postglacial emergence and distribution of late weichselian ice sheet loads in the northern barents and kara seas russia reconstructions of late weichselian glacier coverage on the continental shelves of the russian arctic range from a large ice sheet terminating in northern siberia to isolated ice caps restricted to arctic archipelagos this disparity in glacier reconstructions reflects the lack of chronological control on glacial and deglacial landforms we present new holocene relative sea level data from franz josef land and northern novaya zemlya russia that place the thickest glacier loads in the northern barents sea and not over novaya zemlya radiocarbon ages from shelf and terrestrial areas at the former ice sheet margin support deglaciation of the northern barents sea between 13 0 and 10 3 ka considerably later than inferred from isotopic records for the arctic ocean this analysis indicates that the barents sea ice sheet was a dominant sea level reservoir in northern eurasia and that glacier loading of novaya zemlya was comparatively limited during the last glaciation

late quaternary stratigraphy glacial limits and paleoenvironments of the marresale area western yamal peninsula russia stratigraphic records from coastal cliff sections near the marresale station on the yamal peninsula russia yield new insight on ice sheet dynamics and paleoenvironments for northern eurasia field studies identify nine informal stratigraphic units from oldest to youngest the marresale formation labsuyakha sand kara diamicton varjakha peat and silt oleny sand baidarata sand betula horizon nenets peat and chum sand that show a single glaciation and a varied terrestrial environment during the late pleistocene the kara diamicton reflects regional glaciation and is associated with glaciotectonic deformation from the southwest of the underlying labsuyakha sand and marresale formation finite radiocarbon and luminescence ages of ca 35 000 to 45 000 yr from varjakha peat and silt that immediately overlies kara diamicton place the glaciation 40 000 yr ago eolian and fluvial deposition ensued with concomitant cryogenesis between ca 35 000 and 12 000 cal yr b p associated with the oleny and the baidarata sands there is no geomorphic or stratigraphic evidence of coverage or proximity of the yamal peninsula to a late weichselian ice sheet the nenets peat accumulated over the baidarata sand during much of the past 10 000 yr with local additions of the eolian chum sand starting ca 1000 yr ago a prominent betula horizon at the base of the nenets peat contains rooted birch trees ca 10 000 to 9000 cal yr old and indicates a 200 km shift northward of the treeline from the present limits corresponding to a 2 to 4 c summer warming across northern eurasia

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

the north american late wisconsin ice sheet and mantle viscosity from glacial rebound analyses abstract observations of sea level and crustal response to glacial loading cycles provide constraints on the mantle rheology function e and as well as on the ice load i with the latter being largely free from a priori glaciological or climate assumptions and appropriate therefore for testing any such hypotheses this paper presents new results for both continental mantle e and i for the late wisconsin ice sheet using geological evidence for relative sea level change rsl and tilting of palaeo lake shorelines complemented with loose constraints from observations of present day radial crustal displacement across north america the focus is on evidence from near or within the former maximum ice margins and the resulting earth response is representative of sub continental mantle conditions the inversion of the sea level information has limited resolution for earth rheology and simple three layer models characterized by depth averaged effective lithospheric thickness h and upper and lower mantle viscosities um and um respectively adequately describe the response function yielding parameters earth model e 6 of h 102 85 120 km um 5 1 1020 3 5 7 5 x1020 lm 1 3 1022 0 8 2 8 x1022 where the numbers in parenthesis are 95 confidence limits the details of the ice sheet with one exception are not strongly dependent on the rheological assumptions within this range the exception is the lower mantle viscosity that remains correlated with the magnitude scaling of the ice sheet a link that is largely broken by introducing constraints from glacial loading effects on the earth s rotation and dynamic flattening the difference between the continental um and the comparable estimate of 1 2 5 x1020 for ocean mantle is statistically significant shoreline gradient information from glacial lakes mcconnell agassiz algonquin and ojibway provide strong constraints on the response within the interior of the ice sheet and the resulting ice sheet model lw 6 is characterized by multiple ice domes from at least 17 18 ka onwards the resolution for earlier periods is largely constrained by global ice volume considerations the two principal domes are over southern nunavut the keewatin dome and over quebec labrador both of 3500 m thickness separated by an ice ridge some 1500 m lower than the domes the latter a requirement imposed by the gradient information over western canada ice thickness gradients east of the cordilleras are partly constrained by the shoreline data for former lake mcconnell as is the lateglacial ice thickness of the keewatin dome and indicate that any ice free corridor between the laurentian and cordilleran components is unlikely to have existed before 13ka reconstructions of the glacial lakes are consistent with the locations and timing of the observational evidence for the four major lake systems with the likely drainage routes identified the evolution of the lw 6 ice volume function expressed as equivalent sea level is characterized by a rapid decrease in ice volume from 15 to 14 5 ka corresponding to the bolling allerod period in the main from rapid ice retreat along the southern margin with further contributions from drainage through the st lawrence river valley and from ice retreat from the major northern straits and gulfs but not hudson strait where the rsl data point to late removal of ice after 10 ka the contribution of the drainage of the lakes to global sea level rise is small

lake stratigraphy implies an 80 000 yr delayed melting of buried dead ice in northern russia sediment cores from lakes kormovoye and oshkoty in the glaciated region of the pechora lowland northern russia reveal sediment gravity flow deposits overlain by lacustrine mud and gyttja the sediments were deposited mainly during melting of buried glacier ice beneath the lakes in lake kormovoye differential melting of dead ice caused the lake bottom to subside at different places at different times resulting in sedimentation and erosion occurring only some few metres apart and at shifting locations as further melting caused inversion of the lake bottom basal radiocarbon dates from the two lakes ranging between 13 and 9 ka match with basal dates from other lakes in the pechora lowland as well as melting of ice wedges this indicates that buried glacier ice has survived for ca 80 000 years from the last glaciation of this area at 90 ka until about 13 ka when a warmer climate caused melting of permafrost and buried glacier ice forming numerous lakes and a fresh looking glacial landscape copyright 2003 john wiley sons ltd

effects of change in slab geometry on the mantle flow and slab fabric in southern peru the effects of complex slab geometries on the surrounding mantle flow field are still poorly understood here we combine shear wave velocity structure with rayleigh wave phase anisotropy to examine these effects in southern peru where the slab changes its geometry from steep to flat to the south where the slab subducts steeply we find trench parallel anisotropy beneath the active volcanic arc that we attribute to the mantle wedge and or upper portions of the subducting plate farther north beneath the easternmost corner of the flat slab we observe a pronounced low velocity anomaly this anomaly is caused either by the presence of volatiles and or flux melting that could result from southward directed volatile rich subslab mantle flow or by increased temperature and or decompression melting due to small scale vertical flow we also find evidence for mantle flow through the tear north of the subducting nazca ridge finally we observe anisotropy patterns associated with the fast velocity anomalies that reveal along strike variations in the slab s internal deformation the change in slab geometry from steep to flat contorts the subducting plate south of the nazca ridge causing an alteration of the slab petrofabric in contrast the torn slab to the north still preserves the primary fossilized petrofabric first established shortly after plate formation

structure of the subduction system in southern peru from seismic array data the subduction zone in southern peru is imaged using converted phases from teleseismic p pp and pkp waves and pwave tomography using local and teleseismic events with a linear array of 50 broadband seismic stations spanning 300 km from the coast to near lake titicaca the slab dips at 30 and can be observed to a depth of over 200 km the moho is seen as a continuous interface along the profile and the crustal thickness in the back arc region the altiplano is 75 km thick which is sufficient to isostatically support the andes as evidenced by the gravity the shallow crust has zones of negative impedance at a depth of 20 km which is likely the result of volcanism at the midcrustal level of 40 km there is a continuous structure with a positive impedance contrast which we interpret as the western extent of the brazilian craton as it underthrusts to the west v p v s ratios estimated from receiver function stacks show average values for this region with a few areas of elevated v p v s near the volcanic arc and at a few points in the altiplano the results support a model of crustal thickening in which the margin crust is underthrust by the brazilian shield

the role of ridges in the formation and longevity of flat slabs flat slab subduction occurs when the descending plate becomes horizontal at some depth before resuming its descent into the mantle it is often proposed as a mechanism for the uplifting of deep crustal rocks thick skinned deformation far from plate boundaries and for causing unusual patterns of volcanism as far back as the proterozoic eon for example the formation of the expansive rocky mountains and the subsequent voluminous volcanism across much of the western usa has been attributed to a broad region of flat slab subduction beneath north america that occurred during the laramide orogeny 80 55 million years ago here we study the largest modern flat slab located in peru to better understand the processes controlling the formation and extent of flat slabs we present new data that indicate that the subducting nazca ridge is necessary for the development and continued support of the horizontal plate at a depth of about 90 kilometres by combining constraints from rayleigh wave phase velocities with improved earthquake locations we find that the flat slab is shallowest along the ridge while to the northwest of the ridge the slab is sagging tearing and re initiating normal subduction on the basis of our observations we propose a conceptual model for the temporal evolution of the peruvian flat slab in which the flat slab forms because of the combined effects of trench retreat along the peruvian plate boundary suction and ridge subduction we find that while the ridge is necessary but not sufficient for the formation of the flat slab its removal is sufficient for the flat slab to fail this provides new constraints on our understanding of the processes controlling the beginning and end of the laramide orogeny and other putative episodes of flat slab subduction

