petrologic determination of ascent rates for the 1995 1997 soufriere hills volcano andesitic magma the recent eruption of the soufriere hills volcano in montserrat july 1995 to present october 1997 has produced a hornblende bearing andesitic lava dome it is possible to petrologically estimate changes in ascent rates of amphibole bearing magmas for certain rates of decompression a breakdown rim of fine grained anhydrous reaction products forms where amphibole is in contact with melt the thickness of the rim varies with ascent rate most of the amphibole phenocrysts in the magma storage region lack breakdown rims about 10 have 200 400 m thick coarse grained breakdown rims that are interpreted to be relicts of a past heating event study of a time series of new dome andesites showed that ascent rate increased from december 1995 0 001 m s to july 1996 0 008 m s while eruptive style remained extrusive ascent rate increased to 0 012 m s in august 1996 and the first major explosive eruption occurred on 17 18 september 1996

what controlled mid late miocene long term aridification in central asia global cooling or tibetan plateau uplift a review abstract debate continues over whether global cooling or uplift of the tibetan plateau provided the first order driver for the aridification moisture levels of central asia throughout the mid late miocene between about 17 and 5 ma this review attempts to throw new light on the relations between the aridification and these two key factors this paper examines the evolution of miocene climate both moisture and temperature within five separate regions of eurasia to help understand the large scale controls of long term moisture in central asia the five regions include 1 europe 2 high latitude asia 3 the east asian monsoon region 4 the south asian monsoon region and 5 central asia itself because moisture reaching central asia has to firstly cross at least one of the other four regions temperature proxy data compiled from these five regions correlate with ocean temperatures from the global deep sea oxygen isotope records furthermore compiled moisture proxy data from the four regions surrounding central asia co vary and correlate with each other this can be explained by positive feedbacks between drying and cooling and supports the assumption that global cooling provides a dominant driving factor for the drying of eurasia global cooling reduces the amount of water vapor held in the atmosphere and thereby can cause terrestrial drying however in central asia the moisture evolution shows less similarity with its surroundings the uplift of the tibetan plateau including the tianshan mountains could provide a possible explanation for this difference the changing topography resulting from uplift events over time and space strongly influenced the moisture patterns in central asia during miocene times future research on the spatial timing and amplitude of tibetan plateau uplift should be useful to understand the moisture processes in central asia during the miocene

iron oxide characteristics of the chinese loess red clay sequences and their implications for the evolution of the east asian summer monsoon since the late oligocene abstract the most significant climatic change in east asia involved the formation and evolution of the coupled monsoon arid environmental system during the cenozoic which was dynamically linked to global cooling and the growth of the qinghai tibet plateau however our understanding of monsoon evolution since the late oligocene remains controversial due to differences in the interpretation of various environmental proxies from loess archives in this study the composition and relative abundance of pedogenic iron oxides of two eolian sequences from the lingtai section and the zhuanglang drill core were investigated using rock magnetic optical spectroscopy and geochemical analyses the results indicate that while the composition of iron oxides in the mio pliocene red clay and quaternary loess paleosol sequences is similar the concentrations of magnetite maghemite hematite and goethite are quite different between loess paleosol and red clay the combination of magnetic proxies magnetic susceptibility fed fet and color a values reveals that precipitation and temperature played varying roles in driving pedogenesis and chemical weathering since the late oligocene during the late oligocene to early miocene the occurrence of relatively strong pedogenesis and chemical weathering was likely the result of high temperatures rather than high precipitation the east asian summer monsoon intensified during the middle miocene climate optimum 16 5 14 5 ma in response to global climatic warming subsequently during the middle to late miocene the monsoon intensity gradually weakened due to global cooling after 8 ma the summer monsoon strengthened substantially and reached a maximum during 4 5 2 6 ma possibly as a result of the uplift of the northeastern margin of the qinghai tibetan plateau during the late miocene the monsoon intensity then commenced a decreasing trend in the early pleistocene but then strengthened again after 1 0 ma

evidence for northeastern tibetan plateau uplift between 25 and 20 ma in the sedimentary archive of the xining basin northwestern china the growth history of the tibetan plateau provides a valuable natural laboratory to understand tectonic processes of the india asia collision and their impact on and interactions with asian and global climate change however both tibetan plateau growth and asian paleoenvironments are generally poorly documented in pre pliocene times and reflect limited temporal coverage for different parts of the plateau here we present magnetostratigraphic results from the xining basin at the ne margin of the tibetan plateau precisely dating the record between the earliest oligocene 33 ma to the middle miocene 16 ma the pattern of observed paleomagnetic polarity zones is unequivocally correlated to the geomagnetic polarity time scale gpts indicating relatively constant and low sediment accumulation rates 32 m myr except for a peculiar period of unstable accumulation between 25 3 and 19 7ma at the beginning of this interval a marked permanent increase in magnetite content of the sediments is observed and likely relates to a change in provenance we directly relate this unstable period of sediment accumulation and provenance change to the coeval exhumation recently reported by low temperature thermochronology from the laji shan range which subsequently formed the southernmargin of the xining basin evidence for ne tibet tectonismat 25 20 ma can be associated with widespread deformation over the entire himalayan tibetan orogen at this time which may be linked to the coeval appearance of monsoon climate in eastern asia and the onset of central asian desertification

landform classification and soil distribution in hummocky terrain saskatchewan canada this study presents a classification of distinct three dimensional landform elements and examines the relationship between these landform elements and selected soil morphological properties of udic boroll soils in southern saskatchewan canada the classification is based on defined ranges of three criteria derived from topographic data gradient profile downslope curvature and plan across slope curvature seven landform elements are recognized convergent shoulders divergent shoulders convergent backslopes divergent backslopes convergent footslopes divergent footslopes and level elements all of the elements are easily identified in the field the thicknesses of a horizons and depths to calcium carbonate of the soils were consistently greater in convergent versus divergent elements in the same profile group e g shoulders and showed an overall increase in the sequence shoulders backslopes level footslope elements distinct groups of elements emerge from a statistical analysis of the differences between individual and bivariate means of soil morphological properties associated with each landform element the differences in soil properties among the elements can be largely explained by differences in water movement and distribution in hillslope systems the results of the study highlight the need to consider land surface morphology during the development of research designs for soil related studies

partly pedogenic origin of magnetic variations in chinese loess quaternary sequences of interbedded aeolian silts loess and buried soils palaeosols potentially provide one of the best terrestrial records of past climates1 5 magnetic susceptibility variations in loess and palaeosol sequences from china are strongly correlated with climate induced fluctuations of oxygen isotope ratios in deep sea sediments2 4 6 8 as an explanation of this correlation it has been suggested that the recorded variations in magnetic susceptibility depend primarily on the degree of dilution by non magnetic bulk loess constituents of a uniform magnetic assemblage derived from remote but unknown sources6 8 it has also been suggested that such a model involving changes in the accumulation rate of loess superimposed on a constant flux of magnetic particles provides a relative geological timescale through the control of loess deposition by astronomically modulated climate fluctuations7 here we report a preliminary test of this model using rock magnetic properties especially sensitive to variations in magnetic grain size our results show that palaeosols are characterized by much finer magnetic grain size assemblages than are the intervening loess units this suggests that a simple model based on constant magnetic influx and dilution by variable amounts of non magnetic loess is inadequate our magnetic measurements establish the close comparability of the fine grained magnetic minerals in the palaeosol samples to those in contemporary soils and thus point to a partially pedogenic origin for the magnetic mineral assemblages in the palaeosols

