the m 5 1 1980 arudy earthquake sequence western pyrenees france a revisited multi scale integrated seismologic geomorphologic and tectonic investigation this paper presents a combination of seismic imaging geomorphologic and tectonic data and an interpretation of the m 5 1 1980 arudy earthquake sequence putting in relation the seismicity the inherited faults and the geomorphologic wurm and postwurm markers in this region of the pyrenees since the anticlockwise rotation of the regional compression axis in oligocene time western pyrenees are under a dextral regime and the resulting motion is accommodated along major inherited e w dextral strike slip faults the arudy aftershocks sequence is controlled by antecedent horsetail splay faults built at the boundary between two shallow mesozoic crustal blocks most probably due to their differing rheology this boundary has played the role of a seismic barrier stopping the e w slip motion the arudy earthquake has reactivated the eastern segment of the main e w strike slip fault while the post seismic aftershocks correspond to local relaxation processes in normal tectonic behavior

stressing of locked patches along a creeping fault an analysis is presented of the stressing of locked patches along a fault zone which is creeping elsewhere the model consists of strike slip faulted elastic lithospheric plates loaded in a manner equivalent to imposition of a remotely uniform tectonic shear stress the fault zone has an inhomogeneous strength distribution both depth wise and along strike it is modeled as being composed of locked patches and freely slipping parts treated as cracks the solution is given with the use of the line spring model which analyzes the problem by thickness averaged plane stress theory for lithospheric plates which slip along a discontinuity cut at the plate boundary as a boundary condition thickness averaged stress and slip at each local section along the cut was related to one another by the result of an antiplane strain analysis of slip for the crack or crack pair which describes the slipping and locked depth ranges at that section the analysis indicates that the slip distribution along creeping parts of the fault as well as the stress distribution along locked patches depends strongly on the geometry of these zones the model is used to examine stress concentrations associated with a slip deficient seismic gap along strike and to study the effect of local irregularities in the margin of a locked region it is also used to simulate slip and stressing processes associated with the creeping portion of the san andreas fault in central california between the presently locked zones of the great 1906 and 1857 ruptures and to constrain the nature of an apparently locked zone at the southeastern end which ruptures in characteristic parkfield earthquakes near fault creep and broadscale displacement data along the fault since the 1966 parkfield earthquake and inferences from seismicity distributions are used limitations of the modeling procedure at short spatial wavelengths prohibit an accurate description of the parkfield locked patch but results suggest that it may be localized and occupy a small fraction of area of the normal seismogenic zone an effective remote stressing rate of order 0 3 10 6 shear modulus yr is inferred together with a less well constrained 30 to 40 km lithospheric thickness for consistency with the displacement data results enable estimates of stress accumulation along the locked 1857 rupture zone and the build up of fracture energy release capability of order 107 j m2 in 150 yr at its lower margin

three dimensional velocity image of a barrier associated with a moderate pyrenean earthquake the prominent role of barriers is now well recognized in both the initialization and the extension of the earthquake rupture the mapping of barriers is therefore important in understanding the physical processes by which earthquakes occur we invert 800 p wave travel times from an aftershock sequence following a moderate earthquake mb 5 1 in the pyrenean area france using a stochastic inverse technique to derive a three dimensional velocity image of the structure we have carried out two scales of imagery both scales reveal the presence of a high velocity contrast 8 at the eastern limit of the aftershock zone this velocity anomaly corresponds to a geometric discontinuity in the trace of the major fault and can be mapped down to a depth of 8 km this article contains supplementary material

seismicity induced by gas production ii lithology correlated events induced stresses and deformation a 3d relocation technique permits precise locations of induced earthquakes geostatistical processing using the data of 87 boreholes provides the basis of a precise 3d structure with a dome geometry conventional laboratory mechanical tests performed on deep rock samples 1000 m to 5000 m define the rock properties at depths similar to those of the seismic events 1 ml 4 2 that range from 1 to 7 km

a tomographic study of the lithosphere beneath the pyrenees from local and teleseismic data a tomographic image of the pyrenean lithosphere from the surface down to 200 km is deduced from the propagation of the seismic waves recorded at the regional pyrenean seismograph network crustal and subcrustal models are derived independently from the inversion of two different data sets p and s velocity models for the crust are obtained from the propagation of local earthquakes the upper crust appears very heterogeneous and exhibits sharp velocity contrasts in the uppermost 7 km an east west trending zone with velocities up to 6 higher than the surrounding material is observed all along the pyrenees this zone coincides in most places with the exposure of the paleozoic rocks of the axial zone low velocities are observed on both sides of this high velocity zone at the surface they coincide with well known sedimentary basins but dip down to about 11 km depth toward the axis of the range in the central and eastern pyrenees two large high velocity bodies stand out in the north pyrenean zone and extend down to the base of the upper crust they coincide with large positive gravity anomalies and are interpreted as lower crust or upper mantle blocks uplifted into the upper crust the lower crust is poorly resolved but appears to be less heterogeneous than the upper crust moreover the lower crustal structure is almost uncorrelated with the upper crustal structure the major feature of the lower crust is a low velocity anomaly which stretches along the north pyrenean fault in the western pyrenees and beneath the axial zone in the central pyrenees its location nearly coincides with the distribution of the seismicity in the upper mantle p models are derived from the travel times of the remote events recorded at the pyrenean stations the most important feature is the presence in the eastern and central pyrenees of a vertical low velocity heterogeneity south of the north pyrenean fault down to 80 100 km this structure exhibits a 2 velocity contrast with respect to the surrounding structures and is interpreted as lower crustal material subducted during the pyrenean convergence this heterogeneity is not observed in the western pyrenees suggesting that the crustal shortening has been completely accommodated by several thrusts in the thinned aquitaine crust

iron isotope constraints on fe cycling and mass balance in oxygenated earth oceans the fe isotope composition of proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 fe values that is only slightly more variable than the range of fe isotope com positions measured in terrestrial igneous rocks indicating that chemical weathering sedimentary transport and diagenesis play only a minor role in producing fe isotope variations in environ ments where fe redox conditions have been controlled by current levels of atmospheric oxygen in contrast the fe isotope composi tions of hot fluids 300 8c from mid ocean ridge mor spread ing centers define a narrow range that is shifted to lower d 56 fe values by 0 2 0 5 as compared to igneous rocks these new data allow a conceptual model for the fe isotope composition of the oxic oceans that predicts large ranges in fe isotope composition under conditions of changing aerosol and mor fe fluxes such as during periods of major worldwide glaciation

could bacteria have formed the precambrian banded iron formations banded iron formations bifs are prominent sedimentary deposits of the precambrian but despite a century of endeavor the mechanisms of their deposition are still unresolved interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy on the basis of chemical analyses from bif units of the 2 5 ga hamersley group western australia we show here that even during periods of maximum iron precipitation most if not all of the iron in bifs could be precipitated by iron oxidizing bacteria in cell densities considerably less than those found in modern fe rich aqueous environments those ancient microorganisms would also have been easily supported by the concentrations of nutrients p and trace metals v mn co zn and mo found within the same iron rich bands these calculations highlight the potential importance of early microbial activity on ancient metal cycling

seismotectonics of the easternmost transverse ranges california relevance for seismic potential of the southern san andreas fault earthquake locations depths and focal mechanisms from the southern california regional network 1977 1985 are used to identify the orientation and sense of slip of active subsurface faults in the easternmost transverse ranges etr the etr are separated from the salton trough province by the southernmost strands of the san andreas fault saf much of the seismicity in the etr is concentrated well northeast of the saf at relatively shallow depths under the little san bernardino mountains many of these earthquakes reflect slip on steeply dipping left lateral faults striking northeast to east at relatively high angles to the adjacent saf focal mechanisms in the etr show predominantly strike slip normal or oblique normal faulting and share common near horizontal t axes striking wnw p axes range from near vertical to near horizontal and strike mostly nne in contrast reverse and strike slip focal mechanisms that exhibit persistent north trending near horizontal p axes characterize the san gorgonio pass area immediately to the west these different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the saf this boundary which in the coachella valley may be the mission creek fault is also distinguished by abrupt changes in 1 rate and depth of seismic activity 2 topography 3 quaternary vertical deformation 4 strikes and dips of major branches of the saf and 5 seismic velocities in the crust and upper mantle the preponderance of secondary normal faulting in the etr versus secondary reverse faulting in the san gorgonio pass region suggests that fault normal stress is much less across the saf adjacent to the etr if a friction law where strength is proportional to normal stress applies to the saf then a smaller tectonic shear stress would be required tor slip in large earthquakes along the salton trough segment in this case the southernmost saf may have shorter repeat times smaller average displacements and lower moment release rates than earthquakes that rupture through the major restraining bend in san gorgonio pass

the 1994 northridge earthquake sequence in california seismological and tectonic aspects the m w 6 7 northridge earthquake occurred on january 17 1994 beneath the san fernando valley two seismicity clusters located 25 km to the south and 35 km to the north northwest preceded the mainshock by 7 days and 16 hours respectively the mainshock hypocenter was relatively deep at 19 km depth in the lower crust it had a thrust faulting focal mechanism with a rake of 100 on a fault plane dipping 35 to the south southwest and striking n75 w because the mainshock did not rupture the surface its association with surficial geological features remains difficult to resolve nonetheless its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the transverse ranges the northridge earthquake is part of the temporal increase in earthquake activity in the los angeles area since 1970 the mainshock was followed by an energetic aftershock sequence eight aftershocks of m 5 0 and 48 aftershocks of 4 m 5 occurred between january 17 and september 30 1994 the aftershocks extend over most of the western san fernando valley and santa susana mountains they form a diffuse spatial distribution around the mainshock rupture plane illuminating a previously unmapped thrust ramp extending from 7 10 km depth into the lower crust to a depth of 23 km no flattening of the aftershock distribution is observed near its bottom at shallow depths above 7 10 km the thrust ramp is topped by a dense distribution of aftershock hypocenters bounded by some of the surficial faults the dip of the ramp increases from east to west the west side of the aftershock zoae is characterized by a dense steeply dipping and north northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting these events coincided with the gillibrand canyon lateral ramp along the east side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms the focal mechanisms of the aftershocks were dominated by thrust faulting in the large aftershocks with some strike slip and normal faulting in the smaller aftershocks the 1971 san fernando and the 1994 northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge both earthquakes accommodated north south contractional deformation of the transverse ranges the two earthquakes differ primarily in the dip direction of the faults and the depth of faulting the 1971 north northeast trend of left lateral faulting chatsworth trend was not activated in 1994

