u pb and hf isotopic compositions of detrital zircons from the paragneisses of the quanji massif nw china implications for its early tectonic evolutionary history abstract the quanji massif is a cratonic fragment located on the northeastern margin of the qinghai tibetan plateau in western qinghai province northwest china fig 1 this massif consists of pre neoproterozoic crystalline basement unconformably overlain by mid to late neoproterozoic and cambrian strata the basement is dominantly made up of the delingha complex and the supracrustal dakendaban group the delingha complex is made up of 2 24 2 39 ga granitic gneisses with enclaves of dominant amphibolite and minor felsic gneisses as well as granulites the supracrustal dakendaban group is in tectonic contact with the delingha complex and can be divided into the lower and upper dakendaban sub groups data from detrital zircons show that the protolith rocks of these two sub groups were deposited after 2 32 ga and 2 11 ga respectively the detrital zircon age and hf isotopic data and geological correlations suggest that the quanji massif was possibly fragmented from the tarim craton and the delingha complex was probably uplifted to become the major sedimentary source for the upper dakendaban sub group at 2 11 ga our zircon hf isotope compositions demonstrate important crustal growth at 2 6 2 7 ga and 2 5 ga together with the important magmatic activity at 2 2 2 4 ga and geological data our results seem to suggest that the tarim craton was part of the north china craton in its early evolutionary history but rifted away and joined the yangtze craton prior to the neoproterozoic

deep structures and surface boundaries among proto tethyan micro blocks constraints from seismic tomography and aeromagnetic anomalies in the central china orogen abstract the qinling qilian orogen preserves the records of early paleozoic convergence among the proto tethyan micro blocks in this study we analyze the seismic velocity structure and the aeromagnetic anomalies in the qinling qilian orogen and its adjacent areas showing that the northernmost boundaries of these proto tethyan micro blocks are defined by the guyuan longshoushan fault in the qilian orogen and by the luonan luanchuan fault in the qinling orogen respectively the lithosphere north of the qinling qilian orogen subducted southward under the qinling qilian orogen the boundaries of the micro blocks in the qinling qilian orogen of the proto tethys domain are mostly ew or ne trending combined with early paleozoic geological records our results indicate that the south dipping low velocity anomaly under the alxa block and the south dipping high velocity ones under the north qilian orogen and the north china block might indicate that the alxa block the north qilian oceanic slab and the north china block subducted southward the south dipping high velocity anomaly under the north qilian orogen is interpreted as the remnant of the proto tethys ocean in that area however the north dipping low velocity anomaly under the south qinling and the north qinling orogens the qaidam block and the north dipping high velocity one under the yangtze and the bikou blocks suggest that the south qinling and the north qinling orogens the qaidam the yangtze and the bikou blocks subducted northward the present spatial framework of the qinling qilian orogen is related to early paleozoic convergence among the proto tethyan micro blocks

tectonics and hp uhp metamorphism of northern tibet preface abstract this special issue has been prepared for the 8th international eclogite conference iec to be held in the qilian qaidam regions western china during august 2009 most papers in this issue summarize results of many current petrological geochemical geochronological and tectonic studies of both qilian pacific type and qaidam alpine type lithotectonic assemblages important new occurrences include the qilian lawsonite eclogites which required an extremely cold subduction of the early paleozoic oceanic lithosphere and the mixing of oceanic and supracrustal protoliths in the qaidam continental subduction channel these two early paleozoic sutures in northern tibet provide additional constraints on the regional tectonic evolution of paleo and neo tethys in south central asia

a review of lacustrine paleomagnetic records from western north america 0 40000 years bp abstract a review of 16 lake sediment paleomagnetic records from western north america spanning time intervals over the last 40 000 yr bp provides evidence that many of these lacustrine records have recorded geomagnetic secular variation with wavelengths on the order of a few hundred years the younger studies 0 13 300 yr bp show general morphologic agreement among inclination records but most of the declination records are difficult to correlate among the older records there are major disagreements over the existence of the mono lake excursion specifically five studies contain evidence for the excursion but six studies do not show any record of it because errors in radiocarbon dating will distort down core time scales the younger records were rescaled with time scales that were constructed from extremal correlations of long wavelength 1000 2000 yr high amplitude 10 20 inclination features with these new time scales the excellent temporal and morphologic agreement between the fish lake oregon and blue lake idaho declination records confirms the existence of errors in the radiocarbon time scales the lack of agreement among the other rescaled declination records indicates that the records have been distorted or altered by sedimentary and geologic processes and or by sampling measuring and data analysis procedures in terms of the regional geomagnetic record fish lake and blue lake present a 3300 yr master curve of secular variation for northeastern oregon and southwestern idaho the lack of correlation among the other records however prevents us from drawing definite conclusions about the areal variation of the geomagnetic field in western north america over the past 40000 yr bp

uplift of deep crust during orogenic extensional collapse a model based on field studies in the sogn sunnfjord region of western norway the western gneiss region wgr in norway experienced high pressure metamorphism during silurian devonian continent continent collision the eclogite bearing lower crust is separated from the middle and upper crust by major detachment zones formed during extensional collapse of the orogen formation of the devonian basins is related to the extension the footwall of the detachment zones comprises three structural and metamorphic zones the upper zone zone 1 is characterized by penetrative homogeneous down to the west simple shear developed under retrograde greenschist facies metamorphism zone 2 suffered inhomogeneous simple shear of the same polarity petrography and mineral chemistry data from the lower zone zone 3 show a record of initial eclogite facies metamorphism at 600 c and 16 kbar which was decompressed almost isothermally to amphibolite facies conditions at 550 c and 10 12 kbar both the eclogite and amphibolite facies metamorphism developed in a regime of pure shear with vertical shortening the rapid decompression records an approach of approximately 20 km to the surface related to uplift that was probably the result of the removal of a thickened thermal boundary layer in the mantle lithosphere the pure shear regime which developed initially in the lower crust was truncated by zones of simple shear as the lower crust was uplifted to middle and upper crustal levels extension by simple shear in the upper crust was rooted in the lower crust where extension occurred by pure shear the shear zones in zones 1 and 2 did not penetrate the pure shear regime of the lower crust a considerable amount of tectonic stripping of the orogenic welt predates deposition of the devonian sedimentary basins

generation of peraluminous granitic magma in a post collisional setting a case study from the eastern qilian orogen ne tibetan plateau abstract early paleozoic peraluminous granites are abundant in the eastern part of the qilian orogen northeastern margin of the tibetan plateau a combined study involving geochronology whole rock geochemical and sr nd hf isotopic compositions for three early paleozoic peraluminous granitic plutons jishishan ledu and shichuan plutons from the eastern qilian orogen was carried out to evaluate the causes of chemical variations and generation mechanisms of peraluminous granitic magmas these granitic plutons have magma crystallization ages of 455 427 ma and are moderately to strongly peraluminous a cnk 1 03 1 18 geochemical and sr nd hf isotopic data indicate that they consist substantially of crust derived melts the jishishan and ledu peraluminous granites were dominantly produced by partial melting of precambrian orthogneisses the shichuan monzogranites with low hree contents e g yb 0 80 1 83 ppm and slightly negative end t 5 3 to 2 3 and positive ehf t 1 6 to 3 4 could be derived from immature crustal materials relatively high average zircon saturation temperatures 750 c for each pluton obvious negative eu anomalies eu eu 0 28 0 80 and low pb ba ratios 0 03 0 16 for the jishishan ledu and shichuan granites are consistent with crustal melting involving biotite breakdown under fluid absent conditions our results suggest that compositional variations of moderately to strongly peraluminous granitic magmas are mainly controlled by source compositions and melting conditions while the processes such as mixing with mantle derived magma fractional crystallization restite unmixing and peritectic assemblage entrainment were insignificant or only play secondary roles in their genesis late ordovician to middle silurian crustal anatexis in the eastern central qilian was probably linked with slab break off which may be an important mechanism in addition to lithospheric delamination for the generation of moderately to strongly peraluminous granites in a post collisional setting