the extent of the barents kara ice sheet during the last glacial maximum it has been a long standing discussion whether the barents kara ice sheet expanded onto mainland russia during the last glacial maximum lgm in this paper we describe many well dated by conventional and ams 14 c methods and optically stimulated luminescence sedimentary sequences in the controversial area of northern russia the sequences discussed are not covered by till and yet all predate the lgm the deposits consist mostly of aeolian or lacustrine easily deformable soft silt and fine sand two sites feature frozen mammoth carcasses and three sites contain palaeolithic artefacts and mammalian bones we emphasise that these formations show no sign of having been overridden by an ice sheet at several sites deposition of aeolian sediments and formation of ice wedges took place during the lgm time span these observations present unambiguous proof that the barents kara ice sheet did not cover mainland russia during lgm with a possible exception for the northern tip of the taimyr peninsula r 2001 elsevier science ltd all rights reserved

lithospheric scale structure across the bolivian andes from tomographic images of velocity and attenuation for p and s waves we have developed a three dimensional lithospheric scale model across the bolivian andes at 20 s based on tomographic images of velocity and attenuation for both p and s waves observations of travel time and attenuation for this study are from regional mantle earthquakes in the subducted nazca plate recorded on a portable broadband seismic array broadband andean joint experiment and seismic exploration of the deep andes in bolivia and chile the shallow mantle under the altiplano from 18 s to 21 s is high velocity and moderately high q vp 8 3 vs 4 7 qp 500 and qs 200 suggesting lithospheric mantle high velocity material in the altiplano extends to a depth of 125 150 km the shallow mantle of the western cordillera is characterized by high vp vs 1 83 suggesting a correlation between vp vs and arc volcanism seismic velocity in the western cordillera mantle is on average only slightly reduced from global averages however velocity and attenuation anomalies are locally strong vp 7 8 vs 4 3 qp 200 and qs 100 consistent with partial melt conditions under the los frailes volcanic field in the eastern cordillera shallow mantle velocity and q decrease drastically from the neighboring altiplano vp 7 8 vs 4 3 qp 300 qs 100 however high vp vs is not as pervasive as it is in the western cordillera we believe that slab derived water and perhaps other volatiles strongly influence the western cordillera while the eastern cordillera low velocity region is more affected by partial melt and or compositional changes average velocity and q in the shallow mantle across the bolivian andes where the tomographic images are best resolved are significantly higher than in most mantle wedge environments where corresponding images are available this is likely the result of a compressional back arc setting in the andes this implies that lithospheric shortening and thickening associated with the formation of the andes has profoundly influenced the shallow mantle structure across the range shallow mantle structure is locally influenced by the subduction processes particularly under the western cordillera however the differing volcanism and seismic character under the two cordilleras suggest that the volcanic process in the eastern cordillera may be distinct from arc volcanism tertiary volcanism in the eastern cordillera is located in the region where mantle shortening is suspected to be greatest both the timing and location of volcanism are consistent with upward migration of mantle wedge asthenosphere following the removal of over thickened lithosphere

geophysical constraints on the dynamics and retreat of the barents sea ice sheet as a paleobenchmark for models of marine ice sheet deglaciation our understanding of processes relating to the retreat of marine based ice sheets such as the west antarctic ice sheet and tidewater terminating glaciers in greenland today is still limited in particular the role of ice stream instabilities and oceanographic dynamics in driving their collapse are poorly constrained beyond observational timescales over numerous glaciations during the quaternary a marine based ice sheet has waxed and waned over the barents sea continental shelf characterized by a number of ice streams that extended to the shelf edge and subsequently collapsed during periods of climate and ocean warming increasing availability of offshore and onshore geophysical data over the last decade has significantly enhanced our knowledge of the pattern and timing of retreat of this barents sea ice sheet bsis particularly so from its late weichselian maximum extent we present a review of existing geophysical constraints that detail the dynamic evolution of the bsis through the last glacial cycle providing numerical modelers and geophysical workers with a benchmark data set with which to tune ice sheet reconstructions and explore ice sheet sensitivities and drivers of dynamic behavior although constraining data are generally spatially sporadic across the barents and kara seas behaviors such as ice sheet thinning major ice divide migration asynchronous and rapid flow switching and ice stream collapses are all evident further investigation into the drivers and mechanisms of such dynamics within this unique paleo analogue is seen as a key priority for advancing our understanding of marine based ice sheet deglaciations both in the deep past and in the short term future

the migration history of the nazca ridge along the peruvian active margin a re evaluation abstract the collision zone of the 200 km wide and 1 5 km high nazca ridge and the peruvian segment of the convergent south american margin between 14 s and 17 s is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc this is evident by a narrowing of the shelf a westward shift of the coastline and the presence of marine terraces as the nazca ridge is oblique with respect to both trench and convergence direction of the nazca plate it migrates southward along the active plate boundary for reconstructing the migration history of the nazca ridge this study uses updated plate motion data resulting from a revision of the geomagnetic time scale the new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of 75 mm a before 10 8 ma 61 mm a 10 8 4 9 ma and 43 mm a 4 9 ma to present intra plate deformation associated with mountain building in the peruvian andes since the miocene reduces the relative convergence rate between nazca plate and peruvian forearc taking an intra plate deformation at a rate of 10 mm a estimated from space geodetic and geological data into account does not significantly reduce these lateral migration velocities constraining the length of the original nazca ridge by its conjugate feature on the pacific plate yields a length of 900 km for the subducted portion of the ridge using this constraint ridge subduction began 11 2 ma ago at 11 s therefore the nazca ridge did not affect the northern sites of ocean drilling program odp leg 112 located at 9 s this is supported by benthic foraminiferal assemblages in odp leg 112 cores indicating more than 1000 m of subsidence since at least middle miocene time and by continuous shale deposition on the shelf from 18 to 7 ma recorded in the ballena industrial well at 11 5 s the model predicts the passage of the ridge crest 9 5 ma ago this agrees with the sedimentary facies and benthic foraminiferal stratigraphy of odp leg 112 cores which argue for deposition on the shelf in the middle and late miocene with subsequent subsidence of a minimum of several hundred meters onshore at 12 s the sedimentary record shows at least 500 m uplift prior to the end of the miocene also in agreement with the model

response of the mantle to flat slab evolution insights from local s splitting beneath peru the dynamics of flat subduction particularly the interaction between a flat slab and the overriding plate are poorly understood here we study the seismically anisotropic properties and deformational regime of the mantle directly above the peruvian flat slab we analyze shear wave splitting from 370 local s events at 49 stations across southern peru we find that the mantle above the flat slab appears to be anisotropic with modest average delay times 0 28 s that are consistent with 4 anisotropy in a 30 km thick mantle layer the most likely mechanism is the lattice preferred orientation of olivine which suggests that the observed splitting pattern preserves information about the mantle deformation we observe a pronounced change in anisotropy along strike with predominately trench parallel fast directions in the north and more variable orientations in the south which we attribute to the ongoing migration of the nazca ridge through the flat slab system

thickening the altiplano crust by gravity driven crustal channel flow 1 in the central andes crustal thickness is not well correlated to upper crustal shortening only little shortening is documented in the upper crust of the 60 65 km thick altiplano plateau whose thickening and uplift were delayed with respect to earlier and greater thickening in the adjacent western and eastern cordilleras because crustal thickness variations induce horizontal stress gradients and cause crustal flow a thickness dependent channel flow is modeled here and applied to the central andes in situ thickening is assumed for both cordilleras while the altiplano crustal thickening is generated by lateral flow from these overthickened adjacent domains a 8 1018 pa s viscosity channel is predicted for crustal thicknesses exceeding 45 50 km to match the estimated topographic evolution of the central andes thickening in the cordilleras was sufficient to generate a flow of 6 109 m3 per unit length toward the initially 30 35 km thick altiplano

the lack of correlation between flat slabs and bathymetric impactors in south america flat slab subduction has been attributed to various causes including mantle wedge dynamics overriding by the upper plate age of the subducting plate and subduction of anomalously thick oceanic crust one often favored explanation for flat slabs is the subduction of buoyant features on the oceanic plate in the form of an aseismic ridge or oceanic plateau we show through plate tectonic reconstructions of the marquesas tuamotu and austral plateau assuming that features on the conjugate plate can be used as proxies for subducted bathymetric anomalies that there is very little correlation between the subduction of such anomalies and historic zones of flat subduction in south america it is apparent that subduction of a bathymetric anomaly need not lead to a flat slab and not all flat slabs are associated with the subduction of a bathymetric anomaly

