curie surface of borborema province brazil abstract the curie surface interpreted from magnetic data through spatial frequency domain techniques is used to provide information on the thermal structure of borborema province the borborema province is part of the neoproterozoic collision of an orogenic system situated between the sao francisco congo and sao luis west africa cratons which formed the gondwana supercontinent the curie surface of borborema province varies from 18 to 59 km which reveals the complexity in the crustal composition of the study area the thermal structure shows different crustal blocks separated by the main shear zones which corroborates the evolution model of allochthonous terranes the curie surface signature for the west portion of pernambuco shear zone may indicate processes of mantle serpentinization once the curie isotherm is deeper than mohorovic discontinuity in this region the amplitude of bouguer anomaly decreases which corroborates long wavelength anomaly observed in the magnetic anomaly we interpreted this pattern as evidence of the brasiliano pan africano s subduction collision event earthquakes in the region are concentrated mainly in shallow curie surface regions less resistant crust and in transition zones between warm and cold blocks we calculated the horizontal gradient of the curie depth to emphasize the signature of contact between the thermal blocks these regions mark possible crustal discontinuities and have high correlation with orogenic gold occurrence in the study area

models of crustal thickness for south america from seismic refraction receiver functions and surface wave tomography abstract an extensive compilation of crustal thicknesses is used to develop crustal models in continental south america we consider point crustal thicknesses from seismic refraction experiments receiver function analyses and surface wave dispersion estimates of crustal thickness derived from gravity anomalies were only included along the continental shelf and in some areas of the andes to fill large gaps in seismic coverage two crustal models were developed a by simple interpolation of the point estimates and b our preferred model based on the same point estimates interpolated with surface wave tomography despite gaps in continental coverage both models reveal interesting crustal thickness variations in the andean range the crust reaches 75 km in southern peru and the bolivian altiplano while crustal thicknesses seem to be close to the global continental average 40 km in ecuador and southern colombia despite high elevations and along the southern andes of chile argentina elevation lower than 2000 m in the stable continental platform the average thickness is 38 5 km 1 st deviation and no systematic differences are observed among archean paleoproterozoic cratons neoproterozoic fold belts and low altitude intracratonic sedimentary basins an exception is the borborema province ne brazil with crust 30 35 km thick narrow belts surrounding the cratons are suggested in central brazil parallel to the eastern and southern border of the amazon craton and possibly along the transbrasiliano lineament continuing into the chaco basin where crust thinner than 35 km is observed in the sub andean region between the mid plate cratons and the andean cordillera the crust tends to be thinner 35 km than the average crust in the stable platform a feature possibly inherited from the old pre cambrian history of the continent we expect that these crustal models will be useful for studies of isostasy dynamic topography and crustal evolution of the continent

the northeast brazilian rift system the northeast brazilian rift basins provide important data critical to the understanding of continental rifting processes associated with the opening of the south atlantic these basins represent the locus of intersection of the southern and equatorial branches and some basins yield substantial chronostratigraphic data that constrain the temporal and spatial interaction of the rift phases similar data are not found in its counterpart in africa especially for the neocomian these early cretaceous rift basins of northeast brazil illustrate key three dimensional geometries of intracontinental rift systems mainly controlled by the basement structural framework during the main rift phase syn rift ii neocomian early barremian extensional deformation was distributed over three main rift axes 1 the gabon sergipe alagoas gsa trend 2 the reconcavo tucano jatoba rtj trend and 3 the cariri potiguar cp trend during this phase extensional deformation jumped west from the easternmost basins gsa trend to a series of ne trending intracratonic basins rtj and cp trends characterized by a set of asymmetric half grabens separated by basement highs transfer faults and or accommodation zones these basins are typically a few tens of kilometers wide and trend ne sw roughly perpendicular to the main extension direction during the neocomian preexisting upper crustal weakness zones like the dominantly ne sw trending shear zones of the brazilian pan african orogeny controlled the development of intracrustal listric normal faults internal transverse structures such as transfer faults and accommodation zones were also controlled by the local basement structural framework the megashear zones of pernambuco brazil and ngaundere africa seem to have behaved like a huge accommodation zone accommodating extensional deformation along the rtj gsa trends with simultaneous extension along the cp trend during the late barremian syn rift phase iii a significant change in rifting kinematics occurred when the cp trend was aborted and major rifting initiated at the equatorial branch during the aptian while the equatorial branch and benue trough africa experienced the main rift phase the rtj trend was aborted and the gsa trend developed a transitional phase between the rift and drift stage the gsa trend and the offshore potiguar basin represent the site of continued evolution into passive margin basins following the main rift episode

channeling of plume flow beneath mid ocean ridges we investigate the interaction of mantle plumes with mid ocean ridges in 3 d numerical convection models using a finite volume multigrid technique with local grid refinement we are able to incorporate a much stronger temperature dependence of viscosity than was possible in previous models this allows lateral viscosity variations of more than six orders of magnitude which is realistic in a ridge situation in models of ridge centered plumes with constant flux we determine how the shape and the along axis width of the plume head depends on various model parameters the plume material is significantly channeled along the axis of a slow spreading ridge if the plume viscosity is in the order of 1017 pa s the plume material spreads radially below the ridge if the spreading rate is fast or the viscosity contrast between plume and ambient mantle is small in time dependent calculations we investigate the temporal evolution of short term variations of the plume flux at slow spreading ridges these perturbations propagate in the form of pulses along the ridge with a speed much larger than the spreading rate our results support the hypothesis that pulsations of the iceland plume are the cause for v shaped topography and gravity anomalies at the reykjanes ridge

gravity derived moho for south america crustal structure in south america is one of the least understood among the earth s continental areas variations in crustal thickness are still poorly constrained over large portions of the continent because of scarce or unevenly distributed crustal thickness estimates throughout south america to address this scarce and inhomogeneous data cover we explore the possibility to derive crustal thickness from satellite gravity data in this study we utilize the combined gravity model eigen 6c which is composed of goce and other gravity data the gravity field and steady state ocean circulation explorer goce satellite has a much more uniform spatial resolution than any land based gravity or seismic survey in south america the gravity data inversion is for a simple two layer model with fixed density contrast over the interface the moho the method is not relying on point constraint data and assumes that all of the signal is related to topography of the moho model quality can therefore be assessed by a comparison with point observations on crustal thickness we show that for the stable part of the continent 90 of our estimates are similar within error bounds to seismic observations variations occur in active orogenic zones or regions with suspected non standard moho density contrasts a comparison with seismological models shows a high correlation with the most recent model especially in areas where continental and global models of crustal structure have limitations in terms of wave paths or point constraints the gravity based model provides a unique continuity of crustal structure providing new insights on structure and tectonics and increase our understanding of the earth s structure underneath south america

