neogene and quaternary history of vegetation climate and plant diversity in amazonia the neotropical amazonian and andean plant diversity developed mainly during the tertiary in amazonia miocene oral diversity seems considerably higher than today during the neogene tropical taxa entered newly created montane area and montane taxa entered the tropical lowlands the general decrease of temperature during the upper neogene and especially during the quaternary glacial periods may have caused considerable extinctions in the lowlands representation of pollen of apparently principally montane taxa podocarpus hedyosmum in miocene pliocene and quaternary sediments of amazonia and surroundings is still di cult to interpret in terms of temperature decrease at low elevation changes in precipitation may have profound impact on the composition of vegetation communities ilex and melastomataceae increase signi cantly in many glacial pollen records increase of weinmannia in amazonian pollen records seems the best indicator of downward migration of montane vegetation belts a temperature lowering at sea level of 4 5 13c during the last glacial maximum lgm seems reasonable it may have caused a downslope migration of some 700 m of the lower montane vegetation belt lower montane arboreal species may have been able to grow in higher elevation areas 500 m of amazonia increasing background pollen values of montane taxa in the area di erence between a cool and wet middle pleniglacial 60 28 ka bp and a cold and dry upper pleniglacial 28 14 ka bp thus including the lgm is evident in andean and amazonian records statements about environmental conditions of the ice age amazon should be speci ed chronologically the middle pleniglacial is a time of accumulation of uvial sediments the upper pleniglacial is a time of incision of the rivers in their sediments sedimentation started again in the lateglacial since ca 13 ka bp and the holocene when lake levels rose again based on simpli ed considerations of precipitation changes and evaporation we estimate that lgm rainfall may have been reduced by values of ca 45 10 amazonian and cordilleran lakes dried up dry rain forest was locally replaced by savanna savanna forest or cerrado type vegetation dry rain forest savanna forest and pure savanna was locally replaced by extensive semi desert dune formations lower rio branco area in present day central amazonia the present day centers of higher rainfall 2500 mm surrounded by areas of lower rainfall are refuge areas of the very wet rain forest and of the very high plant diversity 300 plant species per 0 1 ha and they should have been that equally or more during the dry climate intervals plant diversity of drier forests is in the order of 100 150 species per 0 1 ha both extinction and speciation in isolation under precipitation and temperature stress may have taken place in these refugia 2000 elsevier science ltd all rights reserved

amazonia during the last glacial abstract new data on the vegetational history and quaternary geology of amazonia permit an improved reconstruction of past environments in amazonia during the last glacial period although limited data from rondonia carajas and guyana show that in certain areas savanna type vegetation and savanna forest had replaced the rain forest during the late pleniglacial ca 22 000 13 000 yr b p the amazonian forest may have been split up into one major west amazonian and several other medium size forest areas this suggests a decline in rainfall of 500 to 1000 mm a reduction of 25 to 40 temperatures may also have been 2 to 6 c 4 2 c lower than today possibly substancially influencing amazonian vegetation during the humid middle pleniglacial 55 000 26 000 yr b p rivers carried a lot of water and sediment resulting in the deposition of lower terrace sediments with one dry interruption around 40 000 yr b p carajas and katira in carajas there is evidence of dry periods ocurring at about 40 000 yr b p and during the early pleniglacial ca 60 000 yr b p rivers carried little water and incised into the low terrace sediments during the dry late pleniglacial water levels rose during the late glacial 13 000 10 000 yr b p or at the beginning of the holocene 10 000 yr b p sedimentation in and of the present inundation valleys commenced after that

chronostratigraphy and correlation of the pliocene and quaternary of colombia abstract names for the chronostratigraphic units of the pliocene and quaternary of colombia are defined and formalized based on studies of outcrops and deep bore holes in the area of the high plain of bogota chronostratigraphic biostratigraphic and lithostratigraphic sequences have been correlated as far as possible a tentative correlation with the marine oxygen isotope stratigraphy is indicated pliocene chronostratigraphic units are provisionally characterized by abundance first appearance in the pollen record of specific floral elements mauritia zone tequendamian ca 5 5 4 2 ma hedyosmum zone facatativanian ca 4 2 3 5 ma myrica zone sisganian ca 3 5 3 1 ma borreria zone chocontanian ca 3 1 2 7 ma and engativanian ca 2 7 2 4 2 2 ma although precise boundaries cannot yet be established the engativanian is a transitional phase characterized by a continuous temperature decrease in successive cold episodes and may include the latest part of the pliocene and the earliest part of the quaternary quaternary chronostratigraphic units are precisely limited by levels of pronounced climatic change these boundaries are near to the first appearance abundance of juglans plantago funzanian ca 2 4 2 2 1 0 ma alnus fuchanian ca 1000 660 ka and cotanian ca 660 335 ka and quercus subanian ca 335 128 ka bacatanian ca 128 73 ka fuquenian ca 73 10 ka and the holocene the relationship with the officially defined position of the base of the quaternary at ca 1 80 ma is shown the last glaciation in colombia is named the cocuy glaciation

petrology of the axial ridge of the mariana trough backarc spreading center abstract the axial ridge of the mariana trough backarc basin between 17 40 n and 18 30 n rises as much as 1 km above the floor of a 10 15 km wide rift valley physiographic segmentation with minor ridge offsets and overlaps coincides with a petrologic segmentation seen in trace element and isotope chemistry analyses of 239 glass and 40 aphyric basalt samples collected with alvin and by dredging show that the axial ridge is formed largely of olivine hypersthene normative tholeiitic basalt about half of these are enriched in both lil elements and volatiles but are depleted in hfs elements like other rocks found throughout much of the mariana trough the lil enrichments distinguish these rocks from n morb even though nd and sr isotope ratios indicate that much of the crust formed from a source similar to that for n morb in addition to lil enriched basalt there is lil depleted basalts even more closely resembling n morb in major and trace elements as well as sr nd and pb isotopes both basalt varieties have higher al and lower total fe than morb at equivalent mg level mg ranges from relatively primitive e g mg 65 70 to more highly fractionated e g mg 45 50 highest parts of the axial ridge are capped by pinnacles with elongated pillows of basaltic andesite e g 52 56 sio 2 these are due to extreme fractional crystallization of basalts forming the axial ridge active hydrothermal vents with chimneys and mats of opaline silica barite sphalerite and lesser amounts of pyrite chalcopyrite and galena formed near these silicic rocks the vents are surrounded by distinctive vent animals polychaete worms crabs and barnacles isotope data indicate that the mariana trough crust was derived from a heterogeneous source including mantle resembling the morb source and an arc source component the latter was depleted in hfs elements in previous melting events and later modified by addition of h 2 o and lil elements

chemical processes in buoyant hydrothermal plumes on the east pacific rise near 21 n hydrothermal plume water containing suspended particles of precipitated black smoke has been sampled from three hydrothermal vent fields on the east pacific rise near 21 n the samples were taken within the lower 22 m of the buoyant plumes directly above 273 350 c black smoker vents using go flo bottles mounted on the submersible alvin based on li as a conservative tracer of the high temperature endmember solution mixing ratios of 102 to 104 g seawater g vent water are achieved within the lower 22 m of the plumes the particle concentrations sampled in the lower 13 m of the plumes are much larger than predicted for these mixing ratios suggesting that the particles are settling within the lower part of the plume the major minerals in the smoke are pyrrhotite fe rich sphalerite pyrite unidentified fe s si and fe si s phases chalcopyrite amorphous silica sulfur fe oxyhydroxides including goethite and anhydrite present in trace quantities are barite isocubanite wurtzite covellite marcasite and unidentified silicates and alsilicates organic matter is common the composition of the plume solutions indicates that 35 25 of the hydrothermally injected fe remains in dissolved form i e 0 45 m within the lower 22 m of the plume in spite of a 3 to 9 fold excess of h2s in the vent solutions nearly all of the mn and si and most of the ba also remain in solution h2s by contrast has been largely 70 100 removed by precipitation as sulfides and sulfur but mainly by oxidation to dissolved species with intermediate oxidation states its early removal implies that 50 30 of the injected fe will eventually precipitate as primary oxides rather than as sulfides and that fe can be fractionated from the other chalcophile metals during sulfide precipitation compared with the vent water the particles are enriched in cu co ag cd al zn pb and ni by factors of 4 to 20 relative to fe and depleted in ba s si and mn scavenging from seawater may play a role in co and cu enrichment at the ngs field and for ag cd and ni at all three fields but is negligible for pb and zn these results indicate that two distinct metalliferous components are delivered to the distal regions of a plume as a result of reactions in the lower part of the buoyant plume 1 1 a dissolved component that includes about half the fe and all of the mn and will eventually precipitate as oxides and 2 2 a particulate component consisting of fine grained sulfide minerals that are enriched in ore metals by factors of 4 to 20

downdip segmentation of strike slip fault zones in the brittle crust outcrop scale field evidence from the southern san jacinto fault zone in southern california indicates that bedding parallel slip may be an important mechanism for transferring and accommodating slip within strike slip fault zones that cut across heterogeneous stratigraphy steeply dipping strike slip fault segments exposed in conglomeratic sandstones abruptly terminate at weaker shale layers where slip occurs on bedding parallel planes cross sections of fault related seismicity within the southern san jacinto fault zone reveal that events generally cluster within tabular steeply dipping fault parallel streaks that are limited in extent in the downdip direction the depths of the downdip terminations of shallow streaks tend to be roughly coincident with the depths of the updip terminations of deeper streaks these configurations define stepover geometries that range in character from sharp to diffuse the examples described herein provide evidence that downdip segmentation within strike slip fault zones may be common over a wide range of scales