two contrasting accretion v collision orogenies insights from early paleozoic polyphase metamorphism in the altun qilian north qaidam orogenic system nw china

secular variation as recorded in lake sediments a discussion of north american and european results type curves depicting secular variations in declination and inclination through holocene time have been constructed by stacking data from individual cores of lake sediment from east central north america and the u k there is a strong positive correlation in declination with zero phase shift froiti 0 to 4500 14 c years b p but from 4500 to 8500 14 c years b p the correlation is strongly negative which we interpret in terms of fluctuations in intensity of standing geomagnetic sources a source located under the atlantic ocean appears to have been dominant before ca 4500 14c years b p and then another similar source located either to the east or to the west of both observation areas gained dominance however the inclination type curves show optimum positive correlation for a phase shift of ca 400 years suggesting that the geomagnetic sources that caused them were drifting westward at ca 0 2 degrees a year there is no way of adjusting the timescales so as to obviate this apparent paradox secular variations of the total geomagnetic vector have been investigated by noting the sense clockwise anticlockwise of looping of the v g p paths though this cannot be interpreted uniquely in terms of westward or eastward drifting sources in the core as demonstrated by specific radial dipole models

synchronous granite intrusion and e w extension in the cuonadong dome southern tibet china evidence from field observations and thermochronologic results the tethys himalaya sedimentary sequence ths is characterized by n s trending extensional rifts nstr and the north himalayan gneiss domes nhgd the cuonadong dome as one of the nhgd is located at the n s trending cuona rift in the eastern part of the tethys himalaya the dome had undergone four major deformation events d1 characterized by top to s thrust resulting in n s shortening and vertical thickening d2 characterized by vertical thinning and n s extensional deformation d3 approximately e w extensional deformation associated with intrusion of leucogranites and d4 late collapse structural deformation around the core of the cuonadong dome there are numerous granitic dykes at the contact zone between the granite and surrounding rocks in the middle unit of the cuonadong dome field observations show that granite intrusion occurred in part synchronously with d3 e w extensional deformation u pb zircon dating shows that the granite crystallization took place at ca 17 5 15 6 ma while muscovite ar ar geochronological data from the deformed pegmatite and mylonitic schist in the cuonadong dome show that the onset of the e w extensional deformation is between 16 5 and 14 8 ma and suggesting that granite intrusion was synchronous with the e w ductile extensional deformation in the cuonadong dome southern tibet combined with previously published ages of the south tibetan detachment system stds our study shows that granite intrusion was at least locally contemporaneous with both the e w and n s extensional deformation especially with the e w extension

congruent paleomagnetic and archeomagnetic records from the western united states a d 750 to 1450 two independently dated high resolution paleomagnetic records one lacustrine and one archeological record the passage across western north america of the same nondipole feature of the geomagnetic field during the time interval from a d 750 to 1450 although these sequences indicate that correlation between paleomagnetic and archeomagnetic records is feasible under certain conditions differences between the records underscore the difficulty of dating accurately an archeological site by correlation of a single archeomagnetic direction with a secular variation curve

holocene secular variation in western north america paleomagnetic record from fish lake harney county oregon an 8 9 m composite section of lake sediment from fish lake steens mountain southeastern oregon has provided a high resolution record of holocene geomagnetic secular variation in western north america the section is composed of overlapping core segments with stratigraphic control based on six tephra layers and other thin distinct bands and with age control from 37 radiocarbon dates the 455 paleomagnetic samples possess a strong and stable magnetic remanence which is carried by pseudo single domain magnetite paleomagnetic records for individual core segments show low scatter and high serial correlation and where the segments overlap there is excellent agreement between records the azimuthal orientation of the composite section was recovered from the known direction of the geomagnetic field at the time of the climactic eruption of mount mazama tephra which is present in fish lake both internal and external evidence including the correspondence with an archaeomagnetic record from the southwestern united states support the conclusion that the fish lake record represents the history of geomagnetic field behavior during at least the last 10 000 years b p analysis of that record reveals that the average geomagnetic direction can not be distinguished from that of a geocentric axial dipole field and that the overall circularity of the field was clockwise but the detailed behavior was quite complex angular standard deviations of both field directions and virtual geomagnetic poles differ significantly from the values predicted from theoretical models of secular variation but are consistent with those of other holocene records when the record from fish lake is compared to the proposed type curve for east central north america there is excellent agreement for the past 6000 years provided the curves are offset by about 400 years this offset corresponds to a rate of westward drift of about 0 062 yr beyond about 6000 radiocarbon years ago good agreement between the two curves occurs when there is no offset suggesting that the field was standing rather than drifting during this time

chemical signatures of palaeotsunamis a forgotten proxy article i nfo article history a range of diagnostic criteria are required to help identify palaeotsunami deposits although chemical signatures have long been used as indicators of palaeosalinity in sedimentary sequences their use in the studies of palaeotsunami deposits is rarely applied and is yet to be widely recognised the chemical composition of interstitial water in coastal ponds and sedimentary sequences in coastal marshes and lagoons has been used successfullyasanindicatoroftsunamiinundation chemicalsignaturescanalsoprovidecluesaboutthelandward limit of runup of a tsunami beyond the area of sediment deposition like most palaeotsunami proxies however the preservation of chemical signatures varies with environmental conditions

historic and paleomagnetic secular variation and the earth s core dynamo process this paper attempts to summarize briefly the recent and ongoing efforts of the geomagnetism and paleomagnetism community to understand both the earth s magnetic field secular variation and its implications for the core dynamo process secular variation is defined in this paper as the general spatio temporal variation of the field secular change will refer to the first time derivative of the spatial field strict geomagnetic definition of secular variation we will emphasize here the long term 10 year or longer behavior of the historic secular variation hsv for this is the variation that is comparable with the paleomagnetic secular variation psv two important related topics the 1969 1970 geomagnetic jerk in secular acceleration and the spatial geomagnetic field components due to crustal or mantle sources are treated more fully in other reviews

geomagnetic secular variation from recent lake sediments ancient fireplaces and historical measurements in southeastern australia abstract compilations of historical observations archaeomagnetic data from ancient fireplaces and palaemagetic results from short cores of sediment from lakes in southeastern australia particularly lake keilambete provide a detailed record of the geomagnetic secular variation during the last 3000 years the independent sets of data are in good agreement if the radiocarbon time scale for the lacustrine record is about 450 years too old the error is attributed to systematic incorporation of ancient carbon into the lake floor sediments mainly through erosion of sediment on the crater walls at times of low water level a significant lag between deposition and the acquisition of stable magnetic remanence is ruled out inclination has been abnormally steep during the last 500 years but remained fairly close to the axial dipole field value prior to that during the last 1000 years the predominant sense of looping of the magnetic vector corresponds to westward drift of the nondipole field secular variations on a time scale of 100 years can be resolved by the lacustrine record

geodynamics of the indosinian orogeny between the south china and indochina blocks insights from latest permian triassic granitoids and numerical modeling

