late quaternary sedimentological and climate changes at lake bosumtwi ghana new constraints from laminae analysis and radiocarbon age modeling abstract the lake bosumtwi sediment record represents one of the longest and highest resolution terrestrial records of paleoclimate change available from sub saharan africa here we report a new sediment age model framework for the last 45 cal kyr of sedimentation using a combination of high resolution radiocarbon dating bayesian age depth modeling and lamination counting our results highlight the practical limits of these methods for reducing age model uncertainties and suggest that even with very high sampling densities radiocarbon uncertainties of at least a few hundred years are unavoidable age model uncertainties are smallest during the holocene 205 yr and the glacial 360 yr but are large at the base of the record 1660 yr due to a combination of decreasing sample density larger calibration uncertainties and increases in radiocarbon age scatter for portions of the chronology older than 35 cal kyr additional considerations such as the use of a low blank graphitization system and more rigorous sample pretreatment were necessary to generate a reliable age depth model because of the incorporation of small amounts of younger carbon a comparison of radiocarbon age model results and lamination counts over the time interval 15 30 cal kyr agree with an overall discrepancy of 10 and display similar changes in sedimentation rate supporting the annual nature of sediment laminations in the early part of the record changes in sedimentation rates reconstructed from the age depth model indicate that intervals of enhanced sediment delivery occurred at 16 19 24 and 29 31 cal kyr broadly synchronous with reconstructed drought episodes elsewhere in northern west africa and potentially with changes in atlantic meridional heat transport during north atlantic heinrich events these data suggest that millennial scale drought events in the west african monsoon region were latitudinally extensive reaching within several hundred kilometers of the guinea coast this is inconsistent with a simple southward shift in the mean position of the monsoon rainbelt and requires changes in moisture convergence as a result of either a reduction in the moisture content of the tropical rainbelt decreased convection or both

seismic reflection profiling across tertiary extensional structures in the eastern amargosa desert southern nevada basin and range province outcrops shallow well control and coincident geophysical surveys are used to interpret a seismic reflection profile in the amargosa desert within the basin and range province of southern nevada the east west trending 27 km long seismic line crosses all or parts of three tertiary subbasins revealing that basin growth occurred by progressive shifts of basin bounding faults the reflection line images tertiary strata that is rotated by steeply dipping listric faults and that noses into normal faults a shallow less than 100 to 200 m deep laterally continuous flat lying low frequency reflector interpreted as a tertiary basalt flow suggests that little vertical deformation has occurred within the easternmost of the small tertiary basins since the eruption of the flow about 10 million years ago moderately dipping reflections within the pre tertiary bedrock may image mesozoic thrust faults the reflection data indicate that whereas the top of the reflective lower crust shallows to the west possibly in the direction of increasing crustal extension the moho is relatively flat between 30 and 33 km deep apparent bright spot reflections from the lower crust are interpreted as evidence for ductile shearing of the lower crust not for active magma chambers doming of the lower crust resembles that observed elsewhere in the basin and range province and is consistent with ductile flow in the lower crust

basin development and syntectonic sedimentation associated with kinematically coupled strike slip and detachment faulting southern nevada a previously unrecognized 12 10 ma clastic sequence is distributed widely south of the las vegas valley shear zone and north of lake mead in southern nevada structural and stratigraphic evidence from these sedimentary and associated volcanic rocks suggests that basin development was kinematically related to movement along the las vegas valley shear zone major movement along the nearby saddle island detachment also occurred within this time interval we suggest that spatially and temporally related basin development sedimentation and stratal tilting reflect kinematically coupled movement along a combined las vegas valley shear zone saddle island detachment fault system

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

paleomagnetism of middle miocene volcanic rocks in the mojave sonora desert region of western arizona and southeastern california paleomagnetic directions have been obtained from 190 early to middle miocene 12 20 ma mafic volcanic flows in 16 mountain ranges in the mojave sonora desert region of western arizona and southeastern california these flows generally postdate early miocene tectonic deformation accommodated by low angle normal faults but predate high angle normal faulting in the region after detailed demagnetization experiments 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations trm because of the episodic nature of basaltic volcanism in this region the 179 flows yielded only 65 time distinct virtual geomagnetic poles vgps the angular dispersion of the 65 vgps is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation the paleomagnetic pole calculated from the 65 cooling unit vgps is located at 85 5 n 108 9 within a 4 4 circle of 95 confidence this pole is statistically indistinguishable at 95 confidence from reference poles calculated from rocks of similar age in stable north america and from a paleomagnetic pole calculated from rocks of similar age in baja california the coincidence of paleomagnetic poles from the mojave sonora desert region with reference poles from the stable continental interior indicates that 1 significant vertical axis net tectonic rotations have not accompanied post middle miocene high angle normal faulting in this region 2 there has been no detectable post middle miocene latitudinal transport of the region and 3 long term nondipole components of the middle miocene geomagnetic field probably were no larger than those of the recent 0 5 ma geomagnetic field in contrast paleomagnetic data indicate vertical axis rotations of similar age rocks in the transverse ranges the eastern transverse ranges and the mojave block we speculate that a major structural discontinuity in the vicinity of the southeastward projection of the death valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations

miocene crustal extension and uplift in southeastern nevada constraints from fission track analysis the gold butte block of southeastern nevada appears to be a cross section through the upper approximately 17 km of colorado plateau crust tilted eastward and tectonically denuded via top to the west normal faulting in tertiary time the east side of the block exposes a steeply dipping cratonic paleozoic section 2 to 3 km thick proterozoic basement to the west is continuously exposed for an apparent thickness of approximately 15 km structurally below the basal cambrian unconformity apatite fission track dating and confirmed track length distributions have been used to determine the time of unroofing of the gold butte block and hence the timing of one episode of extension in this part of the basin and range province samples collected in as east west transect along the block indicate an abrupt transition from a period of relative thermal and tectonic stability to the onset of rapid cooling the fission track data indicate that the unroofing of the gold butte block was rapid and that the crustal section was brought to within a few kilometers of the surface at ca 15 ma

a model for tectonic rotations about a vertical axis abstract the rotation of a rigid ellipsoidal inclusion within a highly viscous fluid orientated so that two of the principal axes remain horizontal is used as a model for the rotation of crustal inclusions in wide zones of continental deformation this model is also applicable to other geological problems involving the rotation of inclusions in a matrix the pattern of behaviour in such a model is shown to be complex in general the rotation rate of the inclusion is a function of all components of the velocity field of the deforming medium and the horizontal aspect ratio of the inclusion however for a given velocity field this aspect ratio must exceed a critical value before the inclusion can rotate continuously inclusions with lower aspect ratios will rotate for a certain range of orientations in an opposite direction to the sense of shear in the deforming zone the possibility of the inclusion changing shape during rotation adds to the complexity of behaviour

possible linkages of late holocene drought in the north american midcontinent to pacific decadal oscillation and solar activity oxygen isotopic composition d 18 o the d 18 o record of the past 3100 years reveals that droughts of greater severity and duration than during the 20th century occurred repeatedly especially prior to 300 ad drought variability was anomalously low during the 20th century 90 of the variability values during the last 3100 years were greater than the 20th century average d 18 o is strongly correlated with the index of the pacific decadal oscillation pdo during the past 100 years and periodicities of the lateholocene d 18 o record are similar to those of the pdo furthermore time series of d 18 o and atmospheric d 14 ca re generally coherent after 700 ad both the pacific climate and solar irradiance probably played a role in drought occurrence but their effects were non stationary through the late holocene citation tian j d m nelson and f s hu 2006 possible linkages of late holocene drought in the north american midcontinent to pacific decadal oscillation and solar activity geophys res lett 33 l23702 doi 10 1029 2006gl028169

post 12 m y rotation of southwest washington paleomagnetic field directions from the basalt of pack sack lookout are compared to those from the pomona member of the saddle mountains basalt of the columbia river basalt group the pomona crops out over a wide region on the columbia plateau east of the cascade range and the basalt of pack sack lookout crops out well to the west of the cascades about 30 to 60 km east of the washington coast our paleomagnetic results support existing geologic and geochemical evidence that both these units are part of a single 12 m y old flow that erupted in western idaho and flowed to the washington coast the magnetic data further indicate that southwest washington has undergone clockwise rotation with respect to the columbia plateau and stable north america the data suggest that both a large scale regional rotation of 16 and locally complex small scale rotations exist the two being present to different degrees in the eastern and western regions of southwest washington the cascade range appears to coincide with the tectonic boundary separating rotated and unrotated regions of washington state rotation of southwest washington appears to have been associated with the rotation of large pieces of western oregon and southern washington possibly as a result of either a ridge push force from the basin and range province or shear along the pacific north america plate boundary

