late miocene environmental change in nepal and the northern indian subcontinent stable isotopic evidence from paleosols neogene sediments belonging to the siwalik group crop out in the himalayan foothills along the length of southern nepal carbon and oxygen isotopic analyses of siwalik paleosols from four long siwalik sections record major ecological changes over the past 11 m y the carbon isotopic composition of both soil carbonate and organic matter shifts dramatically starting ca 7 0 ma marking the displacement of largely c3 vegetation probably semi deciduous forest by c4 grasslands by the beginning of the pliocene all the flood plains of major rivers in this region were dominated by monsoonal grasslands the floral shift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the latest miocene a similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of pakistan and in terrigenous organic matter from the bengal fan showing that the floral shift was probably continentwide the latest miocene also witnessed an average change of 4 in the oxygen isotopic composition of soil carbonate as observed previously in pakistan the reasons for this are unclear if not diagenetic a major environmental change is indicated perhaps related to that driving the carbon isotopic shift recently described pollen and leaf fossils from the surai khola section show that evergreen forest was gradually displaced by semi deciduous and dry deciduous forest between 11 and 6 ma the gradual nature of this floral shift which culminated in the rapid expansion of c4 grasses starting 7 0 m y ago is difficult to explain by a decrease in atmospheric pco2 alone cerling et al 1993 but fits well with a gradual onset of monsoonal conditions in the late miocene in the northern indian subcontinent himalayan uplift driving both monsoonal intensification and consumption of co2 through weathering may be the common cause behind major late miocene environmental change globally however the decline of effective moisture associated with monsoon development has probably slowed not increased the rate of consumption of co2 by chemical weathering of himalayan sediments

crustal structure of the western new england appalachians and the adirondack mountains we present an interpretation of the crustal velocity structure of the new england appalachians and the adirondack mountains based on a seismic refraction wide angle reflection experiment in eastern north america extending from the adirondacks in new york state through the northern appalachians in vermont and new hampshire to central maine modeling of the eastern portion of the profile within the new england appalachians shows a subhorizontal layered crust with upper crustal velocities ranging from 5 5 to 6 2 km s a midcrustal velocity of 6 4 km s and a lower crustal velocity of approximately 6 8 km s crustal thickness increases from 36 km beneath maine to 40 km in vermont little evidence is seen for structures at depth directly related to the white mountains or the green mountains a major lateral velocity change in the upper and mid crust occurs between the appalachians and the adirondacks this boundary projecting to the surface beneath the champlain valley dips to the east beneath the green mountains and extends to a depth of 25 km below the eastern edge of the connecticut valley synclinorium in vermont the tahawus complex a series of strong horizontal reflections at 18 24 km depth beneath the adirondack highlands is seen to dip eastward beneath vermont upper crustal rocks in the adirondack mountains have poisson s ratios of 0 28 0 01 that can be correlated with the marcy anorthosite pois son s ratios of 0 24 0 01 calculated for rocks of the connecticut valley synclinorium indicate a siliceous upper crust in vermont the lower crust is considered to be best represented by intermediate to mafic granulites a high poisson s ratio 0 26 0 27 tends to support a mafic lower crust in the new england appalachians this seismic refraction wide angle reflection experiment provides further evidence for the obduction of the allochthonous western appalachian units onto grenvillian crust above a zone of detachment that penetrates at least to midcrustal depths and was the locus of successive paleozoic thrusting

arid and humid phases in southern spain during the last 4000 years the zonar lake record cordoba a multiproxy study of sediment cores from zonar lake 37 29 00 n 4 41 22 w 300 m a s l supported by 11 14c ams dates provides the first high resolution centennial scale reconstruction of past h

effect of regional topography and hydrology on the lacustrine isotopic record of miocene paleoclimate in the rocky mountains drummond c n wilkinson b h lohmann k c and smith g r 1993 effect of regional topography and hydrology on the lacustrine isotopic record of miocene paleoclimate in the rocky mountains palaeogeogr palaeoclimatol palaeoecol 101 67 79 oxygen isotopic compositions of upper miocene 9 2 m y lacustrine limestone from northwestern wyoming range from 21 5 to 35 2 0 pdb and are the lightest yet reported for a non marine carbonate sequence these values require that lake water was greatly depleted in 180 relative to modern meteoric precipitation and indicate that most inflow was sourced as meltwater from glaciers in the adjacent gros ventre range assessment of climatic factors influencing the isotopic composition of global meteoric water indicates either that nearly all of miocene precipitation in the southern rocky mountains was derived from large lake systems in the snake river plain and northern great basin to the west or that gros ventre catchment elevations were up to 2300 m higher than at present because erosion rates in modern alpine regions suggest that subaerial denudation could only account for up to 1200 m of post miocene elevation reduction any additional lowering must reflect the influence of post laramide epeirogeny during basin and range extensional tectonism lacustrine isotopic data therefore provide boundary conditions on the timing and magnitude of changes in late cenozoic paleoclimate topography hydrology and tectonism in the western wyoming portion of the rocky mountains

contribution of deep seismic profiling to the knowledge of the lower crust in france and neighbouring areas abstract in france and neighbouring areas the crust is essentially formed of segments of the variscan orogen 380 300 ma and older formations cratonic basins began to form in the permian but extensive subsidence only occurred from the lias in basins such as the paris and aquitaine basins and also along the margin of the alpine tethys ocean the pyrenees and alps range eventually formed during the tertiary as a result of the collision between the european and african continental plates large grabens were also rifted in the rhine and rhone valleys deep seismic profiles show remarkable features common to the crust of the whole area although it underwent such a complicated geological history the most striking feature is the occurrence of conspicuous flat reflections densely distributed in the lower part of the crust below a comparatively transparent upper crust these reflections hereafter called laminations have been found to occur on most profiles within a large area covering western europe although they vary in depth thickness and energy dipping reflections have frequently been observed in the upper crust some of them extend across the lower crust and even penetrate into the upper mantle they may be reasonably interpreted as variscan compressional features by correlation with surface geology and comparison with similar features found in deep seismic profiles from other regions the origin of the flat laminations which generally lie in the lower part of the crust is more controversial this paper attempts to use the well known geological history of the region to place some constraints on the hypotheses which may be advanced to explain the formation of these laminations these features may be explained by the following tectonothermal events 1 1 compression which prevailed during the variscan orogeny may have formed the laminations as a result of delamination and shearing near the bottom of the crust associated with melting intrusion and metamorphism during the different stages of the crustal evolution 2 2 the thermal equilibration of the upper mantle which occurred in the permian at the end of the variscan orogeny may have caused metamorphism and magmatism in the overlying crust 3 3 a regional thermal event which might be associated with subordinate extension occurred during the rifting of the alpine tethys and the initiation of mesozoic cratonic basins in late triassic middle jurassic times it may also have caused metamorphism and magmatism in the lower crust while the first model implies conservation of the crust material the other two suggest that the crust mantle transition has been somewhat uplifted since the end of the variscan orogeny the latter hypothesis is one way to explain the undeniable occurrence of seismic events below the present moho reflection the above three models are not mutually exclusive and all these phenomena may well have affected the crust to some extent however the data at hand are still insufficient to determine what parts tectonic and nontectonic processes have played in the creation of the laminations in the lower crust and the moho further experiments are needed to check these hypotheses

cocorp seismic profiling of the appalachian orogen beneath the coastal plain of georgia a southeastward extension onto the coastal plain of an earlier cocorp traverse which confirmed large scale thin skinned thrusting of crystalline rocks of the southern appalachians has provided some of the most spectacular reflections yet seen in crustal seismic data most of the reflectors can be interpreted as either fault surfaces or as metamorphosed strata of late precambrian early paleozoic age they are consistent with the hypothesis that a major detachment extends eastward beneath this part of the orogen although other interpretations with a more complex pattern of detachments or sutures are also possible large scale overthrusting provides a mechanism for incorporating sedimentary rocks into the lower crust and may help to explain many of the layered features on crustal seismic data reflections from deep beneath the coastal plain indicate that the structural configuration of the rocks is complex and that the remains of a collision zone are being observed several east dipping horizons which bear strong similarities to thrust faults in valley and ridge sedimentary rocks are seen in the basement at shallow and mid crustal levels beneath the coastal plain the augusta fault for example displays a reflection which extends at a low angle some 80 km or more southeast of its surface position in conjunction with surface geologic information these new data demonstrate that late paleozoic compressive deformation was pervasive and resulted in lateral movements in the upper crust extending from the valley and ridge to the crystalline rocks beneath the coastal plain a distance of 400 km or more a large antiform cresting at about 2 3 sec or about 6 km below the surface and other structures beneath the coastal plain of georgia deserve further consideration for petroleum exploration although metamorphism may have eliminated petroleum from these rocks refracted arrivals and fault geometries indicate two triassic rift basins beneath coastal plain sedimentary rocks one of which has apparently not been recognized previously

timing of deglaciation and postglacial environmental dynamics in nw iberia the sanabria lake record abstract the multiproxy study sedimentology geochemistry and diatoms of sediment cores from sanabria lake 42 07 30 n 06 43 00 w 1000 m a s l together with a robust 14c chronology provides the first high resolution and continuous sedimentary record in the region extending back the last 26 ka the development of a proglacial lake before 26 cal ka bp demonstrates the onset of deglaciation before the global last glacial maximum similarly to other alpine glaciers in southern european mountains rapid deglaciation occurred at the beginning of the greenland interstadial gi 1e bolling 14 6 cal ka bp following a short lived episode of glacier re advance 14 4 14 2 cal ka bp gi 1d a climatic improvement at 13 9 cal ka bp suggests the glaciers retreated from the lake basin during the gi 1c another glacier reactivation phase occurred between ca 13 0 12 4 ka starting earlier than the onset of gs 1 younger dryas rapid deglaciation during the early holocene 11 7 10 1 cal ka bp was followed by a period of higher river discharge 10 1 8 2 cal ka bp after 8 2 ka the holocene is characterized by a general decreasing trend in humidity punctuated by the driest phase during the mid holocene ca 6 8 4 8 a wetter interval between 4 8 and 3 3 cal ka bp and a relatively decline of rainfall since then till present with a minor increase in humidity during some phases ca 1670 1760 of the little ice age discrete silt layers intercalated in the organic rich holocene deposits reflect large flooding events of the tera river ca 10 1 8 4 7 5 6 2 5 7 5 6 4 6 4 2 3 7 3 3 3 1 2 7 2 5 and 2 0 cal ka bp their synchronicity with a number of cold and humid events described in the atlantic demonstrates a strong control of nw iberian climate by north atlantic dynamics at centennial millennial scale comparison with western mediterranean records points to similar regional dynamics during the holocene although modulated in the nw iberian peninsula by the stronger atlantic influence

