crustal electrical conductivity in north central saskatchewan the north american central plains anomaly and its relation to a proterozoic plate margin seven recording magnetometers monitored time varying fields at points on a northwest southeast line 280 km long in north central saskatchewan during july 1981 the experiment was designed to test the hypothesis advanced in 1975 by alabi camfield and gough that the electrical conductivity anomaly in the north american central plains links with the wollaston domain in the exposed precambrian shield of saskatchewan from clear reversals in the phase of vertical variations it is evident that the conductor passes between two stations straddling the rottenstone la ronge magmatic belt to the immediate east of the wollaston domain enhanced horizontal variations transverse to the belt at a third intermediate station reinforce this interpretation vertical field response arrows obtained from daytime events in the period range 1 40 min clearly indicate the existence of a major conductor that extends to lower crustal depths beneath the belt to the northwest across the cree lake zone reversals in the directio

a geomagnetic induction anomaly from ims data near hudson bay and its relation to crustal electrical conductivity in central north america summary canadian geomagnetic data collected during the international magnetospheric study are used to investigate the terrestrial electrical conductivity structure of north eastern manitoba and part of the north west territories the computed transfer functions resolve a major conductor trending east west between the communities of gillam and back in manitoba regional trends in the surface geology suggest that this conductor may be linked with the north american central plains electrical conductor twodimensional modelling of the data suggests that the conductor dips to the north from a small depth beneath gillam and may extend to the lowermost cnst

intercalibration of astronomical and radioisotopic time the 40ar 39ar radioisotopic dating technique is one of the most precise and versatile methods available for dating events in earth s history but the accuracy of this method is limited by the accuracy with which the ages of neutron fluence monitors dating standards are known calibrating the ages of standards by conventional means has proved difficult and contentious the emerging astronomically calibrated geomagnetic polarity time scale apts offers a means to calibrate the ages of 40ar 39ar dating standards that is independent of absolute isotopic abundance measurements seven published 40ar 39ar dates for polarity transitions nominally ranging from 0 78 to 3 40 ma are based on the fish canyon sanidine standard and can be compared with apts predictions solving the 40ar 39ar age equation for the age of the fish canyon sanidine that produces coincidence with the apts age for each of these seven reversals yields mutually indistinguishable estimates ranging from 27 78 to 28 09 ma with an inverse variance weighted mean of 27 95 0 18 ma normalized residuals are minimized at an age of 27 92 ma indicating the robustness of the solution

crustal reworking at nanga parbat pakistan metamorphic consequences of thermal mechanical coupling facilitated by erosion within the syntaxial bends of the india asia collision the himalaya terminate abruptly in a pair of metamorphic massifs nanga parbat in the west and namche barwa in the east are actively deforming antiformal domes which expose quaternary metamorphic rocks and granites the massifs are transected by major himalayan rivers indus and tsangpo and are loci of deep and rapid exhumation on the basis of velocity and attenuation tomography and microseismic magnetotelluric geochronological petrological structural and geomorphic data we have collected at nanga parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the indus and tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes the localization of this phenomenon at the terminations of the himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics

the karoo igneous province a problem area for inferring tectonic setting from basalt geochemistry tholeiitic basalts and associated intrusives are the major component of the karoo igneous province they are of mesozoic age and constitute one of the world s classic continental flood basalt cfb provinces it has been argued that most karoo basalts have not undergone significant contamination with continental crust and that their lithospheric mantle source areas were enriched in incompatible minor and trace elements during the proterozoic the only exceptions to this are late stage morb like dolerites near the present day continental margins which are considered to be of asthenospheric origin when data for the southern karoo basalts are plotted on many of the geochemical discriminant diagrams which have been used to infer tectonic setting essentially all of them would be classified as calc alkali basalts cab s or low k tholeiites virtually none of them plot in the compositional fields designated as characteristic of within plate basalts there is little likelihood that the compositions of the karoo basalts can be controlled by active subduction at the time of their eruption and no convincing evidence that a subduction component has been added to the subcontinental lithospheric mantle under the entire area in which the basalts crop out it must be concluded that the mantle source areas for cab s and the southern karoo basalts have marked similarities in contrast the data for northern karoo basalts largely plot in the within plate field on geochemical discriminant diagrams available data suggest that the source composition and or the restite mineralogy and degree of partial melting are different for southern and northern karoo basalts there is no evidence for any difference in tectonic setting between the southern and northern karoo basalts at the time they were erupted this appears to be clear evidence that specific mantle source characteristics and or magmatic processes can vary within a single cfb province to an extent that renders at least some geochemical discriminant diagrams most unreliable for classifying tectonic environment with respect to continental volcanic rocks

inhomogeneous structure of the senegal lithosphere from deep magnetotelluric soundings data from 23 magnetotelluric sites were used to determine electrical conductivities within the crust and upper mantle in senegal west africa along a profile about 600 km long data have been obtained in a variety of tectonic zones the west african craton stable since 1850 250 m y the west african mobile belt with ages in the range of 350 650 m y and the senegal sedimentary basin an earth model is derived from two dimensional modeling the model shows several heterogeneities extending deep in the lithosphere and perhaps more deeply in the asthenosphere to a depth of about 460 km at the stations in the western part of the basin the average resistivity is anomalously low 20 30 ohm m down to about 10 km the basin anomaly in the upper crust is assumed to mark an initial rift zone it may reveal a pattern of crustal weakness in this area beneath this zone the resisitivity is in the range of 1000 3000 ohm m down to at least 300 km in the eastern basin the resistivity is decreasing in the depth range of 20 30 km and the top of the ultimate mantle conductor is at a depth of 300 km at the sites in the marginal part of the west african craton conductive layers can be recognized in the depth range of 30 40 and 80 100 km the resistivity drops to 10 ohm m at a depth of about 460 km the model in this region is compared with available geological and geophysical information and an attempt is made to explain some of the observed discontinuities by plate tectonics it is suggested that the major discontinuity which separates the basin and the mobile belt and extends to about 460 km depth could be interpreted as marking a precambrian subduction slab dipping eastward in the west african craton a low resistivity has been determined for a depth range of 130 150 km the upper 130 km were found to have the rather high resistivities of 1000 3000 ohm m a sharp rise in conductivity occurs at a depth of about 460 km beneath the craton in contrast a surprising feature in the kedougou region in the craton is that the major portion of the lower crust is conductive the existence of a birrimian geosynclinal pair in eastern senegal consisting of a eugeosynclinal marine andesitic volcanic trough particularly volcanics with ophiolite affinity and a miogeosynclinal sedimentary sequence could characterize a modern plate tectonic phase dominated by widespread ocean opening and continental collision the conductive material in the lower crust might be explained by the buried ocean crust

an audiofrequency magnetotelluric traverse across the mana pools basin northern zimbabwe summary audiofrequency magnetotelluric soundings in the frequency range 0 016 128 hz have been conducted to investigate the structure of the mana pools basin in northern zimbabwe a karoo sedimentary basin in the zambezi valley data from nine tensorial soundings along a 40 km n s traverse from the river zambezi to the zimbabwe craton and a further sounding on the escarpment have been processed giving the sounding curves required for preliminary modelling studies a variety of l d inversion algorithms have subsequently been applied to these processed data sets to yield interpretive models an approximate 2 d geoelectrical section has been derived by collating the 1 d models a marked and rapid contrast in subsurface resistivity distribution between the escarpment and the basin is apparent within the basin the results show the presence of a top sequence of variable thickness up to 10 km overlying a more resistive 0 1 s m 1 unit that may represent the geoelectrical basement these top sequences which have tentatively been interpreted as sedimentary formations with different stratigraphic units consist of highly conducting 0 3 s m 1 to 1 s m 1 materials overlain by a slightly more resistive typically 0 2 s m 1 unit in some cases with another good conductor adjacent to the surface the data also suggest a rapid thinning of the top sequences close to the escarpment in contrast to the more gradual decrease of the sediments in previous geophysical models for the area on the escarpment the resistivity is higher at all depths the mana pools basin thus appears to be a continuation of the good conductor running from namibia into western zimbabwe identified by de beer 1982a and provides a link to another beneath the lower zambezi basin to the east of mana pools discovered magnetotellurically by losecke knodel muller 1988 comparisons between the geoelectrical model and other available results for the region are presented

fission track dating of zircon zircons from mont blanc granite french italian alps zircon population in granite rocks are most often heterogeneous with respect to uranium concentrations which can vary from a few hundred ppm to 1 and the typology of the grains for example in the type of dominant prism 100 or 110 making them difficult to date using fission track ft dating the mont blanc zircons which vary greatly in both uranium content and zircon type are typically difficult to date by the fission track method to obtain information about fission track retention characteristics of these minerals experimental isothermal annealing was performed on grains free of alpha damage i e annealing of freshly induced tracks uranium poor grains with a dominant 100 prism were found to be more stable a step etching method sem was used to date the zircons uranium poor zircons with a dominant 100 prism yielded higher fission track ages than did uranium rich ones with a 110 dominant prism thus etching conditions temperature and etching time and type of prism chosen

telluric currents and magnetic anomalies current densities of 2 5 10 4 amp m can be generated by constant groundwater migration in artesian aquifers these dc currents are sufficient to generate magnetic anomalies exceeding 200 nt in both the menindee trough australia and the karoo basin south africa telluric currents associated with ionospheric activity can be detected by variations in magnetic induction but constant offsets associated with streaming potentials and geochemical activity sp anomalies are obscured by noise consequently some regional magnetic anomalies may be wrongly attributed to variations in magnetic susceptiblity with residuals explained by remanence

collision orogen provenance western alps detrital signatures and unroofing trends abstract the alps are perhaps the best studied thrust belt formed during continent continent convergence collision orogen shallow intermediate and deep structural levels can be distinguished within the alpine thrust stack each shedding sediments with distinct petrographic and mineralogical signatures in modern first cycle sands carried by alpine rivers bulk composition rank of metamorphic lithic grains metamorphic index mi and dense mineral assemblages all faithfully mirror the tectono metamorphic history of exposed basement and cover units the shallow structural level widely preserved in the ligurian alps consists of remnant ocean turbidites shedding sedimentary to very low rank metasedimentary detritus documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcomman

neoproterozoic tectono thermal evolution of the gariep belt and its basement namibia and south africa abstract within the pan african gariep belt in southwestern africa a para autochthonous predominantly sedimentary sequence deposited in a rift graben and subsequently on a passive continental margin with a kibaran basement is distinguished from an allochthonous predominantly oceanic terrane that was thrust on top of the former during collision of the rio de la plata and the kalahari plates 40ar 39ar mineral cooling ages presented here set the following constraints after kibaran high grade metamorphism the consolidated basement cooled through 300 c at 1006 ma rifting between 781 and 741 ma was followed by the formation of oceanic crust the lower parts of which cooled through 500 c sometime between 630 and 600 ma after inversion from extension to compression some of the oceanic crustal material was shredded off and incorporated into an accretionary wedge a process that is indirectly dated between 573 576 ma by hornblende plateau ages continent continent collision and thrusting of the internal onto the external zones of the tectonic belt culminated between 547 543 ma as indicated by further hornblende plateau ages muscovite cooling ages for the internal zone are between 526 and 529 ma those for the tectonically deeper external zone are 483 ma in the north and between 495 506 ma in the south of the belt in conjunction with published data these results suggest successive closure of first the northern adamastor ocean kaoko belt followed by the khomas sea intracontinental damara belt and finally the southern adamastor ocean gariep belt

fission track and 40ar39ar evidence for episodic denudation of the gangotri granites in the garhwal higher himalaya india abstract in order to document quantitatively the cooling and denudation history of the higher himalayan granites bordering the tethyan sedimentary zone fission track ft apatite and 40 ar 39 ar mica ages have been determined on the gangotri leucogranites and biotite granites in the garhwal region of india gangotri is the source area of the ganges river and lies in the midst of the highest himalayan peaks in india a total of 15 apatite ages from a vertical profile 2580 4370 m on the gangotri granites yields ft ages in the range of 1 5 0 6 to 2 4 0 5 ma indicating that the rock column with a relief of 1800 m cooled through 130 10 c within only 1 million years during the late pliocene an average denudation rate of 2 mm yr is estimated for the past 2 4 million years from the gangotri granites we also report a muscovite 40 ar 39 ar age of 17 9 0 1 ma and a biotite age of 18 0 0 1 ma these reflect cooling of the rocks through 300 350 c probably related to an early miocene pulse of denudation caused by a basement cover detachment the martoli normal fault above the leucogranites time temperature pathways indicate that the cooling of the rocks in the late pliocene quaternary was five to six times the magnitude of cooling between 18 and 2 ma indicating a distinct pulse of rapid denudation in the late pliocene quaternary we interpret these young apatite ages and fast denudation as a geomorphic response increased erosion and cooling of the rocks to a major pulse of tectonic uplift in the higher himalaya shortly before 2 4 ma the effect of climatic cooling on this denudation is considered secondary to the role of tectonic forcing and indeed produced a positive feedback to the primary cause although our study is confined to the garhwal region it is probable that other granitic bodies of the higher himalaya bordering the tethyan sedimentary rocks and forming the loftiest summits in the himalaya have also experienced episodic denudation one major pulse in the early miocene which was mainly tectonic denudation and another in the late pliocene quaternary which was mainly erosional the latter is well recorded by apatite ft data and is consistent with the hypothesis that rapid uplift and denudation of the himalayan rocks may have influenced the initiation of the ice ages in the northern hemisphere

constraints on the late thermotectonic evolution of the western alps evidence for episodic rapid uplift forty two new apatite and zircon fission track ages are presented for samples from the western alps in southern switzerland northern italy and southeastern france measured ages plotted against assumed closure temperatures yield cooling patterns for the final cooling uplift and exhumation of the western alps similar fission track zircon ages in the penninic gran paradiso massif dent blanche nappe sesia lanzo zone and ivrea zone indicate cooling of all four units to 225 c by 33 ma differences in apatite ages reveal differential cooling of the four blocks between 33 ma and the present in the sesia lanzo zone similarity of apatite ages regardless of elevation together with near volcanic confined fission track length patterns suggest rapid cooling and uplift at 25 ma compared with slow cooling of other western alps units around 12 ma uplift is thus not continuous but episodic often over a short time interval beyond the resolution of other methods such episodes of uplift as revealed here in the sesia lanzo zone may be the rule rather than the exception

tectonic evolution of the high himalaya in upper lahul nw himalaya india the upper lahul region in the nw himalaya is located in the transition zone between the high himalayan crystalline hhc to the sw and the tethyan zone sedimentary series to the ne the tectonic evolution of these domains during the himalayan orogeny is the consequence of a succession of five deformation events an early dl phase corresponds to synmetamorphic ne verging folding this deformation created the tandi syncline which consists of permian to jurassic tethyan metasediments cropping out in the core of a large scale synformal fold within the hhc paragneiss this tectonic event is interpreted as related to a ne directed nappe stacking shikar beh nappe probably during the late eocene to the early oligocene a subsequent d2a phase caused sw verging folding in the hhc this deformation is interpreted as contemporaneous with late oligocene to early miocene sw directed thrusting along the main central thrust in the tethyan zone a d2b phase is marked by a decollement thrust a system of reverse faults and gentle folds associated with sw directed tectonic movements this deformation is related to an imbricate structure characteristic of a shallow structural level and developed in the frontal part of a nappe affecting the tethyan zone units of se zanskar nyimaling tsarap nappe a later d3 phase generated the chandra dextral shear zone cdsz a large scale ductile dextral strike slip shear zone located in the transition zone between the hhc and the tethyan himalaya the cdsz most likely represents a part of a system of early miocene extensional and or dextral strike slip shear zones observed at the hhc tethyan zone contact along the entire himalaya a final d4 phase induced large scale doming and ne verging back folding

a fluvial record of long term steady state uplift and erosion across the cascadia forearc high western washington state six late quaternary river terraces preserved along the clearwater river in northwestern washington state provide a 140 ka record of long term incision and uplift across the western side of the cascadia forearc high terrace ages are constrained by weathering rind and radiocarbon dating and by correlation to dated coastal glacio fluvial deposits and the global eustatic curve the terraces overlie flat bedrock surfaces called straths which represent uplifted segments of the river channel bedrock incision is measured by the height of a strath relative to the adjacent modern river channel the straths along the clearwater show an upstream increase in bedrock incision ranging from 0 at the coast to a maximum of 110 m in the headwaters the incision at any point along the profile increases systematically with strath age the calculated incision rates range from 10 ky the clearwater river valley has maintained a steady state profile defined by a long term balance in the rates of incision and rock uplift upstream divergence of terraces is best explained by an increase in the rate of rock uplift from the coast toward the central part of the range these results are consistent with other evidence indicating a long term steady state balance between the accretionary influx and the erosional outflux for this part of the cascadia subduction wedge since 14 ma these results help show how terrace deposits form in tectonically active landscapes the dominantly fluvial clearwater drainage was forming straths while alpine glaciers were advancing in adjacent drainages in turn the straths were buried during the transition to interglacial times because of increased sediment supply due to local deglaciation and because of eustatic highstands that forced aggradation in the lower reach of the drainage and across the continental shelf as well the fluvial system shows strong forcing by the glacial climate cycle even so the river appears to have returned to the same valley profile during each cycle of strath cutting thus bedrock incision is clearly unsteady at time scales shorter than the glacial climate cycle 100 ky but appears to be relatively steady when averaged over longer time scales a simple kinematic model is used to examine how uplift of the cl our analysis indicates that the accretionary flux into the wedge occurs mainly by frontal accretion and not by underplating if accretion occurred entirely at the front of the wedge the present west coast should be moving to the northeast at 3 m ky relative to a fixed puget sound this prediction is in good agreement with offset of a 122 ka sea cliff preserved at the southwest side of the clearwater valley profile in this case the long term margin perpendicular shortening would account for 20 to 35 percent of the geodetically measured northeast southwest shortening across the olympic mountains

late stages of deformation and exhumation of an orogen constrained by fission track data a case study in the western alps new zircon and apatite fission track data combined with a structural analysis provide constraints regarding the large scale tectonic evolution of part of the french italian alps the observed migration of cooling towardmore external units 5 is interpreted to be due to thrust propagation in the absence ol significant amounts of crustal thinning the exhumation has been dominantly by erosion the most internal unit investigated the penninic houillere zone started to cool at ca 35 ma after an carlier tectonometamorophic event that only affected penninic units fission track data from the cheval noir unit replacing the valaisan unit in the south constrain the age of metamorphism in these frontal penninic units to be older than 32 ma there cooling started with the onset of a second stage of the tectonic evolution i e with top to the west northwest movement along the roselend thrust at ca 32 ma this oligocene to early miocene thrusting led to burial and metamorphism in the external massifs and their cover dauphinois unit during a third tectonic stage thrust related culminations formed in the external massifs between 20 and 15 ma this stage led to the onset of exhumation in the dauphinois and in the northern frontal penninic units valaisan units caused by erosion that followed tilling in the backlimb of these culminations northward increasing amounts of miocene pliocene shortening in front of and below the external massifs led to significantly higher amounts of exhumation in the north finally and in very recent times post 5 ma the frontal thrust of the houillere zone was reactivated as a normal fault with moderate offset 3 km

coupled and decoupled regimes of continental collision numerical modeling useful geodynamic distinction of continental collision zones can be based on the degree of rheological coupling of colliding plates coupled active collision zones which can be either retreating or advancing are characterized by a thick crustal wedge and compressive stresses i e himalaya and western alps while decoupled end members which are always retreating are defined by a thin crustal wedge and bi modal distribution of stresses i e compressional in the foreland and extensional in the inner part of the orogen northern apennines in order to understand physical controls defining these different geodynamic regimes we conducted a 2d numerical study based on finite differences and marker in cell techniques in our experiments we systematically varied several major parameters responsible for the degree of rheological coupling between plates during collision such as convergence rate crustal rheology and effective velocity of upward propagation of aqueous fluids and melts in the mantle wedge low convergence rates and fluids melts propagation velocities favor continuous coupling and convergence between the plates coupled collision zones are characterized by continuous accretion of the weak upper continental crust resulting in the development of a thick and broad crustal wedge by hot temperature in the inner parts of the orogen due to radiogenic heating of the thickened crust by compressive orogenic stresses and appearance of a double seismogenic brittle layer involving upper crust and sub moho mantle in contrast high convergence rates and fluid melt percolation velocities produce efficient weakening of the mantle wedge and of the subduction channel triggering complete decoupling of two plates mantle wedging into the crustal wedge and retreating style of collision the evolution of fully decoupled collision zones are characterized by the disruption of the accretionary wedge formation of an extensional basin in the inner part of the orogen and delamination of the weak portion of the continental crust that is first thrusted toward the foreland and subsequently dissected by extensional tectonics transition from coupled to decoupled regime occurs always at the early stages of continental collision indicating that insertion of rheologically weak crustal material in the subduction channel is critical for the subsequent evolution of the collision zone we found good correlations of our numerical results with some of the major collisional orogens in particular the decoupled retreating collision regime reproduces what is observed in the northern apennines