further sulfur and carbon isotope studies of late archean iron formations of the canadian shield and the rise of sulfate reducing bacteria sulfur and carbon contents and isotope ratios are reported for five archean iron formations helen nakina and finlayson lumby and bending lake areas distributed across 850 km of the canadian shield all 2 7 ga old a 34s profile through a complete stratigraphic column oxide facies excluded of the helen iron formation shows a 34s range of 30 2 mean 34s value of 2 5 and a standard deviation i of 7 3 in sharp contrast to the sulfide and siderite facies the oxide facies in the column shows a uniform 34s value close to zero the 34s values obtained for the other four iron formations are again wide ranging highly variable in the sulfide and pyrite siderite facies but uniform and close to zero for the oxide facies the carbon in the oxide siderite chert facies has 13c values of 2 3 to 1 1 in the range of phanerozoic marine carbonates however the carbonates in the graphite rich sulfide facies have 13c values as low as 7 6 the mixing of reduced carbon with marine carbonate is suggested to explain the light carbonate values the reduced carbon associated with the light carbonate is also relatively light at up to 13corg 33 5 but is in the range of other precambrian values distal high temperature abiogenic sulfate reduction as a source of highly fractionated sulfides in the archean iron formations is ruled out on the basis of both isotopic and geologic evidence it is concluded that only the bacterial reduction of sulfate at low temperatures could produce the wide ranging highly variable 34s values exhibited by these sulfides over large areas

a structural model for the seismicity of the arudy 1980 epicentral area western pyrenees france summary the western pyrenees presents a diffuse and moderate m 5 7 instrumental seismicity it nevertheless historically suffered from strong earthquakes i ix msk the seismic sources of these events are not yet clearly identified we focus on the arudy 1980 epicentral area m 5 1 and propose here the reactivation of early cretaceous normal faults of the iberian margin as a potential source the late cretaceous inversion of this basin first in a left lateral strike slip mode and then in a more frontal convergence resulted in a pop up geometry this flower structure attests of the presence of a deep crustal discontinuity the present day geodynamic arrangement suggests that this accident is reactivated in a right lateral mode this reactivation leads to a strain partitioning between the deep discontinuity that accommodates the lateral component of the motion and shallow thrusts rooted on this discontinuity these thrusts accommodate the shortening component of the strain the distribution of the instrumental seismicity fits well the structural model of the arudy basin whatever the compressive regional context the structural behaviour of the system explains too the extensive stress tensor determined for the arudy crisis if we interpret it in terms of strain ellipsoid indeed numerical modelling has shown that this concomitant activity of strike slip and thrust faulting results in an extensive component that can rise 50 per cent of the finite strain we identify too a 25 30 km long potential seismic source for the arudy area the size of the structure and its potential reactivation in a strike slip mode suggest that a maximum earthquake magnitude of 6 5 could be expected the extrapolation of this model at the scale of the western pyrenees allows to propose other potential sources for major regional historical earthquakes

late pleistocene folding above the mail arrouy thrust north western pyrenees france abstract the western pyrenees have experienced several major earthquakes in the last 400 years herein we investigate the ongoing tectonic activity of the arudy area affected by one of the largest earthquakes m 5 1 1980 registered in the instrumental seismicity catalogue in the western pyrenees folding of alluvial terraces younger than 17 3 ky above the mail arrouy thrust mat attest from a late pleistocene tectonic activity this deformation resulted in a shallow fold with a wavelength of 2800 m and amplitude of 7 to 8 m the mat is rooted above a former normal fault of the cretaceous iberian margin which is a potential seismogenic source such observations suggest that the northern flank of the western pyrenees is still subjected to shortening in response to a compressional stress regime this compression is consistent with the african eurasian plate kinematics and the arudy 1980 earthquake nonetheless recent seismological data suggest the presence of an extensional stress regime in the range this extension is indicated by some seismic events mostly observed in the pyrenean high chain our hypothesis is that these extension driven events could be due to a local stress field induced by the elevation of the range on the contrary the compression associated with the regional stress regime could prevail in the outermost domain of the range

strain accumulation in southern california 1973 1984 repeated surveys of selected lines from five trilateration networks along the san andreas fault in southern california have been used to deduce the 1973 1984 strain accumulation records at five localities the secular rate of engineering shear strain accumulation is about 0 3 rad yr with the plane of maximum shear parallel to the local strike of the san andreas fault the secular rate of accumulation of areal dilatation is negligible the data were examined to detect evidence for fluctuations in the rate of strain accumulation for this examination 19 lines were removed from the data set four because they exhibited an obvious coseismic offset and 15 others because they contained at least one very anomalous measurement that could reasonably be attributed to a survey blunder the incidence of such blunders appears to be one in every 75 measurements the remaining data consist of 104 lines with an average of 10 measurements each although the strain accumulation plots for the five networks may exhibit marginally significant temporal fluctuations we are not convinced that those fluctuations are greater than could be attributed to survey error in particular we are unable to demonstrate that the 1973 1979 southern california strain anomaly reported by savage and others is real given the uncertainty in the random and systematic errors in measurement the strain measurements in southern california are marginally consistent with linear in time strain accumulation the strain accumulation plots for the salton network clearly established that unlike the deformation reported after the 1940 imperial valley earthquake no acceleration in the shear strain rate has yet been observed following the 1979 imperial valley earthquake

oxygen isotopic composition of ferric oxides from recent soil hydrologic and marine environments low temperature synthesis experiments on ferric oxide water systems have resulted in disparate oxygen isotope fractionation temperature t curves in this study recent ferric oxides mostly goethites of holocene age were collected and analyzed from a variety of modern soil stream and marine environments where formation temperature and the oxygen isotopic composition 18o of the water from which ferric oxides precipitated can be independently measured or estimated this allows comparison of experimental t relationships with data from natural systems selective dissolution methods were refined for the pretreatment of fine grained minerals in order to obtain reliable 18o values for pure and crystalline ferric oxides the difference 18o between the 18o value of goethite and that of local mean meteoric water ranges from 1 5 to 6 3 for soil goethites from new jersey indiana michigan iowa south dakota and taiwan we argue that these variations are largely the result of differences between the 18o of formation water and that of local mean meteoric water induced probably by 18o enrichment of soil waters by evaporation or other processes in soil horizons where ferric oxides are forming a marine goethite sample from scotland and a subaqueous bog iron sample from new jersey which can not be biased by evaporative processes provide crucial natural evidence that the difference in 18o between goethite and formation water is 1 5 at 10 c this result is consistent with our prior laboratory synthesis results bao and koch 1999 but in conflict with other experimental calibrations given the highly variable 18o value of soil or other surface water as well as the potential of initially formed ferric oxides for reequilibration with subsurface burial fluids during maturation to crystalline phases an understanding of formational and diagenetic conditions is absolutely essential when attempting to use the oxygen isotope composition of ferric oxides as a paleoclimatic proxy

sequence stratigraphy and plate tectonic significance of the transvaal succession of southern africa and its equivalent in western australia the transvaal succession in the northeastern part of the kaapvaal province of southern africa is the erosional remnant of four 2 7 to 2 1 ga unconformity bounded stratigraphic units or sequences each of which is 102 to 103 m thick and spans 107 to 108 years three of these phanerozoic like cratonic sequences occur in the western part of the kaapvaal province and in the pilbara province of western australia thus the two provinces once were part of the same continent vaalbara the stratigraphically lowest sequence historically assigned to the transvaal succession is part of the ventersdorp supergroup in kaapvaal and the upper part of the fortesque group in pilbara it consists predominantly of arkosic sandstone and basaltic andesite it probably is 2687 2 ma the second sequence contains the quartz arenites dolomites and banded iron formations of the transvaal and griqualand west successions in the kaapvaal and the hamersley group in the pilbara the span of ages 2684 6 to 2432 31 ma lithologies and geographic extent of this sequence imply that the limpopo orogeny which welded the zimbabwe province to vaalbara is 2 47 ga not 2 67 0 05 ga the third sequence the pretoria and postmasburg groups in the kaapvaal and the lower part of the wyloo group in the pilbara probably is the clastic wedge generated by the limpopo orogeny the fourth sequence dominated by the felsites of the rooiberg group is restricted to northeastern kaapvaal sequence stratigraphy lithostratigraphy and lithofacies indicate that the pilbara rifted from the southern edge of western kaapvaal the penge iron formation in northeastern kaapvaal the asbesheuwels iron formation in southwestern kaapvaal and the brockman iron formation in the pilbara are the same formation in which cyclothemic units 0 6 to 15 0 m thick extended 1200 km

