isotopic evidence b c o of deep fluid processes in fault rocks from the active woodlark basin detachment zone we report results from boron carbon and oxygen stable isotope analyses of faulted and veined rocks recovered by scientific ocean drilling during odp leg 180 in the western woodlark basin off papua new guinea in this area of active continental extension crustal break up and incipient seafloor spreading a shallow dipping seismically active detachment fault accommodates strain defining a zone of mylonites and cataclasites vein formation and fluid infiltration syntectonic microstructures and vein fill mineralogy suggest frictional heating during slip during extension and exhumation of moresby seamount low carbon and oxygen isotope ratios of calcite veins indicate precipitation from hydrothermal fluids 13cpdb down to 17 18opdb down to 22 formed by both dehydration and decarbonation boron contents are low 7 ppm indicating high grade metamorphic source rock for the fluids some of the 11b signatures 17 35 parent solutions to calcite vein fills are low when compared to deep seated waters in other tectonic environments likely reflecting preferential loss of 11b during low grade metamorphism at depth pervasive devolatilization and flux of co2 rich fluids are evident from similar vein cement geochemistry in the detachment fault zone and splays further updip multiple rupture and healing history of the veins suggests that precipitation may be an important player in fluid pressure evolution and hence seismogenic fault movement

pore pressure stress increase and fault weakening in low angle normal faulting low angle dip 30 normal faults accommodate much extension of the continental crust they apparently move under low resolved shear stress and are anomalously weak characteristics that they share with the san andreas fault structural textural and geochemical arguments suggest that low angle normal faults are weak in both the ductile and brittle regimes partly or totally due to elevated pore fluid pressure high pore pressure in detachment zones may be contained by upper plate strata mineral precipitation in their hanging walls formation of low permeability microbreccia layers threshold pressure gradients and low permeability mylonites below chlorite breccia mechanical analysis shows that fault weakening may preclude equality of the regional and fault zone stress tensors and predicts reorientation and increase of principal stresses in weak fault zones these changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks fault weakening focuses extension in the upper crust onto low angle normal ductile brittle shear zones in the midcrust promoting propagation of low angle brittle normal faults into the upper crust

the influence of grain size and grain size distribution on methods for estimating paleostresses from twinning in carbonates abstract this study examines the relative differential stresses around a minor thrust fault within the mountain city window tennessee u s a the fault zone developed within dolomite rocks and deformation took place by twinning fracturing pressure solution and the development of fine grained deformation zones grain size reduction is observed from undeformed wall rock to the center of the fault zone and occurred by dynamic recrystallization methods to determine paleostresses in naturally deformed rocks from twinning assume a single coaxial strain inducing event the recrystallization within the center of the fault zone removed the effects of earlier deformation so that the twinning more closely reflects a single coaxial deformation event late in the fault history two methods were used to estimate the relative differential stresses across the fault zone and the results show opposite trends towards the center of the fault zone the different results may be partially explained by the influence of grain size as only one of these methods considers the influence of grain size in addition the grainsize data from this fault zone demonstrate that the tendency for a grain size class to be twinned depends on the grain size distribution the grain size distribution may result in grain to grain stress concentrations that induce twinning thus grain size distribution should also be considered to achieve more accurate estimates of paleostresses

analogies to oceanic behaviour in the continental breakup of the western woodlark basin the woodlark basin d entrecasteaux island region off northeast papua new guinea fig 1 offers a rare glimpse of the propagation of active rifting and continental breakup1 into orogenically thickened and subsequently extended lithosphere2 10 rifting continental breakup and the subsequent formation of oceanic lithosphere by sea floor spreading often involve rupture controlled by propagation of a rift tip11 20 we recognize several tectonic elements in the evolving western woodlark intracontinental system that are close geometric analogues of wholly oceanic propagators and a kinematic development that can be viewed as an extrapolation of oceanic tectonics the response to deformational stresses in the final stages of breakup in a thick relatively hot and hence weak continental lithosphere appears akin to that of oceanic lithosphere

depositional and diagenetic c s fe signatures in early paleozoic normal marine shales the degree of pyritisation dop of cambrian silurian normal marine shales is always less than 0 70 similar to that found previously for sediments of devonian to cretaceous ages this indicates that dop can be used as a paleoenvironmental indicator of deposition from normal marine i e oxygenated bottom waters throughout the phanerozoic record although the range in dop is similar significant variations in normal marine cs ratios can occur relatively low cs ratios 1 5 result from lower reactive iron contents which are also independent of organic carbon variations reactive iron contents and hence normal marine cs ratios are able to vary at a regional or basinal level

specific surface areas in coastal sediments relationships with other textural factors abstract sediment specific surface area can be a controlling variable in sediment biogeochemistry therefore a variety of granulometric techniques have been employed to infer relative and absolute surface areas in sediments this study compares specific surface areas with other textural parameters for a suite of aluminosilicate sediments from the gulf of maine and contiguous estuaries the intragranular porosity of sediment grains increases with grain size and is probably due to varying relative contributions of nanometer and micrometer sized pores poor sorting of sediments causes underestimation of absolute surface areas by geometrical calculation using measures of central tendency underestimation is worse in coarser estuarine sediments owing to a combination of increased roughness factors of coarser grains and poorer sorting relative surface areas among sediments may be accurately assessed by measuring the proportion clay or fine silt means and medians do not provide accurate estimation of relative surface areas unless the samples share similar sorting characteristics an algorithm to predict absolute surface areas from grain size distributions of gulf of maine sediments is presented