river profiles along the himalayan arc as indicators of active tectonics abstract longitudinal profiles along sixteen major transverse himalayan rivers add important constraints to models of active continental subduction and its evolution these profiles are characterized by a zone of relatively high gradient that cannot be associated with differential resistence to erosion in all cases the base of the zone of increased gradients correlates with 1 the topographic front between the lesser and high himalayas 2 the narrow belt of intermediate magnitude thrust earthquakes 3 the main central thrust zone mct these features define a small circle in the central portion of the himalayan arc these correlations suggest that the discontinuity in the river profiles and the other features are controlled by a major tectonic boundary between the rising high himalayas and the lesser himalayas no sharp increases in gradient are observed near the main boundary thrust mbt except on a few rivers such as the jhelum or kundar where the mbt lies close to both the mct and the seismic belt thus it is unlikely that the mbt is a major tectonic boundary the diversion of river courses along the mbt and around anticlines in the sub himalayas has probably been caused by aggradation near the rosion deposition boundary upstream of uplifts in the mahabharat range and sub himalayas a parallel is drawn between the himalayas and new guinea based on the hypothesis that continent arc collision of the type occurring in northern australia preceded continent continent collision in the himalayas the present sedimentary tectonic phase in new guinea resembles the subathu paleocene eocene phase in the himalayas incipient counterparts of the major himalayan structures including the mct and the mbt are recognized in new guinea the drainage patterns in the himalayas and in new guinea bear a similar relation to major structures this suggests that 1 the tectonic evolution of the himalayas has been rather uniform since early stages of collision and 2 the himalayan drainage was also formed at these early stages and is therefore antecedent to the rise of the high himalayas

iso kinematic maps from statistical analysis of ps insar data of piemonte nw italy comparison with geological kinematic trends abstract sar interferometry based on permanent scatterers ps insar is used here to study the present crustal mobility of a large area of nw italy in the piemonte region thirty eight satellite scenes ers sar taken from may 1992 to january 2001 were analysed for detecting more than 2 million ps on the study area continuous velocity surface maps iso kinematic maps ikm were obtained from geo statistical and spatial cluster techniques hot spot analysis of ps short period data to identify relative ground motions and to compare them with long period tectonic mobility trends i e those inferred at regional scale over geological times some million years the comparison was made by individuation of homogeneous kinematic areas represented in the ikm and characterization of the boundaries between them iso kinematic boundaries ikb the ikb were used as tools to asses if the ps insar data on present day crustal mobility could fit with the distribution of real tectonic structures or field geological elements ikm were drawn for uplifting geological sectors of piemonte maritime alps gran paradiso langhe where moderate to very low seismicity is recorded and gravitational instabilities of rock mass on mountain slopes are widespread the land sectors have been chosen in order to test the suitability of ikm in very different geo morphological conditions different types of correspondence between the ikm and the geological kinematic trend were found a first type in which the kinematic trend of short period a decade of years i e the ps insar detection time span is in agreement with a long period tectonic trend some million years and seem to be driven by well known faults subparallel to the ikb these kinematic trends can be hidden by the slope movement due to gravitational instabilities a second type in which the kinematic trend of short period does not strictly correspond to the long period trend but can be considered as minor order uplifting subsidence cycles even if in contrast with the long period kinematic trend alternatively the short period kinematic trends could reflect the action of deep seated geological forces or structures not yet known or inferable at least with the recorded ps insar velocities on the basis of the available geological data and models

a magnetotelluric traverse across the mauritanides orogenic belt west africa summary magnetotelluric measurements with reference magnetometers for study of the anomalous geomagnetic variation field were made in the period range 10 10000 s at 10 sites crossing the main geological zones of eastern senegal west africa the results indicate that the electromagnetic fields are controlled by the dominant structural trends of the mauritanides orogenic belt apparent resistivities reflect the major structural units identified from surface geology data in the principal directions for most of the stations exhibit anisotropic behaviour and at certain sites the magnetotelluric responses are affected by surface distortion effects the study of the geomagnetic variation field indicates the presence of a concentration of telluric current flow beneath the craton a lack of obviously conductive geological structures at the surface suggests that this flow of telluric current may be attributed to increased electrical conductivity in the crust the quantitative interpretation of the magnetotelluric results is given in terms of 1 d and 2 d resistivity models it is shown that the data require the presence of a highly anomalous crustal structure 40 rm dipping to the east from a small depth beneath the outcropping margin of the west african craton in the mauritanides the crust is resistive 3000 rm t o a depth of 30 km and overlies a conducting layer of 10 km thick in the depth range 250 300 km resistivities decrease to less than 50 rm this may coincide with the top of the asthenosphere the geoelectric model is discussed in relation to the general geological background of the region considered

tectonic and climatic controls of denudation rates in active orogens the san bernardino mountains california abstract the relative importance of climatic and tectonic factors in driving rates of denudation in mountain ranges has long been debated with both precipitation and rates of crustal uplift cited as first order controls in a variety of different mountainous settings few studies however have explicitly considered the influence of climatic and tectonic processes on denudation rates during the early stages of orogenesis using basin wide denudation rates derived from in situ cosmogenic 10 be and published modern precipitation rate data the significance of rainfall and snowfall on rates of denudation is evaluated for the san bernardino mountains california denudation rates vary between 52 mm ka 1 in the arid northern regions of the mountains and 2700 mm ka 1 in the more humid southern sector we select three basins where the influence of precipitation on denudation rates can be isolated from the effects of crustal uplift and find that there is no apparent relationship between denudation rates and precipitation denudation rates differ more than five fold on the opposing slopes of mill creek a valley bisected by a major splay of the san andreas fault and we propose that this denudation rate disparity can be best explained by variations in uplift rates across the fault the results suggest that crustal uplift is the mechanism that underpins the 50 fold variability in denudation rates we have measured in the san bernardino mountains but that this must be facilitated by sufficient precipitation to remove valley fill deposits

himalayan inverted metamorphism constrained by oxygen isotope thermometry inverted metamorphic field gradients are preserved in two amphibolite facies metapelitic sequences forming the crystalline core zone of the himalayan orogen in the sutlej valley nw india in the high himalayan crystalline sequence hhcs metamorphic conditions increase upwards from the staurolite zone at the base through the kyanite in and sillimanite in isograds finally to reach partial melting conditions at the top the structurally lower lesser himalayan crystalline sequence lhcs shows a gradual superposition of garnet in staurolite in and kyanite sillimanite in isograds although phase equilibria constraints imply inverted temperature field gradients in both units garnet biotite garb rim thermometry indicates final equilibration at a nearly uniform temperature around t 600 c across these sequences the p t path and garnet zoning data show that this apparent lack of thermal field gradient is mainly the consequence of a resetting of the garb equilibria during cooling in order to constrain peak temperature conditions 20 samples along the studied section have been analysed for oxygen isotope thermometry the isotopic fractionations recorded by quartz garnet and quartz aluminosilicate mineral pairs indicate temperatures consistent with phase equilibria and p t path constraints for metamorphic peak conditions together with barometry results based on net transfer continuous reactions the oxygen isotope thermometry indicates peak conditions characterized by 1 a temperature increase from t 570 to 750 c at a nearly constant pressure around p 800 mpa from the base to the top of the hhcs unit 2 a temperature increase from t 610 to 700 c and a pressure decrease from p 900 to 700 mpa from the base to the top of the lhcs metapelites oxygen isotope thermometry thus provides the first quantitative data demonstrating that the himalayan inverted metamorphism can be associated with a complete inversion of the thermal field gradient across the crystalline core zone of this orogen

continental erosion and large scale relief a worldwide investigation of continental erosion is carried out by the study of large drainage basins on the basis of hydrological data environmental factors and basin relief distribution inside each basin mean geochemical and mechanical denudation rates are defined a multicorrelation analysis shows that the mechanical denudation rates ds are uncorrelated with environmental factors and correlated with mean basin elevation h while chemical denudation rates dd are insensitive to relief but correlated with mean annual precipitation furthermore two linear relationships between h and ds are detected 1 ds m 10 yr 419 10 6 h m 0 245 with v explained variance 95 1 this law concerns basins related to orogenies younger than 250 ma the negative intercept is interpreted as a continental sedimentation rate of 245 m m y an alternative model in which one invokes a critical elevation separating erosion from sedimentation is equally successful and leads to lower sedimentation rates 60 110 m m y for both models one derives from the slope of the adjustments erosion time constants on the order of 2 5 m y 2 ds m 10 yr 61 10 6 h m with v 86 5 this law concerns basins related to older orogenies the null intercept suggests the lack of continental storage because of the more important dispersion of the data the erosion time constant is calculated separately for each basin it ranges from 15 to 360 m y the tectonic implications of these results are discussed in particular the short time constant 2 5 m y agrees with orogenic uplift rates on the order of 1 mm yr observed in active mountain chains

integration of zircon color and zircon fission track zonation patterns in orogenic belts application to the southern alps new zealand an exhumed crustal section of the mesozoic torlesse terrane underlies the southern alps collision zone in new zealand since the late miocene oblique horizontal shortening has formed the northeastern southwestern trending orogen and exhumed the crustal section within it on the eastern side rocks are zeolite to prehnite pumpellyite grade greywacke on the western side rocks they have the same protolith but are greenschist to amphibolite facies of the alpine schist zircon crystals from sediments in east flowing rivers hinterland have pre orogenic fission track ages 80 ma and are dominated by pink radiation damaged grains up to 60 these zircons are derived from the upper f10 km crustal section unreset ft color zone that includes the late cenozoic zircon partial annealing zone both fission tracks and color remain intact and unaffected by orogenesis many zircon crystals from sediments in west flowing rivers foreland have synorogenic ftages and about 80 are colorless due to thermal annealing they have been derived from rocks that originally lay in the reset ftcolor zone and the underlying reset ft colorless zone the reset ft color zone occurs between f250 and 400 jc in this zone zircon crystals have color but reset ft ages that reflect the timing of orogenesis d 2002 elsevier science b v all rights reserved