using palaeoenvironmental dna to reconstruct past environments progress and prospects palaeoenvironmental dna palendna is defined as ancient dna adna originating from disseminated genetic material within palaeoenvironmental samples sources of palendna include marine and lake sediments peat loess till ice permafrost palaeosols coprolites preserved gut contents dental calculus tephras and soils as well as deposits in caves rockshelters and at archaeological sites palendna analysis provides a relatively new tool for quaternary and archaeological sciences and its applications have included palaeoenvironmental and palaeodietary reconstructions testing hypotheses regarding megafaunal extinctions human environment interactions taxonomic studies and studies of dna damage because palendna samples comprise markedly different materials and represent wide ranging depositional and taphonomic contexts various issues must be addressed to achieve robust reproducible findings such issues include climatic and temporal limitations the biological origin and state free versus bound of palendna stratigraphic reliability sterile sampling ability to distinguish modern from adna signals dna damage and pcr amplification dna extraction methods and taxonomic resolution in this review we provide a non specialist introduction to the use of palendna for quaternary and archaeological researchers assess attributes and limitations of this palaeoenvir onmental tool and discuss future prospects of using palendna to reconstruct past environments copyright 2014 the authors journal of quaternary science published by john wiley sons ltd on behalf of quaternary research association

ancient sedimentary dna reveals past tsunami deposits abstract palaeotsunami deposits are the primary sources of information on past large tsunami events and thereby are critical for earthquake and tsunami hazard assessments they usually form sandy layers preserved in coastal sediments and contain indicators of marine origins such as microfossils e g diatoms and foraminifera and geochemical signals of saltwater however these indicators are often modified or erased with time to address this issue we present the first application of a palaeogenetic approach to a series of up to approximately 2000 year old sandy palaeotsunami deposits from a coastal wetland on hokkaido island japan we found the dna of marine foraminiferal species preserved in palaeotsunami deposits in the absence of foraminifera tests the recovery and analysis of the ancient sedimentary dna of marine organisms preserved in coastal zone sediments for millennia represents a new and promising tool for the identification of past tsunamis

lithospheric structure beneath the central and western north china craton and the adjacent qilian orogenic belt from rayleigh wave dispersion analysis abstract we used surface wave tomography to image the lithospheric s wave velocity structure along a linear seismic array across the central and western north china craton ncc and the adjacent qilian orogenic belt qb using waveform data from 8 earthquakes collected at 60 broadband stations rayleigh wave phase velocities were measured at periods of 20 120 s and subsequently inverted for reliable s wave velocities from 60 to 200 km depth distinct lithospheric structures and marked lateral variations were revealed beneath the study region correlating well with regional geological and tectonic features the average s wave velocity from 60 to 200 km depth beneath the array is 4 45 km s 1 lower than that in model ak135 high velocities of 4 45 km s are most pronounced beneath the stable ordos block western ncc however the depth to which the observed high velocity body extends varies significantly laterally it is deepest 200 km under eastern ordos and becomes shallower on both sides along the array the maximum depth of the high velocity body gradually shallows westward and reaches 110 km near the western edge of ordos this depth variation suggests a coexistence of both preserved and modified lithosphere in ordos the depth of high velocities decreases rapidly eastward by 80 km over a lateral distance of

late paleoproterozoic multiple metamorphic events in the quanji massif links with tarim and north china cratons and implications for assembly of the columbia supercontinent abstract the quanji massif preserves important records of late paleoproterozoic metamorphic events that provide insights into the tectonic history of the tarim craton and its relation with the north china craton here we investigate amphibolites from two regions in the quanji massif u pb ages from metamorphic zircons in the amphibolites combined with previously published ages from this region define distinct metamorphic events during paleoproterozoic the earlier medium p t type upper amphibolite facies metamorphic event occurred at ca 1 90 1 96 ga in a collisional setting the second event took place with low p t type amphibolite facies metamorphism in an arc setting at ca 1 82 1 85 ga with a subsequent medium p t type metamorphism during arc continental collision at ca 1 80 1 82 ga our data reveal for the first time that the late paleoproterozoic metamorphic events in the quanji massif are closely comparable with those documented from the khondalite belt within the inner mongolia suture zone and the trans north china orogen in the north china craton we therefore propose that the quanji massif and tarim craton probably shared similar tectonic histories with those of the north china craton in the late paleoproterozoic coinciding with the global assembly of the columbia supercontinent

the application of diatoms to reconstruct the history of subduction zone earthquakes and tsunamis abstract earthquake and tsunami records on centennial and millennial temporal scales are necessary to understanding long term subduction zone behavior and the occurrences of large but infrequent events microfossils such as diatoms incorporated into coastal stratigraphy provide some of the most detailed reconstructions of the history of earthquakes and tsunamis we explore qualitative and quantitative techniques that employ the relation between diatoms and salinity tidal elevation and life form to 1 reconstruct records of vertical land level change associated with large earthquakes and 2 identify anomalous sand and silt beds deposited by tsunamis a global database shows that diatoms have been successfully employed in the reconstruction of earthquake and tsunami histories in chile the indian ocean japan new zealand the north sea the pacific northwest of north america and the south pacific we use case studies from some of these locations to highlight advancements in the field and new capabilities that diatoms have enabled examples from the pacific northwest of north america illustrate the evolution of quantitative diatom based reconstructions of earthquake related land level change in alaska and japan diatoms have documented land level changes throughout the earthquake deformation cycle including possible preseismic land level change signals and postseismic deformation diatoms helped identify coseismic uplift along the central chile subduction zone coast and uplift and subsidence along the alaska aleutian megathrust expanding our knowledge of the variability of slip in megathrust ruptures in tsunami studies diatoms help determine the provenance of anomalous sands and silts found in low energy coastal stratigraphic sequences in japan allochthonous marine and brackish diatoms within sand deposits signaled repeated marine incursions into a coastal lake helping identify a possible predecessor to the 2011 tohoku tsunami in the pacific northwest of north america and chile diatoms were used to estimate tsunami run up beyond the landward limit of tsunami sedimentation examples from the north sea thailand and japan show how the fragmentation and sorting of diatom valves may provide evidence of high energy transport during the rapid turbulent flow of a tsunami to conclude we emphasize the importance of studying the modern diatom response to changes in land level and or tsunami inundation to improve diatom based records of prehistoric earthquakes and tsunamis

petrology and geochronology of eclogites from the western segment of the altyn tagh northwestern china abstract the eclogites from the western segment of the altyn tagh occur as lens or boundins within quartzofeldspathic gneisses or pelitic gneisses characterized by amphibolite facies parageneses the petrographic features and reaction textures testify to three main metamorphic stages 1 the peak eclogite facies stage p 15 kbar t 665 880 c 2 the granulitic facies stage p 11 14 kbar t 670 800 c and 3 the amphibolite facies stage p 6 3 9 5 kbar t 619 738 c during decompression the trace element rare earth element and sm nd isotopic data suggest that most of the eclogites have protolith features resembling transitional type t type mid ocean ridge basalt morb the well preserved eclogite was selected for sm nd and u pb isotopic dating the sm nd isotopic data yield a whole rock garnet omphacite isochron of 500 10 ma age the u pb isotopic measurement of zircons from the same eclogite shows that the four grain populations are near concordant and are well plotted on concordia giving a weighted mean age of 503 9 5 3 ma the two different geochronological methods yield a similar age which is interpreted as the peak metamorphic age of eclogites and reflects the existence of a mountain root related to continental subduction during early paleozoic moreover based on the comparison with similar rocks in the northern margin of the qaidam basin a displacement of 350 400 km for the altyn tagh fault have been estimated

