tertiary extensional faulting and evolving ductile brittle transition zones in the northern snake range and vicinity new insights from seismic data new geologic mapping and high resolution seismic data from east central nevada shed light on the evolution of the northern snake range decollement nsrd and its relation to the present basins and ranges we suggest that the nsrd originated in oligocene time as a flat 20 km wide 7 km deep zone of decoupling between localized ductile stretching below and more widely distributed high angle normal faulting above younger east dipping normal faults cut the western part of the nsrd forming the tilted fault blocks and highly asymmetric grabens that constitute the duck creek range duck creek valley and schell creek range spring valley and folded the nsrd to its present antiformal geometry these youngest faults flatten into pervasive subhorizontal reflectors at a depth of 10 to 15 km that we interpret as the youngest ductile brittle transition

effect of lithospheric thickness on the formation of high and low angle normal faults it is accepted that extension of a homogeneous brittle layer should produce high angle normal faults the rotation of the upper parts of such a fault to a low dip requires fault offset that is greater than the local lithospheric thickness new model calculations indicate that a fault cutting lithosphere thicker than 10 20 km can build up only a few kilometres horizontal throw before a new fault replaces it such a small offset leaves the fault little rotated from its initial high dip angle for thinner lithosphere the fault offset may be much larger leading to rotation of the upper inactive parts of a high angle fault to a low dip angle in continents this can happen where thin upper crustal lithosphere is decoupled from mantle lithosphere by weak lower crust the calculations suggest that normal faults begin with high dip angles and only in areas of very thin lithosphere can they be offset enough to produce low angle fault structures this prediction is consistent with the occurrence of low angle faults in areas with higher than normal heat flow

reconstruction of primary productivity from the barium contents in surface sediments of the south atlantic ocean abstract the aim of the present study is an evaluation of the applicability of biogenic barium as a proxy for productivity for this purpose 190 surface sediment samples from the south atlantic ocean were analysed for their barium and aluminium concentrations biogenic barium is estimated by subtracting the calculated terrigenous barium obtained from the terrigenous ba al ratio and the amount of al in the sample from the total ba content in the sample based on the accumulation rates of biogenic barium export production is estimated using three different algorithms proposed by paleoceanography 7 1992 163 global biogeochem cycles 9 1995 289 geomar report 38 1995 105 primary productivity was calculated from these different export productions and compared with measurements of recent primary productivity in the overlying surface waters only the primary productions calculated on the basis of the algorithm of paleoceanography 7 1992 163 yield productivity values comparable to those existing in ocean surface waters this study further reveals that it is not sufficient to use a constant generally applicable organic carbon biogenic barium ratio as is postulated by global biogeochem cycles 9 1995 289 this ratio has to be assessed regionally for the sediments of the cape basin in the eastern south atlantic ocean a new algorithm is developed which gives plausible primary productivities for the overlying surface waters

research focus significance of large displacement low angle normal faults low angle normal faults lanfs or detachment faults dip 30 are a major class of faults that became widely accepted by earth scientists only in the late twentieth century continental lanfs were mapped for nearly 100 yr e g ransome et al 1910 1 but typically were interpreted as odd

crustal structure of the northwestern basin and range province and its transition to unextended volcanic plateaus 1 the northwestern margin of the basin and range province is characterized by a transition from low magnitude 20 extension in northwestern nevada to relatively unextended volcanic plateaus in northeastern california seismic velocity and potential field modeling provides new control on the mesozoic to present tectonic evolution of this poorly understood portion of the u s cordillera we document 20 crustal thinning associated with basin and range extension from a crustal thickness of 37 km under northeastern california to 31 km under northwestern nevada that is consistent with the amount of extension recorded in the upper crust in northwestern nevada suggesting the crustal response to extension was relatively homogeneous over the entire crustal column our modeling also shows a well defined 80 km wide zone of unusually low upper crustal velocities 5 9 6 1 km s that coincide with the surface location of sparse cretaceous granites locating the elusive northern extension of the sierra nevada batholith through northwestern nevada for the first time in the subsurface combining geological and geophysical data we reconstruct the late cretaceous to present crustal evolution of this region documenting an interplay between magmatic addition to the crust erosional exhumation sedimentation and extension that has reversed the direction of crustal thinning from a west facing continental margin to an east facing interior basin margin over this time interval finally we find no evidence in northwestern nevada for unusually thick crust 40 km prior to basin and range extension

northwest basin and range tectonic deformation observed with the global positioning system 1999 2003 1 we use geodetic velocities obtained with the global positioning system gps to quantify tectonic deformation of the northwest basin and range province of the western united states the results are based on gps data collected in 1999 and 2003 across five new quasi linear networks in northern nevada northeast california and southeast oregon the velocities show 3 mm yr westward movement of northern nevada with respect to stable north america west of longitude 119 w the velocities increase and turn northwest parallel to sierra nevada great valley microplate motion and similar to velocities previously obtained to the south the observations are explained by a kinematic model with three domains that rotate around euler poles in eastern oregon and western idaho northeast california experiences internal dextral shear deformation 11 2 3 6 nstrain yr subparallel to pacific north america motion relative motions of the domains imply 2 5 mm yr approximately east west extension in northwest nevada and 1 4 mm yr approximately north south contraction near the california oregon border the northward decreasing approximately east west extension in northwest nevada is consistent with the northern termination of basin and range deformation faulting and characteristic topography no significant extension is detected in the oregon basin and range the oregon cascade arc moves north at 3 5 mm yr and is possibly influenced by the approximately eastward motion of the juan de fuca plate these results disagree with secular northwest trenchward motion of the oregon forearc inferred from paleomagnetic rotations south of latitude 43 however trenchward motion exists and is consistent with block rotations approximately east west basin and range extension and northwest sierra nevada translation

the continental margin of central east greenland in relation to north atlantic plate tectonic evolution the east greenland continental margin is inferred to have formed in three stages during the tertiary evolution of the adjacent north atlantic ocean the east greenland basaltic volcanism occurred over a period of 2 3 ma which was just prior to the probable date of onset of seafloor spreading in the early eocene shortly before magnetic anomaly 24b 52 ma the associated coastal flexure is interpreted as marking the edge of a zone of stretched and thinned continental crust beneath the inner denmark strait shelf and jan mayen microcontinent this represents a short lived rifting phase just prior to break up according to the evolutionary model the initial break up produced the margins south of the faeroe transform fault 68 5 n and north of the jan mayen fracture zone 72 n but in the intervening region the microcontinent remained attached to east greenland after break up the outer shelf of denmark strait formed from anomalously thick oceanic crust complementary to the iceland faeroe ridge the blosseville kyst margin 68 5 n to 70 n was initiated after anomaly 20 time 42 ma as the jan mayen microcontinent started to separate from greenland with spreading on both sides the new split occupied the line of the pre existing coastal dyke swarm and flexure the liverpool land margin 70 n to 72 n was also initiated at this time but the main separation of the northern part of the microcontinent only commenced after anomaly 7 time 26 ma when a single spreading axis was relocated west of the microcontinent it is suggested that the blosseville lavas subsided during deposition by loading and slight crustal stretching doming occurred near kangerdlugssuaq at the time of break up otherwise the topography of the lavas remained subdued until the relocation of the spreading axis at 26 ma after which the main regional uplift took place

