the nature of shallow water carbonate lithofacies thickness distributions carbonate lithofacies thickness distributions are of fundamental importance to understanding shallow water carbonate deposystems because they record evidence of the lateral distribution and migration of lithofacies elements as well as evidence for the various other intrinsic and extrinsic controls on stratal geometries and accumulation rates previous analyses of lithofacies thickness data led to the suggestion that exponential distributions are ubiquitous in the ancient record this has been interpreted to indicate deposition by stochastic poisson process lithofacies mosaics to further investigate these ideas statistical analysis of 56 outcrop and core examples was performed the kolmogorov smirnov test was used to identify the degree to which measured lithofacies thicknesses are well represented by a theoretical exponential distribution results from this analysis show that 16 of the 56 examples can be confidently shown to be exponential while 28 are very probably not exponential this indicates that stochastic poisson processes are a plausible explanation for many carbonate successions but they do not explain all of those tested here suggesting that other non poisson processes either stochastic or deterministic in nature or both must also be important thus lithofacies planform geometries and the processes controlling vertical stacking in ancient carbonate platform top deposystems were likely more diverse than has been suggested requiring significant further quantitative analysis and numerical forward modeling to properly understand

the peraluminous aurumina granite suite in central brazil an example of mantle continental crust interaction in a paleoproterozoic cordilleran hinterland setting abstract peraluminous granites and tonalites granodiorites of the aurumina suite crop out in the external zone of the brasilia fold belt on the western margin of the sao francisco craton rocks of this suite intrude retrograde medium grade schists and paragneisses of the ticunzal formation and display a magmatic muscovite biotite garnet assemblage and discrete graphite lamellae the suite is divided into six members au1 through au6 which grade from synkinematic to post kinematic in character with ages spanning from 2 11 to 2 16 ga according to zircon u pb data compositional features of the aurumina suite and associated coeval metaluminous mafic to intermediate plutons suggest an origin by source hybridization processes characterized by reaction between metasedimentary rocks and basaltic melts that would have taken place at shallow depths pressure

minor and trace element partitioning between immiscible ocelli matrix pairs from lamprophyre dikes and sills monteregian hills petrographic province quebec many lamprophyre dike and sill rocks in the monteregian hills petrographic province of southwestern quebec contain felsic segregations ocelli which have been interpreted as globules of immiscible liquid philpotts 1976 ocelli and matrix material were separated from a number of these rocks and analyzed for major and trace elements the major element data when plotted on a greig diagram outline a field of possible silicate liquid immiscibility at higher alumina alkali content than that previously mapped in iron rich experimental systems the trace element data support a liquid immiscibility hypothesis for the formation of these ocelli since high charge density cations are preferentially concentrated in the matrix mafic material a result which is consistent with theoretical and experimental studies

structural evolution of the texas orocline eastern australia the texas orocline with a half wavelength of similar to 120 km is the largest and most obvious orocline in the southern new england orogen and is clearly recognised in geological maps and geophysical images in the area of the orocline there is a major unconformity between devonian carboniferous metasedimentary rocks texas beds and the overlying early permian rift related basins detailed structural mapping shows that units both above and below the unconformity are folded around the orocline indicating that at least part of the oroclinal deformation has occurred after the deposition of the early permian rocks in addition early permian 298290 ma granitoids are aligned parallel to the oroclinal structure further indicating that deformation partly occurred during or after the early permian pre oroclinal isoclinal folds f 1 and related axial plane cleavage si are well developed in the texas beds and are curved around the oroclinal structure syn oroclinal structures are characterised by minor kink folds and disharmonic folding f 2 in the core of the orocline a later phase of kink folding corresponding to a similar to n s shortening direction does not seem to be related to the orocline and is interpreted as post oroclinal deformation a secondary penetrative fabric parallel to the axial plane of the orocline was not observed indicating low contractional shortening across the orocline 30 we demonstrate that the observed strain is too low to account for oroclinal bending during dextral transpression as previously proposed we suggest an alternative model involving an initial curved structure probably related to subduction rollback or a pre existing curvature in the palaeomargin of eastern australia which was amplified by dextral transpression and subsequent e w contraction c 2011 international association for gondwana research published by elsevier b v all rights reserved

reconstructing the kinematic evolution of curved mountain belts internal strain patterns in the wyoming salient sevier thrust belt u s a analyses of mesoscopic structural and strain patterns in red beds of the triassic ankareh formation and limestones of the jurassic twin creek formation in concert with complementary paleomagnetic studies constrain the three dimensional kinematic evolution of curved fold and thrust systems in the wyoming salient of the sevier thrust belt spaced cleavage fracture and vein networks minor folds and minor faults in limestones and red beds accommodated early layer parallel shortening lps concentrated in front of the growing thrust wedge along with minor strike parallel extension and wrench shear related to the development of orogenic curvature strain estimated using mass balance relations for cleavage seams crinoid ossicles in bioclastic limestone and reduction spots in red beds displays systematic regional patterns principal shortening directions are subperpendicular to structural trend around the salient reflecting a combination of primary curvature and secondary rotation of early lps fabrics lps magnitudes vary from 5 in central parts of the frontal hogsback thrust system where cleavage is absent to 10 30 in the more interior crawford thrust system where cleavage intensity is moderate to strong strain also increases toward the salient ends internal strain is a significant component of total deformation and should be considered when restoring cross sections strain patterns are consistent with a kinematic model involving curved fault slip and differential shortening that produced progressive secondary curvature during thrusting

