tectono sedimentary characteristics of late miocene quaternary extensional basins of the northern apennines italy abstract extensional and compressional regimes exist at the same time in adjacent parts of the northern apennines italy and the former regime succeeded the latter at the same place as the thrust mountain front moved eastward since miocene times numerous extensional basins have developed west of the present mountain divide the westernmost ones formed over attenuated continental crust since late miocene and have been subjected to several subsidence and uplift events with rates of subsidence generally faster than rates of sedimentation they contain continental deposits overlain by gypsum bearing upper miocene marine sequences and siliciclastic marine pliocene deposits these basins were affected by shallow magmatism and are still experiencing high geothermal gradient and late to post magmatic activities such as in the renowned thermal region of larderello to the east closer to the present mountain divide the extensional basins formed later from pliocene to quaternary contain continental fluvial and lacustrine deposits and have not experienced near surface magmatic activities the sedimentary fill architecture of the apennine extensional basins is similar to that of other european american and african extensional or transtensional systems for instance a common characteristic is the prevalence of large alluvial fan deposits at the hanging wall shoulder of half grabens coarser smaller alluvial fans at the footwall shoulder and some fluvial deposits introduced longitudinally unlike the african rifts though the sedimentary facies distribution of extensional basins which developed in active orogenic zones such as in the apennines may be strongly affected by uplifting steep mountain slopes and can have large thick coarse alluvial fans also on the footwall side of the half grabens

thin lithosphere asthenosphere boundary beneath eastern indian craton the lithosphere asthenosphere boundary lab separates the hard and rigid outer layer of the earth lithosphere and the weaker hotter and deeper part of the upper mantle asthenosphere and plays a pivotal role in plate tectonics however its definitive detection especially beneath the cratons is proving elusive one of the geophysical tools used to map the lab beneath the cratons is through magnetotelluric mt observations the resistivity at boundary falls in the range of 5 25 m and can be explained by the presence of a small amount of water in the asthenosphere possibly inducing partial melt here we report thickness of the lab in one of the oldest dated ancient cratons of india eastern indian craton eic of 3 3 gyr from mt studies the two prominent phase sensitive strike directions one each for crust and mantle and the presence of resistive continental lower crust act as a window to mantle in resolving deeper electrical conductivity structures beneath eic our results show that the lab beneath the eic is at 95 km the region is interesting as the electrical properties of the crust and mantle and the moho depth are similar to those of the slave craton canada 4 0 gyr but the depth of the lab beneath the eic is half that of the slave craton as cratonic signatures depicted by ultrapotassic rocks from gondwana coal fields close to eic are preserved at least till early cretaceous 117 ma it is likely that himalayan orogeny could have played a major role in delamination of the lithospheric roots of the eic in addition to attendant seismicity

signatures of continental collisions and magmatic activity in central brazil as indicated by a magnetotelluric profile across distinct tectonic provinces broadband and long period magnetotelluric soundings were collected along a 560 km e w profile in a region of central brazil subjected to neoproterozoic collision tectonics and archean to cretaceous magmatic events the profile crosses the northeastern portion of phanerozoic sediments and volcanics of the parana basin the southern extension of the neoproterozoic metasedimentary rocks of the brasilia belt locally pervaded by cretaceous alkaline magmas of the alto paranaiba igneous province apip and neoproterozoic sedimentary cover and exposed archean basement of the southern sao francisco craton 2d conductivity structures derived by joint inversions of the te and tm polarization modes and a separate inversion of the tipper components show signatures of the past tectonomagmatic events that affected the area a gravity defined suture zone beneath the parana basin is detected in the models as a subvertical conductor extending from crustal to upper mantle depths which strengthen the interpretation of a neoproterozoic collision of the sao francisco craton and a continental block beneath the sedimentary basin deep underthrusting of organic graphite bearing metasedimentary rocks composing a fossil suture zone is proposed to explain such increase in electrical conductivity a similar conductivity signature beneath the sedimentary covered region of the sao francisco craton can be tentatively interpreted as another similar cryptic suture zone other isolated high conductivity anomalies at midcrustal depths below the parana and apip provinces are interpreted as fossil residues representing precipitated graphite and sulfide derived from percolating volatiles during the emplacement of cretaceous mafic ultramafic volcanics a very high conductivity wedge into the lower lithosphere is highlighted in the deep mantle beneath the apip volcanic complex coincident with a zone of low velocity defined by seismic tomography geochemical evidence indicates that the alkaline magmatism stemmed from a metasomatized upper mantle at only slightly raised temperatures thus interconnected carbonatite melts of low melting point and graphite in the lithospheric mantle are the most likely candidates to explain this high conductivity the seismic and conductivity anomalies are probably triggered by the same source mechanisms incipient melting because they are plausibly related to the same magmatic process conductivity anomalies at crustal and mantle depths in the southern segment of the sao francisco craton suggest that its lithosphere was significantly affected by the several tectonomagmatic episodes it has experienced throughout its geological history consequently the enhanced conductivity in the lower crust can be genetically related to upwelling volatile rich intrusions whereas upper mantle high conductivity can be related to refertilization by infiltrations of low degree carbonatitic melts from deeper sourced metasomatic processes

electrical resistivity structure of the upper mantle beneath northeastern china implications for rheology and the mechanism of craton destruction abstract the north china craton ncc and central asian orogen belt caob in northeastern china experienced a range of tectonic events during the phanerozoic dominated by lithospheric thinning of the eastern ncc in the late mesozoic and cenozoic in order to better understand the tectonic evolution of the ncc and the caob new broadband and long period magnetotelluric data were collected along a north west to south east trending profile that extended from the caob across the yanshan belt the tanlu fault zone to the liaodong peninsula a two dimensional 2 d resistivity model was derived from inversion of the transverse electric mode transverse magnetic mode and vertical magnetic field data in the crust of the caob the resistivity model shows a northwest dipping low resistivity zone beneath the solonker suture that is identified as the suture zone formed by the collision between the siberian and north china cratons the upper mantle of the caob is characterized by moderate resistivity values 300 1000 m that are best explained by the presence of hydrogen dissolved in olivine the water concentration of the caob mantle is comparable to values reported for the asthenosphere and cratons that have been significantly hydrated the ncc upper mantle is generally lower in resistivity than the caob upper mantle and a zone of lower resistivity is observed in the upper mantle at the southeast end of the profile beneath the ncc superimposed on this southeast decrease in upper mantle resistivity three low resistivity zones were imaged 1 below the xilamulun fault 2 close to the north south gravity lineament and 3 between the northern yanshan belt and tanlu fault zone the low resistivities can be explained as regions of partial melts or fluids perhaps caused by asthenospheric upwelling together with seismic imaging results and geochemical data the resistivity model shows that the modification of the lithosphere associated with craton destruction has occurred in a spatially non uniform manner in the region of the ncc investigated in this study

shallow erosion beneath turbidity currents and its impact on the architectural development of turbidite sheet systems erosion by turbidity currents changes the morphology of the sea floor the relief of the scoured surface may affect the dynamics of the flow and thereby the pattern of deposition this could in turn affect flow and deposition patterns in subsequent events this study investigates shallow centimetre to decimetre scale erosion beneath turbidite sheet sandstones of the oligocene macigno formation of north west italy where erosion and deposition are variably coupled at the bed scale in a net aggradational setting the research focus was on i the recognition of scour edges and erosive surfaces ii quantification of spatial differences in the amount of erosion and iii an investigation of how this differential erosion can be compensated by the deposits directly overlying the erosional surfaces where they can be observed scour edges commonly have sills of the overlying sandstone intruding beneath blocks and wings of the substrate that is being eroded a consequence of this de laminating scouring style is that erosional surfaces are bedding parallel when followed away from the scour edges giving the appearance of normal conformable bed bases despite their cryptic nature such bedding parallel scour surfaces can be recognized by comparing serial detailed sedimentary logs here 16 bed parallel scour surfaces were identified in a succession comprising 95 beds different styles of compensation by the overlying turbidite beds are defined based on differential sedimentation inside and outside of the scour relief it is found that differential erosion is on average under compensated by differential sedimentation in some cases the overlying deposits anti compensate being thinner at the location where more erosion has occurred unequal spatial distribution of differential erosion in the study area combines with sedimentary under compensation to result in a trend of accumulating section thickness differences over multiple beds in one ca 25 m thick package the maximum cumulative change in lateral gradient during some 20 events reached 0 17 before being reset by a single event this process can be interpreted either as a lobe compensation effect or as a scour enhancement effect depending on the orientation of the palaeohorizontal datum if allowed to proceed the latter process could force the system past a channellization threshold prompting a change from sheet to channelled architecture this type of shallow substrate scouring and differential deposition is likely to be an important process in the build up of sheet turbidite sandstone units and could play a major role in autocyclic adjustment of local sea floor gradients