active transtensional boundary zone between the western great basin and sierra nevada block western u s cordillera the walker lane and northern part of the eastern california shear zone form a boundary zone accommodating differential motion between the sierra nevada and western great basin within the boundary zone global positioning system velocities show a westward increase from 2 3 mm yr in the central great basin to 14 mm yr in the sierra nevada and a clockwise rotation from west northwest to northwest in the same region incremental extensional strain axes recorded by earthquake focal mechanisms and fault slip inversion show an east to west counterclockwise rotation of 50 from parallel with the velocity field in the central great basin to nearly orthogonal to the velocity field along the eastern flank of the sierra nevada unlike plane strain deformation within the central great basin the progressive deviation between the orientation of extension axes and the velocity field in the boundary zone is a product of nonplane strain the boundary zone records active constriction formed in conditions varying from wrench dominated to extension dominated transtension

identification of blind geothermal resources in surprise valley ca using publicly available groundwater well water quality data abstract geothermal resource exploration is generally limited to areas with surface expressions of thermal activity fumaroles and hot springs or relies on expensive geophysical exploration techniques in this study the hidden subsurface distribution of geothermal fluids has been identified using a free and publicly available water quality dataset from agricultural and domestic water wells in surprise valley northeastern california thermally evolved waters in surprise valley have element ratios that vary in response to ca carbonate and mg silicate mineral precipitation and have elevated total dissolved solids tds the arid climate in surprise valley leads to surface water evaporation in a closed basin producing high tds na cl co3 so4 brines in three ephemeral alkali lakes and in shallow groundwater under elevated soil co2 conditions evaporated fluids in surprise valley follow a chemical divide that leads to ca carbonate and mg silicate mineral precipitation plots of dissolved element ratios can be used to distinguish groundwater affected by evaporation from water affected by thermal water rock interaction however it is challenging to select components for plotting that best illustrate different fluid evolution mechanisms here we use a principal component analysis of centered log ratio transformed data coupled with geochemical models of fluid evaporation and thermal mixing pathways to identify components to plot that distinguish between groundwater samples influenced by evaporation from those influenced by thermal processes we find that groundwater samples with a thermal signature come from wells that define a coherent linear geographical distribution that closely matches the location of known and inferred faults modification of the general approach employed here provides promise for identifying blind geothermal resources in other locations by applying low cost geochemical modeling and statistical techniques to areas where large groundwater quality geochemical datasets are available

geometry of cenozoic extensional faulting dixie valley nevada precise definition of geometric relationships between individual basins and ranges may help to reveal the mechanical processes of basin and range cenozoic extensional faulting at depth previous studies have attempted to identify simple horsts and grabens tilted crustal blocks with planar faulting or tilted crustal blocks with listric faulting in the shallow crust normal faults defining these crustal blocks may root 1 individually in the ductile lower crust 2 in regional or local low angle detachment faults or 3 in igneous intrusions or decoupling surfaces produced by the intrusions the present study in dixie valley west central nevada makes use of a seismic reflection survey gravity models seismograms from earthquakes occurring on december 16 1954 and geometrical block models these data show a structurally asymmetric basin bounded by a single zone of faulting on the northwest and by a downbowed and step faulted floor to the southeast the northwest bounding fault is moderately dipping 50 and planar to a depth of 3 km the southeast boundary is step faulted and altogether the faults indicate an extension of 20 across the valley at the rate of 0 38 mm y for the last 8 my synthetic earthquake seismograms confirm a focal depth of 15 km and fault dip of 62 for the fairview peak earthquake and suggest that the focal depth of the dixie valley earthquake was also 15 km instead of the previously reported 40 km local microearthquakes cluster around 10 15 km the geometrical block models indicate that crustal horst graben faulting and planar high angle normal faults rooted in a low angle detachment surface do not readily account for development of the subsidiary step faults found in dixie valley extension of the crust by intrusion may develop high angle faults and with further intrusion may develop the subsidiary faults and produce a complex sagged asymmetric graben like dixie valley

rift structure evolution and earthquakes in the gulf of corinth from reflection seismic images abstract continental extension is forming the gulf of corinth across the strike of earlier alpine evolution here we present the first deep reflection sections with pre stack depth migration processing across the deep basin of the corinth active rift which image structures unpredicted by current models resolving the infill as a pile of layers that are broken by faults allows one to follow their subsidence and deformation history variation among the profiles suggests that southern normal faults control the rift in a time progression from the east towards its western tip on the central derveni itea transect a 3 km widening of the basin accrued since the initiation of this control that is marked by an unconformity between the two main sedimentary units this is estimated to have occurred 0 5 0 6 myr ago assuming the glacio eustatic sea level changes have controlled the stratigraphy of sediments deposited as a succession of layers on the subsiding hangingwall as they did on the southern footwall in forming the famous flight of marine terraces of corinth a roll over anticline and crestal collapse graben are diagnostic of the control by a normal fault of dip varying with depth the deeper low angle part of this bi planar fault is indeed imaged as a reflector in the basement the occurrence of the collapse with a breakaway at the steep southern basin bounding fault of the hangingwall slab can be estimated 0 12 0 2 myr ago with a marked increase in extension rate that brought it to its present fastest value over 10 mm yr the low angle part of the active fault might also have controlled earlier evolution upslope and in the basin when compared with inferences from earthquake studies this low angle active fault may not appear to be seismogenic but may participate to the seismic cycle possibly in a conditionally stable regime active faults seen as sea bottom scarps merely accommodate deformation of its subsiding hangingwall the footwall of the low angle faults which current seismicity shows to be in extension appears then as being pulled out from beneath the rift in a motion towards the rolling back slab that causes the hellenic subduction retreat

geophysical confirmation of low angle normal slip on the historically active dixie valley fault nevada the december 16 1954 dixie valley earthquake ms 6 8 followed the nearby fairview peak earthquake ms 7 2 by 4 min 20 s waveforms from the fairview peak event contaminate those from the dixie valley event making accurate fault plane solutions impossible a recent geologic study of surface rupture characteristics in southern dixie valley suggests that the dixie valley fault is low angle 30 along a significant portion of the 1954 rupture to extend these observations into the subsurface we conducted a seismic reflection and gravity experiment our results show that a portion of the dixie valley ruptures occurred along a fault dipping 25 to 30 as such the dixie valley event may represent the first large low angle normal earthquake on land recorded historically our high resolution seismic reflection profile images the rupture plane from 5 to 50 m depth medium resolution reflections as well as refraction velocities show a smoothly dipping fault plane from 50 to 500 m depth stratigraphic truncations and rollovers in the hanging wall show a slightly listric fault to 2 km depth gravity profiles conservatively constrain maximum basin depth and define overall geometry extension along the low angle section may have occurred in two phases during the cenozoic current fault motion postdates a 13 to 15 ma basalt imaged in the hanging wall and inherits from a fault formed during an earlier extensional pulse concentrated at 24 2 to 24 4 ma the earlier extension suggests extraordinary slip rates as high as 18 mm yr resulting in the formation of the low angle fault break sections of the dixie valley fault where there is no evidence for current low angle slip correlate well with areas where no pre 15 ma slip has been documented