cenozoic magnetostratigraphy of the xining basin ne tibetan plateau and its constraints on paleontological sedimentological and tectonomorphological evolution abstract the xining basin is well known for its thick and continuous sequence of fine grained cenozoic sediments in addition it contains important xiejia fauna which define the xiejian stage of the early miocene in the standard chinese land mammal zonation and it also provides detailed histories of eolian dust deposition which document the aridification of asia monsoon evolution and the initiation of the yellow river in relation to the uplift of the tibetan plateau however the results of magnetostratigraphic dating of the fossil bearing sequence has yielded conflicting ages for the fauna and stratigraphy hindering the use of the sequence for addressing major paleoenvironmental and paleoclimatic issues here we review the paleontological lithological and magnetostratigraphic record of the xining basin and provide a new long and continuous high resolution magnetostratigraphy from the basin center the results show that the observed magnetic polarity zones from the various sections at different sites in the basin exhibit similar magnetic polarity patterns which can be readily correlated in terms of both magnetic polarity zonation and lithofacies in addition they show that the sedimentary sequence of the basin was sub continuously deposited from 54 ma to 4 8 ma the magnetostratigraphic correlations confirm previous constraint of the xiejia fauna in the late oligocene at 25 ma challenging the current chinese mammal land zonation for the early miocene and we suggest that it provides a superior record of eolian dust deposition and river incision in the late pliocene than previous studies there is a close match of the climatic proxy records of the xiejia section dated by magnetostratigraphy with the global climatic record which corroborates the paleomagnetic correlations the evolution of the sedimentary environment provided by the lithofacies demonstrates a complete cycle of basin formation and termination which records the eastern qilian shan experienced three main phases of uplifts slow episodic uplifts at 54 ma and 22 5 ma and a late rapid episodic uplifts at 8 7 ma and since 4 8 3 6 ma this tectonic uplift in the ne tibetan plateau was mostly nearly synchronous responses to the initial and continuing collision of india and asia since 50 5 ma

review of recent developments in mineral magnetism of the chinese loess mineral magnetism of the chinese loess has been investigated for more than 20 years although there is a consensus that the neoformation of fine grained maghemite particles in the superparamagnetic sp and single domain sd grain size regions accounts for the magnetic enhancement in the chinese paleosols quantitative retrieval of paleoclimatic signals in terms of rock magnetic proxies is still a subject of debate the ambiguities arise from the inherent complexities of magnetic proxies as well as the multiple factors that control the pedogenic processes therefore a better description of the magnetic assemblage including its mineralogy grain size distribution and stoichiometry of two distinct origins pedogenic and eolian can help us better understand mechanisms behind variations in magnetic proxies at different timescales in order to link them to the paleoclimatic processes this review focuses on recent developments in loess magnetism and carefully evaluates merits and limitations of rock magnetic proxies furthermore several currently unsolved problems are addressed

paleomagnetic constraints on the middle miocene early pliocene stratigraphy in the xining basin ne tibetan plateau and the geologic implications the xining basin lies in the transitional zone between the arid asian interior and the east asian monsoon region the continuous cenozoic sediments in the basin provide a unique archive recording the uplift of the tibetan plateau and its environmental effects on central asian aridification and asian monsoon evolution however sediments deposited since the middle middle miocene have not been precisely dated hindering our ability to address these issues here we dated a 336 m thick section containing many late miocene fossil mammals from the eastern basin high resolution paleomagnetism revealed 16 normal and 16 reversed zones that correlate well with chrons 3n to 5ar 1r of the geomagnetic polarity time scale constraining the section to 12 7 4 8 ma the changes in lithofacies from floodplain to braided river at 8 6 ma and to thick alluvial fan at 6 3 ma with predominantly southerly paleocurrent directions occur simultaneously with an increase in the sedimentation rates representing two periods of rapid uplift in the eastern qilian shan to the north our results provide a robust oldest age constraint 4 8 ma on the ancient huang shui and yellow river terraces on the basin sediments and eolian deposits on the terraces the presence of well developed floodplain paleosols shallow lake marls and many large mammals in the lower section suggests that the monsoon climate in the late middle miocene to late miocene was more humid and stronger than that at present

growth of the lava dome and extrusion rates at soufriere hills volcano montserrat west indies 2005 2008 the third episode of lava dome growth at soufriere hills volcano began 1 august 2005 and ended 20 april 2007 volumes of the dome and talus produced were measured using a photo based method with a calibrated camera for increased accuracy the total dense rock equivalent dre volume of extruded andesite magma 306 51 mm3 was similar within error to that produced in the earlier episodes but the average extrusion rate was 5 6 0 9 m3s 1 dre higher than the previous episodes extrusion rates varied in a pulsatory manner from 0 5 m3s 1 to 20 m3s 1 on 18 may 2006 the lava dome had reached a volume of 85 mm3 dre and it was removed in its entirety during a massive dome collapse on 20 may 2006 extrusion began again almost immediately and built a dome of 170 mm3 dre with a summit height 1047 m above sea level by 4 april 2007 there were few moderate sized dome collapses 1 10 mm3 during this extrusive episode in contrast to the first episode of dome growth in 1995 8 when they were numerous the first and third episodes of dome growth showed a similar pattern of low 0 5 m3s 1 but increasing magma flux during the early stages with steady high flux after extrusion of 25 mm3

miocene pollen record of kc 1 core in the qaidam basin ne tibetan plateau and implications for evolution of the east asian monsoon abstract thick cenozoic deposits in the qaidam basin provide great potential for understanding the tectonic history paleoclimatic changes and evolution of the east asian monsoon this study examines the pollen record from the kc 1 core for the interval covering the later early to late miocene 18 5 ma thermophilic taxa percentages are high between 18 and 14 ma and decrease after this time a pattern which fits well with the middle miocene climatic optimum mmco between 18 and 14 ma and global climatic cooling after 14 ma during the same period xerophytic taxa percentages gradually increase and those of the conifers gradually decrease suggesting an aridification process in the qaidam region driven by the gradual strengthening of the east asian winter monsoon eawm and weakening of the east asian summer monsoon easm the global climate cooling process appears to have driven the climatic development of the qaidam basin region throughout the miocene but the uplift of the tibetan plateau also contributed

grain sizes of susceptibility and anhysteretic remanent magnetization carriers in chinese loess paleosol sequences detailed rock magnetic studies show that susceptibility mass specific and anhysteretic remanent magnetization arm of the chinese loess paleosol sequences are carried by almost identical magnetic carriers therefore the ratio arm or equivalently arm where is defined as 0 and 0 is the intercept susceptibility of the plot of versus arm and arm is field normalized arm can be used to quantify the grain size of and arm carriers by determining this ratio for three chinese loess paleosol profiles jiuzhoutai yuanbao and yichuan characterized by different degrees of environmentally controlled pedogenesis and sedimentation rates we show that the lower grain size limit of aeolian magnetic particles in the less pedogenically altered loess units is about 100 300 nm in the finer grained pseudosingle domain psd grain size range in contrast the grain sizes of pedogenically produced magnetic particles for mature paleosols dominantly cover both the superparamagnetic sp and single domain sd ranges on the basis of plots of arm against samples can be divided into four regions i ii iii and iv region i corresponds to the least pedogenically altered primary loess samples with arm of 0 165 0 24 samples in region ii a transition zone between the least altered loess and the onset of development of paleosols have values identical to those in region i but have lower arm of 0 09 0 165 with increasing susceptibility in zone iii arm is positively correlated with indicating the gradually increasing influence of sp particles finally in zone iv with higher than 6 5 10 7 m3 kg 1 arm is independent of the variations in suggesting that arm is totally controlled by the pedogenic finest grained particles and the size distribution of these particles remains almost constant the development of soils in the chinese loess revealed by these three profiles from three sites can be clearly explained by a continuous process of pedogenesis increasing from zone i to zone iv the definition of the pedogenic zones can help to improve our understanding of the underlying mechanisms and variability of pedogenesis and thus could enable more successful and accurate separation of the authentic pedogenic signals from the background signal of the aeolian inputs at different loess sites worldwide

an overview of lava dome evolution dome collapse and cyclicity at soufriere hills volcano montserrat 2005 2007 1 the third episode of lava dome growth at soufriere hills volcano montserrat was characterised by higher average magma discharge rates than either previous dome growth episode at this volcano and yet fewer collapses during sustained dome growth at moderate high average rates 6 m3 s we identified 2 6 week discharge pulses that each supplied c 20 mm3 magma from depth our observations are consistent with some existing models but we explain discrepancies by a combination of higher volatile contents and higher ascent rates cycles of c 11 16 days were evident in rockfall lp rockfall and shallow lp earthquake counts related to dome growth and degassing we speculate that degassing at the conduit margins together with stick slip conduit flow may drive these cycles only one major collapse 10 mm3 occurred during the third episode on may 20 2006 as a new magma pulse entered the dome and coincided with heavy rainfall