geoelectric structure of the dharwar craton from magnetotelluric studies archean suture identified along the chitradurga gadag schist belt summary broad band magnetotelluric data were collected at 50 stations over a 400 km long approximately east west profile over the granite greenstone terrain of dharwar southern india the tensor decomposed data were interpreted using a 2 d inversion scheme the geoelectric model is suggestive of a suture along the chitradurga gadag schist belt formed by the thrusting of the west dharwar craton beneath its eastern counterpart with an easterly dip of 20 30 the thrust proposed here pre dates the formation of these schists which occurred during the late archean 2600 ma the accretionary wedge of the thrust and the depressed part of the west dharwar craton may have controlled the emplacement of the chitradurga gadag and shimoga dharwar schists the subsequent weathering the several episodes of tectonic activity witnessed during the precambrian and the vertical block movements caused during the passage of the indian plate over the reunion hotspot may have modified the crust leading to the present day geological configuration despite its age and several tectonothermal episodes the signature of this thrust is adequately preserved in the dharwar craton several similarities with younger sutures as is evident from the observed relics of the oceanic rocks present along the chitradurga schist belt suggest that the tectonic processes leading to this archean event may have had a close resemblance to those witnessed in recent times magnetotelluric studies also image a zone of low resistivity at a depth of 40 km beneath the west dharwar craton this seems to be a regional feature extending to the north over a distance of at least 250 km beneath the deccan volcanics the low heat flow values and the high density associated with this feature make partial melting an unlikely explanation for the low resistivity the grain boundary graphites and the sulphides deposited in the form of pyrites may have caused the low resistivity in the lithospheric mantle of the west dharwar craton although the fluids generated and trapped in the mantle during the passage of the indian plate over the reunion hotspot in the waning phase of its outburst could also be a possibility the asthenosphere is delineated at a depth of about 100 km beneath the east dharwar craton

electrical conductivity structure of southeastern north america implications for lithospheric architecture and appalachian topographic rejuvenation abstract we present the first three dimensional view of the lithospheric electrical conductivity structure beneath southeastern north america by inverting earthscope long period magnetotelluric mt data we obtain an electrical conductivity image that provides new insights into both the architecture of the appalachian orogen and the cryptic post rifting geodynamic history of the southeastern united states our inverse solutions reveal several elongate electrically conductive features that we interpret as major terrane sutures within the appalachian orogen most significantly we resolve a highly electrically resistive layer that extends to mantle depths beneath the modern piedmont and coastal plain physiographic provinces as high resistivity values in mantle minerals require cold mantle temperatures the mt data indicate that the sub piedmont thermal lithosphere must extend to greater than 200 km depth this firm bound conflicts with conclusions from seismic results the boundary between the anomalously thick resistive sub piedmont lithosphere and the relatively thin moderately conductive sub appalachian lithosphere corresponds within resolution to the modern appalachian topographic escarpment this newly recognized contrast in lithospheric properties likely has important implications for appalachian topographic rejuvenation

paleoenvironmental evolution in a high stressed cold seep system vicchio marls miocene northern apennines italy abstract excellent exposures of miocene seep carbonates enclosed in marine marly sediments vicchio outcrops in the northern apennines italy offer the opportunity to highlight the evolution of a fossil seep ecosystem and the response of benthic communities to high stressed environmental conditions for this purpose seep related facies and molluscs within carbonate bodies have been studied coupled with benthic foraminiferal assemblages and carbon and oxygen isotopes in the enclosing marls the integrated planktonic foraminiferal and calcareous nannofossil biostratigraphy has allowed us to constrain the seepage within well calibrated bioevents and to stress the relationships with paleoclimatic and paleoceanographic variations during the middle miocene in the mediterranean area our biostratigraphic data indicate that the onset of the seepage approximates the mi3b cooling event 13 82 ma and the seepage system lasts for 400 kyr the evolution of the vicchio cold seep system passes through four phases 1 the onset of the seepage characterized by a pervasive flow of methane rich fluids is inferred by 13c depletion of marly sediments and by prevailing benthic foraminifera indicative of suboxic conditions at the sea floor 2 the methane flow becomes focused causing the precipitation of wide pinnacle like carbonate bodies which contain giants lucinids enclosing marls indicate well oxygenated conditions possibly enhanced by paleoceanographic variations connected to the mi3b cooling event 3 the appearance of the vesicomyid christineconcha cf c regab and the absence of lucinids in seep carbonates suggest stable methane rich fluid emissions higher flow rates locally favoured the flourishment of bacterial mats benthic foraminifera show abundance peaks of organic matter depending taxa 4 the reduced intensity of methane rich fluid flows favours the precipitation of stratiform carbonate bodies along strike the macrofauna is characterized by the presence of both vesicomyidae and giant lucinids foraminiferal assemblages in the enclosing marls indicate the restoration of well oxygenated conditions

mass transport deposits in a variscan wedge top foreland basin pisuerga area cantabrian zone nw spain abstract large scale mass transport deposits mtds from two wedge top depocentres in a variscan foreland basin system cantabrian zone iberian massif northern spain have been studied to gain a better understanding of the processes involved in comparable deposits found in modern submarine environments the well exposed outcrops allow documenting the composition internal organization and lateral changes of the mtd units and additionally to know their frequency and significance in a tectonically active depositional setting the synorogenic 5000 m thick succession that contains these resedimented units comprises seven major tectonically controlled sequences whose bounding unconformities were recognised by seismic scale stratal patterns such as fault propagation and growth fold geometries wedging truncation and onlap these major sequences range between 4 and 0 4 myr in duration and between 1300 and 200 m in thickness and display an overall shallowing upward trend from a basal mtd and deep water sediments prodeltaic mudstone wedges turbidites to shallow marine carbonates and deltaic deposits each major sequence consists of one or more transgressive regressive high frequency sequences up to several hundred metres in thickness showing rapid lateral changes of facies associations across the wedge top depocentres the mtd units are composed of megaclasts up to 2 km long and 360 m thick embedded in mudstones matrix and clast supported breccias and conglomerates with variable internal organization and subordinate turbidite sandstones deposited from slides and gravity flows from cohesive debris flows to high density turbidites these en masse deposits originated from the failure of shelf to slope reaches and some of them constitute multiepisodic units resulting from several large scale collapse events these collapses could have been triggered by several factors mainly related to the growth of fault propagation anticlines during thrust emplacement the ensuing tectonic overload would lead to strong flexural subsidence and to the modification of the basin profile both the major sequences and the mtd units are a useful tool for correlating the highly deformed and faulted successions across the three fault bounded synclines and establishing a coherent stratigraphic framework

an empirical electric field model derived from chatanika radar data plasma convection velocity observations obtained with the chatanika alaska incoherent scatter radar have been used to prepare a quantitative empirical model of the convection electric field at auroral latitudes between 58 and 75 vector averaging of the individual days data and smoothing across 2 5 hours of local time were used to reduce substorm perturbations tables of the vector electric field components at 300 km altitude have been prepared for moderately disturbed kp 3 summer conditions resolution of the model is 0 5 of invariant latitude and one half hour of magnetic local time the model field is asymmetric with the afternoon convection cell dominant in magnitude and spatial extent and corresponds to conditions in which the cross polar cap potential is approximately 70 kv

sand rich turbidite system of the late oligocene northern apennines foredeep physical stratigraphy and architecture of the macigno costiero coastal tuscany italy

construction and destruction of the north china craton with implications for metallogeny magnetotelluric evidence from the hengshan wutai fuping region within trans north china orogen abstract the hengshan wutai fuping region within the trans north china orogen is an important corridor that preserves the records of precambrian evolutionary history and phanerozoic magmatic and metallogenic events in the north china craton here we present 2 d and 3 d inversion models of magnetotelluric data collected from the hengshan wutai fuping region and surrounding regions our data reveal nearly horizontal high conductivity layer at lower crustal and upper mantle depths of 35 45 km beneath the hengshan and wutai mountains providing direct evidence for crust mantle decoupling and magmatic underplating in the mesozoic yixingzhai gold deposits and boqiang cu mo au deposits two nearly upright conductors cut through the crust and connect the lower crustal high conductivity layer suggesting frozen channels of fluids and magmas a west dipping slab shaped conductor within the high resistivity lithosphere beneath the wutai complex appears to be the deep extension of the exposed longquanguan ductile shear zone these resistivity anomalies can provide new constraints on the subduction collision events during precambrian tectonic evolution of the trans north china orogen as well as on the phanerozoic tectonic magmatic and metallogenic events in the hengshan wutai fuping region the west dipping slab shaped conductor may indicate a westward oceanic subduction between the wutai and fuping complexes with subsequent collision associated with the formation of the trans north china orogen in the paleoproterozoic we interpret the crustal and upper mantle conductors to be part of an interconnected fluid melt transport channel which probably played an important role for the mesozoic tectonic magmatic and metallogenic events as well as the cenozoic basaltic volcanism in the hengshan wutai fuping region

lithospheric structure of an archean craton and adjacent mobile belt revealed from 2 d and 3 d inversion of magnetotelluric data example from southern congo craton in northern namibia 1 archean cratons and the stitching proterozoic orogenic belts on their flanks form an integral part of the southern africa tectonic landscape of these virtually nothing is known of the position and thickness of the southern boundary of the composite congo craton and the neoproterozoic pan african orogenic belt due to thick sedimentary cover we present the first lithospheric scale geophysical study of that cryptic boundary and define its geometry at depth our results are derived from two dimensional 2 d and three dimensional 3 d inversion of magnetotelluric data acquired along four semiparallel profiles crossing the kalahari craton across the damara ghanzi chobe belts dgc and extending into the congo craton two dimensional and three dimensional electrical resistivity models show significant lateral variation in the crust and upper mantle across strike from the younger dgc orogen to the older adjacent cratons we find damara belt lithosphere to be more conductive and significantly thinner than that of the adjacent congo craton the congo craton is characterized by very thick to depths of 250 km and resistive i e cold lithosphere resistive upper crustal features are interpreted as caused by igneous intrusions emplaced during pan african magmatism graphite bearing calcite marbles and sulfides are widespread in the damara belt and account for the high crustal conductivity in the central zone the resistivity models provide new constraints on the southern extent of the greater congo craton and suggest that the current boundary drawn on geological maps needs revision and that the craton should be extended further south