synchronous radiocarbon and climate shifts during the last deglaciation radiocarbon data from the cariaco basin provide calibration of the carbon 14 time scale across the period of deglaciation 15 000 to 10 000 years ago with resolution available previously only from holocene tree rings reconstructed changes in atmospheric carbon 14 are larger than previously thought with the largest change occurring simultaneously with the sudden climatic cooling of the younger dryas event carbon 14 and published beryllium 10 data together suggest that concurrent climate and carbon 14 changes were predominantly the result of abrupt shifts in deep ocean ventilation

paleomagnetism of the oligocene kalamazoo tuff implications for middle tertiary extension in east central nevada the oligocene kalamazoo tuff 35 ma was sampled for paleomagnetic analysis across a 100 km wide zone of highly extended crust in east central nevada to estimate between site vertical axis rotations and thus the relative importance of strike slip faulting to the mechanism of extension subordinate sampling was also undertaken in a younger hornblende dacite intrusion 35 ma and the overlying tuff of north creek 35 ma in general ash flow tuffs cool and are magnetized within a short period of time relative to geomagnetic secular variation and their eutaxitic structures typically indicate the paleohorizontal these attributes of ash flow tuffs in addition to their generally widespread occurrence make them excellent candidates for combined paleomagnetic and structural studies of regional deformation however in this study a number of problems associated with the paleomagnetism of the kalamazoo tuff are documented they include topography related variations of the eutaxitic structures secular variation during slow cooling postblocking rheomorphic flow and local remagnetization of the tuff by younger igneous activity the tilt corrected data with these sources of error reduced or eliminated exhibit a 28 12 clockwise rotation of the schell creek range relative to the kern mountains region this rotation implies differential extension accommodated by strike slip faulting or n s shortening the paleomagnetic results also suggest that large changes in strike of layered units near faults with presumed strike slip movement need not be the result of oroclinal bending but could result from superimposed sets of orthogonal normal faults

miocene paleomagnetism and tectonic setting of the baja california peninsula mexico a paleomagnetic pole calculated from 64 virtual geomagnetic poles for miocene volcanic rocks of the baja california peninsula 86 1 n 142 6 e 95 3 9 is similar to a reference pole for the north american craton 87 4 n 129 7 e 95 3 0 consistent with the baja peninsula having been adjacent to the mexican mainland in its pre gulf of california position from at least early miocene time to the pliocene opening of the gulf this result is in agreement with marine geophysical and geologic evidence from the gulf of california and baja peninsula and does not support a long period nondipole geomagnetic field in the region during miocene time or 10 of northward transport of the peninsula since then inferred from smaller data sets included in this work by previous workers this study also indicates that a pre gulf of california northward translation 9 of latitude and clockwise rotation 24 of the baja peninsula with respect to north america determined from paleomagnetic studies of cretaceous rocks within peninsular california occurred prior to miocene time paleomagnetic results from late cretaceous to early tertiary basement rocks in western sinaloa mexico are similar to those results from cretaceous rocks of the baja peninsula supporting the early tertiary northward translation of the peninsula between 45 and 34 ma and suggesting that the tectonic boundary separating peninsular california from north america at that time was east of the present day gulf of california

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

large magnitude permian shortening and continental margin tectonics in the southern cordillera the last chance allochthon with displacement estimated to be 55 to 105 km and lateral extent possibly exceeding 300 km was inferred to be middle triassic and thus kinematically distinct from a belt of permian triassic deformation in the western mojave desert correlations of structures forming the death valley thrust belt which includes the last chance allochthon several subjacent structures and their inferred lateral equivalents suggest that prior to tertiary extension individual structures were laterally continuous for distances exceeding 150 km along strike from the nevada test site area southwest to the darwin plateau their inferred mesozoic age and the presence of an early triassic overlap sequence on the darwin plateau precluded projection of these structures farther south in spite of long recognized strain compatibility problems associated with large displacement on thrusts that apparently terminated there without suitable accommodation structures such as tear faults or lateral ramps these problems may be resolved if the last chance allochthon is permian consistent with structural correlations new isotopic data from post tectonic stocks and stratigraphic evidence a large permian turbidite basin near the darwin plateau can be interpreted as a marine foredeep resulting from emplacement of the 5 km thick allochthon rather than a local trough caused by transcurrent faulting during inferred permian triassic continental truncation i propose that anomalous eugeoclinal rocks in the western mojave desert have been emplaced against cratonic strata there by the last chance allochthon which foreshortened an originally narrow continental margin

comparison of varve and 14c chronologies from steel lake minnesota usa annually laminated sediments varves offer an effective means of acquiring high quality palaeoenvironmental records however the strength of a varve chronology can be compromised by a number of factors such as missing varves ambiguous laminations and human counting error we assess the quality of a varve chronology for the last three millennia from steel lake minnesota through comparisons with nine ams 14c dates on terrestrial plant macrofossils from the same core these comparisons revealed an overall 8 4 discrepancy primarily because of missing uncountable varves within two stratigraphic intervals characterized by low carbonate concentrations and obscure laminations application of appropriate correction factors to these two intervals results in excellent agreement between the varve and 14c chronologies these results together with other varve studies demonstrate that an independent age determination method such as 14c dating is usually necessary to verify and potentially correct varve chr

el nino like climate teleconnections in new england during the late pleistocene a glacial varve chronology from new england spanning the 4000 year period from 17 500 to 13 500 calendar years before the present was analyzed for evidence of climate variability during the late pleistocene the chronology shows a distinct interannual 3 to 5 years band of enhanced variability suggestive of el nino southern oscillation enso teleconnections into north america during the late pleistocene when the laurentide ice sheet was near its maximum extent and climatic boundary conditions were different than those of today this interannual variability largely disappears by the young end of the 4000 year chronology with only the highest frequency components roughly 3 year period persisting this record provides evidence of enso like climate variability during near peak glacial conditions

semi automated detection of annual laminae varves in lake sediments using a fuzzy logic algorithm abstract annual laminae varves in lake sediments are typically visually identified measured and counted although numerous attempts have been made to automate this process the reason for the failure of most of these automated algorithms for varve counting is the complexity of the seasonal laminations typically rich in lateral facies variations and internal heterogeneities in the manual counting of varves the investigator acquired and interpreted flexible numbers of complex decision criteria to understand whether a particular simple lamination is a varve or not fuzzy systems simulate the flexible decision making process in a computer by introducing a smooth transition between true varve and false varve in our investigation we use an adaptive neuro fuzzy inference system anfis to detect varves on the basis of a digital image of the sediment the results of the application of the anfis to laminated sediments from the meerfelder maar eifel germany and from a landslide dammed lake in the quebrada de cafayate of argentina are compared with manual varve counts and possible reasons for the differences are discussed

new approach to assessing age uncertainties the 2300 year varve chronology from eklutna lake alaska usa abstract developing robust chronological frameworks of lacustrine sediment is central to reconstructing past environmental changes we present varve chronologies from five sites extending back 2300 years from eklutna lake in the chugach mountains of south central alaska the chronologies are built from image analysis of high resolution photographs and ct scans of sediment cores the age uncertainty of each record is tested by three methods we first present varve chronologies from individual sites and reconcile the difference in varve delimitation from two observers the varve chronologies from each site are then compared to each other using a series of marker beds that can be traced across the lake basin finally using a new bayesian probabilistic model we develop age models that incorporate information regarding age uncertainty from the multiple observer method and the age distribution of marker layers from multiple cores to evaluate the accuracy of the bayesian model output we used seven radiocarbon ages from terrestrial macrofossils and four tephra layers traceable across the core sites the major element geochemistry of the tephra layers and their ages are presented here for the first time the bayesian age model offers a new approach to quantifying age uncertainty in inter correlated cores of varved sediment

