migrated eastwards since the early cretaceous the subduction zone retreated to the current taiwan okinawa trough and the jurassic magmatic arc forearc was transformed to a backarc indicating an eastward migration or jumping of the yanshanian subduction along the continental margin of the scb the tectonic boundary of the mesozoic gscb was also identified through the magnetic anomaly field and the high order gravity field

an analysis of anisotropy of rocks containing shape fabrics of rigid inclusions abstract this paper presents a theoretical basis for estimation of mechanical anisotropy in homogeneous rocks containing shape fabrics of rigid inclusions the analysis is based on two types of viscous models one containing linear fabrics of prolate a b c inclusions cf l tectonite and the other containing planar fabrics of oblate a b c inclusions cf s tectonite models show contrasting bulk viscosities in stretching normal viscosity and shearing shear viscosity parallel to the fabric the axial ratio r a b and the volume concentration v of rigid inclusions appear to be the principal parameters in determining the viscosity contrast in anisotropic models with linear fabrics normal viscosity p increases monotonically with increase in r whereas shear viscosity s increases to a maximum and then drops down to a near stationary value in anisotropic models with planar fabrics the normal viscosity increases little with increasing flatness of inclusions but the variation assumes a steep gradient when the latter is large shear viscosity on the other hand is relatively less sensitive to the shape of inclusions the ratio of normal and shear viscosities conventionally described as anisotropy factor in both the models is always greater than 1 indicating that normal viscosity will be essentially greater than shear viscosity irrespective of the axial ratio of inclusions forming the fabric models with a linear fabric show contrasting normal viscosities in pure shear flow along and across the linear fabric the anisotropy is expressed by the ratio of longitudinal and transverse normal viscosities anisotropic factor it is revealed that the transverse viscosity is essentially less than the longitudinal viscosity as observed in test models

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting