gravitational potential energy and regional stress and strain rate fields for continental plateaus examples from the central andes and colorado plateau abstract the commonly observed extension in areas of elevated and thickened crust is an expected consequence of having excess gravitational potential energy gpe compared to the low gpe of the surrounding crust while this conceptual model is well founded it is less clear how well gpe related stress orientations compare quantitatively to observed stress and strain rate orientations and what any inconsistency tells us about the presence of other competing forces we estimate the gpe distribution for the central andes and the greater colorado plateau area using topography and crustal thickness variations respectively and compare the related stress fields with the world stress map as well as with a geodetic strain rate field for the colorado plateau only in both areas deviatoric stresses associated with gpe variations alone cannot fully account for the observed deformation rate field for the central andes only a combination of deviatoric stresses associated with gpe and relative plate motions can account for the near n s tensional stress observed in the peruvian andes and the margin normal compressional stress along the eastern cordillera and sub andean fold and thrust belt the observed deformation field around the colorado plateau shows e w extension largely inconsistent with the deviatoric stresses associated with gpe variations except for the area east of the rio grande rift the ne sw oriented stress observed on the southwestern colorado plateau is consistent with the orientations of tensional deviatoric stresses associated with gpe variations we argue that this consistency could be haphazard stress observations may not reflect the current state of stress due to inherited structure or could result from the relative high strength of colorado plateau that allows for regional gpe variations and possibly basal shear to be more significant forces than far field plate interactions for the central andes and colorado plateau stresses associated with gpe variations have a strong influence on the total stress field and can thus be used to calibrate the overall level of deviatoric stress acting within the lithosphere

drilling deep into young oceanic crust hole 504b costa rica rift hole 504b is by far the deepest hole yet drilled into the oceanic crust in situ and it therefore provides the most complete ground truth now available to test our models of the structure and evolution of the upper oceanic crust cored in the eastern equatorial pacific ocean in 5 9 m y old crust that formed at the costa rica rift hole 504b now extends to a total depth of 1562 3 m below seafloor penetrating 274 5 m of sediments and 1287 8 m of basalts the site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust as a result the conductive heat flow approaches the value of about 200 mw m predicted by plate tectonic theory and the in situ temperature at the total depth of the hole is about 165 c the igneous section was continuously cored but recovery was poor averaging about 20 the recovered core indicates that this section includes about 575 m of extrusive lavas underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes the latter have been sampled in situ only in hole 504b the igneous section is composed predominantly of magnesium rich olivine tholeiites with marked depletions in incompatible trace elements nearly all of the basalts have been altered to some degree but the geochemistry of the freshest basalts is remarkably uniform throughout the hole successive stages of on axis and off axis alteration have produced three depth zones characterized by different assemblages of secondary minerals 1 the upper 310 m of extrusives characterized by oxidative seafloor weathering 2 the lower extrusive section characterized by smectite and pyrite and 3 the combined transition zone and sheeted dikes characterized by greenschist facies minerals a comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology alteration and porosity into three zones that are analogous to layers 2a 2b and 2c described by marine seismologists on the basis of characteristic seismic velocities many of the logs and experiments suggest the presence of a 100 to 200 m thick layer 2a comprising the uppermost rubbly pillow lavas which is the only significantly permeable interval in the entire cored section layer 2b apparently corresponds to the lower section of extrusive lavas in which original porosity is partially sealed as a result of alteration nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900 1000 m below seafloor within the transition between extrusives and sheeted dikes indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2b and 2c and confirming that layer 2c consists of intrusive sheeted dikes a vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends that between the sheeted dikes of seismic layer 2c and the gabbros of seismic layer 3 which has never been sampled in situ may be within reach of the next drilling expedition to hole 504b therefore despite recent drilling problems deep in the hole current plans now include revisiting hole 504b for further drilling and experiments when the ocean drilling program returns to the eastern pacific in 1991

change from late tertiary compression to quaternary extension in southern tibet during the india asia collision field studies in southern tibet show that subsequent to folding resulting from the india asia collision 50 40 ma reverse faults and backthrusts formed as a result of nne ssw and nnw sse trending compressions they affect conglomerates of oligocene miocene age and are probably contemporaneous with shearing on the himalayan main central thrust and folding of paleogene basins along the northern border of southern tibet 25 16 ma rift systems postdate compressional structures they were active during the pleistocene and probably during the pliocene 5 2 ma change from a compressional to an extensional regime in tibet probably occurred during the late miocene 10 5 ma this change is tentatively correlated with uplift inferred by chinese workers from paleogene to present day variations of the tibetan vegetation recent and active faulting in the high plateau results from a wnw ese striking extension this is in agreement with tibetan focal mechanisms the himalayan lowlands are subjected to a nne ssw trending compression as demonstrated by superficial deformations and seismicity therefore the horizontal maximum compressive stress trajectory is roughly parallel to india asia convergence hmax being 1 in the lowlands and 2 in the high plateau this change results from the increase of the vertical principal stress value due to the weight of the elevated mass zz is 3 in the lowlands and 1 in the high plateau thus the third principal stress axis is 2 in the lowlands and 3 in the high plateau allowing extension to occur in the wnw ese direction extensional tectonics are significantly different in southern and northern tibet southern tibet being constrained between the himalayan syntaxes the e w extensional principal deviatoric stress value 3 is of rather small magnitude and the n s trending 2 value is extensional as demonstrated by analysis of recent faults normal components prevail along all the azimuths in the field this favors formation of rift systems of large axial extension on the contrary north of the himalayan syntaxes northern tibet flows easily eastward the e w extensional principal deviatoric stress value 3 is of higher magnitude and the n s stress 2 is compressional as shown by inversion of focal mechanism data this favors strike slip components on faults which deviate from the n s direction as expressed in the field by shorter n s trending graben linked with normal strike slip faults these faults permit some n s shortening in northern tibet without thickening in response to india asia collision in high plateaus which are subjected to lateral compression such as the andean altiplano and tibet the extensional direction is basically controlled by the convergence direction being roughly orthogonal to the latter

stable isotopic investigation of clay minerals and pedogenesis in an interfluve paleosol from the cenomanian dunvegan formation n e british columbia canada abstract mineralogical and stable hydrogen and oxygen isotopic compositions have been obtained for clay minerals comprising an interfluve paleosol clayhurst ch 2 from the early to middle cenomanian dunvegan formation british columbia canada this paleosol provides a special opportunity to determine the stable isotopic composition of early late cretaceous paleoprecipitation on the western coast of the western interior seaway at a very northerly paleolatitude 65 detrital kaolinite and illite and complex dioctahedral mixed layer clay minerals of pedogenic origin are abundant throughout the paleosol in the d values of 93 to 81 and 18 o values of 12 9 to 11 6 were generally obtained for the ancient soil water and hence paleoprecipitation for one thin interval near the top of the paleosol higher values are indicated for the ancient soil water these results may indicate processes arising from a westward transgression of the western interior seaway

geology of a vigorous hydrothermal system on the endeavour segment juan de fuca ridge a high precision high resolution geologic map explicitly documents relationships between tectonic features and large steep sided sulfide sulfate silica deposits in the vigorously venting endeavour hydrothermal field near the northern end of the juan de fuca ridge water depth in the vent field varies from 2220 to 2200 m location of the most massive sulfide structures appears to be controlled by intersections of ridge parallel normal faults and other fracture fissure sets that trend oblique to and perpendicular to the overall structural fabric of the axial valley the fractured basaltic substrate is primarily composed of well weathered pillow and lobate flows as presently mapped the field is about 200 by 400 m on a side and contains at least 15 large 1000 m3 sulfide edifices and many tens of smaller commonly inactive sulfide structures the larger sulfide structures are also the most vigorously venting features in the field they are commonly more than 30 m in diameter and up to 20 m in height actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller maximum venting temperatures of 375 c are associated with the smaller structures in the southeastern portion of the field highest temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20 30 c lower hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture controlled black smoker activity near the top of the structures a different type of high temperature venting takes place from the upper sides of the structures in the form of overflow from fully exposed quiescent pools of buoyant 350 c vent water trapped beneath overhanging sulfide sulfate silica ledges or flanges these flanges are attached to the upper outer walls of the large sulfide edifices two types of diffuse venting in the endeavour field include a lower temperature 8 15 c output through colonies of large tubeworms and 25 50 c vent fluid that seems to percolate through the tops of overhanging flanges the large size and steep walled nature of the these structures evidently results from sustained venting in a mature hydrothermal system coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development

a numerical method for determining the state of stress using focal mechanisms of earthquake populations application to tibetan teleseisms and microseismicity of southern peru the numerical method described in this paper enables the study in terms of stress of the kinematics of seismic faults provided by focal mechanisms this method assumes a mean state of stress in the source region and is based on the simple mechanical model used for fault population analysis which supposes slip in the direction of the resolved shear stress acting on the fault plane the proposed algorithm first defines compressional and tensional zones resulting from superimposition of the compressional and tensional quadrants limited by the nodal planes this enables one to test the data homogeneity furthermore this restricts the space where the principal stress axes have to be searched then for each principal stress reference whose location is constrained by above confined zones the r value chosen equal to 2 1 3 1 is computed which fits the slip vector on each nodal plane this permits one to select a set of preferred seismic fault planes from a set of auxiliary planes finally a state of stress is computed from the preferred seismic fault plane set using the non linear simplex method already applied to fault populations this algorithm is constructed so as to avoid two major difficulties misleading estimation of the deviatoric stress tensor which may result from excessive emphasis by minimizing the residuals by a least squares method if some of the data are wrong and a lengthy prospection of the stress references over all the space of directions this calculation does not take into account possible changes of the strike and dip of the nodal planes however we use rotations of nodal planes in agreement with the first arrival data to test the compatibility of these data with the computed state of stress tibetan teleseisms and southern peruvian microseisms are analyzed and results are compared with kinematics of superficial recent and active faults measured in field in the same regions this suggests that major seismic events may give a good evaluation of the regional mean state of stress focal mechanisms of the small events are also modelled by a mean stress deviator but this differs significantly from the regional state of stress in southern peru deduced from quaternary faulting this may be due to imprecise location of the small events however some movements are strongly different from those predicted by the computed mean stress deviator this suggests that small fault kinematics surely depend on the regional state of stress but that local effects may control the small deformations

geochemical constraints on chemolithoautotrophic metabolism by microorganisms in seafloor hydrothermal systems mixing of hydrothermal fluids and seawater at the ocean floor combined with slow reaction kinetics for oxidation reduction reactions provides a source of metabolic energy for chemolithotrophic microorganisms which are the primary biomass producers for an extensive submarine ecosystem that is essentially independent of photosynthesis thermodynamic models are used to explore geochemical constraints on the amount of metabolic energy potentially available from chemosynthetic reactions involving s c fe and mn compounds during mixing of hydrothermal fluids with seawater for the vent fluid used in the calculations epr 21 n obs the model indicates that mixing environments are favorable for oxidation of h2s ch4 fe2 and mn2 only below 38 c with methanogenesis and reduction of sulfate or s favored at higher temperatures suggesting that environments dominated by mixing provide habitats for mesophilic but not thermophilic aerobes and thermophilic but not mesophilic anaerobes a maximum of 760 cal per kilogram vent fluid is available from sulfide oxidation while between 8 and 35 cal kg vent fluid is available from methanotrophy methanogenesis oxidation of fe or mn or sulfate reduction the total potential for chemosynthetic primary production at deep sea hydrothermal vents globally is estimated to be about 1013 g biomass per year which represents 0 02 of the global primary production by photosynthesis in the oceans thermophilic methanogens and sulfate and s reducers are likely to be the predominant organisms in the walls of vent chimneys and in the diffuse mixing zones beneath warm vents where biological processes may contribute to the high methane concentrations of vent fluids and heavy 34s32s s ratios of vent sulfide minerals the metabolic processes taking place in these systems may be analogs of the first living systems to evolve on the earth