electrostatic potential in the auroral ionosphere derived from chatanika radar observations a technique is described for determining the latitudinal variation of the electrostatic potential associated with the ionospheric convection electric field using the north south electric field component derived from radar convection velocity experiments the integral of e dl is taken northward along the magnetic meridian starting at low latitudes the radar data consist of up to 40 independent measurements of plasma convection spanning 15 of invariant latitude centered on chatanika alaska 65 with half hour temporal resolution it has been found that 1 the electric field contributions to the potential at and below 60 are small under most circumstances and 2 the latitudinal variation of the potential is smooth and regular permiting the potentials to be contoured across local time it is found from the experiments that the potential often varies uniformly over 10 latitude at dawn and dusk electric fields of 50 mv m are common it is also noted that the latitude of the greatest negative potential in the premidnight sector coincides with the harang discontinuity in ionospheric currents the potentials calculated from the measured plasma drifts exhibit a regular local time variation equipotential contours derived from the latitude local time potential field obtained with the long duration radar experiments while not providing a snapshot of the instantaneous pattern elucidate the large scale diurnal variation of the electrostatic potential at auroral latitudes from such contours it is found that a two cell convection pattern with varying degrees of asymmetry is consistently present at auroral latitudes that a cross polar cap potential drop of 70 120 kv is present in moderately disturbed conditions and that substorms perturb the potential pattern at all local times

ground based studies of ionospheric convection associated with substorm expansion the instantaneous patterns of electric fields and currents in the high latitude ionosphere are deduced by combining satellite and radar measurements of the ionospheric drift velocity along with ground based magnetometer observations for october 25 1981 for this purpose an updated version of the assimilative mapping of ionospheric electrodynamics technique has been used these global patterns are unobtainable from any single data set the period under study was characterized by a relatively stable southward interplanetary magnetic field imf so that the obtained electric field patterns do reflect in general the state of sustained and enhanced plasma convection in the magnetosphere during one of the satellite passes however an intense westward electrojet caused by a substorm intruded into the satellite de 2 and radar chatanika alaska field of view in the premidnight sector providing a unique opportunity to differentiate the enhanced convection and substorm expansion fields the distributions of the calculated electric potential for the expansion and maximum phases of the substorm show the first clear evidence of the coexistence of two physically different systems in the global convection pattern the changes in the convection pattern during the substorm indicate that the large scale potential distributions are indeed of general two cell patterns representing themore southward imf status but the night morning cell has two positive peaks one in the midnight sector and the other in the late morning hours corresponding to the substorm expansion and the convection enhancement respectively 40 refs 12 figs 1 tab less

electrodynamics of the westward traveling surge it is shown that the global convection pattern the ionospheric current and the field aligned current associated with the westward traveling surge in the asymptotic state can be modeled quantitatively as consequences of a blockage of the hall current from closure in the magnetosphere via field aligned currents the conductivity is allowed to increase self consistently with increasing upward field aligned current in the model this inclusion of the self consistent enhanced ionospheric conductivity due to discrete auroral precipitations is found to generate a localized intense westward electrojet on the poleward side of the harang discontinuity the westward electrojet is also found to rotate counterclockwise merging into the eastward electrojet around the leading edge of the surge thus the major features of the westward traveling surge can be reproduced reasonably well in our model

electrical resistivities of crustal fluids results of calculations on the electrical resistivities of crustal fluids as a function of temperature pressure and fluid composition indicate that crustal fluids can range in resistivity from approximately 1 m the dominant contribution to decreasing the resistivities of crustal fluids comes from increased concentrations of chloride and bicarbonate salts and increased temperature and density of the fluids calculations indicate that the sharpest decrease in fluid resistivity occurs at shallow depths 10 km and temperatures 300 c can not be readily explained as a product of variations in resistivities of crustal fluids these results indicate that a transition in mineralogy or pore shape geometry is more likely responsible for the observed decrease in electrical resistivity of the middle crust than a major change in fluid resistivity

foreland basin sequence response to collisional tectonism as structural salients and recesses evolved from reentrants and promontories along the collisional continental margin associated with the taconic orogeny cross strike structural features provided a mechanism for transferring zones of relative subsidence and uplift across the appalachian foreland basin the regional distribution of lower silurian clastic sequences reflects this tectonic influence thick aggradational sequences formed in areas corresponding to salients in response to high rates of sediment supply and creation of sediment accommodation as the rate of sediment supply exceeded the rate of accommodation added shoreline progradation onto the distal foreland ramp produced upward coarsening sequences in areas of structural recesses accommodation was created by erosion during sea level fall and lowstand upward fining sequences formed as the topographic lows were filled during subsequent sea level rise results from this investigation indicate that predictable variations in foreland basin deposition and in the resulting stratigraphic pattern occur along regional tectonic strike as well as in the dip direction the thickness of foreland ramp sequences is greater in areas of salients than in recesses whereas the ratio of sandstone to total thickness is greater in the recesses aggradational sequences grading laterally into upward coarsening progradational sequences of the distal ramp characterize areas of relative subsidence which provides a mechanism for creating sediment accommodation in contrast deep erosion common unconformities and incised valley fills are present in areas corresponding to recesses where the rate of eustatic fall commonly exceeds the subsidence rate these along strike stratigraphic variations in response to collisional tectonism should be considered in the interpretation of other foreland basin successions

heterogeneous crust and upper mantle across the se brazilian highlands and the relationship to surface deformation as inferred from magnetotelluric imaging 1 we provide evidence for lateral structural zoning of the crust and uppermost mantle across the southeast brazilian highlands from two dimensional magnetotelluric mt imaging of a 250 km long nw se profile extending from the archaean sao francisco craton to near the atlantic coast our model shows the presence of an electrically resistive cover layer of variable thickness 2 20 km and an underlying 5 15 km thick conductive marker horizon atop a resistive segmented layer extending to 60 70 km depth two structurally different lithospheric provinces are suggested based on conductor geometry and lateral segmentation in the model the northwestern half of the profile is characterized by flat to shallowly dipping crustal conductors interpreted as a fold and thrust belt while the southeastern half consists of major steeply dipping lithospheric conductors with spatially coincident high amplitude magnetic anomalies interpreted as a strike slip fault belt the boundary between both provinces may be associated with a suture but lies 35 km nw of a previously suggested boundary between the occidental and oriental terranes the marker horizon is coincident with a low p wave velocity layer and appears to be raised by about 5 km across a 85 km wide belt hosting a neoproterozoic magmatic arc in the southeastern province we propose a link between basement heterogeneity and surface deformation in the region

burial and exhumation across the alps apennines junction zone constrained by fission track analysis on modern river sands terra nova 24 221 226 2012 abstract we applied fission track analysis on modern river sands to constrain the cenozoic evolution of the alps apennines junction zone west of the villalvernia varzi ottone fault samples yielded major peaks at 33 36 and 62 65 ma showing that apatites in the eroded wedge top successions were largely unreset to the east samples yielded prominent peaks at 4 0 4 7 6 5 6 7 and 19 ma probably formed by reset ages our results indicate that the alpine orogenic wedge originally tapered out to the south the northern apennines were buried beneath a thick and widespread epiligurian cover now almost completely eroded and experienced major uplift and erosional exhumation since early pliocene times at average rates of 1 km ma 1 a large amount of alpine derived detritus previously stored on top of the orogenic wedge was transferred into the po plain sediment recycling thus overwhelmed any eventual effect of climate change on plio quaternary sedimentation rates south of the alps

3 d inversion of complex magnetotelluric data from an archean proterozoic terrain in northeastern sao francisco craton brazil

electrical conductivity and paleo proterozoic foredeeps sedimentary facies associated with paleo proterozoic foredeeps are apparently highly conductive the conductive lithology appears to be a euxinic carbonaceous shale deposited on preforedeep continental margin facies the shale is overlaid by axial turbidites and often incorporated into the succeeding fold and thrust belt carbonaceous shales can be made highly conducting if graphitized by exposure to temperatures exceeding 400 c although foredeep facies develop diachronously the most conductive unit is usually found in the core of the fold and thrust belt suggesting that deformation may be essential for the complete graphitization of the carbon the foredeep hypothesis offers an attractive explanation for several highly conductive features detected in the upper and middle crust in laurentia and possibly throughout the world the euxinic facies represent highly specific tectonic marker horizons that are readily delineated by using electromagnetic methods

magnetotelluric constraints on subduction polarity reversing reconstruction models for proterozoic australia two dimensional lithospheric scale magnetotelluric imaging in the central australian proterozoic has constrained the large scale architecture of terrane assembly during paleoproterozoic accretion and collision the comparatively conductive north australian craton consisting of rocks between 2500 and 1730 ma in age has been imaged to extend for 150 km under the 1690 1620 ma warumpi province which forms part of a large comparatively juvenile terrane in central southern australia collision between the north australia craton and warumpi province occurred ca 1640 ma the boundary between these domains is modeled to be subvertical at crustal scale but dips south at 45 in the mantle to depths of 150 km we interpret this geometry to reflect lithospheric scale underthrusting of the north australian craton beneath the warumpi province and suggest that it provides a first order constraint on subduction polarity during collision ca 1640 ma in contrast most contemporary models for the evolution of paleoproterozoic australia propose that the north australian craton was located on the overriding plate of a long lived ca 1800 1550 ma north directed subduction system the polarity of these models is not consistent with the lithospheric scale geophysical architecture