andean flat slab subduction through time the analysis of magmatic distribution basin formation tectonic evolution and structural styles of different segments of the andes shows that most of the andes have experienced a stage of flat subduction evidence is presented here for a wide range of regions throughout the andes including the three present flat slab segments pampean peruvian bucaramanga three incipient flat slab segments carnegie guanacos tehuantepec three older and no longer active cenozoic flat slab segments altiplano puna payenia and an inferred palaeozoic flatslab segment early permian san rafael based on the present characteristics of the pampean flat slab combined with the peruvian and bucaramanga segments a pattern of geological processes can be attributed to slab shallowing and steepening this pattern permits recognition of other older cenozoic subhorizontal subduction zones throughout the andes based on crustal thickness two different settings of slab steepening are proposed slab steepening under thick crust leads to delamination basaltic underplating lower crustal melting extension and widespread rhyolitic volcanism as seen in the caldera formation and huge ignimbritic fields of the altiplano and puna segments on the other hand when steepening affects thin crust extension and extensive within plate basaltic flows reach the surface forming large volcanic provinces such as payenia in the southern andes this last case has very limited crustal melt along the axial part of the andean roots which shows incipient delamination based on these cases a palaeozoic flat slab is proposed with its subsequent steepening and widespread rhyolitic volcanism the geological evolution of the andes indicates that shallowing and steepening of the subduction zone are thus frequent processes which can be recognized throughout the entire system

submarine landforms and ice sheet flow in the kvitoya trough northwestern barents sea high resolution geophysical and sediment core data are used to investigate the pattern and dynamics of former ice flow in kvitoya trough northwestern barents sea a new swath bathymetric dataset identifies three types of submarine landform in the study area streamlined landforms meltwater channels and cavities iceberg scours subglacially produced streamlined landforms provide a record of ice flow through kvitoya trough during the last glaciation flow directions are inferred from the orientations of streamlined landforms drumlins crag and tail features ice flowed northward for at least 135 km from an ice divide at the southern end of kvitoya trough a large channel cavity system incised into bedrock in the southern trough indicates that subglacial meltwater was present at the former ice sheet base modest landform elongation ratios and a lack of mega scale glacial lineations suggest that although ice in kvitoya trough was melting at the bed and flowed faster than the likely thin and cold based ice on adjacent banks a major ice stream probably did not occupy the trough retreat was relatively rapid after 14 13 5 14c kyr b p and probably progressed via ice sheet bed decoupling in response to rising sea level there is little evidence for still stands during ice retreat or of ice proximal deglacial sediments relict iceberg scours in present day water depths of more than 350 m in the northern trough indicate that calving was an important mass loss mechanism during retreat

two step deglaciation of the southeastern barents sea marine geologic evidence from the western barents sea shelf leaves little doubt that theareawascoveredbygroundediceduringthelastglaciation butthepatternandtiming of the subsequent deglaciation were not well determined here we reconstruct the timing and mechanism of ice sheet retreat as constrained by seismic stratigraphy and lithostratigraphy and accelerator mass spectrometer 14 c dating of foraminifera and mollusc shellsinsedimentboringsfromthesoutheasternbarentssea 1000kmfromthewestern shelf edge and former ice sheet margin the deposition of ice proximal glaciomarine sediments upon till began at or shortly before 12 7 ka indicating that the ice sheet retreat whichmostlikelycommencedalongthewesternmarginofthebarentsseashelfat15 14 5 ka reached the southeastern part of the shelf in 2 ka the subsequent accumulation of glaciomarinesedimentstookplaceintwodistinctpulses 12 7 12 1kaand 10 5 9 4ka separatedbyanintervalofnondepositionlasting 1 5ka thetwopulsesofsedimentation were coeval with periods of increasing surface water and air temperatures in the northern atlanticregionandacceleratedeustaticsea levelrise suggestingthattheremainingretreat of the barents shelf ice sheet was paced by these factors

flat slab deformation caused by interplate suction force we image the structure at the southern end of the peruvian flat subduction zone using receiver function and surface wave methods the nazca slab subducts to 100 km depth and then remains flat for 300 km distance before it resumes the dipping subduction the flat slab closely follows the topography of the continental moho above indicating a strong suction force between the slab and the overriding plate a high velocity mantle wedge exists above the initial half of the flat slab and the velocity resumes to normal values before the slab steepens again indicating the resumption of dehydration and ecologitization two prominent midcrust structures are revealed in the 70 km thick crust under the central andes molten rocks beneath the western cordillera and the underthrusting brazilian shield beneath the eastern cordillera

late quaternary ice sheet history of northern eurasia the maximum limits of the eurasian ice sheets during four glaciations have been reconstructed 1 the late saalian 140 ka 2 the early weichselian 100 80 ka 3 the middle weichselian 60 50 ka and 4 the late weichselian 25 15 ka the reconstructed ice limits are based on satellite data and aerial photographs combined with geological field investigations in russia and siberia and with marine seismic and sediment core data the barents kara ice sheet got progressively smaller during each glaciation whereas the dimensions of the scandinavian ice sheet increased during the last ice age the barents kara ice sheet attained its maximum size as early as 90 80 000 years ago when the ice front reached far onto the continent a regrowth of the ice sheets occurred during the early middle weichselian culminating about 60 50 000 years ago during the late weichselian the barents kara ice sheet did not reach the mainland east of the kanin peninsula with the exception of the nw fringe of taimyr a numerical ice sheet model forced by global sea level and solar changes was run through the full weichselian glacial cycle the modeling results are roughly compatible with the geological record of ice growth but the model underpredicts the glaciations in the eurasian arctic during the early and middle weichselian one reason for this is that the climate in the eurasian arctic was not as dry then as during the late weichselian glacial maximum

freshwater and atlantic water inflows to the deep northern barents and kara seas since ca 13 14 c ka foraminifera and stable isotopes foraminiferal stable isotopes and assemblages from franz victoria and st anna troughs provide a valuable record of freshwater and atlantic water flows to the northern barents and kara seas from deglaciation to present the d 18 o and d 13 c of planktonic neogloboquadrina pachyderma s and benthic elphidium excavatum were up to1 4 lower than present at ca 13 11 5 and 10 14 c ka global sea level corrected mostly reflecting substantial freshwater inputs coincident with glacial marine sediment deposition cassidulina teretis exceeded 40 of benthic foraminifera ca 13 and 10 14 c ka indicating subsurface penetrations of atlantic water the transition to postglacial marine conditions is marked by a 1 rise in foraminiferal d 18 o and a sharp fall in c teretis soon after 10 14 c ka these changes imply reduced inputs of freshwater and atlantic water subsequent isotopic and foraminiferal assemblage variations reflect changing atlantic water conditions upstream in the nordic seas and shifts between the warm fram strait and cold barents sea branches of atlantic water we hypothesize that glacial isostatically induced deepening by up to 150 m influenced atlantic water inflows to the northern barents sea during deglaciation and the holocene thus effects of isostatic recovery have to be factored into paleoceanographic reconstructions 2001 elsevier science ltd all rights reserved

the innuitian ice sheet configuration dynamics and chronology abstract portrayal of north american ice cover during the last glacial maximum is dominated by the laurentide ice sheet leaving little detail for the adjacent innuitian ice sheet iis four decades of geological fieldwork across the queen elizabeth islands now warrant specific treatment of the iis including its chronology configuration dynamics and retreat this reconstruction is relevant to the sedimentary history of the arctic ocean and to high latitude climate forcing the iis was composed of both an alpine and lowland sector the advance of the alpine sector occurred as recently as 19 14 c ka bp geological evidence configures outflow from alpine and lowland divides that produced several palaeo ice streams one extending northwestward across the canadian arctic archipelago to the polar continental shelf retreat of the iis commenced along its southwest margin 11 6 14 c ka bp however most of the ice sheet remained on the continental shelf during the younger dryas by 10 14 c ka bp marine based ice experienced widespread calving through the western and central archipelago in response to holocene warming and ongoing eustatic sea level rise the sea penetrated the eastern archipelago by 8 5 14 c ka bp gutting the alpine sector of the iis regional isobases record the glacioisostatic signature of the ice sheet and are congruent with the primary geological evidence the delayed buildup of the iis was out of phase with the growth of the laurentide ice sheet that occasioned climatic and glacio eustatic forcing in the innuitian region recent modelling experiments reinforce the hypothesis that growth of the laurentide ice sheet culminated in a split jet stream that temporarily favoured augmented precipitation and growth of the iis

impacts of increasing low level shear on supercells during the early evening transition abstractthe dynamical response of simulated supercells to temporally increasing lower tropospheric vertical wind shear is investigated using idealized simulations these simulations are based upon observed soundings from two cases that underwent an early evening transition during the second verification of the origins of rotation in tornadoes experiment vortex2 mature supercells were simulated in observed afternoon environments with moderate vertical wind shear and then compared to simulated supercells experiencing observed evening increases in lower tropospheric shear the primary effect of the increase in low level shear is to establish larger values of vertical vorticity at lower altitudes in the storm s updraft in turn this leads to a nonlinear increase in the updraft strength due to the enhanced dynamic pressure minimum associated with larger vorticity in the storm s mesocyclone this is particularly important at low levels where it increases the storm s ability to lift cool surface air includ

foreland uplift during flat subduction insights from the peruvian andes and fitzcarrald arch abstract foreland deformation has long been associated with flat slab subduction but the precise mechanism linking these two processes remains unclear one example of foreland deformation corresponding in space and time to flat subduction is the fitzcarrald arch a broad ne sw trending topographically high feature covering an area of 4 105 km2 in the peruvian andean foreland recent imaging of the southern segment of peruvian flat slab shows that the shallowest part of the slab which corresponds to the subducted nazca ridge northeast of the present intersection of the ridge and the peruvian trench extends up to and partly under the southwestern edge of the arch here we evaluate models for the formation of this foreland arch and find that a basal shear model is most consistent with observations we calculate that 5 km of lower crustal thickening would be sufficient to generate the arch s uplift since the late miocene this magnitude is consistent with prior observations of unusually thickened crust in the andes immediately south of the subducted ridge that may also have been induced by flat subduction this suggests that the fitzcarrald arch s formation by the nazca ridge may be one of the clearest examples of upper plate deformation induced through basal shear observed in a flat slab subduction setting we then explore the more general implications of our results for understanding deformation above flat slabs in the geologic past