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

mis 21 and the mid pleistocene climate transition climate and sea level variation from a sediment core in osaka bay japan abstract we report climate and sea level variation for the marine oxygen isotope stage mis 21 encompassing the end of the mid pleistocene climate transition mpt based on pollen diatom and sulfur records from a 50 m thick sequence in a core from osaka bay an extremely warm climate coincided with the sea level highstand of substage 21 5 when the warm temperate element quercus cyclobalanopsis exceeds 40 of total arboreal pollen this was followed by a warm temperate to temperate and humid climate that continued until the end of mis 21 a linear age model shows that climate was dominated by precessional cyclicity with an inverse correlation between temperature and precipitation the postglacial sea level rise reached its highest peak in substage 21 5 when paleo osaka bay reached its maximum extent including kyoto and nara basins at this time pelagic diatoms were dominant in the central part of the bay sea level dropped below the osaka bay sill about 60 m at present during substage 21 4 followed by a rise above the sill in substage 21 3 and a drop at 21 2 sea level remained below the sill during substage 21 1 the thermal maximum and sea level peak occurred just after the rapid postglacial sea level rise after which there was a gradual decline in temperature and sea level accompanied by precession related oscillations these features are typical of the post mpt interglacials dominated by 100 ka cyclicity these features may be a sign of termination of the mpt

crust and mantle evolution in the late archean evidence from a sm nd isotopic study of the north spirit lake greenstone belt northwestern ontario canada sm nd data are presented for volcanic plutonic and sedimentary rocks from the late archean north spirit lake greenstone belt in northwestern ontario the greenstone belt is divided into six lithotectonic assemblages emplaced between 2700 and 3000 ma the earliest assemblage indicative of an island arc setting is characterized by positive initial nd values 0 to 3 whereas shales and granitoids in succeeding assemblages yield increasingly negative nd values as low as 3 this argues strongly that the assemblages formed in close proximity to each other because the younger assemblages likely include recycled crustal material from earlier assemblages through erosion into younger basins and remelting however even the earliest assemblage exhibits decreasing nd values from basaltic to intermediate felsic volcanic strata within the same assemblage this suggests that the initial arc was itself contaminated by either a sedimentary component or remelting of older oceanic crust granitoid samples yield nd values similar to the shales in the belt indicating a common evolutionary history these granitoids may thus represent the roots of the greenstone belt mantle derived rocks such as basalts and komatiites are derived from a variably depleted mantle source with values as high as 3 8 at 2925 ma a 2925 10 8 ma u pb zircon age obtained for a tonalitic pluton on the northern margin of the north spirit lake belt is similar to the age of a volcanic plutonic cycle in the adjacent favourable lake greenstone belt implying that the two belts may have had common orogenic episodes the north spirit lake greenstone belt appears to have nucleated from an initial primitive volcanic arc and through continued volcanism was transformed into a stable platform exhibiting continental features such as intracrustal differentiation and sedimentary recycling to produce the current array of assemblages in this respect the evolution of the north spirit lake greenstone belt resembles the evolution of proterozoic and phanerozoic continents and suggests that archean microcontinents grew in much the same fashion as present day continents

geochemical implications of rare earth element patterns in hydrothermal fluids from mid ocean ridges abstract rare earth element data are reported for vent fluids collected from eight deep sea hydrothermal areas including samples from sediment hosted and bare rock hydrothermal systems the morb normalized ree patterns from these diverse settings are remarkably similar to each other but distinctly different than seawater or basalt being lree enriched and possessing a strikingly positive eu anomaly the ree concentrations making up these patterns vary widely however both on a spatial and temporal basis when plotted against ionic radii these fluid trends are remarkably linear and show the same fractionation patterns exhibited by plagioclase during magma segregation this analysis suggests that the partitioning of the ree between fluid and rock during high temperature hydrothermal alteration of morb is dominated by crystochemical exchange during dissolution and recrystallization reactions at the exposed surfaces of plagioclase phenocrysts a comparison of ree ionic radii in eight coordination indicate that the partitioning of 3ree and 2eu are dominated by chemical substitution for ca 2 and sr 2 respectively these considerations predict that the total 3ree content of vent fluid varies in a systematic way during the transition from an rich morb plagioclase to ab rich hydrothermal plagioclase and that these parameters can be used as indicators of the average extent of alteration inside active hydrothermal systems

late pliocene vegetation and climate of the lake baikal region southern east siberia reconstructed from palynological data abstract palynomorphs from a palaeomagnetically dated late pliocene sediment core 3 6 2 35 ma reflect highly variable climatic conditions and repeated expansion of steppe and boreo alpine vegetation mixed coniferous forests picea tsuga pinus with associated broadleaved taxa quercus corylus ulmus tilia were affected by dry and cold intervals already between 3 48 and 3 39 ma peak records of non arboreal pollen types and spores including subarctic boreo alpine selaginella selaginoides evidence advances of open vegetation which can be correlated to glacial marine oxygen isotope stages a cooling trend occurred from 3 15 ma to 2 5 ma documented by cold dry intervals and increased fluctuations of picea and pinus around 3 2 9 ma the palaeoclimate variation pattern shifted towards cold dry and warm moist oscillations cooling strengthened after 2 89 ma and severely restricted tsuga development cool and oligotrophic limnic conditions are documented by abundant gonyaulax dinoflagellate cysts in the interval 2 65 2 57 ma due to decreasing precipitation extensive areas with open steppe and rock steppe vegetation became permanently established after 2 62 ma dry conditions became a dominant environmental factor in the baikal region coeval in time with the red clay loess shift in northern china this climatic transition can be explained as a consequence of major intensification of northern hemisphere glaciation around 2 75 ma evidenced by marine records of ice rafted debris

late pliocene pleistocene high resolution pollen sequence of colombia an overview of climatic change two long continental pollen records funza i 357 m and funza ii 586 m from the high plain of bogota colombia at 2550 m elevation have been studied palynologically fission track dates on zircon from intercalated volcanic ashes provided a chronological framework and showed the pollen records which correlate with high precision to be continuous over the interval from ca 3 2 ma to ca 27 ka the late pliocene pleistocene history of the montane forests and open alpine paramo vegetation is documented with a temporal resolution of ca 6 5 ka and for the upper 1 1 ma with a resolution of ca 1 2 ka the immigration of the northern hemisphere elements alnus at ca 1 ma and quercus at ca 0 33 ma which travelled along the panamanian landbridge caused significant changes in the composition of the andean montane forests the successive pleistocene glaciations forced the andean vegetation belts to an almost continuous altitudinal movement the upper forest line at present at ca 3200 m shifted between 1800 m glacials and ca 3500 m interglacials corresponding to a variation in temperature between ca 5 and 15 c at 2550 m altitude a provisional land sea correlation funza pollen odp site 677 18o is shown for the upper 1 2 ma stages 3 35 frequency analysis of several time series showed significant periods of the eccentricity 100 ka and precession 23 and 19 ka bands showing orbital forcing and a strong change in climatic variability around 800 ka at ca 2 7 ma a significant cooling of ca 5 c is documented reflecting the classical terrestrial pliocene pleistocene boundary which correlates to the reuverian praetiglian boundary of the nw european stratigraphical climatic subdivision

hydrothermal scavenging at the mid atlantic ridge radionuclide distributions we have carried out a study of u series radionuclide distributions in the neutrally buoyant plume overlying the tag hydrothermal vent field at 26 n mid atlantic ridge limited uptake of u accompanies the formation of fe oxyhydroxide particulate material within the hydrothermal plume the particulate u concentrations are too small for measurable dissolved u anomalies to be produced within the water column at plume height however such uptake followed by deposition in the underlying sediments could account for 10 of the u enrichment that has been reported previously for metalliferous ridge crest sediments particulate 234th 230th and 228th activities in the tag hydrothermal plume represent 65 of total th activities typical of the deep ocean low dissolved th activities coincide with the high particulate th activities indicating that hydrothermal scavenging strongly affects th isotope distributions in the water column close to hydrothermal vents much of the scavenging occurs during the initial formation of fe rich hydrothermal precipitates but some further scavenging continues as the material is dispersed away from its source distributions of particulate 210pb and 231pa activities in the neutrally buoyant plume are also extremely high 7 12 dpm 102 l and 30 160 dpm 106 l respectively like th 210pb and231pa show evidence for scavenging within the hydrothermal plume total th activities at plume height are 50 higher than background this focusing of th into the plume is consistent with the entrainment and re emplacement at plume height of particulate material recycled from beneath the neutrally buoyant plume

pollen based biome reconstructions for the past 450 000 yr from the funza 2 core colombia comparisons with model based vegetation reconstructions abstract the 586 m funza 2 sequence from the high plain of bogota colombia has provided one of the longest pollen based vegetation reconstructions in the world affinity scores to seven biomes are compared to the record of co2 fluctuation from the vostok ice core for the period spanning from approximately 25 000 to 450 000 yr before present bp results are compared to output from the biome 3 vegetation model run under different environmental scenarios at regional and local scales the model is run with temperature and precipitation reductions relative to the present values of up to 10 c and 700 mm yr 1 respectively for this particular investigation we are interested in the role of co2 as a forcer of vegetation change we run the model with concentrations of atmospheric co2 co2 atm within the range of 170 to 340 ppmv during glacial periods the cool grass shrubland biome is highly dominant less so during interglacial periods when the cool evergreen and cool mixed forest biomes become co dominant in addition to this climate driven altitudinal oscillation of the vegetation there is also a signal that indicates longer term evolution of the vegetation the cool grass shrubland and cool evergreen and cool mixed forest biomes become co dominant for the first time over the 450 000 yr record at approximately 170 000 yr bp become completely anti phase during a period of extreme low co2 atm and then become co dominant from approximately 120 000 yr bp until the core top just prior to the last glacial maximum the model results for the funza area indicate that changes in co2 atm temperature and precipitation are inter linked by the vegetation response a shift of 5 5 c is required to lower the cool grass shrubland biome to altitudes about the funza catchment where the andean forest biome dominant at low co2 atm concentrations particularly below 180 ppmv the composition of the high latitudinal tropical vegetation about the funza catchment changed it is suggested that this low co2 atm combined with a period when the climate was characterised by extreme cooling and drying caused a readjustment of the tropical high altitudinal vegetation zonation and the formation of plant communities that are presently recorded