nine unusually large tsunami deposits from the past 4000 years at kiritappu marsh along the southern kuril trench abstract large earthquakes along the kuril subduction zone in northern japan are known to have caused damaging tsunami although there is a little information on historical earthquakes and tsunami in this area because no documents exist before the 19th century that might refer to tsunami events to determine the likely timing and size of future events we need information on their recurrence intervals and to do this for the prehistoric past we have investigated sediments located in the kiritappu marsh in eastern hokaido that we interpret as laid down by tsunami using reliable multiple lines of evidence from sedimentological geomorphological micropaleontological and chronological results we identify 13 tsunami sands two of these lie within a peat bed above a historical tephra ta a ad 1739 the upper one probably corresponds to the ad 1843 tempo tokachi oki earthquake m 8 2 tsunami and the lower to either the ad 1952 tokachi oki earthquake m 8 2 tsunami or the ad 1960 chilean earthquake m 9 5 tsunami underlying are 11 prehistoric tsunami sand beds nine large sand beds and two smaller sand beds deposited during the past 4000 years because of the wide spatial distribution of the large sand beds and inundation distances inland of between 1200 to 3000 m we suggest that they record unusually large tsunamis along the kuril subduction zone according to our analyses these tsunami sands were derived from the coastal area and although they do not show clear graded bedding they commonly have gradational upper boundaries and erosional bases and include internal sedimentary structures such as plane beds dunes and current ripples reflecting bedload transportation based on our results we calculate the recurrence interval of unusually large earthquakes probably m 8 6 along the kuril subduction zone as about 365 553 years and estimate the youngest large event to have occurred in the 17th century

40ar 39ar thermochronological constraints on the retrogression and exhumation of ultra high pressure uhp metamorphic rocks from xitieshan terrane north qaidam china abstract paleozoic ultra high pressure uhp eclogite bearing mafic boudins are exposed in gneiss in the xitieshan terrane north qaidam petrological and thermobarometric studies show that garnet amphibolite and amphibolite rocks record upper amphibolite facies and lower amphibolite facies retrograde metamorphic conditions of 8 1 9 1 kbar 630 670 c and 5 0 7 2 kbar 530 630 c respectively to better constrain the timing of retrogression and exhumation 40ar 39ar stepwise heating analyses have been performed on minerals that were extracted from these uhp mafic rocks and their country rocks amphibole yields disturbed release spectra marked by anomalously old apparent ages in low temperature steps suggesting the presence of extraneous 40ar isochron ages of 425 ma and 419 ma are obtained for amphibole from the garnet amphibolite and amphibolite respectively which are interpreted as the best estimate for the age of amphibolite facies retrogression during exhumation muscovite from the gneiss yields consistent plateau and isochron ages of 409 ma k feldspar from the same gneiss yields rising staircase shaped age spectra which are characterized by younger apparent ages in the first steps 263 281 ma and higher plateau ages in the last segment 294 321 ma indicating partial diffusive loss of argon from the k feldspar samples biotite from schist yields flat age spectra with a plateau age of 397 ma that is interpreted as a cooling age our new 40ar 39ar data together with published la icpms ages of zircon from a coesite bearing amphibolite indicate that the xitieshan uhp metamorphic rocks may have experienced two stages of exhumation during the first stage from 95 31 km depth the uhp metamorphic rocks were exhumed with a relatively rapid velocity of 4 0 km myr resulting from buoyancy forces subsequently the rate of exhumation decreased gradually from 1 1 to 0 1 km myr between 31 7 km depth this later gentle exhumation was probably governed by extension and or erosion

precambrian evolution of the quanji block northeastern margin of tibet insights from zircon u pb and lu hf isotope compositions abstract the quanji block a cratonic fragment has been discovered between the north qaidam hp uhp metamorphic belt and the qilian orogenic belt in northwestern china this fragment has a pre neoproterozoic crystalline basement overlain unconformably by neoproterozoic paleozoic and mesozoic sedimentary strata pervasive anatexis of the basement produced extensive leucosomes in mafic and felsic migmatites zircons from the leucosomes yielded upper intercept and weighted mean 207 pb 206 pb ages of 1950 ma indicating that anatexis occurred in the late paleoproterozoic during regional high grade metamorphism hf model ages t dm2 for these zircons suggest that protoliths of the felsic and mafic migmatites had crustal residence time of 2 80 and 2 65 ga respectively the basement rocks of the quanji block are compositionally similar to those of the yangtze craton and may have been involved in assembly of the late paleoproterozoic global supercontinent columbia

the paleomagnetism and u th dating of mexican stalagmite das2 a 72 cm high stalagmite from mexico das2 has been analyzed for its paleomagnetism and has provided a record of secular variation beginning about 1200 years ago to present the stalagmite did not grow uniformly and a low content of magnetic material has rendered the record less precise in some parts a topographic test involving the magnetic measurement of coeval specimens from the sides of the stalagmite has shown that the record is detectably free from depositional errors age data were provided by the u th method and concordancy checked by14c and226ra methods shallow inclinations at the base of the record are followed by a partial anticlockwise loop in the declination inclination plot there is good agreement between the das2 data and the southwest u s a archeomagnetic record and with a lacustrine record from oregon from 750 to 1400 ad comparison of the record with the present field at the site latitude shows that westward drift of the field has persisted for at least 1200 years at a drift rate of about 0 30 yr 1 a close match in declinations with the present field at the site latitude suggests that the stalagmite may have recorded the passing of the northern edge of the gulf of guinea negative anomaly about 1000 years ago though the corresponding inclination feature is not present in the record an archeomagnetic synthesis by merrill and mcelhinny of worldwide data to estimate positions of the main dipole shows that it was tilted away from mexico from 500 to 1250 ad and this may account for the shallow inclinations seen at the start of the record

the tectonic evolution of the east kunlun orogen northern tibetan plateau a critical review with an integrated geodynamic model abstract the east kunlun orogen is an integral component of the northern qinghai tibet plateau here we synthesize the geologic geochemical geochronological and stratigraphical dataset and propose an integrated geodynamic model to illustrate the tectonic evolution of this orogen this review also provides important insights on the complex phanerozoic plate tectonic processes we envisage the protracted multi stage accretion and collision of a collage of terranes during the subduction and closure of the south kunlun and anemaqen oceans from the neoproterozoic to early mesozoic the east kunlun orogen is tectonically divided into the north kunlun terrane nkt south kunlun terrane skt muz tagh anemaqen terrane mat and hoh xil bayan har terrane hbt from the north to south of which evolution can be divided into six stages from late neoproterozoic early mesozoic age 550 ma 200 ma i ca 540 481 ma ii ca 481 424 ma iii ca 424 370 ma iv ca 370 271 ma magmatic gap v ca 271 224 ma and vi ca 224 201 ma the nkt as an active continental margin initiated at ca 450 ma as a consequence of backarc ocean lithosphere subduction in the late ordovician middle silurian stage ii the closure of back arc basin led to an oblique collision between the nkt and skt during the early middle silurian stage ii and to a post collisional environment in the late silurian middle devonian stage iii the skt and mat developed as a young island arc both carrying large amounts of ocean island arc granitoids oags and ssz supra subduction zone type ophiolites together with island arc tholeiite iat as a consequence of bidirectional subduction of proto tethys ocean stage i during the collision between skt and nkt the skt transformed from a young island arc into a mature island arc with thickening crust and the granitoids geochemically evolved from the oags to cags continental arc granitoids wpgs within plate granitoids in the late ordovician middle silurian stage ii the collision between skt and mat probably occurred after the middle devonian stage iii as evidenced from the absence of the upper devonian strata in the skt the skt and mat is interpreted as an exotic terrane that has been accreted onto the continental margin and contributed significantly to the continental growth in this orogenic belt a magmatic quiescence occurred during ca 370 270 ma stage iv associated with subduction polarity reversal a new subduction zone initiated at the southern margin of the mat after the middle permian stage v which subsequently resulted in extensively development of calc alkaline granitoids in the nkt skt and mat the progressive chronology change of granitoids from nkt to further south maybe the joint results of flat subduction and slab rollback which make magmatism center retrograde migrate towards the trench the final closure of the anemaqen ocean might have occurred in the late triassic stage vi as indicated by the upper triassic babaoshan terrestrial molasse formation and development of cags and wpgs in the hbt which resulted in the accretion of the north qiang tang microcontinent