early jurassic deformation in the pine forest range northwest nevada and implications for cordilleran tectonics triassic and jurassic rocks in the black rock desert of northwest nevada comprise part of the mesozoic volcanic arc successions found throughout the westernmost u s cordillera detailed structural studies in the pine forest range located in the western black rock desert provide new insight into the jurassic structural history of this part of the mesozoic arc system regional jurassic deformation and metamorphism in the pine forest range affected a thick 10 5 km section of lower paleozoic through triassic strata that were tilted after deformation and are now exposed in approximately cross sectional view at high stratigraphic levels deformation produced a slaty cleavage and relatively open folds both of which are concentrated in rheologically incompetent rock types and was accompanied by metamorphism at low grade at deeper stratigraphic levels the regional foliation intensifies and becomes more pervasive folds tighten and metamorphic grade increases through greenschist into amphibolite grade at the deepest stratigraphic levels the rocks are amphibolite grade l s tectonites with a pronounced mylonitic fabric the 2 km thick zone of mylonites is interpreted as a ductile shear zone that formed when the stratigraphic section was still upright whereas the less deformed younger rocks are interpreted to occupy the upper plate of the shear zone and to record a strain gradient away from the shear zone a variety of structural and regional relations indicate that the shear zone is a ductile thrust fault along which younger rocks moved to the northwest in present day coordinates with respect to older rocks the timing of regional jurassic deformation and metamorphism is constrained by relations in and around two plutons dated by the u pb zircon method the 201 1 ma big creek pluton was intruded as a sill like body along the upper levels of the shear zone and is associated with a variety of features indicating syntectonic intrusion including internal textural features in plutonic rocks and metamorphic changes in wall rocks with proximity to the pluton the 185 1 ma theodore pluton which intrudes across part of the shear zone has a thermal aureole which statically overprints the shear zone fabric deformation was thus ongoing at 201 ma and over by 185 ma the early jurassic age of regional shortening deformation in the pine forest range contrasts with the middle to late jurassic 170 150 ma age of regional shortening documented in many other parts of the mesozoic arc system including the klamath mountains sierra nevada and mojave desert regions of california it also contrasts with the record of early jurassic extensional tectonism documented in the sierra nevada mojave desert and western arizona segments of the mesozoic arc system these conflicting relations indicate a complex tectonic evolution for the jurassic u s cordillera plate margin and lend support to the idea that disparate parts of this plate margin were juxtaposed along younger strike slip faults

seismic reflection evidence for seismogenic low angle faulting in southeastern arizona focal mechanisms for large m 6 earthquakes in extensional terranes suggest that seismogenic normal faults have dips that range from approximately 30 degree to approximately 70 degree geologic relations suggest that low angle faults have accommodated large scale upper crustal extension these disparate observations are often reconciled by arguments that low angle faults move aseismically or rotate to low angles from initially high angles seismic reflection data from the tucson basin in southeast arizona image a low angle normal fault the santa rita fault that crops out along the trend of late quaternary fault scarps caused by large magnitude m approximately 6 7 7 6 earthquakes velocity independent dip analysis from shot records of the santa rita fault indicates that it has a true dip of approximately 20 degree to a depth of at least 6km this observation suggests that low angle extensional faults indeed may be seismogenic and that actual mechanisms for accommodation of upper crustal extension depend on local conditions of stress and preexisting geologic structure

cenozoic tectonic evolution of the south ningxia region northeastern tibetan plateau inferred from new structural investigations and fault kinematic analyses abstract the northeastward growth process of the tibetan plateau has been a key issue to be long termly debated which is well documented by the south ningxia region at the northeasternmost periphery of the tibetan plateau due to its cenozoic convex to the northeast arcuate range and basin structures here new structural measurements and fault kinematic analysis together with new geochronological results delineate a two stage cenozoic tectonic evolution in the region providing the new structural evidences to decipher the northeastward growth process of the tibetan plateau the first stage is marked by widespread sedimentary basin formation during the oligocene to late miocene ca 30 10 5 ma under nw se extensional regime and subsequent basin inversion triggered by nw se compression in the late miocene ca 10 5 9 5 ma these events were probably associated mainly with the far field effect of the northwestward subduction of the pacific plate a little related to the plateau growth the second stage largely presents an intensive mountain building process and following re deformation since the late miocene ca 9 5 ma which was predominantly a result of the far field responses to the northeastward growth of the tibetan plateau and partially to the pacific subduction the second stage further comprises three alternating episodes of shortening and extension events the earliest episode of ne sw compression caused intensive crustal shortening in a long period from the late miocene to early pleistocene ca 9 5 1 8 ma leading to a significant mountain building process in this region a gentle ne sw extension subsequently dominated here in the late pleistocene ca 18 ka as manifested by the widespread appearance of paleo lakes along the nw striking boundary faults since the end of the late pleistocene ca 18 ka a youngest tectonic regime affected this region characteristic of a tectonic transpression causing evidently strike slip activities on the primary faults and the development of small scale pull apart basins along the haiyuan fault

winter dynamics on the northern portuguese shelf part 1 physical processes results of a program of observations carried out on the northern portuguese shelf and slope between july 1996 and june 1999 are used to characterise the main physical processes of the winter regime the data set includes time series of waves and currents at mid shelf 86 m depth and of winds atmospheric pressure and sea level at the coast as well as hydrographic and suspended particle matter spm observations carried during three multidisciplinary surveys the wave regime was found to be characterised by highly energetic conditions during the winter with storm events leading to waves of significant heights exceeding 5 m with mean periods of over 10 s prevailing southerly winds force a downwelling regime over the shelf with poleward flow at mid shelf 10 20 cm s and offshore transport in the bottom ekman layer episodic strong upwelling events can occur during this period leading to important changes in the near bottom stratification at mid shelf tidal currents at mid shelf are dominated by the contribution of the m2 tide 3 cm s and are polarised in the cross shelf direction reflecting the guiding effect of the porto canyon

vertical deformation associated with normal fault systems evolved over coseismic postseismic and multiseismic periods vertical deformation of extensional provinces varies significantly and in seemingly contradictory ways sparse but robust geodetic seismic and geologic observations in the basin and range province of the western united states indicate that immediately after an earthquake vertical change primarily occurs as subsidence of the normal fault hanging wall a few decades later a 100 km wide zone is symmetrically uplifted the preserved topography of long term rifting shows bent and tilted footwall flanks rising high above deep basins we develop finite element models subjected to extensional and gravitational forces to study time varying deformation associated with normal faulting we replicate observations with a model that has a weak upper mantle overlain by a stronger lower crust and a breakable elastic upper crust a 60 dipping normal fault cuts through the upper crust and extends through the lower crust to simulate an underlying shear zone stretching the model under gravity demonstrates that asymmetric slip via collapse of the hanging wall is a natural consequence of coseismic deformation focused flow in the upper mantle imposed by deformation of the lower crust localizes uplift under the footwall the breakable upper crust is a necessary model feature to replicate footwall bending over the observed width 10 km which is predicted to take place within 1 2 decades after each large earthquake thus the best preserved topographic signature of rifting is expected to occur early in the postseismic period the relatively stronger lower crust in our models is necessary to replicate broader postseismic uplift that is observed geodetically in subsequent decades