focal mechanisms of moderate size earthquakes from the analysis of single station three component surface wave records abstract the method proposed a few years ago by ekstrom et al 1986 to retrieve the focal mechanism parameters from a set of love and rayleigh waves recorded on a single station is extended to regional intermediate size earthquakes this method is based on the inversion of the source phase and amplitude spectra in terms of the seismic moment tensor components for large teleseismic events the success of the method relies mainly on the possibility to correct the observed surface wave spectra for propagation effects by using globally averaged phase velocities for moderate size earthquakes m s 4 5 to 5 5 the periods of interest are shorter and it is no longer possible to apply this technique due to large regional variations of the propagation parameters in the method we propose here a dispersion analysis of the regional love and rayleigh wave data is first performed in the period range 30 to 100 sec the phase velocities obtained by integration of the apparent group velocities observed on the seismograms themselves and a standard q value are used to correct for the propagation effects the validity of the method is tested on synthetic seismograms and an application to long period records observed at different stations are then presented earthquakes in the pacific ocean the atlantic ocean and the mediterranean regions are used for this purpose the feasibility of the method is also tested with an historical record made on wiechert and galitzin seismometers for an earthquake that occurred in peru in 1946 finally an example of rapid computation of fault plane solution made after the armenia earthquake of 7 december 1988 is presented

the quantum event of oceanic crustal accretion impacts of diking at mid ocean ridges seafloor diking eruptive events represent the irreducible quantum events of upper oceanic crustal accretion they record events by which a large portion of the oceanic crust has formed through geological history since 1993 the u s navy s real time sound surveillance system has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast pacific these diking eruptive events trigger a sequence of related rapidly evolving physical chemical and biological processes magmatic volatiles released during these events may provide nutrients for communities of subseafloor microorganisms some of which thrive in high temperature anaerobic environments many of the organisms identified from these systems are archaea if microorganisms can thrive in the water saturated pores and cracks within deep volcanically active portions of our planet other hydrothermally active planets may harbor similar life forms

world s largest known precambrian fossil black smoker chimneys and associated microbial vent communities north china implications for early life abstract black smoker chimneys and biological vent communities have been identified at many sites on the deep seafloor particularly along oceanic spreading centers we report the largest and oldest known microbe rich sub meter sized black smoker chimneys and mounds from a 1 43 billion year old sulfide deposit in a continental graben in northern china these chimneys are especially well preserved with characteristic morphology internal textures and internal cylindrical mineralogical zonation four main types of chimneys are distinguished on textural and mineralogical criteria exhibiting either zn fe sulfide or pb zn fe sulfide internal cylindrical mineralogical zones the chimneys mark vent sites in submarine grabens indicating focused flow venting processes the fossil chimneys have mineralogical and geological evolutionary features similar to their counterparts on the modern seafloor and other submarine hydrothermal vents black smoker vent fluids and seafloor tectonism played important roles for formation of the massive sulfide deposits in the mesoproterozoic we also report the first known remarkably diverse assemblage of fossil microbialites from around and inside precambrian vent chimneys demonstrating that proterozoic life flourished around submarine hot vents and deep within the chimney vent passages filamentous spherical rod and coccus shaped fossil microbes are preserved preferentially on sulfide precipitates based on the depth and setting of the fossil biota the organisms that produced the microbialites were likely sulfate reducing chemosynthetic and thermophyllic microbes textural and mineralogical evidence shows that biomineralization processes enhanced chimney growth and sulfide precipitation close association of microorganisms with sulfide chimneys in modern deep sea hydrothermal vents and younger ophiolites has sparked speculation about whether life may have originated at similar vents however little is known about fossil equivalents of vent microfossils and black smoker chimneys from earth s early evolution the fossilized microorganisms from the gaobanhe black smoker chimney sulfide deposits include thread like filaments with branching and twisted forms and preserved organic carbon representing fossilized remnants of microbial mats metabolized at high temperatures characteristic of venting fluids the preservation of fossil microorganisms provides evidence that microbial populations were closely associated with black smoker chimneys in earth s early history the microbial population clearly constitutes the site for mediating mineral formation these ancient microbial fossils lead to a much better understanding of early life on the deep seafloor the discovery of the mesoproterozoic microfossils within black smoker hydrothermal chimneys indicates that hydrothermal activity around sea floor vents supported dense microbial communities and supports speculation that vent sites may have hosted the origin of life

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

the mechanical state of the lithosphere in the altiplano puna segment of the andes abstract information about topography the shape of the geoid seismicity neogene deformation and volcanism in the region of altiplano puna of western south america is used to analyse the state of stress across the convergent plate margin in terms of the effects of topography and simple models of its compensation an average elevation near 4 km is consistent with compensation by a yet unresolved combination of crustal root and hot uppermost mantle producing a geoid high of 22 27 meters average horizontal compressive stress in excess of a reference sea level lithostatic value of 390 bars in a 150 km thick lithosphere and an average shear stress of 170 bars along a 30 dipping interplate boundary the basis for these estimates is evidence for a neutral to extensional stress regime within the high plateau contrasted with a compressional regime on the eastern slopes and along the interplate boundary itself comparison with other plateaus in a convergent plate tectonic setting suggests an evolutionary sequence from compressional to extensional tectonics as elevation of the plateau increases

quaternary normal and reverse faulting and the state of stress in the central andes of south peru field studies in the andes of southern peru show that in the high andes and pacific lowlands quaternary and recent faults are normal this extensional tectonics postdates compressional deformations of pliocene early quaternary age in the sub andes the observed deformations are compressional they affect early quaternary deposits some of the faults separate quaternary deposits from the bedrock and thus are clearly of tectonic origin and not landslide effects striations on the fault planes indicate n s trending extension in the high andes and pacific lowlands the total amount of crustal stretching is small probably of the order of 1 during the last 1 2 m y in the sub andes folds and faults affecting neogene and early quaternary deposits result from n s shortening nevertheless it is supposed that this n s shortening is of early quaternary age the present day compression probably strikes e w judging from focal mechanisms in the sub andes of central peru southern bolivia and northwest argentina data from structural analysis of faults and from earthquake focal mechanisms allow us to surmise the state of stress in the andes of southern peru the high andes and pacific lowlands subjected to n s trending extension are bounded by two zones of e w trending compression the sub andes to the east and the contact between the convergent nazca and south america plates to the west in our model the maximum horizontal compressive stress trajectory hmax is roughly parallel with the e w convergence between the two plates hmax corresponds to 1 in the sub andes and to 2 in the high andes the latter situation is caused by the elevated mass of the high andes where zz the vertical stress is inferred to be 1 thus the third principal stress axis being orthogonal to the other two axes it is oriented n s allowing extension to occur in that direction on the other hand in the sub andes zz is 3 and horizontal e w shortening occurs the state of stress in the andean continental crust above the 30 dipping slab appears to be different from that in the andes of central peru situated above the flat subducting segment in this region compressional deformantion affect a wider part of the cordillera

crustal magnetization reveals subsurface structure of juan de fuca ridge hydrothermal vent fields a near bottom geophysical survey on the endeavour segment of the northern juan de fuca ridge shows that regions of well defined low crustal magnetization are strongly correlated with both active and extinct submarine hydrothermal vent sites in particular at the main endeavour field we find discrete magnetization lows associated with each cluster of vents magnetization lows are directly centered beneath the vent clusters and have diameters of 100 m which implies a near vertical narrow pipe like source region located directly beneath the surface expression of the vent edifices lows are also separated from each other by only 200 m which further implies highly focused zones magnetization lows are also associated with inactive and extinct vent areas which indicates that alteration of the magnetic minerals in the crust rather than necessarily temporary thermal demagnetization is the primary process responsible for the low magnetization these narrow pipe like bodies are highly characteristic of alteration pipes found in ophiolites and are indicative of hydrothermal fluid up flow zones thus each magnetization low may define an individual upwelling zone with distinct subsurface plumbing and thermal structure the crustal magnetization patterns provide important constraints on the geometry of the subsurface plumbing beneath these hydrothermal vent systems at the main endeavour field magnetization lows are distributed along the trend of the rift valley in a semiregular pattern with a spacing of 200 m arguing that upward flow may be partitioned into regularly spaced intervals along the axis of the rift valley

the ancash peru earthquake of 1946 november 10 evidence for low angle normal faulting in the high andes of northern peru summary the ancash peru earthquake is the largest and best documented intracontinental normal fault earthquake to have occurred in historically recorded time along the andean margin of south america the fault geometry and the rupture processes of this earthquake provide information on how normal faulting develops in a predominantly compressional regime and on the hazards associated with normal faulting in other parts of the andes a re evaluation of first motion data intensity data and epicentre locations coupled with waveform modelling suggests that the earthquake occurred along a fault that dips 30 10 at a hypocentral depth of 15 17 km geological information also suggests that the earthquake reactivated an older thrust fault that was favourably orientated with respect to the present regional stress regime

evidence of a recent magma dike intrusion at the slow spreading lucky strike segment mid atlantic ridge 1 mid ocean ridge volcanic activity is the fundamental process for creation of ocean crust yet the dynamics of magma emplacement along the slow spreading mid atlantic ridge mar are largely unknown we present acoustical seismological and biological evidence of a magmatic dike intrusion at the lucky strike segment the first detected from the deeper sections 1500 m of the mar the dike caused the largest teleseismic earthquake swarm recorded at lucky strike in 20 years of seismic monitoring and one of the largest ever recorded on the northern mar hydrophone records indicate that the rate of earthquake activity decays in a nontectonic manner and that the onset of the swarm was accompanied by 30 min of broadband 3 hz intrusion tremor suggesting a volcanic origin two submersible investigations of high temperature vents located at the summit of lucky strike seamount 3 months and 1 year after the swarm showed a significant increase in microbial activity and diffuse venting this magmatic episode may represent one form of volcanism along the mar where highly focused pockets of magma are intruded sporadically into the shallow ocean crust beneath long lived discrete volcanic structures recharging preexisting seafloor hydrothermal vents and ecosystems index terms 3035 marine geology and geophysics midocean ridge processes 7280 seismology volcano seismology 8419 8149 tectonophysics planetary tectonics 5475 4259 oceanography general ocean acoustics 9325 information related to geographic region atlantic ocean keywords mid atlantic ridge earthquake hydroacoustic

accurate source depths and focal mechanisms of shallow earthquakes in western south america and in the new hebrides island arc synthetic seismograms are matched to long period p waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms accurate depths are obtained for a large sample of moderate sized 6 ms 7 events which produced relatively simple p waveforms and do not require intensive analysis the estimated error in the determination of depth is about 5 km or less for most of the events considered the procedure is applied to two suites of events one in south america 83 events and one in the new hebrides 61 events in these two areas of contrasting styles of subduction the accurate depths provide new information on the bending of descending plates near trenches the seismicity and tectonics of the upper plate and the geometry and structure of the zone of contact between the upper and descending plates depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with chapple and forsyth s 1979 preferred model of a bending elastic perfectly plastic plate however an unusually deep event occurred beneath the fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late miocene time the calculated depth of the event 48 km places it below the lithosphere asthenosphere boundary derived from thermal models vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin 10 km thick curved zone of contact dipping arcward or landward from the trench axis however a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact these events would be accommodated by an interplate zone 15 km thick in both subduction zones interplate events occur at depths between about 15 and 50 km however the downdip width of the south american interplate zone is 2 to 3 times larger than that in the new hebrides most of the intraplate events near the interplate boundary are located in the descending plate in the upper plate seismicity is more pronounced in the back arc regions of the two subduction zones thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc depths of shallow earthquakes in the eastern andes and sub andean regions of south america range between about 9 and 36 km indicating that both the upper and middle crust are seismically active about 75 of the events occurred in the upper 25 km furthermore the focal mechanisms indicate that from ecuador to argentina the crust of the eastern side of the andean cordilleran is in a state of east west compression oriented nearly parallel to the direction of convergence between the nazca and south american plates the depths the orientations of the earthquake nodal planes and the close relationships to quaternary tectonics imply basement involved crustal shortening in much of the eastern andes and sub andean zones of peru and argentina

the imprint of time on canadian soil landscapes abstract the imprint of time on canadian soil landscapes reflects the strong influence of repeated quaternary glaciations with most soils having formed on late pleistocene glacial sediments and to a lesser extent holocene non glacial sediments soil chronosequences in these youthful landscapes seldom span the full holocene and have been documented principally in areas of coastal isostatic uplift and shoreline evolution as well as on recessional moraine sequences in the cordillera these chronosequences have placed quantitative constraints on rates of podzolization in environments ranging from coastal temperate rainforest to subarctic longer sequences involving soils and relict paleosols on pleistocene morainal and glaciofluvial surfaces in the central yukon territory record more complex legacies of pedogenesis and landscape evolution but with much coarser time control and many potentially confounding influences of climatic change and disturbance processes older weathered bedrock and regolith can make significant contributions to soil parent materials in several settings 1 in favoured locations where local conditions protected these materials from glacial erosion in southern canada 2 beyond the maximum limits of glaciation principally in northwestern canada and 3 in the arctic where cold based non erosive ice allowed preservation of these materials closer collaborations between pedologists and other quaternary scientists will be needed to ensure a more complete understanding of these complex pedogenic legacies in canadian landscapes

three dimensional p and s wave velocity structures of southern peru and their tectonic implications in 1981 an 18 station regional array was operated in southern peru above the area where the dip of the subducted nazca plate changes from virtually horizontal in the northwest to about 30 in the southeast arrival times of compressional p and shear s waves from microearthquakes recorded by this array are used in this paper to investigate the three dimensional velocity structure of the crust and upper mantle in the region between the coast and the cordillera occidental the results suggest a crustal thickness of about 40 km beneath the coast increasing to about 70 km beneath the cordillera occidental this change in thickness occurs abruptly in the northwest but is gradual in the southeast the inverse correlation between the dip of the moho and the dip of the slab suggests a broad scale causal relation between the two at the same time free air gravity anomalies suggest that the steepening of the moho in the northwest may occur only in the region under the array because the crust is thicker in the southeast than originally surmised a suite of earthquakes previously thought to occur in the mantle now appears to be confined to the crust the association of these earthquakes with a shallow dipping moho is suggestive of ongoing crustal deformation to the west of the andes however other seismological and geological observations make the role of these earthquakes in crustal deformation unclear s wave velocities in the mantle between 70 and 130 km depth above the 30 dipping slab are low suggesting the presence of partially melted asthenosphere that may be responsible for the magmatic activity observed in southern peru

stress modeling in the andes constraints on the south american intraplate stress magnitudes regions of high topography which exhibit both compressional and extensional deformation provide a natural laboratory to place constraints on horizontal intraplate stress magnitudes an elastic finite element analysis of the lithospheric stress field in the cordillera bianca region of peru has been used to evaluate the lithospheric stress state and constrain the south american intraplate stress magnitudes the orthogonal orientation of the andean high topography to the east west regional stress field in conjunction with the well documented change in deformation style from compression to extension across the cordillera provides a unique setting for the stress modeling a two dimensional finite element grid consisting of an assembly of isotropic elastic quadrilateral elements in a state of plane strain was used to represent the lithosphere an isostatically balanced crustal density model was used to calculate the stresses associated with the lateral density variations between the andean topography and the brazilian shield in some models horizontal stress was concentrated at midcrustal depths by including vertical variation in young s modulus the modeling results indicate bounds of 10 and 75 mpa for the magnitude of the average horizontal stress averaged over a 100 km thick lithosphere a preferred value of 25 mpa is obtained on the basis of fitting all of the data for the rheological models considered and predicting a transition from extension to compression near the 3000 m elevation contour such a far field average stress is consistent with a ridge push origin because the modeling performed in this study assumes that the topographic load is entirely compensated by the buoyancy of the deflected moho these estimates represent upper bounds the effect of incorporating thermal support for the topographic load of the cordillera bianca would be to further reduce our estimate of the intraplate stress magnitudes analysis of the distribution of calculated stress with depth indicates that previous assumptions about the vertical plane shear stress within the lithosphere being negligible are acceptable for the geometry of the andes

extension related to a high topography results from a microearthquake survey in the andes of peru and tectonic implications summary a microearthquake study performed in 1985 october november in the cordillera blanca western cordillera of northern peru allowed us to make a precise mapping of its shallow seismicity 160 local events are accurately located in the vicinity of an active normal fault system showing a vertical displacement of 4500 m over the last 7 myr part of the seismic activity can be related to this west dipping system but a great number of events indicate a new seismic zone situated under the highest peaks of the chain by using an inversion algorithm of p wave polarities we show a regional deviatoric stress tensor which defines an extensional axis in a direction approximately orthogonal to the andean structures this result is in good agreement with the theoretical distribution of stresses expected in an elongated high topography bordered by tectonic convergent zones moreover we suggest that an absolute uplift of this young batholith occurs on a normal fault system and that this phenomenon is partly due to buoyancy forces originating in its crustal root

micromorphology and development of some central yukon paleosols canada abstract this paper focusses on the wounded moose and diversion creek paleosols which developed in the central yukon territory of northern canada wounded moose paleosols developed during the pre illinoian interglacial period and contain well developed reddish void and grain argillans in the bt horizons diversion creek paleosols developed in the sangamonian period and also contain void and grain argillans but these argillans are weakly developed brownish in colour and occur only rarely both of these paleosols have been exposed to one or more cold periods during which cryogenic processes dominated the soil development these cryogenic processes produced papules embedded grain argillans aggregates silt cappings and oriented fabrics the reddish argillans that are found in some diversion creek paleosols are thought to have originated from the former wounded moose paleosols when the pre reid illinoian glacial ice overrode them and incorporated them into the diversion creek parent material the development of the micromorphological features during the various glacial and interglacial periods is given with reference to the climate of the time

gradients in the composition of hydrothermal fluids from the endeavour segment vent field phase separation and brine loss hydrothermal fluid samples collected in 1984 1987 and 1988 from a large vent field near 47 57 n on the endeavour segment of the juan de fuca ridge jfr have been analyzed for major and minor elements and gases there are of the order of 100 individual smoker vents on 10 large sulfide structures which are localized along faults and fault intersections across the vent field each sulfide structure has a characteristic fluid composition which varies very little from one vent orifice to the next or from year to year on a given structure however there are large gradients in fluid composition across the vent field with endmember chlorinity increasing from 255 mmol kg in the sw to 505 mmol kg in the ne end member concentrations of major elements are well correlated with chlorinity and endmember volatile concentrations in the lowest chlorinity fluids are approximately twice as high as in the highest chlorinity fluids the gradients in composition across the vent field and measured vent fluid temperatures 400 c are consistent with supercritical phase separation and loss of brine phase below the seafloor the factor of 2 variation in co2 and h2s is larger than expected for loss of a very high chlorinity brine concentrations of iron and manganese are not positively correlated with chlorinity suggesting that temperature and ph are more important in controlling metal solubility elevated ammonia and bromide chloride ratios indicate that there has been subseafloor interaction between the hydrothermal fluids and organic matter and high boron concentrations point to a sedimentary source

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

on crustal seismicity of the amazonian foothill of the central peruvian andes a microearthquake survey of crustal seismicity conducted in 1985 on the eastern cordillera and the amazonian foothills in central peru gives a description of the present tectonic activity related to the uplift of the andes hypocenters on the huaytapallana fault are shallower than 10 km and their focal mechanism is in agreement with the fault trace corresponding to the earthquakes of 1969 and suggests a reverse strike slip movement on a plane striking n130 an active tectonic zone is evidenced along a nw se direction passing through the amauta subandean region the vertical distribution of hypocenters and the comparison of shallow and deep focal mechanisms suggest a reverse fault dipping to the west depths vary between 0 and 20 km and never exceed 32 km the huaytapallana cordillera is uplifted by thrust faults on both sides

middle to late pleistocene ice extents tephrochronology and paleoenvironments of the white river area southwest yukon abstract sedimentary deposits from two middle to late pleistocene glaciations and intervening non glacial intervals exposed along the white river in southwest yukon canada provide a record of environmental change for much of the past 200 000 years the study sites are beyond the marine isotope stage mis 2 glacial limit near the maximum regional extent of pleistocene glaciation non glacial deposits include up to 25 m of loess peat and gravel with paleosols pollen plant and insect macrofossils large mammal fossils and tephra beds finite and non finite radiocarbon dates and twelve different tephra beds constrain the chronology of these deposits tills correlated to mis 4 and 6 represent the penultimate and maximum pleistocene glacial limits respectively the proximity of these glacial limits to each other compared to limits in central yukon suggests precipitation conditions were more consistent in southwest yukon than in central yukon during the pleistocene conditions in mis 5e and 5a are recorded by two boreal forest beds separated by a shrub birch tundra that indicate environments as warm or warmer than present a dry treeless steppe tundra dominated by artemisia frigida upland grasses and forbs existed during the transition from late mis 3 to early mis 2 these glacial and non glacial deposits constrain the glacial limits and paleoenvironments during the middle to late pleistocene in southwest yukon

extensional basin evolution in the cordillera blanca peru stratigraphic and isotopic records of detachment faulting and orogenic collapse in the andean hinterland 1 sedimentologic provenance oxygen isotope and 40ar 39ar results provide insights into late cenozoic evolution of an extensional hinterland basin in the peruvian andes the 5 6 5 km cordillera blanca composes the glaciated footwall of a low angle normal fault parallel to active contractional structures in the andean fold thrust belt the 200 km long wsw dipping 19 36 cordillera blanca detachment fault accommodated 12 15 km of dip slip displacement inducing subsidence of the hanging wall supradetachment basin which is filled by 1300 m of conglomerate sandstone siltstone and limited carbonate of the upper miocene pliocene lloclla formation lithofacies associations are attributed to lacustrine fan delta to proximal stream dominated alluvial fan sedimentation provenance data record footwall unroofing of jurassic through miocene volcanic and sedimentary rocks during nw directed axial dispersal of sediment provenance data record the appearance of footwall derived granite clasts from the upper miocene cordillera blanca batholith and a change to transverse wsw directed transport variations in the character and preservation of basin fill suggest along strike propagation of the fault through time initial extension and subsidence is constrained to the latest miocene by an 40ar 39ar biotite age of 5 4 0 1 ma for a basal tuff in the lloclla formation estimation of paleolake water composition shows very negative 18o vsmow values 13 6 to 18 2 indicating that high elevations comparable to modern were already attained in the cordillera blanca during earliest normal faulting and basin evolution these results lend support to models suggesting that substantial shortening crustal thickening and surface uplift were necessary precursor conditions for the generation of the cordillera blanca detachment fault

a novel microbial habitat in the mid ocean ridge subseafloor abstract the subseafloor at the mid ocean ridge is predicted to be an excellent microbial habitat because there is abundant space fluid flow and geochemical energy in the porous hydrothermally influenced oceanic crust these characteristics also make it a good analog for potential subsurface extraterrestrial habitats subseafloor environments created by the mixing of hot hydrothermal fluids and seawater are predicted to be particularly energy rich and hyperthermophilic microorganisms that broadly reflect such predictions are ejected from these systems in low temperature 15 c basalt hosted diffuse effluents seven hyperthermophilic heterotrophs isolated from low temperature diffuse fluids exiting the basaltic crust in and near two hydrothermal vent fields on the endeavour segment juan de fuca ridge were compared phylogenetically and physiologically to six similarly enriched hyperthermophiles from samples associated with seafloor metal sulfide structures the 13 organisms fell into four distinct groups one group of two organisms corresponding to the genus pyrococcus and three groups corresponding to the genus thermococcus of these three groups one was composed solely of sulfide derived organisms and the other two related groups were composed of subseafloor organisms there was no evidence of restricted exchange of organisms between sulfide and subseafloor habitats and therefore this phylogenetic distinction indicates a selective force operating between the two habitats hypotheses regarding the habitat differences were generated through comparison of the physiology of the two groups of hyperthermophiles some potential differences between these habitats include fluid flow stability metal ion concentrations and sources of complex organic matter