cyclical patterns in volcanic degassing revealed by so2 flux timeseries analysis an application to soufriere hills volcano montserrat abstract cyclical patterns of behaviour in timeseries of seismic and geodetic data at volcanoes are frequently observed during lava dome building eruptions and are particularly well documented from the current eruption of the soufriere hills volcano shv montserrat however the discontinuous nature of many so 2 measurements often preclude the identification and quantitative analysis of cyclical patterns in degassing data here using a long so 2 timeseries from shv with continuous measurements since 2002 we explore for the first time degassing behaviour at a resolution comparable to that possible for seismic and deformation datasets timeseries analysis of flux data spanning 2002 2011 reveals that so 2 emissions at shv exhibit complex cyclicity with dominant cycles evident on both multi year and multi week 50 day timescales these cycles persist through phases of both active extrusion and eruptive pause and show close similarities to periodic components previously identified at shv in timeseries of seismicity ground deformation and lava extrusion the strength of expression or amplitude of degassing cycles particularly on multi week timescales shows distinct temporal variation and appears to correlate with the occurrence and nature of explosive activity occurring in 2002 2009 this suggests that the amplitude of surface gas flux cycles is modulated by physical conditions within the conduit direct quantitative comparison between seismicity dome growth and degassing for eruptive phases 2 2002 2003 and 3 2005 2007 reveals that peaks in so 2 flux appear to correspond broadly to enhanced lava extrusion and elevated seismicity within cycles of 30 50 days however time lags of 2 4 and 7 days are observed between initial low frequency seismic swarms and peaks in dome growth so 2 flux and rockfall event rate respectively multi parameter correlations offer valuable insights into the controls on subsurface gas ascent but further research is required to fully explore the contributions of permeability and overpressure as well as other subsurface processes

a 20 ma pollen record of east asian summer monsoon evolution from guyuan ningxia china abstract pollen analysis was conducted on the fluviolacustrine sediments from the east side of the liupan mountains a relatively monsoon sensitive region and provided for the first time a continuous evolutionary history of vegetation and climate in east china during the neogene the pollen record spanning the last 20 ma indicates that the vegetation and thus east asian summer monsoon evolution can be divided into three stages 20 13 14 25 ma 14 25 11 35 ma and 11 35 0 08 ma in spite of general dominance of steppe throughout the neogene during the period 20 13 14 25 ma the pollen assemblage was characterized by high abundance of artemisia and small amounts of temperate to subtropical deciduous broad leafed trees such as juglans carya quercus and betula suggesting that the east asian summer monsoon was generally strong the interval from 14 25 to 11 35 ma was marked by a significant decline in the east asian summer monsoon as indicated by artemisia gradually replaced by humulus and halophytic chenopodiaceae nitraria and ephedra also reached high values in this period since 11 35 ma ago percentages of halophytes dominated by chenopodiaceae and tamaricaceae maintained a high mean value of about 40 revealing a weak intensity of east asian summer monsoon the general weakening of the monsoon circulation during the neogene and the significant monsoon decline during the late middle miocene in particular might link with the global cooling trend probably through two mechanisms cooling and ice sheet expansion over the polar region caused an increase in meridional temperature gradients leading to the southward retreat of the monsoon circulation the global cooling may have also reduced the amount of water vapor held in the atmosphere and led to both additional cooling and further weakening of the east asian summer monsoon the cause for a short lived resumption during the period 1 0 0 8 ma is under investigation

step wise change of asian interior climate preceding the eocene oligocene transition eot abstract understanding the global climate change from greenhouse to icehouse conditions at the eocene oligocene transition eot 34 million years ago requires climatic records from oceanic as well as continental realms of the key late eocene doubthouse period preceding this switch here we report integrated stratigraphic results from well dated late eocene continental mudflat to saline lake paleoenvironments of the xining basin northeastern tibetan plateau western china recording regional and global change cyclostratigraphic analysis strongly suggests continuous dominance of the 41 kyr obliquity cycle in the whole late eocene interval down to the base of polarity chron c18n 2n at 39 ma with additional input of the 100 kyr eccentricity cycle up to the base of chron c13r at 34 7 ma this might imply that high latitude climates dominated the area long before the eot probably related to incipient ice volume fluctuations furthermore our results reveal two paleoenvironmental deterioration steps preceding the eocene oligocene transition the first step occurs in the top of chron c17n 1n at 36 6 ma this age closely corresponds to 1 the high altitude pollen appearance in chron c16 2r at 36 4 ma in the same section 2 the recently dated final retreat of the tarim sea in western china and 3 a shift from precession to obliquity dominance in the atlantic ocean this near co occurrence suggests global change at this time we hypothesize this change is related to an increase in incipient ice sheet volume leading to passing threshold conditions for the high altitude pollen appearance and tarim sea retreat finally leading to decreased moisture availability in the xining basin at the second step in the base of chron c13r at 34 7 ma a substantial increase in clastic sedimentation rates is observed this might relate to increased climate variability preceding the greenhouse to icehouse transition at the eot that prevented landscapes to attain equilibrium configurations

color reflectance of chinese loess and its implications for climate gradient changes during the last two glacial interglacial cycles 1 to examine the relationship between color changes of loess deposits and paleoclimate conditions a north south transect consisting of twelve loess sections were studied color reflectance was measured on loess paleosol samples from the loess transect which accumulated over the last two glacial interglacial cycles the redness value a shows an overall southward increase for both loess and soil units coinciding with the pattern of a southerly increase in pedogenic development and the present north south climatic gradient the redness of loess deposits is linearly correlated with latitude values implying that this parameter is an efficient proxy for the weathering intensity of loess deposits the gradient of weathering intensity is much steeper during interglacials than in glacials by comparison of color records for different time interval

cenozoic tectonic evolution of the elashan range and its surroundings northern tibetan plateau as constrained by paleomagnetism and apatite fission track analyses abstract magnetostratigraphic studies of sediments in the western gonghe and eastern qaidam basins together with apatite fission track aft analyses in the elashan range suggest that the elashan range and its surroundings experienced a two stage evolution a protracted uplift from the cretaceous to the early tertiary which explains the absence of the paleogene strata in the region and an accelerated growth of the elashan range since the latest oligocene caused by extensively distributed thrust faults in the region this later phase of uplift resulted in the subsequent thick sediments within the both easternmost qaidam and gonghe basins basal ages of 21 22 ma in the wulan section of 20 ma in the southern gonghe section and of l2 ma in the chaka section the two phases of thrust motion and range growth mean that the qaidam and gonghe basins were never united during cenozoic times the strike slip is likely to have occurred at l0 ma as shown by the paleomagnetic declination data which show a rapid transition in rotation sense from counterclockwise to clockwise at that time this time is also similar to the 9 ma age of initiation of the wenquan fault determined by backward calculation based on its late quaternary slip rate eventually the qinghai nan shan thrust belt began to develop at 6 ma

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

miocene cooling in the northern qilian shan northeastern margin of the tibetan plateau revealed by apatite fission track and vitrinite reflectance analysis apatite fission track and vitrinite reflectance data from phanerozoic rocks of the northern qilian shan and jiuxi basin at the northeastern margin of the tibetan plateau show a complex thermal history with multiple paleothermal events peak paleotemperatures reached in the middle cretaceous suggest that hydrocarbon generation occurred in the basin at that time paleozoic mesozoic samples show evidence of a middle tertiary cooling episode between 20 and 10 ma this thermal fingerprint is significant because stratigraphic evidence for miocene exhumation is overwhelmed by spectacular pliocene pleistocene deformation and erosion we interpret the miocene cooling as recording initial exhumation of the northern qilian shan earlier than other models have proposed i e after 6 5 ma miocene cooling is broadly coincident with cenozoic uplift and exhumation recorded elsewhere in the tibetan tarim region

coupled subdaily and multiweek cycles during the lava dome eruption of soufriere hills volcano montserrat 1 observations of volcanoes extruding andesitic lava to produce lava domes often reveal cyclic behavior at soufriere hills volcano montserrat cycles with subdaily and multiweek periods have been recognized on many occasions observations clearly show that the period of subdaily cycles is modulated by the multiweek cycle the subdaily and multiweek cycles have been modeled separately as stick slip magma flow at the junction between a dyke and an overlying cylindrical conduit and as the filling and discharge of magma through the elastic walled dyke respectively here we couple these two models to describe the behavior over a period of well observed multiweek cycles with accompanying subdaily cycles from 13 may to 21 september 1997 the coupled model captures well the asymmetrical first order behavior the first 40 of the multiweek cycle consists of high rates of lava extrusion during short period high amplitude subdaily cycles as the dyke reservoir discharges itself the remainder of the cycle involves increasing pressurization as more magma is stored and extrusion rate falls followed by a gradual increase in the period of the subdaily cycles

