subduction induced pore fluid venting and the formation of authigenic carbonates along the cascadia continental margin implications for the global ca cycle abstract pore fluid venting associated with subduction induced sediment deformation causes precipitation of calcium carbonate as prominent carbonate chimneys or cement in the accreted sediments across the active continental margin off oregon and washington a depletion of interstitial ca 2 with a maximum decrease of 50 relative to seawater ca 2 over only 1 5m depth and reduction in porosity in the deformed sediments suggest that interstitial ca 2 is removed to form calcium carbonate cement in contrast the pore waters of the undeformed abyssal plain sediments show no depletion in dissolved ca 2 they are either enriched to a maximum of 5 or show no change in dissolved ca 2 here the background level of caco 3 content in the sediment is only 0 1 to 1 calcium carbonate precipitation in the deformed sediments probably occurs as the result of upward migration and oxidation of biogenic methane and of the increase in carbonate saturation due to release of excess pore pressure during fluid venting upward advection of fluids at rates of 1 28 cm y 1 is predicted from diffusion advection reaction models applied to the downcore concentration profiles of dissolved ca 2 and nh 4 in the tectonically deformed sediments the range of predicted flow rates is related to the type of calcium carbonate lithification i e slow rates generate cement and fast rates generate chimneys carbonate mineral precipitation associated with pore fluid venting requires direct transfer of ca 2 from the oceanic basement to the accretionary complex such a mechanism leads us to propose that the accretionary complexes of the global plate subduction zones are a major sink for crustal ca 2 a global flux of crustal ca 2 that is removed by carbonate mineral precipitation may be as muc3 as the hydrothermal ca input this significant ca flux not previously considered in the global geochemical budget implies that pore fluid venting in subduction zones may also act as a global sink or source for other elements

the fate of the submarine ikaite tufa columns in southwest greenland under changing climate conditions abstract ikaite is considered a metastable mineral forming and stable only at low temperatures and therefore an indicator of low temperature carbonate precipitation often associated with cold marine seeps it is found world wide but most spectacularly in ikka fjord in southwest greenland as submarine carbonate tufa columns here ikaite is formed as a result of submarine spring water mixing with cold seawater as ikaite disintegrates at temperatures above 6 7 c it has been speculated that global warming could endanger this unique habitat as well as other sites in ikka fjord in situ water chemistry in and around an ikaite column measured continuously over two years showed that the column water is alkaline ph 9 10 throughout the year with temperatures of 1 3 6 0 c and conductivities of 5 7 7 9 ms cm 1 favoring year round growth of columns at 4 5 cm per month short term in situ measurements with needle micro sensors from both older dehydrated and calcified parts and more recently formed solid parts of an ikaite tufa column showed similar ph and temperature values including a temperature variation over the tidal cycle in the uppermost recently deposited ikaite matrix spring water escaping at the top causes passive drag of seawater into the porous ikaite matrix leading to a mixing layer several centimeters thick that has ph values intermediate to the spring water in the column and the surrounding seawater we conclude that the main part of the columns consisting of fossilized ikaite inverted to calcite partly sealed by calcifying coralline algae and with year round flow of alkaline freshwater through distinct channels are resistant to warming in the more diffuse top part of the columns the formation of ikaite and thus column growth will be limited in the future due to increased fjord water temperature during the

glendonites and methane derived mg calcites in the sea of okhotsk eastern siberia implications of a venting related ikaite glendonite formation mg calcite cemented bioturbation traces glendonite aggregates of idiomorphic bi pyramidal crystals and porous amber colored carbonate concretions were recovered from a methane dominated cold vent area in 380 m water depth at the northern sakhalin slope sea of okhotsk bioturbation traces consist of mg calcite cemented sediment with 13c values between 37 and 46 peedee belemnite pdb which implicate methane as the carbon source glendonite a calcite pseudomorphosis after ikaite caco3 6h2o and amber colored concretions are both composed of varying amounts of calcite and mg calcite with 13c values between 19 and 34 pdb isotope analyses of an ikaite crystal recovered in the vicinity reveals 13c values between 20 and 22 pdb indicating organic matter as the carbon source microscopic investigations of glendonites and amber colored concretions show a porous fabric of a primary calcite phase that is overgrown by a secondary mg calcite cement as ikaite pseudomorphs to porous calcite and as carbon isotope values are the same for the ikaite and the high end member values of glendonite samples the primary calcite phase is suggested to be a former ikaite phase because they share the equal color fabric mineral and isotopic composition with glendonites the amber colored concretions are also suggested to represent calcite that transformed from ikaite a mixture of pseudomorph calcite 13c 20 pdb which originally formed as ikaite from degraded organic matter and mg calcite 13c 43 pdb which crystallized due to the anaerobic oxidation of methane can explain the varying carbon isotope data of the glendonites and the amber colored concretions the growth of ikaite the transformation of ikaite to calcite and the crystallization of mg calcite indicate changing geochemical conditions within a cold vent environment at different times ideal conditions for the ikaite formation are given during the establishment of a cold vent site when upward migrating methane rich fluids enhance the anaerobe decomposition of organic matter which again increases the phosphate and alkalinity concentrations near the sediment surface lower rates of organic matter decomposition during on going venting decrease these high phosphate but also sulphate concentrations and allows other carbonate phases as mg calcite to form this additional carbonate precipitation and the ikaite formation itself lower the high alkalinity and destabilize ikaite which pseudomorphs to porous calcite triggered by the further upward shifting so4 h2s boundary and increasing methane oxidation rates the typical cold vent methane derived carbonate genesis takes place which cements the sediment pore space and induces the secondary mg calcite crystallization within the glendonite fabric taking this scenario into account ikaite formation should be a common process in the beginning of methane dominated vent activity at cold bottom water temperatures glendonite pseudomorphs can be assumed to represent a typical manifestation at fossil and recent cold vents at high latitudes

flow vorticity in zhangbaling transpressional attachment zone se china abstract plagioclase porphyroclasts with well preserved idiomorphic shapes and zoning and showing limited clast interaction are ubiquitous in the flat lying zhangbaling schist that is exposed east of the tan lu fault in southeast china plagioclase porphyroclasts define rigid particles whose distribution be related to the kinematic vorticity of the schist using the methods of porphyroclast hyperbolic distribution phd and modified rf passchier wallis plot the kinematic vorticity numbers calculated from this approach range from wk 0 72 to 0 82 increasing progressively from south to north along the zhangbaling belt such a wk distribution indicates that the zhangbaling schist was deformed uniformly under simple shear dominated general shear and that the zhangbaling ductile crust experienced relatively even crustal thinning deformation the subhorizontal zhangbaling schist is considered a mid crustal attachment zone that coupled the rigid upper crust to a subvertical wrench shear zone in the lower crust