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

development of a sequential extraction procedure for iron implications for iron partitioning in continentally derived particulates abstract the development of a sequential extraction procedure for iron in modern and ancient sediments is presented the scheme recognizes seven operationally derived iron pools 1 carbonate associated fe fe carb including siderite and ankerite 2 easily reducible oxides fe ox1 including ferrihydrite and lepidocrocite 3 reducible oxides fe ox2 including goethite hematite and akaganeite 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u as such this is the first extraction scheme specifically developed to allow the separate identification of magnetite and the first to allow a complete evaluation of fe carbonate phases such as siderite and ankerite the scheme was developed following tests on pure mineral phases to evaluate the minerals solubilized by each technique and to determine optimum extraction times further tests on mixtures of pure minerals and on grain size separated sediments from two major us rivers and two glacial meltwaters validate the specificity of the scheme for different pools of iron minerals and demonstrate a high degree of reproducibility for each analytical stage the data obtained for the riverine and glacial sediments are additionally discussed in relation to the dominant modes of transport of different iron minerals in fine grained continental sediments

mechanics of low angle extensional shear zones at the brittle ductile transition corresponding to low values of the velocity ratio ve vs the 2 d solution for ve vs 1 0 3 reveals the development of a periodic system of extensional shear bands dipping at 30 toward the shearing direction at a depth of 12 to 14 km shear bands are formed after less than half a million years at the base of the reaction zone defined by the region where feldspar to mica reaction is completed shear bands do not propagate to greater depths because the pressure prevents the feldspar from fracturing and thus the reaction to occur the periodic system of shear bands defines a midcrustal flat weakened zone within which the equivalent shear stress is enhanced by at least a factor of three at the shear band tips brittle fracture could thus occur within the midcrustal flat weakened zone explaining therefore the microseismicity monitored at these depths in regions of active extension index terms 8109 tectonophysics continental tectonics extensional 0905 8159 tectonophysics rheology crust and lithosphere 8164 tectonophysics stresses crust and lithosphere 7218 seismology lithosphere and upper mantle 3230 mathematical geophysics numerical solutions keywords continental extension strain localization detachment fault rheology reaction softening numerical model

deep structure of an arc continent collision earthquake relocation and inversion for upper mantle p and s wave velocities beneath papua new guinea we examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction by analyzing the seismicity and upper mantle velocities in the new guinea arc continent collision earthquakes in the papua new guinea png region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities using p and s wave arrival times recorded by seismographs in png a total of 957 well recorded earthquakes were chosen for use in three dimensional velocity inversions from an initial catalog of 12 960 events based on the stability of initial hypocenter relocations constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters and to exploit a priori knowledge of the geometry of velocity anomalies spherical geometry was used to accurately calculate long ray paths up to 1500 km in length most of the 700 best located hypocenters from the png data set are in the upper mantle beneath the finisterre huon fh ranges the newly accreted island arc terrane beneath the papuan peninsula and in the new britain seismic zone an oceanic subduction system these hypocenters show a well defined seismic zone dipping vertically or steeply to the north beneath the northern fh ranges from 125 to 250 km depth continuous along strike with the new britain seismic zone to the east which shows earthquakes to 600 km depth the steeply dipping intermediate depth seismicity flattens near 100 km depth beneath the fh ranges and forms a subhorizontal seismic zone beneath the fh ranges by contrast seismicity continues upward to the surface beneath new britain where the arc has not yet collided with new guinea the 100 km deep flat zone does not continue south of the fh ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane and no clear evidence for a south dipping seismic zone could be found the relationship of the flat part of the slab 100 km beneath the fh ranges and surface faulting is difficult to discern the apparent surface suture bounding the south side of the fh ranges is 100 km directly above the southern termination of intermediate depth seismicity nowhere in png could clear evidence for arc polarity reversal be found from seismicity all of the well located subcrustal earthquakes beneath the papuan peninsula lie in a narrow horizontal line between 125 and 175 km depth that follows the center of the peninsula and are distinctly separated from the fh seismic zone to the north these events do not require a southward subducting plate beneath the papuan peninsula an alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals allowing for only crustal heterogeneity reduces travel time residual variances by 40 from a simple one dimensional structure while allowing for additional mantle heterogeneity results in 10 further reduction for the various parameterization s attempted here inversions with several different block geometries show low velocities at 35 to 171 km km depth beneath the bismarck sea back arc basin and high velocities just south of the intermediate depth fh seismic zone the pattern of lateral heterogeneity supports the inference from seismicity of north dipping slabs beneath both northeast new guinea and new britain

anisotropy pore fluid pressure and low angle normal faults low angle normal faults lanfs are commonly associated with high angle normal faults high pore fluid pressures and foliated fault cores explanations for lanf slip based on high pore fluid pressure critically depend on stress rotations in the fault core zone which have been based on an assumption of isotropy anisotropy fundamentally changes the coupling between pore fluid pressure and effective stress the response of saturated fault core rocks in lanfs can be explored by combining the effective medium theory with anisotropic poroelasticity the nucleation and evolution of lanfs are investigated in a framework of local changes in rock properties and the interaction of a pressurised pore fluid with intrinsic lithological and extrinsic damage anisotropies this quantitative approach enables comparison with detailed field and laboratory measurements of fault rock properties