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

flat versus normal subduction zones a comparison based on 3 d regional traveltime tomography and petrological modelling of central chile and western argentina 29 35 s

interaction between the nazca and south american plates and formation of the altiplano puna plateau review of a flexural analysis along the andean margin 15 34 s abstract the results of a two dimensional flexural analysis applied to the andean margin which is based on the correlation between topography and bouguer anomaly are here reviewed in order to characterize rigidity variations across and along the forearc arc transition of the central andes and to understand the role of the forearc in the formation of the altiplano plateau the forearc has maximum rigidities between 15 and 23 s forearc rigidity decreases gradually southward and sharply toward the plateau the main orogen elevations higher than 3000 m is very weak along the entire central andes a semi quantitative interpretation of these trends based on the relationship between flexural rigidity and the thermo mechanically and compositionally controlled strength of the lithosphere allows the following conclusions to be made 1 across strike rigidity variations are dominated by the thermal structure derived from the subduction process 2 the forearc constitutes a strong cold and rigid geotectonic element 3 southward weakening of the forearc is directly related to the decreasing thermal age of the subducted slab 4 very low rigidities along the main orogen are caused by the existence of a thick quartz rich crust with a low strain rate to heat flow ratio 5 the strength of the plateau lithosphere is localized in an upper crustal layer whose base at 15 km could be correlated with a p to s seismic wave converter trac1 of yuan et al 2000 yuan x sobolev s kind r oncken o et al 2000 subduction and collision processes in the central andes constrained by converted seismic phases nature v 408 21 28 diciembre p 958 961 6 the forearc plateau rigidity boundary corresponds to a zone of changing thermal conditions eastward increasing crustal thickness and felsic component in the crust and low strain rate deformation which correlates with a west verging structural system at the surface these conclusions suggest that the rigid forearc acts as a pseudo indenter against the weak plateau and allows the accumulation of ductile crustal material that moves westward from the eastern foreland this pseudo indenter is geometrically represented by a crustal scale triangular zone rooted at trac1 this model allows the integration of existing contradictory ideas on the dynamics of forearc plateau interaction that are related to the relative importance of upper crustal compressive structures and lower crustal accumulation below the forearc

seismic evidence for deep water transportation in the mantle we report seismic evidence for the transportation of water into the deep mantle in the subduction zone beneath northeastern japan our data indicate that water is released from the hydrated oceanic crust at shallow depths approximately 100 kilometers and then forms a channel of hydrated mantle material on top of the subducting plate that is the pathway for water into the deep mantle our result provides direct evidence that shows how water is transported from the ocean to the deep mantle in a cold subduction zone environment

holocene relative sea level history of novaya zemlya russia and implications for late weichselian ice sheet loading abstract we present six new radiocarbon dated emergence curves that provide a detailed record of postglacial emergence of northern novaya zemlya and ages which constrain the emergence of vaygach island in the southern archipelago radiocarbon ages on hiatella sp from a lateral moraine in russkaya gavan and abundances of foraminifea in a marine core from nordenskiold bay 300 km south of our study area indicate that coastal deglaciation occurred prior to 10 000 cal yr b p however postglacial emergence commenced 7000 cal yr b p with stabilization of global sea level the total emergence is 13 11 m above sea level asl with apparent uplift rates of 1 2 mm yr for the past 2000 yr indicating modest glacier loads 11 000 cal yr b p deglaciation or both the isobase pattern showing no east west tilt across novaya zemlya and offshore moraines suggest a separate ice dispersal center over novaya zemlya for the later stages of the late weichselian glacial cycle and possibly earlier

a review of postglacial emergence on svalbard franz josef land and novaya zemlya northern eurasia the pattern of postglacial emergence in the barents sea is pivotal to constraining the timing of deglaciation and extent and thickness of the last ice sheet in northern eurasia this review unites records of holocene relative sea level from svalbard franz josef land and novaya zemlya to better understand the geometries of past ice sheet loads emergence data from northern eurasia confine the maximum area of glacier loading to the northwestern barents sea where 100 m of emergence is measured on kongs ya deglacial unloading commenced on western and northern spitsbergen c 13 12 14 c ka ago and by c 10 5 14 ck a on eastern svalbard and more distal sites on franz josef land and novaya zemlya the marine limit phase c 13 12 14 c ka on western and northern spitsbergen is characterized by the construction of spits indicating a dominance of long shore drift over stormgenerated fetch reflecting extensive sea ice coverage of coastal areas at sites in proximity to the ice sheet margin on western and northern spitsbergen there is evidence for a transgressive regressive cycle c 6 4 14 c ka possibly reflecting back migration of displaced mantle material a modern transgression is inferred from the marine erosion of 17th century cultural features and 14 c ages of whalebone and terrestrial peat buried by modern storm gravels that place sea level at its present position by c 2 to 1 ka ago the greatest observed emergence on franz josef land occurs on bell island with a marine limit at 49 m aht formed c 10 14 c ka available emergence data since 9 ka show rising strandlines toward the southwest at b0 3 m km the northern limit of emergence on franz josef land is poorly constrained because relative sea level data is sparse north of 80 30 0 n in contrast to svalbard and franz josef land the marine limit on northern novaya zemlya is only 10 15 m above high tide and formed between 6 5 and 5 0 14 c ka when global sea level was stabilizing all sites show little apparent emergence during the past 2 ka with the youngest raised landforms at identical heights to storm beaches this minimal glacio isostatic signature on novaya zemlya and on vaygach island where deglaciation commenced 10 ka ago indicates ice sheet thicknesses of o1 5 km the spatial variation in emergence for the barents sea indicates that western and northern spitsbergen and novaya zemlya were near the reactive margin of the ice sheet and these areas sustained the briefest ice coverage 2 6 ka and were probably not in isostatic equilibrium in contrast central and eastern svalbard and southern franz josef land were beneath a substantial ice load and probably sustained this load for c 10 14 c ka and achieved isostatic equilibrium isostasy residual from an ice sheet model portrays well the general pattern of uplift and load response at the centre of ice sheets but deviates substantial at the ice sheet margin or areas covered by thin ice like novaya zemlya

the impact of the south american plate motion and the nazca ridge subduction on the flat subduction below south peru 1 flat subduction near peru occurs only where the thickened crust of the nazca ridge subducts furthermore the south america continent shows a westward absolute plate motion both the overriding motion of south america and the subduction of the nazca ridge have been proposed to explain the flat slab segment below south peru we have conducted a series of numerical model experiments to investigate the relative importance of both mechanisms results suggest that the average upper mantle viscosity should be about 3 5 1020 pa s or less and basaltic crust should be able to survive 600 to 800 c ambient temperature before transforming into eclogite to explain the slab geometry below peru the effect of the overriding plate is estimated to be as large or twice as large as that of the plateau subduction

late weichselian glaciation of the russian high arctic a numerical ice sheet model was used to reconstruct the late weichselian glaciation of the eurasian high arctic between franz josef land and severnaya zemlya an ice sheet was developed over the entire eurasian high arctic so that ice flow from the central barents and kara seas toward the northern russian arctic could be accounted for an inverse approach to modeling was utilized where ice sheet results were forced to be compatible with geological information indicating ice free conditions over the taymyr peninsula during the late weichselian the model indicates complete glaciation of the barents and kara seas and predicts a maximum sized ice sheet for the late weichselian russian high arctic in this scenario full glacial conditions are characterized by a 1500 m thick ice mass over the barents sea from which ice flowed to the north and west within several bathymetric troughs as large ice streams in contrast to this reconstruction a minimum model of glaciation involves restricted glaciation in the kara sea where the ice thickness is only 300 m in the south and which is free of ice in the north across severnaya zemlya our maximum reconstruction is compatible with geological information that indicates complete glaciation of the barents sea however geological data from severnaya zemlya suggest our minimum model is more relevant further east this in turn implies a strong paleoclimatic gradient to colder and drier conditions eastward across the eurasian arctic during the late weichselian