application of allochthonous organic carbon and phosphorus forms in the interpretation of past environmental conditions organic matter in sediments for instance carbon nitrogen and phosphorus can be used to reconstruct the paleoecological and pollution history of lakes and their catchment basins in this paper the contents of allochthonous organic carbon allochthonous oc and autochthonous organic carbon autochthonous oc in sediment cores taken from wuliangsuhai lake and daihai lake in northern china are quantified by using a binary model and phosphorus forms in the sediment cores from the two lakes are extracted by sequential extraction techniques the results indicate that the palaeoenvironment and paleoclimate of daihai lake and its catchment basin in the recent 250 years can be well reconstructed based on the content of allochthonous oc the climate was relatively humid and warm in the period of 1865 2005 while relatively dry and cold in the period of 1765 1865 the sedimentary information of allochthonous oc in the 22 42 cm portion of the sediment cores in daihai lake corresponds to the final cold fluctuation of the little ice age that occurred since the middle holocene the difference of phosphorus forms in the sediment cores between the two lakes indicates that phosphorus input to the lakes and the correlation between phosphorus forms and distribution and the changes of environment are influenced by the eutrophication mechanisms and environmental conditions of the two lakes

lake sediment response to land use and climate change during the last 1000 years in the oligotrophic lake sanabria northwest of iberian peninsula abstract high resolution sequential analysis of the upper 380 mm of a sediment core from lake sanabria an oligotrophic freshwater lake in the granitic zone of the nw iberian peninsula shows four lacustrine sedimentary episodes some of which have been interpreted as the little ice age lia radiometric dating of the core c 14 accelerator mass spectrometry ams gives an age of 875 ad for the oldest sedimentary episode subsequent variations are attributed to human influence on the catchment of lake sanabria agricultural activity of the san martin de castaneda monastery during the middle ages the breached vega de tera dam in january 1959 or the designation of lake sanabria as a natural park in 1978

modeling the isotopic composition of precipitation the physics of the stable isotopes of water is incorporated into a two dimensional kinematic bulk cloud microphysical model the model is run for several idealized classical stratiform and convective storm situations and the resulting isotope ratios of precipitation and water vapor are diagnosed and compared to observations for stratiform snow the model produces low isotope ratios that decrease rapidly poleward of the warm front the lowest isotope ratios occur when the atmosphere is cold and when the vertical velocity attains its maximum value high in the troposphere for stratiform rains the model produces much higher isotope ratios without a significant poleward gradient as a result of isotope exchange between the falling rain and the surrounding vapor isotope ratios of rain are lowest when the melting level is near the ground and isotope exchange is minimized for air mass thunderstorms isotope ratios are uniformly high in warm air no matter what the cloud height unless hail approaches or reaches the ground the model also produces a significant amount effect for rain in which isotope ratios decrease with increasing rainfall totals isotope ratios are particularly low when the rain derives from a recirculation process in which air previously charged by vapor from falling rain subsequently rises under such conditions the model sometimes produces isotope ratios that decrease from the periphery to the core of the precipitation shield it is suggested that this recirculation process is responsible for extraordinarily low isotope ratios observed in some hurricanes and organized thunderstorms the dominant cloud microphysical processes can sometimes be inferred from isotope ratios of precipitation the model produces ice pellets with isotope ratios close to those of rain when the pellets are produced by homogeneous freezing of rain and close to those of snow when the pellets are produced by refreezing of partially melted snow a climatology of isotope values that matches the main features of the observed global data set and of a seven year record of storms at mohonk lake new york is generated by running the model for a wide range of conditions this includes the deuterium excess d d 8 18o for antarctic snows that increases markedly as d falls below 300 and the deuterium deficit observed for rain in warm dry regions

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

palaeolimnological approaches to climate change with special regard to the biological record abstract lakes systems respond physically chemically and biologically to changes in climate and these responses are registered in many various ways in lake sediment records this paper focuses principally on the biological record and describes recent research that aims to reconstruct i past variability in effective moisture and ii past variability in temperature changes in effective moisture are manifested especially by changes in lake water level in closed lake basins water level change can cause significant changes in water salinity that is registered in sediments by changes in the composition of fossil assemblages especially diatoms in open freshwater lake basins changes in water level are best reconstructed from the aquatic macrofossil record and potentially from the ratio of open water to littoral species of cladocera ostracods and diatoms the most promising biological method for reconstructing past temperature is chironomid analysis and a range of new chironomid temperature transfer functions are being developed however biological responses to changing temperature are usually indirect as a result of temperature effects on water column stratification nutrient cycling and alkalinity generation and in most cases a correct interpretation of the climatic significance of changes in the fossil record requires an understanding of the processes that are operating in the water column a multi proxy whole lake approach is advocated as the best way forward not only as a means of reconstructing past climate but also as a means of assessing the impact of climate change on lake systems

using dendrogeomorphology and 137cs and 210pb radiochronology to estimate recent changes in sedimentation rates in kushiro mire northern japan resulting from land use change and river channelization abstract the purpose of this study is to describe the history of sediment deposition that has been occurring for several decades along the margins of the kushiro mire into which the kuchoro river drains the dendrogeomorphic technique was used to examine an even aged forest and adventitious roots to assess short term 20 years deposition rates and the frequency of heavy rainfall events for medium 40 years and long term 100 years deposition rates radiochronological analyses using 137 cs and 210 pb were also conducted several instances of high deposition rates caused by flooding events during the last 20 years were identified from the analysis of adventitious roots the average sedimentation rate from 1963 to the present estimated from the 137 cs study is over 1 3 cm year using the 210 pb dating method based on a cfcs model which considered the organic matter content appropriate to wetland sediment three deposition rates in the last 60 years were estimated 2 0 cm year from 1981 to 2000 8 9 cm year from 1975 to 1981 and 0 14 cm year from 1939 to 1975 the analysis of environmental radionuclides indicated that the greatest sedimentation rates occurred in the years 1975 to 1981 when the kuchoro river was extensively channelized and flood and or precipitation events occurred this suggests that land use development and channelization introduced a large amount of sediment into the marginal areas of wetland through the drainage system and this was deposited on top of the peat soil

north atlantic oscillation recorded in carbonate δ18o signature from lagunillo del tejo spain abstract oxygen 18o and carbon 13c isotope compositions of authigenic carbonates measured in lagunillo del tejo sediment document precipitation variability during the last millennium in the iberian range modern water samples show that lagunillo 18o and d plot below the global meteoric water line gmwl sediment samples show a covariant trend between carbonate 18o and 13c indicating that the precipitation evaporation ratio has largely controlled the isotopic composition of this lake this covariant trend is used to extract information about past lake level changes humid periods occurred around ad 1300 1450 ad 1620 1775 and ad 1950 1980 while the driest periods were concentrated around ad 1200 ad 1500 and especially around the transition between the 19th and 20th centuries we compared these inferences with previous studies in this lake and with nao index and discovered strong correspondence of signals this sensitive site can be used to extent records of nao back in time providing a framework for climatic modeling and ecological management

deep seismic reflection profiling and the growth of the continental crust abstract deep seismic reflection profiling provides evidence for the style and magnitude of phanerozoic crustal growth by both lateral and vertical accretion profiles across convergent zones have been interpreted as showing tectonic underplating of the continental crust by fragmented oceanic lithosphere profiling across extensional zones provides evidence for magmatic underplating of the crust in addition to studies over active areas deep reflection profiling over large areas of western europe north america and australia shows that the lower crust is strongly reflective and seismically distinct from the upper crust and upper mantle the layering is detected beneath many different terranes of varying crustal type and basement age from archaean to cenozoic and is often apparently younger than the tectonic or metamorphic age of the exposed upper crust thus continental growth curves estimated from upper crustal measurements alone may underestimate phanerozoic crustal additions and mean crustal ages similarly estimated may be too high

structural evolution of the cape smith thrust belt and the role of out of sequence faulting in the thickening of mountain belts out of sequence thrusting represents a mechanism capable of heterogeneous internal deformation in thrust belts and was largely responsible for internal thickening in the early proterozoic cape smith belt the south verging thrust belt is marked by a complex history of accretion internal deformation thickening induced metamorphism uplift and erosion an overall chronology for thrust belt evolution is built around the timing of thermal peak metamorphism prethermal peak deformation includes 1 initial piggyback stacking of basalt dominated thrust sheets above a basal decollement localized at the basement cover contact 2 greenschist to amphibolite grade metamorphism accompanied by development of a ductile shear zone at the base of the thrust belt and 3 out of sequence thrusting precipitating the metamorphic thermal peak both above and below these faults synthermal peak out of sequence thrusting was responsible for accretion of autochthonous crystalline basement slices and eventual abandonment of the external basal decollement postthermal peak deformation is characterized by the continued development of out of sequence thrusts which appear to young toward the hinterland north internal thickening by development of flattening cleavages or folding appears to be regionally insignificant when compared to that accomplished by the out of sequence faults this suggests that 1 an eroding and accreting thrust wedge may be able to maintain its critical shape by out of sequence thrusting and 2 this style of internal deformation is favored in belts dominated by relatively strong material e g basalt as in the cape smith belt