global variations of surface ocean productivity in low and mid latitudes influence on co2 reservoirs of the deep ocean and atmosphere during the last 21 000 years based on detailed reconstructions of global distribution patterns both paleoproductivity and the benthic 13c record of co2 which is dissolved in the deep ocean strongly differed between the last glacial maximum and the holocene with the onset of termination i about 15 000 years ago the new export production of low and mid latitude upwelling cells started to decline by more than 2 4 gt carbon year this reduction is regarded as a main factor leading to both the simultaneous rise in atmospheric co2 as recorded in ice cores and with a slight delay of more than 1000 years to a large scale gradual co2 depletion of the deep ocean by about 650 gt c this estimate is based on an average increase in benthic 13c by 0 4 0 5 the decrease in new production also matches a clear 13c depletion of organic matter possibly recording an end of extreme nutrient utilization in upwelling cells as shown by sarnthein et al 1987 the productivity reversal appears to be triggered by a rapid reduction in the strength of meridional trades which in turn was linked via a shrinking extent of sea ice to a massive increase in high latitude insolation i e to orbital forcing as primary cause

contrasts in composition and evolution of tertiary cfbs between west and east greenland and their relations to the establishment of the icelandic mantle plume

structural controls on geothermal circulation in surprise valley california a re evaluation of the lake city fault zone faults and fractures play an important role in the circulation of geothermal fluids in the crust and the nature of that role varies according to structural setting and state of stress as a result detailed geologic and geophysical mapping that relates thermal springs to known structural features is essential to modeling geothermal systems published maps of surprise valley in northeastern california suggest that the lake city fault or lake city fault zone is a significant structural feature cutting obliquely across the basin and connecting thermal springs across the valley newly acquired geophysical data audio magnetotelluric gravity and magnetic combined with existing geochemical and geological data suggest otherwise we examine potential field profiles and resistivity models that cross the mapped lake city fault zone while there are numerous geophysical anomalies that suggest subsurface structures they mostly do not coincide with the mapped traces of the lake city fault zone nor do they show a consistent signature in gravity magnetics or resistivities that would suggest a through going fault that would promote connectivity through lateral fluid flow instead of a single continuous fault we propose the presence of a deformation zone associated with the growth of the range front surprise valley fault the implication for geothermal circulation is that this is a zone of enhanced porosity but lacks length wise connectivity that could conduct fluids across the valley thermal fluid circulation is most likely controlled primarily by interactions between n s trending normal faults

the northwestern margin of the basin and range province part 1 reflection profiling of the moderate angle 30 surprise valley fault abstract seismic reflection profiling demonstrates that the active significant offset surprise valley fault that marks the western boundary of the basin and range province in northernmost california dips at a moderate angle only 30 to 2 km depth a nearby seismic refraction wide angle reflection profile albeit of lower resolution shows the fault plane remains approximately planar to at least 7 km depth variably tilted volcanic strata of known ages within the surprise valley basin demonstrate that 7 km of normal slip occurred along the surprise valley fault within the past 8 4 myr at a time averaged slip rate of 1 2 mm yr and that the fault rotated from an initial dip of as much as 60 to its present dip of 30 the rotation of this isolated fault was not accomplished by domino style rotation as seen elsewhere in the basin and range province but rather by flexural rift shoulder uplift aided by mid crustal flow

two phase neogene extension in the northwestern basin and range recorded in a single thermochronology sample we use a combination of apatite 4 he 3 he u th he and fission track thermochronology to date slip on the surprise valley fault in northeastern california by analyzing a single sample from the warner range in the footwall of the fault this sample a granitic clast from a conglomerate yielded a fission track age of 11 6 2 8 ma and a u th he age of 3 02 0 52 ma geologic relationships indicate that this sample was buried to a depth of 3 3 km prior to exhumation during slip on the surprise valley fault fission track age and length data indicate that the sample was fully reset 120 c prior to exhumation which began sometime after 14 ma a single aliquot of nine apatite grains was step heated for 4 he 3 he analysis modeling of the resulting 4 he distribution indicates that cooling from 80 c to 20 c occurred between 3 and 1 ma interconsistent time temperature t t solutions to the combined 4 he 3 he u th he and fission track data require two distinct periods of cooling consistent with non continuous slip on the surprise valley fault early cooling and fault slip took place between 14 and 8 ma followed by more recent fault slip ca 3 ma this timing is consistent with both local geologic relationships and with the regional timing of faulting along the western margin of the basin and range province these data demonstrate the resolving power of combined fission track u th he and 4 he 3 he thermochronometric data to extract low temperature t t information from a single sample close to earth9s surface

the northwestern margin of the basin and range province part 2 structural setting of a developing basin from seismic and potential field data abstract surprise valley in northeastern california offers an ideal opportunity to examine the structural setting of a developing extensional basin due to its late miocene to recent activity in isolation from other major normal fault bound basins seismic velocity and potential field modeling help determine the nature of basin fill and identify intra basin faults based on a detailed gravity and magnetic profile we identify shallow subsurface basalt flows and several faults within the valley that may accommodate hundreds of meters of vertical offset possibly cutting and offsetting the 30 east dipping surprise valley fault that rotated during footwall tilting of the adjacent warner mountains some of these intra basin faults correspond with mapped quaternary fault scarps but others have no surface expression these faults may represent the currently active fault system within the basin if so they would indicate that basin development is transitioning away from the main range front normal fault to a new set of steep intra basin faults that are more favorable for accommodating regional transtensional strain

unusually low rates of slip on the santa rosa range fault zone northern nevada the santa rosa range fault zone srrfz is one of the most topographically prominent normal fault systems in the northern basin and range province of the western united states it has been assigned high rates of vertical slip by others and has been identified as a possible site of the future extension of the central nevada seismic belt cnsb we use detailed trench mapping and luminescence dating to estimate displacements and timing of the last several large magnitude paleoearthquakes on the southern part of the srrfz at a trench site near orovada nevada coseismic vertical displacements ranged from 1 to 2 8 m for each of the last four events luminescence ages provide time limits for the last three events of 125 155 ka 90 108 ka and 11 16 ka these data yield recurrence intervals of 17 65 k y and 74 97 k y and an elapsed time of 11 16 k y since the youngest event slip rate determinations at the orovada site are complicated by multiple fault strands but rates calculated from a variety of data are surprisingly low 0 01 0 16 mm yr given the topographic prominence of the santa rosa range a lack of compelling patterns in a comparison of paleoseismic parameters indicate that the srrfz is no more likely a location for a large magnitude earthquake than previously identified seismic gaps or along faults that lie directly north of the cnsb