tectonic evolution of south america during the late proterozoic abstract this paper discusses the evolution and the late proterozoic assembly of the south american continent i e the west gondwana under the light of global tectonics the northern portion of the continent the amazonian craton is a pre brasiliano domain it may be a fragment derived from laurentia this domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent brasiliano domain the brasiliano domain had a more complex history and composition during the brasiliano cycle middle proterozoic to early paleozoic in the brasiliano domain several late proterozoic continental plates have been identified sao luiz west africa congo kasai sao francisco rio de la plata arequipa antofalla which acted as forelands to the brasiliano orogenic belts that surround and amalgamate these plates smaller fragments of pre brasiliano continental lithosphere are common in the interior of the brasiliano orogenic belts and played diversified roles microplates microcontinents structural highs some of them have acted as true backlands hinterlands flanked by arc magmatism two principal types of brasiliano orogenic belts are recognized between the continental plates a marginal orogenies to the late proterozoic plates with pelitic carbonatic rocks and discrete volcanism which were thrust by collision onto the margin of the plates and which seem to represent final products of previous passive margin sequences b distal orogenics internal belts within the continental masses sometimes a branching system of orogenics that contain varied clastic and minor carbonate sedimentary rocks accompanied by important bimodal volcanism and calc alkaline plutonism for these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized the amalgamation of the continent during the late proterozoic i e the formation of the brasiliano structures indicates the existence of practically synchronous multilateral stresses within lithospheric plates this can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere analogous to modern continental collisions two large and important tectonic zones may be candidates to such a scenario the hijaz mozambique zone site of connection of east and west gondwana and the transbrasiliano lineament which cuts across south america and extends to north africa following the zone of connection between pre brasiliano and brasiliano domains other small ocean openings and closings inside the various brasiliano areas dom feliciano gariep for instance were minor events and complementary to the other two major tectonic zones mentioned above

crustal structure of the eastern borborema province ne brazil from the joint inversion of receiver functions and surface wave dispersion implications for plateau uplift we investigate the crustal structure of the borborema province of ne brazil by developing 44 s wave velocity depth profiles from the joint inversion of receiver functions and fundamental mode rayleigh wave group velocities the borborema province is located in the northeasternmost corner of the south american continent and represents a portion of a larger neoproterozoic mobile belt that formed during the brasiliano pan african orogeny extensional processes in the mesozoic eventually leading to the separation of africa and south america left a number of aborted rift basins in the continental interiors and episodes of diffuse intraplate volcanism and uplift marked the evolution of the province after continental breakup our velocity depth profiles reveal the existence of two crustal types in the province i the thin crustal type which consists of 30 32 5 km thick crust with an upper layer of 3 4 3 6 km s overlying a lower layer of 3 7 3 8 km s and ii the thick crustal type which consists of a 35 37 5 km thick crust with velocities between 3 5 and 3 9 km s down to 30 km depth and a gradational increase in velocity vs 4 0 km s down to upper mantle depths the crustal types correlate well with topography with the thick crustal type being mainly found in the high standing southern borborema plateau and the thin crustal type being mostly found in the low lying sertaneja depression and coastal cuestas interestingly the thin crustal type is also observed under the elevated topography of the northern plateau we argue that the thick crustal type is rheologically strong and not necessarily related to postbreakup mantle processes as it is commonly believed we propose that extensional processes in the mesozoic stretched portions of the brasiliano crust and formed the thin crustal type that is now observed in the regions of low lying topography leaving the rheologically strong thick crust of the southern plateau at higher elevations the crust making the northern plateau would have thinned and subsided during mesozoic extension as part of a greater sertaneja depression to then experience uplift in the cenozoic and achieve its present elevation

mesozoic igneous activity in the maranha o province northern brazil 40ar 39ar evidence for separate episodes of basaltic magmatism low and high ti basaltic rocks of the maranha o province in northern brazil appear to be related to the opening of the atlantic ocean 40ar 39ar dating was carried out on powdered whole rock basalts both washed and unwashed in dilute nitric acid to document this relationship acid treatment does not alter the argon retention properties of retentive phases but is effective in removing alteration phases high ca k degassed at low laboratory temperatures our results identify that at least two periods of volcanic activity formed the maranha o province low ti basalts from the western sector are 200 190 ma and are related to the opening of the north atlantic high ti basalts from the eastern sector are 129 124 ma slightly younger than compositionally similar rocks in the serra geral flood basalt province of southern brazil and are related to the opening of the south atlantic other flood basalts such as the columbia river deccan rajmahal ferrar and siberian provinces were each predominantly formed in 1 2 ma the maranha o basalts remnants of separate igneous events spanning 70 ma should not be classed as a single flood basalt province the temporal complexities noted in the maranha o basaltic rocks appear to be related to the tracks of hotspots that led to the rifting of the atlantic ocean two sets of mesozoic basaltic rocks occur in southern africa india and south america resulting from events leading to the fragmentation and dispersal of sections of gondwanaland in each sub continent the two provinces were formed by events separated by 50 70 ma of these pairs only the maranha o and serra geral show geochemical similarities indicating compositionally distinct upper mantle source regions under sections of brazil separated by 3000 km

timing and mechanisms for the generation and modification of the anomalous topography of the borborema province northeastern brazil abstract the results of apatite fission track analysis in 14 granitic gneissic samples from two regional transects across the borborema plateau northeastern brazil show evidence for two dominant paleothermal events a late cretaceous cooling event beginning sometime between 100 and 90 ma and a second cooling event in the neogene the distribution of the fission track results suggests that the cooling events have a broad regional expression and are consistent with the geologic record in the araripe basin western borborema province which attests to a post albian uplift of the whole region we hypothesize that the first event is due to the uplift and denudation of regional permanent topography generated after the breakup of brazil and africa such topography is predicted by models of continental margin extension in which continental lithosphere thinning is followed by thickening of the adjacent hinterland lithosphere and crust kusznir n j karner g d 2007 continental lithospheric thinning and breakup in response to upwelling divergent mantle flow application to the woodlark newfoundland and iberia margins in karner g d manatschal g pinheiro l eds imaging mapping and modeling continental lithosphere extension and breakup special publication 282 geological society london pp 389 419 in northeastern brazil this extension engendered topography may have been amplified by magmatic underplating related to the saint helena and ascension plumes the miocene cooling event 20 0 ma occurred at a time characterized by the transition from carbonate ramp to progradational clastic systems on the pernambuco paraiba margin and the offshore potiguar basin this same stratigraphic response characterizes the neogene stratigraphy of many passive margins and attests to a global increase in the delivery of clastics to margins the simplest explanation of which is a climate change that accentuated erosion of pre existing topography thus the present rugged landscape of northeastern brazil is interpreted to be a product of this younger denudation event a corollary of this study is that the history distribution and delivery of clastics to the northern and northeastern margins of brazil are a function of the regional development of the continental landscape during the late cenozoic