origin and residence time of groundwater in the tadla basin morocco using multiple isotopic and geochemical tools summary groundwater resources in the tadla basin stem from surface water recharge and different groundwater inflows forming a multilayered aquifer system which hosts one of morocco s most important groundwater reservoirs the hydrodynamic infrastructure i e the relationship between all regional aquifers recharge and the residence time of waters poses a serious challenge for current water management and future exploitation in aiming for a long term sustainable utilization a combined hydrogeologic and isotopic investigation using hydrochemical and isotopic tracers such as 18 o 2 h 3 h 13 c and 14 c was carried out in order to determine the sources of water recharge to the aquifers the groundwater flow system and the residence time of these waters more than one hundred point measurements throughout the study area in varying wells boreholes springs and rivers were investigated chemical compositions of the groundwater indicate an important influence by the host carbonate rocks from each of the tadla aquifers stable isotope results indicate the existence of two groups of groundwater corresponding to the unconfined aquifer in the north and the confined aquifer in the south the 18 o 2 h 3 h and 14 c data indicate that the high atlas mountains in the south and east of the basin which are characterized by high rainfall and low 18 o and 2 h values are currently the major source of recharge for the tadla aquifers a significant recharge zone lies in the northern part of the basin where all the aquifers outcrop the confined zones show depleted 18 o values corresponding to the signature of recharge water from the atlas mountains moreover all isotope data demonstrated clearly that the tassaout springs which are located in the southwest of the basin and were previously interpreted as representing natural outlet of the deep aquifers are comprised of young waters with depleted 18 o and 2 h signatures suggesting a high altitude recharge from the atlas mountain in contrast the unconfined parts of the aquifers show higher values of 18 o indicating an evaporation phenomenon which occurs during infiltration or recharge from irrigation the mixing process of old and recent waters is confirmed by 14 c and 3 h the isotopic data also indicates probable interaction and flow between the different aquifers the isotopic and hydrochemical data is therefore essential in confirming possible mixing relationships between aquifers whereas traditional hydraulic data is not capable to provide a quantitative assessment of these relationships the data generated in this study will certainly encourage the revision and improvement of the current hydrological water resources model for the tadla basin the results provide a framework for development of a comprehensive management plan in which water exploitation shifts towards areas with modern recharge and where young and high quality groundwater is found

hydrochemical and isotopic characterisation of eastern haouz plain groundwater morocco the groundwater of eastern haouz of morocco is an important source for the high density population and intensive agricultural activity in the area the water demand here is increasing year by year groundwater levels measured in the study area show that the water table is quite deep 40 50 m and that the groundwater flow is closely following the surface topography major ion chemistry was made on 40 groundwater samples and about 20 samples among them were used for stable isotope measurements a large variation is found between the minimum and the maximum values of different ions the dominant cation na concentration varied between 18 and 1 159 mg l and the anion cl varied between 53 and 2 176 mg l samples from schist and triassic sediments are relatively more mineralised thus unsuitable for drinking as well as agricultural purposes groundwater within the plains alluvium is relatively less mineralised than the other older formations however hydrochemical values of many samples collected from alluvium are also not in the potable range owing to the natural and the anthropogenic contamination hence hydrochemical data of the study area show geological control on water quality apart from salinity about 25 of the samples have higher nitrate content than the drinking water permissible limit stable isotope measurements show that groundwater recharge to the phreatic aquifer is controlled by local conditions

positive inversion tectonics in foreland fold and thrust belts a reappraisal of the umbria marche northern apennines central italy by integrating geological and geophysical data abstract unraveling the tectonic style in the outer zones of fold and thrust belts is generally puzzling because the basement cover relationships are often hidden in the subsurface as in the outer northern apennines of italy this study aims to reconstruct the deep setting of the northern apennine foreland thrust belt by integrating surface structural geological and subsurface seismic reflection profile and well data corroborated by a gravity magnetic modeling a remarkable mountain ridge the umbria marche apennine ridge umar which corresponds to a prominent area of structural and topographic elevation characterizes the morphology of the northern apennines this mountain ridge is constituted by meso cenozoic carbonates involved in neogene compressive structures and is surrounded by wide exposures of foredeep deposits the basement cover relationships are poorly constrained and both thin and thick skinned tectonic styles have been applied the interpretation of subsurface data allowed recognizing a thick pre jurassic sedimentary sequence filling a late paleozoic triassic symmetric fault bounded extensional basin lying directly below the umar this deep rooted basin underwent positive inversion during the neogene compression and thrust fold development the reconstructed thick skinned inversion tectonic model is consistent with both the modest amount of shortening and the remarkable structural elevation of the umar the outcomes of this study reveal that prominent mountain ridges occurring in foreland thrust belts are most likely related to the deep rooted basement involved positive inversion of pre existing extensional basins