the geochemistry of late archaean microbial carbonate implications for ocean chemistry and continental erosion history trace element concentrations and combined sr and nd isotope compositions were determined on stromatolitic carbonates microbialites from the 2 52 ga campbellrand carbonate platform south africa shale normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive la and y anomalies and in being depleted in light rare earth elements in contrast to modern seawater and microbialite proxies the 2 52 ga samples lack a negative ce anomaly but possess a positive eu anomaly these latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where unlike today hydrothermal fe input was not oxidised and scavenged and rare earth elements were not coprecipitated with fe oxyhydroxides the persistence of a positive eu anomaly in relatively shallow campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated archaean to continental erosion dominated phanerozoic rare earth element flux ratio the dominant hydrothermal input is also expressed in the initial sr and nd isotope ratios there is collinear variation in sr nd systematics which range from primitive values 87sr 86sr of 0 702386 and end of 2 1 to more evolved crustal ratios mixing calculations show that the range in trace element ratios e g y ho and initial isotope ratios is not a result of contamination by trapped sediment but that the chemical and isotopic variation reflects carbonate deposition in an environment where different water masses mixed calculated nd flux ratios yield a hydrothermal input into the 2 52 ga oceans one order of magnitude larger than continental input such a change in flux ratio most likely required substantially reduced continental inputs which could in turn reflect a plate tectonic causation e g reduced topography or expansion of epicontinental seas

iron isotopes in the early marine diagenetic iron cycle measurements of fe oxyhydroxides reactive fe iii in two shallow sediment profiles from reducing and oxic environments on the arabian sea margin demonstrate fe isotope fractionation during early marine diagenesis reactive fe iii has 56 fe values between 0 77 and 0 19 values are lowest at the top of the sediment profile and considerably lower than bulk sediment fe 56 fe 0 preferential reduction and dissolution of light fe isotopes during diagenesis leaves behind an increasingly smaller and heavier reactive fe iii residual initially the isotopic composition of reactive fe iii evolves down core according to the fractionation factor typical of microbial dissimilatory fe reduction deeper in the profile 56 fe values remain unchanged despite further fe reduction here another process with a different fractionation factor becomes dominant probably reduction by dissolved sulfide the 56 fe of the residual reactive fe iii suggests that 25 of the initially present reactive fe iii was reduced by microbial fe reduction when fe is diagenetically recycled between reducing sediments at depth and an oxic top layer the process depending on recycling efficiency may result in the accumulation of light fe in the top layer while the complementary heavier residual is buried fe diffusing from the seafloor back into the ocean should reflect the low 56 fe diagenetic source of dissolved fe

decoupling photochemical fe ii oxidation from shallow water bif deposition abstract oxidized fe minerals in archean paleoproterozoic banded iron formations bifs are commonly taken to indicate the presence of biogenic o 2 or photosynthetic fe ii oxidizing bacteria in the oceans photic zone however at least one viable abiogenic oxidation mechanism has been proposed prior to the rise of atmospheric oxygen and the development of a protective ozone layer the earth s surface was subjected to high levels of ultraviolet radiation bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved fe ii under such conditions dissolved ferrous iron species such as fe 2 and fe oh would have absorbed radiation in the 200 400 nm range leading to the formation of dissolved ferric iron fe iii which in turn would have hydrolyzed to form ferric hydroxide fe oh 3 at circumneutral ph cairns smith a g 1978 precambrian solution photochemistry inverse segregation and banded iron formations nature 76 807 808 braterman p s cairns smith a g and sloper r w 1983 photo oxidation of hydrated fe 2 significance for banded iron formations nature 303 163 164 this process has been invoked to account for bif deposition without need for biology francois l m 1986 extensive deposition of banded iron formations was possible without photosynthesis nature 320 352 354 here we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models the experiments simulate the chemistry of ambient precambrian seawater mixing with fe ii rich hydrothermal fluids with and without uv irradiation we find that if fe ii was effused from relatively shallow seamount type vent systems directly into an anoxic photic zone the photochemical contribution to solid phase precipitation would have been negligible instead most of the fe ii would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite fe 3 si 2 o 5 oh 4 and or the ferrous carbonate siderite feco 3 depending on different simulated atmospheric pco 2 levels conversely in experiments where fe ii was exposed either to phototrophic fe ii oxidizing bacteria or to o 2 ferric hydroxide formed rapidly and the precipitation of ferrous iron phases was not observed if as suggested on mass balance grounds bif deposition requires that fe be sourced from shallow seamount type systems then we are driven to conclude that oxide facies bif are the product of a rapid non photochemical oxidative process the most likely candidates being direct or indirect biological oxidation and that a significant fraction of bif could have initially been deposited as ferrous minerals

origin damping and pattern of development of faults in granite observations of brittle fractures in the granite of the massif de la borne as well as in other rocks allow us to discuss the different hypotheses regarding the origin of faults it seems that all faults originate by shear along a preexisting plane which can be an important fracture joint former fault or a microflaw pore grain boundary the shear on the plane gives rise to an array of en echelon cracks that makes the rock less resistant and permits the lengthening of the fault we also found an arrangement of horsetail fractures damping the movements on the faults we then propose a pattern for the development of the faults they originate on preexisting plane and propagate by initiating en echelon cracks the movements are damped by horsetails which permit by faults connection in relays the creation of larger faults

primary and diagenetic controls of isotopic compositions of iron formation carbonates mineralogic chemical and isotopic compositions have been determined for 97 carbonate microbands in five core segments from the early proterozoic 2 5 ga dales gorge member of the brockman iron formation hamersley basin western australia samples were obtained both from banded iron formation bif macrobands 9 12 at paraburdoo on the southern margin of the basin and from bif macroband 13 at wittenoom 130 km nw at paraburdoo oxygen isotopic compositions of coexisting chert and magnetite microbands were measured and indicated final equilibration temperatures ranging from 60 160 degrees c this range is consistent with observed mineral assemblages and indicates a considerable temperature gradient across the basin cf t approximately 300 degrees c at wittenoom becker and clayton 1976 carbon isotopic compositions of carbonates are near 7 vs pdb at paraburdoo and 10 5 at wittenoom but the greater isotopic depletion at wittenoom appears related to primary or diagenetic processes not metamorphism contents of organic carbon are consistently low isotopic depletion is roughly correlated with iron abundance and together with petrographic observations and chemical balances is consistent with the model of bif deposition introduced by beukes et al 1990 primary siderite delta approximately 5 precipitated from an anoxic water column depleted in 13c additional depletion of 13c is associated with coprecipitation of iron oxides and organic carbon oxygen isotopic abundances of microbanded carbonates are similar to those under and overlying massive marine carbonates ranging from 17 6 to 21 0 vs smow 9 6 to 12 9 vs pdb millimeter scale variations in abundances of 13c and 18o are associated with diagenetic replacement of primary siderite by secondary ankerite and or magnetite it is shown that these isotopic variations cannot result from mineral dependent fractionations metamorphism or the influence of large volumes of water in an open system

iron isotopes constrain biologic and abiologic processes in banded iron formation genesis abstract the voluminous 2 5 ga banded iron formations bifs from the hamersley basin australia and transvaal craton south africa record an extensive period of fe redox cycling the major fe bearing minerals in the hamersley transvaal bifs magnetite and siderite did not form in fe isotope equilibrium but instead reflect distinct formation pathways the near zero average 56fe values for magnetite record a strong inheritance from fe3 oxide hydroxide precursors that formed in the upper water column through complete or near complete oxidation transformation of the fe3 oxide hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of 56fe values 1 addition of marine hydrothermal fe 2 aq 2 complete reduction by bacterial dissimilatory iron reduction dir and 3 interaction with excess fe 2 aq that had low 56fe values and was produced by dir most siderite has slightly negative 56fe values of 0 5 that indicate equilibrium with late archean seawater although some very negative 56fe values may record dir support for an important role of dir in siderite formation in bifs comes from previously published c isotope data on siderite which may be explained as a mixture of c from bacterial and seawater sources several factors likely contributed to the important role that dir played in bif formation including high rates of ferric oxide hydroxide formation in the upper water column delivery of organic carbon produced by photosynthesis and low clastic input we infer that dir driven fe redox cycling was much more important at this time than in modern marine systems the low pyrite contents of magnetite and siderite facies bifs suggests that bacterial sulfate reduction was minor at least in the environments of bif formation and the absence of sulfide was important in preserving magnetite and siderite in the bifs minerals that are poorly preserved in the modern marine record the paucity of negative 56fe values in older early archean and younger early proterozoic bifs suggests that the extensive 2 5 ga hamersley transvaal bifs may record a period of maximum expansion of dir in earth s history

iron and carbon isotope evidence for ecosystem and environmental diversity in the 2 7 to 2 5 ga hamersley province western australia abstract the largest excursion in kerogen 13c and bulk mineral 56fe values yet measured in the ancient rock record occurs in rocks of 2 7 to 2 5 ga age new fe isotope data integrated with previously collected c isotope data on the same samples document the metabolic diversity of microbial communities in the neoarchean hamersley province of the pilbara craton in western australia samples of shales carbonates and mixed carbonate shale lithologies were collected from three drill cores two cores from the depocenter of the province and one from the margin shallow water clastic carbonate rocks deposited in the center of the province tumbiana formation record kerogen 13c values that indicate c cycling by various anaerobic or aerobic methane pathways but the restricted range in 56fe values indicates little or no fe redox cycling deep water sediments deposited contemporaneously in both parts of the hamersley province jeerinah formation record slightly positive 56fe values in the relatively shallower and suboxic margin but strongly negative 56fe values in the deeper euxinic depocenter of the province a pattern consistent with fe cycling via a basin fe shuttle driven by bacterial dissimilatory iron reduction dir kerogen 13c values from these units indicate coupling of microbial fe cycling to aerobic methanotrophy or anaerobic oxidation of methane younger black shales intercalated with iron formation marra mamba iron formation in the depocenter record a shift to near zero 56fe values reflecting an fe budget dominated by hydrothermal and clastic sources however time equivalent fe rich carbonate shale lithologies deposited on the margin of the province carawine dolomite have 56fe values that steadily decrease from near zero to strongly negative values these relatively fe rich carbonates may reflect a carbonate trap of a dir driven fe shuttle similar to the sulfidic trap in the euxinic portion of the jeerinah formation in contrast younger shallow water carbonates deposited by turbidity currents in relatively deep water in the center of the province wittenoom formation have 56fe values that correlate with fe concentrations a pattern that indicates fe cycling by rayleigh fractionation through precipitation of iron oxides from aqueous ferrous iron