magnetotelluric soundings in the canadian rocky mountains summary magnetotelluric soundings have been made at 25 stations in the rocky mountain trench rmt and main ranges near 53 n close to the centre of a major conductivity anomaly which had been mapped in a magnetovariation array study most stations covered the frequency range 0 01 500 hz and three stations 0 0002 500 hz the resistivity tensor shows low to moderate anisotropy in the rmt but is strongly 2 d or 3 d in the rocky mountains apparent resistivities as a function of frequency are displayed in pseudosections along the trench and along a transverse profile across the rmt and into the main ranges in preparation for 2 d modelling 1 d inversions have been used to construct resistivity depth sections satisfying both magnitudes and phases of the mt responses these show very low resistivities in the range 1 10s2m in the upper crust under the rmt and even lower values under the main ranges the latter values give strong confirmation of the northern rockies conductor reported by bingham gough ingham and are in agreement with models of the conductors fitted to long period magnetovariation fields by ingham gough parkinson the mt results here reported add some essential depth and resistivity information it is suggested that the conductors beneath the rocky mountains main ranges and trench constitute a thickening at the edge of the canadian cordilleran regional ccr conductor gough has argued that a wide variety of geophysical and geological parameters indicate high temperatures and partial melting in the mantle under the ccr conductor at the upper crustal depths penetrated in this magnetotelluric study it is considered more probable that the high conductivity is caused by hot saline water of mantle origin rather than silicate melt the ccr in general may have two layers of fluid producing its high conductivity silicate melt below and saline hot water above

mass accumulation rates in asia during the cenozoic summary this work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in asia since the beginning of the cenozoic 66 ma the estimates were derived from isopach maps cross sections and drill holes or stratigraphic columns assuming regional similarity of the strata average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods palaeocene 66 58 ma eocene 58 37 ma oligocene 37 30 and 30 24 ma miocene 24 17 17 11 and 11 5 ma pliocene 5 2 ma and quaternary 2 0 ma these rates shed new light on the geological history of asia since the onset of the collision of india with asia 50 ma the overall average accumulation rates curve for asian sedimentary basins since the beginning of the tertiary shows an exponential form with slow accumulation rates less than 0 5 106 km3 myr 1 until the beginning of the oligocene more than 15 myr after the onset of the collision from the oligocene onwards rates increase quickly in an exponential manner reaching their maximum values in the quaternary more than 1 5 106 km3 myr 1 from these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the india eurasia collision history at smaller scales one may distinguish between independent histories at the subcontinental and basin scales this permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of asia during the cenozoic

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

electromagnetic exploration for fluids in the earth s crust abstract electromagnetic geophysical techniques yield information on the spatial distribution of electrical conductivity a parameter sensitive to fluids in the rocks the paper describes electromagnetic techniques of use through the earth s crust including magnetovariation mv arrays magnetotelluric mt sounding and dc resistivity sounding field studies using these techniques lead to a discussion of various causes of high crustal conductivity including aqueous fluids silicate partial melt and films of graphite or magnetite on grain surfaces water in the crust is proposed as the preferred cause of high conductivity in the juvenile crust of tectonically active regions in particular where the anomalous conductivity is correlated with high heat flow and other geophysical and geological parameters

remagnetizations in late permian and early triassic rocks from southern africa and their implications for pangea reconstructions a paleomagnetic study of late paleozoic and early mesozoic sedimentary rocks from southern africa suggests wide spread remagnetization of these rocks samples of the mofdiahogolo formation in botswana and of the lower beaufort group in south africa were treated using thermal alternating field and chemical demagnetization the mofdiahogolo redbeds show a univectoral decay of the remanence revealing a characteristic direction of

active tectonism in the central alps contrasting stress regimes north and south of the rhone valley an integrated interpretation of seismicity fault plane solutions and deep seismic reflection data suggests that the ne sw to nw se trending rhone simplon fault zone and the gently s dipping basal penninic thrust separate fundamentally different stress regimes in the western swiss alps north of the rhone simplon fault zone strike slip earthquakes on steep dipping faults within the helvetic nappes are a consequence of regional nw se compression and ne sw extension to the south vertical maximum stress and n s extension are responsible for normal mechanism earthquakes that occur entirely within the penninic nappes above the basal penninic thrust such normal faulting likely results from extension associated with southward movements collapse of the penninic nappes and or continued uplift and relative northward displacements of the underlying alpine massifs geological mapping and fission track dating suggest that the two distinct stress regimes have controlled tectonism in the western swiss alps since at least the neogene

geomorphological hazards related to deep dissolution phenomena in the western italian alps distribution assessment and interaction with human activities abstract deep dissolution affects great part of soluble rocks e g gypsum and anhydrite of the western italian alps the related superficial phenomena sinkholes gravity induced processes and a local worsening of geomechanical rock properties are not limited to typical karsts landscape and cause slope instability also affecting populated sites and infrastructures the paper aims to describe general characteristic of dissolution phenomena to interpret their conditioning factors and evolutionary stages and to assess possible hazards due to their superficial effects the search for evidences of deep dissolution leads to the selection of representative sites in the central part of the western italian alps piemonte and valle d aosta region detailed geological and geomorphological studies have been used to classify the selected sites by type size and variable state of activity very different evolutionary stages of dissolution phenomena have been interpreted by comparison of case studies some are early embryonic others are more evolved up to typical sinkholes or even remodelled by other phenomena some cases show an extreme complexity in the interactions between corrosion phenomena and other geomorphic processes slope deformations from one side and karst fluvial and glacial phenomena to the other a wide range of movement rates on slope instabilities induced by deep dissolution have been estimated by topographic and geomorphic data geochemical data on removed rocks by dissolution indicate 0 4 mm year values for local subsidence historical and technical data indicate low frequency of major dissolution induced collapses but highlight widespread damages to tunnels roads and buildings especially along slopes

the nares strait gravity anomaly and its implications for crustal structure a negative free air gravity anomaly is associated with nares strait the waterway that separates ellesmere island and greenland two east west gravity profiles that cross ellesmere island and nares strait were collected a low with values in the range of 100 to 120 mgal 1000 to 1200 m s2 was observed and two dimensional crustal models were created to identify the cause of the anomaly the gravity anomaly cannot be attributed wholly to the bathymetry of the strait or to the sedimentary rocks underlying the strait crustal models that reproduce the anomaly have a m discontinuity that slopes under nares strait towards ellesmere island so that the crust beneath ellesmere island is thickened the anomaly is similar to those associated with ancient and modern suture zones regions of collided continental crust plate reconstructions suggest nares strait is a collisional boundary between the north american plate ellesmere island and the greenland plate the gravity anomaly supports this interpretation

nares strait a suture zone geophysical and geological implications abstract the motion of greenland relative to ellesmere island along nares strait is determined from poles of rotation which provide control for the motion independent of local geology and geophysics the plate kinematics around the north atlantic ocean the norwegian and greenland seas and the eurasian basin of the arctic ocean constrain motion along nares strait these motions are checked by examining the stability characteristics of the triple junctions these junctions are found to be stable the motion along nares strait between anomalies 34 and 13 is a combination of strike slip and compression the regional geology is found to support the plate reconstructions the local geology of the nares strait area is reviewed and found not to refute the predicted motions the geophysical and geological data are interpreted in terms of the wilson cycle the opening and closing of an ocean the nares strait area has the characteristics of a cryptic suture a join between regions of collided continental crust

the oligocene alps domal unroofing and drainage development during early orogenic growth abstract the source of huge volumes of quartzofeldspathic detritus feeding oligo miocene foredeep turbidites of the apennines has long remained an unsolved geological enigma in this article we reinterpret the available sandstone petrography fission track stratigraphic and geomorphological evidence to demonstrate that the adriatic foredeep was dominantly sourced by a himalayan type tectonic aneurysm growing in the central alps since the late oligocene the lepontine dome the lepontine dome is the envelope of distinct subdomes that formed stepwise from east to west during northwestward indentation of adria beneath the axial alps focused erosion of domal culminations cut by antecedent drainage started suddenly in the late oligocene with unroofing of recently emplaced plutons and young amphibolite facies gneisses exhumation was tectonically driven and possibly accelerated by positive feedback between erosion and uplift of hot weak rocks from depth the huge volumes of resulting detritus gonfolite macigno clastic wedge were transported southward for hundreds of kilometers along the rapidly subsiding foredeep formed since middle eocene times in front of the alpine retrobelt and nascent apennine forebelt such massive sediment influx following 15 2 myr of starved foredeep sedimentation testifies to rapid formation of mountain relief in the central alps synchronous to dextral activity along the insubric fault the lepontine dome remains today as the only deeply dissected region in the alps where medium grade metamorphic rocks of the axial belt are extensively exposed the relationships between geological structures and drainage patterns still mirror those formed originally at oligocene times river headwaters have not been significantly displaced since then and their thalwegs constrain finite lateral displacement relative to the southalpine retrowedge to a maximum of 10 20 km our reconstruction has wide implications not only for a better understanding of the tertiary evolution of the alps apennines orogenic couple but also for general relationships among tectonic activity relief formation drainage development erosion distribution and long distance sediment transfer during continental collision and orogenic growth

isotopic constraints on the age and provenance of the lesser and greater himalayan sequences nepalese himalaya the main central thrust of the central himalaya juxtaposes the lesser himalayan sequence footwall with the greater himalayan sequence hanging wall both of which are regarded as components of the northern passive margin of india considerable uncertainty surrounds their respective ages and sedimentary derivation u pb and sm nd isotopic studies of samples from the langtang area central nepal demonstrate important distinctions between the two rock packages the greater himalayan sequence had a sedimentary provenance that included a major source of 0 8 1 0 ga zircons implying a late proterozoic age the source of the precambrian part of the lesser himalayan sequence contained 1 87 2 60 ga zircons and its depositional age is probably middle proterozoic nd isotopic characteristics of the two sequences are different at 21 ma the greater himalayan rocks had end values between 14 6 and 18 5 while their lesser himalayan counterparts had values between 21 4 and 25 9 giving rise to major differences in their model ages the lesser and greater himalayan sequences at langtang are interpreted as correlative to the lower and upper parts of the vindhyan supergroup currently exposed south of the gangetic plain in northern india on the basis of available age data the exposed hanging wall of the main central thrust in central and eastern nepal contains no archean early proterozoic basement if the fault does root into the basement at depth it must do so farther north than recently published cross sections indicate this implies a total displacement on the fault that is substantially greater than the 140 210 km required by tectonic overlap in the eastern himalayas