ch 4 consuming microorganisms and the formation of carbonate crusts at cold seeps to understand the role played by microorganisms in the formation of cold seep carbonates we conducted an integrated microbial mineralogical and organic geochemical study of methane related authigenic carbonate crusts formed on eastern mediterranean mud volcanoes we show that supersaturation with respect to carbonate minerals is induced by microbial anaerobic oxidation of methane combined lipid biomarker analysis and 16s rrna gene surveys identified a highly diversified methane consuming archaeal community possibly comprising novel species implying that the anaerobic oxidation of methane is phylogenetically widespread and directly implicating these organisms in the process of crust precipitation moreover pore water sulphate gradients produced by co occurring methane based sulphate reduction exert the main control on aragonite versus magnesian calcite precipitation we propose that this may be the dominant mode of carbonate crust formation at cold seeps world wide in agreement with aquatic chemistry predictions and explaining carbonate mineralogy

deformation volume and cleavage development in metasedimentary rocks from the ballarat slate belt psammites from the ballarat slate belt in se australia exhibit well developed differentiated or spaced cleavage defined by alternating phyllosilicate and quartz rich domains termed p and q domains respectively strain estimates derived from independent microstructural and chemical observations suggest that the p q fabrics developed in response to plane strain deformation dominated by solution transfer with the principal finite shortening in the p domains approximately twice that in the adjacent q domains significant finite extensions are indicated by ubiquitous quartz albite chlorite overgrowths in both p and q domains while pressure shadow development around syntectonic pyrite porphyroblasts suggest finite extension of at least 100 this estimate is comparable with the extension predicted for constant volume deformation and consequently there appears to have been no significant bulk material loss or gain on the hand specimen scale these observations are consistent with solute transfer scales as little as a few centimetres via diffusion in a stationary fluid and do not require although do not necessarily preclude large scale advective fluid transport through the slate belt during cleavage formation as suggested in previous studies

triassic polyphase deformation in the feidong zhangbaling massif eastern china and its place in the collision between the north china and south china blocks the feidong zhangbaling massif is located between the dabieshan and sulu areas along the tan lu fault in eastern china five tectonic metamorphic events are distinguished there the earliest deformation d1 corresponds to a southward compression that occurred during subduction of the south china block below the north china block top to the south shearing is coeval with late permian early triassic blueschist facies metamorphism and possibly with the development of south verging recumbent folds in the neoproterozoic paleozoic sedimentary cover of the south china foreland the main ductile deformation d2 is an extensional one characterized by top to the north shearing coeval with the early stage of exhumation of the high pressure rocks a top to the nw ductile shearing and microfolds overturned to the northwest belong to a second deformational phase of exhumation d3 which is distinct from the main event d2 previous 40ar 39ar mica dates ranging between 245 and 212 ma suggest late permian early triassic ages for the d1 to d3 events the d4 event produced ne sw trending folds in the sedimentary cover interpreted as gravity collapse structures a late cretaceous brittle extensional event d5 controls the opening and infill of continental half grabens in the study area the tan lu fault is a cretaceous brittle normal fault the lack of ductile deformation presented along the tan lu fault suggests that it did not play a significant role in the exhumation of high pressure metamorphic rocks

highly isotopically depleted isoprenoids molecular markers for ancient methane venting abstract we propose that organic compounds found in a miocene limestone from marmorito northern italy are source markers for organic matter present in ancient methane vent systems cold seeps the limestone contains high concentrations of the tail to tail linked acyclic c20 isoprenoid 2 6 11 15 tetramethylhexadecane crocetane a c25 homolog 2 6 10 15 19 pentamethylicosane pme and a distinctive glycerol ether lipid containing 3 7 11 15 tetramethylhexadecyl phytanyl moieties the chemical structures of these biomarkers indicate a common origin from archaea their extremely 13c depleted isotope compositions 13c 108 to 115 6 pdb suggest that the respective archaea have directly or indirectly introduced isotopically depleted methane derived carbon into their biomass we postulate that a second major cluster of biomarkers showing heavier isotope values 13c 88 is derived from sulfate reducing bacteria srb the observed biomarkers sustain the idea that methanogenic bacteria in a syntrophic community with srb are responsible for the anaerobic oxidation of methane in marine sediments marmorito may thus represent a conceivable ancient scenario for methane consumption performed by a defined two membered bacterial consortium 1 archaea that perform reversed methanogenesis by oxidizing methane and producing co2 and h2 and 2 srb that consume the resulting h2 furthermore the respective organic molecules are unlike other compounds tightly bound to the crystalline carbonate phase the marmorito carbonates can thus be regarded as cold seep microbialites rather than mere authigenic carbonates

mantled porphyroclast gauges we investigate the behavior of the isolated mantled porphyroclast in a shear zone the method employed is a finite element model three distinct phases clast mantle and matrix are present the rheologies are power law with exponents ranging from 1 to 5 and the far field boundary condition is simple shear the effective viscosity of the mantle is assumed to be less than those of the clast and the matrix we show for which parameter sets mantled porphyroclasts reach super horizontal stabilization with respect to the shear plane and sense clasts in natural mylonites frequently exhibit similar orientations which are interpreted as stable inclinations the systematic examination of the matrix mantle clast system allows for the construction of attractor maps that can be directly used as gauges for i the effective viscosity contrast between matrix and mantle ii the production rate of mantle material around the clast as a function of the bulk shear strain and iii for the total shear strain the necessary data required to use the attractor maps are simple geometrical parameters that can be measured in the field i e clast aspect ratio clast inclination versus the shear plane mantle thickness and mantle and clast area this new method successfully reproduces the characteristics of natural porphyroclasts and is in good agreement with data from natural shear zones q 2005 elsevier ltd all rights reserved