hydrothermal fluid venting along a seismogenic detachment fault in the moresby rift woodlark basin papua new guinea we investigate the conditions of hydrothermal alteration occurring along an active seismogenic normal fault dipping 30 in the moresby rift woodlark basin papua new guinea petrological and fluid inclusion studies were conducted in the samples recovered from the ocean drilling program leg 180 that cored the footwall scarp and hanging wall of the fault chemical analyses of the thermodependent si al substitutions in chlorites show that synrift alteration is coeval with a temperature rise from 200 c to 300 c in the footwall of the detachment whereas no thermal event is recorded in the hanging wall in the same samples fluid inclusions indicate that footwall rocks followed an isothermal exhumation path from 5 7 km to the subsurface taken together the results strongly suggest that the 100 c temperature rise results from the upward migration of h2o nacl fluids expelled by dehydrating metamorphic rocks from the lower crust fluid release began with the onset of detachment faulting and is still actively occurring nowadays the fact that low angle earthquakes also occur now at 5 7 km along the fault suggests that the dehydration of the metamorphic rocks and the subsequent circulation of hot hydrothermal fluids may be responsible for the weakening of the middle crust and the formation of a detachment in the moresby rift

deformation fabrics of faulted rocks and some syntectonic stress estimates from the active woodlark basin detachment zone ocean drilling program odp leg 180 investigated in the western woodlark basin off papua new guinea the nature and evolution of continental extension eventually leading to crustal break up and sea floor spreading at moresby seamount the rift related extension is localized at a recently active low angle 30 detachment fault partly buried beneath a pliocene pleistocene sedimentary synrift sequence data from three drillsites sample the detachment fault itself secondary faults in its hanging wall and a steep normal fault cutting the footwall the fault plane itself is manifested as a strongly altered fault gouge deformation of turbiditic sediments in several fault zones in the hanging wall is dominated by brittle mechanisms and accompanied by intensive veining and pervasive diagenetic cementation the metabasic rocks of the footwall below the detachment show an unusual transition from ductile to brittle deformation fabrics with increasing depth many fracture systems show evidence of repeated opening and healing during multistage hydrothermal mineralization syn mylonitic microstructures and vein fill mineralogy suggest exhumation of the detachment footwall from considerable depth in the crust two palaeo piezometers were applied to calcite filled veins that show evidence of plastic deformation differential stress values of similar magnitude and probably close to the rock failure strength are found in both the hanging wall and footwall

sources of iron oxides in reddish brown soil profiles from calcarenites in southern spain abstract we studied the distribution with depth of the different forms of iron in two reddish brown rubified soils developed on hematite free calcarenites in the province of cordoba southern spain hematite appeared in significant amounts in the a and b horizons of the soils and seemed to have originated mainly from the weathering of the fe bearing clay minerals smectites because of their low contents of fe both the non clay fraction and the carbonate of the parent material contributed little to fe oxide formation the distribution with depth of goethite in the clay fractions suggested that part of the initial goethite had been lost and might have been altered to hematite

talc friction in the temperature range 25 400 c relevance for fault zone weakening abstract talc is one of the weakest minerals that is associated with fault zones triaxial friction experiments conducted on water saturated talc gouge at room temperature yield values of the coefficient of friction shear stress effective normal stress n in the range 0 16 0 23 and increases with increasing n talc gouge heated to temperatures of 100 400 c is consistently weaker than at room temperature and talc has a temperature pressure range of stability that extends from surficial to eclogite facies conditions making it of potential significance in a variety of faulting environments talc has been identified in exhumed subduction zone thrusts in fault gouge collected from oceanic transform and detachment faults associated with rift systems and recently in serpentinite from the central creeping section of the san andreas fault typically talc crystallized in the active fault zones as a result of the reaction of ultramafic rocks with silica saturated hydrothermal fluids this mode of formation of talc is a prime example of a fault zone weakening process because of its velocity strengthening behavior talc may play a role in stabilizing slip at depth in subduction zones and in the creeping faults of central and northern california that are associated with ophiolitic rocks

shallow dips of normal faults during rapid extension earthquakes in the woodlark d entrecasteaux rift system papua new guinea earthquakes in the woodlark d entrecasteaux rift system reveal changes in fault geometry along strike as extension progresses from continental to oceanic modes in one segment evidence is found for earthquakes on shallow dipping normal faults to examine these trends locations of 147 earthquakes are redetermined and focal mechanisms for 21 events are derived from waveform inversion where seafloor spreading occurs east of 152 e most earthquakes illuminate a ne sw strike slip fault north of the oceanic rift system along the woodlark rise elsewhere earthquakes occur along a well defined rift zone and focal mechanisms reveal normal faulting with north south t axes and depths less than 10 km most normal faulting earthquakes show high angle fault planes dips 40 50 but several are consistent with slip on moderately low angle planes dipping 15 35 these low angle nodal planes are only found in a region between 150 5 e and 152 5 e where the transition occurs from seafloor spreading to continental rifting a seismic reflection profile near 151 e reveals a major normal fault that dips to the north at 25 35 just landward of the oceanic rift tip that fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied hence normal faults dipping 30 are likely to be seismically active in at least one part of this rift system in this region just landward from the rift tip the highest extension rates are expected 40 mm yr and strain appears localized on a single system of normal faults because shallow dips for normal faults are seen near the rift tip but not elsewhere along strike high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips perhaps high shear traction at the base of the upper crust localized by nearby spreading sufficiently alters the local stress regime