montane forest evolution during the last 650 000 yr in colombia a multivariate approach based on pollen record funza i the core interval of 0 140 m of the 357 m long pollen record from site funza i was analysed using multivariate analysis this long continental record is from the high plain of bogota located at 2550 m altitude in the eastern cordillera of colombia time control is based on a visual match with core funza ii from the same location which includes absolute fission track ages and a match with the 18o record of odp site 677 we reassessed the time frame of funza i compared with previous papers and conclude that the period of ca 650 25 kyr bp is represented in this study 73 fossil taxa present in 4 of the 409 pollen assemblages were analysed using the divisional classification program twinspan we recognised 10 cluster types that form three main cluster zones w warm cw cool and c cold pollen zones based on previous visual inspection were compared with new cluster types which were graphed as a bar code along the pollen diagram the use of two pollen sums standard pollen sum and a zonal pollen sum appeared useful for identifying local effects on the forest composition modern pollen rain data for the study area were summarised we used the concept of indicator taxa that behave as differential taxa to reconstruct climatic change during the period of 650 330 kyr bp alnus hedyosmum weinmannia melastomataceae myrica and podocarpus were the most important taxa of the andean forest the new time frame indicates that the first scattered presence of quercus started 478 kyr bp 102 m core depth the first continuous presence was from 423 kyr bp 92 m and the presence of zonal quercus forest occurred from 330 kyr bp 72 m during the period of 330 135 kyr bp zonal quercus and weinmannia forest developed replacing partly podocarpus dominated forest in the uppermost andean forest belt as a consequence during the last 135 kyr bp a high representation of quercus on the high plain of bogota no longer coincides with warm climatic conditions but now occurs during cool to cold conditions the pollen record was correlated with the 18o record temperature drop during marine oxygen isotope stage 4 and the very first part of marine oxygen oxygen stage 2 is calculated as a minimum of 6 6 to 7 8 c excluding a potential effect of low glacial pco2 the inferred maximum amplitude of the upper forest line during the last 650 kyr was from ca 1900 to 3400 m cf 3200 m at the present day corresponding to a range of ca 1 2 c to 7 8 c compared with today copyright 2000 john wiley sons ltd

the deglaciation of the americas during the last glacial termination this paper reviews current understanding of deglaciation in north central and south america from the last glacial maximum to the beginning of the holocene together with paleoclimatic and paleoceanographic data we compare and contrast the pace of deglaciation and the response of glaciers to major climate events during the global last glacial maximum glgm 26 5 19 ka average temperatures decreased 4 to 8 c in the americas but precipitation varied strongly throughout this large region many glaciers in north and central america achieved their maximum extent during the glgm whereas others advanced even farther during the subsequent heinrich stadial 1 hs 1 glaciers in the andes also expanded during the glgm but that advance was not the largest except on tierra del fuego hs 1 17 5 14 6 ka was a time of general glacier thickening and advance throughout most of north and central america and in the tropical andes however glaciers in the temperate and subpolar andes thinned and retreated during this period during the bolling allerod interstadial b a 14 6 12 9 ka glaciers retreated throughout north and central america and in some cases completely disappeared many glaciers advanced during the antarctic cold reversal acr 14 6 12 9 ka in the tropical andes and patagonia there were small advances of glaciers in north america central america and in northern

extensive volcaniclastic deposits at the mid atlantic ridge axis results of deep water basaltic explosive volcanic activity a recent mapping survey followed by submersible diving investigations revealed the existence of extensive volcaniclastic deposits on three adjoining progressively deeper segments of the mid atlantic ridge south west of the azores the maximum water depth at which subaqueous magmatic and hydro magmatic explosive processes can occur is controversial this discovery provides fundamental constraints on the dynamics of deep subaqueous volcanic eruptions using multibeam bathymetry sonar image interpretations submersible observations volcaniclastic sample descriptions and chemical analysis evidence of extensive deep water up to 1700 m magmatic explosive activity of basalt at the axis of a spreading mid ocean ridge is documented for the first time

a dual origin for the hydrothermal component in a metalliferous sediment core from the mid atlantic ridge a highly metalliferous sediment core collected from tag 26 n mid atlantic ridge has been analyzed for uranium series isotopes rare earth elements ree and radiogenic isotopes two types of deposition of hydrothermal material have been identified and characterized the first is sulfide material which has been rapidly deposited and is sometimes clearly turbiditic this material exhibits high u fe and eu sm ratios and low 230th fe and ree fe ratios it is proposed that u is incorporated into the sulfides by diffusion through the oxidized coating with reductive fixation near the sulfide oxide boundary the second hydrothermal component is particulate fallout from the neutrally buoyant plume which is diluted by the background pelagic sedimentation this component exhibits u fe ratios around 2 10 5 an order of magnitude greater than reported plume particulate levels but the 230th fe ree fe and eu sm ratios are similar to plume values nd and pb isotopes have been used to differentiate the hydrothermal and pelagic end members within this sediment core

high frequency stacking pattern and stages of canyon gully evolution across a forced regressive shelf edge delta front abstract the succession of varying facies tracts filling within a canyon or a deep incised gully cutting across the delta front lobes at the shelf edge requires to be explained by high frequency allocyclic and or autocyclic events a fifth and sixth order sequence stratigraphic model for the shelf edge canyon fill or gully fill is here proposed for the first time the first gelasian cold phase and the renewal of transpressional uplift of basin margin occurring at the same time enforced regressive transit of the inner shelf paleo orinoco delta onto the paleo shelf edge its delta front developed along the shoulders of the major successive basin bounding growth faults the delivery of clastic sediments into the deeper slope and beyond in the columbus basin was through the gullies and canyons that incised and cannibalized across the shelf edge delta the mayaro formation of southeast trinidad furnishes the role of growth faulting during the first phase of delta front development at the paleo shelf edge thereby it serves as an example of several fifth order sequences within a fourth order equivalent episode which is contemporaneous to both the glacio eustatic episode the pretiglian and the renewal of transpressional uplift after a pause during the pliocene thus large incision and subsequent filling within the mayaro formation cover a passage of time fifth order equivalent that is unequivocally shorter than the span of the falling pretiglian eustatic sea level and second renewed phase of transpression tectonism the categorization of sedimentary facies of the canyon fill or gully fill and the encasing delta front outside the canyon gully cut into a series of sixth order genetic facies tracts shows variations of geomorphic stages of the canyon gully evolution from accommodation creation to the stages of filling these stages therefore should be attributed to the sixth order equivalent autogenic changes influenced by high frequency growth tectonics instead of assuming the stages to be related to the relatively lower frequency glacio eustatic allocyclicity of the earliest gelasian

architecture and seismotectonics of a regional low angle normal fault zone in central italy information from surface geology subsurface geology boreholes seismic reflection and refraction profiles and seismicity are used to depict the geometry and the possible seismogenic role of the altotiberina fault af a low angle normal fault in central italy the af extends along the inner umbria region for a length of 70 km with an average dip of 30 and an horizontal displacement up to 5 km it emerges west of the inner border of the tiber basin and deepens beneath the umbria marche carbonate fold and thrust belt to a depth of 12 14 km close to the af surface trace low angle synthetic east dipping normal faults extensively outcrop whereas high angle antithetic west dipping normal faults prevail farther east integrating geological and seismologic information it can be stated that the af behaves as an active extensional fault zone and represents the basal detachment of the west dipping seismogenic normal faults of the umbria marche region the af belongs to a regional ne dipping low angle normal fault system etrurian fault system efs which extends for 350 km from northwestern tuscany to southern umbria early preliminary considerations suggest that the efs may play an important role in controlling active extension and related seismicity in northern central italy

the effect of hydrothermal processes on midwater phosphorus distributions in the northeast pacific abstract the distributions of dissolved particulate and sedimentary phosphorus were measured in the region of the juan de fuca ridge to determine the impacts of hydrothermal processes on the phosphorus cycle in the oceans significant negative dissolved phosphate anomalies ranging from 0 to 60 nmol l were observed in the water column at depths between 1900 and 2300 db the largest anomalies

late quaternary glacier response to humidity changes in the arid andes of chile 18 29 s abstract today no glaciers exist between 18 30 s and 27 s even on mountains much higher than 6000 m these dry high mountain environments are very sensitive to changes in climatic boundary conditions particularly humidity since temperatures are far below freezing here we present a reconstruction of climatic changes since the last glacial maximum for the chilean dry andes of the southern hemisphere we reconstructed regional equilibrium line altitudes ela for three different moraine stages representing extensive past glaciations in this currently unglaciated region comparison of the regional pattern of the ela with modern climate conditions allows us to draw implications about the paleoclimatic conditions during the best preserved moraine stage ii glaciations in the northern as well as in the southern part of the research area our results suggest humid conditions in the northern part 18 24 s during late glacial times with strongly increased convective precipitation during austral summer the temporal coincidence of glaciers in the mountains and high lake levels on the altiplano tauca phase is evident to the south no simple shift of the westerlies is implied by our glacier reconstructions the northern limit of effective moisture for glacier formation did not shift significantly but stayed in the area of the present day northernmost glaciers of the westerlies at about 27 s but further south precipitation increased significantly accompanied by at least 2 3 c colder conditions the age of this glaciation is uncertain but probably of late glacial or last glacial maximum lgm times overall glaciers in the dry andes of south america clearly responded to large changes in the humidity not primarily the temperature regime with all its consequences on the environment hydrology vegetation etc illustrating the different regional responses to global climatic change