upper mantle anisotropy beneath peru from sks splitting constraints on flat slab dynamics and interaction with the nazca ridge a r t i c l e i n f o a b s t r a c t the peruvian flat slab is by far the largest region of flat subduction in the world today but aspects of its structure and dynamics remain poorly understood in particular questions remain over whether the relatively narrow nazca ridge subducting beneath southern peru provides dynamic support for the flat slab or it is just a passive feature we investigate the dynamics and interaction of the nazca ridge and the flat slab system by studying upper mantle seismic anisotropy across southern peru we analyze shear wave splitting of sks ssks and pks phases at 49 stations distributed across the area primarily from the peru lithosphere and slab experiment pulse we observe distinct spatial variations in anisotropic structure along strike most notably a sharp transition from coherent splitting in the north to pervasive null non split arrivals in the south with the transition coinciding with the northern limit of the nazca ridge for both anisotropic domains there is evidence for complex and multi layered anisotropy to the north of the ridge our ks splitting measurements likely reflect trench normal mantle flow beneath the flat slab this signal is then modified by shallower anisotropic layers most likely in the supra slab mantle but also potentially from within the slab to the south the sub slab mantle is similarly anisotropic with a trench oblique fast direction but widespread nulls appear to reflect dramatic heterogeneity in anisotropic structure above the flat slab overall the regional anisotropic structure and thus the pattern of deformation appears to be closely tied to the location of the nazca ridge which further suggests that the ridge plays a key role in the mantle dynamics of the peruvian flat slab system

lithospheric structure beneath the northern central andean plateau from the joint inversion of ambient noise and earthquake generated surface waves the central andean plateau cap as defined by elevations in excess of 3 km extends over 1 800 km along the active south american cordilleran margin making it the second largest active orogenic plateau on earth the uplift history of this high plateau with an average elevation around 4 km above sea level remains uncertain as paleoelevation studies along the cap suggest a complex non uniform uplift history as part of the central andean uplift and the geodynamics of high topography caught project we image the s wave velocity structure of the crust and upper mantle using surface waves measured from ambient noise and teleseismic earthquakes to investigate the upper mantle component of plateau uplift we observe three main features in our s wave velocity model including 1 a positive velocity perturbation associated with the subducting nazca slab 2 a negative velocity perturbation below the subandean crust that we interpret as anisotropic brazilian cratonic lithosphere and 3 a high velocity feature in the mantle above the slab that extends along the length of the altiplano from the base of the moho to a depth of 120 km a strong spatial correlation exists between the lateral extent of this high velocity feature and the relatively lower elevations of the altiplano basin suggesting a potential relationship determining if this high velocity feature represents a small lithospheric root or foundering of orogenic lithosphere requires more integration of observations but either interpretation implies a strong geodynamic connection with the uppermost mantle and the current topography of the northern cap

chilean flat slab subduction controlled by overriding plate thickness and trench rollback how flat slab geometries are generated has been long debated it has been suggested that trenchward motion of thick cratons in some areas of south america and cenozoic north america progressively closed the asthenospheric wedge and induced flat subduction here we develop time dependent numerical experiments to explore how trenchward motion of thick cratons may result in flat subduction we find that as the craton approaches the trench and the wedge closes two opposite phenomena control slab geometry the suction between ocean and continent increases favoring slab flattening while the mantle confined within the closing wedge dynamically pushes the slab backward and steepens it when the slab retreats as in the peru and chile flat slabs the wedge closure rate and dynamic push are small and suction forces generate in some cases flat subduction we model the past 30 m y of subduction in the chilean flat slab area and demonstrate that trenchward motion of thick lithosphere 200 300 km currently 700 800 km away from the peru chile trench reproduces a slab geometry that fits the stress pattern seismicity distribution and temporal and spatial evolution of deformation and volcanism in the region we also suggest that varying trench kinematics may explain some differing slab geometries along south america when the trench is stationary or advances the mantle flow within the closing wedge strongly pushes the slab backward and steepens it possibly explaining the absence of flat subduction in the bolivian orocline

early to mid holocene atlantic water influx and deglacial meltwater events beaufort sea slope arctic ocean abstract holocene high resolution cores from the margin of the arctic ocean are rare core p189ar p45 collected in 405 m water depth on the beaufort sea slope west of the mackenzie river delta 70 33 03 n and 141 52 08 w is in close vertical proximity to the present day upper limit of modified atlantic water the 5 11 m core spans the interval between 6800 and 10 400 14 c yr b p with an 800 yr ocean reservoir correction the sediment is primarily silty clay with an average grain size of 9 the chronology is constrained by seven radiocarbon dates the rate of sediment accumulation averaged 1 35 mm yr stable isotopic data 18 o and 13 c were obtained on the polar planktonic foraminifera neogloboquadrina pachyderma s and the benthic infaunal species cassidulina neoteretis a distinct low 18 o event is captured in both the benthic and planktonic data at 10 000 14 c yr b p probably recording the glacial lake agassiz outburst flood associated with the north atlantic preboreal cold event the benthic foraminifera are dominated in the earliest holocene by c neoteretis a species associated with modified atlantic water masses this species decreases toward the core top with a marked environmental reversal occurring 7800 14 c yr b p possibly coincident with the northern hemisphere 8200 cal yr b p cold event

stable isotopes in benthic foraminiferal calcite from a river influenced arctic marine environment kara and pechora seas oxygen and carbon stable isotope compositions of modern benthic foraminifera in the kara and pechora estuarine regions of the arctic continental shelf were compared with water 1 8 o 1 3 c d i c temperature and salinity the foraminiferal 1 8 o distribution is mostly similar to that of equilibrium calcite primarily controlled by mixing of runoff and seawater in near estuarine areas depth 15 20 m with additional temperature control seaward the 1 3 c composition is controlled by remineralization of organic matter mostly imported by rivers and by water mixing most analyses used the common arctic foraminifer elphidium excavatum forma clavata which yields depleted 1 3 c as expected for an infaunal species isotope records for sediment cores off the pechora and yenisey rivers show a decelerating rise in isotope values of up to 10 in 1 8 o over the last 10 kyr in a pattern similar to that of rising sea level and concomitant retreat of river mouths with local complexities probably resulting from glacio isostatic rebound and changes in sedimentation

seismic structure in southern peru evidence for a smooth contortion between flat and normal subduction of the nazca plate rapid changes in slab geometry are typically associated with fragmentation of the subducted plate however continuous curvature of the slab is also possible the transition from flat to normal subduction in southern peru is one such geometrical change the morphology of the subducted nazca plate along this transition is explored using intraslab earthquakes recorded by temporary regional seismic arrays observations of a gradual increase in slab dip coupled with a lack of any gaps or vertical offsets in the intraslab seismicity suggest warping of the slab concentrations of focal mechanisms at orientations which are indicative of slab bending are also observed along the change in slab geometry the presence of a thin ultra slow velocity layer usl atop the horizontal nazca slab is identified and located the lateral extent of this usl is coincident with the margin of the projected linear continuation of the subducting nazca ridge implying a causal relationship wherein increased hydration of the ridge results in the formation of the usl downdip waveform modelling of the 2 d structure in southern peru using a finite difference algorithm provides constraints on the velocity and geometry of the slab s seismic structure and confirms the absence of any tears in the slab the seismicity and structural evidence suggests smooth contortion of the nazca plate along the transition from flat to normal subduction the slab is estimated to have experienced 10 per cent strain in the along strike direction across this transition

late weichselian deglacial history of the svyataya saint anna trough northern kara sea arctic russia abstract marine sediment core and seismic records from the svyataya saint anna trough provide new insight into the distribution of late weichselian glacial coverage ice retreat pattern and post glacial environments in the northern barents and kara seas these records indicate that the saint anna trough was filled with grounded glacier ice which likely reached the shelf edge during the late weichselian maximum several radiocarbon dates suggest early deglaciation of the deep axial part of the trough prior to 13 3 ka two sandy beds in the deglacial section of the cores imply distinct pulses of iceberg calving and or melting which were probably associated with stepwise retreat of the ice margin morainic ridges and glacial sole markings in the western part of the saint anna trough indicate that the northern central barents sea was a site of a large ice mass during deglaciation a smaller ice cap is inferred for the northern kara plateau at later stages ice retreat on the western flank of the trough was directed towards franz josef land and was presumably facilitated by a separation of the barents sea and novaya zemlya ice domes deglaciation of the saint anna trough was completed by ca 10 ka high post glacial sediment fluxes between 10 and 8 ka were probably related to sea floor coastal erosion and or siberian river discharge during the rising sea level