footwall involvement during arc continent collision ungava orogen northern canada geological evidence and theoretical models are presented to illustrate deformation paths followed by continental crust during underthrusting subduction and collision fieldwork in an early proterozoic collisional orogen indicates that a thrust belt and its autochthonous footwall basement are obliquely exposed in a crustal cross section with significant structural relief the collisional orogen records an evolution in the style and extent of involvement of the archaean footwall from 1 underthrusting without evidence of internal deformation 2 limited thrusting and ductile deformation of footwall rocks to 3 folding of the entire thrust belt and its footwall into cuspate lobate long wavelength structures the structural evolution occurred during hydration of basement rocks coeval with collision related metamorphism lithospheric strength profiles constrained by simple numerical heat conduction models suggest that the observed deformation path for the footwall crust was controlled by weakening due to heating of basement rocks and by changes in the relative strengths of different crustal layers the profiles predict a very weak lower crust and upper mantle at the time of footwall involved folding suggesting that the lower crust and upper mantle deformed by ductile flow concomitant with buckling of the stronger upper crust

structural analysis using thrust fault hanging wall sequence diagrams ogden duplex wasatch range utah detailed mapping and cross section traverses provide the control for structural analysis and geometric modeling of the ogden duplex a complex thrust system exposed in the wasatch mountains east of ogden utah the structures consist of east dipping folded thrust faults basement cored horses lateral ramps and folds and tear faults the sequence of thrusting determined by means of lateral overlap of horses thrust splay relationships and a top to bottom piggyback development is willard thrust ogden thrust weber thrust and taylor thrust major decollement zones occur in the cambrian shales and limestones the tintic quartzite is the marker for determining gross geometries of horses this exposed duplex serves as a good model to illustrate the method of constructing hanging wall sequence diagram a series of longitudinal cross sections that move forward in time and space and show how a thrust system formed as it moved updip over various footwall ramps a hanging wall sequence diagram also shows the complex lateral variations in a thrust system and helps to locate lateral ramps lateral folds tear faults and other features not shown on dip oriented cross sections

geological significance of seismic reflections in collision belts geological problems raised by the interpretation of deep seismic reflections in collision belts result from the structural complexity of the crust and the lack of surface control and well documeted references some of these problems are illustrated by examples coming from various tectonic settings in france and adjacent areas and tentative interpretations are proposed

metamorphic thermal and tectonic evolution of central new england mountain skitchewaug and big staurolite nappes reactivation of a new detailed tectonic model is presented for the acadian orogenic this fabric during thrusting is recorded in some rocks of the big belt of central new england vermont and new hampshire that staurolite nappe by rotated garnets that grew during near isothermal accounts for a wide range of petrological and structural observations loading only the sillimanite isograd crosses the fall mountain three belts are considered the eastern vermont merrimack and skitchewaug nappe boundary metamorphic breaks across the skitintervening bronson hill belts specific observations in eastern chewaug big staurolite nappe boundary at the base of the big vermont that are accounted for in the model include the following staurolite nappe and at the margin of the keene and alstead domes p t paths are clockwise with maximum pressures near the athens require post metamorphic thrusting when p t conditions were in chester and strafford domes of 8 11 kbar but with maximum the greenschist facies these observations can be explained by a pressures decreasing to 3 5 kbar at the boundary with the bronson relatively simple model involving in sequence thrusting from east to hill belt differential exhumation of the vermont domes relative to west commencing in central new hampshire at 400 410 ma the rocks in easternmost vermont is required by the recorded differences preservation of the low grade belt along the vermont new hampin maximum pressure 5 6 kbar 15 20 km and the present day shire border requires that crustal thickening in vermont was not geographical separation 7 10 km specific observations in new caused by emplacement of new hampshire nappes onto eastern hampshire that are explained include the following p t paths in vermont and that the nappes of western new hampshire had time the merrimack belt are counter clockwise with maximum pressures to cool before final juxtaposition against the low grade belt cooling of 4 5 kbar and are related to high regional heat flow and heat ages constrain this final juxtaposition to have occurred in the transfer by early acadian plutons p t paths in the bronson hill carboniferous suggesting that the acadian was a prolonged event belt are intimately associated with structural position an early spanning as much as 100 myr contact metamorphism is evidenced in the skitchewaug and fall mountain nappes near contacts with the early acadian bethlehem gneiss 400 410 ma peak metamorphic temperature rises upwards in the nappe sequence an inverted metamorphic sequence

the andean type gangdese mountains paleoelevation record from the paleocene eocene linzhou basin abstract paleoelevation reconstruction using oxygen isotopes is making a significant contribution to understanding the cenozoic uplift of the himalayas and the tibetan plateau this paper presents new oxygen and carbon isotopic compositions from well dated tertiary paleosols lacustrine calcareous carbonates and marls from the nianbo 60 54 ma and upper pana formations 51 48 ma of the linzizong group in the linzhou penbo basin the sediments of the nianbo formation which are 180 m thick were deposited in alluvial fans braided rivers fan deltas and on nearshore to offshore lacustrine settings whereas those of the upper pana formation are 100 m thick and are comprised predominantly of proximal alluvial fan and braided river deposits correlations between the lithofacies and stable isotopic compositions suggest that the basin was mainly a hydrologically open environment it is confirmed that the 18 o c and 13 c c values from nianbo and pana formations have not yet been reset by late stage diagenesis based on petrographic examination oxygen isotope of the fossil ostracodes and tectonic deformation of strata the paleoelevations are reconstructed using the corrected most negative paleosurface water 18 o psw values these imply that the linzhou area had attained an elevation of 4500 400 m during the period of the indo asian collision i e achieved a near present elevation and may form an andean type mountain range stretching the gangdese arc before collision the gangdese mountains probably maintained high elevations since at least the paleocene and could play a crucial role in the climate change in the interior of the tibetan plateau during the early cenozoic the paleogeomorphic scenario of the eocene tibet is proposed to exist at two high mountains in excess of 4500 m that sandwiched a low elevation basin

a geophysical study of the earth s crust in central virginia implications for appalachian crustal structure a regional seismic reflection line i 64 across the virginia piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region a highly reflective upper crust an allochthonous blue ridge province underlying thrust sheets including the blue ridge master decollement and a basal decollement at a depth of about 9 km 3 s are confirmed on the seismic data immediately east of the blue ridge province appalachian structures plunge to as much as 12 km 4 s depth the evington group hardware terrane and chopawamsic metavolcanic rocks carolina terrane crop out in the piedmont province and numerous eastward dipping reflections originate from these rocks in the subsurface these eastward dipping reflectors overlie a gently west dipping 10 15 highly reflective zone that varies in depth from 1 5 s 4 5 km beneath the goochland terrane to 4 s 12 km beneath the rocks of the evington group some of the overlying eastward dipping reflections apparently root in this zone the zone may include decollement surfaces along which the overlying rocks were transported relatively few reflections originate from within autochthonous grenville basement at the western end of the profile the goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the chopawamsic metavolcanic rocks a broad zone of east dipping 20 45 reflections bounds the goochland terrane on the east these reflections may originate from deformation zones and continue to moho depths they appear to be correlative with similar events seen on other appalachian lines the pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent piedmont gravity high suggest continuity of crustal features along the length of the appalachians a major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in virginia that is the appalachian gravity gradient and the piedmont gravity high the crust thins from about 52 km beneath the appalachian mountains to about 35 km beneath richmond virginia and then rethickens by up to 10 km beneath the zone of east dipping reflections mylonites east of richmond the i 64 seismic data also contain a sequence of reflections at about 9 12 s indicative of lower crustal layering the base of this zone of reflections coincides almost exactly with the mohorovicic discontinuity interpreted from earlier refraction work the layering extends about 70 km west from richmond virginia and is interpreted as a lower crustal transition zone that is believed to persist across most of virginia

a geodynamic framework for interpreting crustal scale seismic reflectivity patterns in compressional orogens summary 2 d plane strain finite element models are used to calculate crustal deformation patterns within model compressional orogens and the results are compared with natural examples the fundamental driving mechanism for model deformation is provided by asymmetric detachment and underthrusting of mantle lithosphere with deformation being rooted at the stress singularity where the mantle detaches deformation steps up into the overlying crust and spreads laterally according to the rhcological properties assumed for the crust the crust is modelled using plastic frictional and viscous rheologies and incorporates the effects of compositional layering and variable geothermal gradients overall deformation is determined by balancing the internal strength of the crust against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming orogen the amount of convergence in the models is limited to 150 km or less and so the results are not strictly comparable with larger orogens involving greater convergence nevertheless larger orogens seem to evolve through stages that may be understood in terms of the models provided that consideration is confined to smaller scale components that are limited in time and space the continuum deformation calculated by the finite element models is qualitatively converted to equivalent pattcrns of discrete faults and shear zones by associating these discrete features with the areas of highest cumulative strain using the results of sandbox experiments as guides significant styles of deformation include 1 coupling between step up shears and subhorizontal detachments at one or more levels 2 localization of upper crustal deformation by subduction of the lower crust and 3 pinning of deformation by surface denudation a partial catalogue of such model styles is presented in which the strength of basal coupling between crust and mantle the number of rhcological layers within the crust the thickness of subducted lower crust and the extent of surface denudation are varied systematically compressional orogens with a wide range of ages are characterized by crustal scale seismic reflectivity similar to patterns of fault shear zones represented in this catalogue of styles this similarity suggests that crustalscale seismic reflection data can be used to constrain the dynamic processes involved in compressional orogcnesis

crustal profile of mountain belt cocorp deep seismic reflection profiling in new england appalachians and implications for architecture of convergent mountain chains relatively shallow reflections from beneath the taconic allochthon and green mountain anticlinorium are suggested to mark a zone of detachment that carries precambrian grenville basement of the green mountains in its upper plate the data suggest that tectonic thickening of the shelf sedimentary and synorogenic flysch sequence beneath the taconic allochthon has occurred and that major thrust fault systems champlain thrust of western vermont which imbricate shelf lithologies project downplunge into the seismic profile eastward dipping reflections beneath the east flank of the green mountains appear to project across the connecticut river valley into new hampshire because some of these reflections can be projected into an exposed zone of probable thrust imbricated precambrian grenville basement and younger metasediments there is a strong implication that similar rock types exist in the deeper crust beneath new hampshire the gross geometry of the eastward dipping zone is a large thrust ramp that facilitated faulting of continental basement in the green mountains to higher structural levels thus suggesting the anticlinorium is a large hanging wall structure or ramp anticlinorium highly deformed satellitic massifs of grenville basement such as the chester dome and sub silurian and silurian devonian rocks of the bronson hill anticlinorium and merrimack synclinorium are probably transported along intracrustal detachments as well data suggest the possiblity of a tectonically buried continental margin transition zone much like that interpreted for the southern appalachians this zone would have been preserved from prior rifting of the proto atlantic ocean and its inherited geometry could have controlled subsequent convergent deformation during appalachian orogenesis although regional deep seismic reflection profiling has not been carried out across the alps existing data imply lateral changes in crustal reflection character similar to those observed in the appalachians deep seismic reflection data from convergent mountain belts raises the possibility of such preserved transition zones beneath transported rocks and provides an alternative interpretation to lithosphere penetrating sutures in explaining lateral geologic and geophysical contrasts in the crust modified journal abstract