asthenospheric imprints on the lithosphere in central mongolia and southern siberia from a joint inversion of gravity and seismology mobal experiment summary we present a joint inversion of gravity and teleseismic data to enlighten the lithospheric structures of the baikal mongolia region an area characterized by high topographic contrasts sporadic cenozoic volcanism extension and large transcurrent faulting in the vicinity of the baikal rift central asia the study uses a 1000 km long seismic transect that cross cuts the main tectonic structures from north to south namely the siberian platform tunka basin hangay dome and gobi altai belt the siberian platform depicts a high velocity lithosphere down to about 150 km we evidence strong velocity contrasts within the crust below the hangay dome and the tunka depression interpreted as a thickened crust a low velocity density region is located at various depths below the hangay dome thanks to the dense spatial coverage of gravity data we are able to define the 3 d geometry of this particular low velocity density anomaly the hangay dome anomalous body extends from 60 to 225 km depth at his largest point and slightly thins to no more than 40 km at its easternmost end a deep low velocity zone below 150 km with only a weak negative density contrast is observed on the eastern part of the transect we propose that the late cenozoic uplift of the whole mongolian plateau and associated rifting magmatism high heat flow and lithospheric thinning are not externally driven by the india asia collision but results from the interaction of two mantle plumes with the overlying lithosphere conversely recent faulting may be the major expression of the india asia collision in this region as well as n s striking rift basins that connect and interact with major strike slip faults such as the bolnai fault the comparison between time residuals of this experiment and the baikal rift seismic data shows an undeniable disproportion for the hangay anomaly the hangay time residuals are far more positive than through the baikal arguing for an asthenospheric signature that is not seen beneath the baikal rift furthermore the southern edge of the thick siberian cratonic lithosphere may favour the rise of a sublithospheric mantle flow at the contact with the baikal mongolia lithosphere

gps measurements of crustal deformation in the baikal mongolia area 1994 2002 implications for current kinematics of asia 1 we present new geodetic results of crustal velocities over a large part of northern asia based on gps measurements in the baikal rift zone and mongolia spanning the 1994 2002 period we combine our results with the gps velocity field for china of wang et al 2001 and derive a consistent velocity field for most of asia we find contrasted kinematic and strain regimes in mongolia with northward velocities and n s shortening in westernmost mongolia but eastward to southeastward motion and left lateral shear for central and eastern mongolia this eastward to southeastward motion of central and eastern mongolia is accommodated by left lateral slip on the e w trending tunka bolnay and gobi altay faults 2 1 2 mm yr 1 2 6 1 0 mm yr 1 and 1 2 mm yr 1 respectively and by about 4 mm yr 1 of extension across the baikal rift zone consequently 15 of the india eurasia convergence is accommodated north of the tien shan by n s shortening combined with dextral shear in the mongolian altay and by eastward displacements along major left lateral strike slip faults in central and eastern mongolia we find a counterclockwise rotation of north and south china as a quasi rigid block around a pole north of the stanovoy belt which rules out the existence of an amurian plate as previously defined and implies 2 mm yr 1 of left lateral slip on the qinling shan fault zone

vs and density structure beneath the colorado plateau constrained by gravity anomalies and joint inversions of receiver function and phase velocity data 1 the colorado plateau is a physiographic province in the western us with an average elevation of 1 9 km where in contrast to neighboring provinces there is little evidence of large scale tectonic deformation or magmatism recent availability of earthscope usarray seismic data allow us to better examine the crust and upper mantle structure beneath the region and test proposed explanations for the plateau s uplift and relative stability using phase velocities for fundamental mode rayleigh waves and p receiver functions we perform over 800 joint inversions for 1 d shear wave velocity vs profiles sampling the plateau and surrounding regions down to 150 km depth we image a sharp change in crustal thickness at the western edge of the colorado plateau with a more gradual increase eastward moving into the rocky mountains a relatively thick 100 km lithosphere beneath the plateau extends into the rocky mountains to the north we use empirical scaling relations to estimate densities from our vs results and predict the associated gravity anomalies which are inconsistent with the observed distribution of the bouguer gravity anomalies we somewhat reconcile the prediction and observations by assuming that lateral density variations below 50 km can be ignored and the lithospheric root is therefore neutrally buoyant while there is some evidence for small scale convection and lithospheric removal at its edges the shape of the lithospheric mantle anomaly is consistent with a large scale uplift of the plateau by heating since removal of the farallon slab we conclude that the lithospheric root is key to the long term stability of the colorado plateau leading to a colder stronger crust

timing of crust formation deposition of supracrustal sequences and transamazonian and brasiliano metamorphism in the east pernambuco belt borborema province ne brazil implications for western gondwana assembly abstract the main structural feature of the central domain of borborema province ne brazil is a network of dextral and sinistral shear zones these shear zones rework an older regionally developed flat lying foliation in orthogneisses and supracrustal belts which in the east pernambuco belt was formed under amphibolite facies conditions this study reports la icp ms u pb zircon ages of metaigneous and metasedimentary rocks aiming to constraint the pre transcurrent tectonothermal evolution in the eastern pernambuco domain ages of 2125 7 and 2044 5 ma in a mafic layer of banded orthogneiss are interpreted as the age of the protolith of the orthogneiss and of high grade transamazonian metamorphism respectively the latter age is consistent with the occurrence of low th u metamorphic zircon xenocrysts dated at 2041 15 ma in the leucosome of a migmatitic paragneiss a granitic orthogneiss dated at 1991 5 ma reflects late to post transamazonian magmatic event a similar age 1972 8 ma was found in rounded zircon grains from a leucocratic layer of banded orthogneiss ages of detrital zircons in a paragneiss sample indicate derivation from sources with ages varying from the archean to neoproterozoic with peak ages at ca 2220 2060 1940 1200 1150 and 870 760 ma detrital zircons constrain the deposition of the supracrustal sequence to be younger than 665 ma magmatic zircons with the age of 626 15 ma are found in the leucosome of a migmatitic paragneiss and constrain the age of the brasiliano high temperature metamorphism a lower intercept age of 619 36 ma from a deformed granodiorite dated at 2097 5 ma and the crystallization age of 625 24 ma of the felsic layer of banded orthogneiss also confirm the late neoproterozoic metamorphism these results show that the present fabric in basement and supracrustal rocks was produced during the brasiliano orogeny paleoproterozoic ages reported in this study are similar to those found in other sectors of the borborema province the cameroon and nigeria provinces and the sao francisco congo craton they show the importance of the transamazonian eburnean event and suggest that these tectonic units may have been part of a larger single continental landmass likewise similarities in post transamazonian metamorphic and magmatic events in the borborema nigeria and cameroon provinces suggest that they shared a common evolution and remained in close proximity until the opening of the atlantic ocean