controls on marine carbonate cement mineralogy review and reassessment abstract commonly cited models for controls on variations in carbonate mineralogy are only partly successful in accounting for natural mineralogical trends this limited success may result in part from a failure to recognize or address the ramifications of assumptions inherent to such models in general changes from calcite to aragonite or vice versa and variations in mgco3 incorporation in calcite are interpreted as resulting from 1 kinetic rather than thermodynamic effects 2 the effects of fluid chemistry rather than hydrologic biological or substrate effects 3 the effects of single parameters of fluid chemistry rather than the interactive or aggregate effects of several components and or 4 inorganic rather than organic parameters of fluid chemistry recent laboratory data suggest the necessity of a new approach to modeling controls on mineralogy these data indicate that no single fluid parameter is likely to be a sole or dominant control on carbonate mineralogy in natural environments in some cases thermodynamic stability differences may be as important as kinetic factors in determining mineralogy furthermore the effects of substrate hydrologic and biological factors and of organic compounds cannot be ruled out for most natural situations although at present reliable data are lacking to quantify the influence of these factors on mineralogy future models of controls on carbonate mineralogy need to consider the aggregate and interdependent effects of all of these factors

testing the utility of the porphyroclast hyperbolic distribution method of kinematic vorticity analysis kinematic vorticity wk is a dimensionless measure of rotation relative to finite stretching and is essential for complete understanding of flow in ductile shear zones the porphyroclast hyperbolic distribution phd method is a widely used technique for estimating wk based on the acute angle between the flow eigenvectors determined from the orientations of back rotated s porphyroclasts the utility of the phd was tested with tertiary ultramylonites from the santa catalina mountains arizona proterozoic ultramylonites from the virgin mountains nevada and a paleozoic ultramylonite from the hylas zone in the virginia piedmont bootstrapping statistics and a computational sieving process were used to analyze the phd data set average standard deviations of the bootstrapped data sets yield a 1s standard error of 9 for a wk value measured as a simple shear sieving results imply that back rotated porphyroclasts may not orient parallel to the extensional eigenvector the phd method is useful for discerning between deformations that are pure shear dominated general shear or simple shear dominated but is not accurate enough to report precise wk values when performed on multiple sections the phd method can identify zones of monoclinic versus triclinic shear 2007 elsevier ltd all rights reserved

origin and paleoenvironmental significance of calcite pseudomorphs after ikaite in the oligocene creede formation colorado abstract pseudomorphs of calcite are present in lacustrine strata and travertine of the oligocene creede formation the sedimentary moat fill of the creede caldera in the tertiary san juan volcanic field the pseudomorphs previously thought to have formed after gaylussite caco3na2co3 5h2o are interpreted to have formed after ikaite caco3 6h2o on the basis of their modes of occurrence pseudomorph morphology and textural and chemical aspects of the calcite replacement the extent of their exposure and degree of preservation in the creede formation allow elucidation of the depositional conditions in which the ikaite formed and was subsequently pseudomorphically replaced the revised paleoenvironmental interpre ation of the creede formation also has implications for understanding of the late oligocene climate of the southwestern united states formation of ikaite requires water temperatures of 0 3 c supersaturation with respect to calcite and the presence of a chemical inhibitor such as dissolved phosphate to prevent growth of calcite or aragonite shearman and smith 1985 geochemical calculations and hydrogeologic considerations suggest that waters with dissolved phosphate contents sufficient to inhibit calcite growth and stabilize ikaite could have developed in the creede moat lake although a chemical remnant of high phosphate content is not observed in the strata the granular texture of the creede pseudomorphs is common to calcite pseudomorphs after ikaite from many localities a model is proposed in which slow pseudomorphic replacement over a period of a day or more results in sand size calcite grains forme directly from the ikaite this model has important implications for the interpretation of isotopic and chemical data obtained from pseudomorphs and thinolitic tufa

application of the rfφ technique to ellipitical markers deformed by pressure solution abstract the applicability of the r f technique of finite strain analysis to elliptical markers whose shape is modified by pressure solution is investigated variably oriented elliptical markers shortened by removing material from one or both sides in open and closed systems are compared to markers shortened homogeneously by the same amount for the models examined it was found that the r f technique correctly predicts the principal directions the initial marker axial ratio and the axial ratio of a circle deformed in a like manner interpretation of strain ratios in terms of percent shortening requires knowing how much volume was lost equations relating the strain ratio to percent shortening are given for both constant volume and volume loss deformation

simultaneous pure and simple shear the unifying deformation matrix abstract any simultaneous combination of finite simple shear and finite pure shear is a linear transformation which can be expressed as a single transformation matrix for two dimensions the matrix is upper triangular with an off diagonal term called the effective shear strain is a simple function of the pure and simple shear components for three dimensions a simultaneous combination of thrusting in the x direction thrusting in the y direction and a wrench in the x direction in addition to 3 orthogonal components of coaxial strain can also be represented by a 3 3 upper triangular matrix here three off diagonal terms xy x y and y z occur xy is a simple function of the horizontal coaxial strain values and thrusting in the x direction yz depends on the coaxial strain components in the y and z directions and the thrusting in the y direction while xz is related to all six strain components the matrix also allows for volume change either homogeneously or preferentially in a single direction a method of decomposing the deformation matrix into a series of incremental deformation matrices where each incremental deformation records the same kinematic vorticity number as the finite deformation is shown the orientation and magnitude of the finite strain ellipsoid ellipse is easily and accurately found at any increment during the deformation

strain and fabric analyses based on porphyroclast interaction abstract two dimensional computer models of the interaction of elliptical clasts in a variety of kinematic conditions are used to study how clast imbrication and shape fabric development relate to strain and strain history the models provide templates for a known clast density and clast ellipticity that correlate clast imbrication angle of collision and clast shape fabric to strain and kinematics of deformed rocks modeling also shows that the mode of clast interaction plays a significant role in the accumulation of imbricated clasts and the development of shape fabric upon collision between clasts the trains formed can remain fixed or rotate if they rotate imbricated clasts can become independent when they are no longer forced together by the flow based upon the mode of clast and train rotation using the march rotation of passive lines and the jeffery rotation of rigid particles as end members three models are defined in an attempt to simulate the conditions of clast interaction under varying conditions the march fixed train march rotating train and jeffery rotating train models simulate solid state intermediate and magmatic conditions respectively modeling shows that the two march models produce similar results but the jeffery model of train rotation always produces a lower proportion of imbricated clasts and a much weaker shape fabric application of the modeling to the study of a syntectonic granite in the sierra nevada suggests that deformation was dominated by simple shear some k feldspar imbrication and fabric development took place initially in the magmatic stage and culminated in the solid state to produce a large proportion of imbricated k feldspar porphyroclasts which define a strong shape fabric this type of analysis provides a rationale to determine syn vs post emplacement deformation in the granite

finite strain analysis of the zhangbaling metamorphic belt se china crustal thinning in transpression abstract finite strain analysis of pebbles in the metatillite and phyllite shows that the deformation of the metatillite phyllite zone is flattening to plane strain dominated results of spo analysis of the rigid plagioclase porphyroclasts in three perpendicular planes indicate monoclinic strain symmetry of the zhangbaling schist and oblate geometry of the clast forward strain modeling of rigid clasts is performed based on the shape fabric of the clast population integrated results of finite strain modeling and finite strain analysis indicate that this midcrustal section experienced at least 18 36 thinning in the oblique convergence between north and south china blocks