volcanic eruptions in the longgang volcanic field northeastern china during the past 15 000 years the longgang volcanic field located in northeastern china is volcanically active with a number of eruptions during the quaternary but the chronology of the eruptions is poorly defined some tephra layers are well preserved in the annually laminated sediments of maar lakes in the region and facilitate the construction of a much improved chronological framework for the volcanic history of the area the results of our investigations reveal that three basaltic explosive eruptions occurred at ad 460 11460 cal yr bp and 14000 cal yr bp respectively the largest explosive basaltic eruption ad 460 produced a thick black scoria layer in the longgang volcanic field including lakes the tephra distribution and chronological data suggest that this eruption is likely to be from the jinlongdingzi volcano two basaltic flood eruptions occurred at jinlongdingzi the earlier basaltic eruption produced a lava flow that spread over a forest and encased standing trees two radiocarbon ages obtained from charcoal samples collected from the burned remains of these trees are 1828 1989 cal yr bp and 2164 2359 cal yr bp in the most recent stage of volcanism the lava flow extended only ca 2 km and flowed into lake dalongwan from the present status of the forest ecosystem which has not yet reached the fully mature successional stage we estimate that this lava is very young ca a few hundreds years old jinlongdingzi is a potentially dangerous volcano monitoring and assessment of the potential hazards in the longgang volcanic field should be carried out in the future

magnetic grain size variations through an ash flow sheet influence on magnetic properties and implications for cooling history rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks provide a basis for interpretation of aeromagnetic data over volcanic terranes and yield insights into the depositional and cooling histories of ash flow sheets a rhyolitic ash flow sheet the miocene aged tiva canyon member of the paintbrush tuff contains both titanomagnetite phenocrysts present in the magma prior to eruption and cubic fe oxide microcrystals that grew after emplacement systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the nevada test site natural remanent magnetization varies from less than 1 10 4 to more than 8 10 4 a m3 kg 1 and in phase magnetic susceptibility varies from less than 1 10 6 to more than 10 10 6 m3 kg 1 the microcrystals which include both magnetite and maghemite have curie points and maximum unblocking temperatures between 580 c and 640 c rock magnetic data including in phase and quadrature magnetic susceptibilities as well as hysteresis parameters demonstrate that these microcrystals are of superparamagnetic and single domain sizes titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15 m thick zones at the top and bottom within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet the decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break possibly represented by a concentration of pumice laboratory heating produces changes in magnetic properties which vary complexly from sample to sample such changes include 1 decrease in saturation magnetization 2 both increase and decrease in in phase and quadrature magnetic susceptibilities and 3 shifts in coercivity spectra the complex sample to sample variation in response to heating is attributed to differences in initial grain size distributions

uniqueness of geological correlations an example from the death valley extended terrain the northern death valley furnace creek fault zone is a major northwest trending fault system in the death valley region but the magnitude of offset and its significance to extensional tectonism in the region are controversial stratigraphic data have provided first order constraints on offset across the fault zone but offer limited resolution previous correlations of mesozoic thrust faults across the fault zone have relied on regional stratigraphic trends to fingerprint structurally similar thrust plates we show that a northeast trending mesozoic backfold within the predominantly east directed fold and thrust belt was probably continuous from the southern panamint range california to the belted range nevada the relative position of this antithetic structure allows uniquely compatible correlations of three mesozoic structures between two range blocks across the northern death valley furnace creek fault zone independent of stratigraphic arguments the offset structures have geometric properties of size vergence order and spacing that indicate a probability less than 10 4 of the mesozoic orogen identically repeating this structural suite in two unrelated places reconstruction of these correlative structures indicates 68 4 km n48 w 6 w of apparent dextral offset between the cottonwood and funeral mountains in general agreement with previous estimates based on stratigraphic observations but of substantially higher precision this reconstruction and the restoration of a 30 km separation between the funeral and grapevine mountains indicated by offset segments of the backfold suggest a simple pre extensional geometry for the mesozoic orogen in the death valley region consistent with observations from unextended blocks and analogous to that of the cordilleran orogen at the latitude of calgary alberta

varve counting reveals high resolution radiocarbon reservoir age variations in palaeolake lisan the laminated lacustrine sediments deposited in the last glacial lake lisan represent annual deposits of primary aragonite and silty detritus that reflect the annual supply of bicarbonate bearing freshwater to the lake a varve counting curve was constructed for the time interval of ca 17 4 22 cal ka bp based on aragonite u th and atmospheric radiocarbon ages of organic debris recovered from the studied section radiocarbon in the primary evaporitic aragonite comprises both atmospheric and old carbon reflecting the reservoir age the aragonite reservoir ages were determined by comparing the aragonite radiocarbon dates to the varve counting curve and are found to lie in the range 1900 600 a and display a continuous decline this opens the possibility for high annual resolution monitoring of the reservoir age similar in quality to tree ring counting during the upper part of marine isotope stage mis 2 our work also demonstrates that a uniform reservoir age correction is inappropriate when determining the chronology of short term climate events in lacustrine environments copyright 2009 john wiley sons ltd

remagnetization and tectonic rotation of upper precambrian and lower paleozoic strata from the desert range southern nevada in the desert range of southern nevada miogeoclinal sedimentary rocks mostly shallow water limestones of latest precambrian through early ordovician age yield three components of magnetization having different blocking temperature spectra 1 a low blocking temperature component near the direction of the present axial dipole field 2 an intermediate blocking temperature component with northerly declination and inclination 60 and 3 a characteristic component with southeasterly declination and inclination 20 combinations of alternating field and thermal demagnetization indicate that the intermediate and characteristic components reside in magnetite the intermediate magnetization probably reflects a viscous partial thermoremanent magnetization vptrm imposed between the late cretaceous and mid cenozoic it was probably acquired when the strata were dipping slightly eastward the characteristic magnetization is probably a vptrm imposed during regional uplift in the late permian the degree of heating required to have destroyed any primary magnetization is consistent with the conodont color alteration index observed in the ordovician rocks additionally the characteristic magnetization in those rocks is younger than bedding disruption caused by major late stylolitization red purple mudstones from the middle member of the wood canyon formation in which a directionally similar characteristic magnetization resides in fine grained hematite also appear to have been remagnetized in these rocks the remagnetization probably reflects partial recrystallization as the blocking temperatures are too high to have been reset by burial heating the sampled sections have undergone relative tectonic rotation about a vertical axis consistent with late tertiary oroflexural bending that had been proposed on independent geologic evidence the characteristic magnetization probably provides a reliable estimate of the magnitudes of the vertical axis rotations as the regional geology suggests the desert range strata were essentially horizontal throughout the paleozoic the total observed relative rotation is 44 5 representing absolute counterclockwise rotation 27 4 of the northern part of the desert range and absolute clockwise rotation 17 5 of the southern part the only unit in the desert range sequence that may retain a primary magnetization is the late precambrian rainstorm member of the johnnie formation the characteristic magnetization of this unit exhibits two polarities and probably resides in specular hematite after correction for 18 of counterclockwise rotation the resulting pole 5 n 151 e is near other late hadrynian poles from north america