single and double exhumation of fault blocks in the internal sesia lanzo zone and the ivrea verbano zone biella italy the combination of magmatic structural and fission track ft data is used to unravel oligocene miocene near surface tectonics in the internal western alps this includes reburial of parts of the already exhumed sesia lanzo zone and their subsequent re exhumation we define blocks mainly on the base of their oligocene miocene cooling history ft data and on published paleomagnetic data the preservation of a paleosurface allows a detailed reconstruction of the exhumation burial and re exhumation of different tectonic blocks near surface rigid block rotation is responsible for the reburial of the lower oligocene paleosurface in part of the sesia lanzo zone the cervo block and for the conjugate uplift of deeper portions of the ivrea verbano zone the sessera ossola block this block rotation around the same horizontal axes produces in the currently exposed portions of the two blocks quite different temperature time paths while the surface of the cervo block is buried the lower part of the sessera ossola block is uplifted the rotation is constrained between the age of emplacement of the biella volcanic suite on top of the sesia lanzo zone 32 5 ma and the intrusion of the valle del cervo pluton 30 5 ma after this relative fast movements the concerned blocks remained in or underneath the partial annealing zone of zircon until in aquitanian times they were rapidly uplifted into the partial annealing zone of apatite the further stage of exhumation out of the partial annealing zone of apatite extends over the entire miocene at that time units of the external western alps underwent fast exhumation external brianconnais valais in addition to the well known post collisional deformation in the axial and external western alps the internal units i e the upper plate hold an apparent stable position in terms of exhumation

why is the electrical resistivity around the ktb hole so low the low resistivity anomaly close to the ktb borehole coincides with both a self potential and a static magnetic anomaly if this coincidence is not accidental it may yield information about the conditions required for the existence of low resistivity anomalies in the deeper crust a possible answer to the question in the title of this paper is that the oxygen fugacity in the crust around the ktb hole must be low enough to stabilize graphite on a grain boundary scale this could partially explain the extremely low resistivities of the graphitized cataclastic zones the formation of magnetic pyrrhotite from 250 to 4000 m deep and the large self potential anomaly the latter requires a large vertical gradient of the oxygen fugacity and a continuous graphitic conductor through this gradient zone

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

seismic reflectors conductivity water and stress in the continental crust the uppermost 10 15 km of the earth s continental crust differs in several geophysical properties from the lower crust the upper crust is electrically resistive seismically transparent contains nearly all intracontinental earthquake hypocentres and responds to stress elastically with brittle fracture the lower crust is electrically conductive contains many seismic reflectors is aseismic and shows ductile response to stress i show here that the characteristics of both regions can be explained if the entire crust contains saline water in separated cavities in the compressively stressed rocks of the upper crust but in the lower crust forming an interconnected film on the crystal surfaces the model may apply principally to technically active parts of the continental crust beneath shields the lower crust may be dry

simple models for the beattie magnetic anomaly in south africa abstract the origin of the approximately 1000 km long beattie magnetic anomaly bma in south africa remains unclear and contentious key issues include the width depth and magnetization of its source in this study we use uniformly magnetized spheres prisms and cylinders to provide the simplest possible models which predict the 1 km altitude aeromagnetic measurements along a profile across the bma the source parameters are adjusted by forward modeling in case of a sphere an inversion technique is applied to refine the parameters our results suggest that two similarly magnetized and adjacent sources with a vertical offset can explain the observed magnetic anomaly the best fitting model corresponds to two highly magnetized 5 a m 1 sheet like prisms extending from 9 to 12 km depth and from 13 to 18 km depth respectively and with a total width reaching 80 km other less preferred models show thicker and deeper magnetized volumes associated magnetizations seem to be mostly induced although a weak remanent component is required to improve the fit we also compare our results with the interpretation of independent magnetotelluric and seismic experiments along the same profile it suggests that the geological sources for the bma are mostly located in the middle crust and may be displaced by a shear zone or a fault contrary to previous models suggesting a serpentinized sliver of paleo oceanic crust within the natal namaqua mobile belt we propose that granulite facies mid crustal rocks within this belt may cause the bma

eohimalayan fold and thrust belt implications for the geodynamic evolution of the nw himalaya india 1 in the himalayas the eocene deformation phase is called the eohimalayan event by means of a forward and backward balanced cross section along a classical traverse through the tethyan himalaya we constrain eohimalayan sw vergent folding and sw directed thrusting in a belt above a basal thrust at 10 km depth this thrust is located at the structural position of the southern tibetan detachment system with a ne directed normal sense displacement suggesting miocene reactivation of a major eocene contractional fault stepwise palinspastic restoration demonstrates that this eocene contractional fault has been subsequently openly folded due to southwestward propagation of the himalayan thrust system

interplay between erosion and tectonics in the western alps bedrock fission track analysis high resolution petrography and heavy mineral analyses of sediments are used to investigate the relationships between erosion and tectonics in the western alps along the aosta valley cross section exhumation rates based on fission track data are higher in the fault bounded western block than in the eastern block 0 4 1 5 vs 0 1 0 3 mm yr 1 erosion rates based on the analysis of bed load in the dora baltea drainage display the same pattern and have similar magnitudes in the relative sub basins 0 4 0 7 vs 0 04 0 08 mm yr 1 results highlight that climate relief and lithology are not the controlling factors of erosion in the western alps the main driving force behind erosion is instead tectonics that causes the differential upward motion of crustal blocks

soil formation on limestones in the arid region of northeast somalia abstract in order to clarify the history of landscape in the arid part of northern somalia the genesis of soils in the darror valley and its surroundings has been investigated more than 70 soil profiles were described in the field and 20 soil profiles were analysed in the laboratory in this paper analytical results of 5 representative soil profiles and 6 limestone samples are presented four soils are developed from limestone and are situated on levelled plateaus whereas the fifth soil is developed from alluvial material in a shallow depression the alteration intensity of these soils have been estimated by means of the fe o content and the si l al l ratio in soils of the elevated areas both the fe o content and the si l al l ratio decreases with depth in soils developed on alluvial material the fe o content as well as the si al ratio show no variation with depth and their values are as high as those for the top soils of the elevated areas this reflects that the parent materials of the soils in the depressions are originated from weathered material of limestones and that the soils are very weakly altered in situ by balancing the carbonate amount in the soils of the elevated area the primary height of their parent rocks limestone was calculated the age of the soils was also calculated through determination of the amount of carbonate lost by weathering these calculations were based on the assumption that the amount as well as the fluctuation of rain during soil formation was similar to the present conditions the results of these calculations indicate that the oldest soil in the landscape is relatively young and that most soils developed since the late pleistocene as a result of a continuous soil erosion since tertiary caused by tectonic uplift of the landscape and or intensive rainfall which occurred during past nonarid climatic conditions

strain partitioning in the axial nw alps since the oligocene 1 this work analyzes the postmetamorphic evolution of the northwestern alps thanks to a multiscale field geology study integrated by fission track analysis it illustrates how the fission track method constraining throws along major faults and age of mesoscale deformation provides interpretation keys to detangle complex structural data sets results are discussed within the framework of the alps apennines knot and give new insights on strain partitioning within transpressional orogenic belts the axial nw alps acted since the late oligocene as an orogen scale shear zone accommodating in the aosta valley dextral strike slip coeval with forward propagation of external thrusts within this transcurrent belt right lateral deformation was partitioned between the fast exhuming western block and the already unroofed eastern block these blocks show contrasting structural characters the former is cut by first order longitudinal structures bounding major tectonostratigraphic units the latter is fragmented by complex patterns of lower order faults interpreted as subsidiary shears the onset of mesoscale extension widespread in the eastern block is diachronous across the belt and migrated through time toward the european foreland transition from syncollisional shortening to orogen scale transcurrence observed in the nw alps corresponds to a regional strain field variation documented in the mid oligocene all around the alps apennines knot which is possibly related to a change of the adria trajectory relative to europe since the late oligocene deformation associated with oblique plate convergence was partitioned between strike slip in the axial belt and thrusting in external areas most of the shortening was accommodated in the helvetic chains 102 km deformation was limited elsewhere to an order of 101 km

magnetotelluric measurements across the beattie magnetic anomaly and the southern cape conductive belt south africa 1 the beattie magnetic anomaly bma and the southern cape conductive belt sccb two of earth s largest continental geophysical anomalies extend across the southern african continent in an east west direction to resolve structural details of the sccb a high resolution magnetotelluric study was conducted in march 2004 along a 150 km long n s profile across the karoo basin in south africa a two dimensional conductivity model at a scale of the entire crust exhibits three distinct zones of high conductivity 1 beneath the surface trace of the axis of the bma we observe a high conductivity anomaly at 5 to 10 km depth however it remains enigmatic whether both geophysical anomalies have the same source 2 a shallow regionally continuous subhorizontal band of high conductivity can be linked to a 50 to 70 m thick pyritic carbonaceous marker horizon intersected in deep boreholes 3 several highly conductive synformal features in the mid crust are newly imaged near the northern extremity of the profile our data generally support the existence of a deeper conductivity belt but allow for a much better definition of crustal conductivity in this region such laterally confined zones of crustal conductivity can only be resolved with a dense site spacing and broadband recordings

microseismic activity and basement controls on an active intraplate strike slip fault ceres tulbagh south africa earthquakes far from plate boundaries are poorly understood because of few well studied examples and uncertainty of what controls the location of these events in 1969 a damaging local magnitude m l 6 3 strike slip earthquake with no surface expression occurred in the ceres tulbagh region in the south african stable continental interior here we present a microseismic study of the ceres tulbagh area conducted over three months in 2012 in which 172 events recorded on at least three stations follow a gutenberg richter relationship for 1 5 m l 0 5 the events delineate a 5 km wide subvertical zone that is microseismically active to a depth of 15 km this fault zone is subparallel to the 1969 aftershock zone and at low angle to the regionally inferred greatest horizontal stress we argue that the microseismically active zone is guided by inherited structures in the basement geology this and similar structures may represent significant earthquake hazard in plate interiors online material tables of station and event parameters and p and s wave arrival times