assessing the magnitude of the 869 jogan tsunami using sedimentary deposits prediction and consequence of the 2011 tohoku oki tsunami article i nfo in this paper the spatial distribution and sedimentological features of the 869 jogan tsunami deposit along the pacific coast of japan are reviewed to evaluate deposit based estimates of the magnitude of the jogan tsunami and the use of tsunami deposits in the prediction of the 2011 tohoku oki earthquake and tsunami inundation of the sendai plain and the offshore wave sources of both tsunamis are compared the jogan tsunami deposit is ubiquitous on the coastal plains of sendai bay whereas to date it is only identified in a few locations along the sanriku and joban coasts this resulted in an underprediction of the size of the wave source of the tohoku oki tsunami the inland boundary of the inundation area of the tohoku oki tsunami on the sendai plain is approximately equivalent to that of the jogan tsunami although many sedimentological and geomorphologic factors make a direct comparison of the tsunamis complicated and difficult the magnitude of the jogan earthquake mw 8 4 which was derived from the tsunami deposit inland extent and numerical inundation modeling was too small to predict the magnitude of the tohoku oki earthquake mw 9 0 9 1 and tsunami additional research is needed to improve deposit based estimates of the magnitudes of past tsunamis and to increase the ability to use tsunami deposits in conjunction with inundation modeling to assess future tsunami hazards

early paleoproterozoic magmatism in the quanji massif northeastern margin of the qinghai tibet plateau and its tectonic significance la icpms u pb zircon geochronology and geochemistry abstract the quanji massif is located on the north side of the qaidam block and is interpreted as an ancient cratonic remnant that was detached from the tarim craton there are regionally exposed granitic gneisses in the basement of the quanji massif whose protoliths were granitic intrusive rocks previous studies obtained intrusion ages for some of these granitic gneiss protoliths the intrusion ages span a wide range from 2 2 ga to 2 47 ga this study has determined the u pb zircon age of four granitic gneiss samples from the eastern central and western parts of the quanji massif cl images and trace elements show that the zircons from these four granitic gneisses have typical magmatic origins and experienced different degrees of pb loss due to strong metamorphism and deformation la icpms zircon dating yields an upper intercept age of 2381 41 2 ma from monzo granitic gneiss in the hudesheng area and 2392 25 2 ma from granodioritic gneiss in the mohe area eastern quanji massif and 2367 12 2 ma from monzo granitic gneiss in the delingha area central quanji massif and 2372 22 2 ma from monzo granitic gneiss in the quanjishan area western quanji massif these results reveal that the intrusive age of the protoliths of the widespread granitic gneisses in the quanji massif basement was restricted between 2 37 and 2 39 ga indicating regional granitic magmatism in the early paleoproterozoic perhaps related to the fragmentation stage of the kenorland supercontinent geochemical results from the granodioritic gneiss from the mohe area indicate that the protolith of this gneiss is characterized by adakitic rocks derived from partial melting of garnet amphibolite beneath a thickened lower crust in a rifting regime after continent continent collision and crustal thickening genetically similar to the ttg gneisses in the north china craton this suggests that the quanji massif had a tectonic history similar to the archean central orogenic belt of north china craton during the early paleoproterozoic we tentatively suggest that the quanji massif and the parental tarim craton and the north china craton experienced rifting in the early paleoproterozoic after amalgamation at the end of the archean the tarim craton and north china craton might have had close interaction from the late neoarchean to the early paleoproterozoic

metamorphism and deformation of blueschist belts and their tectonic implications north qilian mountains china abstract two blueschist belts in the north qilian mountains occur in middle cambrian and lower ordovician strata and strike n30 35 w for about 500 km along the caledonian fold belt on the south west margin of the sino korean plate the styles of metamorphism and deformation are quite different in the two belts the middle cambrian to ordovician rocks in the high grade belt are mainly blueschists and c type eclogites in which six phases of lower and upper crustal deformation have been recognized the rocks contain glaucophane phengite epidote clinozoisite chlorite garnet stilpnomelane piedmontite albite titanite and quartz the estimated p t conditions of eclogites are 340 10 c 8 1 kbar and of blueschist 380 c 6 7 kbar the ordovician rocks in the low grade belt are characterized by ductile to brittle deformation in the middle to upper crust the low grade blueschists contain glaucophane lawsonite pumpellyite aragonite albite and chlorite the estimated p t conditions are 150 250 c and 4 7 kbar k ar and 39ar 40ar geochronology on glaucophane and phengite from the high grade blueschist belt suggest two stages of metamorphism at 460 440 and 400 380 ma which may represent the times of subduction and orogeny the subduction metamorphism of the northern low grade blueschist belt took place approximately at the end of the ordovician

geochemical signatures up to the maximum inundation of the 2011 tohoku oki tsunami implications for the 869 ad jogan and other palaeotsunamis abstract the geochemical signature of the tohoku oki tsunami deposit and underlying soil was assessed two months five months and seven months after the 11 march 2011 tsunami inundated the sendai plain the extent of the recognisable sand deposit was traced up to 2 9 km inland while a mud deposit was found up to 4 65 km inland representing 60 and nearly 95 of the maximum tsunami inundation respectively the limit of tsunami inundation was identified 4 85 km from the shore using geochemical marine markers s and cl two months after the tsunami in the absence of any sedimentological evidence concentrations of other geochemical markers k ca sr indicative of the marine incursion and associated minerals were found to decrease landward 13 c and 15 n and c n ratios suggested a mixture of terrestrial and marine organic sources in the sediment while 34 s of sulphate reflected the marine source of water soluble salts the chemical composition of the 869 ad jogan tsunami sand deposit was characterised by high sr and rb concentrations and was comparable to that of the tohoku oki tsunami deposit suggesting that the sources of sediment may be similar marked decreases in s and cl with time indicated that rainfall resulted in the leaching of salts from the sandy sediments however both s and cl markers as well as sr were still well preserved in the muddy sediments and underlying soil beyond the limit of the recognisable sand deposit seven months after the tsunami this suggests that geochemical indicators may well be useful in identifying the extent of historical and palaeotsunamis by determining the marine origin of fine grained sediments beyond the limit of recognisable sand deposition in particular when marine microfossils are sparse or lacking as is the case on the sendai plain this would allow researchers to redraw palaeotsunami inundation maps and re assess the magnitude of events such as the jogan tsunami and other palaeotsunamis not only on the sendai plain but also elsewhere around the world this has important implications for tsunami risk assessment hazard mitigation and preparedness

origin of devonian mafic magmatism in the east kunlun orogenic belt northern tibetan plateau implications for continental exhumation