episodic caldera volcanism in the miocene southwestern nevada volcanic field revised stratigraphic framework 40ar 39ar geochronology and implications for magmatism and extension the middle miocene southwestern nevada volcanic field swnvf is a classic example of a silicic multicaldera volcanic field in the great basin more than six major calderas formed between 15 and 7 5 ma the central swnvf caldera cluster consists of the overlapping silent canyon caldera complex the claim canyon caldera and the timber mountain caldera complex active from 14 to 11 5 ma and centered on topographic timber mountain locations of calderas older than the claim canyon caldera source of the tiva canyon tuff are uncertain except where verified by drilling younger peralkaline calderas black mountain and stonewall mountain formed northwest of the central swnvf caldera cluster we summarize major revisions of the swnvf stratigraphy that provide for correlation of lava flows and small volume tuffs with the widespread outflow sheets of the swnvf new laser fusion 40 ar 39 ar isotopic ages are used to refine and revise the timing of eruptive activity in the swnvf the use of high sensitivity mass spectrometry allowed analysis of submilligram sized samples with analytical uncertainties of 0 3 1 permitting resolution of age differences as small as 0 07 ma these results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the swnvf major caldera episodes belted range crater flat paintbrush timber mountain and thirsty canyon groups erupted widespread ash flow sheets within 100 300 k y time spans and pre and post caldera lavas erupted within 100 300 k y of the associated ash flows peak volcanism in the swnvf occurred during eruption of the paintbrush and timber mountain groups when over 4500 km 3 of metaluminous magma was erupted in two episodes within 1 35 m y separated by a 750 k y magmatic gap peralkaline and metaluminous magmatism in the swnvf overlapped in time and space the peralkaline tub spring and grouse canyon tuffs erupted early and the peralkaline thirsty canyon group tuffs and stonewall flat tuff erupted late in the history of the swnvf flanking the central volumetrically dominant peak of metaluminous volcanism magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data particularly in the overlapping grouse canyon and area 20 calderas of the silent canyon caldera complex volcanism in the swnvf coincided with the miocene peak of extensional deformation in adjoining parts of the great basin although regional extension was concurrent with volcanism it was at a minimum in the central area of the swnvf where synvolcanic faulting was dominated by intra caldera deformation significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the swnvf between the paint brush and timber mountain caldera episodes larger magnitude detachment faulting in the bullfrog hills southwest of the central swnvf caldera cluster followed the climactic timber mountain caldera episode postvolcanic normal faulting was substantial to the north east and south of the central swnvf caldera cluster but the central area of peak volcanic activity remained relatively unextended in postvolcanic time volcanism and extension in the swnvf area were broadly concurrent but swnvf area were broadly concurrent but in detail they were episodic in time and not coincident in space

paleomagnetic evidence for counterclockwise block rotation in the north nevada rift region paleomagnetic data from mid miocene dikes and flows at three localities within the north nevada rift indicate that some crustal blocks have rotated approximately 19 degree plus minus 7 degree counterclockwise relative to stable north america as one possible consequence a revised mid miocene extension direction for the area is about n89 degree e s89 degree w indicating an approximately 25 degree difference between mid miocene and modern least principal stress directions the rotation may be accommodated by a right lateral component of slip on northwest trending oblique slip faults these as well as data from other parts of the basin and range province indicate that strike slip faulting and associated block rotation contributed to cenozoic extension

variations of younger dryas atmospheric radiocarbon explicable without ocean circulation changes the concentration of radiocarbon 14c in the atmosphere depends on its production rate by cosmic rays and on the intensity of carbon exchange between the atmosphere and other reservoirs for example the deep oceans for the holocene the past 11 500 years it has been shown that fluctuations in atmospheric radiocarbon concentrations have been caused mostly by variations in the solar magnetic field1 2 3 recent progress in extending the radiocarbon record backwards in time4 5 6 7 8 9 10 has indicated especially high atmospheric radiocarbon concentrations in the younger dryas cold period between 12 700 and 11 500 years before the present these high concentrations have been interpreted as a result of a reduced exchange with the deep ocean reservoir caused by a drastic weakening of the deep ocean ventilation7 8 9 11 12 here we present a high resolution reconstruction of atmospheric radiocarbon concentrations derived from annually laminated sediments of two polish lakes lake go ciaz and lake perespilno these records indicate that the maximum in atmospheric radiocarbon concentrations in the early younger dryas was smaller than previously believed and might have been caused by variations in solar activity if so there is no indication that the deep ocean ventilation in the younger dryas was significantly different from today s

dust records from varved lacustrine sediments of two neighboring lakes in northeastern china over the last 1400 years dust storms are natural phenomena described in chinese historical documents as early as 1150 bc these documents provide a way to evaluate proxy data from different natural records the minerogenic clastic content and grain size from two closed maar lakes located a short distance apart in northeastern china were analyzed the area is located in the path of dust storms the minerogenic clastics in the studied maar lakes originate mainly from dust deposition general variations of the minerogenic clastic content in these two neighboring lakes show a good coincidence with dust concentration in a tibet ice core and dust events as recorded in historical documents the minerogenic clastic content in the lake sediments is linked with the climate conditions in the dust source regions and wind strength three periods of intensive dust events ad 1050 1330 ad 1590 1690 and the last 200 years have been identified during the past 1500 years the recent increasing pattern of clastic content could be linked to human activity in the dust source regions significantly most of the medieval warm period mwp was marked by a high frequency of dust storms

the steens mountain oregon geomagnetic polarity transition 3 its regional significance study of the variations of direction and intensity of the geomagnetic field as recorded by the miocene lava flows on steens mountain southeastern oregon has resulted in a detailed description of total field behavior during a reversal in polarity in addition to information about the polarity reversal itself the detailed paleomagnetic record includes several thousand years of geomagnetic history preceding and following the polarity transition at 15 5 ma in order to test the feasibility of using this record as a means of correlation in this part of the western united states comparisons are made of reconnaissance and previously published paleomagnetic records obtained from what has been thought to be the steens basalt or rocks of equivalent age despite the fact that many of these earlier studies were not done in detail and were not intended for correlation purposes convincing similarities among some of the records are evident the steens basalt paleomagnetic record does indeed have potential as a correlation tool during this time of widespread basaltic volcanism additionally paleomagnetic data from flows that were sampled in detail yield a middle miocene paleomagnetic pole at 88 3 n 209 0 95 6 3 for the high lava plains of oregon this pole position is statistically indistinguishable from the earth s rotational axis and implies that no tectonic rotation of this region has occurred since these lava flows were erupted data from selected sites within the coeval part of the columbia river basalt group yield a paleomagnetic pole at 88 7 n 171 6 e 95 4 0 the columbia river basalt group pole is statistically indistinguishable from either the rotational axis or from the high lava plains pole these findings indicate no post 20 ma differential rotation between south eastern washington and south central oregon in contrast to previous interpretations

comment and reply on magnitude of crustal extension in the southern great basin we thank royse for explicitly raising a point that we did not make completely clear in our paper evidence that establishes a tertiary age for the development of the las vegas shear zone this is a problem not only for the shear zone but for the garlock fault as well in the case of both faults the principal matching features across them which demonstrate large displacement and or bending are of mesozoic age or older indicating that displacements could have occurred during the mesozoic

preparation of polished thin sections from poorly consolidated regolith and sediment materials abstract good quality polished thin sections can be prepared from poorly consolidated wet fragile and friable regolith and sediment materials in a standard earth science thin section laboratory with only minor additional equipment materials are dried over an acetone bath impregnated with a polyester resin mix and cut and polished using minor modifications of standard hard rock thin sectioning equipment and techniques the sections produced are suitable for both optical and electron microscopy and electron probe microanalysis

paleomagnetic and structural evidence for middle tertiary counterclockwise block rotation in the dixie valley region west central nevada paleomagnetic data from late oligocene to early miocene ash flow tuffs at four localities in the northern dixie valley region west central nevada indicate that parts of the crust have rotated counterclockwise by at least 25 sup 0 and perhaps significantly more in late cenozoic time field relations in white rock canyon stillwater range suggest that rotation 1 was accommodated by right lateral slip on northwest trending faults 2 spanned ash flow tuff emplacement and 3 probably ceased before eruption of overlying middle miocene basalts accurate estimates of cenozoic extension as well as evaluation of earlier mesozoic structures must include the strain partitioned into rotation in the area