hydrothermal plumes along the east pacific rise 8 40 to 11 50 n particle distribution and composition the discovery of a volcanic eruption at the 9 45 52 n site on the east pacific rise epr led us to investigate how the composition of suspended matter in hydrothermal plumes may be used to provide a better understanding of the relative age and vent fluid characteristics of a ridge crest system during surface ship surveys during the troughs tropical ridge observations of underwater geochemical hydrothermal signals expedition we collected hydrothermal plume particles along a 350 km section extending from approximately 8 40 n to 11 50 n of the epr north and south of the clipperton transform fault over the segment south of the clipperton transform fault the sfe ratio in the particles ranged from about 0 9 to 10 with the highest values immediately over the new vent field at the 9 45 52 n site these enrichments exactly coincide with the elevated 3he heat and ch4mn ratios in the plumes over the same vent field the s rich particles are primarily organic and may be related to the bacteria blizzard associated with the eruption in addition very high cufe and znfe ratios were observed in the hydrothermal plumes south of the clipperton transform fault these high ratios are indicative of the predominance of high temperature h2s rich vents between 9 35 n and 9 50 n in contrast the samples from the segment north of clipperton transform fault had sfe ratios which ranged from 0 04 to 0 82 typical of mature fe rich hydrothermal plumes these results suggest that the gas rich vent fluids at the 9 45 52 n site produced the sulfur enriched particles in the water column gas rich plumes and sulfur enriched plume particles were also observed underneath the 1986 megaplume on the juan de fuca ridge jdfr suggesting a similar mode of formation north of the clipperton transform fault the hydrothermal particles are dominated by fe rich oxyhydroxide particles characteristic of mature high temperature vent fields the plume maximum is centered at 11 08 n the fe oxyhydroxide particles scavenge ca si p v cr and as from seawater for p and v the mean element fe ratios in the epr hydrothermal particles are intermediate between jdfr and tag hydrothermal systems for as the mean element fe ratios of all three systems are about the same new evidence for si scavenging by hydrothermal fe oxyhydroxide particles indicates that this sink for dissolved si in the oceans is small

magnetic polarity and fission track chronology of a late pliocene pleistocene paleoclimatic proxy record in the tropical andes abstract two sections exposing a late pliocene pleistocene sedimentary sequence in the marginal valleys of the bogota basin colombian andes south america were sampled for paleomagnetic analysis magnetostratigraphy and fission track dates of tephra beds provide a detailed geochronologic calibration for the sedimentary basin measurements of magnetic susceptibility complement the regional environmental record provided by lithological and palynological evidence sedimentation in the bogota basin started in the early part of the gauss chron at ca 3 2 myr the oldest recorded sediments belong to the guasca member of the upper tilata formation they were deposited in a lacustrine paludal environment near the end of the tectonic uplift in the bogota area and or under pliocene climatic conditions that were warmer than today repeated climate cooling associated with glaciations in the surrounding mountains resulted in the deposition of a fluvial lacustrine complex referred to as the subachoque formation the first glaciation is placed near the gauss matuyama polarity reversal at 2 6 myr a lithologic change in the subachoque formation marked by coarser grained fluvial deposits and a possible increase in amplitude of the magnetic susceptibility signal occurs near the matuyama brunhes boundary at 0 8 myr indicating a shift toward higher magnitude climate oscillations

middle pleistocene climate change recorded in fossil mammal teeth from tarija bolivia and upper limit of the ensenadan land mammal age fossiliferous middle pleistocene sediments of the tarija basin of southern bolivia contain a classic ensenadan land mammal fauna new carbon isotopic data reported here for 50 specimens of the grazing mammals equus horse and cuvieronius mastodon documented from eight stratigraphic levels at tarija vary significantly in the 13c values of their teeth the pattern of variation appears to reflect the proportion of c3 and c4 grasses eaten during colder more c3 and warmer more c4 times within age limits set by associated magnetostratigraphy the cold periods can be correlated with particular even numbered stages in the marine oxygen isotope record and the warm periods can be correlated with odd numbered stages the oldest fossil teeth analyzed from the tarija section can thereby be assigned to stage 29 and the youngest to stages 17 or 15 that is the teeth range in age from about 1 1 myr to as young as 0 7 myr based on correlation of the upper part of the tarija beds to the isotopic stages the upper limit of the ensenadan land mammal age is between 0 7 and 0 6 myr which is younger than stated in most previous studies

hydrothermal scavenging at the mid atlantic ridge modification of trace element dissolved fluxes abstract we have carried out an icp ms study of the distributions of trace elements fe ca mg al mn cu zn v as cr co pb y mo ag sn cd and be in particulate material collected from the tag hydrothermal plume 26 n mid atlantic ridge to examine the extent to which fluxes of dissolved trace elements emitted from hydrothermal vent fluids are balanced by the precipitation of associated particulate phases comparison of particulate trace element distributions with particulate fe concentrations within the neutrally buoyant plume reveals that the distribution of trace elements within the plume is controlled by the following processes i co precipitation from vent fluids ii scavenging from seawater and iii preferential settling from the plume and oxidative dissolution within the water column for y and be continuous scavenging from seawater appears to be important whereas for v as and cr it appears that conditions suitable for uptake into particles may be restricted to the buoyant portion of the hydrothermal plume from a comparison of trace element fe ratios in the particles and associated mid atlantic ridge vent fluids it appears that for many elements e g cu co ag pb al cd be and zn the flux of dissolved material from vent fluids is matched closely by the precipitation of hydrothermal particulate phases of the various elements studied here hydrothermal activity only appears to act as a significant source for dissolved ca and dissolved mn

fine structure of the rupture zone of the april 26 and 27 1997 northridge aftershocks we investigated the rupture geometry of two m w 4 5 earthquakes april 26 1997 and april 27 1997 that occurred on the western edge of the aftershock zone of the 1994 m w 6 7 northridge earthquake both events have thrust mechanisms with a steep plane dipping 75 se and a shallow plane dipping 45 ne an empirical green s function deconvolution followed by a waveform inversion was used to determine the slip distribution of the two events the inversion results show that the steep plane fits the data slightly better than the shallow plane the background seismicity aftershocks of the 1994 northridge event in the epicentral region shows the existence of a north dipping fault plane similar to that of the 1971 san fernando earthquake however the spatial trend of the two april 1997 events and their aftershocks reveals tightly clustered seismicity on a steep plane dipping south relative relocation of the april 26 and 27 events shows that the april 27 event ruptured 1 4 km n70 e of the april 26 event and at a slightly shallower depth i e almost along strike and on a steep plane dipping south these observations suggest that the steep plane is the fault plane of both events the two events ruptured on a plane which is almost perpendicular to the trend of the regional background seismicity thus the seismogenic structure beneath the transverse ranges exhibits complexity on scales of a few kilometers the rupture area for both the april 26 and 27 events is 1 km 2 with a stress drop of at least 20 to 30 bars

oceanic spreading center generated basaltic crust and associated sulfidic and carbonate rich hydrothermal deposits in the archean ca 3 ga north spirit lake greenstone belt ontario canada detailed mapping of a portion of the north spirit lake greenstone belt reveals a basal basaltic volcanic unit ca 3 ga with a komatiitic component and strong major and trace element geochemical affinity with typical mid ocean ridge basalts e g n morb the overlying transitional unit includes basaltic tuffs which have close geochemical affinity to the underlying basalts and to n morb interbedded with and overlying the tuffs is a thin laterally continuous sulfide rich unit with chert interbeds which is in turn overlain by interbedded carbonate chert and iron rich sedimentary rocks the chemical sedimentary rocks of the transitional unit display rare earth element ree patterns and positive eu anomalies consistent with derivation from fluids associated with submarine hydrothermal vent fields the sulfide unit is interpreted to represent proximal hydrothermal vent deposits derived from black smoker activity near a spreading center the overlying carbonate and chert mounds and beds are interpreted as hydrothermal deposits formed at lower temperatures in a more distal setting relative to the spreading center the carbonate deposits may have formed in an environment analogous to that of the lost city hydrothermal field kelley et al 2001 where abundant chimneys dendrites and beds of carbonate are produced 15 km from the mid atlantic rift axis the basal volcanic unit consequently is interpreted as ocean floor basalts formed at an archean spreading center with the basaltic tuffs representing the last volcanic emanations and with the chemical sediments representing hydrothermal vents deposits both proximal and distal to the spreading center the capping siliciclastic unit likely is part of an accretionary wedge possibly representing ocean closure after a protracted seafloor spreading episode the sedimentary rocks were derived in part from a weathered tonalite trondhjemite granodiorite source the supracrustal rocks of the north spirit lake greenstone belt provide an unusually clear window into some tectonic activity on the primitive earth including evidence for the existence of ocean floor basalts proximal and distal hydrothermal activity and possibly a protracted cycle of ocean opening and destruction