the 1984 abruzzo earthquake italy an example of seismogenic process controlled by interaction between differently oriented synkinematic faults the evolution of the seismogenic process associated with the ms 5 8 sangro valley earthquake of may 1984 abruzzo central italy is closely controlled by the quaternary extensional tectonic pattern of the area this pattern is characterised by normal faults mainly nnw striking whose length is controlled by pre existing mio pliocene n100f10j left lateral strike slip fault zones these are partly re activated as right lateral normal oblique faults under the quaternary extensional regime and behave as transfer faults integration of re located aftershocks focal mechanisms and structural features are used to explain the divergence between the alignment of aftershocks wsw ene and the direction of seismogenic fault planes defined by the focal mechanisms nnw sse of the main shock and of the largest aftershock ms 5 3 the faults that appear to be involved in the seismogenic process are the nnw sse barrea fault and the e w m greco fault there is field evidence of finite quaternary deformation indicating that the normal barrea fault re activates the m greco fault as right lateral transfer fault no surface faulting was observed during the seismic sequence the apparently incongruent divergence between aftershocks and nodal planes may be explained by interpreting the m greco fault as a barrier to the propagation of earthquake rupturing the rupture would have nucleated on the barrea fault migrating along strike towards nnw the sharp variation in direction from the barrea to the m greco fault segments would have represented a structural complexity sufficient to halt the rupture and subsequent concentration of post seismic deformation as aftershocks around the line of intersection between the two fault planes fault complexities similar to those observed in the sangro valley are common features of the seismic zone of the apennines we suggest that the zones of interaction between nw se and nnw sse plio quaternary faults and nearly e w transfer faults extending for several kilometres in the same way as m greco does might act as barriers to the along strike propagation of rupture processes during normal faulting earthquakes this might have strong implications on seismic hazard especially for the extent of the maximum magnitude expected on active faults during single rupture episodes d 2002 elsevier science b v all rights reserved

the iron isotope composition of microbial carbonate abstract in a comparative study of fossil and recent stromatolites and microbialites we have investigated whether reefal stromatolitic limestones are able to preserve fe isotope compositions that potentially serve as proxies of seawater chemistry at the time of their formation it was found that 56 fe values of archean stromatolites vary between 2 1 and 0 5 devonian and carboniferous microbiallites vary between 0 and 1 and modern microbialites from the great barrier reef show a range from 0 12 to 0 15 five lines of evidence support the possibility that these compositions are potentially pristine 1 fe concentrations and fe isotope ratios are not correlated 2 the concentrations of other elements that are potentially mobile during carbonate diagenesis mg sr do not correlate with 56 fe 3 mn concentrations are inversely correlated with 56 fe which might hint at meteoric alteration however if mn concentrations and fe isotope signatures were pristine features they might serve as a record of the reduction of these two metals in seawater 4 dolomitisation of two devonian limestones does not shift their fe isotope compositions 5 rare earth element and yttrium rey patterns in all limestones are similar to seawater rey sources this suggests that the bulk of the fe in the samples occurs in hydrogenous carbonates not in primary or secondary fe and mn hydroxides because the latter have different rey patterns the modern microbialites are unfractionated in fe relative to crustal rocks thus these hydrogenous precipitates either incorporated unfractionated fe of the ambient seawater composition or they modified the fe composition upon incorporation from a fractionated ocean fe reservoir in either case provided diagenetic alteration has not modified the fe isotope composition in a major way a potential open ocean fe isotope proxy may exist that may allow reconstruction of secular variations in seawater fe isotope compositions our preliminary results suggest that neoarchean microbial carbonate fe may have been lighter than today s microbialite fe and that the carbonates fe isotopes potentially record the efficiency of transition metal reduction in ocean basins through geologic time

stable isotope geochemistry of cherts and carbonates from the 2 0 ga gunflint iron formation implications for the depositional setting and the effects of diagenesis and metamorphism abstract 18 o values for all but three of 97 samples of chert along a 220 km transect through the 2 0 ga gunflint iron formation of the animikie basin north america range from 21 3 to 24 7 o there is no correlation between 18 o and the type of chert i e deeper water lutitic chert shallower water arenitic chert and peritidal algal chert the fact that the gunflint iron formation has been exposed to thermal conditions only slightly above burial diagenesis suggests that the values of the chert have not been significantly affected by metamorphism field and petrographic observations and the small 18 o range of all the chert types suggests that silicification lithification and any conversion to quartz chert from a hydrous silica precursor occurred close to the sediment water interface the uniform 18 o values of all the different chert types suggest that all cherts precipitated over a relatively common temperature interval 20 c from a common parent water having a relatively uniform 18 o the isotopic data suggest silica saturation and precipitation was not the result of the mixing of seawater and meteoric water or the result of evaporative processes siderite in the deep water banded facies of the gunflint displays a 18 o range of 6 0 to 2 6 o whereas siderite in the more shoreward organic rich shale facies is significantly enriched in 13 c 13 c 2 5 to 0 5 o 18 c of siderite does not appear to have been altered as a result of neomorphic recrystallization and authigenic microsparitic siderite along stylolites displays the same 13 c range as primary and neomorphosed siderite however neomorphic and authigenic siderite are depleted in 18 o by up to 7 o relative to primary unaltered microspheroidal siderite the lower 13 c of banded facies siderite relative to the shale facies siderite does not seem to be the result of thermal decarboxylation or anaerobic oxidation abiotic or bacterial of organic matter but rather is better explained by primary precipitation in an ocean system that was layered with respect to carbon isotopes the more basinward banded facies siderite precipitated from a 13 c depleted fe 2 rich hydrothermal water mass that was moved by advective upwelling from submarine regions of magmatic volcanic hydrothermal activity onto the continental shelf the siderite of the shale facies precipitated from a more near shore shallow seawater mass that was comparatively enriched in 13 c 13 c 0 2 o ankerite in the gunflint iron formation occurs as a replacement of primary siderite in the banded facies and as a replacement of greenalite and chert in the arenite facies many ankerite samples have 13 c values that are similar to those of the shale facies siderite which suggests that shallow seawater was a major component of the diagenetic fluid later neomorphism of the replacement ankerite at higher burial temperatures and probably in the presence of 18 o depleted waters resulted in a wide range of 18 o values 13 8 22 5 o the unique mineralogy of the upper most gunflint iron formation calcite serpentine and magnetite compared to the majority of the formation the very low values and overlying diabase sills suggest that the non ferroan nature of the carbonate in the upper gunflint is the result of contact metamorphism

fault slip in southern california measurements of slip on major faults in southern california have been performed over the past 18 yr using principally theodolite alignment arrays and tautwire extensometers they provide geodetic control within a few hundred meters of the fault traces which complements measurements made by other techniques at larger distances approximately constant slip rates of from 0 5 to 5 mm yr over periods of several years have been found for the southwestern portion of the garlock fault the banning and san andreas faults in the coachella valley the coyote creek fault the superstition hills fault and an unnamed fault 20 km west of el centro these slip rates are typically an order of magnitude below displacement rates that have been geodetically measured between points at greater distances from the fault traces exponentially decaying postseismic slip in the horizontal and vertical directions due to the 1979 imperial valley earthquake has been measured it is similar in magnitude to the coseismic displacements analysis of seismic activity adjacent to slipping faults has shown that accumulated seismic moment is insufficient to explain either the constant or the decaying postseismic slip thus the mechanism of motion may differ from that of slipping faults in central california which move at rates close to the plate motion and are accompanied by sufficient seismic moment seismic activity removed from the slipping faults in southern california may be driving their relatively aseismic motion

experimental determination of fe isotope fractionation between aqueous fe ii siderite and green rust in abiotic systems abstract iron isotope fractionation between aqueous iron and siderite has been measured using abiotic synthesis experiments at 20 c applying a rayleigh distillation model to three experiments in which siderite feco3 was continuously precipitated from solutions over the course of tens of hours the derived fractionation factor is 103ln fe ii siderite 0 48 0 22 2 this experimentally determined fractionation factor is significantly smaller by 1 5 than that calculated from vibrational frequencies and mossbauer shifts geochim cosmochim acta 64 2000 849 865 geochim cosmochim acta 65 2001 2487 2497 and lies on the high end of that inferred from natural mineral assemblages where the fe ii aq siderite fractionation factor was inferred to lie between 1 7 and 0 3 contrib mineral petrol 144 2003 523 547 the measured fe ii aq siderite fractionation from this study suggests that siderite from archean and proterozoic rocks which have moderately negative 56fe values 1 5 to 0 5 may reflect precipitation from fe ii aq whose origin was hydrothermal in contrast natural siderites that have higher 56fe values apparently require non hydrothermal sources of fe and or different pathways of formation or may reflect the effects of cation substitution on the fe ii aq carbonate fractionation factor