a model for iron deposition to euxinic black sea sediments sulfur isotope and c s fe elemental data from the lower devonian siegenian emsian hunsruck slate of germany allow the conditions that led to the pyritization of soft bodied fossils to be determined the slates preserving pyritized soft tissues generally contain low concentrations of organic matter 0 3 0 4 percent c and pyrite sulfur 0 0 2 percent s respectively but are unusually rich in total and hcl extractable iron concentrations of iron are significantly lower in samples of the hunsruck slate from areas where fossils lack pyritized soft tissues the fossil pyrite is very enriched in s 34 compared to fine grained disseminated pyrite in the adjacent slates delta fossil sediment 10 to 50 permil suggesting that pyritization of the fossils persisted into later stages of authigenic mineralization pyrite formation in the sediments was limited by the low concentrations of metabolisable organic matter leaving residual sulfate and iron available for continued fossil pyritization in organic rich microenvironments a diffusion with precipitation model indicates that a critical control on the initial preservation of readily metabolisable soft tissue in this way is the presence of high concentrations of sediment iron capable of being solubilized during shallow burial diagenesis in association with low concentrations of metabolisable organic matter in the matrix pore waters rich in dissolved iron allow the sulfide generated by the decay of readily metabolisable soft tissue to be trapped efficiently within the carcass

zircon isotopic age from the union ultramafic complex maine ultramafic and mafic rocks of the union complex maine u s a are interpreted as remnants of iapetus oceanic crust based upon geological geochemical and structural considerations these rocks were emplaced along a cryptic acadian suture during north american avalonian collision u pb analyses of zircons from the quartz diorite portion of the middle road unit yield a primary crystallization age of 410 7 ma marking a maximum age for collision and disappearance of iapetus ocean floor other studies have shown a minimum time of peak acadian deformation at 385 400 ma strongly suggesting that the acadian event occurred in a period of less than 20 ma in this portion of the northern appalachians

porewater sulphur geochemistry and fossil preservation during phosphate diagenesis in a lower cretaceous shelf mudstone lower cretaceous mudstones exposed at speeton in north yorkshire uk contain lobsters and burrows preserved in diagenetic phosphate concretions isotopic compositions of sulphur in both diagenetic sulphide and structural sulphate in diagenetic phosphate have been measured in an attempt to constrain diagenetic porewater chemistry the occurrence of phosphatized and pyritized lobsters and similarly preserved burrows allows a detailed comparison of these microenvironments with the host sediments host sediments are extensively bioturbated and characterized by very light sulphide isotopic compositions mean sulphide 34s 48 3 3 1 n 19 and sulphate isotopic compositions that are lighter than lower cretaceous seawater sulphate mean sulphate 34s 8 7 3 2 1 n 19 these isotopic values can be explained by the action of bioturbating macrofauna the introduction of oxygen in ventilating seawater results in the oxidation of early formed isotopically light pyrite resulting in porewater sulphate that is enriched in 32s subsequent pyrite formation via bacteriogenic reduction of isotopically light porewater sulphate leads to the formation of isotopically light pyrite whilst residual porewater sulphate apparently remains relatively enriched in the 32s isotope sulphur isotopic values for the burrows are very different sulphide isotopic compositions average 34 4 0 4 1 n 3 and sulphate isotopic compositions average 14 4 6 6 1 n 4 these isotopic compositions are the result of rapid development of closed system conditions in burrows resulting in the build up of acidity necessary for phosphate precipitation and coprecipitation of isotopically light sulphate formed by oxidation of isotopically light sulphide surrounding the burrows lobster shell isotopic compositions fall between these two groups on the basis of their isotopic compositions some lobsters appear to have died close to the sediment water interface whilst others appear to have been buried in burrows prior to death