quaternary extension in southern tibet field observations and tectonic implications we summarize evidence for quaternary and active faulting collected in the field during three sino french expeditions to southeastern tibet 1980 1982 detailed mapping of quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the himalayas in the last 2 0 5 m y the maximum horizontal principal stress in south tibet appears to be only the intermediate principal stress 2 1 being vertical south of the chord joining the eastern and western syntaxes of the himalayan arc extensional strains are principally localized within seven regularly spaced rift zones three of which have been studied in some detail the extension direction is determined to be n96 e 7 mainly from statistical averaging of strikes of newly formed normal faults throw rates on normal faults are evaluated for different time spans 2 0 5 m y 60 40 kyr and 10 2 kyr b p using structural and topographic reliefs as well as synglacial and postglacial vertical offsets the rate of quaternary extension is about 1 m y 1 along an 1100 km long ese traverse across south tibet this corresponds to a spreading rate of 1 0 6 cm yr this rate and the divergent horizontal projections of slip vectors of earthquakes along the himalayan front constrain the rate at which rigid india underthrusts southern tibet to be 2 1 cm yr although most of the normal faults appear to be independent of and nearly orthogonal to the e w mesozoic tertiary tectonic fabric the yadong gulu rift appears to be guided for over 130 km by the older oblique ne sw nyainqentanglha range and fault zones along it this reactivated zone is the most prominent left lateral strike slip fault system in se tibet excepting this zone and the vicinity of the se extremity of the karakorum fault quaternary strike slip faulting is rare in south tibet i e south of the chord between the syntaxes of the himalayan arc north of the chord the tectonic style is different there minor conjugate strike slip faulting is widespread and appears to control quaternary normal faulting which is more diffuse and subdued than in south tibet along the chord the presence of a major zone of active right lateral en echelon strike slip faults karakorum jiali fault zone probably reflects the greater facility of eastward extrusion in north central tibet in response to the northward push of india the eastern himalayan syntaxis may be an obstacle to such extrusion movements south of the chord

subsurface processes at the lucky strike hydrothermal field mid atlantic ridge evidence from sulfur selenium and iron isotopes at lucky strike near the azores triple junction the seafloor setting of the hydrothermal field in a caldera system with abundant low permeability layers of cemented breccia provides a unique opportunity to study the influence of subsurface geological conditions on the hydrothermal fluid evolution coupled analyses of s isotopes performed in conjunction with se and fe isotopes have been applied for the first time to the study of seafloor hydrothermal systems these data provide a tool for resolving the different abiotic and potential biotic near surface hydrothermal reactions the 34s between 1 5 and 4 6 and se values between 213 and 1640 ppm of chalcopyrite suggest a high temperature end member hydrothermal fluid with a dual source of sulfur sulfur that was leached from basaltic rocks and sulfur derived from the reduction of seawater sulfate in contrast pyrite and marcasite generally have lower 34s within the range of magmatic values 0 1 and are characterized by low concentrations of se 50 ppm for 82se 76se ratios the 82se values range from basaltic values of near 1 5 to 7 the large range and highly negative values of hydrothermal deposits observed cannot be explained by simple mixing between se leached from igneous rock and se derived from seawater we interpret the se isotope signature to be a result of leaching and mixing of a fractionated se source located beneath hydrothermal chimneys in the hydrothermal fluid at lucky strike we consider two sources for s and se 1 the end member hydrothermal fluid with basaltic se isotopic values 1 5 and typical s isotope hydrothermal values of 1 5 2 a fractionated source hosted in subsurface environment with negative 34s values probably from bacterial reduction of seawater sulfate and negative 82se values possibly derived from inorganic reduction of se oxyanions fluid trapped in the subsurface environment is conductively cooled and has restricted mixing and provide favorable conditions for subsurface microbial activity which is potentially recorded by s isotopes fe isotope systematic reveals that se rich high temperature samples have 57fe values close to basaltic values 0 whereas se depleted samples precipitated at medium to low temperature are systematically lighter 57fe values between 1 to 3 an important implication of our finding is that light fe isotope composition down to 3 2 may be explained entirely by abiotic fractionation in which a reservoir effect during sulfide precipitation was able to produce highly fractionated compositions

large massive sulfide deposits in a newly discovered active hydrothermal system the high rise field endeavour segment juan de fuca ridge submersible observations and side scan sonar imagery delineate ten large venting sulfide sulfate silica structures located on an uplifted block of the axial valley floor on the central portion of the endeavour segment juan de fuca ridge this newly discovered vent field lies 1 8 km north of the main endeavour vent field the axial valley a horst and faults controlling sulfide deposition trend parallel to the ridge axis the horst rises 10 to 15 m above the valley floor and offsets a lava lake which extended 500 m in width across the valley the largest sulfide structures are perched along the major horst bounding faults one structure is the tallest single edifice 45 m reported in the pacific ocean smaller less active and inactive sulfide edifices are associated with faulted remnants of the lava lake surface fissured pressure ridges and at the edge of collapse pits based on the relationship between faults and sulfide structures the morphology of sulfide edifices and their venting characteristics we infer that sulfide structures grow vertically by chimney accumulation and laterally by flange development late stage deposition of amorphous silica toughens the large deposits and may be responsible for the unusual size and aspect ratio of endeavour edifices

habitability of the marine serpentinite subsurface a case study of the lost city hydrothermal field the lost city hydrothermal field is a dramatic example of the biological potential of serpentinization microbial life is prevalent throughout the lost city chimneys powered by the hydrogen gas an

use of micromorphology for palaeoenvironmental interpretation of complex alluvial palaeosols an example from the mill creek formation albian southwestern alberta canada abstract field observations are often not sufficient for process based interpretations of palaeosols particularly where they form parts of thick aggradational pedocomplexes within alluvial successions under such conditions micromorphology provides genetic temporal and spatial information on soil forming processes that is critical to an understanding of past environmental conditions thick alluvial successions of the albian mill creek formation contain abundant evidence of pedogenesis but few well developed palaeosol profiles and therefore provide an ideal case study in which to demonstrate the usefulness of micromorphological data for palaeoenvironmental interpretation the micromorphological features of greatest interpretive value are types of clay coatings and ferruginous segregations structure and fabric papules evidence of bioactivity and ferruginous concretions provide information on geomorphic surface stability and assist in reconstructing temporal changes in drainage conditions while individual features can provide some palaeoenvironmental information the relationships of features to one another and assemblages of features provides additional information when analysed hierarchically to establish a sequence of sedimentologic and pedogenic events a common recurring sequence of palaeoenvironmental events subject to local variations can be recognized throughout the mill creek formation the presence of illuvial clay requires that water percolated through the soil and that the soil periodically dried out so that the translocated clay was retained dark reddish clay coatings indicate clay illuviation under freely drained conditions while pale yellow and silty clay coatings suggest that phases of free drainage alternated with phases of poorly drained or saturated soil conditions the presence of iron depletion coatings iron nodules and quasiferrans indicates that these units were at least periodically saturated and the occurrence of multiple overlapping phases within single thin sections demonstrates that redox conditions fluctuated strongly suggesting development in the vadose zone recent soils containing similar assemblages of features develop under warm temperate seasonal climates alternating phases of well drained and saturated conditions on the mill creek floodplains are attributed to changing sediment supply and local palaeogeomorphology rather than to any major regional climate change this type of process based micromorphological analysis should have broad application in other complex pedogenically modified alluvial successions and similar studies would lead to a more detailed understanding of ancient palaeoenvironments

determination of the pore fluid pressure ratio at seismogenic megathrusts in subduction zones implications for strength of asperities and andean type mountain building 1 we construct the differential stress profile across the fore arc in a subduction zone from the force balance between the shear stress at seismogenic megathrust and the lithostatic pressure we assume that is written by 1 n where is the pore fluid pressure ratio is the coefficient of static friction and n is the normal stress given a density structure of the fore arc wedge we determine by comparing calculated fore arc stresses with observed ones as 0 95 0 98 in shikoku miyagi peru north chile and south chile and 0 90 0 93 in south vancouver island and washington the parameter averaged over the seismogenic megathrust is of the order of 10 mpa stress drops of great earthquakes in these zones occupy 14 87 and not a constant fraction of they on the other hand increase linearly with 1 we propose a simple fault model in which the area of asperities as a fraction of the total fault area is proportional to 1 variation of fractional area of asperities thus may explain the observed correlation and the regional variation of assuming that the differential stress at summit of the andean mountains is zero not at the coast as observed at present we determine to be 0 84 in north chile in the mountain building stage such a smaller value of along with 0 9 in subduction zones would suggest that variation of controls the tectonic style of the earth

chemical evolution of mid ocean ridge hot springs abstract chemical analyses yielding elemental concentrations of major and minor elements of four hot springs on the east pacific rise at 21 n and 10 hot springs from the southern trough of the guaymas basin gulf of california provide a basis for thermodynamic modeling of conductive cooling of the hydrothermal endmembers mixing of the hydrothermal endmember with seawater and reaction of an epr type fluid with sediment of bulk chemical composition corresponding to unaltered sediment from dsdp hole 477 in the guaymas basin results of the calculations indicate that conductive cooling of endmember fluids within closed chimneys and conduits accounts for the solution composition of one vent on the east pacific rise and several vents in guaymas basin mixing of the hydrothermal fluids with seawater yields a prediction of mineral assemblages closely approximating those observed in samples of chimneys from the east pacific rise and drill cores in guaymas basin the hypothesis that guaymas basin hydrothermal fluids result from interaction of an epr type fluid with sediment cover in guaymas basin is supported by calculations which predict an increase in ph to a value similar to guaymas basin fluids an order of magnitude decrease in metal concentrations and an excellent agreement between predicted mineral assemblages as a function of extent of interaction with sediment and observed mineral assemblage distribution with depth

evidence for early glaciation of southeastern beringia between the klondike plateau and yukon tanana highlands of yukon and alaska respectively current maps explain glaciated alpine locales and periglacial areas in terms of localized pliocene pleisto

tectonic implications of the microearthquake seismicity and fault plane solutions in southern peru using recordings from 15 portable instruments and six permanent stations operated during two field investigations of microearthquakes in southern peru we determined locations of 888 shallow and intermediate depth earthquakes and 56 fault plane solutions a series of tests evaluating the effects of elevation corrections erroneous velocity structure and random errors in arrival times on calculated locations suggest that the locations of most of the events are uncertain by less than 7 km at shallow depths 60 km the seismicity is concentrated in three belts one offshore one beneath the coast and one beneath the western cordillera there is also scattered but infrequent shallow activity near the altiplano the belts offshore and beneath the coast are apparently associated with slip along the nazca south american plate boundary although the style of deformation is complicated one can infer that beneath the coast the active boundary between the nazca and south american plates is within the mantle rather than at the base of the south american crust the belt beneath the western cordillera is about 30 km in width and dips southwest from near the surface to a depth of 60 km fault plane solutions suggest that crustal shortening is taking place in a northwest southeast direction in this region the significance of this activity is difficult to explain but these earthquakes provide no clear evidence of tectonic erosion in this area a composite fault plane solution from the few events near the altiplano shows normal faulting which is typical of the active tectonics of that region the results of the investigation of intermediate depth earthquakes shows that the downgoing slab in southern peru is warped and not torn in the region where the dip changes from nearly horizontal to a dip of about 30 they also suggest that the axis of the contortion is not perpendicular to the coast but parallel to the direction of plate motion moreover the volcanic line ends at the contortion which suggests that a relatively thick wedge of aesthenospheric material is necessary for the formation of subduction related volcanism fault plane solutions for events between 60 and 100 km depth show a large scatter but some suggest a detachment of a sliver of the downgoing slab solutions for deeper events are more consistent with one another with the t axes for most events being nearly horizontal and generally normal to the projections of the seismic zone

asperities barriers characteristic earthquakes and strong motion prediction recent results from paleoseismology suggest that earthquakes recurrent on a given fault may often have the same characteristic length and amount of slip stable asperities and barriers which survive many earthquakes can explain these results the activity of volcano usu japan since 1977 which produced a dacite dome with a total upheaval of 180 m offered a great opportunity to study the recurrence behavior of a fault seismologists at hokkaido university revealed a family of repeated earthquakes with nearly the same amount of slip generated from the same fault plane on the crater wall along which the dome moved upward this family can be explained by the existence of a stable asperity on the crater wall and is called the asperity type family they found however another type of family of earthquakes which share the same fault plane but show a great range of variation in the amount of slip the behavior of the latter family can be explained by the presence of weak barriers over a fault plane defined by strong stable barriers and this family is referred to as the barrier type family this type of earthquake model has been used successfully in the interpretation of observed strong motion acceleration spectra from california and the results reveal the stability of local stress drop among different earthquakes the agreement found between local stress drops estimated from strong motion data and those inferred from geological observation supports the possibility for predicting earthquake strong motion directly from geological observation of the causative fault