an iron shuttle for deepwater silica in late archean and early paleoproterozoic iron formation iron formations are typically thinly bedded or laminated sedimentary rocks containing 15 or more of iron and a large proportion of silica commonly 40 in the ca 2590 2460 ma campbellrand kuruman complex transvaal supergroup south africa iron formation occurs as a sediment starved deepwater facies distal to carbonates and shales iron minerals primarily siderite define the lamination the silica primarily occurs as thin beds and nodules of diagenetic chert now microcrystalline quartz filling pore space and replacing iron formation minerals and co occurring deepwater lithologies mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production anoxygenic photosynthesis abiotic photochemistry and chemoautotrophy using fe ii as an electron donor the origin and mechanism of silica precipitation in these deposits have received less attention here we present a conceptual model of iron formation that offers insight into the deposition of silica the model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides which precipitate fe oh 3 binds silica and sinks from the surface ocean along with organic matter shuttling silica to basinal waters and sediments fe iii respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters driving precipitation of siderite from fe2 and respiration influenced co2 silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases this model explains many of the mineralogical textural and environmental features of late archean and earliest paleo proterozoic iron formation

intraplate seismicity new seismotectonic data in western europe abstract fault plane solutions for 128 earthquakes which occurred in france and neighbouring areas have been determined stress field information given by these data are discussed in relation to previous tectonic knowledge the western alps are characterized by a compressional regime with a fan shaped distribution of p axes which might be related to the interaction mechanism between europe and the adriatic plate the pyrenean region is mainly submitted to a south north compression tectonic activity in this area seems to be determined by the presence of two subduction zones with reverse dips in the south bay of biscay and in the central pyrenees the transition zone between these two active belts is characterized by significant seismicity with a transpression regime

seismological geological and geophysical constraints for the gualdo tadino fault umbria marche apennines central italy abstract the april 3 1998 mw 5 1 gualdo tadino earthquake central italy was the last significant event in the 6 month long umbria marche seismic crisis this event and its aftershocks occurred in an area where active faulting produces no striking geological and geomorphological effects in this study we investigated the ruptured fault using detailed seismological data and a re processed and re interpreted seismic reflection profile aftershock location and focal mechanisms were used to constrain the geometry and kinematics of the ruptured fault and a comparison was made with the subsurface image provided by the seismic profile we found that the 1998 gualdo tadino earthquake occurred on a wsw dipping normal fault with a length of about 8 km and a relatively gentle dip 30 40 confined between 3 5 and 7 km in depth kinematics of the mainshock and aftershocks revealed a ne trending extension in agreement with the regional stress field active in the northern apennines belt the mw 5 1 earthquake originated above the top of the basement and ruptured within the sedimentary cover which consists of an evaporites carbonates multilayer we hypothesised that the active fault does not reach the surface blind normal fault

glacial to interglacial vegetation changes in the northern and southern pyrenees deglaciation vegetation cover and chronology abstract new palynological investigations have been carried out in the southern northern and mediterranean part of the pyrenean chain pollen percentage and concentration diagrams are compared for a period including the end of the glacial period the late glacial and postglacial periods from the new results and from comparisons with previous published data the steppe and semi desert vegetation of the end of the last glacial period are described as well as the plant communities of the period 15 13 ka a dry climate was responsible for the early glacial retreat before 30 ka deglaciation was continuous up to about 15 14 ka without any readvance around 20 18 ka different aspects of the late glacial vegetation are studied and the first stages of the postglacial are described the younger dryas event is found everywhere but particularly clearly in the mediterranean area the most important 14 c dates are discussed and a plateau of 14 c ages is clearly visible around 10 000 bp

stress near geometrically complex strike slip faults application to the san andreas fault at cajon pass southern california slip on an undulatory strike slip fault induces predictable residual stresses in the adjacent crust elastic analytic and finite element models are developed to quantify these stresses for arbitrary fault geometries across fault parallel planes domains of reverse normal right lateral and left lateral residual stresses are induced near the bends in the fault these residual stresses increase in magnitude from one slip event to the next typically reaching failure level after several events a two dimensional analytic elastic plate model for slip on a sinusoidal fault provides general results on the scales and patterns of slip induced stresses and fourier synthesis allows for the solution with arbitrary but small fault distortions maximum fault normal residual stress is proportional to the square of the wavenumber of the sinusoidal trace and decays exponentially away from the fault reduced to 2 e times the maximum value at a distance equal to 1 wavenumber this fault parallel residual shear stress oscillates between dextral and sinistral along the fault with maximum magnitude adjacent to the maximum excursions in the fault trace fault parallel residual shear stress is maximum at a distance from the fault of 1 wavenumber where its magnitude is 1 e times the maximum normal stress on the fault two and three dimensional finite element analyses extend the analytic model they account for depth decaying fault perturbation amplitude and slip deficit and are valid for large fault undulations application of the model to the san andreas fault in the cajon pass region produces the complex distribution of fault parallel normal reverse right lateral and left lateral structures recognized near the main trace left lateral stresses on planes subparallel to the san andreas fault at the cajon pass well are predicted by this model

effect of bacteria on the elemental composition of early diagenetic siderite implications for palaeoenvironmental interpretations many ancient early diagenetic siderite concretions are thought to have been precipitated by microbially mediated reactions and their trace element chemistry has been used to infer palaeoenvironments of formation e g marine versus freshwater in this study pure cultures of the microorganism geobacter metallireducens were used to precipitate siderite in the laboratory at a range of temperatures 18 40 c magnesium and calcium in three different ratios 3 1 as in seawater 1 1 and 1 3 as in freshwater or manganese were added to some cultures at each incubation temperature to study trace element incorporation the siderite produced exhibited the rhombohedral crystal form typical of concretionary siderite however this microbial siderite did not simply retain the trace element chemistry of the water from which it precipitated as is assumed in palaeoenvironmental interpretations instead manganese and calcium incorporation were found to be inversely proportional to the rate of microbial activity which itself is dependent upon the nutritional status of the microorganisms magnesium incorporation was found to be rate independent and at high mg ca ratios it caused inhibition of calcium incorporation in short the influence of microorganisms on the trace element composition of early diagenetic siderite must be taken into account in order to produce valid palaeoenvironmental interpretations

stress transfer and seismic instabilities in the upper crust example of the western pyrenees abstract when the regional state of stress is close to the condition for earthquakes to occur small stress and strain changes could trigger seismic instabilities stress diffusion 1 2 mpa has been suggested to explain at lithospheric scale space and time correlated earthquakes and surface deformations in subduction zones 100 200 km 10 40 years we test the possible regional mechanisms at the upper crustal scale in the western pyrenees where major regional events m max 5 7 locally induced events and subsurface fluid pressure fluctuations are temporally and spatially correlated in a first phase fluid withdrawal decrease of pore pressure of the aquifer at the gas water interface affects the natural regional seismicity rate the locked fault inhibits small events for a time during which tectonic stresses accumulate major regional events then occur on the pyrenean fault when the energy of deformation is large enough to allow seismic instabilities to occur these major events m 1 5 trigger seismic instabilities m 1 4 in the vicinity of the gas extraction area 30 km away as a result of gas extraction the lacq field zone has local weakness characteristics similar to the neighbouring main active fault the pyrenean earthquakes lead the gas field events by 2 years this correlation is consistent with a viscoelastic model where salt rock acts as a viscoelastic channel below an elastic bed the possible connections proposed here may give a new insight worldwide in understanding the role of fluids in seismic instabilities and seismic risk assessment in the neighbouring of man made fluid pressure fluctuations when natural seismic activity is high

photosynthetic microbial mats in the 3 416 myr old ocean recent re evaluations of the geological record of the earliest life on earth have led to the suggestion that some of the oldest putative microfossils1 and carbonaceous matter were formed through abiotic hydrothermal processes2 3 similarly many early archaean more than 3 400 myr old cherts have been reinterpreted as hydrothermal deposits rather than products of normal marine sedimentary processes2 4 5 here we present the results of a field petrographic and geochemical study testing these hypotheses for the 3 416 myr old buck reef chert south africa from sedimentary structures and distributions of sand and mud we infer that deposition occurred in normal open shallow to deep marine environments the siderite enrichment that we observe in deep water sediments is consistent with a stratified early ocean6 7 we show that most carbonaceous matter was formed by photosynthetic mats within the euphotic zone and distributed as detrital matter by waves and currents to surrounding environments we find no evidence that hydrothermal processes had any direct role in the deposition of either the carbonaceous matter or the enclosing sediments instead we conclude that photosynthetic organisms had evolved and were living in a stratified ocean supersaturated in dissolved silica8 9 3 416 myr ago

fe isotope variations in the modern and ancient earth and other planetary bodies iron the fourth most abundant element in the earth s crust has four naturally occurring stable isotopes 54fe 5 84 56fe 91 76 57fe 2 12 and 58fe 0 28 and the natural mass dependent isotope variations of fe in the rock record span a range of 4 per mil in 56fe 54fe ratios fig 1 the field of fe isotope geochemistry is relatively new but has received considerable attention because it may allow us to gain a better understanding of how fe is cycled in different environments iron typically occurs as either reduced ferrous fe in oxygen poor environments or as oxidized ferric iron in oxygen rich environments notably only the reduced species is soluble in oxygenated aqueous solutions unless the ph is low in the archean and early proterozoic the earth may have been relatively oxygen poor e g kasting et al 1979 grandstaff 1980 holland 1994 suggesting that there may have been significant quantities of fe 0 9 millimolar dissolved in the oceans as fe ii aq e g ewers 1983 sumner 1997 the extensive iron formations of archean to early proterozoic age may have been deposited from such fe ii rich oceans e g beukes and klein 1992 in the modern oxic oceans however fe contents are exceedingly low 1 nanomolar in the open oceans e g martin and gordon 1988 bruland et al 1991 martin 1992 johnson et al 1997 and it is now recognized that marine productivity is fe limited in parts of the open oceans e g martin and fitzwater 1988 martin et al 1989 1994 the differences in the behavior of fe with redox state and the significant isotope fractionations 1 or more in 56fe 54fe that are associated with redox conditions suggests that fe isotope studies will be extremely useful for tracing