chemical and physical characteristics of iron oxides in riverine and glacial meltwater sediments abstract the speciation and grain size distribution of iron have been examined in several riverine and glacial sediments in order to evaluate controls on the concentrations and modes of occurrence of iron oxides in weathering derived particulates three operationally defined fractions have been recognised relating to the reactivity of iron towards dissolved sulphide 1 highly reactive iron fe hr consisting of ferric oxides 2 poorly reactive iron fe pr consisting of magnetite and some sheet silicate iron and 3 unreactive residual silicate iron fe u the finer fractions of each riverine and glacial suite are enriched in fe hr and usually also have higher fe hr fet ratios however coarser fractions may also have elevated fe hr contents and relationships of fe hr to both surface area and al indicate that iron oxides dominantly occur in association with the surfaces of aluminosilicate minerals which are common in finer grained fractions but may also be relatively enriched in coarser fractions a similar range of fe hr surface loadings is evident in the riverine and glacial meltwater suites from 0 6 to 1 8 mg fe m 2 for the riverine sediments and 0 5 to 1 5 mg fe m 2 for the glacial meltwater sediments despite wide variations in catchment bedrock lithology and climatic conditions significantly lower iron oxide loadings 0 1 mg fe m 2 were found in subglacial sediments most fe hr is closely associated with organic c although the latter occurs as blebs or smears rather than continuous coatings e g ransom et al 1997 ransom b bennett r h baerwald r shea k 1997 tem study of in situ organic matter on continental margins occurrence and the monolayer hypothesis mar geol 138 1 9 electron microscope observations show that iron oxides may occur as nanoparticulate 10 20 nm spheres at the edges of sheet aluminosilicate minerals as crystalline oxides infilling surface pits in coarser grained aluminosilicate minerals and occasionally as discrete crystalline oxides the similar occurrences of iron oxides in both riverine and glacial environments indicates that the delivery of reactive iron oxides to the oceans during glacial interglacial cycles will be affected by the masses and grain size characteristics of sediments delivered by glacial and riverine fluxes rather than by differences in iron mineralogy these delivery rates will be modified by behaviour of the oxide nanoparticles during transport from the continents

effects of finite rifting times on the development of sedimentary basins abstract most thermo mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post rift development of the basin this assumption greatly simplifies the mathematical treatment but is not in accord with what is found in nature where 10 to 50 m y rifting events commonly accompany the formation of sedimentary basins and continental margins the effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer this technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced the effect of a finite duration extension event is that heat is lost during rifting increasing the syn rift subsidence at the expense of the post rift lateral heat flow which was not included in previous studies of the effect of finite rifting times has a significant effect on the subsidence history distribution of sediments and thermal history in particular the post rift subsidence is decreased by more than 25 for a 20 m y rifting event and by more than 10 15 for a rifting event as short as 10 m y this will significantly decrease the subsidence rates in the post rift stage and implies that inferences concerning the structure development and thermal history of the basin derived from using curves to interpret backstripped subsidence can be greatly in error variations in syn rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors such as the flexural response of the lithosphere to sediment loading to affect the final width of the basin the total amount of sediments that accumulate and the basin stratigraphy

palaeostress estimation using calcite twinning experimental calibration and application to nature abstract by means of high temperature deformation experiments on calcite rocks of different grain sizes we have established the relationships between differential stress and twinning incidence twin density and volume fraction and the way that these relationships depend on grain size both twinning incidence and volume fraction increase with grain size at a constant level of differential stress log twin density is proportional to stress but is independent of grain size at a given grain size volume fraction of twinning increases non linearly with differential stress twinning activity measured by twinning incidence increases linearly with stress the behaviour of each of these parameters is independent of temperature strain and strain rate these relationships can be used as a basis for the estimation of palaeostress values in naturally deformed calcite rocks this is illustrated by a study of twinning activity in limestones close to thrust faults in the cantabrian zone of the variscan orogenic belt in northwestern spain high peak differential stresses were measured ranging from 150 to 250 mpa using the turner dynamic analysis technique however it is clear that shear stresses resolved parallel to the fault planes were very low the attainment of high stress differences when the resistance to faulting was low is attributed either to bending forces induced during the motion of the non planar surfaces relative to each other or to the stress peak being attained prior to the localization of deformation onto the fault surfaces

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

continental rifting and metamorphic core complex formation ahead of the woodlark spreading ridge d entrecasteaux islands papua new guinea 1 we evaluate the role of a metamorphic core complex mcc on normanby island in the woodlark rift located 1 km thickness of blueschist derived mylonites formed in a midcrustal shear zone during the pliocene at 400 500 c this top to the north zone appears to have reactivated the gently dipping base of the papuan ophiolite papuan ultramafic body pub and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the mcc and rapid subsidence of the woodlark rise mylonites in the mcc s lower plate have been exhumed along a detachment as a result of 50 km of slip at rates of 12 mm yr the inactive back tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the plio pleistocene plate motion a second se vergent detachment has been established on the opposite flank of this rolling hinge style mcc probably since 0 8 at depth and provide a sufficient mechanism for activating low angle normal faults in the rift mcc inception was not localized to the tip of the woodlark mor instead extreme crustal thinning near the mcc preconditioned later continental breakup the lower crust appears to be weak thickening beneath unloaded footwalls to uplift mccs above sea level and flowing laterally to even out regional crustal thickness contrasts on a 1 6 m y timescale deep seated transforms separate rheologically distinct domains in which extension has been localized along the weak pub to cause mcc formation vs those in which slip is distributed across an imbricate zone of more uniform strength normal faults the trobriand fault connects in the eastern woodlark rift to the owen stanley fault in the papuan ranges which is probably moving at nearly the full plate velocity