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

sedimentary environments and mechanisms of organic matter enrichment in the mesoproterozoic hongshuizhuang formation of northern china abstract the mesoproterozoic hongshuizhuang formation in jixian northern china is a hydrocarbon source unit containing abundant organic matter we investigated the mechanisms controlling this organic matter accumulation by analyzing the sedimentology and geochemistry of the qinghe section in the kuancheng region of northern china trace element geochemistry suggests that organic matter accumulation was controlled primarily by marine productivity which was the main determinant of redox conditions elevated marine productivity and consequent anoxic sulfidic bottom water conditions were coincident with accumulation of the organic rich toc 1 0 lower part of the hongshuizhuang formation deposition of the organic lean upper hongshuizhuang formation appears to have been coeval with diminished productivity and a transition to oxic suboxic redox conditions enhanced organic matter accumulation in the lower hongshuizhuang formation was promoted by a warm humid paleoclimate and submarine hydrothermal activity average rb sr ratios 3 42 and negative carbon isotope excursion reflect a warm climate likely caused by the release of volcanogenic co 2 warm conditions would have promoted continental weathering and increased the flux of nutrients into the oceans enhancing marine productivity an increased supply of nutrients and enhanced marine productivity would have depleted oxygen in the bottom water environment and favored the proliferation of sulfate reducing bacteria leading to anoxic sulfidic conditions the relative stability of 13 c carb values suggests that organic matter enrichment in the hongshuizhuang formation reflects changes in surface water productivity and redox conditions caused by climate induced sea level fluctuations shale normalized rare earth element ree distributions show a depletion in light rees and enrichment in heavy rees positive eu anomalies and mean y ho and ba sr ratios of 33 2 and 9 91 respectively suggesting that deposition of the hangshuizhuang formation was accompanied by submarine hydrothermal activity

low temperature alteration of serpentinized ultramafic rock and implications for microbial life abstract alteration serpentinization of ultramafic rocks is known to generate hydrogen through oxidation of ferrous iron in olivine or pyroxene and reduction of water hydrogen oxidation can act as a primary source for chemolithotrophic microbial communities and it is suggested that hydrogen availability is essential for the existence of a deep subsurface biosphere in this study we focus on ongoing low temperature water rock processes in partially serpentinized dunite from the leka ophiolite complex mid norway we aim to determine whether these reactions release reduced compounds and nutrients that can be utilized by the associated microbial communities the study is based on the chemical and physical alteration features of a 51 m deep rock core as well as the chemical composition of groundwater from the core s borehole the borehole intersects three main fracture zones observations during drilling indicated that water infiltration largely takes place along the deepest zone the rock in the fracture zones is nearly completely altered to serpentine brucite magnetite and minor calcite an increase in ph and decrease in mg si and alkalinity of the groundwater along the borehole from the deepest to the outermost fracture zone reflect ongoing water rock reactions where dissolution of brucite serpentine and calcite and precipitation of chrysotile hydromagnesite and iron hydroxide are most likely to occur detection of both h 2 and o 2 in the groundwater influenced by the outermost fracture zone suggests that the local development of highly reducing conditions in the narrow fractures is due to low water rock ratios and to dissolution of fe ii containing brucite and serpentine resulting in the formation and successive diffusion of h 2 into the oxic borehole oxidation of h 2 reduced fe and mn nh 4 and ch 4 are all potential energy yielding reactions that could be utilized by chemolithotrophic microorganisms using dissolved organic or inorganic carbon as possible carbon sources methane could result from microbial oxidation of h 2 and reduction of co 2 or from the degradation of organic matter the results demonstrate that low temperature water rock reactions in serpentinized ultramafic rock lead to production of reduced chemical species that could support primary producers in diverse microbial communities

fossil cryogenic features in paleosols of southern italy characteristics and paleoclimatic significance the work presents the results of a study of two paleosols located at 1350 m a s l on the sila massif in southern italy the purpose of this research is to examine paleoenvironmental conditions during the last glaciation based mainly on soil morphological micromorphological and sedimentological data these two paleosols contain climatically sensitive cryogenic features including a sand wedge an ice wedge cast silt veins silt lenses reticulate silt veins and matrix lenses these features are interpreted to indicate that during the last glacial maximum this area in southern italy experienced conditions characterized by very low temperature little snow cover and great aridity

deposition and pedogenesis of periglacial sediments and buried soils at the serpentine hot springs archaeological site seward peninsula ak abstract soil micromorphology is an excellent tool for relating quantitative laboratory data to soil development in complex pedogenic settings this study utilizes micromorphology scanning electron microscopy bulk soil geochemistry clay mineralogy and particle size analysis to reconstruct the depositional and pedogenic history of the serpentine hot springs fluted point site on the seward peninsula alaska sediment deposition occurred via colluviation which dominated during the late glacial period and early to middle holocene and aeolian processes which dominated during the younger dryas yd and late holocene two buried soils are present one dating to the holocene thermal maximum htm and the other dating just before the neoglacial period pedofeatures fall into four categories 1 cryogenic features 2 clay illuviation features 3 podzolization redox features and 4 anthropogenic features the spatial relationships among these features provides insight into the pedogenic history of the soil the htm soil is characterized by clay illuviation suggesting warm mildly acidic well drained soil conditions the pre neoglacial soil is characterized by incipient placic horizon development and podzolization indicative of moist acidic variably drained conditions cryogenic features associated with late holocene neoglaciation dominate the modern soil despite cryogenic activity and the presence of permafrost the cultural stratigraphy of the site remains intact

evidence for strain rate variation and an elevated transient geothermal gradient during shear zone evolution in the cordillera blanca peru abstract the cordillera blanca shear zone cbsz occupies the footwall of the cordillera blanca detachment fault an active low moderate angle detachment fault extending for 200 km along the western margin of the cordillera blanca in central peru recrystallized quartz paleopiezometry two feldspar thermometry on asymmetric strain induced myrmekite and structural analysis from the cbsz provide evidence for shear zone evolution within a transient elevated geotherm during syn convergent extension shear zone width varies by a factor of six along strike with widths of up to 400 m within the central segment of the shear zone microstructurally derived strain rates calculated from quartz paleopiezometry and two feldspar thermometry results are on the order of 10 14 to 10 12 s 1 and are broadly consistent with macrostructurally derived strain rates of 10 14 to 10 13 s 1 calculated from shear zone thickness and previously determined exhumation rates within the context of existing thermochronological data constraints on batholith emplacement and previously determined offset along the cordillera blanca detachment our data suggest that ductile deformation in the cbsz occurred within a transient elevated geothermal gradient of at least 50 c km and potentially as high as 100 c km which deviated from a 30 c km gradient for an unspecified duration of time we discuss the evolution of the cordillera blanca where observed deformation fabrics are related to strain localization along an exhuming fault as well as thermal weakening from the interplay between faulting and magmatism

formation of h2 ch4 and n species during low temperature experimental alteration of ultramafic rocks abstract hydrogen methane and ammonium are important chemical species for chemolithotrophic microorganisms that sustain subsurface endolithic communities it is well known that production of h 2 through serpentinization of the primary fe ii containing minerals olivine and pyroxene in ultramafic rocks and reduction of co 2 to ch 4 are processes that occur at higher temperatures knowledge is however limited about these reactions under low temperature conditions and how they are affected by fe ii containing secondary minerals such as brucite and serpentine which are commonly found in these environments in this experimental study we explored the formation of h 2 ch 4 and nh 4 during low temperature 25 c reactions between deionised water and 1 unaltered 2 medium altered and 3 highly altered dunites over a period of 99 days hydrogen was detected in all three experiments and the concentration increased over time the o 2 concentration decreased over time but o 2 was still present at significant levels when h 2 started to form this suggests that the h 2 was formed locally at the surface of mineral grains rather than in the hypoxic solution the highly altered dunite gave the highest and the unaltered dunite the lowest h 2 concentrations and while the o 2 concentration decreased fast with the medium altered dunite significant amounts of o 2 still remained with the unaltered dunite at the end of the experiments both the medium and highly altered dunite gave o 2 concentrations close to or below the detection limit during the last part of the experiment factors that likely affected the h 2 production are fe content mineralogy dissolution rates and surface areas of the minerals as well as the presence of catalytic minerals such as chromite and magnetite methane was also detected and increased with time in all experiments the results indicate that ch 4 formation also occurred locally on the surface of minerals detection of both nh 4 and no 3 in the aqueous solutions indicates that previously absorbed n species were leached out during the experiment increasing nh 4 and decreasing no 3 concentrations over time suggest no 3 reduction likely catalysed by mineral surfaces the results imply that low temperature water rock reactions in moderately to highly altered ultramafic rocks provide reduced chemical species that can be utilized as electron donors in energy yielding metabolic processes by subsurface microorganisms such reactions could play an important role in sustaining subsurface microbial communities particularly in ophiolites and near seafloor parts of the ultramafic oceanic lithosphere at slow spreading mid ocean ridges and possibly also in other rocks and sediments the results suggest that brucite and possibly also serpentine may be more important sources of fe ii and thus h 2 formation than olivine in low temperature ultramafic systems formation of h 2 in reducing microenvironments on mineral surfaces surrounded by oxic conditions increases the extent of h 2 formation and the subsurface habitat for aerobic h 2 oxidizing microorganisms

mid ocean ridge sulfide deposits evidence for heat extraction from magma chambers or cracking fronts numerous seafloor observations show that the sizes of high temperature hydrothermal sulfide edifices vary dramatically with spreading rate on fast spreading ridges venting occurs from spindly chimneys which reach heights of about 10 m while on slow spreading ridges vents are frequently located near the tops of large sulfide mounds whose volumes may reach 105 106 m3 we argue that such variations are the result of a profound difference in the nature of hydrothermal heat extraction between fast spreading ridges where spreading occurs predominantly by magmatism and slow spreading ridges where there is a significant component of tectonic extension along fast spreading ridges a steady state axial magma chamber can be insulated by a relatively thick conductive boundary layer because heat extraction is limited by low permeability in such systems episodes of vigorous venting are linked to diking events not only because these introduce a significant heat source into the upper crust but also because the intrusion of a dike is the primary source of increased permeability near the ridge axis over a time scale of the order of a decade mineral deposition tends to clog the reaction and upflow zones and there is a high probability that young sulfide structures will be buried by subsequent eruptions on slow spreading ridges tectonic extension maintains the fluid pathways necessary to support vigorous convection in the waning stages of magmatism hydrothermal circulation driven by a downward migrating cracking front can cool the entire crust leading to the formation of very large sulfide deposits as the depth of circulation increases overburden pressures reduce the permeability and the hydrothermal heat fluxes decrease progressively once hydrothermal fluids penetrate the moho serpentinization clogs the fluid pathways and high temperature venting ceases until it is reactivated by a fresh magmatic intrusion