short scale variations of tectonic regimes in the western european stress province north of the alps and pyrenees the data of the european part of the world stress map wsm are used to study the western european stress province north of the alps and pyrenees the study area is characterised by an almost homogeneous n 145 degrees e orientation of the maximum horizont

stress orientation profile to 3 5 km depth near the san andreas fault at cajon pass california a detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1 75 3 46 km in the cajon pass drill hole located 4 2 km from the san andreas fault in southern california the profile is based on the analysis of stress induced well bore breakouts and consists of 32 616 orientation determinations at a basic sampling interval of 4 cm the azimuth of the apparent maximum horizontal compressive stress around the borehole is 057 s d 19 depth dependent variations in breakout azimuths from a few degrees to as much as 100 occur over depth intervals of several centimeters to hundreds of meters in the borehole with wavelengths showing a self similar distribution over a range of depth intervals the variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole assuming linear elastic behavior perturbations in stress magnitude and orientation were computed for predrilling slip on these faults fault stress drops limited to the ambient shear stress i e up to total stress drop along with occasional local extreme stress drops and geometrical complications can explain the variations in breakout orientations in the drill hole the penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in cajon pass which indicates left lateral shear on planes parallel to the san andreas fault may not be representative of the state of stress near the fault at depth in this region rather the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right lateral shear on planes parallel to the san andreas combining these observations with the highly variable stress state in shallow boreholes in the western mojave desert it is suggested that the heterogeneous stress field in this region representing a spectrum of seismic events may extend over substantial distances across and along the san andreas fault zone if the average measured breakout orientation in cajon pass is representative of the state of stress near the san andreas in this region then substantial left lateral shear stress is resolved on planes parallel to the fault in contradiction to the sense of its long term motion we use an elastic dislocation model to compute the net stress change near the cajon pass drill site since just prior to the large 1812 earthquake taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the san andreas and san jacinto faults the model suggests that the net change in fault parallel shear stress during the current earthquake cycle in the cajon pass area is nearly negligible the difference between the measured and computed results requires that additional left lateral shear stress be superimposed on a generally fault normal maximum horizontal compressive stress the results suggest that during the great fort tejon earthquake of 1857 left lateral shear stress parallel to the san andreas fault in the region of cajon pass may have played a role in terminating the southeastward extent of the rupture propagation

lherzolites and the western chainons bearnais french pyrenees structural and paleogeographical pattern abstract field observations of the chainons bearnais in the area of oloron sainte marie reveal two families of units the first one contains a well developed jurassic cretaceous series it structurally underlies the second one the second one contains a thin jurassic cretaceous series deposited on a basement including metamorphic paleozoic terrain and lherzolites the structural arrangement as well as the sedimentological characteristics suggests a southern origin for these highest units the tectonic association between metamorphic and ultrabasic rocks of the basement was probably formed during the hercynian orogeny or in a late hercynian period however it clearly predates fossiliferous sediments attributed to the callovo oxfordian pic de saraille lourdios or still undated terrains presenting a typical triassic facies tos de la coustette lourdios

soil gas profiles as a tool to characterise active tectonic areas the jaut pass example pyrenees france a new method to investigate active tectonic structures using soil gas composition at faults suggests relevant information about regional stress conditions which can be obtained rapidly and at relatively low cost in 1995 we carried out geochemical profiles around the epicentre of the m 5 1 1980 earthquake near arudy in the french pyrenees where the presence of minor fractures is evident in the field and confirmed using satellite spot imagery fractures are conduits facilitating fluid migrations in the crust and are also pathways for the release of deep seated gases to the atmosphere in order to investigate the implication of these fractures in the present deformation i e if they are connected to the hercynian substratum at a depth of about 1800 m soil gases were measured along four traverses crossing the observed structures gases determined were 222rn co2 and 4he each of them for their characteristic source 222rn has essentially a shallow origin due to its short half life whereas co2 is the major soil gas component with a mainly surficial biogenic source however there could additionally be co2 from crustal or mantle degassing which would also be the principal sources of he data analysis clearly reveals anomalous values for each gas at specific positions along the traverses two sets of fractures corresponding to different observed trends are distinguished the one characterised by he anomalies accompanied by other gases and the second with few identified he anomalies the agreement between the geochemical data and the field observations leads us to propose a deformation model for the area studied analogous to a pull apart system located in a right lateral shear zone

rare earth elements in pore waters of marine sediments abstract the rare earth elements rees were measured in pore waters of the upper 25 cm of sediment from one site off peru and three sites on the california margin the pore water ree concentrations are higher than sea water and show systematic down core variations in both concentration and normalized pattern from these analyses and from comparison to other chemical species measured dissolved fe mn ba oxygen nitrate phosphate it is suggested that pore water rees can be grouped into three categories those that are from an fe source those that are from a poc source and cerium oxide rees from the fe source appear where anoxia is reached they have a distinctive middle ree mree enriched pattern the concentrations in this source are so elevated that they dominate ree trends in the fe oxide reduction zone the net result of flux from the poc source is relative enrichment of heavy rees hrees over light rees lrees reflecting remineralizing poc and complexation with doc a common linear ree pattern seen in both oxic and anoxic sediments is associated with this poc source as well as a hree enriched pattern that is seen in surficial sediments at the peru site overall the pore water results indicate that mn oxides are not an important carrier of rees in the oceans a ree biogeochemical model is presented which attempts to reconcile ree behavior in the water and sediment columns of the oceans the model proposes that poc fe oxide and ce oxide sources can explain the ree concentration profiles and relative abundance patterns in environments ranging from oxic sea water to anoxic pore water the model is also consistent with our observation that the ce anomaly of pore water does not exceed unity under any redox condition

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

seismic activity at the western pyrenean edge abstract the present day seismicity at the westernmost part of the pyrenean domain reported from permanent networks is of low to moderate magnitude however it is poorly constrained due to the scarce station coverage of the area we present new seismic data collected from a temporary network deployed there for 17 months that provides an enhanced image of the seismic activity and its tectonic implications our results delineate the westward continuity of the e w pyrenean band of seismicity through the variscan basque massifs along the leiza fault ending up at the hendaya fault this seismicity belt is distributed on a crustal scale dipping northward to almost 30 km depth other relevant seismic events located in the area can be related to the central segment of the pamplona fault and to different e w thrust structures

geochemistry and sedimentology of a facies transition from microbanded to granular iron formation in the early proterozoic transvaal supergroup south africa abstract a transition from microbanded kuruman to granular griquatown iron formation is described in terms of sedimentological petrographic and geochemical characteristics major minor trace and rare earth elements as well as whole rock carbon and oxygen isotopic compositions transitions between microbanded and granular iron formation are uncommon in the global stratigraphic record with microbanding absent from most known granular iron formations the kuruman griquatown transition zone differs from other granular iron formation sequences in being non oolitic i e peloidal and magnetite rich instead of hematite rich it is concluded that the coarsening upward microbanded granular iron formation units in the kuruman griquatown sequence represent upward shallowing storm dominated deposits that is primary microbanded relatively deep water fair weather deposits having been intermittently reworked by storm wave action form wavy sharp based graded and poorly sorted irregular interbeds of granular iron formation this is in contrast to typical hematitic oolitic iron formation units which represent true shoal deposits geochemical results strongly support the storm reworking hypothesis microbanded and granular iron formation beds have similar ree patterns with negative ce anomalies and depletion in light ree reflecting deposits in a marine setting below the zone of shallow surface water positive eu anomalies in the ree patterns suggest admixture of a hydrothermal component of the ocean water carbonates in the iron formation are depleted in 13c indicating involvement of organic matter during diagenetic alteration of the iron formations there are also indications that total dissolved carbon dioxide in deeper water was more depleted in 13c than in shallower water deposition of the iron formations was sporadically interrupted by influx of tuffaceous material now represented by centimeter thick shard and lapilli bearing stilpnomelane lutite beds these beds differ chemically from the iron formations in being enriched in elements such as al sc y zr nb hf ta th u and the lanthanides and displaying ree patterns with strong negative eu and slight positive ce anomalies the composition of the parent magma of the volcanics remains uncertain because of probable post depositional alteration of the volcanic glass however it was most probably of intermediate to acidic composition as concluded from the widespread distribution of the ash beds typical of plinian type deposits chemical variations in the minor trace and ree compositions of the iron formations are closely correlated with the amounts of admixed volcanic ash material most of the iron formations appear chemically very uncontaminated being highly depleted in all minor trace and ree and organic carbon contents the iron formations were therefore most probably deposited as a chemical precipitate on a submerged shallow cratonic platform with volcanic ash as the only exogenic material sporadically reaching the depository