the last glacial maximum of svalbard and the barents sea area ice sheet extent and configuration abstract the timing extent and configuration of the late weichselian barents ice sheet has been debated for several decades this debate has arisen largely because of the limited or conflicting field evidence on which most models have been based in particular reconstruction of the marine parts of the former barents ice sheet has been controversial this paper aims to review the geological observations and interpretations regarding the size and timing of the late weichselian ice sheet combined with numerical modelling of its formation in order to produce a reconstruction of ice sheet extent and behaviour sub glacial till with overlying glacimarine deposits dated to the late weichselian is found over most of the barents sea floor and the continental shelf west of svalbard glacially induced debris flow deposits on the large bjonoya and isfjorden trough mouth fans strongly support the idea of ice sheet extension to the shelf edge during maximum glaciation isobase maps show a centre of post glacial uplift in the north central barents sea and glaciological and isostatic modelling suggest that the ice sheet was 2000 3000 m thick in this area the ice sheet was confluent with ice over the kara sea but the interaction between the barents and kara ice sheets is not yet fully understood the deglaciation of the barents ice sheet started ca 15 ka probably by calving within the deeper troughs by 12 ka most of the central barents sea was ice free and ice remained over the svalbard franz josef land and novaja zemlya archipelagos and adjacent shallow shelf areas the coasts and fjords of these islands were ice free by 10 ka

new constraints on the limits of the barents kara ice sheet during the last glacial maximum based on borehole stratigraphy from the pechora sea a new 14 c verified borehole stratigraphy provides the first age controlled reconstruction of the late quaternary glacial history of the pechora sea southeasternmost barents sea a complete glaciation of the pechora sea is confirmed for middle weichselian time prior to ca 35 40 ka composition of glacial erratics indicates that ice was moving from or across southernmost novaya zemlya and vaygach island after a brief interstadial period with normal marine conditions the pechora sea was affected by a drop in sea level and a drier climate subsequently the late weichselian barents kara ice sheet occupied the northwestern part of the pechora sea but did not reach the coast of the pechora lowland as previously believed these data provide a new constraint on the last glacial maximum lgm ice sheet limits in the eurasian arctic the inferred direction of the last glacial maximum ice movement in the pechora sea was from the northeast but with a stronger northern component than the penultimate glaciation the ice sheet retreated early ca 13 ka after which the shallow pechora sea was subjected to strong erosion during the postglacial sea level rise

nature of crustal terranes and the moho in northern costa rica from receiver function analysis 1 the central american subduction zone in northern costa rica shows along strike variations in both the incoming and overriding plates by analyzing the subducting oceanic moho m1 and the upper plate moho m2 with receiver functions we investigate the variability in the hydration state of the subducting cocos plate and the nature of crustal terranes within the overriding caribbean plate we calculate high quality p and pp wave receiver functions using broadband data of the global seismology network geoscope project and the crseize pocosol and corisubmod experiments in addition we estimate the depth h and vertically averaged vp vs to moho and present a sensitivity study to explore the effects of a dipping interface on receiver functions and the h and estimates our results are consistent with a drier oceanic mantle subducting beneath the southernmost part of the nicoya peninsula as compared to a serpentinized oceanic mantle subducting beneath the northern part in the caribbean plate we describe the nature of the mesquito nicoya and chorotega terranes by integrating new and published vp vs estimates both the nicoya and chorotega terranes display high vp vs 1 80 1 92 consistent with their oceanic character in contrast the oceanic mesquito terrane mostly displays lower vp vs 1 62 1 80 more compatible with continental crust which may indicate that subduction zone magmatism is modifying the crust to display continental character our estimates show that the deepest m2 42 km is observed in the volcanic arc region whereas the shallowest m2 27 33 km is observed in parts of the fore arc and back arc regions

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

intermediate depth earthquake faulting by dehydration embrittlement with negative volume change earthquakes are observed to occur in subduction zones to depths of approximately 680 km even though unassisted brittle failure is inhibited at depths greater than about 50 km owing to the high pressures and temperatures it is thought that such earthquakes particularly those at intermediate depths of 50 300 km may instead be triggered by embrittlement accompanying dehydration of hydrous minerals principally serpentine a problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2 4 gpa 60 120 km depth above which brittle fracture mechanics predicts that the instability should be quenched here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine grained solid reaction products formed during dehydration this phenomenon was observed under all conditions tested pressures of 1 6 gpa temperatures of 650 820 degrees c independent of the sign of the volume change of reaction although this result contradicts expectations from fracture mechanics it can be explained by separation of fluid from solid residue before and during faulting a hypothesis supported by our observations these observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth as long as there are hydrous minerals breaking down under a differential stress

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

late quaternary stratigraphy of western yamal peninsula russia new constraints on the configuration of the eurasian ice sheet ice sheet reconstructions for the last glacial maximum in northern eurasia range from nearly complete coverage by a contiguous marine based ice sheet to large unglaciated areas stratigraphic records from yamal peninsula russia provide new insight on the eastern limit of the eurasian ice sheet in the kara sea radiocarbon and infrared stimulated luminescence ages from coastal cliff sections date the emplacement of the kara diamicton as older than ca 40 ka reflecting regional glaciation ice wedge growth peat accumulation and eolian and fluvial deposition characterized the past 40 k y and do not support coverage of yamal peninsula by an ice sheet or proximity to a glacier margin thus the late weichselian eurasian ice sheet was largely confined to barents sea and its global sea level contribution is reduced to 7 m of sea level equivalent

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

lateglacial and early holocene environments of novaya zemlya and the kara sea region of the russian arctic pollen and radiocarbon data from novaya zemlya and the kara sea region suggest that the hypo thetical panarctic ice sheet denton and hughes 1981 never existed in this area at least during the last 16 000 years lateglacial tundra environments were slightly cooler and drier than the present ones but there were also warmer intervals such as the allerod which were rather favourable for vegetation development better conditions existed during the early holocene when warm atlantic air masses and sea currents gradually penetrated eastwards to the kara region

glacial geology of outer bjornoyrenna southwestern barents sea the upper cenozoic glacigenic succession in outer bjornoyrenna bear island trough southwestern barents sea is investigated utilizing data from six shallow boreholes 143 m below sea bed and seismic profiling overdeepened troughs in the lowermost part of a large plio pleistocene wedge at the shelf margin are interpreted as glacial erosion forms this suggests that the overlying succession which basinward attains a thickness of several km was deposited during a period with glaciations large scale possible sand ridges in the stratigraphic lower part of this succession are confined to two north south trending zones parallel to the present shelf break they probably formed in a continental shelf coastal environment with exceptionally high sediment supply and fluctuating relative sea level related to glaciations a core from a channel fill in the lower part of the succession consists of possibly glaciofluvial gravelly sand intercalated with till most of the other cores are dominated by middle and upper pleistocene muddy massive partly fissile diamictons interpreted to be mainly tills undrained shear strength of these cores ranges from about 20 kn m 2 to more than 300 kn m 2 and both underconsolidated intervals compared to the present burial depth are found an underconsolidated interval comprising eemian interglacial biourbated marine sediments overlain by laminated weichselian glaciomarine sediments on the outher shelf is covered by 50 100 m of muddy diamicton this diamicton is suggested to be mainly till deposited by a late weichselian ice sheet which was grounded to more than 500 m below present sea level earlier weichselian ice expansions did not reach the outer shelf based on the stratigraphy in the shallow cores and seismic interpretation we suggest that there have been five major expansions of grounded ice to the shelf break since ca 440 ka the present study suggests that glacial erosion was a major process in the late cenozoic denudation of the barents sea and in large areas perhaps as much as 200 250 m of preexisting strata could have been eroded by glaciers since about 440 ka

the svalbard barents sea ice sheet historical current and future perspectives abstract the history of research on the late quaternary svalbard barents sea ice sheet mirrors the developments of ideas and the shifts of paradigms in glacial theory over the past 150 years since the onset of scientific research there in the early 19th century svalbard has been a natural laboratory where ideas and concepts have been tested and played an important but rarely acknowledged role in the break through of the ice age theory in the 1870 s the history of how the scientific perception of the svalbard barents sea ice sheet developed in the mid 20th century also tells a story of how a combination of fairly scattered and often contradictory observational data and through both deductive and inductive reasoning could outline a major ice sheet that had left but few tangible fingerprints since the 1980 s with increased terrestrial stratigraphical data ever more marine geological evidence and better chronological control of glacial events our perception of the svalbard barents sea ice sheet has changed the first reconstructions depicted it as a static concentric single domed ice sheet with ice flowing from an ice divide over the central northern barents sea that expanded and declined in response to large scale late quaternary climate fluctuations and which was more or less in tune with other major northern hemisphere ice sheets we now increasingly perceive it as a very dynamic multidomed ice sheet controlled by climate fluctuations relative sea level change as well as subglacial topography substrate properties and basal temperature in this respect the svalbard barents sea ice sheet will increasingly hold the key for understanding the dynamics and processes of how marine based ice sheets build up and decay