how flat is tibet high resolution digital topography three arc second grid for most of tibet provides new information to characterize the relief of the highest and largest plateau on earth the arid to semiarid central and northern part of the plateau interior has low relief average slopes of over 250 m windows and a mean elevation of 5023 m above sea level at moderate wavelengths of m relief is or less for most of tibet as opposed to the much higher relief of up to 6 km on the plateau edges where glacial and fluvial dissection is greater because of higher levels of precipitation the only faults manifesting significant topographic relief are the relatively small scale generally north trending graben systems primarily in southern tibet and several large scale fault systems near the edges of tibet the flatness of tibet implies that 1 there has been little deformation especially shortening of the uppermost crust north of the graben systems during the late cenozoic and 2 shallow crustal isostatic compensation has been acting to level the surface of the plateau

geological interpretation of dekorp 2 s a deep seismic reflection profile across the saxothuringian and possible implications for the late variscan structural evolution of central europe abstract a 250 km long deep seismic section the first of its kind in west germany s dekorp project is interpreted in terms of late variscan collisional tectonics the structural model used is based on the intracontinental thrust tectonics concept of modern collision orogens the crustal profile across the saxothuringian zone seems to indicate north facing ramps antiformal stacks and culminations within the crystalline basement of the internal mid european variscides that have been only partially overprinted by permo mesozoic crustal stretching

crustal laminations in deep seismic profiles in france and neighbouring areas summary remarkable crustal features appear on the ecors profiles carried out in northern france and the bay of biscay as well as on the swat profiles shot in the western channel and the celtic sea the most striking one is the occurrence of flat laminations in the lower crust dipping events and laminations are also present in the upper and lower crust especially in the swat profiles they can readily be related to tectonic events variscan in age some of them identified in the field the flat laminations in the lower crust are at first interpreted as resulting from delamination shearing magmatism and metamorphism at the crust mantle transition during the variscan orogeny this interpretation raises some difficulty concerning the space and time correlation of the laminations with the variscan orogeny they seem to have been emplaced after the permian triassic infilling of the plymouth bay basin and before the early cretaceous opening of the bay of biscay an early to middle jurassic age is suggested a period when large cratonic basins were formed without noticeable extension heat flow increase and magmatism are proposed as a second hypothesis for the formation of the lower crust laminations choosing between orogenic and non orogenic causes of these laminations will require further deep seismic profiles together with good velocity determination

climatic changes vs catastrophic events in lacustrine systems a geochemical approach abstract a geochemical study of oligotrophic lake sanabria nw iberian peninsula allows us to distinguish the geochemical response of a lacustrine system to catastrophic events and climatic events the main major elements analysed were k na mg ca ti al fe mn p and s the organic matter content in the sediment of lake sanabria is determined by the rainfall regime of the region periods when sediments were enriched in ti mg k and al are indicative of relatively wet paleoenvironmental conditions whereas intervals when sediments were enriched in s and p associated with organic matter are indicative of dry conditions a factor analysis of the geochemical data shows a progressive temporal displacements from organic rich conditions relatively high values of s and p to inorganic rich conditions relatively high values of ti mg k al and na maximal inorganic content occurred at the beginning of two discrete sedimentary levels the lol2 cal 1160 1230 yr ad and the lol1 cal 1510 1680 yr ad horizons the lol2 and lol1 horizons constitute two discrete events related to climatic changes during the beginning and end of the little ice age respectively in contrast the geochemical analysis provides evidence that the samples associated with the 1959 catastrophic event that occurred in the catchment area of lake sanabria do not follow the long term geochemical evolution paths of the climate driven system

seismic signature and tectonic cross section of the variscan crust in western france deep seismic reflection profiling ecors program vertical and wide angle survey coupled with field and subsurface geology are used to propose a complete nne ssw section of the variscan belt in western france from belgium to southern brittany on this 800 km long profile which probably crosses two paleozoic sutures the variscan belt appears as a broad fanlike orogen characterized by large 100 km northward and southward overthrusts with polyphase deformation and metamorphism the central part of the belt central armorican zone suffered a simpler dextral shearing parallel to the strike of the belt with only green schist metamorphism deep reflection profiling shows that all the large thrusts visible at the surface root deeply in the lower crust or even crosscut the moho in the same way all the large strike slip faults parallel to the belt appear at depth as narrow discontinuities which crosscut the whole crust and in some cases the moho the seismically well layered crust develops essentially at depth in the most deformed and metamorphic internal parts of the belt in contrast with the relatively transparent brabant foreland basement which escaped the variscan deformation the moho appears as a relatively flat boundary at an average depth of 35 km with different characters it is sharp and strongly reflective both by wide angle and vertical seismics below the internal thrust zones much less distinct in the central armorican zone and only defined by low frequency signals at wide angle below the brabant foreland all these characteristics led us to consider the layering of the lower crust and the moho itself as initially tectonometamorphic features essentially produced by shearing and crust mantle decollement during the variscan stacking of the crust by intracontinental lithospheric subduction

structural evolution of an a type subduction zone lofoten rombak area northern scandinavian caledonides the climactic phase of the caledonian orogeny in scandinavia involved collision of the baltic and greenland cratons in late ordovician to silurian time we infer that the collision resulted in large scale subduction of the leading edge of the baltic craton beneath an andean type greenland continental margin deep erosional levels in the lofoten rombak area norway sweden 69 n permit direct examination and evaluation of the structural evolution of a continental or a type subduction zone the early stages of collision involved en bloc underthrusting of the continental margin to depths of at least 30 km during this process deformational effects in the lower plate were restricted to its upper few hundred meters ultimately en bloc a type subduction should be limted by buoyancy effects as convergence continued the lower plate became involved in a series of imbricate thrusts with the same vergence as the intial subduction zone the temporal and spatial relationships between these basement involved thrusts are similar to those observed in foreland fold and thrust terrains field and geophysical data bearing on lower plate behavior elsewhere in the caledonldes and in other collisional orogens suggest that the structural features observed in the lofoten rombak area may be generally characteristic of a type subduction in continent continent collisional settings

continental collisions and seismic signature processes active during continental collisions and orogenic collapse create distinct structures which are imaged by reflection seismic profiling examples are shown and discussed

crustal structure of the external variscides in northwest spain from deep seismic reflection profiling abstract recent deep crustal seismic reflection profiling escicantabrica 1 provides a detailed image of the external parts of the variscan belt in northwest spain these seismic data together with surface geological information provide constraints on the crustal structure of the foreland thrust and fold belt cantabrian zone and the transition to the hinterland areas narcea antiform area the image in the cantabrian zone includes strong inclined reflections in the upper 5 s that correspond to mappable paleozoic rocks at the surface which have been deformed by thrusting and folding during the carboniferous some of the major thrust faults imaged show a ramp flat geometry at depth and merge into a highly reflective band between 5 and 6 s that corresponds to the cantabrian zone detachment the precambrian basement is almost transparent underneath the cantabrian zone detachment there are a few subhorizontal discontinuous reflections at 11 s that may correspond to the reflection moho at the transition to the hinterland the seismic profile includes short inclined reflections grouped in several reflective bands in the upper 6 s these correspond to the narcea antiform which comprises at the surface a stack of thrust sheets containing precambrian rocks the narcea antiform is placed above a crustal scale ramp imaged by strong w dipping reflections between 6 and 9 s these reflections merge upwards into the cantabrian zone detachment at 6 s and downwards into a reflective lower crustal zone located between 9 and 12 s these midcrustal reflectors are interpreted as a major variscan shear zone that marks the transition between the thin skinned tectonics of the external areas and the thick skinned tectonics of the hinterland areas the lower crust is highly reflective below the narcea antiform area and reflectivity fades out to the east beneath the foreland thrust and fold belt the different seismic responses at middle and lower crustal levels in the foreland and hinterland areas suggest that variscan tectonics strongly affected deep crustal levels in the hinterland areas whereas they remained undeformed beneath the foreland

a deep seismic profile of 800 km length recorded in southern queensland australia summary in 1984 the australian bureau of mineral resources and the geological survey of queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the eromanga basin to the beenleigh block east of the clarence moreton basin a relatively transparent upper crustal basement with an underlying more reflective lower crust is characteristic of much of the region prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the clarence moreton basin and the adjacent beenleigh block and provide some of the most interesting features of the entire survey a wide angle reflection refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the nebine ridge the only clear deep reflectors are interpreted as p to sv or sv to p converted reflections from a mid crustal boundary at a depth of about 17 km the combined nebine ridge data provide well constrained p and s wave velocity models of the upper crust and suggest a crustal structure quite different from that beneath the adjacent mesozoic basins

orogen scale decollements the continental lithosphere responds to stress by deforming as a generally layered medium deep seismic reflection data coupled with a variety of ancillary geological and geophysical data are interpreted to provide images of fault zones that tend to form moderately dipping ramp structures in mechanically rigid layers and flat detachments in mechanically weak layers this geometry is similar to ramp and flat structures observed at smaller scales in thrust and fold belts and leads to the interpretation that most orogens are underlain by orogen scale decollements in a manner that is analogous to so called thin skin deformation in sedimentary rocks decollements may occur within the crust for example near the base of a sedimentary section or in the middle crust near the moho in the subcrustal lithosphere or in the asthenosphere even intracratonic basement cored thick skin uplifts that occasionally occur in foreland regions such as the wyoming province are probably large crustal scale versions of ramp flat features observed in supracrustal rocks and are thus likely caused by the same fundamental tectonic processes