plio quaternary vertical motion of the northern apennines insights from dynamic modeling we test the effects of different geodynamic mechanisms on the pliocene to present day dynamics and in particular on the vertical motions of the northern apennines system by means of two dimensional finite element models we show that the pliocene features of the northern apennines exhumation of deep rocks in western italy and fast subsidence rates in the adriatic foredeep can be adequately reproduced by a model slab characterized by the passive sinking of a negatively buoyant adriatic slab and by the upwelling of buoyant asthenosphere beneath western italy model slab is however not able to account for the quaternary and present day pattern of vertical motion of eastern italy and of the adriatic basin a deep configuration of the system characterized by a detached slab model detach may explain the vertical motions of these areas constrained by uplift of the eastern portions of the apenninic chain by an eastward shift of the foredeep depocenter and by lower subsidence rates in the adriatic basin the tectonic and stratigraphic data showing major differences between the tertiary and the quaternary evolution of the northern apennines are discussed

semi allochthonous sedimentation in the apenninic thrust belt abstract on thrust sedimentation is not so common in mobile belts outside the apennines one of the relevant contributions to apenninic geology by the florentine giovanni merla in the 1950 s is the recognition of these traveller or floating sequences this concept of semi allochthon as merla called it is reviewed here and discussed in the light of recent advances in stratigraphy and basin analysis several unconformities are detected in the sequence which make it possible to split it into cycles or depositional sequences at variance with classical formations correlations have been attempted with sediments deposited behind and in front of the thrust belt in comparison with foredeep flysch sequences semi allochthonous ones show better preserved stratigraphical relations vertical stacking and less deformation which emphasizes their possible role as reference sections for syn orogenic sedimentation in the apennines

archean and paleoproterozoic crustal evolution and evidence for cryptic paleoarchean hadean sources of the nw sao francisco craton brazil lithochemistry geochronology and isotope systematics of the cristalândia do piaui block abstract the cristal ndia do piaui block located in the northwestern margin of the sao francisco craton represents the basement of the rio preto fold belt it is composed of archean orthogneisses of ca 3 2 ga reworked at 2 81 and 2 68 ga with juvenile to moderately juvenile ehf values between 1 51 and 8 07 and high k syenogranites dated at 2 65 ga with crustal ehf values between 10 37 and 19 54 both with model ages tdmc varying from 3 57 to 4 33 ga indicating cryptic paleo to eoarchean and even hadean sources metamafic ultramafic rocks iron formations metacherts and graphite schists occur in association with the archean orthogneiss the whole set is intruded by paleoproterozoic ca 2 2 ga metagranitoids with compositions varying from granodioritic with sanukitoid type signatures to monzogranitic and alkali feldspar granitic with crustal signatures they are related to the rhyacian orosirian orogeny responsible for the complex deformation patterns printed in the archean basement orosirian metasedimentary rocks are represented by garnet biotite paragneiss with maximum depositional age of ca 1 95 ga intrusive mafic dikes in the complex show ages of ca 2 07 ga and isotopic features of mantle derived magmas considering the presented data the cristal ndia do piaui block represents a metacratonic domain corresponding to part of the guanambi correntina paleoplate wich had been involved in crustal accretion and reworking from the archean to the paleoproterozoic many of the elements of the evolutionary stages wich are present in the sao francisco congo paleocontinent can be recognized suggesting an evolution of this crustal segment amounts to the eoarchean era and disclosing the existence of cryptic paleoarchean or even hadean nuclei reworked in at least three metamorphic events during the rhyacian orosirian orogeny

high and highly anisotropic electrical conductivity of the asthenosphere due to hydrogen diffusion in olivine experimental studies on electrical conductivity in hydrated olivine had been made only at relatively low temperatures the extrapolation of these data to asthenosphere temperatures shows that explaining high and highly anisotropic conductivity by hydrogen is difficult we present new experimental results on the electrical conductivity in hydrated olivine single crystals measured under a broader temperature range than before t temperature 573 1373 k at p pressure 4 gpa the new results agree well with the previous results at low temperatures t 1000 kt 1000 k substantial deviations from the previous results are observed i anisotropy in conductivity increases with temperature and ii activation energies at high temperature regime are substantially higher than those at lower t the high temperature behavior is consistent with a model invoking diffusion of two protons at m site vacancy whereas the low temperature behavior suggests conduction by more mobile species such as free proton the present results i explain the discrepancy between different previous studies conducted under different temperature regimes and ii show that most of geophysical observations on the electrical conductivity in the asthenosphere including the high and highly anisotropic conductivity can be explained by high water content in the asthenosphere that is consistent with the geochemical model combining with a previous model of low seismic wave velocity of the asthenosphere a subsolidus model invoking the role of hydrogen provides a unified explanation for the geophysical anomalies of the asthenosphere

earthquakes as a triggering mechanism for debris flows and turbidites on the calabrian ridge abstract debris flows and turbidites have been described in piston cores bottom photographs and side looking sonar records from basin floors amid the rugged irregular topography of the calabrian ridge a sediment transport process is inferred which began with a rotational slump on the steep basin wall subsequently transformed into a debris flow and turbid cloud such events occurred approximately every 1500 years because the debris flow turbidite deposits are correlative between basins some relatively widespread mechanism such as an earthquake is required to initiate the slope failure the estimated intensity of ground shaking as a result of historical earthquakes on the nearby calabrian and hellenic arcs has been used to develop a relationship between frequency of occurrence and intensity of ground shaking extrapolation of this relation to infrequent large magnitude events shows that the 1500 yr event is small enough to be geologically plausible but large enough to cause slope failure on basin walls it is possible that the historical record fortuitously contains one of these large magnitude long repeat time events which may have caused the most recent of the observed debris flows in 1903

the araa uaa west congo orogen in brazil an overview of a confined orogen formed during gondwanaland assembly abstract the neoproterozoic adamastor brazilide ocean was generated during the breakup of the rodinia supercontinent and remnants of its oceanic lithosphere have been found in the brasiliano pan african orogenic system that includes the aracuai west congo brasilia ribeira kaoko dom feliciano damara and gariep belts the aracuai and the west congo belts are counterparts of the same neoproterozoic orogen the first belt comprises two thirds of the aracuai west congo orogen this orogen is rather unique owing to its confined nature within the embayment outlined by the sao francisco and congo cratons in spite of this the presence of ophiolitic remnants and a calc alkaline magmatic arc indicate that the basin orogen evolution comprise both oceanic spreading and consumption it is assumed that coeval paramirim and sangha aulacogens played a key role by making room for the aracuai west congo basin sedimentary successions record all major stages of a basin that evolved from continental rift when glaciation related sedimentation was very significant to passive margin rifting started around 1 0 0 9 ga the oceanic stage is constrained by an ophiolitic remnant dated at 0 8 ga if the cratonic bridge that once linked the sao francisco and congo palaeocontinental regions did not hinder the opening of an ocean basin it certainly limited its width as a consequence only a narrow oceanic lithosphere was generated and it was subducted afterwards this is also suggested by orogenic calc alkaline granitoids occuping a small area of the orogen geochronological data for pre syn and late collisional granitoids indicate that the orogenic stage lasted from 625 ma to 570 ma a period of magmatic quiescence was followed by intrusion of postcollisional plutons at 535 500 ma the features of the aracuai west congo orogen suggest the development of a complete wilson cycle in a branch of the adamastor ocean which can be interpreted as a gulf with limited generation of oceanic lithosphere

turbidites in foreland and on thrust basins of the northern apennines abstract turbiditic sediments are associated with all stages of development of the apennines orogen not only the classical flysch of oligo miocene age macigno cervarola marnoso arenacea but also the coeval smaller bodies based on thrust sheets loiano of ranzano sequence and the younger molasse of the plio pleistocene foredeep except for more recent ones quaternary turbidite units are affected to some degree by thrusting and later block faulting the equivalent marginal deposits were also deformed uplifted and eroded to a great extent however identification and correlation of sequences and paleoenvironments across the orogenic belt can be helped by a deposits with rather strong eustatic imprint high stands and b preservation of marginal sediments by early remobilization cannibalism into deeper depocentres the following categories of basins can be recognized main foreland basin or foredeep in front of thrust sheets based on undeformed or previously rifted continental crust thrust top basins meso autochthonous auct in absence of a well defined major foredeep satellite basins thrust based but inwardly associated with a foredeep and of much smaller size linear dimensions in the order of kilometers and ten of kilometers volume of fill in the order of tens to hundreds cubic kilometers the foredeep persisted for the whole orogenic cycle but changed its character and was more or less segmented two main stages can be recognized conventionally labelled as the flysch stage up to the late miocene appenines mostly submerged and the molasse stage late miocene to recent present relief already in existence satellite basins floating on the ligurian sheet epi ligurian persisted up to the mid pliocene but received turbidites only in the oligocene and miocene others were created later on thrusts of different style involving the foreland sedimentary cover piggyback basins of ori and friend 1984 turbidite lithosomes of the flysch group are the largest ones volumes up to 30 000 km 3 with maximum size and lateral continuity of individual beds megaturbidites they are not seen as overlapping units filling coexisting and adjacent basins but as distinct superposed isochronous units within which only time transgressive sedimentation could occur macigno thus stands for all oligocene turbidites of the northern apennines foredeep cervarola for early miocene ones and marnoso arenacea for mid late miocene ones it is assumed that these units reflect sedimentary cycles of some order correlatable with global charts and splittable into depositional sequences sensu vail and collaborators