frictional properties of low angle normal fault gouges and implications for low angle normal fault slip abstract the mechanics of slip on low angle normal faults lanfs remain an enduring problem in structural geology and fault mechanics in most cases new faults should form rather than having slip occur on lanfs assuming values of fault friction consistent with byerlee s law we present results of laboratory measurements on the frictional properties of natural clay rich gouges from low angle normal faults lanf in the american cordillera from the whipple mts detachment the panamint range front detachment and the waterman hills detachment these clay rich gouges are dominated by neoformed clay minerals and are an integral part of fault zones in many lanfs yet their frictional properties under in situ conditions remain relatively unknown we conducted measurements under saturated and controlled pore pressure conditions at effective normal stresses ranging from 20 to 60 mpa corresponding to depths of 0 9 2 9 km on both powdered and intact wafers of fault rock for the whipple mountains detachment friction coefficient varies depending on clast content with values ranging from 0 40 to 0 58 for clast rich material and 0 29 0 30 for clay rich gouge samples from the panamint range front detachment were clay rich and exhibit friction values of 0 28 to 0 38 significantly lower than reported from previous studies on fault gouges tested under room humidity nominally dry conditions including samples from the same exposure samples from the waterman hills detachment are slightly stronger with ranging from 0 38 to 0 43 the neoformed gouge materials from all three localities exhibits velocity strengthening frictional behavior under almost all of the experimental conditions we explored with values of the friction rate parameter a b ranging from 0 001 to 0 025 clast rich samples exhibited frictional healing strength increases with hold time whereas clay rich samples do not our results indicate that where clay rich neoformed gouges are present along lanfs they provide a mechanically viable explanation for slip on faults with dips p f 3 overpressures and or correcting for 5 of footwall tilting furthermore the low rates of frictional strength recovery and velocity strengthening frictional behavior we observe provide an explanation for the lack of observed seismicity on these structures we suggest that lanfs in the upper crust depth

low angle normal faults and seismicity a review although large low angle normal faults in the continental crust are widely recognized doubts persist that they either initiate or slip at shallow dips 30 because 1 global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30 and 2 andersonian fault mechanics predict that normal faults dipping less than 30 cannot slip geological reconstructions thermochronology paleomagnetic studies and seismic reflection profiles mainly published in the last 5 years reinforce the view that active low angle normal faulting in the brittle crust is widespread underscoring the paradox of the seismicity data for dip slip faults large enough to break the entire brittle layer during earthquakes m w 6 5 consideration of their surface area and efficiency in accommodating extension as a function of dip suggests average recurrence intervals of earthquakes r tan assuming stress drop rigidity modulus and thickness of the seismogenic layer do not vary systematically with dip if the global distribution of fault dip normalized to total fault length is uniform the global recurrence of earthquakes as a function of dip is shown to be r tan sin this relationship predicts that the frequency of earthquakes with nodal planes dipping between 30 and 60 will exceed those with planes shallower than 30 by a factor of 10 in good agreement with continental seismicity assuming major normal faults dipping more than 60 are relatively uncommon revision of andersonian fault mechanics to include rotation of the stress axes with depth perhaps as a result of deep crustal shear against the brittle layer would explain both the common occurrence of low angle faults and the lack of large faults dipping more than 60 if correct this resolution of the paradox may indicate significant seismic hazard from large low angle normal faults

continental rifting and initial sea floor spreading in the woodlark basin our understanding of the processes by which continents rift and sea floor spreading initiates is derived primarily from studies either of old passive margins and oceanic crust or of young regions of intra continental extension where spreading has not yet started it has been thought that continental rifting ceases when sea floor spreading begins1 2 that oceanic fracture zones develop from transfer or transform faults within continental rifts3 4 and that linear magnetic anomalies correlate with the onset of sea floor spreading during times of magnetic reversals5 6 here we present a marine geophysical survey of one of the few active examples of continental rifting and spreading initiation the western woodlark basin papuan peninsula region of new guinea which shows that in detail these assumptions do not hold the data confirm models of the rifting to spreading transition that invoke both ridge propagation and nucleation of discrete spreading cells7 10 and provide an unambiguous example of a spreading centre reorienting by synchronous jumping rather than propagation

mineralogy porosity and fluid control on thermal conductivity of sedimentary rocks summary estimates of thermal conductivity of the main non clay and clay sedimentary minerals are presented especially for the case of argillaceous ones these estimates are averaged estimates based on the interpretation of laboratory conductivity porosity and mineralogy measurements performed on small volumes of the order of 100cm3 of water saturated moist and air saturated samples the sampling is representative of the main sedimentary rocks sandstones carbonates evaporites and shales and it is composed of samples considered as structurally isotropic material first we experimentally verify the first order control of mineralogy porosity and fluid content on bulk conductivity and we demonstrate that such influences may be predicted accurately using a geometric mean model as long as isotropic samples are used then we interpret the data using an inverse method in order to estimate the average mineral conductivities of the main non clay and clay minerals which give the best fit to the individual laboratory measurements through the geometric mean model the analysis is based on measurements on 82 non clay samples and 29 clay samples taken on oil well cores or cuttings on outcrops and on artificially recompacted samples estimates of average mineral conductivities returned by the inversion process for the main non clay minerals are similar to generally admitted values 7 7 w m k for quartz 3 3 w m i k for calcite 5 3 w m l k for dolomite and 6 3 w m k for anhydrite values obtained for the clay minerals are systematically lower than those obtained for non clay ones they are of the order of 1 9 w m k for illite and smectite 2 6 w m k for kaolinite and 3 3 w m l k for chlorite these estimates combined with porosity and mineralogy data are used with the geometric mean model in order to verify its first order validity for predicting thermal conductivity of small volumes of porous isotropic sedimentary rocks the bulk conductivity is predicted with an accuracy of the order of f10 per cent for moist or water saturated samples and of the order of f20 per cent for air saturated samples