vegetation and climatic changes during the middle miocene in the wushan basin northeastern tibetan plateau evidence from a high resolution palynological record abstract there remains no detailed record of the middle miocene vegetation and climatic changes which occurred in central asia and their possible driving mechanisms this is because there is still a lack of high resolution records here we present a sporopollen record from the wushan basin on the northeastern tibetan plateau central asia spanning the period 16 1 13 6 ma the sporopollen record shows that a dense mixed forest growing in rather warm and humid climatic conditions was affected by a general drying trend during the period 16 1 15 ma it demonstrates that although the climate was generally warm and humid during the middle miocene climate optimum mmco it was also a time of climatic instability the dense forest gave way to an open forest in response to a less humid climate between 15 and 14 4 ma dense mixed forest made a return with an increasingly humid climate during 14 4 13 8 ma this vegetation and climatic succession could be associated with global cooling or more particularly a higher temperature rebound set against the background of a long term cooling trend a more open forest appearing in response to drier and colder climatic conditions dominated the study area during the 13 8 13 6 ma period this could be compared to the rapid global cooling event mi 3b this significant global cooling event exerted a major impact on terrestrial vegetation climate and biota our high resolution sporopollen record demonstrates that global climate changes could have been the first order driving force for the middle miocene vegetation and climate changes seen in the wushan basin in central continental asia with the tectonic uplift of the tibetan plateau probably playing a subordinate role

mesozoic cenozoic evolution of the xining minhe and dangchang basins northeastern tibetan plateau magnetostratigraphic and biostratigraphic results 1 accurate stratigraphic ages are crucial to understanding the deformation history of the tibetan plateau prior to and during the indo asian collision efforts to quantify mesozoiccenozoic ages are hindered by limited fossils and a paucity of volcanic horizons and regionally correlative strata magnetostratigraphic and biostratigraphic results from the xining minhe longzhong basin complex and dangchang basin provide an improved chronology of nonmarine basin development over a large region of the northeastern tibetan plateau 34 37 n 101 105 e analyses of 171 magnetostratigraphic levels and 24 palynological assemblages 120 species indicate late jurassic early cretaceous to mid tertiary deposition although magnetic polarity zonation is incomplete independent palynological age control partially restricts possible correlations to the geomagnetic polarity timescale the sediment accumulation record basin provenance structural geology and published thermochronological data support a history of jurassic exhumation late jurassic early cretaceous fault related basin initiation and cretaceous paleogene reduced accumulation these patterns which are compatible with late jurassic early cretaceous extension and cretaceous paleogene postrift thermal subsidence were disrupted at about 40 30 ma when shortening related to the indo asian collision induced localized range uplift vertical axis rotation and amplified subsidence indexterms 1520 geomagnetism and paleomagnetism magnetostratigraphy 8102 tectonophysics continental contractional orogenic belts 9320 information related to geographic region asia 9604 information related to geologic time cenozoic 9609 information related to geologic time mesozoic keywords tectonics magnetostratigraphy tibetan plateau cenozoic mesozoic sedimentary basins

tibetan uplift prior to the eocene oligocene climate transition evidence from pollen analysis of the xining basin uplift of the tibetan plateau and the himalayas since the onset of the indo asia collision is held responsible for asian aridifi cation and monsoon intensifi cation but may also have gradually cooled global climate leading to the 34 ma eocene oligocene transition to unravel the interplay between tibetan uplift and global climate proxy records of asian paleoenvironments constrained by accurate age models are needed for the paleogene period here we report the 38 ma appearance of high altitude vegetation recovered from palynological assemblages in precisely dated lacustrine sediments from the xining basin of the northeastern tibetan plateau region this result confi rms previous evidence for important regional uplift in the central and northern tibetan plateau regions during the early stage of the indo asia collision this is consistent with the idea that the associated increase in rock weathering and erosion contributed to lowering of atmospheric co 2 leading to the eocene oligocene transition

magnetostratigraphy of cenozoic sediments from the xining basin tectonic implications for the northeastern tibetan plateau 1 the xining subbasin of the longzhong basin holds the longest continuous cenozoic stratigraphic record at the margin of the northeastern tibetan plateau despite a rich biostratigraphic content including the xiejia mammal fauna the tectonic evolution of the basin is largely unconstrained in this study we present stratigraphic biostratigraphic and magnetostratigraphic results that provide a basis for reconstructing the cenozoic tectonic evolution of the xining basin with respect to adjacent regions of the northeastern tibetan plateau magnetostratigraphic analysis from three red bed sections in the xining basin indicates continuous deposition at low and nearly constant accumulation rates average 2 2 cm kyr from 52 0 to 17 0 ma we interpret this result to indicate that no major regional tectonic event implying large sediment accumulation variations has affected the xining basin deposition during this considerable time window in detail accumulation rate variations outline a three stage evolution with 1 8 cm kyr from 52 0 ma to 34 5 ma 4 1 cm kyr from 34 5 to 31 0 ma and 2 3 cm kyr from 31 0 to 17 0 ma the second order increase between 34 5 and 31 0 ma can be interpreted to result from either a distal tectonic event or to be of climatic origin although the region was tectonically quiescent for much of the cenozoic tectonic activity occurred during basin initiation or reactivation at circa 55 0 52 5 ma and during intense basin deformation after 17 0 ma

the middle miocene climatic transition east antarctic ice sheet development deep ocean circulation and global carbon cycling abstract the middle miocene represents a major change in state in cenozoic climatic evolution following the climax of neogene warmth in the late early miocene at 16 ma the early stage of this climatic transition from 16 to 14 8 ma was marked by major short term variations in global climates east antarctic ice sheet eais volume sea level and deep ocean circulation in the later stage from 14 8 to 12 9 ma climatic developments included major growth of the eais and associated antarctic cooling a distinct increase in the meridional temperature gradient large fluctuations in sea level followed by a global sea level fall and important changes in deep water circulation including increased production of southern component water east antarctic ice sheet growth and polar cooling also had large effects on global carbon cycling and on the terrestrial biosphere including aridification of mid latitude continental regions increased stability of the eais after 14 8 ma represents a crucial step in the establishment of late neogene global climate systems what controlled these changes in polar climates and the east antarctic ice sheet deep ocean circulation changes probably played a major role in the evolution and variation in polar climates as they have throughout the cenozoic oxygen and carbon isotopic evidence for warm saline deep water production in the eastern tethyan northern indian ocean indicates that meridional heat transport to the antarctic inhibited cenozoic polar cooling and eais growth during the early middle miocene from 16 to 14 8 ma inferred competition between warm low latitude sources derived from the eastern tethyan northern indian ocean and a cold high latitude source southern component water from 16 to 14 8 ma may have been associated with instability in the antarctic climate and cryosphere reduction of warm saline deep water flow to the southern ocean at 14 8 ma may have decreased meridional heat transport to the antarctic cooling the region and leading to increased production of southern component water these middle miocene climatic and cryospheric changes in the antarctic had profound effects on marine and terrestrial climates as the meridional surface temperature gradient increased boundaries between climatic zones strengthened leading to increased aridification of mid latitude continental regions in australia africa and north and south america enhancing the development of grasslands and stimulating the evolution of grazing mammals