large scale patterns of auroral ionospheric convection observed with the chatanika radar ionospheric convection at auroral latitudes has been examined during a series of long duration experiments with the chatanika alaska incoherent scatter radar these experiments have been carefully designed to obtain maximum latitudinal coverage 56 75 while maintaining a temporal resolution of 30 min in order to resolve the effects of individual substorms on the convection pattern design criteria for the experiments are described together with presentation of observational data acquired during 400 hr of radar operations during various levels of geophysical disturbance the data accentuate the repeatability of the gross features of the auroral convection and its basic conformity to the two cell pattern predicted from the large scale magnetospheric circulation for moderate to active geomagnetic conditions uniform sunward convection with velocities of 800 1200 m s spans the 62 to 72 latitude band at both dawn and dusk in quieter circumstances the sunward convection continues in this region but has smaller speeds and is centered at higher latitudes little evidence of a throat in the dayside convection pattern is seen at latitudes below 75 except in very disturbed circumstances during several experiments coordinated operation of the chatanika and millstone hill massachusetts radars permitted the simultaneous observation of the auroral convection pattern at two different local times substorms affect the convection at all local times and appear to generally enhance the rate of convection without seriously changing its direction near midnight latitudinal displacements and expansions of the convection pattern during substorms may result in a local rotation or reversal of the direction of convection while near dawn and dusk convection velocities are enhanced and the region of sunward flow expands to lower latitudes

magnetotelluric evidence of the tectonic boundary between the rio de la plata craton and the pampean terrane chaco pampean plain argentina the extension of the transbrasiliano lineament abstract the transbrasiliano lineament represents a continental shear belt that transversely intersects the south american platform from nne to ssw much evidence of this lineament exists to the north but it remains uncharacterized in the distal andean foreland to the south where it is associated with the tectonic boundary between the rio de la plata craton and the pampean terrane this tectonic boundary is mostly unexposed in the chaco pampean plain argentina debate continues about the existence of an east or west dipping subduction zone and a later collisional event between the terranes here we report the results of a magnetotelluric survey along a w e profile at 27 s between 63 45 and 60 30 w to characterize the geoelectric structure the magnetotelluric data were processed the dimensionality and distortion of the geoelectrical medium were analyzed and a 2 d inversion model was developed the distribution of the resistivities at the lithospheric scale indicates a highly resistive crust 20 000 m on the east side of the profile that is correlated with the rio de la plata craton on the west side of the profile a less resistive but still highly resistive crust 12 000 m is correlated with the pampean terrane the highly resistive blocks are separated by an east dipping conductive anomaly 150 250 m that is correlated with the transbrasiliano lineament this conductive feature is consistent with the east dipping subduction model and can be explained by the presence of graphite in paleosutures in long stable geological terranes oblique convergence between the terranes may have developed the transpressional shear belt the results provide new geophysical evidence of the tectonic boundary between the rio de la plata craton and the pampean terrane beneath the chaco pampean plain that extends southward into the transbrasiliano lineament the study also improves the knowledge of the amalgamation of western gondwana

the conductivity structure across the trans european suture zone from magnetotelluric and magnetovariational data modeling abstract to achieve an electrical conductivity model of the trans european suture zone tesz magnetotelluric measurements in the period range of 10 20 000 s along two almost parallel profiles with a total of 65 sites starting from the north german polish basin crossing the tesz and ending on the east european craton were conducted magnetovariational responses were combined with other magnetotelluric transfer functions because for large periods they are free of near surface galvanic distortions and thus lead to more reliable resolution of deep structures following the results of strike and dimensionality analysis input data for two dimensional inversion were created by rotating all transfer functions to the common strike direction models derived from different combinations of data were compared and several sensitivity analyses were carried out the results show a highly conductive layer of sediments saline aquifer and the tesz as a broad region of enhanced conductivity it was also clarified that the lower crust and upper mantle of the east european craton eec are more resistive than the paleozoic platform pp underlying the northeast german basin and this resistivity extends to the greater depths

average characteristics and activity dependence of the subauroral polarization stream 1 data from the millstone hill incoherent scatter radar taken over two solar cycles 1979 2000 are examined to determine the average characteristics of the disturbance convection electric field in the midlatitude ionosphere radar azimuth scans provide a regular database of ionospheric plasma convection observations spanning auroral and subauroral latitudes and these scans have been examined for all local times and activity conditions we examine the occurrence and characteristics of a persistent secondary westward convection peak which lies equatorward of the auroral two cell convection individual scans and average patterns of plasma flow identify and characterize this latitudinally broad and persistent subauroral polarization stream saps which spans the nightside from dusk to the early morning sector for all kp greater than 4 premidnight the saps westward convection lies equatorward of l 4 60 invariant latitude spans 3 5 of latitude and has an average peak amplitude of 900 m s in the predawn sector saps is seen as a region of antisunward convection equatorward of l 3 55 spanning 3 of latitude with an average peak amplitude of 400 m s

deep conductivity structure beneath the northern brasilia belt central brazil evidence for a neoproterozoic arc continent collision abstract the brasilia belt in central brazil experienced a broad range of tectonic events including several periods of compressional and extensional deformations during the proterozoic magnetotelluric data were collected at 27 sites along a profile crossing the northern part of the belt strike analysis and distortion decomposition indicated that most of the data are two dimensional 2d and fit a regional geoelectric strike angle of n20e parallel to the tectonic grain 2d conductivity model is constrained by available gravity and seismic refraction data and reveals the cumulative effects of the multiple tectonomagmatic episodes that affected the area in the past on current crustal and upper mantle structure the results show that the resistive lithospheric root of the sao francisco cratonic plate is restricted to the easternmost part of the profile and does not extend westward beneath the brasilia belt the external zone of the fold belt composed of slightly metamorphosed rocks at the surface presents crustal conductors that can be tentatively interpreted as regions with fossil fluid flows at mid crust depth beneath a gold mineralization zone and as a very shallow mineralized zone within the metamorphic rocks the external zone is separated to the west from the goias massif by a strong conductivity contrast indicative of a suture zone along the rio maranhao fault system crustal conductivity structure under the niquel ndia complex is distinct from what is observed in other parts of the goias massif in agreement with the proposal of the complex being an allochthonous block in spite of the large tectonothermal events that affected the goias magmatic arc there are no significant conductive anomalies at crustal depths in its eastern side the most prominent feature detected in the geoelectric model is a huge high conductivity anomaly below the entire goias massif in the central part of the belt mainly concentrated at the uppermost mantle but also affecting mid to lower crustal depths it can be interpreted as associated with thermal events either related to mesoproterozoic and or neoproterozoic continental rifting processes or to delamination and detachment processes at the end of the evolution of the belt in the neoproterozoic effects related to oceanic lithosphere subduction during the collision and accretion of the goias massif and goias magmatic arc to the western passive margin of the sao francisco cratonic plate in the neoproterozoic may alternatively explain the anomaly

mass transfer from the alps to the apennines volumetric constraints in the provenance study of the macigno modino source basin system chattian aquitanian northwestern italy a quantitative provenance approach is applied to the late oligocene early miocene macigno modino turbidite complex which accumulated in the foreland basin developed in front of the growing northern apennines but was supplied from the rapidly rising western alps at the northwestern end of the basin as well as the traditional recognition of source rock lithologies and their gross paleogeographic location the provenance approach adopted here evaluates the relative contributions to the clastic complex from different types of source rock and estimates the volume of material transferred from the alps to the apennines within the macigno modino source basin system and the overall volume of rock assumed to have undergone erosion in order to supply it these elements are used to constrain the possible source of the clastics and to infer the main paleogeological features of the source area it is concluded that an estimated volume of ca 17 20 103 km3 of basement mainly metamorphic rocks together with ca 103 km3 of intermediate composition volcanic rocks was transferred as detritus from the western alps area to the northern apennines foreland basin creating the 250 300 km long 50 km wide and up to 2 7 km thick macigno modino clastic complex lithology and paleogeography combined with volume estimates and current knowledge of the regional geology of the western alps consistently suggest that the bulk of the sediments funnelled into the macigno modino complex were derived from the uplifted ivrea crustal block which formed a crystalline massif some 5000 km2 in extent that rose ca 3 km during the oligocene forming one of the main elements of western alpine geology during that time

small scale spatial variability in turbidity current flow controlled by roughness resulting from substrate erosion field evidence for a feedback mechanism abstract analysis of turbidity flow processes from turbidite outcrops remains difficult in this paper the distribution of orientations of paleoflow indicators around scours a few centimeters to a few decimeters deep is analyzed two examples are presented one from the gres de champsaur formation in southeastern france and one from the macigno formation in northern italy the orientations of paleoflow data sets on either side of the scours are analyzed and differ 10 15 over horizontal distances of only a few meters this mismatch is argued to represent a rotation of the flow whilst it flowed over the scour the reconstructed rotation is a new example of a feedback mechanism involving interaction between the current and its substrate paleoflow indicators only record flow direction at the base of the flow however the flow substrate interaction can be expected to affect the complete turbulent velocity structure of the flow so that erosion by frontal parts of the current influences the suspended sediment transport processes of hindward regions of the current