active detachment faulting above the peruvian flat slab the cordillera blanca detachment fault in the peruvian andes is to our knowledge the first active detachment to be documented above a modern flat slab crustal detachment has unroofed the ca 8 ma cordillera blanca batholith now the backbone of the highest mountain range in peru large magnitude slip along the fault was thermally enhanced by emplacement of the batholith the penultimate magmatic event prior to flattening of the nazca slab however extensional models based on arc magmatism and crustal thickening alone do not adequately explain the scale or structural asymmetry of a series of young deep seated west dipping normal faults across peru here we show that the onset of detachment faulting coincided with subduction of the aseismic nazca ridge and consequent flattening of the nazca slab we propose that slab buoyancy from ridge subduction triggered extensional collapse of the prethickened continental crust and that this buoyancy drove footwall uplift that exceeds basin subsidence the west dipping asymmetry of late cenozoic extensional faults in peru may be controlled by a preexisting crustal anisotropy older thrusts and or formation of riedel like shears kinematically linked to the flat nazca slab

geological constraints on the kinematics and fault plane solution of the quiches fault zone reactivated during the 10 november 1946 ancash earthquake northern peru the ancash earthquake of 10 november 1946 mw 6 8 produced surface faulting along the quiches normal slip fault field studies reported discontinuous nw se to nnw sse striking seismic scarps that displace quaternary glacial moraines and outwash as well as mesozoic sedimentary rocks structural analyses show that the 1946 normal displacement was accompanied by a small left lateral slip component morphological analysis of the fault scarp and observations in a trench excavated across the scarp show evidence of a single normal fault displacement before the 1946 seismic event that offset late pleistocene glacial moraines this indicates an approximate recurrence interval of more than 13 000 1 000 years for the major events suggesting a rate of net vertical displacement on the fault of less than 0 25 mm yr kinematic analysis along the quiches fault in comparison with other regional studies suggests that the state of stress in the high andes is regionally and locally homogeneous with only minor changes in the stress directions adjacent to major structures such as the maranon thrust and fold belt the minimum horizontal stress 3 axis is oriented n10 25 e the intermediate horizontal stress 2 axis is oriented n100 115 e and the maximum stress 1 axis is vertical we suggest that the 60 sw dipping quiches surface fault plane is connected at depth to a low angle dipping fault plane this seismic fault is a reactivated thrust fault of the tertiary maranon thrust and fold belt

dissolved carbon species in associated diffuse and focused flow hydrothermal vents at the main endeavour field juan de fuca ridge phase equilibria and kinetic constraints 1 despite significant advances in the understanding of hydrothermal processes at mid ocean ridges the linkage between focused flow and diffusively flowing vent fluids has remained elusive here we report the distribution of dissolved carbon species in fluids issuing from associated low moderate and high temperature vent systems collected in 2005 at the main endeavour field mef juan de fuca ridge excess co aq measured in the moderate temperature diffuse flow fluids indicates redox disequilibria between co aq and co2 aq suggesting hydrothermal circulation of short residence time in near seafloor mixing zones according to geochemical modeling the chemical variability of mef fluids is consistent with a combination of conductive cooling reaction transport and subsequent seawater mixing of a single source fluid the similar distribution of c1 c3 alkanes observed in diffuse and focused flow fluids likely indicates a minimal fingerprint of biological and methanogenic metabolism on organic fluxes consistent with a possible short lived hydrothermal near seafloor circulation accordingly dissolved carbon species in low temperature vent fluids can serve as geochemical proxies to distinguish the extent of compositional change in subsurface microbial habitats in the mef hydrothermal fluids and elsewhere part of this reflects conditions unique to the near surface diffuse flow environment but the flux of components from deeper seated hydrothermal processes is important as well in comparison with earlier measurements 1999 and 2000 of high temperature vent fluids at mef the 2005 data set exhibits lower li cl ratios and h2 aq concentrations indicative of higher fluid rock mass ratios and more oxidizing and or lower temperature conditions respectively

habitability of europa s crust the role of tidal tectonic processes jupiter s satellite europa has been identified as one of the most likely sites for life in the solar system the tidal tectonic processes that appear to have governed europa s geology seem to require interaction with an ocean under only a very thin crust providing a variety of evolving environmental niches the mutually dependent relationship between orbital evolution and tidal processes in turn controls europa s rotation heating and stress surface lineaments are correlated with global stress patterns demonstrating that they form by crustal cracking but only if a substantial ocean is present to give adequate tidal amplitude tidal driving of strike slip faulting indicates that cracks penetrate to a fluid layer which is possible only with a very thin ice crust the characteristic ridge sets that cover tectonic terrain are likely built by tidal pumping of fluid and slush to the surface on a daily basis widespread tectonic dilation creates new surface as material rises from below chaotic terrain has morphology and other characteristics indicative of melt through from below surface colorants correlate with locations such as along large scale ridge systems and around chaotic terrain where ocean water reached the surface this model implies that as a result of tides liquid water regularly bathed crustal cracks and surfaces with heat and whatever nutrients are included in the oceanic chemistry creating a variety of habitable environments the processes were recent and thus most likely continue today longer term evolution of environmental conditions provided the need for adaptation and opportunity for evolution

the andean orogenic period and its major structures in central and northern peru the andean orogenic period began in the albian and consisted of at least 6 discrete compressional phases which are albian to late miocene in age deformation migrated progressively toward the amazonian foredeep the major andean structures in central and northern peru are related to the eocene incaic early miocene quechua 1 and late miocene quechua 3 phases the maranon thrust and fold belt mtfb which bounds the western cordillera to the ne rose in the eocene it was reactivated partly by the quechua 1 phase which also folded the altiplano farther e and possibly part of the eastern cordillera the sub andean thrust and fold belt stfb is related to the quechua 3 phase in both mtfb and stfb the imbricate thrusts merge at depth in large decollement faults gravity sliding does not play a dominant role for these faults have to be rooted westward in the pre andean basement the corresponding crustal shortening played a significant part in the creation of the sialic root of the andes

chemosynthetic microbialites in the devonian carbonate mounds of hamar laghdad anti atlas morocco abstract veins cutting across the devonian conical mounds of the hamar laghdad ridge eastern anti atlas morocco were the carriers of geofluids although not completely determined the composition of these fluids seems related to hydrothermal and seepage fluxes that were the primary contributors to the establishment of sub seafloor cryptic environments linking deeper fluid sources to the seafloor the optical and scanning electron microscope investigation of the laminated carbonate infill of these veins has revealed that they consist of stromatolite and oncolite like fabrics in the early and middle devonian mounds the laminated carbonates contain morphologies that are attributable to microbially mediated processes that in turn suggest a direct biological contribution to the deposition of the veins infill microbial evidence include rod shaped and cocci clusters embedded in amorphous membranes iron rich filaments organized as mat like and biodictyon filamentous aggregates or isolated filaments microbial clotted textures and peloids moreover the oncoids from the veins of the middle devonian hollard mound have been interpreted as the mineral replacement of crypto microorganisms as frutexites other microbial evidence are biominerals and include carbonate with spheroids and pyrite framboids in which typifying hydrocarbon seep environments the hollard mound which is the only one with a well documented origin by hydrocarbon seepage shares a number of geomicrobiological evidence with other early devonian mounds these evidence which are supplemented by 13 c depleted data suggest that the infill of the veins system in the hamar laghdad mounds would be the product of chemotrophic chemosynthetic microbial communities that are adapted to crypto habitats sustained by hydrocarbon and other components fluid fluxes

contemporary recent extension and compression in the central andes abstract although extension in the high andes vs compression in the lowlands has already been widely discussed in the literature for the first time we recognized both extensional and contractional structures that developed contemporaneously during late pliocene quaternary times in a wide area of the central andean chain about 90 000 km 2 where crustal earthquake data are missing this area comprises north eastern chile south western bolivia and north western argentina and extends from the puna plateau to the altiplano volcanic belt late pliocene quaternary folds with hinge lines trending nne ssw to n s are mostly located along the westernmost part of the volcanic belt and the eastern part of the western cordillera locally there are coeval reverse faults parallel to the folds which reach up to the surface particularly the miscanti ridge tolocha fault and la casualidad ridge may be the morphostructural expression of tens km long fault propagation folds which locally show topographic scarps hundreds of meters high north and east of the contractional structures we found evidence of late pliocene quaternary normal faults striking n s in the southern part of the study area and nw se in the northern part well developed grabens are present in the higher areas of the volcanic belt and in the transition zone with the puna plateau the surface rupture zones of normal fault swarms range 8 24 km in length with single fault strands up to 18 km long which are typical of tectonic structures the distribution in space and time of the studied contractional and extensional structures indicates that they originated in the same time period we thus address the challenging question regarding the possible origin of the stress sources by analysing possible causes such as volcanotectonics high topography orogeny collapse and gravitational spreading of the orogen in relation also with the role played by inherited structures we finally analyse the relations between the different structures and magma upwelling and the potential for seismic hazard

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

the inverse problem in microtectonics and the separation of tectonic phases abstract the algorithm of inverse problems is applied to three cases in microtectonics a collection of striations measured on pre existing faults may be processed automatically and interpreted in terms of tectonic stress a comparison is made between two different methods formerly proposed by the authors both methods are tested with field data the theoretical results agree with independent field criteria for a separation of tectonic phases

felsic igneous rocks within the 3 3 to 3 5 ga barberton greenstone belt high crustal level equivalents of the surrounding tonalite trondhjemite terrain emplaced during thrusting felsic igneous rocks within the circa 3 5 ga ophiolitic onverwacht group of the barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession these felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic ultramafic rocks of the onverwacht group rather the data indicate that these felsic units are high level equivalents of the widespread gneissic to isotropic trondhjemite tonalite plutons which surround and intrude the greenstone belt the felsic igneous rocks are predominantly shallow level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the onverwacht group by 40 50 ma the felsic intrusions were preferentially emplaced along thrust zones cutting the ophiolite basement and its overlying sedimentary cover structural and chemical data imply that the hydrated ophiolitic rocks of the barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting such structural imbrication of simatic crust can be best incorporated in a model advocating archean intraoceanic obduction associated with shallow angle subduction similar to that documented for the neotectonics of the western aleutians

modeling microbial reaction rates in a submarine hydrothermal vent chimney wall the fluids emanating from active submarine hydrothermal vent chimneys provide a window into subseafloor processes and through mixing with seawater are responsible for steep thermal and compositional gradients that provide the energetic basis for diverse biological communities although several models have been developed to better understand the dynamic interplay of seawater hydrothermal fluid minerals and microorganisms inside chimney walls none provide a fully integrated approach to quantifying the biogeochemistry of these hydrothermal systems in an effort to remedy this a fully coupled biogeochemical reaction transport model of a hydrothermal vent chimney has been developed that explicitly quantifies the rates of microbial catalysis while taking into account geochemical processes such as fluid flow solute transport and oxidation reduction reactions associated with fluid mixing as a function of temperature the metabolisms included in the reaction network are methanogenesis aerobic oxidation of hydrogen sulfide and methane and sulfate reduction by hydrogen and methane model results indicate that microbial catalysis is generally fastest in the hottest habitable portion of the vent chimney 77 102 c and methane and sulfide oxidation peak near the seawater side of the chimney the fastest metabolisms are aerobic oxidation of h2 and sulfide and reduction of sulfate by h2 with maximum rates of 140 900 and 800 pmol cm 3 d 1 respectively the maximum rate of hydrogenotrophic methanogenesis is just under 0 03 pmol cm 3 d 1 the slowest of the metabolisms considered due to thermodynamic inhibition there is no anaerobic oxidation of methane by sulfate aom these simulations are consistent with vent chimney metabolic activity inferred from phylogenetic data reported in the literature the model developed here provides a quantitative approach to describing the rates of biogeochemical transformations in hydrothermal systems and can be used to constrain the role of microbial activity in the deep subsurface