aspects of three dimensional strain at the margin of the extensional orogen virgin river depression area nevada utah and arizona the virgin river depression and surrounding mountains are neogene features that are partly contiguous with the little strained rocks of the structural transition to the colorado plateau province this contiguity makes the area ideally suited for evaluating the sense magnitude and kinematics of neogene deformation analysis along the strain boundary shows that compared to the adjacent little strained area large magnitude vertical deformation both uplift and depression locally exceeding 10 km greatly exceeds extensional deformation and that significant amounts of lateral displacement approximately parallel the province boundary extensional kinematics the directions of lateral displacements and the position and orientation of uplifts and one major depression are such that isostatic rebound following tectonic denudation is an unlikely direct cause of the strong vertical structural relief adjacent to the strain boundary instead the observed structures are first order features defining a three dimensional strain field produced by approximately east west extension vertical structural attenuation and extension normal shortening all major structural elements of the strain boundary strain field are also found in the adjacent basin and range especially evident is a discontinuous and inhomogeneous distribution of vertical structural relief resulting from uplift and tilting of the footwalls on convex upward normal faults combined with formation of extension parallel folds deformation resulted in a bewilderingly complex mixture of coeval folds normal faults strike slip faults and in some areas reverse faults thrust faults and thrust faults reactivated as extensional detachment faults these complexly mixed structures exhibit a tectonically important aspect of scale independence ranging from micro structures to mountain range scale many intensely extended domains appear to reflect a process of surface relief minimization above uplifted and tilted blocks some intensely extended zones are depth limited by relatively undeformed rocks above and intact tilted blocks below and are best viewed as subhorizontal zones of accommodation for the uplift and tilt of subjacent blocks whether or not the intense deformation extends to the surface its depth distribution commonly reflects lithologic or paleostructural controls whatever the cause for the concentration of extensional strain many gently dipping zones are more accurately viewed as local features recording small to moderate extension parallel translations than a regionally interconnected detachment faults recording enormous extension parallel translations although the total neogene strain field in the nevada utah arizona tricorner area has unique aspects other parts of the basin and range contain generally similar structures that presumably formed from similar processes these processes include lateral mass transfer probably by viscoelastic flow of the material needed to inflate areas of strong uplift and to translate blocks laterally by tectonic rafting directions of mass transfer vary but are commonly both parallel and normal to the direction of extension or parallel to the boundaries of extended areas much of the strong vertical structural relief probably reflects boundary condition or localized perturbations in the form of restraints or releases of the lateral flow system these conclusions are consistent with modern concepts of the flexural strength and rheological stratification of the crust in regions undergoing extension

current sierra nevada north america motion from very long baseline interferometry implications for the kinematics of the western united states we use geodetic measurements from very long baseline interferometry to estimate the motion of the sierra nevadan microplate which is composed of the sierra nevada and the great valley the motion of the sierra nevadan microplate relative to the north american plate is described by a right handed rotation of 0 61 m y about lat 32 n long 128 w this euler pole lies only 10 southwest of the sierra nevadan microplate and predicts a significant counterclockwise rotation about a local vertical axis it further predicts a velocity of the eastern edge of the sierra nevada at 38 0 n 119 3 w relative to stable north america of 11 1 mm yr toward n36 3 w quoted uncertainties are plus or minus one standard error which accounts for about one fourth of the velocity between the pacific and north american plates and is 25 clockwise of many prior estimates the velocity nearly parallels the boundary between the sierra nevada and the great basin which implies that current motion within the great basin results in a rotational noncoaxial deformation we use this velocity to estimate how motion is distributed across the broad deforming zone taking up pacific north america plate motion we find that the vector sum of strike slip along the san andreas fault and motion of the sierra nevada relative to stable north america taken up by deformation within the great basin differs little from the pacific north america plate velocity the difference can be described at 36 n along the san andreas fault by a vector of 6 mm yr directed toward n20 w this vector resolves into components of 5 mm yr parallel to the fault and 2 mm yr perpendicular to the fault with 95 confidence intervals of 0 to 10 mm yr and 1 to 5 mm yr respectively the component perpendicular to the fault is several times smaller than found in prior studies and places a small upper bound on fault perpendicular shortening we conclude that motion previously inferred to be taken up by deformation other than strike slip along the san andreas fault or deformation within the great basin is much smaller than previously thought

abrupt changes in north american climate during early holocene times recent studies of the greenland ice cores have offered many insights into holocene climatic dynamics at decadal to century timescales1 2 3 despite the abundance of continental records of holocene climate few have sufficient chronological control and sampling resolution to compare with the greenland findings4 butannually laminated sediments varves from lakes can provide high resolution continental palaeoclimate data with secure chronologies here we present analyses of varved sediments from deep lake in minnesota usa trends in the stable oxygen isotope composition of the sedimentary carbonate indicate a pronounced climate cooling from 8 9 to 8 3 kyr before present probably characterized by increased outbreaks of polar air decreased precipitation temperatures and a higher fraction of the annual precipitation falling as snow the abrupt onset of this climate reversal over several decades was probably caused by a reorganization of atmospheric circulation and cooling of the arctic airmass in summer that resulted from the final collapse of the laurentide ice near hudson bay and the discharge of icebergs from the quebec and keewatin centres into the tyrell sea the timing and duration of this climate reversal suggest that it is distinct from the prominent widespread cold snap that occurred 8 200 years ago in greenland and other regions1 5 6 no shifts in the oxygen isotope composition of sediment carbonate occurred at 8 2 kyr before present at deep lake but varve thickness increased dramatically probably as a result of increased deposition of aeolian dust taken together our data suggest that two separate regional scale climate reversals occurred between 9 000 and 8 000 years ago and that they were driven by different mechanisms

experimental fracture strain and subsidence patterns over en echelon strike slip faults implications for the structural evolution of pull apart basins abstract claybox experiments show that with progressive left lateral displacement on basal left stepping en echelon strike slip faults an extremely complicated zone of evolving fracture strain and subsidence patterns develops in the overlying clay the fracture pattern is more complex than predicted by theoretical models and consists of a progressively widening array of riedel shears conjugate riedel shears and tension gashes above basal master faults conjugate riedel shears rotate anticlockwise and lengthen forming s shaped fractures riedel shears rotate both anticlockwise and clockwise those that rotate anticlockwise eventually open as tension gashes other tension gashes open where the ends of riedel and conjugate riedel shears link strain contours in the deformed clay show an asymmetrical z shaped pattern over the stepover area between basal en echelon faults in the early stages of displacement strain is distributed penetratively over a progressively broadening zone in later stages of displacement strain is partitioned into a narrow middle zone maximum strain is sustained always in the middle of the sheared zone and increases exponentially with progressive displacement subsidence is accommodated in the stepover area by oblique slip on many riedel and conjugate riedel shears distributed throughout the stepover area subsidence y exhibits a linear relationship with master fault displacement x described by y 0 36 x 1 4 the experimental fracture strain and subsidence patterns in clay over en echelon faults can be used as interpretive and predictive tools with which to constrain interpretations of static field examples especially in pull apart basins on the crustal scale where many similarities with the models exist

the relationship between strain rates crustal thickening palaeomagnetism finite strain and fault movements within a deforming zone the deformation of continental lithosphere generally does not occur on a single fault but is distributed over a zone up to a few hundred kilometers wide if the deformation rate is constant throughout the zone and if the velocity is continuous everywhere and does not change with time the deformation gradient tensor can be obtained analytically this tensor directly determines the amount of crustal thickening and is related in a complicated manner to the finite strain tensor the declination of remnant magnetization measured within the zone can also be rotated by the deformation because palaeomagnetic measurements are generally made on rigid blocks which have not been deformed there is no simple relationship between such rotations and the finite strain expressions for all these quantities are obtained in terms of the velocity gradients which are taken to be constant the history of western north america is used to illustrate the relationship between the present deformation by strike slip and thrust motions the rotations of the remnant magnetization vectors and the shape of accreted terranes at the surface distributed deformation is taken up by motion on faults which separate the zone into blocks the displacement rate on the faults their strike and the finite rotations of the blocks are all controlled by the velocity gradients analytic expressions relating these quantities are obtained a deforming zone containing normal faults which crosses central greece takes up both strike slip and extension and illustrates the relationship between the structures within and the motion across the zone a similar but more complicated zone in northeastern iran involves both strike slip and shortening