seismoturbidites a new group of resedimented deposits abstract turbidite beds of exceptionally large volume and areal extent occur in both modern and ancient deep water basins these beds which may reach individual volumes in excess of 100 km3 are apparently the product of catastrophic gravity flows triggered by earthquakes along the margins of highly mobile basins most commonly elongate flysch troughs turbidite beds produced by these catastrophic events are generally characterized by very distinctive geometry internal structures and composition and are termed herein seismoturbidites characteristically these sediments lack time persistent features of deep sea fans such as channels and lobes seismoturbidites may occur as scattered intercalations diluted within otherwise normal turbidite sequences thus forming generally excellent markers for basin wide correlations or as closely spaced repetitive units comprising the bulk of the sedimentary fill of certain flysch basins in both cases they may offer a tremendously useful tool for a better understanding of the distribution of paleoseismic zones in time and space

dynamic equilibrium among erosion river incision and coastal uplift in the northern and central apennines italy erosion river incision and uplift rates in the northern and central apennines italy since 0 9 ma are determined from new cosmogenic nuclide data beryllium 10 concentrations in modern and middle pleistocene sediments indicate erosion rates from 0 20 to 0 58 mm yr these rates are similar to estimates of sediment yield 0 12 0 44 mm yr river incision 0 35 mm yr and uplift 0 01 1 0 mm yr rates inferred from other methods that integrate landscape process rates since the early pleistocene these rates of landscape change are significantly lower than long term exhumation rates of 1 2 mm yr since ca 4 5 ma inferred from thermochronometry collectively these data suggest that hillslope erosion and river incision rates in the northern and central apennines have balanced local uplift rates for 1 my but that exhumation rates have slowed significantly since emergence of the mountain chain in the pliocene this condition of dynamic equilibrium was potentially achieved within ca 3 ma similar to some model predictions of hillslope and fluvial system adjustment

magnetotelluric investigations of the lithosphere beneath the central rae craton mainland nunavut canada new magnetotelluric soundings at 64 locations throughout the central rae craton on mainland nunavut constrain 2 d resistivity models of the crust and lithospheric mantle beneath three regional transects responses determined from colocated broadband and long period magnetotelluric recording instruments enabled resistivity imaging to depths of 300 km strike analysis and distortion decomposition on all data reveal a regional trend of 45 53 but locally the geoelectric strike angle varies laterally and with depth the 2 d models reveal a resistive upper crust to depths of 15 35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries surface projections of the conductive layer coincide with areas of high grade archean metasedimentary rocks tectonic burial of these rocks and thickening of the crust occurred during the paleoproterozoic arrowsmith 2 3 ga and trans hudson orogenies 1 85 ga overall the uppermost mantle of the rae craton shows resistivity values that range from 3000 m in the northeast beneath baffin island and the melville peninsula to 10 000 m beneath the central rae craton to 50 000 m in the south near the hearne domain near vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of hudsonian granites a regional decrease in resistivities to values of 500 m at depths of 180 220 km increasing to 300 km near the southern margin of the rae craton is interpreted as the lithosphere asthenosphere boundary

electrical conductors in archean mantle result of plume interaction 1 from long period magnetotelluric data across the gawler craton we obtain a three dimensional resistivity image of the lithosphere and provide constraints on tectonothermal events dating back to the proterozoic contrary to common observations of archean cratons displaying high electrical resistivity in the mantle lithosphere the magnetotelluric data show low resistivity of around 10 m at 80km depth underneath the 1595ma gawler range volcanics a silicic large igneous province the resistivity distribution appears to be a signature of plume modified orogenesis with low degree partial melting at the base of the lithosphere in a back arc setting the enhanced conductivity is explained through higher hydrogen and iron content in the crystal lattice and also along the grain boundaries of the mantle constituting minerals older arc related magmatism to the southwest across 1620 1610ma st peter suite does not display enhanced conductivity and suggests a depleted mantle lithosphere the data show that yellowstone type mantle plume analogues are preserved through time and still display an elevated electrical signature in the lithosphere today

magmatic sulfide formation by reduction of oxidized arc basalt the hidaka metamorphic belt in southeastern hokkaido japan provides insights into how magmatic sulfide deposits may form through magma mixing deep within arcs here a near complete cross section of arc crust is exposed with large mafic igneous complexes preserved at deeper levels magmatic sulfide mineralization occurs within the opirarukaomappu gabbroic complex ogc which preserves a record of crustal contamination of mafic magmas via assimilation and magma mixing involving introduction of crust derived tonalite assimilation fractional crystallization modelling suggests that the gabbro evolved through a combination of approximately 10 wt mixing and 14 wt fractional crystallization magmatic sulfides and associated gabbros diorites and tonalites at this locality contain graphite with carbon isotope signatures consistent with derivation from the surrounding partially melted carbonaceous shales this indicates that crust derived carbon was added to the mafic magma through the magma mixing and assimilation process sulfur isotope data suggest that sulfur was also added from crustal sources during assimilation and magma mixing the relationships observed in the ogc suggest that intrusion of basalt into a segment of deep arc crust drove partial melting of carbonaceous metamorphic rocks producing graphite bearing felsic magmas with high reducing potential redox budget modelling shows that mixing of only small proportions of these magmas is sufficient to lower the oxidation state of oxidized basaltic magmas enough to induce sulfide saturation and consequent exsolution of immiscible sulfide melt magmatic sulfide deposits are likely to form by this reduction induced sulfide saturation mechanism deep within other arcs where magma mixing is thought to be common

deep sea fan deposits in the macigno formation middle upper oligocene of the gordana valley northern apennines italy abstract facies characteristics are described for a 2300 m thick turbidite succession of the macigno formation middle upper oligocene which crops out as a continuously exposed section in the gordana valley northern apennines in the study area the macigno formation consists of two sandy members separated by a shaly member the sandy members contain lithic associations characteristic of submarine fan deposits and form distinct fan complexes the middle shaly member represents a period of basin starvation three main facies associations are recognized in the macigno fan deposits fan fringe outer fan lobe and braided midfan facies associations the fan fringe deposits consist of monotonous or small scale cyclically arranged thin bedded turbidites the outer fan lobe deposits are composed of medium to very thick bedded classical turbidites internally organized in thickening upward cycles with minor intercalations of thin bedded sandstone beds two distinct types of outer fan lobe deposits distal and proximal are recognized on the basis of rock type internal organization and vertical facies relationships the midfan deposits consist of amalgamated massive sandstone and pebbly sandstone which display repeated thinning upward cycles the three main facies associations are superposed t define major thickening and coarsening upward cycles reflecting distinct episodes of fan progradation the relationships between the facies associations of the macigno deposits and the existing models of submarine fan sedimentation are discussed after the deposition of the fan complexes fan sedimentation in this part of the basin was replaced by movements of allochthonous units heralded by large olistostromes

electrical lithosphere beneath the kaapvaal craton southern africa 1 a regional scale magnetotelluric mt experiment across the southern african kaapvaal craton and surrounding terranes called the southern african magnetotelluric experiment samtex has revealed complex structure in the lithospheric mantle large variations in maximum resistivity at depths to 200 250 km relate directly to age and tectonic provenance of surface structures within the central portions of the kaapvaal craton are regions of resistive lithosphere about 230 km thick in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography but thinner than inferred from seismic body wave tomography the mt data are unable to discriminate between a completely dry or slightly damp a few hundred parts per million of water structure within the transitional region at the base of the lithosphere however the structure of the uppermost 150 km of lithosphere is consistent with enhanced but still low conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for kaapvaal xenoliths the electrical lithosphere around the kimberley and premier diamond mines is thinner than the maximum craton thickness found between kimberley and johannesburg pretoria the mantle beneath the bushveld complex is highly conducting at depths around 60 km possible explanations for these high conductivities include graphite or sulphide and or iron metals associated with the bushveld magmatic event we suggest that one of these conductive phases most likely melt related sulphides could electrically connect iron rich garnets in a garnet rich eclogitic composition associated with a relict subduction slab

magnetotelluric images of paleoproterozoic accretion and mesoproterozoic to neoproterozoic reworking processes in the northern sao francisco craton central eastern brazil abstract broadband and long period magnetotelluric mt data were collected along an east west oriented 580 km long profile across the northern sao francisco craton where extensive proterozoic and phanerozoic sedimentary cover and lack of deep probing geophysical surveys have prevented to establish unequivocally the cratonic character of the archean paleoproterozoic lithosphere following dimensionality analyses the mt dataset was interpreted using both 2 d and 3 d inversion procedures the near surface structure is better resolved in the 2 d model due to its finer resolution a huge upper crustal conductor is found all along the shallow early neoproterozoic irece basin in the central domain of the craton extending laterally for approximately 150 km and restricting signal propagation below the basin its high conductance is explained by a combination of high porosity and high fluid salinity in the sedimentary package another upper crustal conductor is observed on the west side of the profile interpreted as fractured metasedimentary rocks of the rio preto belt thrusted on top of the craton basement during neoproterozoic marginal collision the 3 d model explains significantly better the measured data related to deep structure contrary to what is expected for a stable cratonic block the geoelectric model shows pronounced electrical complexity and heterogeneity an indication that the cratonic lithosphere was multiply reworked in the past by tectonothermal events different lithospheric resistive blocks bounded by major conductive zones are identified constrained by geochemical and isotopic data these vertical conductive interfaces are interpreted as cryptic suture zones due to large scale amalgamation of continents and microcontinents leading to the assembly of the sao francisco craton in the paleoproterozoic the conductivity enhancement is more likely explained by emplacement of sulfides along previous suture zones during mafic magmatism at upper mantle depths high conductivity observed below most of the profile indicates that metasomatism or refertilization processes with incompatible elements caused by the paleoproterozoic subducting slabs and mesoproterozoic to neoproterozoic upwelling of deep fluids and melts reworked this portion of the craton mantle