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

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

methane derived carbonate cementation of marine sediments from the kattegat denmark geochemical and geological evidence abstract methane derived carbonate cementation of holocene marine sediment occurs at several locations in the kattegat denmark generally the carbonate cemented sandstones occur at the seafloor as individual slabs or widely distributed thinly lithified pavements in some areas the lithified sandstones take the form of large and spectacular vertical pillar and mushroom like bodies standing up from the seafloor the carbonate cement consists of aragonite high mg calcite and dolomite the high mg calcite is the most widespread carbonate solid phase found aragonite is primarily associated with shell accumulations in the sediments so far dolomite has only been recorded as cement in the large vertical lithified sandstone bodies the carbonates are remarkably depleted in 13c i e generally in the range of 55 to 45 pdb and are therefore interpreted as being the result of methane oxidation by sulphate reducing bacteria in the anaerobic diagenetic zone of the sediments however the oxygen isotopic composition and the concentrations of minor elements indicate that carbonate precipitation took place under relatively constant temperature and salinity conditions in normal marine pore fluids the habit and mineralogy of the carbonate cemented sandstone observed most likely are controlled by the effect of bacterial activity and upward gas migration on the interstitial geochemical microenvironment the lithified sediments were subsequently exposed as spectacular sandstone formations on the seafloor by submarine erosion data from onshore exploration wells reveal that methane is encountered in eemian early weichselian late quaternary marine deposits the chemical composition and the isotopic signatures of the methane suggest a gas of microbial origin and it is thought that the gas is generated within the host marine quaternary sediment the locations of submarine gas seeps along with those of the carbonate cemented sandstone formations and the onshore gas seeps show a linear distribution which largely coincides with the extension of the fennoscandian border zone thus the seeps may be controlled by the structural framework in the northeastern part of the danish subbasin

thermochronologic constraints on deformation and cooling history of high and ultrahigh pressure rocks in the qinling dabie orogen eastern china the hong an block is the best place to study the exhumation of high pressure hp and ultrahigh pressure uhp metamorphic rocks in the qinling dabie orogen of eastern china because it lacks the extensive cretaceous tectonic and thermal overprint observed in the dabie shan we measured timing of deformation and rate of cooling of the hp uhp rocks by 40ar 39ar analyses of synkinematic minerals from tectonites of key structural zones in the hong an and tongbai shan normal sense shear along the north dipping huwan detachment at the northern edge of the hong an block occurred between 237 and 231 ma this detachment facilitated the bulk of the exhumation of the hp uhp rocks our new 40ar 39ar ages combined with u pb zircon and sm nd ages of 245 240 ma suggest that exhumation of uhp rocks from mantle depths occurred at rates of 5 25 mm yr from 245 to 230 ma the mountain range is a warped extensional footwall within which white mica cooled from 225 to 205 ma locally younger extension is recorded by white mica recrystallization at 198 194 ma after which the entire block had cooled to below 300 c early cretaceous 40ar 39ar ages from the tongbai shear zone indicate that dextral shear along the southwest boundary of the orogen was contemporaneous with normal to sinistral oblique slip along the xiaotian mozitan fault along the northern boundary coeval dextral and sinistral shear zones along the northern and southwestern margins of the hong an and dabie shan would have caused eastward lateral extrusion of these two blocks perhaps driven by collision of the lhasa block with eurasia

significance of aragonite cements around cretaceous marine methane seeps abstract detailed petrography and geochemistry of carbonate precipitates in cretaceous cold seep mounds from the canadian arctic show spectacular early diagenetic products some still preserved splays and isopachous layers of fine acicular aragonite and large botryoids and crusts of low magnesium calcite showing unusual entanglement of former fibrous calcite and aragonite the latter mineralogy is suggested by clear flat terminated cathodoluminescence patterns interpreted as ancient crystal growth steps and the former by rhombohedral terminations the early cement phases very likely precipitated in cold arctic water dominated by bicarbonates derived from bacterially oxidized methane these cements have d13c values around 44 and d18o values of 1 8 to 0 1 pdb coexistence of calcite and aragonite early cements in the cretaceous seep mounds is unusual because precipitation occurred in high latitude cold water settings and during a so called calcite sea mode as in modern marine hydrocarbon seeps the chemistry of the cretaceous system was apparently controlled by chemosynthetic bacterial activity resulting in high ahco3 that promoted precipitation of carbonates we suggest that locally fluctuations in ahco3 aso42 resulted in oscillating aragonite or calcite supersaturation and hence controlled the mineralogy of the early precipitates

ar ar geochronology of ultrahigh pressure metamorphism in central china the dabie shan of china contain the largest areal exposure of ultrahigh pressure regional metamorphic rock known on earth the thermal history of these unusual rocks is central to understanding the tectonic processes responsible for their creation preservation and exhumation published ages of ultrahigh pressure metamorphism and subsequent cooling range from archean to jurassic by analyzing 21 dabie hornblende phengite and biotite samples by the 40ar 39ar method we find that 1 cooling from peak metamorphic temperatures to 300 c occurred between about 206 and 178 ma 2 widespread cretaceous ages reflect reheating by a postultrahigh pressure magmatic extensional episode 3 40ar 39ar and k ar ages older than 230 ma and perhaps 210 ma are geologically meaningless due to the incorporation of excess 40ar and 4 dabie ultrahigh pressure metamorphic rocks are temporally related to blueschist facies rocks farther west in the suture zone

effect of shape and orientation on rigid particle rotation and matrix deformation in simple shear flow the rotation behaviour of rigid particles from a wide range of symmetry classes was analysed in a series of three dimensional analogue experiments in which the particles were embedded in a transparent linear viscous matrix deformed to large shear strains comparison of the measured trajectories with theoretical periodic paths for rigid ellipsoids shows that quadratic slightly orthorhombic and monoclinic particles have trajectory paths and rotation rates closest to theory orthorhombic and monoclinic particles with a minimum aspect ratio 1 5 develop non periodic trajectories the results establish that the simplified theoretical model for ellipsoidal forms is a good approximation for a wide range of natural particle shapes the relationship between the rotation of a rigid particle and deformation of the adjacent matrix was also determined in a series of experiments by placing the particle at different depths relative to an imprinted grid the results are well illustrated by the surface passing through the centre of the particle and initially parallel to the shear plane two deformed regions in the matrix are recognised in the first region the matrix rotates with the particle together forming an approximately ellipsoidal object the second region is dominated by folds of limited extent along the rotation axis and greatest extent in a direction that rotates asymptotically toward the shear direction the folds rotate more slowly than the principal direction of maximum finite stretch