geophysical data reveal the crustal structure of the alaska range orogen within the aftershock zone of the mw 7 9 denali fault earthquake geophysical information including deep crustal seismic reflection magnetotelluric mt gravity and magnetic data cross the aftershock zone of the 3 november 2002 mw 7 9 denali fault earthquake these data and aftershock seis micity jointly interpreted reveal the crustal structure of the right lateral slip denali fault and the eastern alaska range orogen as well as the relationship between this structure and seismicity north of the denali fault strong seismic reflections from within the alaska range orogen show features that dip as steeply as 25 north and extend downward to depths between 20 and 25 km these reflections reveal crustal structures probably ductile shear zones that most likely formed during the late cretaceous but these structures appear to be inactive having produced little seis micity during the past 20 years furthermore seismic reflections mainly dip north whereas alignments in aftershock hypocenters dip south the denali fault is nonre flective but modeling of mt gravity and magnetic data suggests that the denali fault dips steeply to vertically however in an alternative structural model the denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the alaska range orogen modeling of mt data indicates a rock body having low electrical resistivity 10 xm that lies mainly at depths greater than 10 km directly beneath aftershocks of the denali fault earthquake the maxi mum depth of aftershocks along the denali fault is 10 km this shallow depth may arise from a higher than normal geothermal gradient alternatively the low electrical resistivity of deep rocks along the denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the after shock zone

heat flow and structure of the lithosphere in the eastern canadian shield twenty two new determinations of heat flow and radiogenic heat production in the superior and grenville provinces of the canadian shield are presented the new data and previously published measurements strongly constrain the thermal structure of the eastern canadian shield in the abitibi greenstone belt heat flow gradually increases from 29 mw m 2 near the grenville front to 44 mw m 2 east of the kapuskasing uplift this heat flow variation is interpreted in terms of crustal thickening and increased thickness of a tonalitic layer with average heat production of about 1 1 w m 3 this interpretation based on estimated heat production of major rock types in the region is consistent with crustal models derived from recent seismic reflection and refraction studies it also leads to an estimate of about 12 mw m 2 for the mantle heat flow beneath the area the average heat flow in the grenville province 41 10 mw m 2 is the same as that of the superior province this similarity and the lack of significant variation of heat flow across the grenville front indicate that the crust on both sides of the front has similar heat production and thus composition in the western part of the grenville province heat flow reaches high values in the vicinity of the boundary between the allochtonous polycyclic and monocyclic belts where enriched granitic plutons are found in the crystalline terranes in the central part of the grenville province heat flow and heat production are related to each other the parameters of the linear heat flow heat production relationship qr 30 2 in mw m 2 and d 7 1 1 7 km are close to those of the much younger appalachian province implying that the higher appalachian heat flow is due solely to higher heat production in the upper crust the data provide no evidence for variation of mantle heat flow between the superior grenville and appalachian provinces whose tectonic ages range between 2700 and 400 ma the small value of the mantle heat flow about 12 mw m 2 implies that the depth to the 450 c isotherm which controls the effective elastic thickness of the lithosphere is very sensitive to crustal heat production

sensitivity of eurasian climate to surface uplift of the tibetan plateau experiments with climate models are used to illustrate how earth s climate could be expected to change if surface uplift occurred the experiments contrast two climate simulations one with all continents at sea level and one with present day mountains and plateaus the simulations in the eurasia sector show that surface uplift increased elevation produces marked increases in the intensity of summer and winter monsoons of asia with development of wetter climates to the south and east of the tibetan plateau and drier climates to the north and west of the plateau there is a general cooling in northern polar latitudes comparison of simulations from two different models shows that the magnitude of the simulated changes depends upon how the models calculate the land surface hydrology and the ocean temperature the simulated changes produced by surface uplift agree in a general way with many geologic indicators of late cenozoic climate change the results suggest that surface uplift of mountains and platea

some consequences of palinspastic reconstruction in the southern appalachians palinspastic reconstructions of alleghanian compressive structures in the valley and ridge place severe constraints on acceptable models of crustal structure in the southern appalachians these reconstructions in conjunction with available deep seismic reflection data imply that the carolina slate belt arc was transported westward relative to north american cratonal basement by at least 120 km during the late paleozoic at present it is not possible to determine unequivocally whether this juxtaposition took place along subhorizontal intracrustal detachments or whether it was the result of the steepening of a pre existing detachment surface to produce a high angle lithosphere penetrating boundary

late cretaceous unroofing of the white mountains new hampshire usa an episode of passive margin rejuvenation the growing recognition that many passive margins have experienced periods of post rift uplift has raised new questions about the nature of the tectonic processes that drive such uplift the eastern north america margin is of particular interest because it is a classic atlantic style margin that has long been suspected of tectonic rejuvenation yet the timing of post rift uplift events remains elusive to address this question we present apatite u th he and fission track ages from a 1300 m vertical transect in the white mountains of new hampshire usa including a 910 m deep bore hole thermal modeling suggests a period of accelerated denudation from ca 85 to 65 ma this timing is broadly similar to the timing of accelerated exhumation and compression on passive margins elsewhere in the atlantic raising the question of whether a common mechanism such as a change in lateral stresses changes in mantle convection or climate change could be responsible

a new approach to stable isotope based paleoaltimetry implications for paleoaltimetry and paleohypsometry of the high himalaya since the late miocene the change in oxygen isotopic composition of precipitation is modeled using equilibrium fractionation during rayleigh distillation linked to the thermodynamics of atmospheric ascent and water vapor condensation the primary controllers of the vertical variation in isotopic composition with elevation are the low elevation temperature and relative humidity as these control the vertical distribution of condensation an empirical fit of precipitation versus model condensation based on alpine stations is derived this fit is represented in the model as the weighted mean composition of condensation within a 1000 m thick air parcel 1500 500 m above the ground surface and is used for all other regions comparison of model versus observed modern precipitation reveals a close fit particularly of more highly elevated sites comparison of modern waters in the himalayas and southern tibet with model predictions particularly as revealed by comparison of observed and predicted hypsometry provides additional support to the validity of the model finally application of this model to estimates of paleo waters in the himalayas and southern tibet reveals that this region had already achieved its present hypsometry by the late miocene about 10 ma ago fl 2001 published by elsevier science b v

geodynamic models of convergent margin tectonics transition from rifted margin to overthrust belt and consequences for foreland basin development a quantitative geodynamic model for overthrusting of a passive continental margin during attempted continental subduction demonstrates the mechanical and thermal coupling between overthrust loads the lithosphere and the associated foreland basin the model treats the lithosphere as a two dimensional nonuniform elastic plate whose strength is controlled thermally the thermal and flexural evolution of a margin is followed from initial rifting and passive margin development through overthrusting and foreland basin deposition to postdeformational erosion our models indicate the following 1 foreland basin geometry is relatively insensitive to the age of the preexisting passive margin prior to overthrusting sensitivity decreases the farther the overthrust load is empl ced onto the craton 2 foreland basin depth is strongly influenced by the size of the overthrust load maximum depths in excess of 10 km 6 mi can occur given overthrust topography similar to the present day himalayas 3 a few tens of kilometers of postdeformational erosion can occur across topographically high orogenic belts before moderate topography is attained very little of this eroded material is deposited in the foreland basin most must bypass the basin because it also is subjected to erosion and associated flexural uplift 4 thick overthrusts as interpreted from cocorp profiling are consistent with low topographic expression because they were emplaced on preexisting oceanic and stretched continental lithosphere

some comparisons of the structure and evolution of the southern appalachian ouachita orogen and portions of the trans european suture zone region abstract many aspects of the tectonic history of the appalachian ouachita orogen and the trans european suture region are similar in a tectonic sense they are geographically linked because at the end of the paleozoic one could travel from texas to poland following a continuous orogenic belt much of each of these orogenic belts is however buried by younger strata and thus geophysical studies are a key element of efforts to better understand them many more geophysical data are needed to provide a comprehensive picture of the deep structure but in several areas new models of the lithospheric structure are emerging in this paper we review some of these models and introduce two new ones for the southern appalachians in addition we explore the implications of these models for the evolution of these orogenic belts drawing attention to similarities and differences in their structure and evolution the appalachian and caledonian orogens represent temporally parallel development along irregular late proterozoic continental margins but the crustal structure is different because of different accretionary elements and complications in the geometry of the original margins lateral transport via thrusting can confuse efforts to tie geologic structures identified on the surface with deep structures for example in the appalachian ouachita orogen the thrust related transport distance for major features such as the blue ridge piedmont sheet and the benton uplift is at least 200 km on a crustal scale deformation in the appalachian ouachita orogen varies greatly in style and intensity in the ouachita orogen the late proterozoic continental margin seems to be preserved while in the northern appalachians crustal scale deformation is intense along the us appalachians the major crustal structure variation is the thin dense crust of avalonia abutting the relatively thick crust of laurentia along the ouachita orogen several large crustal blocks can be identified that are unlike anything seen along the appalachians

himalayan tectonics weathering processes and the strontium isotope record in marine limestones the time evolution of the isotopic composition of seawater strontium the ratio of strontium 87 to strontium 86 over the last 500 million years has the form of an asymmetric trough the values are highest in the cambrian and recent 0 7091 and lowest in the jurassic 0 7067 superimposed on this trend are a number of smaller oscillations consideration of the geochemical cycle of strontium and the dynamics of weathering shows that only himalayan style continental collisions can influence the isotope ratio on the scale observed the contemporary himalayan orogeny is by far the largest since the late precambrian pan african event that produced the high in the cambrian

climate palaeohydrology and land use change in the central iberian range over the last 1 6 kyr the la parra lake record a multiproxy study of a 7 m long sedimentary sequence from lake la parra 39 50 948 1 52 1014 m a s l supported by 11 14c ams and 210pb 137cs dates provides a robust high resolution hydrological and environmental variability record for the last 1600 years of the las torcas sinkhole complex in the central iberian range the succession of depositional environments in lake la parra sinkhole is controlled by both changes in the regional water table and by the balance between sedimentary input through ephemeral creeks and in lake production of carbonates and organic matter although synergetic links with climate are likely phases of increased sediment delivery to the lake at c ad 500 700 c ad 1000 ad 1450 1500 ad 1550 650 and since 1700 till recent times are driven primarily by human impact in the watershed prior to c ad 300 the sinkhole was dry then became a lake at the end of the roman period ad 350 when the doline was flooded and it has not dried out during the last 1600 years moderate