carbonate petrography kerogen distribution and carbon and oxygen isotope variations in an early proterozoic transition from limestone to iron formation deposition transvaal supergroup south africa the transition zone comprises campbellrand microbialaminated replacing cryptalgalaminate limestone and shale with minor dolomite conformably overlain by the kuruman iron formation of which the basal part is characterized by siderite rich microbanded iron formation with minor magnetite and some hematite containing units the iron formation contains subordinate intraclastic and microbialaminated siderite mesobands and was deposited in deeper water than the limestones the sequence is virtually unaltered with diagenetic mineral assemblages reflecting a temperature interval of about 110 degrees to 170 degrees c and pressures of 2 kbars carbonate minerals in the different rock types are represented by primary micritic precipitates now recrystallized to microsparite early precompactional sparry cements and concretions deep burial limpid euhedral sparites and spar cements precipitated from metamorphic fluids in close contact with diabase sills paragenetic pathways of the carbonate minerals are broadly similar in all lithofacies with kerogen intimately associated with them kerogen occurs as pigmentation in carbonate crystals as reworked organic detritus in clastic textured carbonate units and as segregations of kerogen pigment around late diagenetic carbonate crystals locally kerogen may also be replaced by carbonate spar carbon isotope compositions of the carbonate minerals and kerogen are dependent on their mode of occurrence and on the composition of the dominant carbonate species in a specific lithofacies integration of sedimentary petrographic geochemical and isotopic results makes it possible to distinguish between depositional early diagenetic deep burial and metamorphic effects on the isotopic compositions of the carbonate minerals and the kerogen in the sequence major conclusions are that deep burial thermal decarboxylation led to 13c depletion in euhedral ferroan sparites and 13c enrichment in kerogen organic carbon metamorphic sparites are most depleted in 13c carbonates in oxide rich iron formations are more depleted in 13c than those in siderite rich iron formation whereas the kerogens in oxide banded iron formations bif are more enriched this implies that the siderite rich iron formations were not derived from oxide rich iron formation through reduction of ferric iron by organic matter organic matter oxidation by ferric iron did however decrease the abundance of kerogen in oxide rich iron formation and led to the formation of isotopically very light sparry carbonates siderite and calcmicrosparite both represent recrystallized primary micritic precipitates but differ in their 13c composition with the siderites depleted in 13c by 4 6 per mil on average relative to calcmicrosparite this means that the siderites were precipitated from water with dissolved inorganic carbon depleted in 13c by about 9 per mil relative to that from which the limestones precipitated this implies an ocean system stratified with regard to total carbonate with the deeper water from which siderite rich iron formation formed depleted in 13c iron formations were deposited in areas of very low organic matter supply depletion of 13c may therefore derive not from degradation of organic matter but from hydrothermal activity a conclusion which is supported by 18o composition of the carbonate minerals and trace element and rare earth element ree compositions of the iron formations

coupled fe and s isotope evidence for archean microbial fe iii and sulfate reduction direct fossil evidence for early microbial life on earth is rare microbiological data indicate that sulfate and iron reduction are both among the earliest forms of microbial respiration and direct evidence for the early origin of sulfate reduction comes from sulfur isotopic anomalies in ancient sediments fe isotope geochemistry potentially provides a new way of identifying microbial iron reduction early in earth9s history we present fe isotopic data for sedimentary pyrite from the 2 7 ga belingwe sedimentary basin in zimbabwe isotopically light fe and a remarkable covariation between fe and s isotopes provide strong evidence for coexisting fe and s reduction our results are consistent with an early origin for sulfate reduction and provide direct geochemical evidence for the antiquity of bacterial fe reduction the covariation of fe and s isotopes may provide a useful new tracer of microbial evolution early in earth history

deposition of banded iron formations by anoxygenic phototrophic fe ii oxidizing bacteria the mechanism of banded iron formation bif deposition is controversial but classically has been interpreted to reflect ferrous iron fe ii oxidation by molecular oxygen after cyanobacteria evolved on earth anoxygenic photoautotrophic bacteria can also catalyze fe ii oxidation under anoxic conditions calculations based on experimentally determined fe ii oxidation rates by these organisms under light regimes representative of ocean water at depths of a few hundred meters suggest that even in the presence of cyanobacteria anoxygenic phototrophs living beneath a wind mixed surface layer provide the most likely explanation for bif deposition in a stratified ancient ocean and the absence of fe in precambrian surface waters

isotopic constraints on the late archean carbon cycle from the transvaal supergroup along the western margin of the kaapvaal craton south africa few existing studies illuminate the operation of the carbon cycle before the rise of atmospheric oxygen circa 2400 million years ago stable carbon isotopic measurements of shallow stromatolitic carbonates 0 vpdb and basinal carbonate minerals 6 in iron formation have been used to infer a strong isotopic depth gradient in archean ocean basins from new diamond drill cores obtained by the agouron drilling project from the griqualand west structural basin in the northern cape province south africa we present 13 c data from carbonates and organic matter that offer fresh insights into the late archean carbon cycle three drill cores cover the development progradation and ultimate demise by drowning of the campbellrand carbonate platform ca 2590 2500 ma one captures the platform top shallow marine and intertidal paleoenvironments the other two run through slope and basinal sections deposited adjacent to the platform margin increasing inwater depth likely to 1 km both shallowand deep water carbonates precipitated on the seafloor consistently show 13 c values around 0 5 incompatible with a strong late archean isotopic depth gradient a mathematical model suggests that these isotopic data are consistent with a reduced biological pump increased dissolved inorganic carbon in seawater due to higher atmospheric p co2 or both certain horizons do show distinct isotopic variability such areas are commonly shaly and they tend to be organic and or iron rich strong c isotopic variations occur on a cm scale and most likely stem from diagenetic remineralization of organic matter in sediment starved areas where iron formation developed siderite tends to be 13 c depleted sometimes by as much as 14 these observations suggest a carbon cycle in which iron respiration played a conspicuous role carbon isotope ratios from organic matter in shales are commonly 1 lighter than stratigraphically contiguous carbonates but there is no clear water depth trend in the organic carbon isotopic data taken as a whole the 13 c of organic matter can be explained by several non unique sets of processes including different autotrophic mechanisms of carbon fixation heterotrophic recycling including fermentation and methanotrophy and post depositional diagenesis the most striking feature is the occurrence of organic 13 c values 40 a feature that appears to be commonplace in late archean successions framed in the context of carbon cycle isotopic mass balance both organic and carbonate carbon isotopic data suggest that the proportion of carbon buried as organic matter was not radically different before the appearance of free environmental oxygen

dynamics of fault creep observations along the surface traces of active faults in california and elsewhere indicate that tectonic displacement can occur as seismic slip or as aseismic fault creep fault creep occurs as secular displacement and as displacement episodes called creep events which last a few hours to days and include from a millimeter or less to a few tens of millimeters of displacement instrumental measurements of displacement versus time during creep events show that many events have a characteristic simple form including a steep beginning followed by a gradual decay in the rate of displacement some creep events display small amounts of precursory slip and many seem to be composed of several discrete simple events propagation of creep events along the san andreas calaveras and imperial faults in california has been inferred from the nearly simultaneous observation of creep events at nearby creepmeters and from kinematic modeling of continuous measurements of strain during creep events fault creep can be viewed as arising from the interplay of three factors stress applied to the fault from external sources stress caused by the geometry and distribution of displacement on the fault arising from the elastic response of the surrounding medium to the displacements within the anelastic fault zone itself and the constitutive relations characterizing the resistance to slip on the fault analytic solutions are presented for a simple rectangular dislocation quasi static model of nonpropagating fault creep assuming viscous and quasi plastic power law creep fault zone rheology and for a multi element composed of several rectangular or striplike dislocation surfaces quasi static model assuming a viscous rheology viscous rheology implies solutions composed of exponentials that decay with time the simple one element quasi plastic model is in agreement with the form of creep events observed at melendy ranch california a matrix formulation is presented to calculate dislocation stresses on a fault zone in a half space or plate the formulation involves the division of the dislocation surface into strips or rectangles and the use of published solutions to calculate the stress resulting from displacement on each individual element results from the matrix method agree with analytic results for equilibrium displacements on stress free cracks the matrix formulation is then used as the basis for a numerical method to simulate one and two dimensional propagating creep assuming a quasi plastic rheology on the fault zone results from this method agree reasonably well with observations of afterslip from the 1966 parkfield 1975 oroville and 1979 imperial valley california earthquakes and of a propagating creep event observed along the southern calaveras fault central california during july 1977 this method of analysis and the assumption that creep at each point on the fault zone follows a power law rheology after the applied stress reaches a spatially varying yield stress can explain the secular and episodic nature of fault creep afterslip the simple shape of some individual events and the multiple composite shape of others and the propagation of creep events on the southern calaveras fault observations of propagating creep indicate a yield stress near zero at the surface and throughout much of the fault zone however they also indicate the presence of a zone at a depth of about 0 5 km and about 5 km long with a yield stress of about 15 bars 1 5 mpa the methods of solution presented here should be applicable to many problems in the dynamics of faulting

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

the pyrenean earthquake of february 29 1980 an example of complex faulting abstract an m b 5 1 earthquake occurred on february 29 1980 in the western pyrenees near arudy france the telemetred network of the i p g p operating in this area since 1978 allowed a good localization of this earthquake 43 4 21 n 0 24 59 w depth 4km during the two years preceding this earthquake the seismic activity exhibited a gradual decrease a foreshock of magnitude 1 6 was recorded three hours before the main shock a temporary network set up in the epicentral area a few days after the main shock permitted a precise study of the aftershock sequence fifty fault plane solutions for earthquakes ranging from magnitude 1 5 4 were obtained a complex pattern of faulting was revealed with both strike slip and normal faulting however a regional tectonic stress tensor can be proposed from a detailed investigation of the aftershock sequence this stress tensor is in agreement with previous results in this area

speculations on the geometry of the initiation and termination processes of earthquake rupture and its relation to morphology and geological structure earthquake initiation and termination processes are commonly described in terms of barriers and asperities barriers fall into two classes geometric barriers are associated with places where the orientation of a failure surface changes and relaxation barriers where stress is low because aseismic creep processes outpace tectonic loading geometric barriers fall into conservative and nonconservative subgroups according to whether finite fault motion can proceed without the creation of new structures or whether it demands the creation of new faulting or void space the multiple faulting or fragmentation associated with some nonconservative barriers can disrupt fault planes and form asperities by means of selected examples it is shown that a description in terms of these barriers can help one to visualise the processes of earthquake rupture and its relation to the geological environment