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

a pedogenic model for the magnetic enhancement of late miocene fluvial lacustrine sediments from the xining basin ne tibetan plateau

a late eocene palynological record of climate change and tibetan plateau uplift xining basin china climate models suggest that asian paleoenvironments monsoons and continental aridification were primarily governed by tectonic uplift and sea retreat since the eocene with potential contribution of global climate changes however the cause and timing of these paleoenvironmental changes remain poorly constrained the recently well dated continental mudflat to ephemeral saline lake sedimentary succession situated in the xining basin at the northeastern margin of the tibetan plateau nw china provides a unique opportunity to develop additional proxy successions in this area that are placed accurately in time here a palynological record from this succession is reported high abundances of desert and steppe desert taxa such as ephedripites and nitrariadites nitraripollis are found which can be differentiated by the presence of broad leaved deciduous forest taxa in the lower part of the section particularly up to 36 4 ma magnetochron c16r and a sudden increase of pinaceae pinuspollenites piceaepollenites and abiespollenites which is dated at 36 1 ma c16n 2n coexistence approach coa indicates that from 39 9 to 36 4 ma c17n 1n regional climate was warm and wet while from 36 4 to 33 5 ma c16n 2n c13r climate tends to be cooler and drier the data indicate that paleoenvironmental and palynological changes on the ne part of the tibetan plateau resulted from a combination of long term tectonic uplift forcing and long and short term climate changes the increase of taxa such as piceaepollenites and abiespollenites indicates not only a cooling and drying trend prior to the eocene oligocene e o boundary but also the existence of high altitude mountain habitats in the periphery of the xining basin the sudden pinaceae event correlates closely in time with a marked aridification step as viewed from the lithology of the xining basin that was linked to the sea retreat out of the tarim basin

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

aridification in continental asia after the middle eocene climatic optimum meco global climate cooling from greenhouse to icehouse conditions occurred across an enigmatic transitional interval during the eocene epoch characterized by incipient polar ice sheet formation as well as short lived warming events of which the middle eocene climatic optimum meco is most noticeable understanding this critical period requires high resolution records that are being gathered in marine basins but are still lacking in the terrestrial realm here we provide a precisely dated terrestrial record crossing the meco time interval from the xining basin nw china we document a rapid aridification step and the onset of obliquity dominated climate cyclicity indicated by lithofacies and pollen records dated at 40 0 ma at the base of magnetochron c18n 2n this shift is concomitant within error with the meco peak warming in ocean drilling program site 1258 for which we reassessed the magnetostratigraphic age at 40 0 ma also at base of magnetochron c18n 2n the rapidity of the shift observed in the xining basin and the region wide aridification and monsoonal intensification reported around 40 ma suggests asian paleoenvironments were responding to global climate changes associated with the meco however the xining records show only the permanent shift but not the transient peak warming observed in marine meco records we thus relate this permanent aridification to occur during the post meco cooling we propose the mechanisms linking global climate to asian paleoenvironments may be eustatic fluctuations driving the stepwise retreat of the proto paratethys epicontinental sea or simply global cooling reducing moisture supply to the continental interior in any case eocene global climate cooling from greenhouse to icehouse conditions seem to have played a primary role in shaping asian paleoenvironments

temporal trends in lava dome extrusion at santiaguito 1922 2000 we present a means of extracting lava dome extrusion rates from data provided by the thematic mapper and enhanced thematic mapper plus instruments flown on the landsat satellites extrusion rates derived from the landsat data for santiaguito dome complex guatemala during november 1987 and january 2000 of 0 72 0 08 and 0 48 0 09 m3 s 1 compare favorably with ground based estimates of 0 63 0 28 and 0 48 0 17 m3 s 1 a satellite data derived set of 18 extrusion rate estimates between 1987 and 2000 along with ground based volume estimates prior to 1987 allow us to infer volumes of magma erupted since 1922 these data reveal cyclic extrusion at santiaguito each cycle begins with a 3 6 year long phase of high extrusion rate 0 5 2 1 m3 s 1 followed by a longer 3 11 years phase of low extrusion rate 0 2 m3 s 1 the 8th cycle began in 1996 and was still in its high extrusion rate phase during january 2000 with time however the durations of the low extrusion rate phases have increased peak extrusion and time averaged extrusion rates for each cycle have decreased and the difference between extrusion rates during the high and low extrusion rate phases of each cycle has decreased these trends may be the result of depressurization and exhaustion of the source in this case the current eruption may be expected to end around 2020 alternatively santiaguito may be entering a period of persistent activity maintained by a stable magma supply in this case we may expect extrusion to become less cyclic and to stabilize at 0 2 0 4 m3 s 1 however a 2 wt decrease in sio2 content of erupted products since 1970 appears to be consistent with chamber exhaustion as well as an increase in block lava flow lengths during 1970 2000

multiple timescales of cyclical behaviour observed at two dome forming eruptions article i nfo cyclic behaviour over a range of timescales is a well documented feature of many dome forming volcanoes but has not previously been identified in high resolution seismic data from volcan de colima mexico using daily seismic count datasets from volcan de colima and soufriere hills volcano montserrat this study explores par allels in the long term behaviour of seismicity at two long lived systems datasets are examined using multiple techniques including fast fourier transform detrended fluctuation analysis and probabilistic distribution analysis and the comparison of results from two systems reveals interesting parallels in sub surface processes operating at both systems patterns of seismicity at both systems reveal complex but broadly similar long term temporal patterns with cycles on the order of 50 to 200 days these patterns are consistent with previously published spectral analyses of so2 flux time series at soufriere hills volcano and are attributed to variations in the movement of magma in each system detrended fluctuation analysis determined that both volcanic systems showeda systematic relationship betweenthe numberofseismic eventsand the relative roughness ofthetime series and explosionsatvolcande colimashoweda1 5 2 yearcycle neither observation hasaclear explanatory mechanism atvolcandecolima analysisofreposeintervalsbetweenseismiceventsshowslong termbehaviour that responds to changes in activity at the system similar patterns for both volcanic systems suggest a common process or processes driving the observed signal but it is not clear from these results alone what those processes may be furtherattempts to modelconduitprocessesateach volcanomustaccountfor thesimilarities anddiffer ences in activity within each system the identification of some commonalities in the patterns of behaviour dur ing long lived dome forming eruptions atandesiticvolcanoes provides a motivation for investigating further use of time series analysis as a monitoring tool

saturation regions in catchments and their relations to soil and topographic properties abstract criteria are derived for the existence of saturated areas on draining hillslopes in natural catchments these are given in terms of soil transmissivity the hillslope gradient and its wetness state characterised by the baseflow discharge from the catchment analyses are made for idealised hillslopes with uniform soils to obtain expression for the width of the saturated zone the size of saturated zones on undulating hillslopes with gradiational soils is shown to depend strongly on topographic convergence or divergence the effects of soil transmissivity hillslope geometry and drainage flux density are lumped into a dimensionless parameter whose value determines the location of seepage boundaries on slopes of specified geometry scaling relations for seepage area in catchments of different size are shown to be generally nonlinear and depend primarily on local hillslope gradients relative predictions of seepage zone size are confirmed by observed hydrographs from three catchments with different physical characteristics

particle size separation and evidence for pedogenesis in samples from the chinese loess plateau spanning the past 22 m y a comparison between particle size distributions determined by laser diffraction and the pipette method confi rms previous studies that point to a variable underestimate of the clay fraction using laser diffraction relative to the values obtained by pipette analysis in order to yield four particle sized fractions 8 m for further analysis the pipette method was used on loess and paleosol samples from sites on the chinese loess plateau spanning the past 22 m y in all cases a subsidiary mode in the 8 m fraction provides a basis for evaluating the relative contributions of pedogenic and detrital components pretreatment using acetic rather than hydrochloric acid for carbonate removal results in no signifi cant loss of ferrimagnetic minerals and allows magnetic characterization of the separate fractions this confi rms the isolation of almost all the fi nest pedogenic ferrimagnetic grains in the clay fraction and their negligible contribution to the magnetic properties of the coarse fractions in all the samples from early miocene time onward because similar conclusions may be drawn from previous studies of late pleistocene samples from the east central and extreme western parts of the chinese loess plateau we conclude that this approach provides a basis for separately characterizing eolian deposition and pedogenesis throughout the region and for the whole time span of loess accumulation

cyclic extrusion of a lava dome based on a stick slip mechanism abstract lava dome eruptions are sometimes characterised by large periodic fluctuations in extrusion rate over periods of hours that may be accompanied by vulcanian explosions and pyroclastic flows we consider a simple system of nonlinear equations describing a 1d flow of lava extrusion through a deep elastic dyke feeding a shallower cylindrical conduit in order to simulate this short period cyclicity stick slip conditions depending on a critical shear stress are assumed at the wall boundary of the cylindrical conduit by analogy with the behaviour of industrial polymers in a plastic extruder the elastic dyke acts like a barrel and the shallower cylindrical portion of the conduit as a die for the flow of magma acting as a polymer when we applied the model to the soufriere hills volcano montserrat for which the key parameters have been evaluated from previous studies cyclic extrusions with periods from 3 to 30 h were readily simulated matching observations the model also reproduces the reduced period of cycles observed when a major unloading event occurs due to lava dome collapse