age erosion constraints on an early pleistocene paleosol in yukon canada with profiles of 10be and 26al evidence for a significant loess cover effect on cosmogenic nuclide production rates abstract wounded moose type paleosols developed on remnant deposits of late pliocene to early pleistocene pre reid cordilleran ice sheet cis glaciations in central yukon canada it is an important regional soil geomorphic marker at the boundary between early cis advances and the non glaciated regions of yukon and alaska yet at present its age is poorly constrained between the reid 0 2 ma and earliest 2 84 ma cis advances here we apply depth profiles of in situ produced cosmogenic 26al and 10be to obtain both a minimum exposure age 1 12 0 44 0 36 ma 2 and maximum erosion rate 1 1 0 9 0 5 m myr 1 for the wounded moose paleosol our results show that this soil formed under exceptionally stable conditions max erosion rate similar to polar bedrock erosion rates and that it pre dates the emergence of the 100 ka eccentricity climate cycle contrasting our results from single and joint nuclide depth profile models reveals a significant discrepancy between calculated and effective 10be and 26al production rates 40 65 of expected values we interpret this discrepancy as the result of intermittent loess cover with a time averaged depth between 60 and 110 cm which significantly reduced apparent exposure ages obtained from the single nuclide model the observation of such a significant loess cover effect on cosmogenic nuclide production has implications for exposure dating in glacial and periglacial environments a multi nuclide sampling strategy is required to quantify this effect

influence of subsurface biosphere on geochemical fluxes from diffuse hydrothermal fluids hydrothermal vents along mid ocean systems host unique highly productive biological communities based on microbial chemoautotrophy that thrive on the sulphur metals nitrogen and carbon emitted from the vents into the deep ocean geochemical studies of vents have centred on analyses of high temperature focused hydrothermal vents which exhibit very high flow rates and are generally considered too hot for microbial life geochemical fluxes and metabolic activity associated with habitable lower temperature diffuse fluids remain poorly constrained as a result little is known about the extent to which microbial communities particularly in the subsurface influence geochemical flux from more diffuse flows here we estimate the net flux of methane carbon dioxide and hydrogen from diffuse and focused hydrothermal vents along the juan de fuca ridge using an in situ mass spectrometer and flowmeter we show that geochemical flux from diffuse vents can equal or exceed that emanating from hot focused vents notably hydrogen concentrations in fluids emerging from diffuse vents are 50 to 80 lower than predicted we attribute the loss of hydrogen in diffuse vent fluids to microbial consumption in the subsurface and suggest that subsurface microbial communities can significantly influence hydrothermal geochemical fluxes to the deep ocean

stress field variations in the swiss alps and the northern alpine foreland derived from inversion of fault plane solutions 1 this study is devoted to a systematic analysis of the state of stress of the central european alps and northern alpine foreland in switzerland based on focal mechanisms of 138 earthquakes with magnitudes between 1 and 5 the most robust feature of the results is that the azimuth of the minimum compressive stress s3 is generally well constrained for all data subsets and always lies in the ne quadrant however within this quadrant the orientation of s3 changes systematically both along the structural strike of the alpine chain and across it the variation in stress along the mountain belt from ne to sw involves a progressive counterclockwise rotation of s3 and is most clear in the foreland where it amounts to 45 50 this pattern of rotation is compatible with the disturbance to the stress field expected from the indentation of the adriatic block into the central european plate possibly together with buoyancy forces arising from the strongly arcuate structure of the moho to the immediate west of our study area across the alps the variation in azimuth of s3 is defined by a progressive counterclockwise rotation of about 45 from the foreland in the north across the helvetic domain to the penninic nappes in the south and is accompanied by a change from a slight predominance of strike slip mechanisms in the foreland to a strong predominance of normal faulting in the high parts of the alps the observed rotation can be explained by the perturbation of the large scale regional stress by a local uniaxial deviatoric tension with a magnitude similar to that of the regional differential stress and with an orientation perpendicular to the strike of the alpine belt the tensile nature and orientation of this stress is consistent with the spreading stress expected from lateral density changes due to a crustal root beneath the alps

changes in the tectonic regime above a subduction zone of andean type the andes of peru and bolivia during the pliocene pleistocene this paper adresses the changes in the tectonic regime in the peruvian and bolivian andes that have occurred since the upper miocene when the present day elevation of the cordillera above sea level has been almost reached the stress patterns are deduced essentially from a field study of fault kinematics and a numerical inversion of the slip vector data measured on the fault planes the cuzco fault system in southern peru is chosen as an example to illustrate the methodology used in this region striations on both active and holocene faults are in agreement with a n s extension but faults affecting early pleistocene deposits exhibit two families of striations the younger results from the previous n s extension the older involving reverse motions results from either an e w or a n s compression faults affecting pliocene formations often show an oldest family of striations resulting from a ne sw or an e w trending extension thus three tectonic regimes are demonstrated which are also supported by regional unconformities and sedimentological data 1 a pliocene extensional regime 2 a lower pleistocene compressional regime and 3 a mid pleistocene present day extensional regime similar analyses conducted in the pacific and sub andean lowlands allow sketching of the successive pliocene pleistocene stress patterns in the central andes the quaternary and present day stress pattern is characterized by a n s extension in the high andes and in the pacific lowlands and by an e w compression in the sub andean lowlands and at the contact between the nazca and south american plates this stress pattern is interpreted at a large wavelength 100 km as an effect of compensated topography this model supposes that the vertical lithospheric stress zz increases with the topography the crustal thickness and the low density mantle beneath and that the lithospheric maximum compressional horizontal stress hmax trending e w roughly parallel to the convergence is fairly constant on both edges of the andes the tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then the e w trending hmax is 2 and hmin trending n s is 3 allowing extension to occur in this direction the pliocene stress pattern was characterized by a ne sw or an e w trending extension in the high andes in the pacific lowlands and possibly in the sub andean lowlands this stress pattern was clearly different from the present day one because the e w trending stress was hmin this required a weak push or eventually tractional boundary forces acting on the south american lithosphere it is suggested that this might result from a strong slab pull due to a long steeply dipping slab which decreased the value of the xx stress transmitted to the overriding plate the early pleistocene state of stress was compressional since the elevation of the andes had not markedly decreased during this period this required an increase of the e w trending stress value this resulted from a strong coupling between the two lithospheric plates possibly due to a rupture of a long slab under its own weight other spatial changes in the stress pattern are related to the particular situation of the forearc to the subduction of the buoyant nazca ridge and to the different dips of the slab extension in the high andes is of small magnitude of the order of 1 during the last 1 2 m y in a few basins it may have attained 40 during the pliocene 5 3 m y

establishing a new era of submarine volcanic observatories cabling axial seamount and the endeavour segment of the juan de fuca ridge abstract at least 70 of the volcanism on earth occurs along the 65 000 km network of mid ocean ridge mor spreading centers within these dynamic environments the highest fluxes of heat chemicals and biological material from the lithosphere to the hydrosphere occur during volcanic eruptions however because underwater volcanoes are difficult and expensive to access researchers are rarely if ever in the right place at the right time to characterize these events therefore our knowledge is limited about the linkages among hydrothermal chemical and biological processes during seafloor formation and crustal evolution to make significant advancements in understanding the evolution of mor environments the united states and canada have invested in the first plate scale submarine cabled observatory linked through the global internet spanning the juan de fuca tectonic plate these two networks include 1700 km of cable and 14 subsea terminals that provide 8 10 kw power and 10 gbs communications to hundreds of instruments on the seafloor and throughout the overlying water column resulting in a 24 7 365 presence in the oceans data and imagery are available in real to near real time the initial experimental sites for monitoring volcanic processes include the mor volcanoes called axial seamount and the endeavour segment that are located on the juan de fuca ridge axial a hot spot influenced seamount is the most robust volcano along the ridge rising nearly 1400 m above the surrounding seafloor and it has erupted twice in the last 15 years in contrast the endeavour segment is characterized by more subdued topography with a well defined axial rift and it hosts one of the most intensely venting hydrothermal systems known a non eruptive spreading event lasting 6 years was documented at endeavour between 1999 and 2005 hydrothermal venting intensity chemistry and temperature as well as associated biological communities at both sites were significantly perturbed by the magmatic and intrusive events this paper presents the similarities and differences between the axial and endeavour volcanic systems and identifies reasons why they are ideal candidates for comparative studies the u s has made a 25 year commitment for sustained observations using the cabled infrastructure the highly expandable nature of submarine optical networking will allow for the future addition of novel experiments that utilize ever evolving advancements in computer sciences robotics genomics and sensor miniaturization comprehensive modeling of the myriad processes involved will continue to assimilate and integrate growing databases yielding a new understanding of integrated processes that create the seafloor in the global ocean basins

volcanic eruption of the mid ocean ridge along the east pacific rise crest at 9 45 52 n direct submersible observations of seafloor phenomena associated with an eruption event in april 1991 in april 1991 we witnessed from the submersible alvin a suite of previously undocumented seafloor phenomena accompanying an in progress eruption of the mid ocean ridge on the east pacific rise crest at 9 45 n 52 n the volume of the eruption could not be precisely determined although comparison of pre and post eruption seabeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were 10 m high effects of the eruption included 1 increased abundance and redistribution of hydrothermal vents disappearance of numerous vent communities and changes in characteristics of vent fauna and mineral deposits within the eruption area since december 1989 2 murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates 3 destruction of a vent community by lava flows mass wasting and possible hydrovolcanic explosion at a site known as tubeworm barbecue in the axial summit caldera asc at 9 50 6 n 4 near critical temperatures of hydrothermal vent fluids ranging up to 403 c 5 temporal variations over a 2 week interval in both temperatures and chemical isotopic compositions of hydrothermal fluids 6 unusual compositions of end member vent fluids with ph values ranging to a record low of 2 5 salinities ranging as low as 0 3 wt nacl one twelfth that of seawater and dissolved gases reaching high concentrations 65 mmol l for both co2 and h2s 7 venting at temperatures above 380 c of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices 8 disorganized venting of both high temperature fluids black and gray smoke and large volumes of cooler diffuse hydrothermal fluids directly from the basaltic seafloor rather than from hydrothermal mineral constructions 9 rapid and extensive growth of flocculent white bacterial mats species unknown on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid and 10 subseafloor downslope migration of magma normal to the ridge axis in a network of small scale 1 5 m diameter lava tubes and channels to distances at least 100 200 m outside the asc we suggest that in april 1991 intrusion of dikes in the eruption area to 200 m beneath the asc floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor rich hydrothermal fluids through the overlying rubbly cavernous lavas low salinities and gas rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid vapor phase separation boiling and phase segregation hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and or hydrovolcanic explosions these fissures and pits were lined with white mats of a unique fast growing bacteria that was the only life associated with the brand new vents the prolific bacteria which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids may thrive on high levels of gases in the vapor rich hydrothermal fluids initially escaping the hydrothermal system white bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic blizzard up to 50 m above the bottom the bacterial proliferation of april 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling and or by grazing as other organisms are re established in the biologically devastated area

micromorphology of periglacial sediments from the tayara site qikirtaq island nunavik canada abstract soil micromorphology provides an excellent resource for investigating palaeoeskimo site formation processes in the low arctic periglacial environment in this study micromorphology was used to answer questions concerning the composition structure origin and depositional agencies of the tayara site active layer s sediments as well as to distinguish between natural geogenic biopedogenic and cultural anthropogenic processes five groups of microfeatures were documented these include 1 biopedological features soil organic matter accumulation mesofaunal and fungal activities 2 transport inherited features related to depositional processes solifluction and stream sedimentation 3 frost induced microstructures and cryogenic features such as major post depositional changes ice segregation particle rearrangement frost shattering cryoturbation 4 bone preservation in the active layer at the microscopic level and 5 anthropogenic features related to palaeoeskimo activities the sediments from the three archaeological levels i ia and ii appear to have been very little influenced by anthropogenic factors cultural processes are micromorphologically observed in level ii with some evidence of feeding bone presence burning wood charcoal heated bone and possibly trampling however the effects of natural processes on the sediments are more pronounced than anthropogenic ones