228ra 226ra and 226ra ba ratios in the western mediterranean sea barite formation and transport in the water column 226ra 228ra and ba distributions as well as 228ra 226ra and 226ra ba ratios were measured in seawater suspended and sinking particles at the dyfamed station in the western mediterranean sea at different seasons of year 2003 in order to track the build up and fate of barite through time the study of the 228raex 226raex ratios raex ra activities corrected for the lithogenic ra of suspended particles suggests that baex baex ba concentrations corrected for the lithogenic ba mostly barite formation takes place not only in the upper 500 m of the water column but also deeper i e throughout the mesopelagic layer temporal changes in the 228raex 226raex ratios of sinking particles collected at 1000 m depth likely reflect changes in the relative proportion of barite originating from the upper water column with a high 228ra 226ra ratio and formed in the mesopelagic layer with a low 228ra 226ra ratio 228raex 226raex ratios measured in sinking particles collected in the 1000 m trap in april and may suggest that barite predominantly formed in the upper water column during that period while barite found outside the phytoplankton bloom period february and june appears to form deeper in the water column combining ratios of both the suspended and sinking particles provides information on aggregation disaggregation processes high 226raex baex ratios were also found in suspended particles collected in the upper 500 m of the water column because celestite is expected to be enriched in ra bernstein r e byrne r h and schijf j 1998 acantharians a missing link in the oceanic biogeochemistry of barium deep sea res ii 45 491 505 acantharian skeletons may contribute to these high ratios in shallow waters the formation of both acantharian skeletons and barite enriched in 226ra may thus contribute to the decrease in the dissolved 226ra activity and 226ra ba ratios of surface waters observed between february and june 2003 at the dyfamed station

vertical particle flux in the northeast atlantic ocean pomme experiment 1 in the framework of the programme ocean multidisciplinaire meso echelle pomme experiment a 1 5 year record february 2001 june 2002 of downward particle flux at 400 m and 1000 m was measured by sediment traps at four moorings located in the northeast atlantic between 39 43 n and 17 19 w thorium 230 was used to estimate sediment trap efficiency revealing values ranging from 18 5 to 55 the lowest trapping efficiency was observed for the trap having experienced the highest currents significant interannual variability between 2001 and 2002 was clearly linked to the differences observed in the mixed layer depth at some sites particulate organic carbon poc export was higher up to a factor of 1 6 during summer than during the spring event this could be related to the occurrence of short wind events that deepened the thermocline along with the presence of anticyclonic eddies yielding an input of new nutrients the average percentage of poc exported compared to the primary production of organic carbon in the surface waters ranged between 1 3 and 5 0 with higher export efficiency during the spring finally although the area was shown to present a relatively high mesoscale activity that might impact the geochemistry poc export was rather homogeneous over the pomme area 4 9 1 6 gc m 2 yr 1 were exported below 1000 m between february 2001 and february 2002 therefore a large fraction of the new production may be exported through convection and mode water circulation rather than by particle settling

a conceptual geochemical model of the geothermal system at surprise valley ca abstract characterizing the geothermal system at surprise valley sv northeastern california is important for determining the sustainability of the energy resource and mitigating hazards associated with hydrothermal eruptions that last occurred in 1951 previous geochemical studies of the area attempted to reconcile different hot spring compositions on the western and eastern sides of the valley using scenarios of dilution equilibration at low temperatures surface evaporation and differences in rock type along flow paths these models were primarily supported using classical geothermometry methods and generally assumed that fluids in the lake city mud volcano area on the western side of the valley best reflect the composition of a deep geothermal fluid in this contribution we address controls on hot spring compositions using a different suite of geochemical tools including optimized multicomponent geochemistry geot models hot spring fluid major and trace element measurements mineralogical observations and stable isotope measurements of hot spring fluids and precipitated carbonates we synthesize the results into a conceptual geochemical model of the surprise valley geothermal system and show that high temperature quartz na k na k ca classical geothermometers fail to predict maximum subsurface temperatures because fluids re equilibrated at progressively lower temperatures during outflow including in the lake city area we propose a model where hot spring fluids originate as a mixture between a deep thermal brine and modern meteoric fluids with a seasonally variable mixing ratio the deep brine has deuterium values at least 3 to 4 lighter than any known groundwater or high elevation snow previously measured in and adjacent to sv suggesting it was recharged during the pleistocene when meteoric fluids had lower deuterium values the deuterium values and compositional characteristics of the deep brine have only been identified in thermal springs and groundwater samples collected in proximity to structures that transmit thermal fluids suggesting the brine may be thermal in nature on the western side of the valley at the lake city mud volcano the deep brine meteoric water mixture subsequently boils in the shallow subsurface precipitates calcite and re equilibrates at about 130 c on the eastern side of the valley meteoric fluid mixes to a greater extent with the deep brine cools conductively without boiling and the composition is modified as dissolved elements are sequestered by secondary minerals that form along the cooling and outflow path at temperatures

moderate rates of late quaternary slip along the northwestern margin of the basin and range province surprise valley fault northeastern california 1 the 86 km long surprise valley normal fault forms part of the active northwestern margin of the basin and range province in northeastern california we use trench mapping and radiocarbon luminescence and tephra dating to estimate displacements and timing of the past five surface rupturing earthquakes on the central part of the fault near cedarville a bayesian oxcal analysis of timing constraints indicates earthquake times of 18 2 2 6 10 9 3 2 8 5 0 5 5 8 1 5 and 1 2 0 1 ka these data yield recurrence intervals of 7 3 4 1 2 5 3 2 2 7 1 6 and 4 5 1 5 ka and an elapsed time of 1 2 0 1 ka since the latest surface rupturing earthquake our best estimate of latest quaternary vertical slip rate is 0 6 0 1 mm a this late quaternary rate is remarkably similar to long term 8 14 ma minimum vertical slip rates 0 4 0 5 0 3 mm a calculated from recently acquired seismic reflection and chronologic and structural data in surprise valley and the adjacent warner mountains however our slip rate yields estimates of extension that are lower than recent campaign gps determinations by factors of 1 5 4 unless the fault has an unusually shallow 30 35 dip as suggested by recently acquired seismic reflection data coseismic displacements of 2 4 5 1 m documented in the trench and probable rupture lengths of 53 65 km indicate a history of latest quaternary earthquakes of m 6 8 7 3 on the central part of the surprise valley fault