regional deformation near palmdale california 1973 1983 the tehachapi trilateration network spans the intersection of the san andreas and garlock faults in southern california in the big bend region of the san andreas fault analysis of data from 1973 1983 shows strain differences between the northwest and southeast regions of the network and slip at depth on both faults the palmdale network spanning the san andreas fault entirely within the tehachapi network showed increases of about 1 strain in both east west and north south extension in late 1979 the tehachapi strains also jumped at this time but the magnitude of the increase was only about one third that of palmdale the principal strain rates for tehachapi over the time interval 1973 1983 are e 1 0 08 0 01 strain yr and e 2 0 12 0 01 strain yr with the 1 axis directed n76 e strains were also computed for two subregions the principal strain rates for the southeast tehachapi region are e 1 0 14 0 02 strain yr and e 2 0 16 0 02 strain yr with the 1 axis directed n73 e this result differs significantly from the principal strain rates at palmdale e 1 0 21 0 02 strain yr e 2 0 17 0 01 strain yr with the 1 axis directed n71 e for the northwest tehachapi region e 1 0 11 0 02 strain yr and e 2 0 06 0 02 strain yr with the 1 axis directed n93 e

sedimentation rates basin analysis and regional correlations of three neoarchaean and palaeoproterozoic sub basins of the kaapvaal craton as inferred from precise u pb zircon ages from volcaniclastic sediments abstract calculation of sedimentation rates of neoarchaean and palaeoproterozoic siliciclastic and chemical sediments covering the kaapvaal craton imply sedimentation rates comparable to their modern facies equivalents zircons from tuff beds in carbonate facies of the campbellrand subgroup in the ghaap plateau region of the griqualand west basin transvaal supergroup south africa were dated using the perth consortium sensitive high resolution ion microprobe ii shrimp ii dates of 2588 6 ma and 2549 7 ma for the middle and the upper part of the nauga formation indicate that the decompacted sedimentation rate for the peritidal flat to subtidal below wave base stratifera and clastic carbonate facies southwest of the ghaap plateau at prieska was of up to 10 m ma when not corrected for times of erosion and non deposition dates of 2516 4 ma for the upper gamohaan formation and 2555 19 for the upper monteville formation indicate that some 2000 m of carbonate and subordinate shale sedimentation occurred during 16 ma to 62 ma on the ghaap plateau for these predominantly peritidal stromatolitic carbonates decompacted sedimentation rates were of 40 m ma to over 150 m ma bubnoff units the mixed siliciclastic and carbonate shelf facies of the schmidtsdrif subgroup and monteville formation accumulated with decompacted sedimentation rates of around 20 b for the kuruman banded iron formation a decompacted sedimentation rate of up to 60 b can be calculated thus for the entire examined deep shelf to tidal facies range archaean and phanerozoic chemical and clastic sedimentation rates are comparable four major transgressive phases over the kaapvaal craton followed by shallowing upward sedimentation can be recognized in the prieska and ghaap plateau sub basins in griqualand west and partly also in the transvaal basin and are attributed to second order cycles of crustal evolution first order cycles of duration longer than 50 ma can also be identified the calculated sedimentation rates reflect the rate of subsidence of a rift related basin and can be ascribed to tectonic and thermal subsidence comparison of the calculated sedimentation rates to published data from other archaean and proterozoic basins allows discussion of general precambrian basin development siliciclastic and carbonate sedimentation rates of archaean and palaeoproterozoic basins equivalent to those of younger systems suggest that similar mechanical chemical and biological processes were active in the precambrian as found for the phanerozoic particularly for stromatolitic carbonates matching modern and neoarchaean sedimentation rates are interpreted as a strong hint of a similar evolutionary stage of stromatolite building microbiota the new data also allow for improved regional correlations across the griqualand west basin and with the malmani subgroup carbonates in the transvaal basin the nauga formation carbonates in the southwest of the griqualand west basin are significantly older than the gamohaan formation in the ghaap plateau region of this basin but are in part correlatives of the oaktree formation in the transvaal and of parts of the monteville formation on the ghaap plateau

predicted variation of stress orientation with depth near an active fault application to the cajon pass scientific drillhole southern california preliminary measurements of the stress orientation at a depth of 2 km in the cajon pass scientific drillhole about 3 6 km from the san andreas fault in southern california indicate that the local direction of maximum compression is nearly normal to the fault zone the measurements have been interpreted to indicate that the regional orientation of the maximum compression is normal to the fault and taken as evidence for a very weak fault the orientation of the regional maximum compression expected from plate tectonic arguments is about 66 ne from the strike of the fault geodetic data indicate that the orientation of maximum compressive strain rate is about 43 ne from the strike of the fault and show nearly pure right lateral shear acting parallel to the fault these apparent conflicts in the inferred orientation of the axis of maximum compression may be explained in part by a model in which the fault zone is locked over a depth interval in the range of 2 5 to 15 km but is very weak above and below that interval an analytic solution for this model predicts orientations of the principal stresses that are nearly parallel or normal to the fault for observations made close to the fault and above and below the locked zone but an orientation consistent with plate tectonic estimates at depths corresponding to that of the locked interval this solution does require however a few mm yr of creep at the surface on the san andreas or nearby sub parallel faults such as the san jacinto which has not yet been observed or a shallow zone near the faults of distributed deformation

fe c and o isotope compositions of banded iron formation carbonates demonstrate a major role for dissimilatory iron reduction in 2 5 ga marine environments abstract combined fe c and o isotope measurements of 2 5 ga banded iron formation bif carbonates from the kuruman iron formation and underlying bif and platform ca mg carbonates of the gamohaan formation south africa constrain the biologic and abiologic formation pathways in these extensive bif deposits vertical intervals of up to 100 m were sampled in three cores that cover a lateral extent of 250 km bif fe carbonates have significant fe isotope variability 56fe 1 to 1 and relatively low 13c down to 12 and 18o values 18o 21 in contrast gamohaan and stratigraphically equivalent campbellrand ca mg carbonates have near zero 13c values and higher 18o values these findings argue against siderite precipitation from seawater as the origin of bif fe rich carbonates instead the c o and fe isotope compositions of bif fe carbonates reflect authigenic pathways of formation in the sedimentary pile prior to lithification where microbial dissimilatory iron reduction dir was the major process that controlled the c o and fe isotope compositions of siderite isotope mass balance reactions indicate that the low 13c and low 18o values of bif siderite relative to those expected for precipitation from seawater reflect inheritance of c and o isotope compositions of precursor organic carbon and ferric hydroxide that were generated in the photic zone and deposited on the seafloor carbon fe isotope relations suggest that bif fe carbonates formed through two end member pathways low 13c low 56fe fe carbonates formed from remobilized low 56fe aqueous fe2 produced by partial dir of iron oxide whereas low 13c high 56fe fe carbonates formed by near complete dir of high 56fe iron oxides that were residual from prior partial dir an important observation is the common occurrence of iron oxide inclusions in the high 56fe siderite supporting a model where such compositions reflect dir in place in the soft sediment in contrast the isotopic composition of low fe carbonates in limestone dolomite may constitute a record of seawater environments although our petrographic studies indicate that the presence of pyrite in most low fe carbonates may influence the fe isotope compositions the combined fe c and o isotope data from kuruman bif carbonates indicate that bif siderites that have negative near zero or positive 56fe values may all record biological fe cycling where the range in 56fe values records differential fe mobilization via dir in the sediment prior to lithification our results demonstrate that the inventory of low 56fe marine sedimentary rocks of neoarchean to paleoproterozoic age although impressive in volume may represent only a minimum of the total inventory of fe that was cycled by bacteria

microbial influence on the oxygen isotopic composition of diagenetic siderite abstract numerous early diagenetic siderite concretions previously described have been interpreted as the results of microbially mediated reactions interpretation of oxygen isotope data for such material requires an understanding of the effect of temperature on fractionation processes however whilst equilibrium fractionation of oxygen isotopes between siderite and water has been measured down to 33 c extrapolation to lower temperatures may be invalid furthermore inorganic measurements may not be applicable to microbial systems the specific iron reducing microorganism geobacter metallireducens has been cultured anaerobically in the laboratory using acetate as an organic substrate and amorphous feooh as an electron acceptor the acetate is oxidised to co2 with concurrent iron reduction and extracellular siderite precipitation rhombohedral siderite crystals up to 25 m in size have been precipitated over a range of temperatures 18 40 c stable isotopic analysis of these crystals and the solutions from which they precipitate shows that this microbial precipitation of siderite has an associated isotopic fractionation different from the published equilibrium and which is not simply a function of temperature in all cases 18o values of siderite are lower than predicted by inorganic equilibrium fractionation data this may explain the numerous anomalously low 18o values reported for early diagenetic marine siderites and previously attributed to mechanisms such as mixing with meteoric water sediment water interaction recrystallisation or variable isotopic fractionation despite a lack of supporting evidence

structural analysis of hypocentral distribution of an earthquake sequence using anisotropic wavelets method and application 1 spatial organization of earthquake sequences is investigated to localize active rupture planes and to reconstruct the geometry of the inferred rupture zone we have developed a new approach the normalized optimized anisotropic wavelet coefficient noawc method to extract from a set of hypocenters the active ruptures planes our approach permits the detection of organized structures within a plane regardless of its size location shape anisotropy and orientation it includes the determination of a system of three perpendicular sections minimizing the effects of projections of the hypocenters and intrinsically accounts for uncertainty in the location of the seismic events the accuracy and the effectiveness of the noawc procedure are illustrated on both synthetic and real data an application to the m 5 1 arudy french pyrenees aftershock sequence shows how a combination of the possible mathematically reconstructed geometries can be combined with the available fault plane solutions and geomorphological markers to determine the active rupture planes and to propose and validate a local tectonic model this new multitool approach lends itself to quantitative and computer assisted analyses of large data sets