pyritization of iron and trace metals in anoxic fjord sediments nordasvannet fjord western norway the accumulation and storage of trace metals in coastal sediments is an environmental concern it is therefore important to understand better how these metals are bound or released under different redox conditions this study of fe and trace metal fixation under continuously anoxic conditions in the bottom sediments and the lower water column of the nordasvannet fjord in western norway contributes further to such understanding it allows investigation of both an end member redox state and one important mechanism of fe and trace metal accumulation in sediments the pyritization of fe and trace metals pyrite formation occurs both in the water column and in the sediments of the nordasvannet fjord and favours the fixing of fe and trace metals in the bottom sediments of the fjord thus these sediments act as a continuous sink for fe and trace metals the dop and the degrees of trace metal pyritization for mo ni and cr correlate with organic matter content while it is generally thought that fe is the factor limiting pyrite formation in anoxic environments this study found that degrees of pyritization of fe dop are clearly below 100 and the availability of metabolizable organic matter is limiting pyrite formation this is an important finding because it indicates that increased supply of organic and mineral matter by higher runoff from land would further enhance the fixation of these metals in the fjord sediments as would higher organic matter availability from increased productivity due to higher nutrient supply the metals stored in the bottom sediments could be released into the biogeochemical cycle if redox conditions were to change from anoxic to suboxic or oxic the fjord would then become a source rather than a sink for these metals

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

critical re evaluation of differential stress estimates from calcite twins in coarse grained limestone abstract comparison of two techniques to estimate differential stress magnitude based on calcite twinning reveals significantly different calculated values of differential stress for limestone experimentally and naturally deformed at low temperature this inequality may be due to fundamental differences between the rock type marble versus limestone and deformation temperatures above and below 200 c for samples used in technique calibration versus samples used in testing and application a technique that was empirically derived from marble experimentally deformed at 200 800 c yields values of differential stress that are factors of 2 to 9 times larger than maximum differential stress measured during experimental deformation of limestone at low temperature

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

rates of reaction between silicate iron and dissolved sulfide in peru margin sediments porewaters in site 680 peru margin sediments contain dissolved sulfide over a depth of approximately 70 m which at a sedimentation rate of 0 005 cm yr 1 gives a sediment exposure time to dissolved sulfide of about 1 4 106 yr reactions with dissolved sulfide cause the site 680 sediments to show a progressive decrease in a poorly reactive silicate iron fraction defined as the difference between iron extracted by dithionite fed at room temperature and that extracted by boiling concentrated hcl feh normalised to the total iron content fet straight line plots are obtained for 1n feh fed fet against time of burial from which a first order rate constant of 0 29 10 6 yr 1 equivalent to a half life of 2 4 106 yr can be derived for the sulfidation of this silicate iron comparable half lives are also found for the same poorly reactive iron fraction in the nearby site 681 and 684 sediments this silicate fe fraction comprises 0 8 1 0 fe only 30 60 of which reacts even with 1 5 3 million years exposure to dissolved sulfide diagenetic models based on porewater concentrations of sulfate and sulfide and solid phase iron contents at site 680 are consistent in indicating that this poorly reactive iron fraction is only sulfidized on a million year time scale silicate iron not extracted by hcl can be regarded as unreactive towards dissolved sulfide on the time scales encountered in marine sediments

sedimentary pyrite formation in the arabian sea abstract in this study pyrite formation in relation to environmental conditions was investigated in sediment records from the northern arabian sea considering the low bottom water oxygen concentrations in the oxygen minimum zone omz and the high rates of primary productivity on the oman and pakistan margins sedimentary pyritic s concentrations are unexpectedly low on average 0 4 wt compared to organic contents sediments located deeper than the omz exhibit a good correlation between organic carbon and reduced sulphur conversely the c versus s distribution for omz sediments indicates a partial decoupling between organic carbon accumulation and pyrite formation early post depositional sulphidisation resulted in the diagenetic sulphur enrichment of organic poor intervals this is evidenced by the presence of pyrite enrichments located below organic rich intervals pyrite formation in the sediments within the arabian sea omz was limited by the availability of reactive iron oxides this was caused by the low oxygen concentrations of the omz which resulted in the reduction of iron oxides in the water column or at the sediment water interface however on the basis of the relatively high sedimentary c s ratios and the presence of bioturbation it is clear that no euxinic conditions developed in the omz in the arabian sea during the last 120 kyr