alkaline volcano of paleocene age on the southern guinean margin mapping petrology 40ar 39ar laser probe dating and implications for the evolution of the eastern equatorial atlantic abstract a segment of the southern guinea plateau margin s g p m was surveyed during the 1988 equamarge ii cruise of the r v jean charcot detailed seabeam mapping reveals a huge 3000 m high volcano which was called the nadir seamount culminating at 840 m below sea level and 6 adventive cones volcanics dredged between 2100 and 1300 m of water depth consist of hyaloclastite breccias and subaerial alkali basalts recovered as both massive blocks and eolian pebbles with a desert varnish the data are consistent with the growth of a volcano up to emergence and thereafter its dismembering and subsidence volcanological petrographic and geochemical features ree of the volcanics present similarities with those observed 1 in early basalts from fuerteventura canary islands and maio cape verde islands 2 in lamprophyric volcanism from the sierra leone rise and 3 in volcanics from the gorringe bank 40ar 39ar laser probe dating was performed on two biotite phenocrysts from a massive basalt using step heating and spot fusion procedures both experiments concordantly display a plateau age of 58 6 0 3 ma and an integrated age of 58 6 0 7 ma respectively the nadir seamount provides the third evidence of paleocene alkaline volcanism in the eastern atlantic after the gorringe bank and the northern sierra leone rise krause seamount these volcanoes are presumed to have been controlled by fractures zones the ages and compositions of the nadir and krause seamounts respectively suggest a connection between the volcanic chains of both the sierra leone rise and guinea f z which may have recorded the same hotspot track

petrology of the alkaline rocks of the macau volcanic field ne brazil the macau volcanic field mvf in the borborema province ne brazil contains multiple centres of volcanic activity of early to late cenozoic ages we present element and sr nd pb isotope geochemical data for four of the few most prominent basalt types of this volcanic field serrote preto type serra aguda type pico do cabugi type and serra preta type in order to assess their magmatic history from source to crystallization and the evolution of the mantle beneath the borborema province the basalts are basically sodic nephelinitic basanitic alkali olivine basalts enriched in lile and in nb ta the serra preta cabugi and serra aguda types demonstrate compositions close to primitive characteristics with 10 mgo 15 wt and 200 ppm ni 500 ppm and experienced limited fractional crystallization of olivine clinopyroxene plagioclase oxides with negligible wall rock assimilation rb sr and ba rb constraints support the generation of sio2 undersaturated magmas from mantle melting of amphibole bearing peridotites with minor phlogopite the source for the basanites and alkali basalts is estimated to be a garnet bearing domain around the lithosphere asthenosphere boundary 80 93 km deep while the nephelinites are derived from the adiabatic asthenosphere at 105 km with temperatures of 1480 c their incompatible trace element patterns and sr nd pb isotopic compositions are similar to fozo and em type oib magmas from the comparison of data with those of the ceara mirim dyke swarm we propose that there is a ubiquitous fozo reservoir in the sclm beneath the borborema province this fozo signature characterized the upwelling asthenosphere during the lithospheric extension and thinning at the opening of the equatorial atlantic and is clearly represented in the mesozoic olivine tholeiites of ceara mirim the upwelled asthenosphere cooled as a rigid sclm since the cretaceous and has preserved its fozo signature evident in the macau cenozoic basalts the em signatures in mvf lavas probably represent a deep component derived from metasomatizing fluid melts that have interacted with an originally old oib type mantle to produce metasomatic minerals such as k rich amphibole mica with highly incompatible trace elements contents which enhanced the generation of alkaline magmas through mantle melting

cenozoic uplift and subsidence in the north atlantic region geological evidence revisited abstract the topographic evolution of the passive margins of the north atlantic during the last 65 myr is the subject of extensive debate due to inherent limitations of the geological geomorphological and geophysical methods used for studies of uplift and subsidence we have compiled a database of sign time and amplitude where possible of topographic changes in the north atlantic region during the cenozoic 65 0 ma our compilation is based on published results from reflection seismic studies aft apatite fission track studies vr vitrinite reflectance trends maximum burial sediment supply studies mass balance calculations and extrapolation of seismic profiles to onshore geomorphological features the integration of about 200 published results reveal a clear pattern of topographic changes in the north atlantic region during the cenozoic 1 the first major phase of cenozoic regional uplift occurred in the late palaeocene early eocene ca 60 50 ma probably related to the break up of the north atlantic between europe and greenland as indicated by the northward propagation of uplift it was preceded by middle palaeocene uplift and over deepening of some basins of the north sea and the surrounding areas 2 a regional increase in subsidence in the offshore marginal areas of norway the northern north sea the northern british isles and west greenland took place in the eocene ca 57 35 ma 3 the oligocene and miocene 35 5 ma were characterized by regional tectonic quiescence with only localised uplift probably related to changes in plate dynamics 4 the second major phase of regional uplift that affected all marginal areas of the north atlantic occurred in the plio pleistocene 5 0 ma its amplitude was enhanced by erosion driven glacio isostatic compensation despite inconclusive evidence this phase is likely to be ongoing at present