ree controls in ultramafic hosted mor hydrothermal systems an experimental study at elevated temperature and pressure a hydrothermal experiment involving peridotite and a coexisting aqueous fluid was conducted to assess the role of dissolved cl and redox on ree mobility at 400 c 500 bars data show that the onset of reducing conditions enhances the stability of soluble eu 2 species moreover eu 2 forms strong aqueous complexes with dissolved cl at virtually all redox conditions thus high cl concentrations and reducing conditions can combine to reinforce eu mobility except for la trivalent ree are not greatly affected by fluid speciation under the chemical and physical condition considered suggesting control by secondary mineral fluid partitioning lree enrichment and positive eu anomalies observed in fluids from the experiment are remarkably similar to patterns of ree mobility in vent fluids issuing from basalt and peridotite hosted hydrothermal systems this suggests that the chondrite normalized ree patterns are influenced greatly by fluid speciation effects and secondary mineral formation processes accordingly caution must be exercised when using ree in hydrothermal vent fluids to infer ree sources in subseafloor reaction zones from which the fluids are derived although vent fluid patterns having lree enrichment and positive eu anomalies are typically interpreted to suggest plagioclase recrystallization reactions this need not always be the case

the oak ridge fault system and the 1994 northridge earthquake the 17 january 1994 northridge earthquake moment magnitude m w 6 7 in the san fernando valley in southern california illuminated a hitherto unrecognized blind reverse fault with a moderate southward dip 1 this fault lies beneath the active north dipping santa susana fault system and uplifted both the footwall and the hanging wall of the santa susana fault during the earthquake1 here we argue that footwall uplift on the santa susana fault before the earthquake could have been used to identify the northridge blind thrust as an active fault moreover we propose that the northridge earthquake occurred on a continuation of the oak ridge fault system which reaches the surface in the ventura basin to the west slip rates on the western part of this fault system 2 3 are nearly three times larger than on the northridge blind thrust 4 increasing the probability of a n or t bridge sized earthquake in the heavily populated ventura basin

the impact of milankovitch scale climatic forcing on sediment supply abstract stratigraphy is the main technique to predict the occurrence of potential reservoirs and seals in the oil industry stratigraphic tools used are stratigraphic observations from drilled wells rock samples and wireline logs and seismic interpretation nowadays computer modelling allows scenario forecasting of the sequence to be expected application of these tools indicates that the key input parameters are initial basin topography subsidence eustasy and sediment supply the first three parameters are usually well constrained or can readily be deduced from well and seismic data however the fourth sediment supply is loosely defined and because of that it is used to calibrate the model in the present article the controls on sediment supply are reviewed and the impact of in particular milankovitch scale climatic forcing on sediment supply is tested on the neogene of the niger drainage and delta system sediment supply is controlled by a complex interplay of geographic and climatic factors such as 1 relief or mountain height 2 the size and temporal storage capacity of the drainage area 3 overall climate 4 amount and seasonality of precipitation and 5 vegetation cover studies in geomorphology have indicated that although complex computer models can be made to predict erosional product for the present a large scale sediment supply prediction should be based on the combination of palaeogeographical palaeoclimatological and erosion potential maps more detailed sediment supply predictions should include milankovitch scale variations the climatic impact of these variations is mainly felt when we are in a transition between two climates then the area will experience regular climatic changes from e g wet rainforest to dry monsoonal climate or low to high sediment supply respectively this concept was tested in the forward modelling package stratagem on the neogene of the niger delta four scenarios were evaluated using various combinations of large and milankovitch scale sediment supply curves combined with global or milankovitch scale eustatic sea level curves the results confirm the impact of both large scale and milankovitch scale climatic forcing on sediment supply however in an icehouse world the various scenarios also suggest that the effect of eustatic sea level fluctuations overshadows the impact of milankovitch scale sediment supply by contrast in a greenhouse world the impact of milankovitch scale sediment supply is expected to be more significant in the absence of significant sea level fluctuations

particle geochemistry in the rainbow hydrothermal plume mid atlantic ridge we report the analysis of 18 large volume 500 1500 l in situ filtered samples of particulate material from the largest hydrothermal plume on the mid atlantic ridge overlying the ultramafic hosted rainbow hydrothermal field at 36 deg 14 n measured particulate iron concentrations reach 614 nm high concentrations of particulate fe oxyhydroxides result from the extremely high fe concentration 24 mm and fe h2s ratio 24 of the vent fluids and persist to at least 10 km away from the vent site due to the advection of plume material with the ambient along axis flow two of the nine pairs of pump deployments appear to have intercepted the buoyant or otherwise very young portion of the hydrothermal plume these samples are characterized by anomalously compared to neutrally buoyant plume samples high concentrations of mg u and chalcophile elements and low concentrations of mn ca v y and the rare earth elements ree within the neutrally buoyant plume elemental distributions are largely consistent with previously observed behaviors preferential removal of chalcophile elements conservative behavior of oxyanions p v and u and continuous scavenging of y and the ree this consistency is particularly significant in light of the underlying differences in fluid chemistry between rainbow and other studied sites chalcophile elements are preferentially removed from the plume in the order cd zn co cu phosphorus iron and vanadium iron ratios for the neutrally buoyant plume are consistent with global trends with respect to the concentration of dissolved phosphate in ambient seawater comparison of buoyant and neutrally buoyant plume ratios with data from hydrothermal sediments underlying the rainbow plume cave et al 2002 indicates however that while p fe ratios are indeed constant v fe ratios increase progressively from early stage plume particles to sediments ree distributions in the buoyant and neutrally buoyant plume appear most consistent with a continuous scavenging process during dispersion through the water column

earthquake focal mechanisms in the eastern transverse ranges and san emigdio mountains southern california and evidence for a regional decollement earthquake focal mechanisms obtained from p wave first motions are presented for the eastern transverse ranges and the san emigdio mountains in southern california the former region shows a predominance of strike slip faulting whereas quaternary faults in the region show thrust motion we suggest that the observed strike slip mode of deformation cannot continue indefinitely without the occurrence of more thrust faulting fault deformation in the san emigdio mountains inferred from focal mechanisms is in accord with displacements across quaternary faults in the area this study and a search of the literature has yielded 19 mechanisms with shallow dipping nodal planes previous workers have interpreted such mechanisms as evidence for a regional decollement if such a regional decollement exists these data give some indication of its regional extent slip directions inferred from the focal mechanisms with shallow dipping nodal planes show some regional consistency but this pattern cannot be entirely explained with current tectonic models a comparison of the stress drop of an event having a shallow dipping nodal plane with an event with steeper planes gave inconclusive results

vertical axis rotation controlled by upper crustal stress based on force balance analysis a case study of the western transverse ranges of california abstract this paper evaluates driving mechanisms of vertical axis rotation using data from the western transverse ranges in southern california simple force balance considerations and comparison of torque applied to a rotating block indicate that shear forces applied to the base of the block are not strong enough to produce the motions and deformation observed at the surface for the measured dimensions of the crustal blocks and crustal viscosities in southern california stresses transmitted through the upper crust are one to three orders of magnitude stronger than forces generated in the ductile lower crust these results suggest that the kinematics of crustal blocks in continental deformation zones are primarily controlled by forces within the upper crust rather than a flow field beneath

paleoclimate changes during the last 100 000 yr from a record in the brazilian atlantic rainforest region and interhemispheric comparison abstract a long terrestrial record colonia co 3 from the atlantic rainforest region in brazil 23 52 s 46 42 20 w 900 m a s l registrates variations in the forest expansion during the last 100 000 yr the 780 cm depth core was analyzed at 2 cm intervals and arboreal pollen frequencies were compared to nearby speleothem stable isotope records and neighboring marine records from the tropical atlantic to evaluate regional versus global climate forcing our record was compared with greenland and antarctic ice core records these comparisons suggest that changes in temperature seen in polar latitudes relate to moisture changes e g to changes in the length of the dry season in tropical and subtropical latitudes during glacial as well as interglacial times these climatic changes result from changes in the frequency of polar air incursions to these latitudes inducing a permanent cloud cover and precipitation this is an important result that should help define paleoclimatic features in the southern hemisphere for the last glaciation

temporal variability of the hydrothermal plume above the kairei vent field 25 s central indian ridge 1 continuous profiling of the nonbuoyant plume above a hydrothermal vent site can be used to determine the interaction between the plume formed by venting high temperature fluids and changing background temperature salinity and current fields over a tidal cycle we have conducted a 12 hour monitoring of the nonbuoyant plume above the kairei vent field 25 s central indian ridge the depth of the plume particle maximum varies significantly in the water column from 2150 m to 2350 m but remains strongly associated with the 2 36 987 potential density isopycnal we have used the interaction between two unrelated physical oceanographic phenomena to isolate the section of the time series record most suitable for determining the heat flux of the kairei source on the basis of the rise height of the plume to neutral buoyancy at the start of the time series internal waves are observed with a period similar to the calculated buoyancy period n 1 about 3 hours no consistent relationship is observed between the rise height of the plume and the calculated heat flux for the last 6 hours of the time series isopycnal surfaces are compressed above a strongly developed nonbuoyant plume such that the calculated buoyancy period is less than that of the internal waves a more constant heat flux can be calculated on the basis of the plume rise height from this part of the record