40ar 39ar dating of strike slip motion on the tan lu fault zone east china abstract ne trending sinistral ductile shear belts in the southern part of the tan lu fault zone east china are exposed along the eastern edge of the northern dabie belt and southern zhangbaling belt three hornblende three phengite and 12 biotite separates from greenschist facies mylonites in the shear belts were dated by the 40ar 39ar method in this study a hornblende plateau age of 143 3 ma is interpreted as representing the time of sinistral motion on the zhangbaling part of the tan lu fault zone seven biotite samples from the same part yield 40ar 39ar cooling ages from 137 to 125 ma a phengite plateau age of 138 8 ma from the eastern edge of the dabie belt is considered as being close to the time of deformation in the tan lu strike slip zone five biotite 40ar 39ar ages from 120 to 110 ma and one phengite age of 121 ma from the same area represent cooling ages after the sinistral faulting it is concluded on the basis of these 40ar 39ar ages that the sinistral ductile shear belts in the southern part of the tan lu fault zone were produced in earliest early cretaceous this sinistral motion on the tan lu fault zone is considered to be related to high speed highly oblique subduction of the pacific plate at this time

finite strain heterogeneity and volume loss in slates of the taconic allochthon vermont u s a abstract slates of the taconic allochthon have ellipsoidal reduction spots which have been used to determine the finite state of strain at seven sites the strain is highly heterogeneous at all scales ranging from several centimeters to many kilometers variations in x y ratios is small but variations in y z ratios is large and this pattern is seen at all scales the heterogeneity can be used to determine a strain path which is nearly horizontal on a flinn diagram we propose that uniaxial flattening associated with late stage cleavage development was accommodated by volume loss and that the most highly strained sites have experienced an average 55 volume loss at a minimum explaining our results without volume loss requires a subhorizontal extension and creates strain compatibility problems consideration of the strain history of these rocks suggests that folding related strains have been modified by later cleavage related events the last of which was uniaxial flattening accommodated by volume loss the finite strains are reflective almost exclusively of the latest cleavage producing strains

calcium carbonate hexahydrate from organic rich sediments of the antarctic shelf precursors of glendonites large euhedral crystals of calcium carbonate hexahydrate were recovered from a shelf basin of the bransfield strait antarctic peninsula at a water depth of 1950 meters and sub zero bottom water temperatures the chemistry mineralogy and stable isotope composition of this hydrated calcium carbonate phase its environment of formation and its mode of precipitation confirm the properties variously attributed to hypothetical precursors of the glendonites and thereby greatly expand their use in paleoceanographic interpretation

strain insensitive preferred orientation of porphyroclasts in mont mary mylonites abstract the shape preferred orientation spo of porphyroclasts was determined in high temperature mylonites the porphyroclasts approach rhomboidal sillimanite or elliptical garnet plagioclase sillimanite shapes and exhibit aspect ratios r as high as 11 particles with r 3 are dominantly rhomboidal the long axis of the best fit ellipse defines a very strong spo inclined at 5 10 to the mylonitic foliation in an antithetic sense with respect to the shear direction this angle is independent of r the inclination of the long sides of rhomboidal sillimanite increases from 10 to 20 with decreasing r in contrast the short sides have a constant orientation of 15 to 17 irrespective of r and are parallel to extensional crenulation cleavage low aspect ratio mainly elliptical objects show low intensity spo close to the shear plane the two spos appear strain insensitive in the case of r r 3 a stable position is acquired this is not explained by any of the current theoretical and experimental models of spo

s wave attenuation structure on the western side of the nankai subduction zone implications for fluid distribution and dynamics we estimated the s wave attenuation structure in southwestern japan and the western nankai trough by analyzing maximum s wave amplitudes at 4 8 8 16 and 16 32 hz with a correction term for apparent amplitude attenuation due to multiple forward scattering because the estimated attenuation q 1 in our tomographic study was much larger than q 1 due to wide angle scattering our estimated q 1 was composed mainly of intrinsic attenuation high attenuation areas q 1 1 300 at 4 8 hz were imaged beneath quaternary volcanoes and south off shikoku low 1 1500 at 4 8 hz or moderate q 1 1 500 1 1000 at 4 8 hz was imaged beneath shikoku and nonvolcanic areas of chugoku high and moderate q 1 in and around shikoku are located near the top of subducting philippine sea plate this correspondence implies that these high and moderate q 1 reflect fluid in the subducting slab by applying a theoretical model of attenuation in water saturated porous random media we examined wave induced fluid flow induced by lower frequency 1 hz seismic waves that may be related with triggering of nonvolcanic tremor by surface waves even though q 1 structure in this study cannot fully explain the tremor triggering by wave induced fluid flow large uncertainties of q 1 in tremor zone suggest that high resolution imaging of q 1 and random inhomogeneities would give some constraints for the spatial variation of permeability and other medium properties

monoclinic model shear zones abstract although many ductile shear zones are supposed to have developed by approximately simple shear flow some must have formed under different conditions a few types of such non simple shear zones have been proposed in the literature such as transpression transtension and stretching shear zones this paper presents a full three dimensional kinematic model of shear zones with monoclinic flow geometry monoclinic shear zone types can be classified according to flow parameters and according to the geometry and orientation of accumulating finite strain modelling of finite strain accumulation shows that a number of unusual and potentially problematic structural features may develop in monoclinic shear zones these include stretching lineations normal to the displacement direction foliations normal to the shear zone boundary switching of the position of principal finite strain axes x y and z during progressive deformation and strain gradients along shear zones that may give rise to obliqueness of fabric elements of the same age in adjacent rock volumes