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

a coastal transect of mcmurdo dry valleys antarctica snow and firn marine and terrestrial influences on glaciochemistry samples of snow and firn from accumulation zones on clark commonwealth blue and victoria upper glaciers in the mcmurdo dry valleys 77 788 s 161 1648 e antarctica are evaluated chemically and isotopically to determine the relative importance of local site specific factors vs regional scale influences in defining glaciochemistry spatial variation in snow and firn chemistry confirms documented trends within individual valleys regarding major ion deposition relative to elevation and to distance from the coast sodium and methylsulfonate ms for example follow a decreasing gradient with distance from the coast along the axis of victoria valley 350 119 gl 1 for na 33 14 gl 1 for ms a similar pattern exists between commonwealth and newall glaciers in the asgaard range when comparing major ion concentrations e g na m s c a 2 or trace metals e g al fe among different valleys however site specific exposures to marine and local terrestrial chemical sources play a dominant role because chemical signals at all sites respond to particulates with varying mixtures of marine and terrestrial sources each of these influences on site glaciochemistry must be considered when drawing temporal climate inferences on regional scales

fate of mantle plume material trapped within a lithospheric catchment with reference to brazil 1 local regions of thin lithosphere act as catchments of hot buoyant plume material unless replenished the trapped plume material cools by convection to the mantle adiabat by several tens of million years in particular currently hot material from the 130 ma parana starting plume head is unlikely to supply 85 ma to recent volcanism on the mainland of brazil and the martin vaz and fernando hot spots rather a plume tail may now underlie southern brazil where tomographic studies detect conduit shaped velocity anomaly through the upper mantle if the tomographic study in fact found a plume tail the track crossed the amazon rift at 85 ma and since then lateral flow along thin regions of the lithosphere from the tail fed widespread feeble volcanism including the flow line hot spots of martin vaz and fernando

numerical modeling of weathering erosion sedimentation and uplift in a triple junction divergent margin the majority of numerical models of landscape evolution in divergent margins are focused on the simulation of margins with simplified lithological control on landscape erosion however this approach is insufficient to study the evolution of margins where chemical weathering is an important element increasing rock resistance to physical erosion one example of this margin is the borborema province northeastern brazil where postrift marine sediments are now preserved at elevations 700 800 m in up to 1 km high plateaus capped by duricrust layers the landscape evolution of these uplifted sedimentary basins still eludes explanation here we use numerical models that couple weathering erosion sedimentation sea level changes flexural isostasy and thermal effects due to lithospheric stretching to simulate the tectonosedimentary evolution of the borborema province since the onset of continental stretching during the lower cretaceous these numerical experiments reveal that nearly 70 of the postrift regional uplift observed in the borborema province can be explained by differential denudation of the continent and flexural rebound of the lithosphere the remaining 250 m of uplift can be explained by thermal uplift induced by partial erosion of the base of the continental lithosphere under the borborema province due to edge driven convection in accordance with the anomalously thin continental lithosphere observed under the borborema province additionally the numerical results can explain the regional pattern of fission track ages by the combined effect of differential denudation and flexural rebound in this geometrically complex margin

small scale convection at the edge of the colorado plateau implications for topography magmatism and evolution of proterozoic lithosphere the colorado plateau of the southwestern united states is characterized by a bowl shaped high elevation late neogene quaternary magmatism at its edge large gradients in seismic wave velocity across its margins and relatively low lithospheric seismic wave velocities we explain these observations by edge driven convection following rehydration of colorado plateau lithosphere a rapidly emplaced cenozoic step in lithosphere thickness between the colorado plateau and adjacent extended rio grande rift and basin and range province causes small scale convection in the asthenosphere a lithospheric drip below the plateau is removing lithosphere material from the edge that is heated and metasomatized resulting in magmatism edgedriven convection also drives margin uplift giving the plateau its characteristic bowl shape the edge driven convection model shows good consistency with features resolved by seismic tomography

lateral flow and ponding of starting plume material many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle flood basalts radial dike swarms and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere the buoyant plume head material ponds at the base of the lithosphere and then flows laterally relief on the base of the lithosphere acts as an upside down drainage pattern with enclosed catchments for this flow that is plume material preferentially collects beneath regions where the lithosphere is locally thin such as thermally subsiding sedimentary basins an example is uplift associated with the iceland starting plume which inverted the mesozoic irish sea basin additional relief at the base of the lithosphere is associated with rifting and seafloor spreading in the case of the northern north sea material from the iceland starting plume ponded beneath a basin with locally thin lithosphere rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head an example is the volcanic passive margin off nova scotia which is 2000 km from the center of the bahama starting plume head numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 pa s

geomorphology and structural development of the nested summit crater of lascar volcano studied with terrestrial laser scanner data and analogue modelling abstract many volcano summits host craters that are partially overlapping the formation of such nested craters has been commonly interpreted as vent migration here we present an additional mechanism that may explain the geometry of nested craters at volcanoes lascar volcano the most active volcano of the central volcanic zone in the chilean andes hosts ene wsw trending summit craters that are partially overlapping nested details on the evolution and interaction between the different craters remain unclear to create a robust dataset terrestrial laser scanner tls data were collected at the summit of lascar in 2013 the resulting topographic data set consisting of more than 15 million data points with centimetre sampling allows visualising almost the complete eastern edifice of the volcano s summit from the tls data a digital elevation model dem and a slope map were generated allowing us to create a lineament map and quantify the observed morphological and structural features to further improve our understanding of the processes responsible for the formation of the craters and geomorphology we designed sandbox analogue models results suggest that one of the craters is a parasite crater formed as a consequence of ongoing activity in the adjacent crater our data suggest that the nested craters have all been modified since the last major eruption in 1993 by near surface effects associated with cooling compaction and gravitational sliding of the crater floor infill as the active crater deepens the adjacent inactive crater extends and partially slumps towards the active one understanding the structural development of these nested craters is relevant for assessing potential future eruption sites thus making lascar a dynamic target for a detailed morphology study these findings may similarly be applied to other volcanoes where nested craters have developed

continental deformation in asia from a combined gps solution 1 after decades of research on continental tectonics there is still no consensus on the mode of deformation of continents or on the forces that drive their deformation in asia the debate opposes edge driven block models requiring a strong lithosphere with strain localized on faults to buoyancy driven continuous models requiring a viscous lithosphere with pervasive strain discriminating between these models requires continent wide estimates of lithospheric strain rates previous efforts have relied on the resampling of heterogeneous geodetic and quaternary faulting data sets using interpolation techniques we present a new velocity field based on the rigorous combination of geodetic solutions with relatively homogeneous station spacing avoiding technique dependent biases inherent to interpolation methods we find 1 unresolvable strain rates 3 109 yr over a large part of asia with current motions well described by block or microplate rotations and 2 internal strain possibly continuous limited to high elevation areas