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

what forced the collapse of european ice sheets during the last two glacial periods 150 ka b p and 18 ka cal b p palynological evidence article i nfo we present a new marine palynological record from the temperate north eastern atlantic core md03 2692 celtic armorican margin in the area of influence of the fleuve manche paleoriver during glacial times the runoff of this paleoriver was connected to the glacial history of european ice sheets including the british irish ice sheet biis our study conducted on dinoflagellate cysts assemblages over the last 200 ka associated with quantification of palynological remains reflecting continental influence pre quaternary dinocysts and micro algae pediastrum spp coenobia yields detailed insights into the sea surface paleoenvironments of this sector more specifically mechanisms responsible for the origin of unusual deposits of laminated sequences found at the end of marine isotope stage mis 2 and within mid mis 6 are here constrained we provide evidence of genetic similarities between fluvial discharges occurring before and during times of deposition of the laminated deposits our study clarifies the causes of the collapse of european glaciated systems at the end of mis 2 prior to the deposition of the laminated deposits pleni oceanic influence characterized by high concentrations of quaternary dinocysts prevailed and a dinocyst species tracing the penetration of the warm north atlantic drift into the bay of biscay is recorded however this scenario did not recur prior to the deposition of laminated sediments of mid mis 6 in addition contrary to the laminated deposits found at the end of mis 2 and directly associated with termination i mis 6 laminae appeared 20 ka earlier than termination ii our work shows that during the penultimate glacial stage the collapse of mid latitudes ice sheets around 170 ka b p may have immediately followed the mis 6 5 warming phase then the most important melting event around 150 ka b p may be linked to a peak in insolation at 65 n though less important than those during mis 6 5 and termination ii

scanning micro x ray fluorescence elemental mapping a new tool for the study of laminated sediment records 1 the utility of elemental mapping by scanning x ray fluorescence xrf in the study of annual laminated sedimentary records was investigated on eight annually laminated sediment types the examples were chosen to illustrate the potential of this approach in environments dominated by terrigenous biological and chemical deposition individual laminae were identifiable in elemental maps of all sediment types and were enhanced through the use of data reduction techniques e g principal components transformation laminae were least apparent in clastic dominated systems with no seasonal changes in sediment sources in biologically dominated systems element maps provided insights into the composition of the varve subcomponents related to alternating terrigenous and biologically dominated seasonal periods of deposition chemically precipitated structures were more prevalent than expected from visual investigations alone and may provide an underutilized paleoenvironmental signature of changing limnological conditions elemental mapping offers a valuable tool for the study of laminated records that complements existing techniques e g sem digital image analysis

characteristics of sedimentary varve chronologies a review abstract we constructed a varve data base vdb to study the worldwide distribution of varved sediment records and the fidelity of their varve chronologies clusters of lakes with varved sediments exist in central north america in the canadian arctic and in northern and central europe among the 108 sites included in the vdb varved records are typically 200 500 cm long and cover a period of 1000 2000 years their varve chronologies are often based on counting of annual layers from fresh sediment surfaces and photographs or from epoxy embedded sediment blocks and thin sections the vdb indicates that chronological errors associated with counting sedimentary varves fall generally between 1 and 3 but only 57 of published records are providing quantitative error estimations for their chronologies with a careful documentation of varves and by applying radiometric dating methods to surface sediments as well as using historical events as time markers there is the potential of having centennial long varved records with a very precise age depth control however it is unrealistic to expect errors significantly below 1 from several millennia long sections we found no indication that varve chronologies would have been substantially more accurate and precise in some parts of the world than in others or statistically dependent on varve thickness or temporal extension of a varved sequence according to the vdb close to 90 of the published varve chronologies have been cross checked with some independent dating methods

fault model for the 2015 leucas aegean arc earthquake analysis based on seismological and geodetic observations the 17 november 2015 m w 6 6 earthquake in leucas leukas lefkas or lefkada island in the ionian sea western aegean arc was modeled using teleseismic long period p and sh waveforms and global positioning system gps slip vectors detailed fault modeling in this region characterized by intense seismicity and deformation rates usually assigned to the cephalonia transform fault is a challenge because of the unfavorable observation system to overcome this problem we independently analyzed seismological and geodetic data and then jointly evaluated the results the adopted model indicates that the 2015 earthquake can be assigned to a shallow strike slip fault with a minor component of thrusting along the southwest coasts of leucas and with relatively high slip for the area additionally mostly low angle fault solutions satisfying geodetic observations were identified but were not further investigated the preferred fault model permits recognition that recent m w 6 0 earthquakes in the area some marked by extreme peak ground accelerations are associated with a string of strike slip or oblique slip occasionally overlapping fault segments with variable characteristics along or close to the west coasts of leucas and cephalonia keffalinia kefalonia islands whereas the catastrophic 1953 m w 7 2 cephalonia and other previous major earthquakes were associated with thrust faulting electronic supplement 1 table of the global positioning system gps derived displacements and figures of p wave first motion polarities comparison of earthquake source parameters gps time series 2d projections of geodetic solutions the variance covariance matrix of the geodetic solution and the geodetic variable slip model 1 http www bssaonline org lookup suppl doi 10 1785 0120160080 dc1

varves in lake sediments a review abstract downcore counting of laminations in varved sediments offers a direct and incremental dating technique for high resolution climatic and environmental archives with at least annual and sometimes even seasonal resolution the pioneering definition of varves by de geer 1912 had been restricted to rhythmically deposited proglacial clays one century later the meaning of varve has been expanded to include all annually deposited laminae in terrestrial and marine settings under favourable basin configurations and environmental conditions limnic varves are formed due to seasonality of depositional processes from the lake s water column and or transport from the catchment area subsequent to deposition of topmost laminae the physical preservation of the accumulating varved sequence requires the sustained absence of sediment mixing for example via wave action or macrobenthic bioturbation individual sub laminae in varved lake sediments typically express contrasting colours always differ in terms of their organic chemical and or mineralogical compositions and often also differ with regard to grain size various predominating climatic and depositional conditions may result in clastic biogenic or endogenic incl evaporitic varved sediments and their mixtures to reliably establish a varve chronology the annual character of laminations needs to be determined and verified in a multidisciplinary fashion sources and influences of possible errors in varve chronologies are best determined and constrained by repeated varve counts and by including radioisotopes and correlation with historically documented events a well established varve chronology greatly enhances the scientific value of laminated limnic archives by securely anchoring the wealth of multi proxy palaeoenvironmental information in the form of time series for multidisciplinary investigations applications of varved records are discussed with special reference to advances since the 1980s these span fields like calibrating radiometric dating methods reconstructing past changes of the earth s magnetic field or detecting fluctuations in solar forcing once a varve chronology is established it can be applied to precisely date events like volcanic ash layers earthquakes or human impact as well as short and long term climate temperature precipitation wind hydroclimatic conditions or flooding and environmental changes eutrophication pollution due to their exceptional high temporal resolution and in combination with their robust and accurate internal time scale in calendar years annually laminated sediments can be regarded as one of the most precious environmental archives on the continents these records are necessary to extend temporally limited instrumental records back in time as such they have societal relevance with regard to risk assessments related to natural hazards arising from e g flooding or volcanic eruptions

a 5000 yr record of climate change in varved sediments from the oxygen minimum zone off pakistan northeastern arabian sea the upper holocene marine section from a kasten core taken from the oxygen minimum zone off karachi pakistan at water depth 700 m contains continuously laminated sediments with a sedimentation rate of 1 2 mm yr and a unique record of monsoonal climatic variability covering the past 5000 years our chronostratigraphy is based on varve counts verified by conventional and ams 14 c dating individual hemipelagic varve couplets are about 0 8 1 5 mm thick with light colored terrigenous laminae a deposited mainly during the winter monsoon alternating with dark colored laminae b rich in marine organic matter coccoliths and fish debris that reflect deposition during the high productivity season of the late summer monsoon august october precipitation and river runoff appear to control varve thickness and turbidite frequency we infer that precipitation decreased in the river watershed indicated by thinning varves after 3500 4000 yr b p this is about the time of increasing aridification in the near east and middle east as documented by decreasing nile river runoff data and lake level lowstands between turkey and northwestern india this precipitation pattern continued until today with precipitation minima about 2200 1900 yr b p 1000 yr b p and in the late middle ages 700 400 yr b p and precipitation maxima in the intervening periods as documented by spectral analysis the thickness of varve couplets responds to the average length of a 250 yr cycle a 125 yr cycle the gleissberg cycle of solar activity 95 yr and a 56 yr cycle of unknown origin higher frequency cycles are also present at 45 39 29 31 and 14 yr the sedimentary gray value also shows strong variability in the 55 yr band plus a 31 yr cycle because high frequency cyclicity in the enso band ca 3 5 and 5 yr is only weakly expressed our data do not support a straightforward interaction of the pacific enso with the monsoon driven climate system of the arabian sea