rock magnetic constraints for the mid miocene climatic optimum from a high resolution sedimentary sequence of the northwestern qaidam basin ne tibetan plateau abstract the thick sequence of cenozoic deposits in the qaidam basin of the ne tibetan plateau has great potential for understanding the tectonic and climatic evolution of the region in this study we performed detailed rock magnetic analyses of a well dated 20 7 11 2 ma upper cenozoic sedimentary sequence from the huatugou htg section in the nw qaidam basin the magnetic susceptibility of the sequence is relatively low before 17 4 ma high during 17 4 14 5 ma and lower after 14 5 ma again this pattern is consistent with the occurrence of the mid miocene climatic optimum mmco during 17 4 14 5 ma with global climatic cooling thereafter measurements of multiple rock magnetic parameters indicate a peak in the concentration of fine grained magnetic minerals during 17 4 14 5 ma this feature demonstrates that the enhancement of the magnetic susceptibility is due to the pedogenic production of superparamagnetic sp and single domain sd magnetite suggesting the occurrence of relatively high effective precipitation and high temperature in the qaidam basin at this time comparing with regional and global climatic records the warm and humid climatic conditions coincide with the mmco implying that global climate changes were responsible for the climatic evolution in the qaidam basin throughout the mmco however after 14 5 ma the rapid uplift of the northern tibetan plateau caused it to reach a critical elevation which blocked the moisture supply to the qaidam basin and may also have contributed to the observed climatic changes

late miocene intensified tectonic uplift and climatic aridification on the northeastern tibetan plateau evidence from clay mineralogical and geochemical records in the xining basin

chronological sequence and subdivision of chinese neogene mammalian faunas six land mammal ages are recognized for the neogene of east asia the ages are based upon updated large and small mammal faunas and replace or redefine the land mammal stages erected by li et al 1984 comparisons and correlations are made to faunas in europe northern and southern asia and north america in addition methods of utilizing mammalian faunal data to define and recognize geochronologic ages are discussed

early cenozoic faulting of the northern tibetan plateau margin from apatite u th he ages models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary yet a growing body of evidence in the tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen s history and thus our current mechanical understanding of orogenic plateau development is incomplete new apatite u th he ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures accelerated erosion is used as a proxy of fault timing and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins helium results are synthesized with published geologic thermochronometric and sedimentologic data from which a growing picture of regional compressional deformation in middle to late eocene time in northern tibet emerges we relate the early cenozoic period of deformation to the initiation of collision between india and eurasia despite the fact that the plate boundary was located 3000 km to the south regardless of whether or not high topography was built simultaneously as a result of this deformation early cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time

lithologic and rock magnetic evidence for the mid miocene climatic optimum recorded in the sedimentary archive of the xining basin ne tibetan plateau abstract thick cenozoic deposits in the xining basin provide great potential for understanding the tectonic and climatic evolution of the ne tibetan plateau however the middle miocene climatic optimum mmco in this basin has not been reported up until now due to the poor exposure of the uppermost strata covering this interval in this study a 194 m deep core of flood plain to channel flood deposits was obtained from the tashan flats above the xiejia section in the xining basin magnetostratigraphic dating of the core determines its age at ca 18 5 13 5 ma lithological and rock magnetic analyses demonstrate that the most prominent changes recorded in the present study occurred between 17 and 14 ma during this period the sedimentary sequence mostly consists of gray white or bluish gray claystone and muddy siltstone intercalated with some thin red claystone the rock magnetic parameters show a large variability with higher values in thin red claystone beds and lower values in gray white or bluish gray layers these features demonstrate that due to a warm and wet climate extensive rivers had developed in the mid miocene and that a reducing sedimentary environment prevailed these favorable climatic conditions coincide with the mmco as based on global deep sea deposit records indicating that this warm event exerted a major impact on regional climate change in the ne tibetan plateau during the mid miocene

mineral magnetic record of the chinese loess and paleosols variations in the magnetic susceptiblity of the chinese loess and its interbedded paleosols correlate strikingly well with the deep sea oxygen isotope record low susceptibility values are found for the loess layers and high values are found for the soil horizons two interpretative models have been advanced to account for these magnetic variations both of these models discount any significant in situ formation of magnetite during soil forming periods instead they infer relative concentration of detrital or atmospheric magnetite at these times we critically examine the assumptions made in these models and on the basis of some new mineral magnetic data and fromalized magnetic flux calculations offer an alternative explanation of the loess magnetic record our model identifies pedogenic formation of magnetite as the major contributor to the high magnetic susceptibility of the paleosols the formation and preservation of this pedogenic magnetite is dependent on soil froming conditions and hence reflects the regional climate

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

the fast response of volcano seismic activity to intense precipitation triggering of primary volcanic activity by rainfall at soufriere hills volcano montserrat abstract one minute resolution time series of rainfall and seismic data from the soufriere hills volcano montserrat are analysed to explore the mechanism of external forcing of volcanic eruptions by rainfall over three years of activity the real time seismic amplitude rsam shows a narrow statistically significant peak within 30 min after the start of intense rainfall events and a much broader peak with a lag of 6 40 h the classified seismic events indicate that the volcanic response to rainfall begins at the surface and gradually penetrates deeper into the dome as there is an increase in the pseudo magnitude of surface rockfall events including pyroclastic flows with lags from the first 30 min to 40 h long period rockfalls from shallow degassing at lags of 4 and 14 h and long period and hybrid events source depth approximately 1 km with lags at 14 and 24 h after the start of rainfall events there was no rainfall related change in deeper volcano tectonic activity there was no change in the frequency of any type of classified event indicating that the rainfall acts to modulate existing internal processes rather than generating new events itself these robust results are due to many 229 different rainfall events and not just to a few large magnitude cases the rainfall triggered volcanic activity examined here is consistent with a model of fast shallow interactions with rainfall at the dome surface after which a deeper dome collapse follows

termination of fluvial alluvial sedimentation in the xining basin ne tibetan plateau and its subsequent geomorphic evolution the xining basin in northern china lies topographically in the transitional zone between the tibetan plateau and the loess plateau and climatically in the transitional zone between humid east asia and the arid asian interior during the cenozoic the basin accumulated a thick continuous sequence of fluviolacustrine dominated red beds and subsequent eolian loess the termination of sediment deposition in the xining basin is the key to understanding its subsequent evolution the development of the modern yellow river and the history of dust deposition here we present the results of a detailed paleomagnetic study of the upper sedimentary sequence of the xining basin below the river terraces and in addition we review previous studies of river evolution development our results indicate that the age of the uppermost part of the continuous sequence of fluviolacustrine dominated red beds in the xining basin ranges from ca 20 ma to 4 8 ma suggesting that basin sedimentation ended at the earliest at 4 8 ma and they also provide an earliest age constraint for the subsequent river terrace development and eolian dust deposition in addition these results demonstrate that the termination of sedimentation in the xining basin and the nearby linxia basin was approximately coeval suggesting the rapid regional tectonic uplift of the ne tibetan plateau and resulting reorganization of the landforms and development of the paleo yellow river in the late pliocene c 2017 elsevier b v all rights reserved

mid miocene climatic optimum clay mineral evidence from the red clay succession longzhong basin northern china abstract the mid miocene climatic optimum mmco is one of earth s most recent prolonged warming events recorded in both the deep ocean and mid to high latitude continents thick cenozoic deposits in the longzhong basin northwest china provide great potential for characterizing the mmco but have not been well documented in this study we carry out clay mineralogical and geochemical analyses of the miocene red clay succession from the eastern longzhong basin employing x ray diffraction and geochemistry and discuss the climatic characteristics of the mmco the results reveal that the mmco occurred at 16 14 ma and is associated with relatively high kaolinite and smectite contents high illite crystallinity values and kaolinite illite chlorite ratio and lower chlorite and illite contents these clay mineral assemblages confirm a warm and moist climate during the mmco furthermore the sio2 al2o3 ratio declined with increasing al2o3 indicative of greater precipitation and intensified weathering during this period when compared with local and global records we find that our clay mineral records reflect regional climatic change superimposed upon global climatic trends during the mmco it is suggested that a high concentration of co2 and associated global warming may have been responsible for the mmco rather than the uplift of the tibetan plateau