sedimentary basins formed and carried piggyback on active thrust sheets small sedimentary basins that formed on the apennine southern margin of the po basin complex northern italy have been studied using borehole and seismic reflection information and their evolution can be dated using the largely marine biostratigraphy of the miocene to quaternary sediments these studies show that the basins formed and were filled while being carried on moving thrust sheets and they are therefore named piggyback basins each of these basins corresponds to the active phase of a different thrust front ramp on the pyrenean northern margin of the ebro basin complex northern spain similar piggyback basins formed between eocene and mid miocene time they have since been uplifted and eroded and may therefore be studied in outcrop in northern spain much of the basin fill sediment was deposited in nonmarine environments

early exhumation of underthrust units near the toe of an ancient erosive subduction zone a case study from the northern apennines of italy apatite fission track aft analyses were performed on 16 sandstone samples from a tectonic melange unit exposed in three tectonic windows located near the inferred front of the early miocene subduction system of the northern apennines of italy the tectonic windows display a block on block tectonic melange present under the ligurian units the melange is formed by portions of the upper plate incorporated in the plate boundary shear zone as a consequence of a mechanism of frontal tectonic erosion during the aquitanian early miocene aft and structural data together with stratigraphic constraints allow the reconstruction of a complete deformation cycle with a phase of underthrusting followed by underplating and early exhumation of the tectonic melange the exhumation in particular took place at 10 20 km from the original subduction front moreover the analysis suggests that multiple faults were active at the same time in the frontal part of the subduction zone

archean and paleoproterozoic crust of the sao francisco craton bahia brazil geodynamic features abstract recent geological geochronological and isotopic studies allow the identification of four important crustal segments in the basement of the sao francisco craton in bahia the oldest is the gaviao block in the wsw part of the studied area and comprises granitic granodioritic and migmatitic continental crust including remnants of 3 4 ga ttgs which are amongst the oldest rocks in south america and are associated with archean greenstone belts the youngest segment is exposed in the itabuna salvador curaca belt which extends from se bahia along the atlantic coast to salvador then northwards into ne bahia it is mainly composed of a low k calc alkaline plutonic suite and also contains strips of intercalated metasediments and ocean floor back arc basin gabbro and basalt in the ssw part of the area the jequie block comprises granulitic migmatites with inclusions of supracrustal rocks intruded by many charnockite plutons in the ne the serrinha block is composed of orthogneisses and migmatites which form the basement for paleoproterozoic greenstone belts during the paleoproterozoic transamazonian cycle these four crustal segments collided resulting in the formation of an important mountain belt the regional metamorphism resulting from the crustal thickening associated with the collision occurred at around 2 0 ga major mineralizations were formed during the evolution of the four archean blocks and also during and after the paleoproterozoic collision

plasma convection in the vicinity of the dayside cleft experiments conducted with the chatanika alaska incoherent scatter radar observed the ionospheric convection pattern equatorward of the noontime cleft there plasma transport is characterized by low speed convergence toward a meridian centered somewhat before noon and rotation of the flow into a poleward direction at higher latitudes for undisturbed conditions there is only weak convection at noon at these latitudes but the eastward electric field reaches a maximum of 15 mv m 2 4 hours before and after noon at latitudes between 70 and 74 during active conditions at the peak of the solar cycle the ionospheric cleft was located within the chatanika field of view on several occasions a longitudinally narrow region of rapid poleward convection near noon was not observed rather poleward flow occurred over a 3 4 hour spread of local time for disturbed conditions the prenoon and postnoon regions of plasma entry into the polar cap were enhanced and an eastward electric field of 25 mv m was seen across the 2 hours of local time around noon high density f region plasma was observed convecting poleward through the cleft from a source at lower latitudes in the afternoon sector such plasma seen at very high latitudes within the polar cap serves as a tracer of the convection pattern away from the cleft

evolution of a miocene carbonate shelf northern apennines italy revealed through a quantitative compositional study abstract the evolution of the miocene san marino carbonate shelf torriana outcrop developed on the accretionary prism of the northern apennines has been interpreted through a stratigraphic and compositional study modal analysis allowed to quantify the framework components and to identify four microfacies through which the main steps of the carbonate ecosystem were traced the healthy phase of the carbonate shelf dominated by bryozoans and echinoids originated in a high energy transgressive setting and evolved during a warm period characterized by a progressive increase of nutrients the transitional stage is marked by a reduction of carbonate productivity and by terrigenous intermittent pulses associated with bioclast fragmentation the drowning succession corresponds to deepening upward facies formed by fine grained hybrid arenites to sandy marls with abundant planktonic foraminifera glauconitic grains and clay matrix the demise of the carbonate shelf might have resulted from a combination of regional and global factors that interplayed controlling the detrital input the nutrient budget and the deepening of the basin synsedimentary tectonics triggered subsidence of the basin and enhanced terrigenous discharge moreover the superposition of paleoclimatic and paleoceanographic events monterey and middle miocene climate optimum could have contributed with the intense weathering and remarkable detrital and nutrients supply

geoelectric response of archean lithosphere in the western superior province central canada abstract magnetotelluric mt soundings were made at more than 230 sites in the western superior province in central canada as part of the lithoprobe western superior transect investigations the superior province forms the nucleus of the north american continent its western part contains a regional pattern of east west trending 100 200 km wide granite greenstone and metasedimentary belts or subprovinces the objectives of the mt study are to define the electrical resistivity structure of the crust of the subprovinces and their boundaries and to determine if there exists a signature in the upper mantle related to the formation of the craton the exceptionally large mt database for the western superior province provides a three dimensional sampling of the resistivity structure in this paper the primary mt responses from the data set including geoelectric strike apparent resistivity and phase are used to define first order geoelectrical structures in the region apparent resistivity responses indicate the ca 3 0 ga north caribou terrane contains extremely resistive crust crust to the south accreted at ca 2 7 ga is significantly more conductive with resistivities similar to in the southeast superior province the results suggest that the supracrustal component of the north caribou terrane provides minimal enhancement of the crustal conductivity at the southern margin of the former north caribou superterrane induction arrows define a conductivity anomaly that extends in an east west direction for at least 600 km and causes a reversal of arrows in a region 100 200 km wide this anomaly is interpreted to be caused by a conductive component of the 2 7 ga metasedimentary rocks of the english river subprovince the observations demonstrate the ability of electromagnetic methods to map some neoarchean continental margins in the southwest superior province geoelectric strike azimuths for periods corresponding to signal penetration into the upper mantle are aligned with the trend of subprovince boundaries and are interpreted to be related to fossil structure in the mantle lithosphere this result provides support to the interpretation that the lithospheric mantle of the western superior province has remained intact since the 2 7 ga accretion of the subprovinces the mt responses from the western superior province have thus defined a number of large scale resistivity structures providing information on tectonic structure and history and confirming the value of electromagnetic imaging of archean lithosphere

tectonic and sedimentary evolution of the frontal part of an ancient subduction complex at the transition from accretion to erosion the case of the ligurian wedge of the northern apennines italy subduction can be associated either with accretion or with removal of material from the overriding plate these two processes can either coexist or alternate in time along the same margin their inception has the potential to change the dynamic equilibrium of a marginal wedge resulting in the development of out of sequence thrusts normal and strike slip faults or large submarine landslides in the frontal part of the subduction zone in this work we investigate the effects of the transition from frontal accretion to frontal erosion on the stability of a subduction complex through the study of an ancient example from the northern apennines ligurian subduction complex new structural data suggest that in the early neogene aquitanian the removal and underthrusting of the toe of the wedge formed by both the accreted oceanic sediments and the overlying wedge top basin fill i e the subligurian units implied a process of frontal tectonic erosion the presence on top of the subduction complex of a complete succession of mid late eocene to late miocene slope apron sediments i e the epiligurian succession facilitates a reconstruction of the sedimentary response to this event in the aquitanian large areas of the wedge were denuded from the lower slope sedimentary cover through extensive gravitational mass movements the subsequent deposition of a thick body of submarine debris flows is documented the mass wasting deposits are interpreted as the sedimentary response to the underthrusting of the frontal part of the ligurian subduction complex formed by the subligurian units

the miocene of the hatay area s turkey transition from the arabian passive margin to an underfilled foreland basin related to closure of the southern neotethys ocean abstract miocene sediments of the hatay area document the distal southerly part of an underfilled peripheral foreland basin related to the closure of the southern neotethys the basin is underlain by latest cretaceous and eocene shallow marine carbonates interpreted as the youngest part of the arabian passive margin succession unconformably overlying lower miocene conglomerates sandstones and palaeosols up to 250 m thick represent braided river deposits derived from an uplifted area to the south during the middle miocene carbonate sediments accumulated in a variety of shallow marine environments dominated by a northward sloping ramp with non marine peritidal lagoonal and coral reef deposition in local areas the upper miocene succession is mainly deeper water hemipelagic marl with clastic interbeds messinian evaporites were deposited near the depocentre the early miocene coarse fluvial sediments above the regional unconformity reflect the development of a flexural bulge or re activated basement structure the middle miocene carbonates and upper miocene marls are interpreted as the lower and middle units of an underfilled trinity considered typical of peripheral foreland basins similar lower miocene coarse clastic sediments and mid miocene shallow water carbonates unconformably overlie the arabian platform succession c 75 km to the north where the succession continues upwards into thick shallow marine to non marine terrigenous sediments upper unit of the trinity sourced from the overthrust load comparable coarse clastic sediments are absent from the distal foreland basin in the hatay area probably reflecting its more southerly position structural barriers in the foreland and the westward bypassing of sediment towards the mediterranean sea regional convergence halted by the end of the miocene and was followed by strike slip and westward tectonic escape as a result the miocene foreland basin was dismembered to form the plio quaternary transtensional hatay graben this like many other foreland basins shows some features which are not accommodated by ideal models of foreland basin evolution