mongolian plateau evidence for a late cenozoic mantle plume under central asia the 2500 x 700 km mongolian plateau average elevation 2000 m is situated between the altai orogen and the siberian craton and occupies much of mongolia and transbaikalia in russia the plateau is characterized by 1 basin and range topography and two major domes hentai 600 x 300 km and hangai 800 x 550 km where altitudes reach 3905 m 2 lithosphere that is thinner than adjacent areas minimum 50 km 3 elevated heat flow up to 120 mw m2 4 dominantly alkaline basaltic volcanism in the form of cones lava fields and volcanic plateaus mostly of miocene quaternary age and 5 rifts including baikal main evolution in the pliocene quaternary tunka oligocene early miocene and hobsogol pliocene quaternary existing models explain these features in terms of diapiric upwelling of a mantle asthenolith below the main rifts and or as a long distance effect of the india asia collision we propose that the late cenozoic uplift of the whole mongolian plateau and associated rifting magmatism high heat flow and lithospherec thinning are not externally driven by the india asia collision but are the expression of the interaction of a mantle plume with overlying lithosphere some rifts link and interact with major strike slip faults such as the bolnai such faults may be the major expression of the india asia collision in this region

reply to comment by doran et al on el nino suppresses antarctic warming 1 doran et al commented on our recent paper el nino suppresses antarctic warming bertler et al 2004 suggesting that our interpretation of their article antarctic climate cooling and terrestrial ecosystem response doran et al 2002 was incorrect instead they assert that their paper does not state that 1 antarctica as whole is cooling since 1966 nor that 2 the mcmurdo dry valleys data were used to verify this finding or represent this longer term cooling 2 we are especially pleased that doran et al agree with us that the dry valleys do not show a longer term cooling however we cannot find any fault in our interpretation of their original paper owing to

continental lithospheric thinning and breakup in response to upwelling divergent mantle flow application to the woodlark newfoundland and iberia margins abstract depth uniform stretching is not the dominant deformation process for thinning continental lithosphere leading to breakup it cannot explain the observed depth dependent lithosphere stretching and mantle exhumation at rifted continental margins depth dependent lithosphere thinning in which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust has been observed at many non volcanic and volcanic rifted continental margins including conjugate margin pairs passive continental margins show a paucity of brittle deformation in the upper crust during continental lithosphere thinning leading to breakup and sea floor spreading initiation a new model of rifted continental margin formation has been developed that assumes that deformation and thinning of continental lithosphere leading to breakup occurs in response to an upwelling divergent flow field within continental lithosphere and asthenosphere and that this deformation evolves into sea floor spreading the new model successfully predicts depth dependent stretching of continental margin lithosphere for both non volcanic and volcanic margins and mantle exhumation at non volcanic margins both of which are observed but are not explained by existing depth uniform continental lithosphere stretching models the new model provides a balance of extensional strain supplies an explanation for the paucity of synrift brittle deformation and offers a simple transition from prebreakup lithosphere thinning to sea floor spreading the observed diversity of rifted continental margin structure and width of the ocean continent transition can be explained by variability in the form of the upwelling divergent flow field the new upwelling divergent flow model of continental lithosphere thinning leading to continental breakup successfully predicts the observed bathymetry and margin geometry for the most recent segment of sea floor spreading initiation in the woodlark basin in the western pacific and the observed bathymetry and free air gravity anomaly for the newfoundland and iberian margins

proterozoic history of the borborema province ne brazil correlations with neighboring cratons and pan african belts and implications for the evolution of western gondwana 1 geological and geochronological correlations between borborema province ne brazil and neighboring cratons and brasiliano pan african belts indicate that the amazonian west african and sao francisco congo cratons and the basement of the araguaia borborema nigerian and cameroon provinces were part of the atlantica supercontinent this continent was established at the end of the transamazonian eburnean cycle 2 0 ga and apart from ubiquitous taphrogenesis in the 1 8 1 7 ga interval remained largely unaffected for the following 1 ga around 1 ga an important magmatic event in borborema province correlates with rifting episodes and anorogenic magmatism in the sao francisco congo and amazonian cratons these events are interpreted as failed attempts to break up atlantica which at this time may have been part of the larger rodinia supercontinent renewed extensional conditions in borborema province during the middle and late neoproterozoic are attributed to far field stresses transmitted to the interior of atlantica by outwardly dipping subduction zones that encircled its northern present day coordinates portion the rarity of petrotectonic assemblages typical of subduction zone environments indicates that extension did not evolve enough to form large oceans basins and thus that the borborema province essentially includes reworked intracontinental domains regional deformation and metamorphism starting at 650 640 ma and shear zone development beginning at 590 595 ma were continuously developed through time and were synchronous throughout most of the borborema araguaia cameroon and nigerian provinces postorogenic conditions were reached 540 530 myr ago while active deformation was still occurring in other belts that accreted around atlantica to form western gondwana

the bahia seamounts test of a hotspot model and a preliminary south american late cretaceous to tertiary apparent polar wander path abstract detailed bathymetric and magnetic data collected over the bahia seamounts in the western south atlantic brazil basin are analyzed six paleomagnetic poles are determined one from three seamounts sharing a ridge and five from isolated seamounts two poles fall near a continental late cretaceous south american pole and are assumed to be of late cretaceous age another seamount has a preliminary 40 ar 39 ar age of 62 4 ma we check this age and estimate the ages of other seamounts by rotating their poles into north american coordinates and comparing them with north american paleomagnetic reference poles these ages and the characteristic magnetic polarities of all bahia seamounts are used to test hotspot models of formation of the seamount group the northern and central bahia seamount chains could have formed from a single hotspot the southern bahia chain may require a second hotspot the two postulated hotspots were probably concurrent and separated by 150 to 200 km the paleomagnetic poles trace out a smooth curve from the late cretaceous poles to a pole that is indistinguishable from the spin axis which we estimate as having an early eocene age the pole ages remain preliminary until better age dates are found and rocks are recovered from more seamounts