a comparison of reflection and refraction processing and interpretation methods applied to conventional refraction data from coastal maine abstract in 1984 the u s geological society collected 850 km of refraction data in lines parallel and perpendicular to regional strike of the appalachian orogen in maine and southeastern quebec a 100 km long strike line in coastal maine west of penobscot bay is here interpreted on the basis of both refracted and reflected phases techniques used in seismic reflection processing including normal movement velocity corrections and common mid point stacking were applied to the refraction data in order to enhance the visibility of reflected phases the geometries of the most laterally continuous reflections as defined by these techniques can be correlated with reflections seen on a crossing near vertical reflection profile conventional ray tracing analysis of refracted and wide angle reflected phases in the refraction data constrains the velocity structure to about 14 km depth and provides evidence for high veiocity material 6 5 km sec between 4 and 7 km depth and for lower velocities

deep crustal structure beneath the central and inner west carpathians abstract the 150 km long deep seismic transect 2t crossing the central part of the slovakian west carpathians has revealed an important whole crustal seismic suture zone dipping to the south this zone is dominated by six large and several minor reflection events because of our good knowledge of the surface geology and a large number of comparative studies of the eastern alps it is clear that the upper reflections are due to overthrust sheets that originated during compression of the apulian passive margin tectonically deeper reflections are interpreted as being due to ligurian oceanic igneous and sedimentary rocks sheared in ductile domain during continental collision with the brianconnais passive margin and continental masses north of the central apulian west carpathians cwc behind a significant strike slip fault are the cenozoic structures of the pieniny klippen belt and the magura rhenodanubian flysch south of the cwc the inner tethyan west carpathians iwc are thrust over the cwc the iwc originated during the late jurassic early cretaceous closure of the tethyan ocean and the seismic image of the iwc is different from the highly reflective cwc suture these three pre neogene nappe systems amalgamated as one overthrust system during the neogene subduction of the moldavian sea as the moldavian accretionary wedge the vepor gemer block of the west carpathians where the cretaceous suture is observed has been untouched by any major cenozoic deformation and has remained relatively rigid this unique structure therefore enables us to observe seismically the cretaceous suture zone which elsewhere in the european alpine belt has been almost completely destroyed more general data about the beginning of subduction and subsequent collision processes are presented significant compression and partial subduction of the apulian upper plate passive margin lasted from the hauterivian to the beginning of the turonian for about 25 30 ma subduction of the ligurian ocean occurred from turonian to campanian times after the early campanian the collision of the apulian and brianconnais plates took place

pre syn and post imbrication deformation of carbonate slices along the southern quebec appalachian front implications for hydrocarbon exploration thrust imbricated shelf carbonate slices form a wide but poorly understood part of the southernmost quebec appalachian structural front comprehensive structural analysis of two slices exposed at surface the saint dominique and philipsburg slices shows that pre and post imbrication structures are important in defining the final architecture of the slices the dominant structural style is characterized by thrusts and associated asymmetrical folds tear faults oblique ramps and incipient backthrusts developed during wnw ese shortening a forward breaking piggy back sequence of thrusting is recognised as well as minor out of sequence thrusting the complexity and diversity of contractional structures is directly influenced by lithology bed thickness and shale content bedding parallel slip planes are important in the concentration activation and reactivation of deformation in that there are the loci for veining faulting and folding recognition of lithostructural units provides guidelines for t

tibet uplift and erosion abstract the 5 km high tibetan plateau is an outstanding topographic feature on the earth today its horziontal extent elevation and location cause significant effects on modern atmospheric circulation and climate so the history of uplift of the surface of tibet is linked to cenozoic climate changes at local regional and perhaps global scales geological and geophysical studies of the plateau are contributing data on the present and past deformation of the tibetan lithosphere that has formed the plateau primarily during the cenozoic the principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history different parts of tibet probably had different uplift histories but presently available data are not sufficient for distinguishing many contrasts in one scenario cretaceous and early cenozoic north south distributed shortening of tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early miocene thinning of the high density mantle portion of the lithosphere during the miocene may then have allowed the thick tibetan crust to rise close to its present elevation perhaps higher before 8 ma since then slow east west extension of tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late cenozoic erosion of tibet unlike narrow mountain belts has been unable to match the uplift of a broad plateau orographic precipitation and efficient river networks concentrate erosion on the edges while the interior is protected from significant erosion despite its lofty elevation the southern edge of the plateau the himalaya has suffered a minimum of 25 km of denudation since the miocene while central tibet shows little or no sign of major erosion since that time the gangdese arc in southern tibet was rapidly eroded during the mid miocene when 4 km of rock apparently were removed from the surface as shown by mineral cooling ages this pulse of erosion was probably caused by a combination of local thrust system movement and changes in base level and precipitation due to relative elevation changes between the gangdese and the himalaya to the south the modern long wavelength flatness of tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface

composition of the crust in the grenville and appalachian provinces of north america inferred from v p v s ratios we use the ratios between p and s wave velocities vp vs derived from seismic refraction data to infer the composition of the crust in the grenville and the appalachian provinces of north america the crust exhibits vp vs increasing with depth from 1 64 to 1 84 there is a clear distinction between the grenville province average vp vs 1 81 and the appalachian province average vp vs 1 73 which persists at all depths the boundary between these provinces is east dipping extending for 100 km east of the champlain thrust in the appalachian province the increase in vp vs ratios with depth from 1 67 to 1 74 0 02 may reflect a normal decrease of silica content in the continental crust in the grenville province beneath the central granulite terrane an anomalous vp vs ratio of 1 82 0 02 is observed extending to a depth of 10 km this correlates with the abundance of ca plagioclase in the marcy anorthosite at greater depth 15 20 km where seismic lamination and high electrical conductivity is observed vp vs is 1 84 0 02 and correlates with the tahawus complex a layered mafic intrusion within the 25 km thick lower crust of the grenville province the vp vs is 1 84 0 02 and p velocity is 7 0 0 1 km s which are typical for plagioclase bearing rocks gabbro norite the high vp vs ratio in the grenville province has not been reported in crust of any other age since the grenville province contains 75 of the world s known anorthosites high vp vs ratio is related to high plagioclase we suggest that the composition of the grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid proterozoic

dip domain method applied to the mesozoic connecticut valley rift basins this paper presents a new approach to structural analysis of u s east coast mesozoic basins a contoured compilation of numerical values of dip data for the connecticut valley rift basins reveals distinct domains related to their broader tectonic development additional more standard geologic constraints are then used to interpret the dip domain patterns most of the tilt deformation involved and hence postdated early jurassic basaltic flow units and overlying jurassic thick alluvial deposits normal drag characterizes the attitude of mesozoic strata along the western border fault of the main hartford basin whereas reverse drag is common along its larger listric eastern border fault strata within the basin mostly along its eastern margin include several subgrabens and or strongly tilted zones associated with early subbasins marked by local changes in strike of the early border fault the meriden cross graben may be a similar structure that extended much farther southwestward across the basin within the hartford basin linear domains of steeper dips are interpreted as surface expressions of the edges of a 5 to 15 km wide late stage central graben that trends n15 e diagonally across the basin possibly a reflection of very late strike slip stress systems

application of revised ray tracing migration to imagine lateral variations of seismic fabric corresponding to different tectonic styles in the northern apennines abstract migration of zero offset seismic sections of deep crust can be done with methods based on ray tracing we modify the classical ray tracing migration method rtm introducing a consistency check to control whether back propagated rays satisfy the condition of strict normal incidence at the migrated reflector a synthetic test shows the effectiveness of the method in particular the control of normal incidence allows elimination of physically inconsistent reflectors from the migrated section then rtm is applied to a crustal seismic profile acquired in central italy using a velocity model obtained from wide angle data that reproduces the gross structures of the apenninic crust the lateral variation of the seismic fabric shown from the migrated section reveals the presence of coexisting extensional and compressional tectonic regimes kinematic diffraction modelling gives additional information about both the distribution of seismic velocities and major active geodynamic processes in the upper lithosphere the migrated section supports the subdivision of northern apennines in two tectonic regions a stretched upper plate tuscany and northern thyrrenian supported by a rise of the asthenosphere and a downwarped lower plate adria subducted below the mountain belt

lithoprobe east onshore offshore seismic refraction survey constraints on interpretation of reflection data in the newfoundland appalachians abstract combined onshore offshore seismic refraction wide angle reflection data have been acquired across newfoundland eastern canada to investigate the structural architecture of the northern appalachians particularly of distinct crustal zones recognized from earlier lithoprobe vertical incidence studies a western crustal unit correlated with the grenville province of the laurentian plate margin thins from 44 to 40 km and a portion of the lower crust becomes highly reflective with velocities of 7 2 km s in central newfoundland beneath the central mobile belt the crust thins to 35 km or less and is marked by average continental velocities not exceeding 7 0 km s in the lower crust further east in a crustal unit underlying the avalon zone and associated with the gondwanan plate margin the crust is 40 km thick and has velocities of 6 8 km s in the lower crust explanations for the thin crust beneath the central mobile belt include 1 post orogenic isostatic readjustment associated with a density in the mantle which is lower beneath this part of the orogen than beneath the margin 2 mechanical thinning at the base of the crust during orogenic collapse perhaps caused by delamination and 3 transformation by phase change of a gabbroic lower crust to eclogite which seismologically would be difficult to distinguish from mantle except for a single profile in western newfoundland velocities in the crust are of typical continental affinity with lower crustal velocities less than 7 0 km s this indicates that there was no significant magmatic underplating under the newfoundland appalachians during mesozoic rifting of the atlantic ocean as proposed elsewhere for the new england appalachians a mid crustal velocity discontinuity observed in the newfoundland region does not coincide with any consistent reflection pattern on vertical incidence profiles however we suggest that localized velocity heterogeneities at mid crustal depths correspond to organized vertical incidence reflections