magnetometer array studies earth structure and tectonic processes this review presents results from magnetovariation fields recorded by two dimensional arrays of magnetometers the emphasis is on the conductive structures mapped and studied and their tectonic implications eleven arrays were operated in north america between 1967 and 1985 a region of highly conductive uppermost mantle and or lower crust extending at least from 33 n to 54 n has been shown to extend beneath much of the continent west of the rocky mountains two recent investigations are discussed in more detail those of conductive structures under the canadian rockies and emslab electromagnetic sounding of the lithosphere and beyond array results for washington and oregon including a conductive strip beneath the cascades volcanoes correlations with high heat flow and seismic parameters make it reasonable to attribute these regional conductors to partial melting and or hot saline water within the craton the north american central plains conductor is discussed this narrow crustal feature is associated with a major fracture zone and age boundary which may be a plate boundary of proterozoic time an array beneath the auroral electrojet for study of the external currents is noted results from four array studies in southern africa are considered two of these relate to the southern cape conductive belt which correlates not with high heat flow but with a large static magnetic anomaly the magnetovariation magnetic and gravity anomalies and geological data can be accounted for if an accumulation of ophiolitic rocks including serpentine is present in the lower crust such an accumulation may have formed at a subduction in proterozoic time two other arrays delineated a conductive belt associated with the southwestern end of the african rift system in namibia subsequent resistivity soundings located a conductor of resistivity 10 ohm m at less than 4 km depth two array studies in india follow one in the north and one in the south the array in the himalayan foothills and indo gangetic plain mapped a conductor transverse to the regional strike this may be a fracture zone produced in the interplate collision with a rise of asthenospheric material such as fluids into the fractures the southern indian array reveals crustal conductors under the indo sri lankan graben and another under the comorin ridge southwest of the tip of the subcontinent array studies in central and southeastern australia and in fennoscandia in regions without large local conductivity anomalies have been used to study conductivity variation with depth a note on the carpathian crustal anomaly is included as an example of combination of response parameters secured at different times to form a virtual array

the baltica laurentia connection sveconorwegian grenvillian metamorphism cooling and unroofing in the bamble sector norway the sveconorwegian bamble sector of south norway is a segment of the high grade metamorphic basement of the baltic shield generally considered an integral piece of the global grenvillian orogenic belt in particular the bamble sector is generally directly linked to the grenville province of north america in middle proterozoic tectonic reconstructions however few high precision data exist on the metamorphic chronology of the bamble sector to test such models in this paper high precision u pb data for monazite allanite and titanite indicate that rocks from the bamble sector near arendal underwent granulite facies metamorphism at 1140 ma a second phase of metamorphism may be indicated by u pb titanite ages of 1105 ma or alternatively these may simply record delayed cooling or growth of titanite below temperatures for significant lead diffusion hornblende 40ar 39ar ages indicate cooling of the same rocks below 550 c between 1090 and 1080 ma metamorphic cooling rates of 3 8 c ma between 725 an

genesis of cracks in sandy soils of central east sahara a hypothesis abstract polygon cracks occur in almost all soils of the extreme arid part of the sahara annual precipitation due to daily and seasonal soil temperature fluctuation the dry soil particles mainly sand could get reorganized increasing the bulk density of the soil the soil particles are then able to get bound together through cohesion force and build a coherent soil mass shrinkage of the coherent soil mass by decreasing temperature could drag it into polygon cracks the enlargement of cracks can be explained by repeated expansion and shrinkage of the soil due to temperature fluctuation

soils of the darror valley northeast somalia summary the darror valley is located in the arid part of northeast somalia its soils are highly influenced by their parent material which consists mainly of evaporites limestone conglomerate and alluvial deposits the most common soils in this area are calcic solonchaks gypsic solonchaks gypsic calcisols haplic calcisols eutric leptosols and calcic vertisols all soils have a neutral to weakly alkaline ph value and are very rich in carbonates often rich in gypsum often saline and their moisture content at the end of the dry season is much lower than the moisture content of the permanent wilting point most of these soils have a colour of 7 5 yr which becomes more red in the subsoil analytical data indicate that the topsoils are usually enriched in carbonate whereas the subsoils are enriched in gypsum and or salt the soil salinization is mainly due to tertiary evaporite formations

tectonic geomorphology of the triglav lakes valley easternmost southern alps nw slovenia abstract we present a study on the impact of litho structural setting and neotectonic activity on meso and macro scale relief production in alpine areas the topography of the high alpine triglav lakes valley nw slovenia was studied by means of detailed mapping and stratigraphic study of the valley the triglav lakes valley is characterised by a generally asymmetric transverse e w profile a very steep eastern slope a relatively flat valley and a relatively gentle western slope on the transverse profile the valley floor is essentially flat gently dipping towards the east in the longitudinal cross section however the valley floor is marked by sharply defined fault blocks extending in a w e to nw se direction additionally the highest block elevations 2100 m is in the northern part of the valley the lowest elevations 1600 m in the southern part of the valley our research shows that the triglav lakes valley directly represents the topographic expression of paleogene neogene thrusting and faulting having recorded the following geomorphologic evolutionary stages 1 an oligocene to early miocene w vergent thrusting phase with steep w facing slopes of the eastern part of the valley directly representing the thrusting front and 2 a neogene to present strike slip faulting in nne ssw direction with two bifurcating right lateral strike slip systems we show that the triglav lakes valley almost perfectly mimics the wedge shaped damage zone located between these faults

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

giant non catastrophic landslides and the long term exhumation of the european alps landslides influence local slope morphology affect sediment flux from hillslopes to rivers and mass wasting in response to tectonics and climate forcing however the links between giant non catastrophic landslides known as deep seated gravitational slope deformations dsgsds and the long term evolution of orogenic landscapes are almost unknown we explore these links in the european alps using the first orogen scale inventory of dsgsds 900 over an area 105 km2 and a dataset of published apatite fission track ages 1000 that provides an estimate of the long term exhumation patterns of the orogen we show that dsgsds are more widespread than previously considered and exhibit an orogen scale distribution not explained by well known local lithological and structural controls we test the hypothesis that this orogen scale distribution correlates to the long term evolution of the alps by subdividing the study area into 37 square sub areas 50 50 km classified according to combinations of long term exhumation and mean annual rainfall on each sub area we perform a morphometric analysis of topography hypsometry relief slope excluding local and regional controls due to rock type and structure dsgsds tend to cluster in areas with intermediate exhumation rates fission track age between 10 and 40 ma where large scale topography is less dissected and incision is localised along major valleys here dsgsd abundance correlates positively with the degree of valley incision and related relief instead dsgsds lack in areas which underwent either low exhumation rates resulting in insufficient relief production or high exhumation rates associated to rapid uplift or higher erosional dissection of topography negative correlation between dsgsd abundance and mean annual rainfall suggests that effective hydrological surface processes contribute on the long term timescale to the development of large scale topography unfavourable to dsgsds especially in areas of high exhumation rates where dsgsds are abundant long lasting slope deformations effectively adjust post glacial relief by reducing slope inclination values and are thus expected to significantly contribute to the long term denudation of active orogens

late neogene erosion of the alps a climate driver as with many mountain belts globally the alps have seen a large increase in sediment yield in the late neogene the hypothesis that this increase results from climate change impacts on erosion rate over the past 5 ma is testable given the extensive work completed in the alps sediment budgets thermochronology based cooling rates and estimates of modern rock uplift and erosion in the western alps are reviewed to search for correlation in space and time that might fingerprint climatic events major increases in sediment yield are apparent starting in the late messinian 5 5 mya and accelerating toward the present day this suggests the occurrence of a series of climatic changes that increased erosion rates even as tectonic processes appear to have slowed or stopped at present tight correlations in time are difficult to establish and there remain open questions about the role of tectonics in the coupled system

erosion in the western alps dora baltea basin 2 quantifying sediment yield abstract the present work shows how sediment composition can be used to constrain bed load sediment budgets in mountain catchments a database of high resolution petrographic and dense mineral analyses of detritus carried by the dora baltea river and its tributaries allowed us to partition bed load sediment budgets obtained from data available from a public authority between two different sets of sediment sources major tributaries and major geological units relative and absolute budgets of sediment bed load thus obtained are more robust than those derived from traditional empirical methods alone total detrital flux in the dora baltea drainage basin is 1 000 000 ton year bed load 590 000 ton year suspended load 410 000 ton year with denudation rates of 0 12 mm year data from italian public authority progetto di piano stralcio per l assetto idrogeologico pai parma supplemento straordinario della gazzetta ufficiale 166 more than 75 of the dora baltea total bed load 453 000 9000 ton year is derived from tributaries dora di ferret dora di veny savara grand eyvia buthier lys sourced from the highest granitoid peaks 4000 m a s l m bianco m rosa m cervino gran paradiso massifs which drain 36 of the whole catchment the m bianco massif with its granitoid lithology extreme relief and heavily glaciated drainage basins occupies only 3 of total basin area but it produces 294 000 6000 tons of arkosic sands per year which represents 50 of total dora baltea bed load the dora di veny 2 of total dora baltea mountain catchment exhibits the highest denudation rate of the whole dora baltea mountain basin 0 73 mm year the values calculated in this study compare well with measured modern river loads of partly glaciated mid latitude mountain regions

a magnetovariational study of a geothermal anomaly summary during 1980 september and october an array of 31 threecomponent magnetometers was operated over a known geothermal anomaly in the rocky mountains foothills of alberta the purpose of the array was to investigate the possibility of the existence of a geomagnetic induction anomaly coincident with the geothermal anomaly examples of magnetograms and fourier transform anomaly maps are presented and show that although there is some indication of an anomaly this is to a great extent masked by the presence of large external source field gradients a detailed transfer function analysis of the data is presented both single station transfer functions and a solution of the induction matrix are calculated the latter involves the representation of the normal fields by least squares fitted planes and a justification of the validity of this method is given contour plots of the response of the anomalous vertical field to horizontal hypothetical events and pseudosections of iza hni for traverses across the array are shown these indicate the existence beneath the geothermal anomaly of a body of anomalously high electrical conductivity by treating the observed values of the in phase part of iza ha i along a traverse across the anomaly as due to vertical dipoles it is shown that the top of the conductive body probably lies at 10 20km depth the pseudosections suggest that the body extends at least to lower crustal depths the correlation of the conductivity and geothermal anomalies supports the inference that both are due to the presence of partial melt in the crust

neogene kinematics of the central and western alps evidence from fission track dating zircon and apatite fission track analysis has established the continuation of a zone of rapid change in mineral ages extending from the simplon fault zone in the simplon alps through the rhone valley and along the southeastern side of the mont blanc and belledonne massifs apatite fission track ages to the northwest of this zone generally range from 1 4 to 6 ma whereas those to the southeast range from 6 to 13 ma corresponding values for zircon are 11 13 and 16 26 ma respectively the zone corresponds to the position of the frontal pennine thrust in the mont blanc belledonne region this structure a prominent feature on recent deep seismic profiles may have been reactivated in the neogene with an important normal fault component this is suggested by apatite and zircon fission track ages in the footwall that are younger than those in the hanging wall and by the observed elimination of the lower pennine or simplon nappes along this fault zone in the rhone valley mont blanc belledonne region resulting in the juxtaposition of north penninic and helvetic ultra helvetic nappes at the current erosion level the distribution of fission track ages reflects this movement and the related neogene exhumation of the mont blanc and belledonne external massifs which has continued until less than 1 4 m y ago i e it is almost certainly still active and has tilted the overlying nappe sequence