a teleseismic study of the altiplano and the eastern cordillera in northern bolivia new constraints on a lithospheric model the altiplano one of the world s great plateaus is the most significant geomorphological unit of the central andes knowledge of its lithospheric structure is essential to the understanding of the mountain building in this region in order to study this we performed a teleseismic field experiment in northern bolivia during a 4 month period 34 short period seismic stations were installed along a 320 km long profile crossing the altiplano and the eastern cordillera from the volcanic arc zone to the sub andean zone perpendicular to the main structural direction a careful study of relative travel time residuals led us to propose a qualitative model related to the structural patterns the maximum amplitude of these residuals along the profile reaches 3 s for p phases and have a strong azimuthal dependence the residuals show a noticeable sudden increase clearly associated with the fault system bordering the eastern cordillera to the west the cordillera real fault system the high quality of the data allows us to perform a velocity inversion and to calculate at what depth lateral variations occur the origin of the perturbations in the upper crust is reasonably accounted for by the depth variations of the sedimentary fill with the maximum thickness found under the altiplano basin the velocity perturbations in the lower crust are interpreted as variations of the moho depth which decreases from about 60 km below the altiplano to 50 km below the eastern cordillera in the upper mantle a high velocity zone is observed below the eastern cordillera the major feature of this model is a subvertical boundary dipping slightly to the southwest which separates two strongly contrasting velocity units throughout the model from the surface down to 140 km this boundary coincides with the cordillera real fault system and is interpreted as an old suture as the altiplano is characterized by low velocities in the crust the high velocities beneath the eastern cordillera down to 120 km may correspond to the brazilian craton in the region studied the western limit of the underthrusting of the craton corresponds to the western border of the eastern cordillera

constraints on the late weichselian ice sheet over the barents sea from observations of raised shorelines abstract direct evidence for late weichselian grounded glacier ice over extensive areas of the barents sea is based largely on indirect observations including elevations of old shorelines on svalbard and arguments of isostatic rebound such isostatic models are discussed here for two cases representing maximum and minimum ice sheet reconstructions in the former model the ice extends over the kara sea whereas in the latter the ice is limited to the barents sea and island archipelagos comparisons of predictions with observations from a number of areas including spitsbergen nordaustlandet edgeoya kong karls land franz josef land novaya zemlya and finnmark support arguments for the existence of a large ice sheet over the region at the time of the last glacial maximum this ice sheet is likely to have had the following characteristics conclusions that are independent of assumptions made about the earth s rheological parameters i the maximum thickness of this ice was about 1500 2000 m with the centre of the load occurring to the south and east of kong karls land ii the ice sheet extended out to the western edge of the continental shelf and its maximum thickness over western spitsbergen was about 800 m iii to the north of svalberg and frans josef land the ice sheet extended out to the northern shelf edge iv retreat of the grounded ice across the southern barents sea occurred relatively early such that this region was largely ice free by about 15 000 bp v by 12 000 bp the grounded ice had retreated to the northern archipelagos and was largely gone by 10 000 bp vi the ice sheet may have extended to the kara sea but ice thicknesses were only a fraction of those proposed in those reconstructions where the maximum ice thickness is centered on novaya zemlya models for the palaeobathymetry for the barents sea at the time of the last glacial maximum indicate that large parts of the barents sea were either very shallow or above sea level providing the opportunity for ice growth on the emerged plateaux as well as on the islands but only towards the end of the period of fennoscandian ice sheet build up

facies system of the eastern barents sea since the last glaciation to present abstract twenty two sediment cores raised from the central and eastern parts of the barents sea have been studied to reconstruct the evolution of the facies system since the late weichselian glaciation multiproxy records reveal four lithostratigraphic units which reflect major development stages of paleoenvironments age control is provided by 23 ams 14 c dates for holocene sections of four cores continental moraine deposits of the last glaciation are overlain by proximal glaciomarine facies of the initial deglaciation phase during this phase the barents sea ice sheet detached from the ground resulting in seawater penetration into troughs iceberg calving deposition of ird and fine grained glacier meltwater load in newly formed marine basins the main deglaciation phase is characterized by pulsed sedimentation from various gravity flows resulting in accumulation of distal glaciomarine facies comprising laminated clay and sand sequences with minor ird redistribution of fine grained suspended matter by bottom currents and brine induced nepheloid flows combined with biogenic processes and minor ice rafting caused facies diversity of the holocene marine sediments the holocene facies of shelf depressions reflect rather high but variable productivity responding to climate changes and variations of atlantic water inflow into the barents sea

sediment accumulation in low sedimentation wave dominated glaciated inlets abstract many fjords situated in lowland areas of resistant bedrock are characterized by a comparatively low fluvial sediment flux and are often dominated by wave processes several such fjords in eastern canada are used as examples to develop a generalized sediment distribution model physiographically the fjords can be subdivided into nearshore platforms basin slopes and basin floors where there has been marine transgression the principal sediment source is shoreline erosion of glacial till in areas of crustal rebound late glacial and early holocene muds deposited on highs are eroded as they come within the zone of wave erosion with the falling sea level storm waves resuspend fine grained sediment in water depths of many tens of metres and produce near bottom turbid water zones within basins fine sand and silt settle out rapidly leading to the deposition of graded beds on the basin floors residual sand and gravel accumulate on the near shore platforms while silt and mud prograde over or are advected beyond basin slopes burying older basin fill sediment

the crust and uppermost mantle structure of southern peru from ambient noise and earthquake surface wave analysis abstract southern peru is located in the northern central andes which is the highest plateau along an active subduction zone in this region the nazca slab changes from normal to flat subduction with the associated holocene volcanism ceasing above the flat subduction regime we use 6 s to 67 s period surface wave signals from ambient noise cross correlations and earthquake data to image the shear wave velocity v s v structure to a depth of 140 km a mid crust low velocity zone is revealed and is interpreted as partially molten rocks that are part of the andean low velocity zone it is oblique to the present trench and possibly indicates the location of the volcanic arcs formed during the steepening of the oligocene flat slab beneath the altiplano plateau the recently subducted slab beneath the forearc shows a decrease in velocity from the normal to flat subduction regime that might be related to hydration during the formation of the nazca ridge which in turn may contribute to the buoyancy of the flat slab the mantle above the flat slab has a comparatively high velocity which indicates the lack of melting and thus explains the cessation of the volcanism above a velocity contrast from crust to uppermost mantle is imaged across the cusco vilcanota fault system and is interpreted as the boundary between two lithospheric blocks

the nature of orogenic crust in the central andes 1 the central andes 16 22 s are part of an active continental margin mountain belt and the result of shortening of the weak western edge of south america between the strong lithospheres of the subducting nazca plate and the underthrusting brazilian shield we have combined receiver function and surface wave dispersion results from the banjo seda project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure the major results are as follows 1 the crust supporting the high elevations is thick and has a felsic to intermediate bulk composition 2 the relatively strong brazilian lithosphere is underthrusting as far west 65 5 w as the high elevations of the western part of the eastern cordillera ec but does not underthrust the entire altiplano 3 the subcrustal lithosphere is delaminating piecemeal under the altiplano ec boundary but is not completely removed beneath the central altiplano the altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation leading to lower crustal flow and flat topography in contrast in the high relief inland sloping regions of the ec and sub andean zone the upper crust is still strongly coupled across the basal thrust of the fold thrust belt to the underthrusting brazilian shield lithosphere subcrustal shortening between the altiplano and brazilian lithosphere appears to be accommodated by delamination near the altiplano ec boundary our study suggests that orogenic reworking may be an important part of the felsification of continental crust

lithospheric evolution of the andean fold thrust belt bolivia and the origin of the central andean plateau we combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central andean plateau between 188 and 208 s from late cretaceous to present by integrating geophysical results of upper mantle structure crustal thickness and composition with recently published structural stratigraphic and thermochronologic data we emphasize the importance of both the crust and upper mantle in the evolution of the central andean plateau four key steps in the evolution of the andean plateau are as follows 1 initiation of mountain building by 70 ma suggested by the associated foreland basin depositional history 2 eastward jump of a narrow early fold thrust belt at 40 ma through the eastward propagation of a 200 400 km long basement thrust sheet 3 continued shortening within the eastern cordillera from 40 to 15 ma which thickened the crust and mantle and established the eastern boundary of the modern central andean plateau removal of excess mantle through lithospheric delamination at the eastern cordillera altiplano boundary during the early miocene appears necessary to accommodate underthrusting of the brazilian shield replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the eastern cordillera and altiplano at 24 23 ma and further volcanism recorded by 12 7 ma crustal ignimbrites 4 after 20 ma deformation waned in the eastern cordillera and interandean zone and began to be transferred into the subandean zone long term rates of shortening in the fold thrust belt indicate that the average shortening rate has remained fairly constant 8 10 mm year through time with possible slowing 5 7 mm year in the last 15 20 myr we suggest that cenozoic deformation within the mantle lithosphere has been focused at the eastern cordillera altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination d 2005 elsevier b v all rights reserved