petrology and zircon u pb geochronology of metamorphic massifs around the middle segment of the tan lu fault to define the boundary between the north and south china blocks abstract the tan lu fault tlf is a significant linear structure of eastern china the middle segment of the tlf in the northern anhui province separates the north china block ncb and south china block scb lack of geological work makes the plate boundary here ambiguous detailed field survey and laboratory analyses have been conducted to constrain the formation and evolution of the tlf to the west of the tlf the wuhe massif underwent a granulite facies metamorphism with t 778 836 c p 1 0 1 2 gpa during 2 41 2 43 ga according to la icp ms u pb zircon dating on the contrary to the east of the tlf the zhangbaling massif suffered a greenschist to lower amphibolite facies metamorphism with the average metamorphic temperature at around 340 440 c with the protolith igneous ages of ca 750 ma these differences indicate that the two massifs belong to the different blocks furthermore the granitic gneiss in the central part of tlf gives three populations of zircon u pb age at 667 504 ma 228 2 ma and 211 2 ma respectively the later two groups of spectra correspond to the periods of subduction and exhumation of the dabie massif 245 200 ma combined with previous study the precise plate boundary between ncb and scb has been delineated as the zhuding shimenshan fault and the initial time of the motion for the tlf is also discussed

stable positions of rigid objects in non coaxial flow a study in vorticity analysis abstract distortions of a homogeneous fabric which occur around rigid objects in rocks deformed by non coaxial ductile flow are useful to determine the sense of shear but also contain information on other aspects of the flow rigid objects in a non coaxial flow fall into two categories those which are permanently rotating and those which can reach stable positions at high finite strain the orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow the ratio of instantaneous stretches and the axial ratio of the object in naturally deformed rocks immobilized rigid objects may be recognized from the geometry of the surrounding fabric tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow and the shape of tails reflects the rotational behaviour of the porphyroclast the axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain in practice such an analysis is as yet difficult but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the french pyrenees

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

analogue modelling of the influence of shape and particle matrix interface lubrication on the rotational behaviour of rigid particles in simple shear abstract the rotational behaviour of a rigid particle embedded in a linear viscous matrix undergoing cylindrical simple shear couette flow was studied in 2d rock analogue experiments the influence of particle shape elliptical vs monoclinic aspect ratio and the nature of the matrix particle interface lubricated vs unlubricated was investigated both matrix pdms and lubricant liquid soap were linear viscous with a viscosity ratio of ca 10 4 without lubricant the rotational behaviour of all particles closely approximates the jeffery theory lubricated monoclinic particles with the long diagonal initially parallel to the shear direction show back rotation and approach a stable position lubricated elliptical particles initially parallel to the shear direction also show back rotation but only transiently stabilize weak planar zones in the matrix adjacent to unlubricated elliptical particles do not induce backward rotation in general for elliptical particles rotation rate as a function of orientation depends on axial ratio and thickness of the lubricant mantle for thick mantles initially 10 of the volume of the particle rotation rates are faster than jeffery theory for very thin mantles they are markedly slower compared with thick mantles particularly when the long axis is nearly parallel to the shear direction rotation rates are never strictly zero so true stabilization does not occur however for more elongate particles axial ratio 6 rotation rates are so slow that a very strong shape preferred orientation would develop in a lubricated elliptical particle population in experiments the volume of lubricant is constant and the thickness adjacent to the long side of the particle progressively decreases with increasing strain in natural examples of porphyroclast systems the weak mantle continually develops by recrystallization and or cataclasis of the rigid clast core and a steady state between production and thinning could be attained potentially leading to true stabilization for particles with a high axial ratio

2 d shape preferred orientations of rigid particles in transtensional viscous flow ghosh and ramberg tectonophysics 34 1976 1 70 studied the two dimensional 2 d rotational behaviour of individual rigid particles embedded in a viscous medium subject to simple shear and transpression sr 0 at low medium shear strains we now extend this theoretical study to transtension sr 0 by deriving a new analytical solution to non interacting populations of rigid particles and to high shear strains we used different initial orientations of particles in the same graph to simulate populations of particles in an originally isotropic rock our results show that 1 shape preferred orientations spos developed in viscous simple shear flow are transient and cyclical except for particles with aspect ratio or material lines which tend to the shear direction 2 stable spos can develop in transpression they dip towards the shear sense but when defining an s c structure with the shear foliation this composite fabric is always antithetic and thus indicates the wrong sense of shear 3 stable spos can also develop in transtension they dip opposite to the shear sense but when defining an s c structure with the shear foliation this composite fabric is always synthetic and thus indicates the correct sense of shear 4 spos produced in transpression or transtension can be used as vorticity gauges only when they represent stable orientations which must be demonstrated a priori

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

influence of early diagenesis on geotechnical properties of clay sediments romania black sea abstract the geotechnical properties of clay sediments were investigated using laboratory and in situ measurements as part of the geohazard assessment in the romanian sector of the black sea affected by landslides and seafloor deformation features the sediments were characterized as predominantly high plastic silty clay of high compressibility low undrained shear strength low cohesion and moderate sensitivity a shallow increase in shear strength exceeding the general trend could have been reconciled with evidence for the precipitation of iron sulfides and calcium carbonates related to early diagenetic reactions of sulfate reduction e g cryptic sulfur cycle and anaerobic oxidation of methane comparison of the compression behaviour of natural and reconstituted samples indicated that precipitation of these materials at particle contacts produced cementation the analysis of deeper sediments sharing similar geochemical properties illustrated that the early precipitation of even low quantity of sulfur and carbonate cements allowed clays to develop sufficient strength to preserve an abnormal high water content even under tens of meters burial depth because this cementation process was observed to be associated with an enhanced sensitivity and a strain softening response to undrained shearing it was concluded that such early diagenesis should be considered as a key environmental factor leading to weak layer formation that may influence the susceptibility of seafloor to failure