terrain controls on depressional soil distribution in a hummocky morainal landscape in hummocky morainal landscapes soil distribution in well drained landscape positions tends to follow a consistent pattern soils in depressions however are more difficult to predict reliably this study had two objectives 1 to determine the parent material and landscape properties controlling the formation of the different depressional soils and 2 to use these controls to identify quantitative terrain based predictors of soil type in depressions only two terrain attributes specific dispersal area sda and elevation relative to open water bodies were required to distinguish three main soil groups gley recharge non gley recharge and discharge soils specific dispersal area is the downslope area draining flow from a given grid cell gley recharge soils occur primarily at points with sda of less than 2 m2 m 1 regardless of elevation within a given site because most of the runoff flowing to a point with very low sda values will pond or infiltrate vertically rather than flow downslope non gley recharge soils and discharge soils both occur at points with sda of greater than 2 m2 m 1 the majority of the non gley recharge soils occur above 5 m elevation relative to an open water body and the majority of the discharge soils occur below 5 m elevation relative to an open water body reflecting the importance of solute cycling in the development of discharge conditions buried and depositional soils could not be predicted from current terrain attributes because their profile characteristics were derived from the paleosurface

cenozoic organic carbon isotope and pollen records from the xining basin ne tibetan plateau and their palaeoenvironmental significance marine oxygen isotope records show that the cenozoic global climate has undergone remarkable fundamental changes however how terrestrial ecosystems respond to these changes remains unclear the xining basin on the northeastern tibetan plateau holds continuous fine fluvial lacustrine sediments from the early eocene to the early miocene between similar to 52 ma and 17 ma this paper first presents total organic carbon isotope delta c 13 toc and pollen records from the xiejia section in the xining basin as indicators of the long term terrestrial paleoecosystem and paleoclimate changes in the asian interior of nw china the delta c 13 toc record shows a long term persistently decreasing trend from similar to 20 parts per thousand to 23 5 parts per thousand superimposed with obvious cycles of five distinctive phases of heavier and lighter delta c 13 toc values along with short pulses of especially low values occurring at similar to 32 5 ma 23 ma 21 5 ma and 18 ma these variations generally correlate with changes of gymnosperm content or the ratio of gymnosperm to angiosperm g a

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

control of magma flow in dykes on cyclic lava dome extrusion 1 lava dome eruptions are commonly characterized by large fluctuations in discharge rate with cyclic behaviour on time scalesranging from hours to decades examples include bezymianny volcano russia merapi java santiaguito guatemala mt st helens usa mt unzen japan and soufriere hills volcano montserrat previous models have assumed simple cylindrical conduits for magma transport but extrusions are mainly fed by dykes with cylindrical geometries developing only at shallow levels the widths of dykes embedded in an elastic medium are influenced by local magma pressure affecting flow rates and system dynamics strongly we develop a model for magma flow in dykes which predicts intense pulsations of magma extrusion for the case of a constant source pressure the period time scale is determined by the elastic deformation of the dyke walls and the length to width ratio of the dyke the dyke acts like a volumetric capacitor storing magma as pressure increases and then releasing magma in a pulse of extrusion for the soufriere hills volcano cyclic extrusions with time scales of a few weeks are predicted for dykes 300 500 m long and 3 6 m wide matching observations the model explains the sharp onset of tilt pulsations and seismic swarms citation costa a o melnik r s j sparks and b voight 2007 control of magma flow in dykes on cyclic lava dome extrusion geophys res lett 34 l02303 doi 10 1029 2006gl027466

water flow in a hummocky landscape in central saskatchewan canada i distribution of water and soils abstract water flow in a hummocky landscape is highly variable in time and space the purpose of the study is to use a classification system based on landscape surface form to delineate areas with similar hydrologic properties the study was conducted in a hummocky landscape with a closed drainage pattern in the black soil zone of saskatchewan the sites consisted of three closed drainage basins 80 150 m across surrounding individual temporary sloughs the distribution of water in the landscape was inferred from in situ electrical conductivity and soil morphological properties including thickness of a and eluviated horizons and depth to calcium carbonate in situ electrical conductivity was strongly correlated with the depth of water in the top 1 2 m of the soil the sites were divided on the basis of landscape surface form into lower slope upper slope and saddle positions which accounted for 30 50 and 20 of the cultivated areas of the basins respectively the lower slope positions were well leached of soluble salts and had high water contents thick a horizons gleyed and or eluviated horizons and thick zones of calcium carbonate depletion the upper slope positions had limited leaching of soluble salts low water contents thin a horizons lacked eluviated horizons and calcium carbonate was present either at shallow depths or at the soil surface the properties of the saddle positions were intermediate between the upper and lower slope positions the spatial distribution of water in the landscape persisted over time due to the redistribution of water in the landscape by topography

consistent grain size distribution of pedogenic maghemite of surface soils and miocene loessic soils on the chinese loess plateau abstract there is agreement that ultra ne maghemite grains 100nm are responsible for themagnetic enhancement of chinese loess recent studies show that grain size distribution of ultra nepedogenic maghemite grains within the aeolian sequences on the central chinese loess plateaudeposited in the last 8 ma is consistent however whether the observed grain size distribution ofultra ne pedogenic maghemite grains can be observed in the western chinese loess plateau andmodernsoilsisnotentirelyclear herewe ndthatyoungsurfacesoilsamplesacrossthechineseloessplateau and early neogene loessic soils from the western chinese loess plateau also show consistentgrain size distribution with that of ultra ne pedogenic maghemite grains on the central chinese loessplateau the fact that young surface soil samples show apparent magnetic enhancement suggests thatthe ultra ne pedogenic maghemite grains derive from oxidation of ultra ne magnetite grains copy right 2009 john wiley sons ltd keywords loess plateau magnetic susceptibility arm miocene loess maghemite

pulsed miocene range growth in northeastern tibet insights from xunhua basin magnetostratigraphy and provenance author s lease ro burbank dw hough b wang z yuan d abstract sedimentary rocks in tibetan plateau basins archive the spatiotemporal patterns of deformation erosion and associated climate change that resulted from cenozoic continental collision despite growing understanding of basin development in northeastern tibet during initial india asia collision as well as in the late miocene holocene surprisingly little is known about the intervening period a time when the plateau may have undergone fundamental tectonic changes to fill this gap we present new magnetostratigraphy from a g2300 m thick fluviolacustrine succession that spans ca 30 9 3 ma an integrated analysis of sedimentology subsidence and provenance from this section reveals the sequential pulsed erosion of multiple ranges bordering the xunhua basin emergence of the wnw trending laji shan is highlighted by a doubling of sediment accumulation rates between 24 and 21 ma and a transition to coarse allu vial facies at 20 3 ma detrital zircon u pb age spectra show that these coarse sediments came from basement terranes within the laji shan together these observations suggest accelerated growth of the laji shan and its coupled foreland basin at ca 22 ma the most rapid accumulation rates in xunhua basin occur within the finest grained strata and suggest an underfilled basin during the fastest interval of laji shan deformation growth of the laji shan occurred northward of the contemporaneous plateau margin which had been defined since ca 45 50 ma by the west qinling lying 60 km farther south hence following 20 25 m y of apparent stability the deformation front in this region jumped 60 km to the north at ca 22 ma subsequently growth of the north trending jishi shan occurred at ca 13 ma and is highlighted by an acceleration in xunhua basin accumulation rates between 12 and 9 ma as well as by a significant change in detrital zircon provenance of nearby linxia basin deposits by 11 5 ma initial growth of the wnw trending laji shan in the early miocene and subsequent growth of the north trending jishi shan 10 m y later support interpretations of a middle mio cene kinematic reorganization in northeastern tibet 2012 geological society of america