calcite twins their geometry appearance and significance as stress strain markers and indicators of tectonic regime a review twin lamellae are a widespread feature in calcites of any type and origin twinning in calcite is an important intracrystalline deformation mechanism with low critical resolved shear stress and virtually no temperature dependence in contrast to most slip systems in rock forming minerals twinning therefore permits intracrystalline deformation at very low temperature and very little confining pressure twinning alone however cannot lead to large strains because only one independent slip system is effectively available twins lead to discontinuous shearing on the grain scale which gives rise to considerable strain incompatibilities at grain boundaries twinning consequently leads to strain hardening detailed observations of the geometry of twin lamellae show that incompatibilities at grain boundaries are eliminated by pressure solution on the grain scale at low temperature and dynamic recovery processes at higher temperatures twins are found in various tectonic environments and record very small strains in rocks deformed under very little cover different twinning strain and twinning stress methods and palaeopiezometers are critically reviewed all methods are limited to strains of less than 15 shortening or extension by twinning twinning nevertheless is an important although not directly measurable contribution to the total strain in highly strained calcite rocks at least up to middle greenschist facies under which conditions it is the most effective contributor to the formation of strong lattice preferred orientations in calcite tectonites the appearance of calcite twin lamellae changes systematically as a function of deformation temperature micro twins and straight narrow 1 5 m but fewer twins are developed above approximately 200 c curved twins twinned twins and completely twinned grains indicate the progressive importance of other slip systems and larger intracrystalline strains are possible at ca 250 c and above ancient straight twin lamellae are modified into irregular geometries by recrystallization and grain boundary migration the appearance of calcite twins is proposed as an approximate but rapid and easy to use geothermometer in low temperature environments

the geochemistry of river particulates from the continental usa major elements river particulates have been collected from twenty three rivers from throughout the continental usa the rivers drain mostly large basins basin areas range from 2 1 x 10 3 to 2970 x 10 3 km2 composed of mixed lithologies and spanning a wide range of climatic conditions as evidenced by a large variation in air and ground water temperatures precipitation and runoff suspended particulates have been analyzed for organic and inorganic carbon as well as the major elements al fe mn k si ca mg and na also determined were suspended sediment loads at the time of sampling particle surface areas and grain size distributions five year average river water chemistry and suspended sediment concentrations obtained from usgs water supply reports are also included as supporting information particle chemistry systematically varies with rates of runoff with high runoff rivers transporting the most heavily altered particulates and low runoff rivers carrying the least altered degrees of alteration are indicated by the extent to which the refractory nonmobile elements al and fe are concentrated into the particulates and the extent to which the most easily weathered elements na ca and mg have been leached overall the susceptibility of elements towards leaching is consistent with numerous previous descriptions of element mobility on weathering a simple predictive model has been developed to explore and explain the observed trends between particle composition and river runoff thus the elemental composition of riverine particulates may be given by the expression pcoi propi tm dcoi ss where pcoi is the concentration of element i as the oxide in a riverine particulate propi is the proportion of i as the oxide in unweathered rock dcoi is the dissolved concentration of i as the equivalent oxide tm is the total rock derived mass dissolved in river water and ss is suspended sediment concentration to utilize this equation a model was first developed to predict the concentration of river solutes with the input of temperature precipitation and a limited number of weathering parameters combining this solute model with the above predictive equation for particle composition it was discovered that the observed trends between particle composition and runoff require the special circumstance of decreasing ss with increasing runoff hence the composition of river particulates depends both on the climate parameters of runoff and temperature as they control dissolved river chemistry and the nonclimate parameters including elevation relief tectonics and basin area that control the ss load of rivers

surface area control of organic carbon accumulation in continental shelf sediments the relationship between organic carbon oc and grain size found in most continental shelf sediments is here reinterpreted in terms of the surface area of the sediments cores from many north american shelf environments show downcore decreases in oc to similar refractory background concentrations if expressed relative to the surface area of the sediments this consistent concentration is 0 86 mg oc m 2 which is equivalent in concentration to a monolayer of organic matter coating all mineral surfaces a more global collection of sediment water interface samples show that this relationship is even more extensive with exceptions occurring in areas of very high riverine sediment input organic pollution or low oxygen water columns density separations indicate that organic matter is largely adsorbed to mineral grains the microtopography of surfaces was examined with n2 sorption and most surface area was found to be inside pores of 10 nm width these data lead to a hypothesis that organic matter is protected by its location inside pores too small to allow functioning of the hydrolytic enzymes necessary for organic matter decay such protection would likely work in concert with other protection mechanisms such as humification this consistent surface area correlation with oc concentration may explain control of spatial and temporal variations in oc burial rates by sedimentation rates the pore protection hypothesis provides a causal mechanism for this observed control

how continents break up insights from papua new guinea the woodlark basin in the western pacific forms a continuous system of active continental rifting evolving to well developed seafloor spreading thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin magnetic isochrons indicate a single euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins in agreement with rigid plate tectonic models continental rifting initiated geologically synchronously at 6 ma along the length of the protomargins within a deforming plate boundary zone strain localization and seafloor spreading however developed in a time transgressive fashion from east to west within this zone of deformation spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the solomon and coral seas the transition to spreading occurred after a rather uniform degree of continental extension 200 40 km both early and late stage rifting involved high and low angle normal faults we identify distinct styles in the transition from rifting to spreading which we refer to as nucleation propagation and stalling these breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating the initial spreading segments achieved much of their length at nucleation within rift basins separated along strike by accommodation zones with subsequent lengthening by spreading propagation into rifting continental crust this early propagation and the subsequent development of transform faults between initially nontransform spreading segment offsets produced rift and spreading segmentation boundaries that are not simply correlated the spreading centers nucleated approximately orthogonal in strike to the opening direction but as the protomargins were oblique to this direction nucleation jumps occurred in order to maintain the new spreading centers within the protomargins thus stepwise spreading nucleation in order to remain within a rheologically weak zone rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis is the dominant form of the rifting to spreading transition in the woodlark basin