oxygen isotopes in meteoric calcite cements as indicators of continental paleoclimate because meteoric water 18 o values decrease with decreasing ambient temperature and increasing latitude 18 o values of meteoric calcite cement should exhibit a similar relation with paleolatitude and be an indicator of continental paleotemperatures to test this we compiled isotopic and paleolatitude data for 20 meteoric cements and nine speleothems ranging in age from devonian to modern and in paleolatitude from 3 5 to 83 mean 18 o values for meteoric cements and speleothems both show the same negative correlation with paleolatitude the 18 o vs latitude trend for these carbonates is almost identical to that predicted for modern inland environments but differs from the trend for coastal environments this suggests that the ground water controlling the ultimate composition of meteoric cement is derived predominantly from inland recharge if it is assumed that the modern meteoric water 18 o vs temperature relation is valid for the past and that insignificant evaporation occurred prior to carbonate precipitation then coastal and inland paleotemperatures can be calculated from the 18 o values of meteoric calcite 18 o mcl and seawater 18 o sw by using the equations t coastal 13 3 32 6 0 231 0 0613 18 o mcl 18 o sw and t inland 17 8 16 2 0 572 0 1233 18 o mcl 18 o sw where t is temperature in c calcite precipitated from coastal meteoric water at temperatures between 0 and 25 c will exhibit a narrow range in 18 o 6 to 4 where 18 o sw 0 the 18 o of calcite precipitated from inland meteoric water will be sensitive to paleotemperature ranging from 14 to 5 where 18 o sw 0 for temperatures of 0 to 25 c

mid crustal partitioning and attachment during oblique convergence in an arc system palaeoproterozoic ketilidian orogen southern greenland abstract the ketilidian orogen in se greenland contains a continental magmatic arc julianehab batholith and forearc which formed in the overriding plate of a palaeoproterozoic oblique subduction boundary between c 1854 and 1795 ma during uplift and deformation of the forearc between c 1795 and 1780 ma its sedimentary sequences were invaded by arc type magmas and metamorphosed at ht lp upper amphibolite facies conditions which led to widespread anatexis and generation of abundant s type granites transpression during early deformation d 1 was partitioned into arc parallel sinistral strike slip in the magmatic arc rocks of the batholith and arc normal contraction in the sedimentary and volcaniclastic sequences of the proximal forearc psammite zone subsequently d 1 fabrics in the psammite zone were overprinted and transposed in an intense flat lying d 2 ductile shear zone with arc parallel stretching lineations and top ne kinematic indicators the kinematic significance of the d 2 shear zone is that it accommodated vertical differences in strain and displacement and permitted deformation to be partitioned differently at different levels in the crust as such it is an attachment or coupling zone that provided kinematic linkage between deformation at different crustal levels during ne transport of a forearc sliver overprinting of the upper crustal domain of d 1 strike slip partitioned transpression in the forearc during d 2 is attributed to migration of the attachment zone to a higher crustal level this migration was a consequence of an enhanced geothermal gradient in the proximal part of the forearc due to the emplacement of batholith related plutonic suites during d 2

the influence of matrix rheology and vorticity on fabric development of populations of rigid objects during plane strain deformation abstract the influence of vorticity and rheology of matrix material on the development of shape preferred orientation spo of populations of rigid objects was experimentally studied experiments in plane strain monoclinic flow were performed to model the fabric development of two populations of rectangular rigid objects with object aspect ratios rob 2 and 3 the density of the rigid object populations was 14 of the total area objects were dispersed in a newtonian and a non newtonian power law matrix material with a power law exponent n of 1 2 the kinematic vorticity number wn of the plane strain monoclinic flow was 1 0 8 and 0 6 with finite simple shear strain of 4 6 3 0 and 0 9 respectively in experiments with rob 3 the spo is strongly influenced by wn and the material properties of the matrix deformation of a power law matrix material and low wn resulted in a stronger spo than deformation of a linear viscous matrix and high wn strain localization coupled with particle interaction plays a significant role in the development of a shape preferred orientation high strain simple shear zones separate trains of rigid objects that are surrounded by low strain zones with wn lower than the bulk wn in fabrics involving populations of objects with rob 2 rheology of the matrix materials does not systematically influence the intensity of the spo

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

ikaite caco36h2o precursor of the thinolites in the quaternary tufas and tufa mounds of the lahontan and mono lake basins western united states the thinolites in the quaternary tufas and tufa mounds of the lahontan and mono lake basins of the western united states are identified as being pseudomorphous after crystals of the mineral ikaite caco36h2o ikaite is a metastable mineral in the sedimentary realm it crystallizes from solution at temperatures close to zero celsius but rapidly decomposes at warmer temperatures and becomes pseudomorphed in calcite by in situ redistribution of the calcium and carbonate ions the likely implication is that the thinolites record ice cold episodes during deposition of the tufas

experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites for axial ratios r not vert similar3 porphyroclasts from three mylonites all show a very strong spo with the long axis of the best fit ellipse at an antithetic angle of 5 10 to the shear direction this is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear the cause of this divergence is investigated in a series of high simple shear strain 15 analogue experiments performed in a ring shear machine couette flow using a linear viscous matrix pdms the rotational behaviour of elongate r not vert similar5 rigid particles with elliptical and rhomboidal shapes comparable with the natural examples is modelled for both coherent and slipping particle matrix interfaces interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction but not stabilisation interface slip does result in stabilisation of the rhomboidal particle with the long diagonal oriented at a small antithetic angle to the shear direction for monoclinic particles mirror image shapes referred to here as types 1 and 2 show different rotational behaviour for the type 1 particle with a shape comparable to porphyroclast systems and mica fish the long side rotates asymptotically into parallelism with the shear direction natural examples of type 1 particles such as hornblende and olivine porphyroclasts measured from the finero mylonites southern alps show a very strong preferred orientation for r not vert similar3 with the long side parallel or at a small 5 antithetic angle to the mylonitic foliation for type 2 particles the short side stabilises close to the shear direction or at a small synthetic angle as also observed for sillimanite porphyroclasts from the mont mary mylonites western alps in this natural case stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation the analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles in natural examples decoupling from the matrix may be affected by extensional crenulation cleavage or c planes in s c fabrics

glendonites as a paleoenvironmental tool implications for early cretaceous high latitude climates in australia glendonites calcite pseudomorphs after the metastable mineral ikaite caco3 6h2o occur in the late aptian interval of the bulldog shale in the eromanga basin australia and in other early cretaceous basins at high paleolatitudes ikaite precipitation in the marine environment requires near freezing temperatures not higher than 4 c high alkalinity increased levels of orthophosphate and high pco2 the rapid and complete transformation of ikaite to calcite at temperatures between 5 and 8 c provides an upper limit on the oxygen isotopic composition of the pore waters 2 6 w 3 4 smow if it is assumed that these pore waters are representative of the shallow eromanga basin the calculated w can be used to reassess belemnite fossil oxygen isotopic paleotemperatures temperature recorded by fauna living in the basin at the time of ikaite precipitation data previously reported as 11 to 16 c assuming w 0 0 smow yield paleotemperatures ranging from 1 to 5 c squarely in the range of ikaite stability the low w indicates hyposaline conditions most likely caused by mixing high latitude meteoric waters with seawater the 18o depleted low temperature waters suggest that the region was at least seasonally colder than previously accepted