quaternary and upper pliocene glaciations and forest development in the tropical andes evidence from a long high resolution pollen record from the sedimentary basin of bogota colombia pollen analysis of 1230 samples from a 357 m long core of lake sediments from the sedimentary basin of bogota eastern cordillera colombia alt 2550 m above sea level has provided a high resolution record of changes in the andean vegetation belts and long range climatic fluctuations during the last 3 5 ma direct time control is provided by k ar datings and fission track datings of 14 intercalated volcanic ash layers the resulting pollen diagram has been divided into 55 pollen zones representing 27 complete major climatic cycles glacial interglacial cycles a tentative correlation with the marine oxygen isotope chronology and the european stratigraphical climatic subdivision could be established the altitudinal position of the forest line which corresponds with the 9 5 c isotherm shifted between minimal c 1700 1800 m and maximal c 3400 3700 m alt at present situated at c 3100 m alt which rendered the altitudinal position of the high plain of bogota the dry bed of a former lake at 2550 m a sensitive recorder of temperature changes the estimated fluctuations in mean annual temperature at the level of the high plain of bogota are from 15 c a distinct change in the nature of the climatic cycles occurs around 2 5 ma b p the interval of 3 5 2 5 ma b p shows high frequency low amplitude climatic oscillations whereas the last 2 5 ma is characterized by low frequency high amplitude climatic oscillations with a superposed high frequency oscillation it is assumed that these oscillations correspond mainly with the variations in orbital eccentricity and obliquity the major constituents making up the andean forest have been subjected to considerable changes in the course of time in the early pliocene forests vallea and myrica are prominent elements weinmannia was almost absent the role of polylepis in the timberline forests increased markedly around 3 1 ma b p and also migrated by adaptation from the upper andean forest line to higher elevations the closure of the isthmus of panama during the pliocene enabled northern hemisphere elements to migrate into south america alnus reached the area of bogota around 2 7 ma b p and dramatically changed the vegetation especially on the flats around the lake of bogota where alnus swamp forests replaced myrica carr around 2 9 ma b p weinmannia became an important constituent of the upper andean forest but slightly later slope forests of alnus replaced parts of the area which had previously been covered by weinmannia forest the marked role of vallea in the andean forest diminished definitively after 2 5 ma b p quercus migrated more recently to south america and reached the area of bogota 1 ma b p it changed for the last time drastically the aspect of the andean forest all palynologically recognizable elements of the modern andean forest were present from now on and the subsequent adaptation and speciation of quercus and many other elements must have mainly taken place at the species level

evidence for possible horizontal faulting in southern california from earthquake mechanisms we find that 36 of the 505 fault plane solutions m 3 0 1981 1990 in southern california have a nodal plane dipping no more than 30 with the assumption of the low angle nodal planes being the fault planes four cross sections are constructed to show the possible horizontal faults in the middle and upper crust more than half of these low angle faults are located within or adjacent to the transverse ranges the focal depths vary from 1 km in the southern end of the sierra nevada and the southwestern mojave desert to 20 km in the transverse ranges the slip directions are also diverse in general east west extensional movements are dominant in the boundary between the southern sierra nevada extending to the san emigdio mountains and the western mojave desert whereas north south compressional movements are dominant in the transverse ranges in the peninsular ranges and the salton trough both the slip directions and focal depths vary these features suggest that seismically active low angle faults in southern california may exist at different depths and slip in various directions our data do not support the existence of a regional scale seismically active detachment in southern california only in the western transverse ranges is there some suggestion of a large detachment surface at a depth of about 13 to 14 km

crustal imaging in southern california using earthquake sequences abstract an inexpensive means to further understand the geometry of active faults in southern california arises from the use of aftershock recordings to image crustal structures the advent of regional seismic networks that record digital seismograms from hundreds of stations makes this crustal reflectivity profiling possible even in the absence of conventional active source seismic data we show that it is feasible to image fault structure using three dimensional wide angle prestack kirchhoff migration we achieve this with the use of aftershock traces recorded on the short period vertical stations of the southern california seismic network this work complements seismicity and focal mechanism work by imaging reflectivity volumes and cross sections rather than having to associate events with certain faults further it can image below the seismogenic zone to resolve current geologic controversies on how proposed faults extend below focal depths we demonstrate the validity of these images as showing reflective structures and the ability to use clipped high gain seismograms as sign bit data to yield valid geometric imaging work with data from the 1991 sierra madre earthquake sequence images the prominent lower crustal reflective zone observed beneath most of the san gabriel mountains by the los angeles region seismic experiment line 1 aftershocks of the 1994 northridge earthquake allow us to image a north dipping structure that may represent the fault plane of a crustal penetrating blind thrust the images serve as a test for the existence and geometry of thrust ramps and detachments proposed from balanced section reconstructions of shallow crustal profiles and borehole data our results are more consistent with a thick skinned tectonic regime in the vicinity of the northridge earthquake rather than a thin skinned model

the beginning of the ice age in europe and its major subdivisions abstract the time range of the quaternary is discussed in terms of the view that this period is essentially the one in the earth s history in which during certain intervals inland ice developed in the northern hemisphere this process first became evident about 2 3 ma bp since then the quaternary thus defined has witnessed many climatic variations although for practical reasons it is not feasible to assign the status of stage to each distinclty warm or cold phase climatic change is nevertheless the most appropriate basis for world wide correlation it seems possible to use both climatic change and other correlation parameters to arrive at a practicable subdivision of the quaternary period into stages before about 1 ma vegetational succession during interglacials in western europe differed from that seen in later times a possible cause for this divergence is discussed in the later part of the quaternary after about 1 ma two distinct types of warm temperate intervals occurred one associated with high sea levels and an oceanic climate the other showing lower sea levels and a cooler and more continental climate the difference between these two types is shown by comparison of the distribution of abies during these time intervals

stratigraphy tectonic setting and age of archean supracrustal rocks in the north central part of the north spirit lake greenstone belt nw superior province canada abstract supracrustal rocks in the north central part of the north spirit lake greenstone belt may be divided into three major stratigraphic units the basal unit is a thick succession of mafic and ultramafic subaqueous lavas with minor tuffaceous and sedimentary rocks overlying the lava dominated succession there is a tuffaceous unit that includes a variety of chemical sedimentary rocks such as bedded pyrite chert deposits bedded carbonate cherts large carbonate mounds and magnetitic iron formation the tuffs and chemical sedimentary rocks are overlain by an extensive and thick cover of siliciclastic rocks displaying evidence of emplacement by mass flows and turbidity currents the contact between the tuffaceous and siliciclastic units appears to be conformable but it is generally poorly exposed some of the siliciclastic rocks are quartz rich and contain clasts indicating the existence of an older cratonic source land the siliciclastic unit displays striking lateral variation from a conglomerate dominated facies in the north to turbidites and submarine channel fill deposits in the south and southeast of the study area stratigraphic and sedimentological data support geochemical evidence presented elsewhere nesbitt h w young g m bosman s a longstaffe f j in press oceanic spreading center generated basaltic crust and associated sulfidic and carbonate rich hydrothermal deposits in the archean 3 ga north spirit lake greenstone belt ontario canada geological society of america bulletin suggesting that the lower part of the succession represent part of an ancient ocean floor that was buried beneath a thick siliciclastic succession under highly unstable tectonic conditions a synthesis of u pb geochronological and sm nd isotopic data suggests that volcanic rocks on the north shore of north spirit lake in the core of the prospair anticline in the peridotite bay area and to the east may all have formed at around 3 1 ga succeeding hydrothermal chemical deposits are conformable with and genetically related to the volcanic rocks much younger 2 77 ga crystal tuffs and associated subvolcanic intrusive rocks may represent a separate much younger volcano plutonic cycle if the siliciclastic unit is part of this younger cycle a question that can only be resolved by further geochronological investigations then the time between ocean opening and closure must have been on the order of 270 ma casting doubt on the commonly held notion that archean plate tectonics involved small rapidly moving plates

u pb zircon ages in supracrustal and plutonic rocks north spirit lake area northwestern ontario the north spirit lake greenstone belt in the sachigo subprovince of the superior province comprises parts of three sequences of volcanic and sedimentary rocks the main youngest is separated from the older sequences by an unconformity to disconformity the belt is bounded by large granitoid batholiths and was metamorphosed under greenschist to locally hornblende cordierite facies u pb zircon dating was performed on volcanic sedimentary and plutonic rocks in order to establish an absolute chronology for the evolution of the area a tuff breccia in the lowermost supracrustal sequence is dated at 3023 2 ma zircons from a tuff in the middle sequence show complex u pb relationships although they do not allow a precise age determination the data suggest that the tuff formed sometime between 2950 and 2800 ma ago zircons from a quartz arenite also in the middle sequence yield a simple data pattern and define an age of suggesting derivation of the sediment from a uniform source of this age a zirco

the geochemistry of atlantic hydrothermal particles particles were collected from the dilute portion of neutrally buoyant hydrothermal plumes from four mid atlantic ridge sites mark 23 n tag 26 n broken spur 29 n lucky strike 37 n comparison of data from proximal portions of the tag atlantic german et al 1991 this study and north cleft pacific feely et al 1994 plumes show that oxyanion e g v scavenging is more efficient at tag possibly due to a higher proportion of fe removed as sulfides at north cleft and or the more vigorous mixing in the high energy tag buoyant plume chalcophile elements e g cu show two stage removal they are precipitated as sulfides during initial mixing of vent fluids with seawater and are sedimented from the buoyant plume in the dilute plume they are scavenged from seawater by fe oxyhydroxides the ree show continued scavenging in the neutrally buoyant plume and lower levels in 1993 samples compared to 1988 samples german et al 1990 suggesting that the amount of scavenging is related to particle recycling