dimensionality imprint of electrical anisotropy in magnetotelluric responses abstract dimensionality analysis of magnetotelluric data is a common procedure for inferring the main properties of the geoelectric structures of the subsurface such as the strike direction or the presence of superficial distorting bodies and enables the most appropriate modeling approach 1d 2d or 3d to be determined most of the methods currently used assume that the electrical conductivity of individual parts of a structure is isotropic although some traces of anisotropy in data responses can be recognized in this paper we investigate the imprints of anisotropic media responses in dimensionality analysis using rotational invariants of the magnetotelluric tensor we show results for responses generated from 2d synthetic anisotropic models and for field data that have been interpreted as showing the effects of electrical anisotropy in parts of the subsurface structure as a result of this study we extend the wal dimensionality criteria to include extra conditions that allow anisotropic media to be distinguished from 2d isotropic ones the new conditions require the analysis of the strike directions obtained and take into account the overall behavior of different sites in a survey

an ellipsoidal harmonic representation of earth s lithospheric magnetic field to degree and order 720 1 while high degree models of the earth s gravity potential have been inferred from measurements for more than a decade corresponding geomagnetic models are difficult to produce the primary challenge lies in the estimation of the magnetic potential which is not completely determined by available field intensity measurements and cannot be computed by direct integration described here is the methodology behind the third generation of the national geophysical data center s degree 720 magnetic model key issues are 1 the ellipsoidal harmonic representation of the magnetic potential 2 the reduction of ambiguities by a suitable penalty function and 3 the use of an iterative method to estimate the model coefficients the ngdc 720 model provides the lithospheric magnetic field vector at any desired location and altitude close to and above the earth s surface anticipated uses are in geological and tectonic studies of the lithosphere as a local correction for magnetic navigation and heading systems and the calibration of ground marine airborne and spaceborne magnetometers the ngdc 720 model is available at http geomag org models ngdc720 html and for long term archive at http earthref org cgi bin er cgi s erda cgi n 989

lithospheric and upper mantle structures results of electromagnetic soundings in europe abstract information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric mt and magnetometer data the latter include both magnetometer array studies mv and geomagnetic depth soundings gds locally controlled source data e g data from d c and very low frequency vlf resistivity surveys frequency soundings and airborne electromagnetic mappings provide information on near surface structures e g on exposures of deeper conductors the variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres thus both the mapping of large scale crustal conductivity structures and a detail study of the anomalous structures are possible the electrical conductivity of the crust and upper mantle is described within the plate tectonic framework examples of structures which become electrically conductive at different phases of the process are shown using the results of electromagnetic soundings in various tectonic environments in europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers these include structures from the fennoscandian shield precambrian extensional basins and collisional zones the carpathians scotland and ireland suture zones and the pannonian basin extensional basin and thrust zones as well as conductors detected along the european geotraverse e g the terrain boundaries and the decollement of the variscan thrusting an example which considers the lower crustal conductivity from the fennoscandian shield shows rather large lateral variations in conductance of the lower crust varying from 1 s to several hundreds of siemens estimates of the depth of the asthenospheric conducting layer within the european territory also vary considerably shallowest depths are detected beneath extensional regions the pannonian basin and the rhinegraben 40 80 km whereas depths of 200 km or over or the absence of a conducting layer are detected beneath the alps and old precambrian cratons among the key areas where electromagnetic investigations should play an essential role are the deep structure of the east european platform boundaries of archaean and proterozoic terrains the structure of the teisseyre tornquist zone the nature of the present day continental lower crust and the characteristics of the electrical asthenosphere beneath europe

composition and provenance of the macigno formation late oligocene early miocene in the trasimeno lake area northern apennines abstract sandstone petrography and mudstone mineralogy and geochemistry of the late oligocene early miocene terrigenous deposits of the macigno fm of the trasimeno lake area central italy provide new information on provenance paleoenvironment palaeoclimate and geodynamics during the early stages of the northern apennines foreland basin setting sandstones are rich in trace fossils and are quartzofeldspatic with various crystalline phaneritic mostly granitoids and medium low grade metamorphic rock fragments volcanic and sedimentary lithic fragments are rare the mudstone mineralogy contains a large amount of phyllosilicates quartz and feldspars and small amount of calcite which increases in the mid part of succession palaeoweathering indices chemical index of alteration with and without cao value cia and cia respectively suggest a source area that experienced low to moderate weathering and low recycling processes on average cia 66 4 and cia 69 7 furthermore very low and homogeneous values of rb k ratios the sandstone and mudstone composition reflects a provenance derived from uplifted crystalline rocks the different amount in feldspars the variety of lithic fragments the occurrence of mafic and carbonate input coupled with evidence of multi directional flows suggest a provenance from different source areas the geochemical proxies indicate a provenance from both felsic and mafic sources predominantly for the maesta section that shows cr v values ranging from 1 15 to 3 36 typical of source areas composed of both felsic and mafic rocks the western central alps are inferred to be the main source area of the macigno foreland system but significant signals from the mesomediterranean microplate are also testified these new data suggest that the macigno fm was probably located in a peculiar area which received either distal fine turbidite flows from the northernmost alpine area and residual sandy debris flows coming from the westernmost alps

contained reflected turbidity currents from the middle ordovician cloridorme formation quebec canada an alternative to the antidune hypothesis the parautochthonous cloridorme formation is a syn orogenic flysch succession that was deposited in an elongate foredeep basin as mainly lower middle fan outer fan and basin floor deposits the basin floor deposits about 1 5 km thick are confined to members 1 2 and 1 and are characterized by graded thick 1 10 m mud rich calcareous greywacke beds previously interpreted as deposits of concentrated muddy unidirectional turbidity currents that locally generated backset antidune lamination in internally stratified flows the dominant flow directions were from east to west but west to east transport also occurred as seen in the orientation of ripples climbing ripples flutes consistently overturned flames and grain imbrication we believe that the flows that deposited these thick calcareous greywacke beds reversed by roughly 180 one or more times during deposition of the lower sandy part of the beds flow reversals are consistent with the sharp grain size breaks and mud partings within sandy divisions measurement of grain fabric relative to stratification in the most celebrated antidune bedforms indicates flow from west to east thus the bedforms were produced by west to east migration of megaripples not by the upcurrent migration of antidunes the thick muddy beds were deposited by large volume muddy flows that were deflected and reflected from the side slopes and internal topographic highs of a confined basin floor much like the contessa and similar beds of the italian apennines large quantities of suspended mud were ponded above the irregular basin floor and settled to produce the thick silty mudstone caps seen on each bed because of their mode of emplacement we propose that these beds be called contained turbidites

a magnetotelluric study from over dharwar cratonic nucleus into billigiri rangan charnockitic massif india abstract the electrical resistivity structure of the crust beneath the dharwar craton in southern india was investigated by magnetotelluric method in the present study a northwest southeast oriented 220 km long profile of 18 stations with a station spacing of 10 15 km is used the profile extends from dharwar cratonic nucleus in the north to billigiri rangan charnockitic massifs in the south time series data are processed to get the apparent resistivity and phase the dominant geoelectric strike direction 75 was calculated in a period range of 0 01 1000 s the data are rotated to 75 strike direction two dimensional inversion is carried out by using the non linear conjugate gradient scheme for both apparent resistivity and phase our inversion results show a conductor in the northern side of the profile and two distinct prominent conductors in southern part of the profile the mid lower crust in southern part of the profile shows less resistive 2 flushed terranes regional scale carbonation occurred in late archaean is associated with chitradurga shear zone and in late proterozoic is associated with salem attur shear zone the co 2 rich fluids derived during that time might have exhausted in dehydration reactions later events such as the indian plate passing over several hotspots and the metasomatized fluids associated with the cretaceous tertiary magmatism in the region is the reason for observed low resistivity near billigiri rangan massif and surrounding regions in the south

the dali ash island of rhodes greece a problem in interpreting submarine volcanigenic sediments in the southern part of rhodes greece rhyolitic subaqueous pyroclastic deposits are interbedded with tertiary deep water marine sediments the lowermost and best exposed of these deposits the dali ash is described here the deposit has been previously described as a deep water welded subaqueous ignimbrite this paper shows that there is no evidence of welding and texture previously reported were misidentified the dali ash consists of a lower massive unit 5 m thick overlain by a sequence of ash turbidites 2 5 m thick the lower unit was deposited by a high concentration turbidity current and the ash turbidites by dilute turbidity currents foraminifera are dispersed throughout the deposit and indicate that all the sedimentary gravity flows were cold water particulate systems a palaeomagnetic study also suggests they were deposited cold