a comparison of geomagnetic secular variation as recorded by historical archaeomagnetic and palaeomagnetic measurements palaeomagnetic methods can extend the documentary record of changes in the earth s magnetic field far into the past tolerable agreement is found between various methods demonstrating the geophysical value of palaeomagnetic experiments combining results from the different approaches of investigating secular change can lead to a better perspective and to superior models of geomagnetic field behaviour lake sediments have recently been found to hold remarkably detailed signatures of past field changes a mathematical approach to formulating an empirical description of global geomagnetic field behaviour is proposed and applied to palaeomagnetic data spanning the last 10 ka

timing and mechanism of formation and exhumation of the northern qaidam ultrahigh pressure metamorphic belt abstract the qilian caledonian orogenic belt on the north margin of the qinghai tibet plateau were formed by convergence and collision of the alxa terrain qilian terrain and qaidam terrain the 350 km long wnw ese trending north qaidam ultrahigh pressure uhp metamorphic belt lying between the qilian and qaidam terrains was formed between 495 and 440 ma by deep subduction of the south qilian sea and the qaidam continental crust beneath the qilian terrain the uhp belt was exhumed by a process of oblique extrusion during transformation from normal to oblique intracontinental subduction between the qilian and qaidam terrains exhumation began at 470 460 ma and was completed by 406 400 ma exhumation structures are well preserved in the uhp rocks and record extensive retrograde metamorphism

provenance record of a foreland basin detrital zircon u pb ages from devonian strata in the north qilian orogenic belt china abstract devonian strata in the north qilian belt of the qilian orogen accumulated in a foreland basin formed through collision between the north china craton and the central qilian belt u pb age data for 393 detrital zircons from four sandstone samples from the lower to middle devonian laojunshan formation and upper devonian shaliushui formation range in age from 3 6 ga to 0 4 ga and indicate input from a multi component source detrital zircons older than the early mesoproterozoic 1 4 ga were likely derived from the north china craton whereas late mesoproterozoic to neoproterozoic zircons were derived from the central qilian belt early paleozoic zircons were derived from the north qilian continental arc and associated ophiolites the occurrence of neoproterozoic zircons indicates that their source the central qilian belt was likely once part of the yangtze block of the south china craton prior to its separation from the craton in the late neoproterozoic

the acquisition of post depositional detrital remanent magnetization in a variety of natural sediments summary post depositional detrital remanent magnetization pdrm is the primary means whereby many sediments acquire their palaeomagnetic signal we have studied the acquisition of this magnetization in a variety of natural sediments our technique involves determining the magnetic direction recorded by a sediment as a function of the water content present in the sediment when the sediment experiences a change in the direction of the applied magnetic field most of the sediments used in this study were collected wet from natural environments and were preserved in their original state until they were used in the experiments grain sizes were measured by the settling tube method which led to the determination of the clay silt and sand fractions in each sediment isothermal remanent magnetization acquisition studies indicated that the predominant magnetic carriers were magnetite in the pdrm acquisition studies two distinct modes of behaviour were found for sediments with a sand content less than 60 per cent the original direction of magnetization was preserved regardless of the water content such behaviour is not consistent with a theoretical model which assumes that at high water contents the magnetic carriers remain mobile within fluid filled voids and hence are able to realign along a new magnetic field direction for sediments with a sand content in excess of 60 remagnetization along a new magnetic field direction occurred as expected provided the sediments were sufficiently wet studies of natural sediments and corresponding samples of dried and reconstituted sediments have demonstrated that the magnetic characterization of a sediment can be reliably determined even for older desiccated sediments

combined dna and lipid analyses of sediments reveal changes in holocene haptophyte and diatom populations in an antarctic lake preserved ribosomal dna of planktonic phototrophic algae was recovered from holocene anoxic sediments of ace lake antarctica and the ancient community members were identified based on comparative sequence analysis the similar concentration profiles of dna of haptophytes and their traditional lipid biomarkers alkenones and alkenoates revealed that fossil rdna also served as quantitative biomarkers in this environment the dna data clearly revealed the presence of six novel phylotypes related to known alkenone and alkenoate biosynthesizing haptophytes with isochrysis galbana uio 102 as their closest relative the relative abundance of these phylotypes changed as the lake chemistry particularly salinity evolved over time changes in the alkenone distributions reflect these population changes rather than a physiological response to salinity by a single haptophyte using this novel paleo ecological approach of combining data from lipid biomarkers and preserved dna we showed that the post glacial development of ace lake from freshwater basin to marine inlet and the present day lacustrine saline system caused major qualitative and quantitative changes in the biodiversity of the planktonic populations over time

aperiodic recurrence of geologically recorded tsunamis during the past 5500 years in eastern hokkaido japan 1 along hokkaido s pacific coast near the town of kiritappu sandy deposits in a muddy lagoon and on a nearby beach ridge plain provide evidence for 15 tsunamis between 200 and 6000 years ago additional sand beds at the lagoon probably represent the historical tsunamis of a d 1843 and 1894 we observed the sequences of sandy deposits in continuous slices 2 to 4 m deep some of the deposits consist of just a single sand bed whereas others contain multiple units of sand muddy sand or sandy mud and mud caps including plant detritus we also found at the lagoon a 17th century tsunami deposit that thickens and thins regardless of elevation or distance inland we bracketed the ages of most of the inferred tsunamis by radiocarbon dating of detritus mainly seeds and leaves at the lagoon and charcoal at the beach ridge plain from pretsunami and posttsunami beds tsunami dates computed from the bracketing ages commonly have uncertainties spanning 2 to 4 centuries within these uncertainties the inferred sequence of 15 prehistoric tsunamis at the lagoon beginning almost 6000 years ago can be matched tsunami by tsunami with the inferred history at the beach ridge plain 15 km away the sand sheet extents suggest that most of these tsunamis were larger than any generated at hokkaido in the last 200 years the intervals between these inferred outsized tsunamis average nearly 400 years but range widely from about 100 to about 800 years

archaeomagnetic investigations of geomagnetic secular variations bulgarian archaeomagnetic directions and intensities show a satisfactory agreement with those for ukraine and paris bauer plots of declination against inclination show both clockwise and anticlockwise looping through the last eight millennia the necessity for accurate dating of samples studied is stressed

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

syn collisional granitoids in the qilian block on the northern tibetan plateau a long lasting magmatism since continental collision through slab steepening in this paper we present a new model that can explain the large zircon age spectrum of similar to 510 420 ma within a single sample from the gangcha gcha biotite granodiorite and the huangyuan hy two mica monzogranite on the northern tibetan plateau the large age spread recorded in zircons is characteristic of granitoid samples from the studied region which is best explained by the long lasting magmatism since the onset of continental collision at similar to 500 ma followed by slab steepening and the ultimate slab break off at similar to 450 ma these granitoids have a large major and trace element compositional variation but limited initial sr i sr 450 0 709 to 0 715 nd epsilon nd 450 6 5 to 3 7 hf epsilon hf 450 4 3 to 1 5 and pb pb 206 pb 204 450 17 70 to 17 17 pb 207 pb 204 450 15 60 to 15 69 pb 208 pb 204 450 38 04 to 38 73 isotopic variation the small negative whole rock epsilon nd 450 and epsilon hf 450 values are most consistent with the granitoid source being dominated by subducted seafloor materials the inherited zircons with large negative epsilon hf 450 values e g 50 are indicative of input from the lower continental crust and subducted sediments the correlated variations among major elements trace elements and radiogenic isotopes are best interpreted as reflecting melting induced mixing of a compositionally heterogeneous source with superimposed effect of varying extent of fractional crystallization and crustal assimilation the inherited zircons of palaeo proterozoic age and the archean crustal model ages signify the involvement of ancient basement rocks c 2016 elsevier b v all rights reserved