geochronology structure and basin range tectonism of the warner range northeastern california the warner range of northeastern california is a north trending horst at the northwest margin of the great basin the 3 000 m thick bedrock section exposed in the southern part of the range is tertiary in age and it dips homoclinally 20 to 25 west southwest much of the section consists of volcanic rocks whose k ar ages range from about 32 m y near the base of the section to about 14 m y at the top rocks older than about 26 m y are principally andesitic lahars those between 26 and 17 m y are andesitic lava flows and rhyolitic welded ash flow tuffs and those younger than about 17 m y are principally basaltic and andesitic lava flows the homoclinal character of the warner section indicates that block faulting responsible for uplift and tilting of the range began after emplacement of the youngest exposed rocks drilling in the deeply alluviated graben adjacent to the eastern face of the range indicates that aggregate vertical offset across the range front fault is at least 3 600 m the difference in elevation between alluvium in the bottom of the deepest bore hole and the crest of the nearby part of the range thus vertical displacement has occurred at a minimum average rate of 26 cm 1 000 yr during the past 14 m y offset of alluvium along the eastern base of the range records the continuation of block faulting into holocene time the onset of block faulting somewhat less than 14 m y ago and the change in character of volcanic rocks from principally andesitic to principally basaltic in mid miocene time are similar to volcano tectonic events of a much broader region proposed by previous workers the warner range is remarkable for the nearly complete record of regional late cenozoic volcanic and tectonic events found within a single horst

advective excess ba transport as shown from sediment and trap geochemical signatures abstract we report the results of a geochemical study of sediment and trap material major and trace elements zr ba rare earth elements and th were analyzed on bulk sedimentary material collected along the ne atlantic margin our aim is to test the widespread use of ba barite as a proxy for paleoproductivity in a continental margin area this environment is of great interest because atmospheric oceanic exchanges are important in sediments the geochemical signatures remain close to an upper crust reference with flat shale normalized rare earth elements patterns and constant elementary ratios the calculated biogenic fraction of ba or excess ba 20 45 remains lower than the excess ba record in trap material 80 99 the evolution of the geochemical signature along the margin reflects variable dilution of a detrital post archean australian shale like component by a biogenic carbonaceous seawater derived component the trap material displays a wide range of variation in its trace element content e g ba 150 3000 ppm zr 2 100 ppm except for the abyssal site which is characterized by constant signature in the two other sites all of the trace element contents increase with water depth and present pronounced seasonal changes at each sampled water depth the amount of excess ba also increases in the deepest traps and its evolution throughout the year mimics the change of the other analyzed trace elements in contrast its relationships with particulate organic carbon are not obvious in term of fluxes two periods of enhanced excess ba fluxes are observed 1 excess ba flux increases with the detrital like elements like th especially during winter and 2 excess ba flux is enhanced without any change for the other trace elements during spring to explain the first case a supply through lateral advection is proposed such transient input of significant excess ba flux will have a great impact on the yearly averaged estimation of the export production indeed only the second case reflects a bloom in the biological productivity of the water column and must be taken into account in a mean calculation of the export production finally a normalization of the excess ba by detrital like element like th will help to discriminate between a real increase of the excess ba due to local productivity change high excess ba and high excess ba th ratio 10 000 and any input due to advection process high excess ba but low excess ba th ratio

reconstructing export production at the ne atlantic margin potential and limits of the ba proxy barium ba aluminium al and zirconium zr were measured in sediment trap material deployed at two margin settings of the ne atlantic the bay of biscaye at goban spur and the nw iberian margin the particulate organic carbon poc ba ratios of the trapped material in both margin environments are clearly higher compared to the open ocean although lateral advection of poc may partly explain these higher poc ba ratios for margin systems it is clear that the yield of authigenic particulate ba during organic matter degradation in the water column is lower in margin environments in order to assess export production in margin settings we optimised transfer functions based on trapped ba fluxes that were originally elaborated for open ocean settings calculations of export production based on trapped ba flux and poc ba ratio were compared with calculations based on trapped poc flux only export production based on ba flux show greater internal consistency amongst traps along the same mooring suggesting that this approach has advantages over the one based on poc flux only estimated export productions are of the same order of magnitude as estimates of new production but systematically fall short of the latter this systematic discrepancy needs further investigation

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

particulate barium fluxes in the ross sea we present particulate barium data collected using sequencing sediment traps in the ross sea region of the southern ocean data were collected in 1991 92 1994 95 and 1996 98 from two separate field programs samples were collected at 200 m depth and 75 m above the seafloor 450 765 m depth ba fluxes in the shallow traps at all sites are uniformly low and the barium to organic carbon ba corg ratios in these traps typically agree within a factor of two both spatially and temporally deeper trap fluxes are as much as an order of magnitude larger than shallow trap fluxes in addition the ba corg ratio in the deeper traps varies by more than an order of magnitude with the ratio decreasing with increasing rates of organic carbon flux increases in the deeper trap fluxes may result from horizontal transport of material from other regions of the ross sea resuspension of previously deposited sediment and increases in biogenic ba formation with depth data presented here and within the contemporary literature lead us to speculate that much of the biogenic ba may be formed in the water column at depths greater than 200 m also the factor of 10 range in the ba corg ratio in combination with the depth dependence of biogenic barium formation leads us to conclude that biogenic barium formation may depend on the rate at which particles sink independent of the causal mechanism the variations in this ratio imply that using an algorithm that relates barium export to carbon export may be imprudent in regions of highly variable carbon flux

nature of the crust beneath northwest basin and range province from teleseismic receiver function data received 30 july 2007 revised 12 april 2008 accepted 2 july 2008 published 14 october 2008 1 we utilized teleseismic receiver function techniques complemented by active source refraction seismology to study the crustal structure and continental rift processes responsible for the development of the northwest corner of the basin and range province in northwest nevada our passive seismic array of 28 short period stations spanning 70 km west to east and 5 broadband usarray transportable array stations that extended our aperture to 230 km provided data on crustal properties independent of the results of our active source refraction experiment combining data from the two experiments provides better constraints on moho depth vp vs ratio and the structure and composition of the crust beneath the northwestern basin and range province than possible from either experiment alone our new data indicate moho depths that vary from 29 5 on the east to 36 5 km in the west along the passive source array and vp vs ratios that systematically span a wide range from 1 68 to 1 83 velocity modeling of data collected from the 300 km wide angle refraction reflection survey provides comparable moho depths of 32 37 km under the same region decreasing vp vs is correlated with decreasing crustal thickness with vp vs ratio 1 80 in northeast california and vp vs 1 74 in the thinner crust of the basin and range province we associate the high vp vs of northeast california with mafic additions to the crust during formation of the modoc plateau in a region of accreted paleozoic island arc terrans similar to those exposed in the eastern klamaths and northwestern sierra nevada unusually low vp vs in conjunction with low absolute vp and vs is observed beneath the central part of our array and may be due to the roots of the sierra nevada batholith now dismembered by basin and range faults back azimuthal stacking of our receiver function data suggests a northwest oriented crustal anisotropy fast direction beneath this region parallel to the northwest to southeast oriented cenozoic extension experienced by the basin and range province and suggestive of a similar flow direction in the middle to lower crust