late holocene erosion in nw anatolia from sediments of lake manyas lake ulubat and the southern shelf of the marmara sea turkey abstract this paper deals with modern and ancient sedimentation in fresh water lakes and the marine shelf of the southern marmara region nw anatolia turkey most of the information has been obtained from monitoring of suspended load discharged into two lakes manyas and ulubat in the last 45 years and from 8 to 11 m thick lacustrine sediments in addition to radiocarbon dated shelf sediments this allows a holistic approach to the drainage basin denudation over time the results show that the sedimentation rates in the lakes were low 0 22 cm year 1 from 4000 to 2000 years bp and then they increased 0 29 cm year 1 up to sub recent times and reached 0 44 cm year 1 in the last century it is suggested that deforestation created high rates of sedimentation in the basins and or strong denudation of the region during the late holocene this study also shows that for shallow freshwater lakes the calculation of sedimentation rates must include fine particles lost by the outlets and coarse grained bed load deposited on their shores in addition a high rate of sedimentation has been created by short but repetitive intense depositions during the last century particularly during the last 45 years the rate of sedimentation or denudation has increased dramatically in nw turkey the two world wars and mismanagement of the land had important local effects by increasing deforestation and resulting in the present erosive conditions

crustal structure and evolution of the trans hudson orogen results from seismic reflection profiling a 400 km long deep seismic reflection transect across northeastern montana and northern north dakota reveals the crustal scale structural fabric of the early proterozoic trans hudson orogen beneath the williston basin comparison with deep seismic reflection data across the canadian portion of the same orogen 700 km to the north reveals first order similarities in crustal architecture but documents significant along strike variation in orogenic evolution both transects display a broad crustal scale antiform axial to the orogen in the north geologic data suggest that this antiform is cored by an archean microcontinent in the south west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the archean wyoming craton within the wyoming craton the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the early proterozoic trans hudson orogen as interpreted from potential field and drill core data on the basis of subsurface geochronologic data from the crystalline basement and by analogy with the glennie domain within the exposed trans hudson orogen in canada we suggest that the southern antiform is cored by an archean crustal fragment that was caught up in the terminal collision of the wyoming and superior cratons during hudsonian orogeny the eastern side of the trans hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections we interpret the easterly dip of these reflections as evidence that the superior province was thrust westward over the interludes of the orogen during terminal collision although juvenile early proterozoic terranes characterize the exposed segment of the trans hudson orogen in canada limited drill core information within the dakota segment of the orogen shows a predominance of granulitic archean age crust this difference in basement lithologies along strike within the orogen may indicate the following either juvenile crust comparable to that exposed in the northern trans hudson was never present in the south or it was removed by progressive over thickening erosion and or faulting postorogenic extensional collapse may be responsible for preservation of juvenile terranes in the north

geochemical evidence supporting t c chamberlin s theory of glaciation in 1899 t c chamberlin proposed that the co sub 2 content of the atomsphere decreased during times of enhanced continental erosion ultimately resulting in glacial epochs he ascribed the increase in the rate of chemical weathering relative to the rate of supply of co sub 2 from earth s interior to increased orogenic activity and globally higher average elevations which promoted rapid chemical erosion of silicates the oceanic record of strontium isotopes preserved in marine sediment supports his suggestion that glacial climates during the phanerozoic are in part linked to increases in the rate of global chemical erosion relative to outgassing from earth s interior further the close correspondence of the major tectonic episodes of the late cenozoic to times of increased continental erosion and glaciation suggests that chamberlin s hypothesis of the cause of glacial periods should be revived

thrust tectonics and crustal shortening in the external french alps based on a seismic cross section abstract duplex geometry has an important bearing on understanding the development of orogenic zones based on a published seismic section through the french alps the external massifs are considered to form a duplex structure as a result of basement thrusting the duplex is restored to a uniform basement thickness giving a model estimate of shortening of 45 km the interpretation of this figure in terms of early alpine crustal thinning is discussed

huascaran δ18o as an indicator of tropical climate during the last glacial maximum the interpretation of the isotopic composition of andes tropical mountain glaciers is reconsidered using a simple rayleigh distillation model it is shown that andes snowline depression and huascaran ice core 18o are consistent with a 3 c drop in sea surface temperature during the last glacial maximum accompanied by an increase from 40 at present to 60 during the lgm in the annual mean proportion of atmospheric water lost to continental runoff broecker s interpretation of huascaran 18o as a paleo hygrometer is discussed

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

marsh benthic foraminifera response to estuarine hydrological balance driven by climate variability over the last 2000 yr minho estuary nw portugal abstract a high resolution study of a marsh sedimentary sequence from the minho estuary provides a new palaeoenvironmental reconstruction from nw iberian based on geological proxies supported by historical and instrumental climatic records a low salinity tidal flat dominated by trochamminita salsa haplophragmoides spp and cribrostomoides spp prevailed from ad 140 1360 roman warm period dark ages medieval climatic anomaly this sheltered environment was affected by high hydrodynamic episodes marked by the increase in silt clay ratio decrease of organic matter and poor and weakly preserved foraminiferal assemblages suggesting enhanced river runoff the establishment of low marsh began at ad 1380 this low salinity environment marked by colder and wet conditions persisted from ad 1410 1770 little ice age when foraminiferal density increased significantly haplophragmoides manilaensis and trochamminita salsa mark the transition from low to high marsh at ad 1730 since ad 1780 the abundances of salt marsh species jadammina macrescens trochammina inflata increased accompanied by a decrease in foraminiferal density reflecting climate instability when droughts alternate with severe floods sw europe marsh foraminifera respond to the hydrological balance controlled by climatic variability modes e g nao and solar activity thus contributing to the understanding of ne atlantic climate dynamics

limited basement involvement in caledonian deformation hinnoy nnorth norway and tectonic implications abstract structural and petrographic relationships on east hinnoy north norway indicate that the caledonian nappes which are exposed directly to the east were emplaced over an older precambrian crystalline terrane lofoten terrane while at amphibolite facies metamorphic conditions caledonian structural and metamorphic effects disappear structurally downward away from the basal thrust of the nappe stack the limited involvement of the basement in caledonian deformation is explained by the limited availability of water which was introduced from external sources the external water source is hypothetically the autochthonous and allochthonous cover and the precambrian austerfjord group metasedimentary rocks within the basement which underwent prograde metamorphic dehydration during caledonian orogenesis the mechanisms by which addition of water concentrated strain in the basement probably include reaction induced ductility and hydrolytic weakening of the constituent silicate minerals there are two main tectonic implications 1 in collisional metamorphic belts the lithosphere of the underthrusted plate may remain rigid below 15 25 km depth and 2 the common phenomenon of detachment of crystalline thrust sheets at mid crustal levels may be controlled in some cases by the limited access of water to pre existing crystalline rocks rather than by thermal structure

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

geochemical evidence c n and pb isotopes of recent anthropogenic impact in south central chile from two environmentally distinct lake sediment records in this paper we compare the elemental and isotopic c n pb geochemistry of lake sediments from two contrasted environments in south central chile the first lake laguna chica de san pedro lcsp is situated in the urbanised area of the biobio region 36 s the second lake lago puyehue 40 s is located 400 km to the southeast of lcsp and within an andean national park our aim is to identify environmental impacts associated with increasing industrial activities and land degradation during the last 150 a in lcsp shifts in c n atomic ratios 13c and 15n from 1915 1937 to the late 1980s are attributed to successive land degradation episodes in the lake watershed based on a pb isotopic mixing model we estimate that up to 20 of lead in lcsp sediments is supplied from urban atmospheric pollution by contrast human impact in the watershed of lago puyehue is very limited we observe no change in organic geochemistry during the last 150 a and lead contamination remains lower than 5 even during the last decades although contamination levels are much higher in lcsp than in lago puyehue a peak in anthropogenic pb is recorded during the same period 1974 1976 at both sites this maximum contamination level is consistent with increased industrial activity in the vicinity of concepcion copyright 2010 john wiley sons ltd

crustal velocity and moho topography in central new hampshire the white mountain batholith in central new hampshire is a major plutonic topographic and gravimetric feature superimposed on the ne sw trending regional pattern of accreted terranes in northeastern north america compressional and shear wave arrival times from a two dimensional seismic array experiment are used to constrain a one dimensional crustal velocity model and moho topography for central new hampshire our deployment a piggyback on the ontario new york new england refraction experiment onynex recorded 10 explosive sources using 20 three component digital seismometers the crust can be modeled as 34 5 km thick and with three layers having linear gradients without midcrust discontinuities for this model p velocities are 6 1 6 2 km s in the upper crust and 6 2 6 8 km s in the lower crust s velocities are 3 5 3 7 km s in the upper crust and 3 7 3 9 km s in the lower crust because moho reflections comprise our principal constraints on lower crustal velocities there is a tradeoff between lower crustal velocity and crustal thickness in particular an average crustal thickness of 39 40 km reported elsewhere in new england is not precluded by our data the poisson s ratio for the crust varies between 0 23 0 26 in the lower range of values reported for crustal minerals our dataset is consistent with 2 lateral variation in the upper crust underlying the new hampshire and white mountain plutonic series but larger variations deeper than 15 km if modeled in terms of moho topography pmp traveltime residuals and pmp p residuals suggest thicker crust 36 37 km under the white mountain batholith with thinner crust 33 34 km to the east and southwest of the white mountains such topography of the moho correlates well with both the surface elevation and the bouguer gravity anomaly in the region gravity data do not favor the simplest alternative model a thick low velocity granitic lower crust beneath the batholith using a simplified density model roughly 30 35 of the gravity anomaly in the white mountain region can be modeled as the result of the inferred moho depression requiring only a 5 km low density root for the white mountain batholith the short wavelength of the moho topography is consistent with a very low flexural rigidity for the lithosphere 1021 nt m suggesting that the plutons were intruded into a weak lithosphere a rapid transition from positive to negative pmp residuals falls along a line that parallels the onshore extension of the kelvin seamounts and may correspond to the line of weakness hypothesized by sykes 1978 to have been tectonically active during the opening of the north atlantic ocean at 100 200 ma