seismicity and structure of nazca plate subduction zone in southern peru abstract we define subducting plate geometries in the nazca subduction zone by re locating intra slab earthquakes in southern peru 14 18 s and using previously published converted phase analysis results to clarify the slab geometry and inferred relationships to the seismicity we also provide both p and s wave velocity models of the subducting nazca plate and mantle layer above the slab using double difference tomography to understand upper plate volcanism and subduction zone process the double difference constraints for determining the hypocenters and velocity model ensure high accuracy of the relative location of earthquakes with respect to velocity structure the relocated seismicity shows a smooth contortion in the slab dip transition zone for 400 km between the shallow 30 to flat dipping interface to the northwest and the 30 dipping interface to the southeast we find a significant slab dip difference up to 10 between our results and previous slab models along the profile region sampling the horizontal slab at a depth of 85 95 km robust features in both p and s wave tomography inversions are both arc normal and along arc velocity variations in the arc normal direction all profile results show that the slab velocities beneath the forearc down to a depth of 100 km transition to higher velocities beneath the backarc at 100 140 km depth in the along arc direction velocities of the subducting flat slab are shown to be depressed compared to those of the normal dip slab in addition high shear wave velocities and low vp vs are detected in the mantle layer above the flat slab indicating its cold and dry environment such differences in the velocity structures for the slab and mantle wedge between the two regions may indicate different hydration states which greatly affects the upper plate structure of southern peru

structure of the los angeles basin from ambient noise and receiver functions a velocity v s and structure model is derived for the los angeles basin california based on ambient noise surface wave and receiver function analysis using data from a low cost short duration dense broad band survey lassie deployed across the basin the shear wave velocities show lateral variations at the compton los alamitos and the whittier faults the basement beneath the puente hills san gabriel valley shows an unusually high velocity 4 0 km s 1 and indicates the presence of schist the structure of the model shows that the basin is a maximum of 8 km deep along the profile and that the moho rises to a depth of 17 km under the basin the basin has a stretch factor of 2 6 in the centre grading to 1 3 at the edges and is in approximate isostatic equilibrium

the influence of environmental low level shear and cold pools on tornadogenesis insights from idealized simulations abstractidealized dry simulations are used to investigate the roles of environmental vertical wind shear and baroclinic vorticity generation in the development of near surface vortices in supercell like pseudostorms a cyclonically rotating updraft is produced by a stationary cylindrical heat source imposed within a horizontally homogeneous environment containing streamwise vorticity once a nearly steady state is achieved a heat sink which emulates the effects of latent cooling associated with precipitation is activated on the northeastern flank of the updraft at low levels cool outflow emanating from the heat sink spreads beneath the updraft and leads to the development of near surface vertical vorticity via the baroclinic mechanism as has been diagnosed or inferred in actual supercells that have been simulated and observed an intense cyclonic vortex forms in the simulations in which the environmental low level wind shear is strong and the heat sink is of intermediate strength relative to the

coalescent greenland and innuitian ice during the last glacial maximum revising the quaternary of the canadian high arctic abstract this paper constitutes a fundamental revision of the author s earlier research and resolves a long standing debate concerning the late quaternary history of the canadian high arctic supporting its inundation by glaciers during the last glacial maximum lgm a transect along east ellesmere island demonstrates that ice advanced northward and southward along the axis of nares strait during the late wisconsinan debouching from a central saddle in kane basin the configuration of ellesmere island and greenland ice is based on moraines meltwater channels and erratics the youngest ams 14c date obtained on shelly till suggests ice buildup after 19 ka bp whereas many other dates are younger than 30 ka bp furthermore the lowest youngest detectable amino acid ratios on shells from till postdate the robeson aminozone 70 ka bp and are separated from holocene ratios by a narrow gap wholly consistent with the occupation of the strait by late wisconsinan ice re entry of the sea throughout nares strait is shown by a series of paleogeographic maps based on geomorphic evidence and radiocarbon dates on shells associated with marine limit deglaciation at the north end of the strait occurred by 10 1 ka bp and at the south end by 9 0 ka bp nares strait may still have been blocked by ice north of kane basin at 8 ka bp however by 7 5 ka bp it provided an unobstructed seaway from the arctic ocean to baffin bay the prominent ice margins within the fiords of eastern ellesmere island previously considered to mark the last ice limit record regional stabilization of land based ice after breakup of preceding marine based margins other implications of this study are that ice buildup occurred post 30 ka bp and possibly post 19 ka bp during an interval widely assumed to be constrained by severe aridity in the high arctic and that coalescent greenland and ellesmere island ice during the lgm promoted thickening of the eastern ellesmere island ice divide causing enhanced westward flow across the island to eureka sound sizeable postglacial emergence 80 120 m on eastern ellesmere island is associated with the retreat of thick ice 1 km from the strait greater emergence in eureka sound which extends southward into the central arctic islands confirms the previously proposed axis of the innuitian ice sheet further clarification of the extent geometry and retreat of the innuitian ice sheet will provide new opportunities for glaciological and geophysical modelling and help to identify late quaternary sediment sources for the arctic ocean and baffin bay

nazca south america interactions and the late eocene late oligocene flat slab episode in the central andes 1 the most prominent features of the andean range are the altiplano and puna plateaus which were constructed by crustal shortening and uplift over the past 45 myr the early construction of these plateaus may have controlled subsequent growth of the orogen proposed models have suggested that an abrupt acceleration in relative motion between the nazca plate and the south american plate at 30 ma may have led to compression of the continent however the major plate motion change occurred at 25 23 ma and paleomagnetic rotations and crustal shortening of the andean forearc require that the arica bend formed prior to about 25 ma inferred history of flat slab subduction along the altiplano section of the andean margin and the structure of the adjacent south american cratonic shield combine to suggest an alternate scenario based partly upon geodynamic models of oceanic continental plate interactions in subduction zones we propose that central andean tectonism may have been controlled by two distinct regimes of subduction 1 oblique subduction along the central andean margin during the late eocene and oligocene accompanied by downdip alignment with the center of the amazonian shield flat slab activity in this phase of orogenesis may have been caused by a combination of cratonic root enhanced tectonics and oceanic plateau subduction and 2 an abrupt transition to trench normal subduction after 25 ma toward the more distal sao francisco craton was accompanied by a return to normal angle subduction similar interactions are hypothesized to have occurred during the laramide orogeny in western north america

the postglacial pleistocene of the northern russian mainland abstract the paper summarizes the results of recent studies of late pleistocene postglacial geological events which occurred in the flatlands of northern russia stretching from the timan ridge to the laptev sea coast the available sedimentary record includes thawed and perennially frozen subaerial sediments and in places lacustrine and fluvial formations hundreds of geochronometric measurements by radiocarbon and luminescence techniques place this record within the time span of 50 11 ka bp which confirms an early mid weichselian age of the underlying glacial complex first established by the queen programme the spectrum of proxies includes sedimentary facies pollen data plant macrofossils oxygen isotope values megafauna remains insect assemblages traces of dna paleosols and permafrost properties together they indicate a continental progressively deteriorating climate with very cold winters and warmer than present summers through the second half of the late pleistocene minor climatic fluctuations are largely due to slightly changing humidity during mis 3 these changes are more pronounced in periglacial european russia in the vicinity of the barents kara ice sheet where the mid weichselian biotic maximum contrasts with conditions in the late weichselian polar desert in siberia the periglacial environments were more stable and beneficial for biota inspite of the increased continentality the recent data accumulated by various researchers are incompatible with the conventional correlation of the karginsky interglacial strata of siberia with mis 3 or any part of the european pleniglacial

a 3 d geodynamic model of lateral crustal flow during andean mountain building although the andes are believed to have resulted mainly from crustal shortening the shortening history remains debated and appears to require lateral along strike crustal flow three dimensional viscous flow modeling shows that within geological uncertainties the andes may have been produced by either neogene shortening alone or with significant pre neogene shortening these scenarios require major along strike crustal flow and predict significantly different histories of uplift and crustal motion

structure of the subduction transition region from seismic array data in southern peru data from three seismic arrays installed in southern peru were analysed using receiver functions from p pp and pkp wave phases in order to image the subducted nazca slab the arrays cover the transition region from flat slab subduction in central peru to normal subduction with an angle of about 30 further south a previous study used data from the first array in the normal subduction region to image the moho depth and slab and showed the existence of a mid crustal structure at 40 km depth that is suggested to be a possible underthrusting of the brazilian shield here we discuss new observations from the other two arrays that span the transition between the two subduction regimes and the flat subduction region the results provide an image of the flattened slab from the coast to approximately 300 km inland and also across the transition region from flat to 30 subduction which appears to be a bend rather than a tear in the slab in the flat slab region the slab is well defined near the coast and flattens out at 100 km depth beneath the altiplano the slab appears to start flattening some 400 km in advance of the subduction of the nazca ridge and the flattening is maintained for 1300 km after its passage the moho begins at a depth of around 30 km near the coast and has a maximum depth of 75 km beneath the altiplano consistent with the results of the other arrays both arrays also show a positive impedance mid crustal structure at 40 km depth which if explainable by underthrusting of the brazilian shield would add further support to the observations from the normal subduction region and show the northward and westward extent of the signal the underthrusting hypothesis would explain the missing crust from the shortening budget needed to support the altiplano the vp vs ratios for both arrays exhibit average values between 1 73 and 1 75 indicating a lesser likelihood that there is a high degree of partial melting or magma bodies at depth in this region the receiver function results provide new imaging of the flat slab and transition from normal to flat slab subduction which allows for comparison of different subduction regimes