geologic context of geodetic data across a basin and range normal fault crescent valley nevada geodetic strain and late quaternary faulting in the basin and range province is distributed over a region much wider than historic seismicity which is localized near the margins of the province in the relatively aseismic interior both the magnitude and direction of geodetic strain may be inconsistent with the holocene faulting record we document the best example of such a disagreement across the ne striking 55 nw dipping crescent normal fault where a nw oriented 70 km geodetic baseline records contemporary shortening of 2 mm yr orthogonal to the fault trace in contrast our geomorphic paleoseismic and geochronologic analyses of the crescent fault suggest that a large extensional rupture occurred during the late holocene epoch an excavation across the fault at fourmile canyon reveals that the most recent event occurred at 2 8 0 1 ka with net vertical tectonic displacement of 4 6 0 4 m at this location corresponding to the release of 3 m of accumulated nw se extension measured alluvial scarp profiles suggest a minimum rupture length of 30 km along the range front for the event implying a moment magnitude m w of at least 6 6 no prior event occurred between 2 8 ka and 6 4 0 1 ka the 14 c calender age of strata near the base of the exposed section assuming typical slip rates for basin and range faults 0 3 mm yr these results imply that up to one third or 1 m of the extensional strain released in the previous earthquake could have reaccumulated across the fault since 2 8 ka however the contemporary shortening implies that the fault is unloading due to a transient process whose duration is limited to between 6 years geodetic recording time and 2 8 ka the age of the most recent event these results emphasize the importance of providing accurate geologic data on the timescale of the earthquake cycle in order to evaluate geodetic measurements

kinematics of the northern walker lane an incipient transform fault along the pacific north american plate boundary in the western great basin of north america a system of dextral faults accommodates 15 25 of the pacific north american plate motion the northern walker lane in northwest nevada and northeast california occupies the northern terminus of this system this young evolving part of the plate boundary offers insight into how strike slip fault systems develop and may reflect the birth of a transform fault a belt of overlapping left stepping dextral faults dominates the northern walker lane offset segments of a w trending oligocene paleovalley suggest 20 30 km of cumulative dextral slip beginning ca 9 3 ma the inferred long term slip rate of 2 10 mm yr is compatible with global positioning system observations of the current strain field we interpret the left stepping faults as macroscopic riedel shears developing above a nascent lithospheric scale transform fault the strike slip faults end in arrays of n striking normal faults suggesting that dextral shear diffuses into extension in the great basin coeval extension and dextral shear have induced slight counterclockwise fault block rotations which may ultimately rotate riedel shears toward the main shear zone at depth thus facilitating development of a throughgoing strike slip fault

on the oxidation and burial of organic carbon in sediments of the iberian margin and nazare canyon ne atlantic abstract as a contribution to the ec omex ii program sediment carbon and nitrogen budgets are presented for the iberian margin northeastern atlantic the budgets for degradable organic carbon and associated nitrogen were calculated from sediment and pore water properties using a steady state version of a numerical coupled diagenetic model omexdia data were collected throughout the major upwelling period along five transects four of which were located on the open margin and one positioned in a major submarine canyon the nazare canyon a comparison of in situ oxygen profiles measured with monocathodic microelectrodes and with clark type microelectrodes showed that monocathodic electrodes overestimate the oxygen concentration gradient near the sediment water interface this artifact probably results from the loss in sensitivity of the monocathodic microelectrode during profiling shipboard time course measurements with clark type electrodes demonstrated transient conditions upon sediment retrieval on deck and indicated enhanced rates of oxygen consumption in the surface sediment presumably as a result of lysis or exudation of oxidisable substrates by infauna as a result oxygen fluxes calculated from shipboard oxygen profiles overestimated in situ fluxes by up to a factor of 5 for water depths 1000 m the sediments from the canyon and from a depositional area on the shelf were enriched in organic carbon 3 4 5 wt relative to the open margin stations 0 5 2 wt and showed c n ratios exceeding redfield stoichiometry for marine organic matter indicating there was deposition of organic carbon of terrestrial origin in these areas the oxidation of organic carbon on the open margin declined from 11 gcm 2 y 1 on the shelf to 2 gcm 2 y 1 at 5000 m water depth and was dominated by aerobic oxidation the reactivity of the degradable organic carbon at the time of deposition was 1 on the shelf and declined to 1 offshore the burial of refractory organic carbon at the stations along the open margin transects also declined with increasing water depth from 5 gcm 2 y 1 on the shelf to 2 y 1 at 2000 m depth whereas the burial of particulate inorganic carbon declined from 20 gcm 2 y 1 to 2 y 1 a comparison of the estimated total organic carbon deposition and predicted delivery for the shelf suggest that 58 to 165 gcm 2 y 1 is oxidized in the water column laterally advected or focused into one of the canyons anaerobic oxidation denitrification and therefore total oxidation of organic carbon was enhanced within the canyon relative to the open margin total organic carbon oxidation decreased with water depth from 22 gcm 2 y 1 at the head of the canyon to 3 gcm 2 y 1 over its fan the reactivity of the organic carbon deposited in the canyon was lower than those of the shelf stations suggesting that the canyon is being enriched in older laterally advected organic matter the burial of refractory organic carbon in sediments from the nazare canyon was considerably higher than in the sediments from the open margin it also decreased with depth from 20 gcm 2 y 1 at 343 m to 2 5 gcm 2 y 1 at 4298 m water depth the burial of particulate inorganic carbon was slightly lower than that of refractory organic carbon the burial of refractory organic carbon and the deposition of degradable organic carbon were both positively correlated with the sedimentation rates for the iberian margin and indicated burial efficiencies were 0 6 to 48 a single trend for burial efficiency versus sedimentation rate for both the canyon and the open margin indicates that the sedimentation rate was the master variable for the geographical distribution of organic carbon oxidation and carbon preservation on the nw iberian margin

the geochemistry and tectonic significance of pre metamorphic minor intrusions of the central metasedimentary belt grenville province canada abstract more than 250 samples collected from dykes and sills intruded into the grenville supergroup have been analysed by xrf techniques for their major and trace element contents the ree contents of 43 of the samples have been determined by inaa using these data the rocks have been identified and placed in seven suites each characterized by its own trace element signature there are four tholeiitic suites the low zirconium suite which comprises a bimodal series of basalts and minor dacites the intermediate zirconium suite which is made up of basalts andesites dacites and rhyolites the high zirconium suite consisting of high iron basalts and rare highly fractionated tholeiites which comprise basalts characterized by high mg number high ni and cr contents and with strongly fractionated trace element patterns in addition a calc alkaline suite made up of basalts and andesites a transitional suite of andesites and trachyandesites and an ultra potassic suite of leucitites have been identified on chondrite normalized multi element plots all of the suites show negative nb and ti anomalies similar ranges of rock types with similar anomalies occur within volcanic arcs and back arc basins formed at collisional plate margins where the nb and ti anomalies are interpreted as subduction signatures and also in some continental flood basalt sequences in which the anomalies are commonly suggested to result from crustal contamination the geology of the central metasedimentary belt leads us to favour an interpretation in which the minor intrusions were emplaced in supracrustal rocks deposited upon attenuated continental crust in a back arc basin during a collisional orogeny