from subduction to collision in the northern tibetan plateau evidence from the early silurian clastic rocks northwestern china abstractthe qilian orogen records early paleozoic collisional suturing of the qaidam block and the central qilian block to the north china craton the composition and u pb age of detrital zircons and the composition of cr spinels from the early silurian lujiaogou and angzanggou formations in the northern part of orogen indicate derivation from evolving oceanic and continental source terranes heavy mineral chemistry indicates the incorporation of suprasubduction zone type ophiolitic detritus in addition to continent derived material integrating these chemical and age data with regional data on the duration of subduction related magmatic activity syn and postcollisional granitic rocks and high pressure metamorphic rocks constrains the transformation from oceanic subduction to continental collision to 450 440 ma the collision resulted in a flood of detritus into the northern part of the orogen from the central qilian block which masked input from the intervening magmatic arc implying rapid exposure o

a 3500 year paleomagnetic record of late holocene secular variation from blue lake idaho three meters of sediment from blue lake idaho have provided a 3500 year paleomagnetic record with low scatter and good serial correlation comparison of the blue lake record with the record from fish lake oregon reveals differences of up to 963 years for secular variation extrema based on the separate radiocarbon chronologies for blue lake and fish lake to achieve a common time scale the blue lake record was redated using the fish lake ages for five major inclination extrema with this adjusted time scale the excellent correspondence between the declination records confirms the extremal correlations and demonstrates that paleomagnetic correlations may provide more accuracy for relative stratigraphy than radiocarbon dating the morphological agreement between the smoothed records documents secular variation features with wavelengths of 1000 2000 years and amplitudes of 10 20 in inclination and 20 30 in declination examination of the unsmoothed records indicates that features with wavelengths of less than a few hundred years and amplitudes of less than 5 are probably noise

pulsed growth of the west qinling at 30 ma in northeastern tibet evidence from lanzhou basin magnetostratigraphy and provenance the development of cenozoic basins in the northeast margin of the tibetan plateau is central to understanding the dynamics of plateau growth here we present a magnetostratigraphy from the lanzhou basin dating the terrestrial deposits from the eocene 47 ma to the middle miocene 15 ma the stratigraphic observation palocurrent and seidment provenance analysis suggest that the lanzhou basin subbasin of the longzhong basin probably initiated as a topographically enclosed depression during eocene to early oligocene 47 30 ma we suspect that right lateral transtensional deformation inherited from the cretaceous may result in formation of the lanzhou basin at the eocene subsequently changes in paleocurrent sandstone and conglomerate compositions and detrital zircon provenance reflect the pulsed growth of the west qinling at 30 ma which triggered not only the formation of new flexural subsidence to the north of the west qlinling but also renewed subsidence of lanzhou basin into this broad foreland basin system we compare this growth history with major ne tibet deformation and suggest it may result from eastward extrusion of the tibetan plateau due to the onset of altyn tagh fault activity at oligocene

stratigraphy distribution and petrographic properties of holocene tephras in hokkaido northern japan abstract thirty one widespread visible holocene tephras in hokkaido northern japan were identified and their stratigraphic relationships were examined among these three tephras from komagatake volcano ko c1 ko c2 and ko g three from tarumae volcano ta a ta b and ta c us 1663 from usu volcano ma b from mashu volcano and b tm from baegdusan volcano on the boundary between north korea and china cover a large area of southern and eastern hokkaido and are thus useful tephras for correlating sedimentary records and holocene phenomena all tephras were identified on the basis of their mineral composition and the refractive indices and major elements of their glass shards the volcanic glass samples were dehydrated before refractive index measurement by heating at 400 c for 12 h most holocene tephras in hokkaido can be easily identified on the basis of the refractive indices and major element compositions of volcanic glass

a simple model for calculating tsunami flow speed from tsunami deposits abstract this paper presents a simple model for tsunami sedimentation that can be applied to calculate tsunami flow speed from the thickness and grain size of a tsunami deposit the inverse problem for sandy tsunami deposits where grain size and thickness vary gradually in the direction of transport tsunami sediment transport is modeled as a steady spatially uniform process the amount of sediment in suspension is assumed to be in equilibrium with the steady portion of the long period slowing varying uprush portion of the tsunami spatial flow deceleration is assumed to be small and not to contribute significantly to the tsunami deposit tsunami deposits are formed from sediment settling from the water column when flow speeds on land go to zero everywhere at the time of maximum tsunami inundation there is little erosion of the deposit by return flow because it is a slow flow and is concentrated in topographic lows variations in grain size of the deposit are found to have more effect on calculated tsunami flow speed than deposit thickness the model is tested using field data collected at arop papua new guinea soon after the 1998 tsunami speed estimates of 14 m s at 200 m inland from the shoreline compare favorably with those from a 1 d inundation model and from application of bernoulli s principle to water levels on buildings left standing after the tsunami as evidence that the model is applicable to some sandy tsunami deposits the model reproduces the observed normal grading and vertical variation in sorting and skewness of a deposit formed by the 1998 tsunami

the formation of pangea abstract the making of pangea is the result of large scale amalgamation of continents and micro continents which started at the end of the neoproterozoic with the formation of gondwana as pieces were added to gondwana on its south american antarctica and australia side ribbon like micro continents were detached from its african and south chinese side cadomia in the late neoproterozoic avalonia and hunia in the ordovician galatia in the devonian and cimmeria in the permian cadomia was re accreted to gondwana but the other ribbon continents were accreted to baltica north china laurussia or laurasia finding the origin of these numerous terranes is a major geological challenge recently a global plate tectonic model was developed together with a large geological geodynamic database at the lausanne university covering the last 600 ma of the earth s history special attention was given to the placing of gondwana derived terranes in their original position using all possible constraints we propose here a solution for the variscan terranes another paper deals with the altaids the galatian super terrane was detached from gondwana in the devonian during the opening of paleotethys and was quickly separated into four sub terranes that started to by pass each other the leading terranes collided at the end of the devonian with the hanseatic terrane detached from laurussia in the carboniferous gondwana started to impinge onto the amalgamated terranes creating the variscan chain and the pangean super continent east of spain paleotethys remained opened until the triassic subducting northward under laurasia roll back of the paleotethyan slab triggered the collapse of most of the european variscan orogen which was replaced by series of permian rifts some of them becoming oceanized back arc basins during the triassic major force changes at the pangean plate limits at the end of the triassic provoked its break up through the opening of the proto caribbean central atlantic alpine tethys oceanic seaways

early palaeozoic north qaidam uhp metamorphic belt on the north eastern tibetan plateau and a paired subduction model an early palaeozoic uhp metamorphic belt was recently discovered in the north qaidam mountains in the north eastern part of the tibet plateau the belt contains abundant eclogite with lesser amounts of garnet peridotite and gneiss the paragneiss contains abundant zircon with coesite inclusions whereas the eclogite has garnet with coesite pseudomorphs and omphacite with exsolved quartz the calculated peak metamorphic conditions for the eclogite are t 730 c and p 2 8 gpa the garnet peridotites are magnesium rich with mg s 100 mg mg fe up to 92 olivine in the peridotites is highly magnesian fo 91 5 and tio2 poor and is typical of low t garnet peridotites calculated peak metamorphic conditions for the peridotites are t 837 c and p 2 5 gpa u pb and sm nd isotope dating indicate that continental subduction started during the early ordovician 495 ma the north qaidam uhp together with the north qilian hp belt about 300 km to the north constitute a pair of genetically related early palaeozoic subduction zones along the northern margin of the tibet plateau