pedostratigraphy and pedological and geochemical characterization of las carreras loess paleosol sequence valle de tafi nw argentina abstract the las carreras loess paleosol sequence is considered the most complete quaternary pedostratigraphic record in south america it consists of 32 paleosols formed during the past 1 15 ma chronological control is provided by magnetostratigraphy sediment and paleosol characterization is based on a variety of methods including soil chemistry geochemistry and micromorphology the main pedogenic processes identified are carbonate leaching and reprecipitation ped formation and translocation of clay and organic matter in many cases the boundaries between individual paleosols are not clearly defined due to indistinct transitional zones based on both pedological and sedimentological characteristics the sequence was subdivided into three units with paleosols in units i and iii designated as luvisols and paleosols in unit ii tentatively classified as luvic kastanozems this subdivision reflects environmental changes associated with the mid pleistocene transition overall geochemical variability is low and largely related to pedogenesis the homogeneity suggests a rather constant provenance that seems to be linked to the central andes pursuant to the chaco model of iriondo 1997 models of deposition of loess and loessoids in the upper quaternary of south america journal of south american earth sciences 10 71 79 and includes direct volcanic ash falls and local sources in a subordinate manner in a paleoclimatic context loess paleosol alternation is interpreted to reflect climatic cycles that were primarily caused by the variability of the south american monsoon system

absolute chronology of the pliocene quaternary sediment sequence of the bogota area colombia abstract a revised geochronological framework is presented for the sequence of unconsolidated sediments present in the bogota area eastern cordillera colombia this is based on 11 fission track dates on zircons that were obtained both from exposed ash layers and from a series of ashes from the funza ii borehole which reached 586 m below the surface of the high plain the 3 dates obtained from the exposed ash layers provide control for the older part of the sequence 6 2 5 ma the 8 dates from the funza ii core give control for the younger part of the sequence 3 0 ma acceptance of these new fission track dates on volcanic zircons means that many of the tephra dates obtained from the funza i core that included fission track dates on glass shards and k ar dates using the mineral fractions in the ashes published in 1984 are rejected it also confirms earlier climate stratigraphical dating the neogene quaternary sediments of the bogota area span a period of at least the last 6 ma the fluvial lacustrine sediment record registers major tectonic uplift of the eastern cordillera for the period between 5 and 3 ma the development of the large sedimentary basin of bogota after 3 5 ma with an important phase of tectonic adjustment at about 1 ma and a long period of strong climatic fluctuations that started shortly after 2 7 ma

neogene and quaternary development of the neotropical rain forest the forest refugia hypothesis and a literature overview abstract the upheaval of the northern andes in miocene and pliocene time changed the drainage system in northern south america significantly and caused the present day rain forest areas of choco and the lower magdalena valley became separated from amazonas plant diversity may have reached the highest level in the miocene or pliocene and excessive present day phytodiversity may be regarded as a legacy of the tertiary rather than an evolutionary product of the quaternary in the quaternary strong temperature oscillations related to the series of ice ages were superposed on the late tertiary forest dynamics which included river displacement and latitudinal migrations of the equatorial rain belt caloric equator with the rhythm of the precession cycle of orbital climate forcing the hypothesis that claims a permanent rain forest cover all over the amazon basin during the last glacial is in contrast with the forest refugia hypothesis which accepts replacement of rain forest by savanna or savanna forest during dry climatic intervals both scenarios have been evidenced by pollen records in this paper it is suggested that both hypotheses are not necessarily conflicting and apparently did occur in different parts of the amazon basin and in different periods depending on the climatological constraints a compilation of the most important literature concerning the vegetational climatic and environmental history of the rain forest areas of amazonas and choco and surrounding dry ecosystems has been included

geology lithogeochemistry and u pb geochronology of the aberdeen lake area nunavut new insights into the neoarchean tectonic evolution of the central rae domain abstract the aberdeen lake supracrustal belt is one of several neoarchean greenstone belts preserved within the rae domain of the western churchill province we divide the belt into 1 a lower sequence of ca 2750 ma komatiite ttg tonalite trondhjemite granodiorite intrusions and ttg contaminated fe tholeiitic mafic gneiss 2 a middle sequence of ca

a cross section of the los angeles area seismically active fold and thrust belt the 1987 whittier narrows earthquake and earthquake hazard retrodeformable cross sections across the los angeles area interpret the pliocene to quaternary deformation to be a developing basement involved fold and thrust belt the fold and thrust belt is seismically active as evidenced by the 1987 whittier narrows earthquake ml 5 9 and the 1971 san fernando earthquake mw 6 6 the structural geology of the los angeles area is dominated by three major compressional uplift trends 1 the palos verdes anticlinorium and western shelf 2 the santa monica mountains anticlinorium and 3 the verdugo mountains san rafael hills and the san gabriel mountains these trends result from major thrust ramps off a detachment s at 10 15 km depth thrusts of the verdugo mountains san rafael hills and the san gabriel mountains reach the surface the other two uplifts are associated with blind thrusts compressional seismicity is concentrated along these thrust ramps the 1987 whittier narrows earthquake probably occurred on the elysian park thrust which underlies the santa monica mountains anticlinorium the thrust interpretation accounts for the geometry of the anticlinorium the seismological characteristics of the earthquake and the geometry of coseismic uplift the earthquake and aftershocks occurred within a structurally complex narrow zone of miocene and pliocene northwest trending faults that cross the anticlinorium at a high angle these northwest trending faults are interpreted to be reactivated faults now behaving as tears in the elysian park thrust and not the result of active right lateral deformation extending into the whittier narrows area our analysis suggests the whittier narrows earthquake sequence occurred within a structurally weakened zone along the elysian park thrust we also suggest that the whittier fault is not an important quaternary structure and may not be seismogenic the regional cross section is a nonunique solution and other possible solutions are considered multiple solutions arise from the presence of two intersecting compressional belts in the los angeles area the transverse ranges and the northern peninsular ranges the belts may be due to one or more regional detachments and the northern peninsular ranges may be northeast or southwest vergent the deformed top of the crystalline basement along the regional cross section requires a minimum of 15 0 km of north south convergence between the palos verdes hills and the san andreas fault regardless of the structural solution restoration of our cross section solution requires 21 4 km of north south convergence on top of the crystalline basement including 6 4 km of slip continuing offshore to make structures of the continental borderland and 29 7 km of convergence on the basal detachment geologic relationships suggest major shortening started between early and late pliocene time 2 2 4 0 ma which yields a minimum convergence rate of 3 8 6 8 mm yr between the palos verdes hills and the san andreas fault convergence rates for our solution range from 5 4 to 13 5 mm yr between the edge of the continental borderland and the san andreas fault slip rate estimates for the elysian park thrust along the eastern portion of the santa monica mountains anticlinorium are 2 5 5 2 mm yr if the 1987 whittier narrows earthquake is the characteristic earthquake along the 170 km long anticlinorium then recurrence intervals are estimated to be 5 6 12 1 years this recurrence is higher than the historic seismic record along the santa monica mountains anticlinorium and suggests that 1 our slip rates are too high 2 some crustal shortening is taken up aseismically and or 3 earthquakes larger than the 1987 whittier narrows earthquake will occur along the anticlinorium but less frequently

evidence for vertical partitioning of strike slip and compressional tectonics from seismicity focal mechanisms and stress drops in the east los angeles basin area california we have synthesized the characteristics of the seismogenic zone in the east los angeles basin by analyzing earthquake data recorded during the past 30 years 1981 2010 the seismicity is distributed along the whittier fault with the majority of earthquakes located adjacent to the south side in the depth range from 0 to 9 km with b value of 1 1 0 05 and mostly normal and strike slip faulting within the depth range of 9 12 km the seismicity is scattered uniformly across the region the b value is 1 0 0 05 and all three faulting styles are present at the deepest depths 12 18 km seismicity is sparse and primarily limited to a few clusters striking north these deeper earthquakes primarily have reverse fault motion and the b value is 0 78 0 04 inversion of high quality focal mechanism data for the orientation of the regional stress field showed that the direction of maximum compressional stress rotates from n12 w at shallow depth to due north at the bottom of the seismogenic zone similarly a depth dependence is observed in stress drops calculated from p wave source spectra which indicate stress drop generally increases from 7 mpa at shallow depth 3 km to 53 mpa at the base of the seismogenic zone 17 km overall our results provide new evidence for the vertical partitioning of styles of deformation and state of stress within this complex fault system in the east los angeles basin

recognizing distinct portions of seamounts using volcanic facies analysis examples from the archean slave province nwt canada abstract archean volcanic rocks in the mafic dominated ca 2 66 2 69 ga point lake and beaulieu river belts slave province northwest territories are significant in demonstrating the facies that characterize specific portions of pillow volcanoes or seamounts irrespective of tectonic setting three distinct localities mapped in detail display facies consistent with 1 proximal deep water 2 medial to distal deep water and 3 medial shallow water seamount settings the proximal facies in the point lake belt include a 55 m thick non vesicular pillowed sequence cut by numerous mafic dykes and sills dykes contain multiple chilled margins indicating successive magma pulses which contributed to edifice construction abundant feeder conduits in addition to the absence of fragmental facies and vesicles are typical of the central deep water portion of seamounts where growth is initiated the medial to distal deep water facies in the point lake belt are represented by a 30 80 m thick assemblage of disorganized pillow breccia and pillowed and massive flows with 5 27 vesicularity massive non vesicular hyaloclastite intermingled with sedimentary material fluidal peperite in addition to thin shale units interstratified with pillow breccia and hyaloclastite indicate that sedimentation and volcanism were contemporaneous an increase in fragmental units and vesicularity relative to the proximal deep water facies is suggestive of the medial to distal part of a seamount in shallower water bedded tuffs laterally along strike with massive flows are the results of turbidity current deposition immediately following localized subaqueous eruptions a medial shallow water seamount setting is represented in the beaulieu river belt by a 5 85 m thick sequence of vesicular lobate pillowed and massive flows stratified pillow breccia and hyaloclastite and mafic dykes vesicularity ranges from 21 49 in pillowed flows 5 40 in massive flows and 20 35 in pillow breccia and hyaloclastite stratified pillow breccia developed along steep flow fronts in shallow water whereas bedded hyaloclastite formed during reworking and redeposition of autoclastic hyaloclastite on seamount flanks in shallow water the volcanic facies associations in the study areas are analogous to those of modern seamounts associated with the mid atlantic ridge and east pacific rise as well as mesozoic cenozoic seamounts in the canary islands fiji southwest japan the sea of japan and cyprus volcanological studies in the point lake and beaulieu river volcanic belts and subsequent comparisons with phanerozoic analogues demonstrate the manner in which distinct portions of ancient seamounts can be recognized in similar archean terranes