magma chamber properties from integrated seismic tomography and thermal modeling at montserrat it is widely believed that andesitic magmas erupted at arc volcanoes are stored in shallow reservoirs prior to eruption but high resolution images of focused regions of magma in the shallow crust are rare we integrate seismic tomography with numerical models of magma chamber growth to constrain the magma chamber beneath soufriere hills volcano montserrat our approach reveals the characteristics and dynamics of the magmatic system with a level of detail that no single method has yet achieved the integrated analysis suggests that a magma chamber of 13 km3 with over 30 melt fraction formed between 5 5 and at least 7 5 km depth a significantly higher melt fraction than inferred from the seismic data alone the magma chamber may have formed by incremental sill intrusion over a few thousand years and is likely to be a transient geologically short lived feature these volume and geometry estimates are critical parameters to model eruption dynamics which in turn are key to hazard assessment and eruption forecasting

asian monsoons in a late eocene greenhouse world the strong present day asian monsoons are thought to have originated between 25 and 22 million years myr ago driven by tibetan himalayan uplift however the existence of older asian monsoons and their response to enhanced greenhouse conditions such as those in theeocene period 55 34myrago are unknownbecause of the paucity of well dated records here we show late eocene climate records revealing marked monsoon like patterns in rainfall and wind south and north of the tibetan himalayan orogen this is indicated by low oxygen isotope values with strong seasonality in gastropod shells and mammal teeth from myanmar and by aeolian dust deposition in northwest china our climate simulations support modern like eocene monsoonal rainfall and show that a reinforced hydrological cycle responding to enhancedgreenhouse conditions counterbalanced the negative effect of lowertibetanrelief onprecipitation these strong monsoons later weakened with the global shift to icehouse conditions 34myr ago

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

water flow in a hummocky landscape in central saskatchewan canada iii unsaturated flow in relation to topography and land use the importance of unsaturated flow in groundwater recharge and soil development in the interior plains region of north america is poorly understood the purpose of this study was to obtain quantitative estimates of unsaturated water flow rates in a hummocky upland area and to relate the flow rates to topography and land use the sites consisted of closed drainage basins 80 150 m across surrounding individual temporary sloughs in the black soil zone in central saskatchewan and were instrumented with tensiometers and neutron access tubes the sites were divided into lower slope upper slope and saddle positions on the basis of landscape surface foam the magnitude of the unsaturated flux was commonly 10 8 ms 1 in the lower slope positions 10 9 to 10 8 ms 1 in the saddle positions 10 10 10 9ms 1 in the upper slope positions following summer following and 10 10ms 1 or less in the upper slope positions following three consecutive years of cropping downward flow was of similar magnitude to upward flow inthe lower slope positions and two to five fold higher than upward flow in the upper slope and saddle positions in a year with 30 above average rainfall up to 25 mm of downward flow and 5 mm upward flow occurre across the bottom of the root zone in the saddle positions the unsaturated downward flux penetrated to greater than 2 5 m depth and was sufficiently large to be of importance to soil development and crop growth deep drainage was greater in saddle then in upper slope positions greater following summer fallow than following three years of cropping and least under native vegetation

tibetan plateau aridification linked to global cooling at the eocene oligocene transition the eocene oligocene transition about 33 5 million years ago was a major global climate event the end of the eocene was unusually warm with no significant ice on antarctica but the oligocene saw the arrival of a permanent antarctic ice sheet two papers this week relate to the continental effects of this global change dupont nivet et al examined sedimentary records from the tibetan plateau and find a drop in atmospheric water which caused cooling and aridification coincident with antarctic cooling previous studies attributed this phenomenon to the rapid uplift of the tibetan plateau but this new work suggests that regional tibetan climate was influenced by global events in an unrelated paper on the same climate transition zanazzi et al explore the cooling in north america at the time using stable isotope measurements from fossil teeth and bones to create a proxy temperature record they find a large drop in mean annual temperature of 8 2 c a greater fall than seen in the oceans this continental transition may explain why many cold blooded reptiles and amphibians became extinct whereas mammals able to regulate their body temperature escaped relatively unscathed precisely dated evidence details that aridification on the tibetan plateau occurred at the time of the eocene oligocene transition in the marine records this suggests a direct link between the palaeontological and palaeoenvironmental changes recorded in continental asia at the time and global climate cooling continental aridification and the intensification of the monsoons in asia are generally attributed to uplift of the tibetan plateau and to the land sea redistributions associated with the continental collision of india and asia1 2 3 whereas some studies suggest that past changes in asian environments are mainly governed by global climate4 5 6 the most dramatic climate event since the onset of the collision of india and asia is the eocene oligocene transition an abrupt cooling step associated with the onset of glaciation in antarctica 34 million years ago7 8 9 however the influence of this global event on asian environments is poorly understood here we use magnetostratigraphy and cyclostratigraphy to show that aridification which is indicated by the disappearance of playa lake deposits in the northeastern tibetan plateau occurred precisely at the time of the eocene oligocene transition our findings suggest that this global transition is linked to significant aridification and cooling in continental asia recorded by palaeontological and palaeoenvironmental changes10 11 12 and thus support the idea that global cooling is associated with the eocene oligocene transition13 14 15 we show that with sufficient age control on the sedimentary records global climate can be distinguished from tectonism and recognized as a major contributor to continental asian environments

environmental magnetic evidence for enhanced aridification in the tarim basin since 5 3 ma nw china abstract the tarim basin located in the rain shadow of high asia is an ideal basin for studying the long term aridification history and the interplay between climate and regional tectonics here we report new environmental magnetic records from the foreland basin of southern tian shan our new magnetic records together with the results of color parameters suggest that the paleoenvironment in the study area changed abruptly since 5 3 ma this result is consistent with other paleoclimatic records from different parts of the tarim basin suggesting a basin wide extreme aridification prevailed in the tarim basin since the beginning of the pliocene the long term cooling trend of the climate in the tarim basin since the late miocene was generally controlled by first order global climate however the extreme aridification since 5 3 ma was dominantly controlled by decreased moisture transport caused by the final collision between pamir and tian shan as a response to the northward displacement of the pamir salient

source location of long period seismicity at volcan de colima mexico this paper presents an analysis of seismicity associated with the volcanic activity of volcan de colima mexico and recorded in the period november 2005 april 2006 during a field survey by the istituto nazionale di geofisica e vulcanologia ingv osservatorio vesuviano the observatorio vulcanologico de colima of colima university and the instituto andaluz de geofisica university of granada three different types of volcanic earthquakes have been identified on the basis of their spectral properties type a 0 3 1 hz type b 1 5 hz and type c 3 4 hz results of polarization analysis applied to type a events show a predominance of radial motion indicating that the wavefield comprises compressional waves p and shear waves polarized in the vertical plane sv while the signal always begins with a negative polarity type a b and c earthquakes have been located using both a flat layered model and a 3d model including topography hypocentre distributions indicate that the source of type a signals is very shallow and confined to a small volume lying about 1 km below the crater in contrast the source of type b and c events is significantly deeper with most hypocentres located in a volume of about 1 km3 centred at 2 5 3 km depth a cluster analysis based on the cross correlation among the waveforms of different events recorded at the same station was applied to type a earthquakes only two clusters which include only a small percentage of events were found indicating that earthquake families were uncommon during the period of our survey

global warming and rainfall lessons from an analysis of mid miocene climate data abstract the mid miocene climatic optimum mmco has been regarded as an analog for future global warming due to its scale of warming being equivalent to the warming predicted for the next century by the international panel on climate change ipcc s fourth report however little is known about how the mmco influenced paleoprecipitation in central china in this study two pollen floras from the eastern longzhong basin northern central china were quantitatively analyzed to reconstruct the climatic conditions likely during the mmco the paleoclimatic parameters obtained by applying the coexistence approach coa to these two palynological assemblages showed that the mean annual temperature mat 16 6 c for the mmco was 5 7 c higher than the present day mat 10 9 c and that mean annual precipitation map 955 4 mm was 413 5 mm higher than at present 541 9 mm indicating a much warmer and wetter climate the higher mean temperature for the coldest month cmt and the minimum monthly precipitation mimp values also indicate a warmer and wetter coldest and driest month respectively and thus suggest the prevalence of a more balanced seasonal paleoclimate in northern central china these climatic conditions are equivalent to higher global temperatures of 5 6 c during the mmco this finding would imply that precipitation in northern central china will increase in the context of ongoing global warming