annually laminated lake sediments and the study of quaternary environmental changes a review abstract annually laminated lake sediments non glacial varves have in recent years been discovered in increasing abundance especially since the development of in situ freezer samplers so far the majority of examples have been found in three main areas eastern north america northern and central europe and east and central africa a much more widespread distribution seems highly probable however the formation of annual laminations in freshwater lakes is governed by a number of factors the most important of which appears to be morphometry seasonality of sediment supply which may be controlled by physical biological or even cultural processes in the lake or in its catchment is also clearly significant four main types of laminations here termed ferrogenic calcareous biogenic and clastic have so far been recorded although it is quite clear that others exist special field and laboratory techniques are needed in order that the particular properties of varved sediments may be fully exploited annually laminated sediments have so far been applied to the investigation of a number of aspects of the quaternary these include the study of a geochronology b climatic change c vegetation history especially the role of fire in forest ecosystems d the calculation of rates of sediment influx to lakes and of rates of erosion from their catchments and e the monitoring of processes in the modern environment such as heavy metal pollution eutrophication and acid rain annually laminated lake sediments combine an internal independent chronology with the preservation of discrete visible increments of seasonal and annual sedimentation given this unique property and the likelihood that many more examples will be discovered the prospects for further applications of studies of these sediments to the investigation of the quaternary seem very good indeed

a 900 year pollen inferred temperature and effective moisture record from varved lake mina west central minnesota usa abstract drought is endemic to the north american great plains causing severe economic consequences however instrumental climate data only exist from ca ad 1890 and limited tree ring paleolimnological archeological and eolian records document the last two millennia to address this lack of monitoring and paleoclimatic data the pollen preserved in the varved sediments of lake mina minnesota on the northeastern border of the great plains were analyzed may and february mean monthly temperatures and annual precipitation minus potential evapotranspiration were reconstructed at a 4 year resolution using a pre settlement pollen climate calibration set the period of the so called little ice age lia ad 1500 1870 was colder than the medieval climate anomaly mca ad 1100 1500 in west central minnesota winter temperatures in the lia declined more than summer ones the pollen record suggests that the lia occurred in three phases an initial cold phase from ad 1505 to ad 1575 a warmer phase and then a very cold phase from ad 1625 to ad 1775 there were severe droughts detected in the lake mina record from ad 1660 to ad 1710 and ad 1300 to ad 1400 suggesting that high resolution pollen records can detect events previously defined from the tree ring records this latter century scale drought is concurrent with the widely reported ad 1250 1400 mega drought which exceeds the severity of 20th century droughts

deglacial laminated facies on the nw european continental margin the hydrographic significance of british irish ice sheet deglaciation and fleuve manche paleoriver discharges we have compiled results obtained from four high sedimentation rate hemipelagic sequences from the celtic sector of the nw european margin ne atlantic to investigate the paleoceanographic and paleoclimatic evolution of the area over the last few climatic cycles we focus on periods characteristic of deglacial transitions we adopt a multiproxy sedimentological geochemical and micropaleontological approach applying a sampling resolution down to ten microns for specific intervals the investigation demonstrates the relationships between the bay of biscay hydrography and the glacial deglacial history of both the proximal british irish ice sheet biis and the western european continent we identify recurrent phases of laminae deposition concurrent with major biis deglacial episodes in all the studied cores evidence for abrupt freshwater discharges into the open ocean highlights the influence of such events at a regional scale we discuss their impact at a global scale considering the present and past key location of the bay of biscay versus the atlantic meridional overturning circulation amoc

ams radiocarbon dating of annually laminated sediments from lake holzmaar germany ams radiocarbon ages have been determined on terrestrial macrofossils selected from the annually laminated sediments of lake holzmaar germany the radiocarbon chronology of this lake covers the last 12 6 ka comparison of the radiocarbon dated varve chronology with tree ring data shows that an additional 878 years have to be added to the varve chronology the corrected c 14 varve chronology of holzmaar reaches back to ca 13 8 ka cal bp and compares favourably with the results from soppensee switzerland hajdas et al 1993 the corrected ages for the onset and the end of the younger dryas biozone are 11 940 cal bp and 11 490 cal bp respectively the ash layer of the laacher see volcanic eruption is dated at 12 201 224 cal bp and the ulmener tephra layer is dated at 10 904 cal bp

paleomagnetic directional dispersion produced by plastic deformation in a thick miocene welded tuff southern nevada implications for welding temperatures two voluminous rhyolitic to quartz latitic ash flow sheets the tiva canyon and topopah spring members of the paintbrush tuff were erupted from the miocene claim canyon cauldron in southern nevada although lithologically similar these units differ greatly in their recording of the ancient geomagnetic field the reversely magnetized tiva canyon member yields remanent directions that are well grouped both within and between sites the normally magnetized topopah spring member however yields remanent directions that vary both laterally and vertically remanent directions from a 100 m thick section of nonwelded to partially welded topopah spring are well grouped precision parameter k 200 and give a mean direction d 293 i 69 that is probably a good approximation of the geomagnetic field at the time of emplacement and cooling in three 300 m thick sections of more densely welded tuff inclinations from the upper parts of the topopah spring are steep 70 but flatten with depth to about 20 and then steepen abruptly near the base of the unit in general the directional dispersion increases with the thickness of the section k 30 for sections that are about 300 m thick much of the directional variation in the topopah spring can be attributed to an inclination error produced by subblocking temperature rotation of the magnetic carriers during welding the possibility of subblocking temperature deformation in an ash flow sheet is enhanced by 1 high blocking temperature spectra of the primary magnetic phases 2 relatively low emplacement temperatures and 3 great stratigraphic thickness thermal demagnetization results from the topopah spring member indicate that welding may have continued to temperatures as low as 475 c in the basal vitrophyre of the ash flow sheet remanent directions from the tiva canyon member were unaffected by the welding process because this unit possesses significantly lower blocking temperatures and does not attain the great thicknesses of the topopah spring

kinematic role of transverse structures in continental extension an example from the las vegas valley shear zone nevada analysis of the slip budget of the las vegas valley shear zone southern nevada leads to a model that explains many of the kinematic complexities that characterize transverse structures in continental extensional settings including 1 abrupt terminations of these structures along strike 2 large lateral translations across relatively short faults 3 apparent lack of strike slip offset or contradictory evidence of shear sense along some transverse structures 4 significant variation in slip magnitude along strike and 5 juxtaposition of differentially extended terranes on opposite sides of faults many major transverse structures may function largely as transfer faults that either link or form passive boundaries between differentially extended terranes

paleomagnetism and rotation constraints for the middle miocene southwestern nevada volcanic field middle miocene rocks of the southwestern nevada volcanic field swnvf lie across the projection of the walker lane belt within the basin and range province and thus provide an interesting opportunity to test for late cenozoic vertical axis rotation paleomagnetic data from individual ash flow sheets document no significant relative vertical axis rotation among localities within central swnvf an area of relatively low stratal tilts and widely spaced faults a time averaged mean paleomagnetic direction d 351 4 i 52 7 95 4 5 calculated from data from numerous separate rock units suggests that the central swnvf underwent minimal counterclockwise vertical axis rotation r 7 1 6 6 with respect to the north american craton no clockwise vertical axis rotation is found to support projection of dextral faults of the walker lane beneath the central swnvf clockwise rotation of variable magnitude is common at numerous sites from southern and western margins of the field these clockwise rotations probably reflect dextral shear strain developed at the interface between the little extended central swnvf block and more strongly extended areas to the south and southwest of the field negligible rotation of 11 45 ma to 13 25 ma tuffs relative to the central swnvf was found at the southeast margin of the field where 90 clockwise rotation at the northwest termination of the las vegas valley shear zone had been postulated any clockwise rotation in this area must predate 13 25 ma and thus dextral shear within this part of the walker lane belt was not synchronous or connected across the southern margin of the field small counterclockwise vertical axis rotation relative to the craton as found for the central swnvf block might be a regional feature in the western great basin