review of earth science research using terrestrial laser scanning abstract the application of advanced remote sensing technologies including terrestrial laser scanning tls to the earth sciences has increased rapidly in the last two decades improving the spatial and temporal resolution of data terrestrial laser scanning units have evolved into a common tool in studies of spectral and structural geology seismology natural hazards geomorphology and glaciology special consideration of the advantages and limitations of tls in each of these fields is discussed in depth in the context of important work published in each field the workflow used in a tls survey is crucial to the success of the survey and field specific earth science workflows are therefore also discussed products based on tls data such as triangulated irregular networks tin and digital surface models dsm are commonplace tools throughout the earth sciences and the use of these tools to measure slip distributions fault geometries aeolian transport river bed morphologies and flows and other research problems is expanded on where appropriate the review concludes with a discussion of recent trends in tls instrument development and their potential impact on the use of tls in the earth sciences in the future

upper mantle structure of the borborema province ne brazil from p wave tomography implications for rheology and volcanism data were collected with support of the instituto nacional de ciencia e tecnologia em estudos tectonicos inct et of the centro nacional de desenvolvimento cientifico e tecnologico cnpq grant number 57 3713 2008 01 and the borborema deep electromagnetic and seismic experiment bodes cnpq grant number 400743 2014 0 flsn acknowledges support from the coordenacao de aperfeicoamento de pessoal de nivel superior capes through a 4 yr scholarship to complete his phd at the universidade federal do rio grande do norte ufrn jj thanks cnpq for his research fellowship cnpq grant number 304421 2015 4 ms acknowledges support by brazilian science without border program grant number 40 2174 2012 7

an updated crustal thickness map of central south america based on receiver function measurements in the region of the chaco pantanal and parana basins southwestern brazil

numerical modeling of flexural stresses in the upper crust of ne brazil implications for stress regimes in intraplate settings abstract we model the contribution of the stresses generated by the topography and density heterogeneities in the stress field of ne brazil and the interplay between these and regional stresses our model consists of a horizontally layered elastic slab the lithosphere that floats above an inviscid fluid the asthenosphere where the surface load is generated by the topography and bathymetry and the internal loads are calculated from the bouguer anomalies the deviatoric flexural stresses are calculated in the fourier domain our results showed local flexural stresses with magnitudes comparable to those of the plate wide stresses and mainly controlled by the long wavelength bouguer anomalies scenarios for the total stress field were calculated by superposing different values for an e w oriented regional compression we predict the stress axes orientations and stress regimes for the equatorial margin and the pernambuco shear zone pesz region however we highlight the importance of spreading stresses not considered here in rotating the maximum horizontal compression shmax predictions the prediction of stress regime and stress axes orientation in pesz using low to zero regional compression suggest that a decoupling of the shallow stresses from the plate wide stresses might occur in this region another possible explanation found was that the spreading stresses might counterbalance the regional stresses we conclude that the superposition of principally flexural and plate wide stresses explains reasonably the observed stress regimes and shmax orientations in a great part of ne brazil furthermore flexural stresses are playing an important role in the reactivation of the pesz possibly controlling the stress field

the poisson ratio of the australian crust geological and geophysical implications the poisson ratio which depends on the vp vs ratio provides much tighter constraints on the crustal composition than either the compressional or the shear velocity alone the crustal poisson ratio can be determined from the joint analysis of the travel times of waves converted at the moho and of crustal multiples reflected at the top of the moho we have analyzed the records of the permanent stations installed on the australian continent complemented by the data of the skippy experiment the results reveal substantial variations in the poisson ratio in the different tectonic units for the proterozoic crust an increase of the poisson ratio with increasing crustal thickness is systematically observed while for the phanerozoic crust the poisson ratio tends to decrease for increasing crustal thicknesses these observations are in remarkable agreement with the results of the deep seismic soundings that were performed in the former soviet union the variations observed in the proterozoic provinces can perhaps be explained by underplating of mafic materials at the base of the crust fl 2000 elsevier science b v all rights reserved

crustal architecture of the borborema province ne brazil from receiver function ccp stacks implications for mesozoic stretching and cenozoic uplift abstract we have investigated the crustal architecture of the borborema province of ne brazil by constructing common conversion point ccp receiver function stacks from teleseismic p waveforms recorded at 64 seismic stations in the region the borborema province represents the western portion of a larger neoproterozoic mobile belt that experienced extension in the mesozoic leading to the formation of a number of intra continental rift basins and eventually continental breakup after continental breakup episodes of uplift in the province perhaps related to coeval episodes of cenozoic volcanism helped shape the high topographies of the borborema plateau our receiver function ccp stacks image clear p to s conversions at the crust mantle boundary and confirm independent evidence for a 36 38 km thick crust under the southern portion of the plateau and a thinner 30 32 km thick crust in the surrounding regions including the northern plateau the cross sections also reveal the presence of an intra crustal discontinuity at 9 18 km depth under the regions of thin crust that fades away under the thick southern plateau we argue that the thin crust in the borborema province is the result of mesozoic crustal stretching and that the intra crustal discontinuity represents a low angle detachment zone that helped accommodate extension in the crust the thick crust under the southern plateau would then represent a rheologically stronger portion of the borborema province that resisted deformation by mesozoic extension while the thin crust under the northern plateau would be a portion of formerly depressed thin crust that was uplifted during the cenozoic

fracturing and calcite cementation controlling fluid flow in the shallow water carbonates of the jandaira formation brazil the shallow marine carbonate rocks of the jandaira formation have been subject to significant permeability variations through time due to various events of fracturing and calcite cementation as a consequence the jandaira formation accommodated fluid flow only during specific moments in time we reconstructed these episodes of fluid flow based on isotope characterizations and microscope characteristics of calcite veins and host rock cements the jandaira formation which belongs to the post rift sequence of the potiguar basin in northeast brazil was deposited from the turonian onward until a marine regression exposed it in the campanian due to the subaerial exposure meteoric waters flushed out marine connate waters leading to an event of early diagenesis and full cementation of the jandaira formation fluid flow through the resulting impermeable carbonate formation appears to be closely related to fracturing fracturing in the late cretaceous induced a drastic increase in permeability giving rise to extensive fluid circulation host rock dissolution associated to the circulating fluids led to calcite vein cementation within the fracture network causing it to regain an impermeable and sealing character in the research area fluid flow occurred during early burial of the jandaira formation at estimated depths of 400 900 m this study documents the first application of fluid inclusion isotope analysis on vein precipitates which allowed full isotopic characterization of the paleo fluids responsible for calcite vein cementation the fluid inclusion isotope data indicate that upwelling of groundwater from the underlying acu sandstones provided the fluids to the fracture network in miocene times renewed tectonic compression of a lower intensity created a secondary fracture network in the jandaira formation the density of this fracture network however was too low to induce a new episode of fluid circulation as a result this tectonic event is associated with the development of barren extensional fractures