continental subduction and a mechanism for exhumation of high pressure metamorphic rocks new modelling and field data from oman abstract the physical model presented reveals two principal regimes of continental subduction a highly compressional hc regime and a low compressional lc regime characterised by high and low pressure between the overriding and subducting plates respectively the pressure is inversely proportional to the pull force which depends on the difference between average density of the subducting lithosphere and density of the mantle the subducting continental crust reaches a maximum depth which is proportional to the strength of the crust inversely proportional to the interplate pressure and is ca 200 km on average the crust then fails forming a major crustal thrust the location of failure depends on the regime of subduction on the interplate pressure in the hc regime failure occurs near the front of the subduction collision zone whereas the crust fails at greater depth under the base of the overriding plate in the lc regime the failure is followed by buoyancy driven uplift of the subducted crustal slice while the lithospheric mantle keeps subducting the uplift causes a normal sense displacement formation of a normal fault along the upper surface of the crustal slice for the lc regime failure of the crust near the base of the overriding plate a rapid spontaneous crustal uplift intrusion into the interplate zone brings the deeply subducted crust to shallow depths under the pressure of this crust the frontal part of the overriding plate undergoes local extension and then fails forming a tectonic window the rising material the high pressure rocks is exhumed within this window in the hc regime uplift of the subducted crust after its failure in front of the overriding plate is possible only with erosion of the relief which provides an unloading effect allowing the subducted crustal slice to rise up the exhumation depth is generally smaller in this regime but the volume of the exhumed material is larger the hc subduction regime has been shown earlier to match the himalayan situation the lc regime fits the situation in the oman mountains considered in this paper

alkenone distribution in lake van sediment over the last 270 ka influence of temperature and haptophyte species composition abstract fossil long chain alkenones have been used for several decades to reconstruct past ocean surface water temperatures and gained recent interest as a paleotemperature proxy for continental lake settings however factors besides temperature can affect alkenone distributions in haptophyte algae and alkenone compositions can differ between haptophyte species alkenone biosynthesizing haptophyte algae are genetically much more diverse in lakes than in the marine realm and species level variations in alkenone compositions could have implications for alkenone paleothermometry here we performed a paired analysis of alkenone distributions and haptophyte species compositions using ancient dna in up to 270 ka old sediments of lake van in turkey to reveal a possible species effect on fossil alkenone distributions and paleotemperature estimates the same predominant haptophyte in lake van today prevailed also since the last 100 ka however a calibration of alkenone paleotemperature especially in the oldest analyzed intervals is complicated due to a more complex haptophyte species composition predominated by a haptophyte lvhap 6 which is phylogenetically different from sequences recovered from currently existing lakes including lake van and from haptophyte species existing in culture the predominance of lvhap 6 coincided with the presence of alkenone mec38 3 and relatively high mec37 3 4 2 4 and mec38 4 5 ratios 3 0 uk37 index values in the sediment core over the last 270 ka reflect relative changes in past temperature and are additionally linked to haptophyte species composition a sustained period of high salinity as indicated by pore water salinity measurements could potentially have triggered the succession of haptophytes as sources of alkenones in lake van

microfossils from coastal environments as indicators of paleo earthquakes tsunamis and storms abstract coastal risk assessment and hazard mitigation require datasets on centennial and millennial temporal scales to capture natural variability and multiple occurrences of the largest but least frequent events coastal sediments from low energy depositional environments archive geologic evidence of paleo earthquakes tsunamis and storms many of the best reconstructions of these events are derived from changes in microfossil diatoms foraminifera and pollen assemblages in this review we explain how microfossils are used to reconstruct records of paleoearthquakes by quantifying the amount of coseismic and interseismic vertical land movements along tectonically active coastlines examples from the united states alaska and the pacific northwest japan and chile show that microfossil based transfer functions may provide continuous records of vertical land movement during earthquake deformation cycles we discuss how microfossil habitat preferences and taphonomic character are used to constrain sediment provenance e g beach nearshore or offshore sources and identify overwash deposits and how this information can be used to reconstruct the recurrence of tsunamis and storms analysis of overwash deposits from thailand and malaysia indicates the ability of microfossils to resolve individual waves within tsunami sediments and an example from the sendai coastal plain in japan uses foraminifera to ascribe a beach to nearshore provenance for the 2011 tohoku tsunami deposit finally we present recent examples from the gulf of mexico on the use of foraminifera to estimate the volume and distance of transport of storm overwash from hurricanes

the early paleozoic xitieshan syn collisional granite in the north qaidam ultrahigh pressure metamorphic belt nw china petrogenesis and implications for continental crust growth abstract syn collisional magmatism produced by partial melting of subducted oceanic and continental crust during the continental collision plays an important role in understanding the orogenic evolution and crustal growth this contribution reports zircon la icpms u pb ages and hf isotopic compositions whole rock major and trace elements and sr nd isotopic compositions of syn collisional xitieshan intrusion within the north qaidam ultrahigh pressure metamorphic uhpm belt to study its petrogenesis and contribution to continental crust growth la icpms zircon u pb dating of the xitieshan granite yields magmatic crystallization ages of 441 2 ma and 442 2 ma which are consistent with the peak age of ultrahigh pressure eclogite facies metamorphism in the xiteishan terrane the temporal correlation between them confirms that the xitieshan granite was products of syn collisional magmatism during the continental collision between qaidam and qilian blocks these granites show high k calc alkaline and slightly peraluminous signature low zirconium saturation temperatures and high contents of k2o with the s type characteristics they have rare earth element and trace element patterns resembling those of bulk continental crust with strong fractionation of light and heavy rare earth elements la yb n 19 26 moderately eu negative anomalies eu 0 65 0 71 and the obviously nb ta p and ti negative anomalies the xitieshan granite also exhibits remarkable sr nd isotopic differences initial 87sr 86sr 0 70920 0 71080 and end t 4 54 4 11 from the contemporaneous granites within north qaidam uhpm belt combined with the positive ehf t 0 5 5 3 and ages of inherited zircons 475 518 ma the magmatism is best explained as resulting from melting of subducted oceanic and continental crust during continental collision isotopic mixing calculations suggest that ca 28 35 ocean crust and ca 65 72 continental materials contribute to the origin of the xitieshan granite thus the syn collisional felsic magmatism represents juvenile crust with input of oceanic crustal materials derived from the depleted mantle and the hypothesis continental collision zones are primary sites for net continental crust growth is applicable in the north qaidam

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas

secular variation of archaeomagnetic direction in the american southwest a d 750 1425 oriented archaeomagnetic samples were collected from 158 in situ features at 33 archaeological sites in the southwestern united states seventy three independently dated features were used for analysis of secular variation a moving window technique with outlier rejection was developed to compute a smoothed secular variation curve this technique incorporates weighted fisher statistics to account for imprecision in both the age of remanence acquisition and the direction of magnetization for each feature a mean direction and measure of dispersion is generated for each window of time the secular variation record covers the period a d 750 1425 during which time the virtual geomagnetic pole vgp moves in a generally counterclockwise fashion from 86 n 83 e at a d 750 to the lowest latitude position of 74 n 192 e at a d 1075 and then to 85 n 236 e at a d 1425 the median rate of vgp movement is 0 036 yr 1 similar to present day rates the southwestern record shows close correspondence to other north american archaeomagnetic limnomagnetic and speleomagnetic records the archaeomagnetic record can serve as the master curve for the derivation of archaeomagnetic dates