barium accumulation in the atlantic sector of the southern ocean results from 190 000 year records extensive investigations of sedimentary barium were performed in the southern south atlantic in order to assess the reliability of the barium signal in antarctic sediments as a proxy for paleoproductivity maximum accumulation rates of excess barium were calculated for the antarctic zone south of the polar front where silica accumulates at high rates the correspondence between barium and opal supports the applicability of barium as a proxy for productivity within the antarctic zone north of today s average sea ice maximum interglacial vertical rain rates of excess barium are high with a maximum occurring during the last deglaciation and early holocene and during oxygen isotope chronozone 5 5 during these periods the maximum silica accumulation was supposedly located south of the polar front glacial paleoproductivity instead was low within the antarctic zone north of the polar front significantly higher barium accumulation occurs during glacial times the vertical rain rates however are as high as in the glacial antarctic zone therefore there was no evidence for an increased productivity in the glacial southern ocean

the batbjerg complex east greenland a unique ultrapotassic caledonian intrusion batbjerg is an ultrapotassic igneous complex of caledonian age 445 ma at the western extremity of kangerdlugssuaq fiord east greenland the part of the arcuate complex not covered by kangerdlugssuaq glacier extends 7 km with a 1000 m relief the dominant intrusive member is alkaline pyroxenite grading to jacupirangite and ijolite but containing leucite as well as nepheline dominant mineralogy is clinopyroxene magnetite nepheline and leucite no feldspar is present except as fingerprint intergrowths of nepheline with potassium feldspar intergrowths of kalsilite with potassium feldspar are thought to be derived from leucite by subsolidus breakdown leucite has not previously been reported from a plutonic intrusion and kalsilite only once other intrusive members are olivine gabbro dikes of syenite to granite and diatremes of appinitic character cutting the basement gneiss outside the alkaline pyroxenite a partial screen of limestone is the cauldron subsided remnant of a former paleozoic sediment

the mlindi ring structure an example of an ultrapotassic pyroxenite to syenite differentiated complex the mlindi ring complex in southern malawi dated at 495 ma is intrusive in the proterozoic migmatite basement composed mainly of hornblende and biotite bearing paragneiss and dolomitic marble three main lithologic facies have been distinguished 1 syenite which crops out in the outer part of the complex as an oval shaped ring dike 7 km 9 km and as a rim of massive bodies cutting the mafic rocks of the central part 2 two types of diopside biotite gabbro syenogabbro and gabbronorite and 3 a banded phlogopite pyroxenite cumulate intruded by the diopside biotite gabbro

two phase westward encroachment of basin and range extension into the northern sierra nevada 1 structural geophysical and thermochronological data from the transition zone between the sierra nevada and the basin and range province at latitude 39 n suggest 100 km westward encroachment of basin and range extensional deformation since the middle miocene extension accommodated primarily by east dipping normal faults that bound west tilted range forming fault blocks varies in magnitude from 150 in the wassuk and singatse ranges to the east geological and apatite fission track data from exhumed upper crustal sections in the wassuk and singatse ranges point to rapid footwall cooling related to large magnitude extension starting at 14 15 ma farther to the west geological and thermochronological data indicate a younger period of extension in the previously unextended pine nut mountains the carson range and the tahoe truckee depression initiated between 10 ma and 3 ma and incipient post 0 5 ma faulting to the west of the tahoe truckee area these data imply the presence of an extensional breakaway zone between the singatse range and the pine nut mountains at 14 15 ma forming the boundary between the sierra nevada and basin and range at that time in addition fission track data imply a miocene preextensional geothermal gradient of 27 5 c km 1 in the central wassuk range and 20 5 c km 1 in the singatse range much higher than the estimated early tertiary gradient of 10 5 c km 1 for the sierra nevada batholith this might point to a significant increase in geothermal gradients coupled with a likely decrease in crustal strength enabling the initiation of extensional faulting apatite fission track geophysical and geological constraints across the sierra nevada basin and range transition zone indicate a two stage coupled structural and thermal westward encroachment of the basin and range province into the sierra nevada since the middle miocene

coral barium calcium record of mid holocene upwelling activity in new caledonia south west pacific abstract a variety of inimical factors i e sea surface temperature nutrient input nature of substrates were invoked previously to explain the retardation in reef settlement in new caledonia during the holocene in order to clarify the respective role played by temperature and nutrient level in the growth of fringing reefs along the southwest coast of the island trace element ratios i e sr ca and ba ca were used respectively as a paleothermometer and as a tracer of nutrient load from modern and mid holocene about 6000 calendar years old massive porites coral colonies the results obtained from the modern coral record indicate that near the outer barrier reef upwelling activity is controlled by southeast winds blowing in summer by contrast along the southwest coastline the fringing reefs are subjected to barium input mainly derived from weathering of barite rich rocks cropping out in the adjacent mainland comparison between the fossil and modern coral sr ca and ba ca records shed light on three different aspects of the mid holocene climate and coral reef evolution in new caledonia 1 the variability of paleo sea surface temperature sst seems to have been similar to the modern one although the island probably experienced sst about 1 c higher than at present 2 upwelling activity appears to have been greater than at present barium being supplied from the open sea to coastal zones by strong wind generated upwelling currents in relation to cooling events 3 the onset of reef development in the southwest areas of new caledonia was delayed due to the stronger upwelling of nutrient enriched waters during the early middle holocene the flowing of upwelled waters over the southwest coastal areas began at 8200 calendar years bp and ceased at the time when the outer barrier reef crest reached present day sea level about 5500 to 5000 calendar years ago

the northern walker lane refraction experiment pn arrivals and the northern sierra nevada root abstract in may 2002 we collected a new crustal refraction profile from battle mountain nevada across western nevada the reno area lake tahoe and the northern sierra nevada mountains to auburn ca mine blasts and earthquakes were recorded by 199 texan instruments extending across this more than 450 km long transect the use of large mine blasts and the ultra portable texan recorders kept the field costs of this profile to less than us 10 000 the seismic sources at the eastern end were mining blasts at barrick s goldstrike mine the goldstrike mine produced several ripple fired blasts using 8000 44 000 kg of anfo each a daily occurrence first arrivals from the larger goldstrike blasts are obvious to distances of 300 km in the raw records first arrivals from a quarry blast west of the survey near watsonville ca located by the northern california seismic network with a magnitude of 2 2 can be picked across the recording array to distances of 600 km the watsonville blast provides a western source nearly reversing the goldstrike blasts a small earthquake near bridgeport ca also produced pickable p wave arrivals across the transect providing fan shot data arrivals from m5 events in the mariana and kuril islands also appear in the records this refraction survey observes an unexpectedly deep crustal root under the northern sierra nevada range over 50 km in thickness and possibly centered west of the topographic crest pn delays of 4 6 s support this interpretation at battle mountain nevada we observe anomalously thin crust over a limited region perhaps only 150 km wide with a moho depth of 19 23 km pn crossover distances of less than 80 km support this anomaly which is surrounded by observations of more normal 30 km thick crust a 10 km thick and high velocity lower crustal pillow is an alternative hypothesis but unlikely due to the lack of volcanics west of battle mountain large mine and quarry blasts prove very effective crustal refraction sources when recorded with a dense receiver array even over distances exceeding 600 km new elastic synthetic seismogram modeling suggests that pn can be strong as a first arrival easing the modeling and interpretation of crustal refraction data fast eikonal computations of first arrival time can match pickable pn arrival times