are the alps collapsing abstract orogenic collapse is a process that transfers gravitational potential energy from regions of high potential energy to regions of lower potential energy this transfer is classically considered to be accomplished by extension in the orogenic core and by synchronous shortening in foreland regions of the orogen not all extensional features in collisional mountain belts need however reflect orogenic collapse normal faulting thrust faulting and strike slip faulting are all active in different parts of the alps today and reflect complex local responses to ongoing europe adria convergence the western alps is the only area today where extension and shortening radial to orogen trend occur synchronously and where orogenic collapse may be an important process elsewhere in the alps normal faults are oriented at a high angle to orogen trend and were primarily active in oligocene and miocene time most present day activity in the central and eastern alps is on strike slip faults that are accommodati

magnetotelluric soundings and the geological structure and tectonics of the senegalo mauritanian basin in northern senegal west africa the results of the magnetotelluric soundings provide new insights on the geological structure and tectonics of the onshore senegalo mauritanian basin in northern senegal west africa electrical resistivities can be correlated with the major geological units in the upper 1000 m resistivity variations indicate a transition from freshwater saturated sediments near the surface to saline water saturated sediments above the basement the resistivity of the sediments is dominated by their porosity and the resistivity of the contained fluids the resistivity of the cretaceous sediments is unexpectedly low especially in the western part of the basin the top of the metamorphic granitic basement cannot be located on the west portion of the profile because of the feeble resistivity contrast between the impervious compacted aptian jurassic limestones and the metamorphic crystalline rocks the magnetotelluric estimates of the depth of the basement range from 600 m in the east to 4000 m in the west based on the magnetotelluric interpretation and other independent geophysical and geological information a cross section through the senegalo mauritanian basin along the magnetotelluric traverse is presented a zone of high electrical conductivity is observed in the crust at depths of 4 6 km on the eastern margin of the deep basin this conductive structure may be the extension of one further south that was revealed by an earlier investigation the entire conductive zone which runs north south between longitudes 15 and 16 w may be a major fracture zone related to the opening of the atlantic ocean however its origin remains to be established

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

the southern cape conductive belt south africa its composition origin and tectonic significance abstract magnetometer array studies have led to the discovery and mapping of the southern cape conductive belt sccb crossing the southern tip of africa from west to southeast coasts the sccb lies just south of the namaqua natal belt of cratonic rocks remobilized about 1000 m y b p it is shown that it coincides with a zone of weakness which has been exploited by three major geosynclinal accumulations over some 600 m y relationships between the sccb and the basement geochronology geology and tectonics are considered in detail these relationships support the view that the conductive belt was formed by an accumulation of marine sediments and oceanic lithosphere at the top of a proterozoic subduction which stopped about 1000 to 800 m y b p associated with this subduction we propose a proterozoic range of andean mountains whose roots are now exposed in the namaqua natal belt later subduction further south near the present south coast is proposed to account for the intrusion between the south coast and the sccb of the cape granites in the time interval 600 500 m y b p there is some evidence for a third yet more distant subduction episode off permian gondwanaland after outlining this tectonic history the paper turns to a closer examination of the hypothesis that the southern cape conductive belt consists of partly serpentinized basalt accumulated at the top of a proterozoic subduction a large static magnetic anomaly which correlates with the sccb over most of its length is well fitted by a model which strongly supports this hypothesis bouguer gravity anomalies along western and central profiles likewise support the hypothesis a discussion follows of the process of formation of the proposed block of serpentinized marine rocks beginning with serpentinization of the crust near oceanic ridges by reaction of warm porous newly extruded basalt with seawater convecting through it the serpentinized basalt is stable at crustal temperatures and pressures and so is transported in the seafloor until it reaches a subduction where it accumulates because of its low density examples of such accumulations are cited finally it is shown that any iron in the olivine and pyroxene in the original basalt precipitates when these silicates are hydrated to serpentine as magnetite which is the dominant mineral conferring high electrical conductivity and high magnetic susceptibility on serpentinites in particular the beattie static magnetic anomaly requires in our model calculation a very high susceptibility readily attainable in basalt at 15 20 serpentinization the authors know of no other rock able to provide this high susceptibility a similar percentage of serpentinization provides the density required to model the gravity anomalies

geoelectrical structure of the northern part of the senegal basin from joint interpretation of magnetotelluric and geomagnetic data magnetotelluric mt and differential geomagnetic sounding dgs surveys were made in the senegal basin west africa to investigate the conductivity structure of the crust and upper mantle magnetic and electric variations were recorded at 11 sites along a east west profile 220 km in length the period range of the measurements was 10 10 000 s for the western stations the mt response displays low skew with moderate anisotropy the dgs data indicate the presence of an anomalous geomagnetic variation field in the deep basin and on the eastern margin of our study area a telluric current concentration in the thick conductive sediments of the deep basin might cause this anomaly both mt and dgs data are then compared with two dimensional model results to explain the observed features along the profile results reveal a strong anomalous crustal structure in the central part of the profile involving a conductive zone 2 ohm m at depths in the range 4 23 km within the deep basin the results of the two dimensional modeling are not conclusive acceptable fits between observed and calculated data have been obtained with alternate models a continuous conductive zone 0 7 2 ohm m may exist beneath the uppermost layers at a depth of approximatively 1 km but the observed data can be explained equally well by poorly conducting material intruded into the highly conductive sedimentary cover this ambiguity is inherent in the two dimensional models themselves both models show that the sediments increase westward to reach a thickness greater than 4000 m when approaching the coast it is suggested that the flow of current observed in this study is largely controlled by strong lateral variation in the resistivity of the basin and or by variation of the sedimentary thickness the models also indicate the existence of two decreased resistivity layers in the eastern part of the profile one at the base of the crust and the second at 80 km with a resistivity of 20 ohm m no such layers were detected at these depths below the deep basin at some depth greater than 150 km there is a general trend toward lower resistivities from 2000 to about 2 ohm m across the entire profile this may indicate the presence of a small degree of partial melt

trace element and nd isotope systematics in detrital apatite of the po river catchment implications for provenance discrimination and the lag time approach to detrital thermochronology abstract detrital thermochronology is often employed to assess the evolutionary stage of an entire orogenic belt using the lag time approach i e the difference between the cooling and depositional ages of detrital mineral grains preserved in a stratigraphic succession the impact of different eroding sources to the final sediment sink is controlled by several factors including the short term erosion rate and the mineral fertility of eroded bedrock here we use apatite fertility data and cosmogenic derived erosion rates in the po river catchment alps apennines to calculate the expected percentage of apatite grains supplied to the modern po delta from the major alpine and apenninic eroding sources we test these predictions by using a cutting edge dataset of trace element and nd isotope signatures on 871 apatite grains from 14 modern sand samples and we use apatite fission track data to validate our geochemical approach to provenance discrimination we found that apatite grains shed from different sources are geochemically distinct apatites from the lepontine dome in the central alps show relative hree enrichment lower concentrations in ce and u and higher 147 sm 144 nd ratios compared to apatites derived from the external massifs derived provenance budgets point to a dominant apatite contribution to the po delta from the high fertility lepontine dome consistent with the range independently predicted from cosmonuclide and mineral fertility data our results demonstrate that the single mineral record in the final sediment sink can be largely determined by high fertility source rocks exposed in rapidly eroding areas within the drainage this implies that the detrital thermochronology record may reflect processes affecting relatively small parts of the orogenic system under consideration a reliable approach to lag time analysis would thus benefit from an independent provenance discrimination of dated mineral grains which may allow to proficiently reconsider many previous interpretations of detrital thermochronology datasets in terms of orogenic wide steady state

precursor seismicity foreshocks and aftershocks of the uttarkashi earthquake of october 20 1991 at garhwal himalaya abstract the uttarkashi earthquake m b 6 6 of october 20 1991 occurred in a complicated geological tectonic setting at the main central thrust mct zone of the himalaya the main shock was preceded by a precursory earthquake activity which continued for about eight years and then there was a quiescent period for about three years three foreshocks m 4 0 occurred 48 h before the main shock two aftershocks m 4 0 occurred within 12 h of the main shock which was then followed by another two aftershocks m 4 0 after about seven days on october 27 1991 the largest aftershock m 5 2 occurred within 12 h the aftershocks magnitude less than 4 0 continued for about two months and it decayed at a rate t 0 87 about 1000 earthquakes magnitude less than 1 0 and above were recorded by a closely spaced five station temporary microearthquake meq network out of these 124 aftershocks which occurred inside the network are located by homogeneous station method spatial variation of the aftershocks with time was observed fault plane solutions of the main shock and aftershocks indicate dominantly thrust faulting based on the hypocentral data depth estimate of the plane of detachment is made and a fault interaction model is presented the model shows that the main shock occurred at the juncture of mct zone plane of detachment and the basement thrust the north dipping uttarkashi fault a parallel fault at the mct zone has been activated by the tectonic stress generated due to relative movement of the tibetan plate over the indian plate

miocene to present differential exhumation in the western alps insights from fission track thermochronology 1 analysis of cooling age patterns yielded by low temperature thermochronometers provides key information about the role played by tectonic discontinuities during the late stages of exhumation of metamorphic belts in the western alps fission track data published so far are heterogeneously scattered and concentrated in few structural domains preventing analyses at the scale of the whole belt the new apatite fission track data reported in this work obtained with the external detector method as well as the population method in very low u content samples fill this gap they constrain the postmetamorphic evolution of the western alps along two transects from the foreland to the retroforeland unraveling the effective role played by some major faults during the exhumation of the belt at shallow crustal levels a clear regional pattern characterized by decreasing ages moving from the axial sector to the european external sector of the belt and by an along strike gradient with increasing ages from north to south has been unraveled evident breaks in this age pattern have been detected in correspondence of faults that are near parallel to the trend of the belt pointing to the occurrence of active tectonics during and after exhumation the most apparent breaks have been observed in the axial sector of the belt where the postmetamorphic deformation would have been negligible according to classic tectonic models faults located in the axial sector split the belt into two major blocks eastern and western since the miocene the western block experienced higher exhumation rates than the eastern one such differential exhumation was accommodated in the northern portion of the belt thanks to reverse motion along the internal houiller fault which occurred in a convergent transcurrent framework to the south it was accommodated instead by normal reactivation of the brianconnais front and by activity of the longitudinal fault system which occurred in a divergent transcurrent framework the tectonic activity affecting the axial sector of the belt in a context of regional dextral strike slip is coeval with the forward propagation of the external thrusts and of similar magnitude we suggest that the contrasting kinematic regimes i e convergent versus divergent transcurrence observed in the western alps moving along strike were responsible of the increasing exhumation rates toward the north revealed in both blocks by the along strike age gradient the higher exhumation rates recognized northward would be related to an increasing importance of crustal shortening that promoted erosion during the late stages of exhumation of the belt