upper mantle anisotropy of the borborema province ne brazil implications for intra plate deformation and sub cratonic asthenospheric flow abstract the geological record of the borborema province northeast brazil documents tectonic events that characterised the precambrian formation and mesozoic breakup of gondwana large scale shear zones and associated granitic plutons that developed during the neoproterozoic brasiliano pan african orogeny and major sedimentary basins of mesozoic age indicate significant deformation across the region however whether or not the shear zones resulted from precambrian terrane accretion or are simply the result of episodes of subsequent intra plate deformation is debated also poorly understood is the effect of the thick sao francisco mantle keel on present day asthenospheric flow to address these issues we have performed a teleseismic shear wave splitting study of mantle seismic anisotropy from a new broadband seismograph network centred on the borborema province shear wave splitting parameters t reveal a lack of plate scale anisotropic fabrics associated within the continental interior perhaps supporting models of formation and evolution of the borborema province involving minimal deformation of the lithospheric mantle delamination of anisotropic lithosphere during the development of cenozoic volcanism that eroded older fossil lithospheric fabrics is unlikely because the widespread cenozoic magmatism required to achieve this is absent in the geological record instead the apparently low levels of seismic anisotropy observed in the interior of the borborema province may simply reflect depth dependent anisotropy nulls low t observations may be the subtractive result of orthogonal fast directions in the lithosphere and asthenosphere towards the brazilian coast t 1 s and fast directions are sub parallel to stretching fabrics formed during the opening of the south atlantic this may imply that the mantle lithosphere was deformed but not completely destroyed during gondwana breakup however a more complete backazimuthal coverage of splitting measurements across the region is needed to confirm or refute these hypotheses unambiguously

el nino suppresses antarctic warming here we present new isotope records derived from snow samples from the mcmurdo dry valleys antarctica and re analysis data of the european centre for medium range weather forecasts era 40 to explain the connection between the warming of the pacific sector of the southern ocean jacka and budd 1998 jacobs et al 2002 and the current cooling of the terrestrial ross sea region doran et al 2002a our analysis confirms previous findings that the warming is linked to the el nino southern oscillation enso kwok and comiso 2002a 2002b carleton 2003 ribera and mann 2003 turner 2004 and provides new evidence that the terrestrial cooling is caused by a simultaneous enso driven change in atmospheric circulation sourced in the amundsen sea and west antarctica

bulk crustal properties of the borborema province ne brazil from p wave receiver functions implications for models of intraplate cenozoic uplift abstract we investigate variations in crustal thickness and bulk vp vs ratio across the borborema province of ne brazil by analyzing teleseismic p to s conversions recorded at 52 seismic stations in the province the borborema province represents the western portion of a larger neoproterozoic mobile belt that resulted from the assembly of gondwanaland and that split from the african continent during mesozoic times the evolution of the province after continental breakup was marked by episodes of diffuse intraplate magmatism perhaps leading to uplift of the borborema plateau in the cenozoic a number of geodynamic models have been proposed to explain coeval cenozoic magmatism and uplift in the province which invoke either thermal anomalies under the plateau and related mantle upwellings channeling along lithospheric thin spots from a distant mantle plume and small scale convection at the continental edge alternatively plateau uplift might have resulted from thickening of the crust after depth dependent stretching of the continental lithosphere in the mesozoic most of the models imply mafic underplating of the plateau s crust in order to fully explain its elevated topography but the volume of such mafic underplate varies among them our results show that i the crust is 32 40 km thick under the borborema plateau ii the crust is generally thinner about 30 33 km under the lower topographies surrounding the plateau and iii vp vs ratios are in the 1 68 1 80 range for both regions of higher and lower topography no apparent correlation is observed between vp vs ratio and crustal thickness our results suggest that compositional differences between thick and thin crust across the borborema province are minimal and that models of plateau uplift involving a small volume of mafic underplate provide a more plausible explanation for the observed topography of the borborema plateau

local lithospheric relief associated with fracture zones and ponded plume material 1 buoyant plume material ponds at the base of oceanic lithosphere it is a natural probe for the relief on the base of the lithosphere owing to plate age because it flows laterally along an upside down drainage pattern geomorphic and geochemical studies of near ridge and on ridge hot spots indicate as expected that the oceanic lithosphere thickens as it ages corridors of thin lithosphere channel plume material along the axis while the thick lithosphere at the end of ridge segments dams this along axis flow channel hot spots extend from plumes toward ridge axes however a good correlation between local lithospheric thickness and local age is not evident at fracture zones within the old ocean basins such a correlation would be expected if the lithosphere cooled only by conduction then pressure release melting and volcanoes would occur preferentially near fracture zones where plume material cascades from thick old to thin young lithosphere this concentration of volcanic edifices does not occur this lack of correlation between lithosphere thickness and plate age is an expected consequence of small scale convection at the base of the lithosphere scaling relationships and numerical models indicate that modestly vigorous convection of several milliwatt per meter squared is sufficient to perturb local lithospheric relief beneath fracture zones without changing the gross correlation of lithospheric thickness with plate age typically the step in lithospheric thickness moves toward the old side of the fracture zone

seismic velocity anomalies beneath se brazil from p and s wave travel time inversions 1 we present the result from teleseismic travel time inversions for p and s wave data mainly recorded at portable broadband stations in se brazil the stations were deployed at 45 sites within an area 1000 1700 km during the years 1992 1999 more than 10 000 relative p and s wave arrival times including core phases were obtained from the waveforms using a new coherence functional these p and s relative phase times are independently inverted for slowness perturbations earthquake relocations and station corrections the final models represent the least amount of structure required to explain the residuals within a defined standard error the robust and consistent features in the velocity anomaly models are interpreted and their resolution is tested with synthetic case inversions we confirm the existence of a cylindrical low velocity anomaly beneath the parana basin which has been interpreted by vandecar et al 1995 as the fossil conduit through which the initial tristan da cunha plume head traveled to generate the parana etendeka flood basalts about 130 ma we now show that this low velocity cylindrical structure seems to be confined to the upper mantle beneath the upper mantle the velocity anomalies show a n s oriented pattern which we interpret as due to the nazca plate subducted slab at lithospheric depths the archean southern part of the sao francisco craton shows high velocities down to 200 300 km all areas with late cretaceous postrift alkaline intrusions are characterized by low velocities at lithospheric depths

thomas l wright and fred w klein two hundred years of magma transport and storage at kilauea volcano hawai i 1790 2008