tectonic significance of basement cover relationships in the green mountain massif vermont middle proterozoic basement rocks of grenvillian age form the core of the green mountain massif in southern vermont and two distinctive cover rock sequences of late proterozoic to middle ordovician age flank the massif on the west and east the eastern cover sequence is dominated by metamorphosed graywacke silt stone shale and mafic volcanics and unconformably overlies the basement rocks along the eastern margin and in the northern interior of the massif the western boundary of the massif south of clarendon vermont is an unconformity between middle proterozoic basement and western cover sequence rocks composed of metamorphosed conglomerate quartz sandstone dolomite and limestone the western boundary north of clarendon however is a thrust which carried grenville basement and eastern cover rocks over the western cover sequence the structural interpretation of these basement cover contacts and the contrast in lithology and inferred depositional environment of the western and eastern cover seq

revealing climatic variability of the last three millennia in northwestern iberia using pollen influx data climatic variability of the last 3 millennia in nw iberia has been documented using high resolution pollen analysis of vir 18 core retrieved from the ria de vigo 42degrees14 07 n 8degrees47 37 w the depth age model is based on two accelerator mass spectrometry c 14 dates and three historically dated botanical events in galicia the expansion of juglans and pinus as well as the introduction of eucalyptus during the last 3000 years the relative pollen record demonstrates the occurrence of an open deciduous oak forest indicating a humid and temperate climate in northwestern iberia two step forest reduction since 975 cal bc suggests climate as the main cause rather than major socio economic changes documented in historical archives absolute pollen influx has been compared with instrumental summer and winter temperatures and tentatively used as a proxy of short decadal scale and low amplitude similar to 1degreesc temperature variations this new approach allows us to detect for the first time in nw iberia the millennial scale climatic cyclicity suggested by north atlantic records challenging the apparent climatic stability reflected by the relative pollen record the little ice age is recorded as low pollen influx values between 1400 and 1860 cal ad with a cold maximum at 1700 cal ad maunder minimum the roman and medieval warm periods are detected through high pollen influx values at 250 cal bc 450 cal ad and 950 1400 cal ad respectively c 2003 elsevier science b v all rights reserved

paleogene high elevations in the qiangtang terrane central tibetan plateau abstract the timing of uplift of the tibetan plateau remains controversial with estimates varying from eocene to more recent than pliocene in particular the paleoaltimetry of the qiangtang terrane on the central tibetan plateau is completely unknown here we present new stable isotope results of fluvial lacustrine carbonate cement pedogenic carbonate and marl from the kangtuo and suonahu formations deposited between 51 and 28 ma in the heihuling bamaoqiongzong area of the northern qiangtang terrane the lithofacies associations indicate that the kangtuo formation was deposited in alluvial fan and fluvial floodplain environments and the suonahu formation was deposited in near shore lacustrine playa lake and channelized fluvial environments carbon and oxygen isotope values coupled with the sedimentary facies interpretations point to evaporation and low respiration rates in the eocene oligocene paleosols suggesting an arid climate in the high qiangtang area at the time the 18opsw values of paleo surface water reconstructed from the lowest i e the least evaporated 18oc pdb values of the unaltered authigenic carbonates are used to make the minimum estimates of the average paleoelevation of the drainage basin the paleoelevation of the northern qiangtang terrane is reconstructed as above 5000 m by at least the middle oligocene 28 ma similar to the present elevation in this area the aridity and the positive shift in oxygen isotope values of surface waters in our study area may suggest that the high lhasa terrane established by the middle oligocene blocked the northward transport of tropical moisture maintenance of high elevation 5000 m from at least the oligocene to the present suggests that the qiangtang crust was already thickened by that time and that underthrusting of india beneath asia since then has continued to provide additional material while at the same time driving asian lower crust eastward by crustal flow

reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals the paleorelief of mountain belts can be estimated from the 18 o value of authigenic minerals development of relief during mountain building often creates lee side rain shadows in which precipitation is depleted in 18 o and d the magnitude of this rain shadow effect is strongly correlated to relief a compilation of 18 o data from surface waters throughout the globe shows a linear relationship between net elevation change and 18 o r 2 0 79 through the use of this relationship we investigated the timing and magnitude of elevation change in the southern alps of new zealand and the sierra nevada of california the 18 o values of kaolinites from new zealand show an 6 decrease in the early pliocene that corresponds to an 2 km elevation change in the southern alps the 18 o of smectites from the sierra nevada show little change since 16 ma suggesting that these mountains have been a long standing topographic feature

master decollement root zone beneath the southern appalachians and crystal balance a root zone of the southern appalachian master decollement is postulated to exist beneath kings mountain belt on the basis of an alternative interpretation of cocorp seismic profiles palin spastic restoration of the entire crust and earlier geological and geophysical interpretations mildly deformed cambrian metased imentary rocks beneath the decollement end beneath the piedmont in the vicinity of danielsville georgia reflections farther southeast are probably from ultramylonites weak subhorizontal reflections continue southeasterly to the kings mountain belt where a series of east dipping 30 reflections dominate the section to a depth of 20 km beneath the charlotte belt reflections southeast of the root zone are possibly all related to the structure in an accreted island arc

a collisional crustal fabric pattern recognised from seismic reflection profiles of the appalachian caledonide orogen abstract deep seismic reflection profiles across the appalachians and across the once contiguous caledonides around britain and ireland show a broadly similar pattern of crustal reflection fabrics the pattern is of two adjacent fabric domains each characterized by dips towards the other bounded by an interface which dips in conformity with one of the fabrics the domain interface strikes parallel to the orogen suggesting a genetic relationship between the fabrics and the orogen from the north sea to the long island platform the domain boundary dips to the northwest in the southern appalachians the pattern appears with less consistency between the charlotte and carolina slate belts the pattern appears with the same polarity as further north but below the coastal plain it occurs with opposite polarity the interface dips to the southeast an attractive but not exclusive hypothesis for explaining the fabric domain contrast is that it results from detachment of all or most of the crust from underlying mantle lithosphere during collisional orogenesis

palaeo altimetry of the late eocene to miocene lunpola basin central tibet the elevation history of the tibetan plateau provides direct insight into the tectonic processes associated with continent continent collisions here we present oxygen isotope based estimates of the palaeo altimetry of late eocene and younger deposits of the lunpola basin in the centre of the plateau which indicate that the surface of tibet has been at an elevation of more than 4 kilometres for at least the past 35 million years we conclude that crustal but not mantle thickening models combined with plate kinematic solutions of india asia convergence are compatible with palaeo elevation estimates across the tibetan plateau

depth of geological contrast across the west african craton margin anomalies in teleseismic arrivals at stations astride the west african craton margin in senegal are large and systematic for rays which have passed beneath the craton margin lateral variation in seismic velocity structure beneath the margin persists to several hundred kilometres depth the major relative delay time and slowness azimuth anomalies fit a simple model of a steep boundary in the mantle aligned with the major gravity anomaly associated with the craton margin the mantle beneath the mauritanide orogenic belt has lower p velocity than the adjacent craton between 80 and 220 km but higher average velocity both above and below that depth range

reconstruction of holocene precipitation patterns in europe using pollen and lake level data abstract lake level data can be used to refine palaeoclimate reconstructions based on pollen data this approach is illustrated for the european holocene estimates of p pet precipitation minus potential evapotranspiration were first inferred from modern pollen analogues the pollen based estimates were then compared with the status of lakes within a 5 radius analogues with p pet anomalies inconsistent with the lake level changes were rejected the constrained sets of analogues were used to estimate continental scale patterns of annual mean temperature and annual precipitation at 3000 yr intervals estimated temperature anomalies differed only slightly from the unconstrained reconstructions estimated precipitation anomalies however showed improved spatial coherence and increased regional contrast and were occasionally reversed in sign the effect of the constraint was to impose a rational selection among almost equally similar modern pollen analogues with similar temperatures but widely varying moisture regimes the resulting maps showed clear spatially coherent patterns of change in precipitation as well as temperature suitable for comparison with climate model results further improvement of these maps will become possible as a more extensive coverage of lake level data is obtained

depositional response to foreland deformation in the carboniferous of eastern kentucky the appalachian basin is a typical foreland basin that has subsided episodically under the loads of successive thrust sheets the dynamics of basin subsidence and upwarping of basin margin arch systems reflects the flexural properties of the lithosphere quantitative models of the lithosphere depend on assumed rheological properties this study attempts to test these geodynamic models in a part of the appalachian basin mississippian deposition is correlated with waning acadian tectonism orogenic quiescence coincided with basin overdeepening and uplift of the cincinnati arch or forebulge although the foreland basin was mudstone dominated shoaling along the crest of the basin margin arch system resulted in reworking and wedges of unconformities in contrast the onset of alleghenian orogenesis during the pennsylvanian is reflected in a thick wedge of terrigenous clastic sediments the pennsylvanian basin fluctuated between underfilled restricted marine and overfilled nonmarine conditions rapid response of the basin to thrust sheet loading is inferred these characteristics of mississippian and pennsylvanian deposition support a viscoelastic model of the lithosphere

crustal structure and orogenic material budget in the west central pyrenees surface and subsurface data are combined to construct a crustal scale cross section of the western central pyrenees france and spain at the boundary between the european and iberian plates the position of moho reflections in the ecors arzacq reflection profile suggests a tectonic wedge of european crust and upper mantle had indented the iberian plate at lower crustal levels the european wedge is overlain by an upper iberian wedge thus constituting a double stacked wedge geometry the upper wedge was delaminated and deformed giving rise to the pyrenean orogenic prism manifested as a bivergent fan in upper crustal levels the underthrust lower iberian plate has been imaged to depths of 55 60 km but crustal budget considerations based on a palinspastic reconstruction require that this continental root subducted to depths up to 90 km total orogenic contraction calculated from surface structures is about 75 80 km which was accomplished between the latest cretaceous and the early miocene at an averaged rate of 1 2 mm yr consistent with these moderate values exhumation of the orogen is much less than in the more shortened eastern parts of the range paleozoic basement and preorogenic mesozoic rocks are little eroded and much of the exhumation involved cannibalization of early foreland basins which once covered the entire poorly emergent orogen at this transect this resulted in a continuous process of sediment recycling and coupled with a considerable lateral arrival of material a bulk negative erosion sedimentation budget