attachment formation during partitioning of oblique convergence in the ketilidian orogen south greenland subhorizontal attachment zones provide coupling between lithospheric layers in orogenic belts a mid crustal attachment zone is exposed in the palaeoproterozoic ketilidian orogen south greenland which formed as a result of north directed oblique convergence at a cordilleran type margin rifting c 2 1 ga and compressional deformation and magmatism 1850 ma on the continental margin was followed by an extended sinistral transpression from 1850 to 1730 ma now separated into three episodes or peaks of activity the first episode was focused on the back arc region and was followed by the main arc construction phase during which transpression was partitioned into strike slip and contraction components despite the longevity of this active margin system individual tectonic events took place rapidly e g development of fore arc d1 d3 and accompanying high temperature low pressure metamorphism took place over c 12 ma we explain the fore arc and batholith evolution by the upward migration of an underlying attachment structure through the upper crustal partitioned blocks this migration may be attributed to an increase in the geothermal gradient accompanied by or followed by exhumation of the mid crust the partially molten hence weak attachment zone solidified and strengthened during cooling before emplacement of the post orogenic rapakivi suite during the third distinct phase of mild sinistral transpression

ikaite tufa towers in ikka fjord southwest greenland their formation by mixing of seawater and alkaline spring water abstract ikka fjord in southwest greenland is the type locality for ikaite a metastable hexahydrate of calcium carbonate here ikaite forms submarine tufa columns rising up to 18 m above the fjord bottom the columns are spectacular examples of abiogenic carbonate precipitation in a cold seawater environment and so far represent the only known formation of ikaite in seawater we have analyzed the 2h 13c 14c 18o and 87sr contents and major element and minor element compositions of ikaite and of the different water types in and around ikka fjord in order to understand the formation of the ikka columns water from inside the columns is fresh but alkaline rich in ions of sodium bicarbonate and carbonate and has a stable isotope composition identical to precipitation falling on the nearby gronnedal ika igneous complex and we conclude that the tufa columns form over submarine springs issuing from the bottom of ikka fjord dissolution of unidentified sodium carbonate minerals in carbonatites in the igneous complex probably accounts for the peculiar chemistry of the springs ikaite supersaturation and precipitation is controlled by mixing of spring water and seawater at the cold 6 c fjord bottom and precipitation is accelerated by the formation of hydrated caco3o aq ion pairs while phosphate ions in the spring water act as inhibitor for precipitation of anhydrous carbonates we propose that the spring water seeps in ikka fjord literally create their own conduits in the form of the vertical chimney like columns with ikaite forming continuously at the tops of the columns the only limits to upward growth are winter ice and the fresh water layer capping the fjord during summer

the solubility and stabilization of ikaite caco3 6h2o from 0 to 25 c environmental and paleoclimatic implications for thinolite tufa we determined the solubility of ikaite from 0 to 25 c to model its saturation state in natural waters and test the hypothesis that it is the precursor of the calcite pseudomorphs in thinolite tufa of quaternary lake lahontan reversible solubility at buffered p co 2 yields the following expression for the dissolution constant of ikaite log k ikaite 0 15981 2011 1 t where t circ k and 0 circ c leq t leq 25 circ c derived standard state properties are delta g ikaite circ 2541 9 kjmol 1 pm 0 66 delta h ikaite circ 2973 4 kjmol 1 pm 1 02 s ikaite circ 306 6 jt 1 mol 1 pm 1 2 modeling shows that ikaite is undersaturated at all temperatures in seawater and in alkaline lakes but that it rapidly approaches saturation near 0 c its precipitation in near freezing marine sediments requires large additions of hco 3 to pore fluids from the diagenetic decomposition of organic matter its crystallization in tufas of alkaline lakes howeve

submarine columns of ikaite tufa in the small ikka fjord in southwestern greenland we have studied remarkable submarine tufa columns forming over alkaline springs by abiotic precipitationof the metastable cold water mineral ikaite caco3 6h2o 1 3 the columns which support a rich marine life are up to 20 m high with annual growth of more than 50 cm we wish to nominate the locality which seems to be unique for classification as a geological world heritage site

ikaite pseudomorphs in the zaire deep sea fan an intermediate between calcite and porous calcite translucent brown aggregates of calcium carbonate crystals have been found in cores from the zaire deep sea fan west equatorial africa the aggregates are well preserved but very friable upon storage they become yellowish white and cloudy and release water chemical mineralogical xrd petrographical crystal morphological and stable isotope data demonstrate that the crystals have passed through three phases 1 an authigenic carbonate phase probably calcium carbonate which is represented by the external habit of the present crystals 2 a translucent brown ikaite phase caco 3 6h 2 o unstable at temperatures above 5 c and 3 a phase consisting of calcite microcrystals that are poorly cemented and form a porous mass within the crystal form of the morphologically unchanged first phase the transformation from the first phase into ikaite was probably a kinetic replacement the transformation from ikaite into the third phase occurred because of storage at room temperature the presence of ikaite is indicative of a low temperature anaerobic organic carbon rich marine environment ikaite is probably the precursor of a great number of porous calcite pseudomorphs and possibly also of many marine authigenic microcrystalline carbonate nodules

glendonites from an early jurassic methane seep climate or methane indicators abstract an early jurassic late pliensbachian section margaritatus to spinatum ammonite zone northeast of cremlingen germany revealed three horizons with abundant glendonites calcite pseudomorphs after ikaite caco3 6h2o as well as authigenic carbonates these pseudomorphs have previously been used as indicators for organic and phosphate rich paleoenvironments with near freezing bottom water temperatures stable isotope measurements clearly show that the ikaite glendonite was precipitated at a methane seep location with methanotrophic sulfate reduction as the triggering geochemical process for ikaite formation at the sulfate methane interface the termination of methane seepage probably led to the disintegration of the primary mineral ikaite and successive pseudomorph formation reconstructed paleo bottom water temperatures ca 10 c from belemnites and ostracods indicate temperatures well above the previously observed range of ikaite stability near freezing raising the question if methane seeps are distinct geochemical sites promoting ikaite formation at higher temperatures