fission track and numerical thermal modeling of differential exhumation of the glarus thrust plane switzerland abstract geometry and differences in metamorphic grade along the glarus thrust external european alps can be explained by a restoration of the hanging wall some 10 km toward the south and back rotation of the thrust plane into a ramp flat ramp geometry the apatite fission track ft age pattern from the area suggests that isotherms were parallel to the thrust plane during cooling of the rocks along the thrust plane below the apatite closure temperature two forward modeling programs that both produce time temperature paths are used to constrain the thermal and structural evolution of differential exhumation along the glarus thrust a a two dimensional finite difference thermal model is used to examine differential exhumation of the thrust plane starting from an assumed horizontal position of the middle part of the glarus thrust at some 10 ma when parallel to the paleo isotherms five locations in the crustal section corresponding to locations on a final horizontal and vertical sampling traverse record their time temperature t t history during exhumation along particle paths the near surface temperature distribution is strongly influenced by the development of a simplified step like topographic relief which causes all modeled samples to reach the surface at the end of the modeled time period b the calculated t t paths of these five modeled samples were used to forward model apatite ft data based on an existing annealing model by comparison between synthetically generated ft data and actual measured ft samples the exhumation history of the glarus alps was evaluated with respect to its thermal and temporal brackets several parameters such as the location of the rotation point the thermal gradient and the topography have significant influence on model results best fit models produce results that are consistent with independent geological and geophysical information

rates of continental erosion and mountain building the first objective of this work was to obtain values for the rates at which continental erosion can smooth out or remove the topographic expression produced by orogeny the dominant part is played by mechanical erosion which acts most strongly in regions of large topographic expression chemical erosion depends strongly on precipitation or runoff in individual river drainage basins but because most continents have very similar average rainfall chemical erosion is fairly uniform for continental sized areas and will succeed in planing down all continents to a level peneplain if given enough time the exception to this rule is australia which has a very low chemical erosion rate because of its dryness the time constants for mechanical and chemical erosion so obtained vary between about 30 and 300 my depending on the continent and the assumptions made mountain building occurs throughout the geological time scale but at a non uniform rate although there will not be a balance between erosion and mountain building over a short time scale due to the non uniform rate of mountain building the long term situation must be that the two phenomena should balance out it is shown that the freeboard of continents will respond to the long term balance between mountain building and erosion an expression has been derived for the average continental elevation in which the rate of mountain building depends on the rate of radiogenic heat production within the earth it is shown that relatively small changes in average elevation above sea level of a few hundred metres are predicted to have occurred since the beginning of the proterozoic as mountain building is predicted to decrease on average with time because of the reduction in internal heat generation and as erosion is dependent on the average elevation this average elevation will decrease slowly through time the opposite of what some workers have predicted a more complicated model of mountain building is then investigated in which one component of mountain building has a sinusoidal signal the oscillations in average elevation depend on the period of the sinusoid being smaller for shorter periods finally an average continental elevation is derived using a list of real orogenic events although this list of orogenies is incomplete there is some indication that the actual continental elevation as seen in the flooding history of the continents is similar to that derived in this paper

tectonic interpretation of an inverse gradient of zircon fission track ages with respect to altitude alpine thermal history of the gran paradiso basement extensive sampling along vertical and north south cross sections in the gran paradiso basement internal crystalline massif western alps reveals an inverse gradient of zircon fission track ages with respect to altitude which is explained by an inverted metamorphism during eocene time subsequent to the obduction of the schistes lustres nappe high pressure eoalpine phase the gran paradiso massif cooled down 90 80 myrs ago the austro alpine nappe of monte emilius dent blanche overthrust the gran paradiso massif 38 40 myrs ago inducing in its basement an inverted metamorphism or more precisely a nappe emplacement metamorphism greenschist facies the last cooling below 100 c occurred as early as 35 40 myrs ago in the southern gran paradiso massif in the northern part this last cooling occurred 20 myrs ago accompanied by an important uplift phase 2 mm year

electromagnetic geophysics and global tectonics lateral variations of electrical conductivity in the lithosphere and below it can be informative concerning both contemporary and ancient global tectonics the methods of investigation include classical four electrode resistivity depth probing which is of special value in continental shields magnetotellurics long used on land and recently in the pacific to show that in electrical terms the lithosphere is thicker near an oceanic ridge than had been expected and magnetometer arrays which are useful primarily in mapping highly conductive structures examples of the use of all three methods will be reviewed with emphasis on the last in conjunction with other geophysical geological and geochemical data magnetometer arrays have been useful in mapping partial melt under the western united states in discovering an extension of the southern end of the african rift system to the atlantic coast in detecting a major fracture zone in central north america which may be a proterozoic interplate collision feature and in discovering a conductive belt under southernmost africa which may result from accumulation of serpentinized marine crustal rocks at the top of a subduction under proterozoic gondwanaland inherent difficulties of quantitative modeling limit the utility of magnetometer arrays largely to discovery and mapping of two and three dimensional conductive structures this remains valuable however particularly when other physical properties can be used to limit the range of possible interpretations of the conductive structures recent array studies now in progress in western canada have defined two new conductivity anomalies one of these may represent a local partial melt concentration under the rocky mountains trench the other a precambrian rift valley in the lower crust previously suggested by others on the basis of seismological gravity and static magnetic data both are being investigated by more concentrated arrays

staircase 40 ar39ar spectra of fine grained white mica timing and duration of deformation and empirical constraints on argon diffusion staircase 40 ar 39 ar spectra of 2 to 6 m white micas in ductile carbonate mylonites morcles doldenhorn nappe switzerland record ages between 32 and 13 ma these ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold thrust nappe we interpret these data to record protracted white mica neocrystallization and growth during low temperature ductile deformation such that the differences between the maximum and minimum ages in the 40 ar 39 ar spectra record the time span over which mica formed and grew we have modeled the argon concentrations in these micas taking into account the thermal history of the nappe mica growth and diffusional loss of argon our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite and that the retentivity of argon in white mica is greater than generally thought

the aosta ranzola extensional fault system and oligocene present evolution of the austroalpine penninic wedge in the northwestern alps post nappe brittle structures across the austroalpine penninic wedge are representative of two principal deformation stages the former d1 developed during the late oligocene the latter d2 from the miocene to the present major d1 features are the e w aosta ranzola half graben and the ne sw ospizio sottile subvertical normal fault system oligocene extension was also characterized by hydrothermal activity and intrusion of calc alkaline to ultra potassic dikes from the miocene onwards d2 the ospizio sottile system was re activated as a se sinistral boundary of the pennine graian alps block of nappes bodily moving away from the simplon detachment to the sw inside the escaping block a number of d2 slightly diverging sublatitudinal splays aosta piccolo san bernardo system was generated as well as a regular set of nw trending normal faults from the simplon to the gran paradiso massif instead the aosta ranzola system was poorly reworked in that time more generally the short lived oligocene extension d1 was associated to slab break off of lower plate wedge rebound rapid uplift and thermal perturbation from the early miocene onwards the oligocene extension was replaced by renewed compression allowing the vertical extrusion of the lepontine dome and lateral escape of the overlying pennine graian alps block

erosion in the western alps dora baltea basin 1 quantifying sediment provenance abstract sourced in the m bianco the highest mountain of the alps the dora baltea river cuts across the arc of the western alps drains all major alpine tectonic units from the helvetic domain to the south alpine domain and finally joins the po river petrographic composition of clastic detritus carried by the dora baltea river and its tributaries faithfully mirrors the complex geology of the western alps high resolution modal analysis of dora baltea sands allowed us to distinguish seven end member metamorphoclastic provenances river sands from the m bianco external massif are dominated by quartz feldspars and hornblende detritus from basement and cover rocks of the brianconnais zone includes low grade metasedimentary lithic fragments epidote chloritoid and garnet sands from the internal penninic massifs include quartz and subordinate feldspars garnet and epidote abundant metabasite and serpentine schist lithic grains along with epidote actinolite and few pyroxenes characterize detritus from the calcschists and meta ophiolites of the oceanic piemonte zone sands from the austroalpine zone include quartz abundant metamorphic lithic fragments garnet and locally significant sillimanite dent blanche kinzigites the relative contribution of each end member to the dora baltea sediment flux was assessed with forward modelling of end member mixing our estimates calculated from the entire petrographic and mineralogical data set indicate that the main sources of bed load in the dora baltea mountain basin are the granitoid to gneissic rocks exposed in the m bianco massif and in the austroalpine zone in order to make allowance for the contribution of a particular bedrock constituent in a compound source area to the derivative sand the original concept of sand generation index sgi was modified to assess the relative contribution of the seven end member sources of detritus the highest total basin sgi indices are calculated for the m bianco massif sgi 16 4 and austroalpine domain sgi 1 4 the m bianco massif which represents only 3 of the whole catchment has the highest capacity of sand sized bed load production owing to its granitoid lithology and extreme relief this actualistic provenance study demonstrates that sediment composition can be markedly influenced by source rocks that occupy only a minor portion of the drainage area

an integrated geophysical study of the beattie magnetic anomaly south africa abstract the source of the beattie magnetic anomaly bma still remains unclear with several competing hypotheses here we add a piece to the puzzle by investigating available potential field data over the anomaly filtered magnetic data show the bma as part of a group of linear magnetic anomalies as the linear anomaly north of the bma is associated with exposed supracrustals migmatites and shear zones within the natal thrust terranes we assume a similar source for the bma this source geometry constrained by seismic and mt data fits potential field data over the bma and other magnetic linear anomalies in the south central and south western karoo in these models the bodies deepen from 5 km towards the south with horizontal extents of 20 60 km and vertical extents of 10 15 km densities range from 2800 to 2940 kg m 3 and magnetic susceptibilities from 10 to 100 10 3 si these magnetic susceptibilities are higher than field values from supracrustal rocks 10 60 10 3 si but could be due to the fact that no remanent magnetisation was included in the model the lithologies associated with the different linear anomalies vary as is evident from varying anomaly amplitudes the strong signal of the bma is linked to high magnetic susceptibility granulite facies supracrustals 10 50 10 3 si as seen in the antarctic where the mobile belt continued during gondwana times