diversity and distribution of subseafloor thermococcales populations in diffuse hydrothermal vents at an active deep sea volcano in the northeast pacific ocean received 30 august 2005 revised 28 july 2006 accepted 5 september 2006 published 20 december 2006 1 the presence diversity and distribution of a key group of subseafloor archaea the thermococcales was examined in multiple diffuse flow hydrothermal vents at axial seamount an active deep sea volcano located in the northeast pacific ocean a polymerase chain reaction pcr approach was used to determine if this group of subseafloor indicator organisms showed any phylogenetic distribution that may indicate distinct subseafloor communities at vents with different physical and chemical characteristics targeted primers for the thermococcales 16s rrna small subunit ribosomal rna gene and intergenic transcribed spacer its region were designed and applied to organisms filtered directly from a variety of diffuse flow vents thermococcales were amplified from 9 of 11 samples examined and it was determined that the its region is a better phylogenetic marker than the 16s rrna in defining consistent groups of closely related sequences results show a relationship between environmental clone distribution and source vent chemistry the most highly diluted vents with elevated iron and alkalinity contained a distinct group of thermococcales as defined by the its region suggesting separate subseafloor thermococcales populations at diffuse vents within the axial caldera

granitoid emplacement and deformation along a major crustal lineament the cordillera blanca peru abstract the cordillera blanca batholith and ignimbrites of the intermountain callejon de huaylas basin represent the last magmatic event in the andean cycle of central northern peru this most spectacular example of an extensional regime is characterised by acid magmatism spanning some 10 ma with both magma emplacement and subsequent deformation being fundamentally controlled by a ca 300 km lineament known as the cordillera blanca fault complex the western margin of the batholith has been strongly deformed by the cordillera blanca fault resulting in a series of ductile to brittle structures that increase with intensity away from the relatively undeformed core towards the margin the late stage brittle fabrics are coeval with extensional structures in the callejon de huaylas basin and relate to uplift and extension along the fault zone the more ductile fabrics as indicated by combined k ar and 40 ar 39 ar mineral dates formed earlier in the intrusion history and are consistent with emplacement into an active dextral strike slip shear zone the rapid uplift and high erosion rates seen from the late miocene onwards and the current state of stress in this region of thickened crust are due to gravitational instability body forces set up at high topographical levels and buoyancy forces resulting from the intrusion of young acid magmas at high crustal levels

expanding frontiers in deep subsurface microbiology abstract the subsurface biosphere on earth appears to be far more expansive and physiologically and phylogenetically complex than previously thought here several aspects of subsurface microbiology are discussed molecular and biogeochemical data as well as characteristics from new isolates suggest that ecosystems below deep sea hydrothermal vents are inhabited primarily by thermophilic archaea and bacteria the void spaces and conduits in basalt at mid ocean ridges and even more so at sediment covered hydrothermal vent sites represent promising hunting grounds for novel chemosynthetic archaea and bacteria as examples we highlight the subsurface microbial communities within the basalt flanks of the juan de fuca ridge off british columbia and the hydrothermal sediments at the guaymas basin in the gulf of california and at middle valley on the juan de fuca ridge in the deep continental subsurface microbial studies have primarily targeted aquifers in basaltic and granitic rock with an almost exclusive emphasis on h2 driven chemolithoautotrophy for example in the subsurface water from lidy hot springs idaho usa h2 consuming methanogenic archaea represent the most populous organisms comprising 95 of all cells besides h2 other potential sources of chemical energy reducing power are considered including abiotically synthesized organic matter and reduced sulfur compounds in addition the well established sequences of redox couples in microbial metabolism are investigated at elevated temperatures using a thermodynamic approach values of peo for 11 common reduction half reactions are tabulated at 25 50 100 and 150 c although many modes of metabolism may be employed by subsurface biota sulfate reduction appears to be one of the more ubiquitous strategies recent studies of sulfate reducing archaea and bacteria in marine and continental systems are reviewed

distribution of porosity in a section of upper oceanic crust exposed in the troodos ophiolite the porosity structure of a section of old oceanic crust was characterized by a detailed study of the volcanic sequence of the troodos ophiolite the distribution and abundance of porosity was compared to the amount of secondary minerals in order to calculate how porosity created by crustal accretion is modified by alteration as a section of crust ages eight sites with one dominant lithology and contrasting alteration histories were studied two types of porosity were quantified macroscopic macrofractures vesicles vugs interpillow zones and total macroscopic and laboratory measured microscopic porosity crustal accretion produced a vertically zoned porosity structure due to differences between pillows that formed during the main and waning stages of crustal construction mean initial macroscopic porosity ranges from 14 to 17 in uppermost pillows and from 6 to 10 in the underlying pillows and massive flows macroscopic porosity was reduced by 10 11 where alteration was most pervasive i e 250 m of the paleoseafloor beneath this zone pillow and flow outcrops showed decreases of 1 3 and 2 5 respectively the combined effects of volcanic morphology and alteration resulted in no systematic variation with depth in the final macroscopic porosity 4 8 the greatest decrease in macroscopic porosity was localized where abyssal hill topography or volcanic edifices and sedimentation rates facilitated the growth of the seafloor weathering zone which comprises 10 of the volcanic pile thus in modern ocean basins alteration would have the greatest effect on crustal porosity in areas of rough topography where crests of abyssal hills remain unsedimented for long periods of time porosity data derived from deep sea drilling project ocean drilling program cores and borehole logs show similar relationships which implies that the evolution of upper crustal porosity is controlled by the local environment and that the evolution of upper crustal porosity cannot be uniquely described by seismic data

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

time and mode of exhumation of the cordillera blanca batholith peruvian andes the cordillera blanca batholith 12 5 myr forms the highest peruvian summits and builds the footwall of the cordillera blanca normal fault cbnf even if several models have been proposed the processes driving both the exhumation of the cordillera blanca and extensional deformation along the cbnf are still debated here we quantify the emplacement depth and exhumation of the batholith of the northern peru arc from the late miocene to present based on a compilation of crystallization ages and new thermobarometry data in the cordillera blanca batholith we propose that the batholith was emplaced at a depth of 3 km in successive sills from 14 to 5 ma by contrast the younger rocks exposed at the surface were emplaced the deepest i e 6 km and are located close to the cbnf suggesting post 5 ma tilting furthermore a formal inversion of the thermochronologic data indicates an increase of the exhumation rates in the cordillera blanca during the quaternary the higher predicted exhumation rates correlate with areas of high relief both in the northern and central part of the cordillera blanca suggesting that quaternary valley carving by glaciations have a significant impact on the latest stage of the cordillera blanca exhumation 2 0 ma

quaternary paleosols tephra deposits and landscape history in south kamchatka russia abstract paleosols developed in various glacial and glaciofluvial deposits of potentially different ages in the southern kamchatka peninsula of russia are covered by multiple layers of volcanic ash and have been affected by permafrost weathering indices and iron fractionation data indicate slight differences in soil development between the paleosols on terminal moraines and associated terraces of two different ages in the central part of the region suggesting that the two depositional phases were separated by an interstadial rather than an interglacial according to the extent of weathering and the age of the oldest tephra overlying the paleosols ca 8000 bp both sets of deposits are of late pleistocene age glacial deposits on the west coast of the peninsula differ from those on the east in containing ice wedge casts also the paleosol here is less developed and is covered by lake sediments as well as tephra units the oldest tephra dated to ca 40 000 bp is present only in the west suggesting that the western glacial deposits date from an older middle pleistocene glaciation

the regional intraplate stress field in south america a compilation of lithospheric stress directions for continental south america and the inferred major patterns of the regional intraplate stress field are presented stress orientations are based primarily on earthquake focal mechanisms and quaternary fault slip inversion published in the literature four new focal mechanisms based on short period p wave modeling and selected centroid moment tensor solutions published by u s geological survey consistent with p wave first motions at south american and other world wide standard seismograph network stations are also included in the data base the observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the south american plate in the andean plateau altitudes greater than 3000 m n s extensional stresses predominate e w compressional stresses are observed in the sub andean and platform regions extending up to about 1000 km east of the andes the maximum horizontal stress shmax is uniformly oriented in the e w direction throughout western south america averages of the shmax direction at grid points spaced 2 5 o were taken as representing the regional field the e w direction of this regional field is not affected by the change of strike of the andean chain nor by the contact of a flat subducted slab underneath the regional shmax direction is oriented 15 o more clockwise than the direction of the nazca plate convergence the difference between the regional shmax orientations and the absolute plate motion may be only about 6 o this may indicate that contact forces with the nazca plate may not be the only major contributor to the intraplate stresses in western south america the eastern limit of this andean stress province seems to coincide with aseismic regions in the upper amazon basin and in the paraml basin in the central amazonian region seismicity and stress data suggest a seismic province with n s compressional stresses the origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle amazon basin in northeastern brazil seismicity is characterized by upper crustal strike slip earthquakes bordering the potiguar marginal basin a model is proposed for this region in which the stress field is the result of a superposition of regional e w compressional stresses and local extensional stresses oriented perpendicular to the continental margin possibly related to density contrasts and sediment loading in the continental shelf

seismicity fault plane solutions depth of faulting and active tectonics of the andes of peru ecuador and southern colombia the intracontinental seismicity of the andes of peru ecuador and southern colombia is concentrated along the easternmost flank of the cordillera beneath the western margin of the sub andes the focal depths and fault plane solutions of the largest events were constrained by comparing the observed long period p waves with synthetic waveforms in general the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest southeast and reflect crustal shortening perpendicular to the range probably in response to the subduction of the nazca plate to the west earthquakes in the sub andes occur at depths of between 8 and 38 km indicating that much of the crust deforms in a brittle manner the seismicity seems to reflect antithetic underthrusting of the brazilian shield beneath the eastern margin of the andes however the earthquakes are too deep and their nodal planes are too steep to be associated with the thin skinned decollement of the sedimentary cover instead they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub andes hence the earthquakes on the east side of the andes may reflect the style of deformation in the hinterland of fold and thrust belts such as in the canadian rockies while much of the decollement farther east occurs aseismically in contrast normal faulting occurs in the high andes on planes parallel to the strike of the mountain belt thus there is a delicate balance between the compressive stress applied to the andes in the direction of subduction which causes thrust faulting in the sub andes and the gravitational body forces acting on the topographically higher parts and the crustal root of the andes which may cause normal faulting at high elevation

floodplain palaeosols of the cenomanian dunvegan formation alberta and british columbia canada micromorphology pedogenic processes and palaeoenvironmental implications

deep sea smokers windows to a subsurface biosphere since the discovery of hyperthermophilic microbial activity in hydrothermal fluids recovered from smoker vents on the east pacific rise the widely accepted upper temperature limit for life based on pure culture data has risen from below the boiling point of water at atmospheric pressure to approximately 115 degrees c many microbiologists seem willing to speculate that the maximum may be closer to 150 degrees c we have postulated not only higher temperatures than these under deep sea hydrostatic pressures but also the existence of a biosphere subsurface to accessible seafloor vents new geochemical information from the endeavour segment of the juan de fuca ridge indicative of subsurface organic material caused us to re examine both the literature on hyperthermophilic microorganisms cultured from deep sea smoker environments and recent results of microbial sampling efforts at actively discharging smokers on the endeavour segment here we offer the case for a subsurface biosphere based on an interdisciplinary view of microbial and geochemical analyses of endeavour smoker fluids a case in keeping with rapidly evolving geophysical understanding of organic stability under deep sea hydrothermal conditions

deformation induced microstructures in soils affected by mass movements the soil deformation produced by mass movements gives rise to specific microscopic features the study of some markers of deformation such as the orientation of rigid clasts and the morphology of fine grained intercalations indicates that in most cases mass movements can be related to simple shear the development of these features varies according mainly to the type of mass movement solifluction debris flow or earth slide different factors play a significant role in their formation intrinsic soil characteristics such as the grain size of the matrix water content and matrix element viscosity ratios and external factors such as the intensity and homogeneity of deformation a careful study of the microscopic features leads to a better understanding of the dynamics of mass deposits