a chronological framework for the holocene vegetational history of central minnesota the steel lake pollen record abstract paleorecords from minnesota and adjacent areas have often been used to evaluate large scale climatic processes in the mid continent of north america however most of these records are compromised by chronological flaws making problematic any comparisons with climatic interpretations based on other records e g gisp2 in greenland we report here a high resolution pollen record with a secure chronology constrained by 26 14 c dates on terrestrial macrofossils from steel lake central minnesota about 11 2 ka cal bp 11 000 calibrated yr before present the late glacial picea forest near steel lake was succeeded abruptly by pinus banksiana and or resinosa the pinus forest began to open 9 4 ka cal bp with the expansion of prairie taxa and a pine parkland or savanna prevailed until about 8 ka cal bp when quercus replaced pinus to become the dominant tree in the prairie areas for 4500 yr the close chronological control permits the correlation of key vegetational changes with those at other reliably dated sites in the eastern dakotas and in minnesota suggesting that the abrupt decline of the spruce forest was time transgressive from southwest to northeast during 2000 yr and that the development of prairie was time transgressive in the same direction over 2600 yr correlation of key pollen horizons at steel lake with those in the high resolution pollen profiles of elk lake ca 50 km northwest of steel lake suggests that the well known elk lake varve chronology for the early holocene is about 1000 yr too young

continuum calculations of continental deformation in transcurrent environments a thin viscous sheet approximation is used to investigate continental deformation near a strike slip boundary the vertically averaged velocity field is calculated for a medium characterized by a power law rheology with stress exponent n driving stresses include those applied along boundaries of the sheet and those arising from buoyancy forces related to lateral differences in crustal thickness calculations are performed with either velocity or stress prescribed on the transcurrent boundary exact and approximate analytic solutions for a region with a sinusoidal strike slip boundary condition are compared with solutions for more geologically relevant boundary conditions obtained using a finite element technique the across strike length scale of the deformation is approximately 1 4 n 1 2 times the dominant wavelength of the imposed strike slip boundary condition for both the analytic and the numerical solutions this result is consistent with length scales observed in continental regions of large scale transcurrent faulting an approximate linear relationship between displacement and rotation is found that depends only on the deformation length scale and the rheology calculated displacements finite rotations and distribution of crustal thicknesses are consistent with those observed in the region of the pacific north america plate boundary in california

tephrostratigraphical investigation of lake sediments and a peat bog in northeastern china since 20 000 years this is a detailed tephrostratigraphical investigation of late quaternary deposits in the longgang and changbaishan volcanic fields of northeastern china a total of 45 reference samples which were collected from either side of the chinese korean border showed very similar geochemical characteristics to the millennium eruption of tianchi volcano through comparing the published data of the glass shards detected in gushantun with these reference samples further description is that the glass shards in the sediment of gushantun came from the tianchi volcano eruption in ad 1702 1668 and 1597 a basaltic tephra layer found in the sediment of hanlongwan associated with an eruption of the jinlongdingzi volcano which happened in 1500 2100 cal yr bp by comparing with the published data from sihailongwan and xiaolongwan tianchi and jinlongdingzi volcano are both active and erupted several times during the historical period reference samples and the tephra layers detected in hanlongwan sihailongwan gushantun

tertiary extension north of the las vegas valley shear zone sheep and desert ranges clark county nevada detailed mapping reveals the presence of high angle extensional faults and low angle gravity slides on the west side of the sheep range three major high angle faulting events each account for 20 of eastward rotation and accommodate extension between the las vegas range and the desert range low angle faults represent surficial slides in response to topography produced by extension on the high angle faults faulting took place during the miocene synchronously with deposition of the horse spring formation and with displacement on the las vegas valley shear zone the extension in the sheep range took place without volcanism intrusion or metamorphism of the paleozoic sedimentary rocks offset thrust faults suggest that the area west of the sheep range extended almost 100 during the miocene while the corresponding area south of the las vegas valley shear zone did not extend significantly the shear zone bounded the extending terrane on the south acting as a transform fault this extension west of the sheep range may in part balance that mapped by anderson 1971 in the eldorado mountains the las vegas valley shear zone and the lake mead fault system may have acted together to compensate for areas of localized extension between the colorado plateau and the vicinity of the specter range

holocene precipitation seasonality captured by a dual hydrogen and oxygen isotope approach at steel lake minnesota abstract middle holocene 8 to 4 ka bp warmth and aridity are well recorded in sediment archives from midcontinental north america however neither the climatic driver nor the seasonal character of precipitation during this period is well understood because of the limitations of available proxy indicators for example an important challenge is to distinguish among the interacting effects of evaporation temperature or precipitation seasonality in existing 18 o records from the region here we combine hydrogen isotopes of palmitic acid and oxygen isotopes of carbonate to derive lake water isotopic values during the holocene at steel lake in north central minnesota in combination these data enable us to separate variations in evaporation from variations in the isotopic composition of input waters to lake variations in evaporation are used as a proxy for aridity and lake water input isotopic values are used as a proxy for the isotopic values of meteoric precipitation our results suggest that lake water input isotopic values were more negative during the middle holocene than at present to test whether these more negative values are related to temperature or precipitation seasonality we compare pollen inferred temperatures and the expected isotopic value of precipitation resulting from these temperatures to the reconstructed precipitation isotopic values results suggest that middle holocene warmth and aridity were associated with increased evaporation rates and decreased summer precipitation these inferences are consistent with climate simulations that highlight the role of seasonal insolation and sea surface temperatures in driving variations in precipitation seasonality during the holocene results also suggest that changes in holocene precipitation seasonality may have influenced the expansion of the prairie forest border in minnesota as well as regional variations in grassland community composition this study demonstrates the efficacy of the dual hydrogen and oxygen isotope approach for distinguishing changes in evaporation and precipitation seasonality in the paleolimnological record

millennial scale east asian monsoon variability of the last glacial deduced from annually laminated sediments from lake sihailongwan n e china abstract lake sihailongwan in jilin province ne china provides the first continuous and almost entirely seasonally laminated sediment record on the east asian mainland comprising the complete holocene the late glacial period and large parts of the last glacial sediment and palynological proxy data provide a finely resolved regional environmental history of the east asian monsoon a varve based chronology shl vc2 has been established for the last 65 000 years and allows a detailed comparison with other long regional and global palaeoclimate records vegetation density of the study area depends on the long run on precessionally forced insolation changes with superimposed millennial scale variability during the last glacial periodic increase of organic carbon content and thermophilous tree species like ulmus and fraxinus and contemporary decrease of shrub alnus precisely mirror millennial scale climatic variations primarily known from greenland ice cores as dansgaard oeschger cycles as well as late glacial period climate changes percentages of trees shrubs pollen and in particular lake productivity related data reveal substantial differences between interstadial intensities with those between 50 and 60 ka bp being more pronounced than the following ones

paleomagnetic evidence for the age and extent of middle tertiary counterclockwise rotation dixie valley region west central nevada paleomagnetic data obtained from oligocene to lower miocene igneous rocks and middle miocene basaltic rocks at fifteen localities from a region surrounding dixie valley in west central nevada indicate that parts of the area experienced counterclockwise vertical axis rotation and these data provide constraints on the extent and timing of rotation counterclockwise vertical axis rotation probably exceeding 30 is indicated for oligocene to lower miocene rocks in the central part of the study area calculated rotations increase with increasing age of the oligocene to lower miocene rocks indicating that vertical axis rotation was concurrent with ash flow deposition about 23 33 ma paleomagnetic data indicate that middle miocene basaltic rocks about 10 17 ma postdate significant counterclockwise vertical axis rotation although this interpretation is complicated because the rocks were erupted episodically in pulses that apparently were short lived with respect to geomagnetic secular variation counterclockwise rotation was related to a deformation event that predated development of the present basin and range physiography of the area the total amount of crustal rotation in tertiary time in the area is poorly known rotation estimates calculated from time averaged mean directions that incorporate data from a broad range of the oligocene to lower miocene units probably underestimate total tertiary rotation because the mean directions include data from units that postdate much of the rotation for example a rotation estimate for a composite sequence from the central part of the study area is 23 15 whereas estimates from older and younger halves of the sequence are 37 21 and 11 16 respectively paleomagnetic data indicate that oligocene to lower miocene rocks at some localities in the northern and southern parts of the study area e g the golconda canyon locality probably did not experience significant tertiary counterclockwise rotation although the paleomagnetic data alone are insufficient to fully define the geographic area subjected to tertiary counterclockwise rotation the data suggest that a discrete subregion was affected rather than the entire basin and range province