frequency spectra and paleoclimatic variability of the high resolution 30 1450 ka funza i pollen record eastern cordillera colombia the evolution of orbital periodicities during the quaternary has been studied in the 30 1450 ka funza i pollen record from cores through the sedimentary basin of bogota eastern cordillera colombia the 1178 pollen samples analysed between 2 90 and 340 00 m generate 5 time series with an average time resolution of ca 1200 years our geochronological framework is based on fission track ages on zircon in combination with correlation of the pollen record with the oxygen isotope record of odp site 677 from the eastern pacific the 5 data sets were analysed in the frequency domain with maximum entropy spectrum analysis mesa and thomson multi taper spectrum analysis tmt using a moving 400 ka window with steps of 20 ka frequencies closely related to the milankovitch theory were detected in all 5 data sets the 100 ka period in the arboreal pollen record is restricted to the last ca 800 ka of the quaternary at a confidence level largely higher than 90 periodicities close to 23 ka occur throughout the record at a high significance level above 90 and are strongest in the subparamo data a statistically significant period centred at ca 40 ka also occurs in the funza i record four frequency bands centred at ca 30 ca 15 ca 13 and ca 10 ka were also detected the 5 principle data sets from the funza i pollen record reveal an evolution of orbital frequencies in the middle and late quaternary which are in general agreement with other marine e g imbrie et al 1984 and continental e g kukla et al 1990 records

the rainbow hydrothermal plume 36 15 n mar the rainbow hydrothermal plume was discovered during a recent geophysical survey along 200 km of the mid atlantic ridge mar sw of the azores triple junction in which at least seven new sites of hydrothermal activity were identified here we present the first hydrographic study of the rainbow plume 36 15 n the strongest of the features located during that survey the plume is detectable from real time in situ nephelometer anomalies and extends 10 15km w e in a non transform discontinuity ntd between two adjacent ridge segments maximum anomalies in the rainbow plume indicate particle enrichments at least as large as those seen directly above the trans atlantic geotraverse tag vent field 26 n mar analysis of hydrographic data indicates a vent source at rainbow with a thermal output of up to 98 mw representing 140 of the thermal output previously attributed to tag both lines of evidence indicate the rainbow plume to be the strongest such feature yet found on the mar

hydrothermal scavenging on the juan de fuca ridge 23othxs 10be and rees in ridge flank sediments abstract we have investigated the geochemistry of a hydrothermally enriched sediment core recovered from the western flank of the n cleft segment juan de fuca ridge 8km west of the megaplume area previously identified near 45 n the core contains varying biogenic lithogenic and hydrothermal components as reflected in caco3 al and fe contents respectively horizons of pronounced hydrothermal input in core top sediments and at depth exhibit increased concentrations of fe cu zn and pb and shifts in pb isotopic compositions toward nonradiogenic morb hydrothermal values ree concentrations co vary with hydrothermal fe down core and shale normalised ree distribution patterns exhibit both negative ce anomalies and positive eu anomalies indicative of input from plume particle fall out unsupported 230thxs activities down core are consistent with continuous slow sediment accumulation rates of 0 54 cm ky for 200 ky since the deposition of the deeper fe rich horizon 10be 0 and 9be isotope concentrations also co vary with hydrothermal fe down core and exhibit 10 be o 9 be ratios which approach that of pacific ocean deep water indicative of a seawater scavenging source 10 be 0 230 oth xs 0 ratios throughout most of core gc88 6 are greater than mean pacific ocean values indicating that hydrothermal scavenging can lead to significant net removal of dissolved 10be into ridge flank sediments

the rainbow vent fluids 36 14 n mar the influence of ultramafic rocks and phase separation on trace metal content in mid atlantic ridge hydrothermal fluids fluids were collected from the rainbow vent field 36 14 n on the mid atlantic ridge mar during the 1997 diving flores cruise this vent field in ultramafic rocks at a depth of 2300 m is composed of 10 black smokers emitting acidic ph 2 8 fluids at 365 c the low ph of the hot temperature rainbow fluids likely results from seawater ultramafic rock interaction that releases h ions into reducing hydrothermal fluids fluid chemistry is strongly influenced by phase separation generating cl rich brines clem 750 mm strongly enriched with mn fe co ni cu zn ag cd cs pb y and rare earth elements ree ree and transition metal abundance particularly fe and mn in the rainbow fluids is dramatically higher than in other mar fluids the abundance of trace element and ree enrichment is due to the greater solubility of these elements that is strongly favored by cl complexation at low ph and high temperature conditions chondrite normalized ree patterns show strong lree enrichment with evidence of the typical eu anomaly this ree partitioning suggests that either 1 ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and or 2 that the unique rainbow fluid temperature ph and redox state issued from the ultramafic character of leached substratum can produce unique ree partitioning

phosphate removal by oceanic hydrothermal processes an update of the phosphorus budget in the oceans abstract we present a compilation of dissolved phosphate and solid phase p data from the oceanic crust to evaluate the effects of hydrothermal processes on the oceanic budget of p concentrations of phosphate in fluids that emanate from ridge axis hydrothermal systems are less than that in bottom seawater the extent of removal in these fluids is at least 30 and in some hydrothermal systems dissolved phosphate is removed completely from the circulating fluid evidence for the removal of phosphate in each of six ridge flank hydrothermal systems is based on systematic variations in porewater profiles of phosphate and speeds of porewater flow the extent of removal is 80 in these ridge flank systems these removal processes are recorded in the basaltic crust as an increase in p concentration that coincides with an increase in extent of alteration and content of ferric iron phosphate also is removed in hydrothermal plumes by coprecipitation with fe oxyhydroxide particles which eventually deposit on the seafloor each of these hydrothermal processes results in a flux of p into the oceanic crust bottom seawater flow through ridge axis hydrothermal systems removes at most 0 4 of the preindustrial dissolved riverine flux of p while ridge flanks remove at least 5 but less than 50 of the dissolved riverine flux consistent with p data from deep sea drilling project dsdp sites 417 and 418 removal of phosphate by coprecipitation with fe rich particles in hydrothermal plumes along ridge axes accounts for 18 33 of the dissolved riverine flux thus hydrothermal systems remove about 50 of the preindustrial dissolved riverine flux of phosphate we have included these new estimates in a revised budget for oceanic p

40ar 39ar dating on tephra of the velay maars france implications for the late pleistocene proxy climatic record abstract pollen contents and magnetic records of the pleistocene sedimentary sequences cored in the praclaux and lac du bouchet maars velay region french massif central demonstrated that they have high potential as detailed climatic and environmental archives tephra layers interbedded in these sequences have been studied in order to perform isotopic dating first petrographic studies and 40 ar 39 ar ages have shown that these tephra layers are the witness of the same eruption from the sancy volcano centre located 100 km to the north according to fourteen 40 ar 39 ar ages carried out on sanidine and biotite grains this eruption occurred 275 5 ka ago this result allows to construct a time scale for a composite high resolution record of the climate covering the last 300 000 years and to establish an absolute chronology for the palaeoclimatic events recorded in these sequences these tephra layers are interbedded in organic layers deposited during an interstadial stage called the amargiers interstadial which according to its age can be considered as equivalent to isotopic stage 9a moreover the geographical extension of the tephra and its well constrained age give to this tephra the interest of a chronostratigraphic marker for upper quaternary sequences in southeastern france

the coso geothermal field a nascent metamorphic core complex investigation of the coso range using seismicity gravity and geochemistry of rocks and fluids supports the interpretation that the structure hosting the geothermal resource is a nascent metamorphic core complex the structural setting is a releasing bend in a dextral strike slip system that extends from the indian wells valley northward into the owens valley this tectonic setting results in nw directed transtension which is accommodated by normal and strike slip faulting of the brittle upper 4 6 km of the crust and shearing and ductile stretching below this depth accompanied by shallow igneous intrusions focal mechanisms of some small earthquakes that have occurred from 1996 to the present beneath the coso range exhibit depth dependent rotation of seismic p and t axes indicating that the local orientations of the principal stresses likely favor resolved shear stress on low angle faults these small earthquakes occur near the base of seismicity which we interpret as coincident with the brittle ductile transition geochemical results show a significant asthenospheric influence in the isotopic composition of rocks and fluids indicating that the crust is thinned within the coso structure thinned upper crust is underlain by a more dense mafic body at depths of 10 km or less this is consistent with observed gravity anomalies and models the mafic body may represent cumulates left over from the fractional crystallization of rhyolite which occurs as endogenous domes at coso or it could be a sheeted dike complex in the upper mid crustal area transtension began at 2 3 ma and continues today using a long term crustal deformation rate of 2 mm yr we infer that the basal detachment fault commonly observed in fully exhumed metamorphic core complexes will reach the surface in two to four million years