crustal structure of the sheared southwestern barents sea continental margin abstract eighteen expanding spread profiles from the southwestern barents sea continental margin were studied with a variety of seismic interpretation techniques and in conjunction with seismic reflection data the velocity depth profiles from the slope intercept herglotz wiechert and sum methods were compared and contrasted to the velocity depth curve calculated from the iterative forward modelling of the amplitudes the velocity depth profiles derived from the sum and forward modelling of the amplitudes were used to produce synthetics in both the x t and p domains to choose the technique that most accurately represented the data the results showed the oceanic crust at the senja fracture zone to be different from standard oceanic crust with a higher than average velocity for layer 2 of 5 7 6 4 km s 1 and slightly smaller crystalline crustal thickness the continent ocean transition at crustal depths was shown to be narrow in the range of 10 20 km our results were consistent with two different crustal provinces east of the senja fracture zone the tertiary marginal basin and the cretaceous basin region the velocity depth curves enabled us in a gross sense to suggest the depths to deep sedimentary interfaces and the top of the crystalline crust that were normally not observed in the reflection profiles depths down to 20 km were inferred for the base of the sedimentary section in the southwestern barents sea the crust thinned gently towards the continent ocean boundary at the senja fracture zone whereas the crustal thickness changed abruptly below the hornsund fault zone at the margin farther north we associated the change in crustal structure with cenozoic in the thick crust beneath the svalbard platform in the north the depths to mantle determined on the shelf were consistent with that observed on a deep multichannel seismic line that crosses the area

kinematics of orocline and arc formation in thin skinned orogens the regional traces of folds faults and foliations found in many fold thrust belts are bent in plan view it proves valuable to distinguish between two types of bent orogens based on their kinematic evolution oroclines or rotational arcs are bent orogens in which segments of the orogen change strike during the evolution of the bend and nonrotational arcs are bent orogens in which segments of the orogen do not change strike during development of the bend the kinematic evolution of a bend that forms in a thin skinned orogen can be described in terms of three parameters the displacement path trajectories followed by points along the strike of the orogen the magnitude and distribution of tangential extension along the strike of the orogen and the change in position of the endpoints of the orogen with respect to a reference line only slight differences in the displacement path trajectory pattern determine whether an orogen evolves as a nonrotational arc or as an orocline thus the distinction between these two types of bends is not always of major tectonic significance compressional deformation along irregular continental margins or the impact of indentors on continental margins during collisional orogenies more likely leads to formation of nonrotational arcs as can be simulated with a sand wedge model interaction between a fold thrust wedge with obstacles in the foreland or with a wrench fault more likely leads to orocline formation orocline formation is also associated with noncoaxial reactivation of thrust faults tangential extension is a necessary consequence of certain displacement path trajectory patterns and can accompany development of either nonrotational arcs or oroclines two examples one from the umbrian apennines and one from the makran range demonstrate how movement on cross strike fault arrays can accommodate tangential extension in thin skinned bends

oroclines thick and thin an orocline is a thrust belt or orogen that is curved in map view due to it having been bent or buckled about a vertical axis of rotation two distinct types of oroclines are recognized progressive and secondary progressive oroclines are restricted to the scale of a thrust sheet to thrust belt are thin skinned and develop during thrust sheet emplacement secondary oroclines are larger occurring at the scale of an orogen and are plate scale features that affect crust and lithospheric mantle unlike progressive oroclines which develop during initial orogenesis and in response to the same orogen perpendicular stress responsible for thrust sheet emplacement secondary oroclines are extra orogenic developing after initial orogenesis and in response to an orogen parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development we present case studies of the wyoming salient a progressive orocline that characterizes the sevier thrust belt of the western united states and the coupled cantabrian and central iberian oroclines which are linked secondary oroclines affecting the variscan orogen of iberia the vertical axis rotations involved in progressive and secondary orocline formation are most readily quantified through paleomagnetic analysis detailed three dimensional palinspastic restoration that incorporates translation rotation and strain can distinguish the role if any of primary curvature in progressive oroclines the use of tectonic vectors such as paleocurrent directions provides a means of recognizing and characterizing the initial geometry of secondary oroclines because secondary oroclines involve the entire lithosphere detailed studies of coeval metamorphism and magmatism provide a means of constraining the fate of the mantle lithosphere during oroclinal buckling

role of decollement material with different rheological properties in the structure of the aljibe thrust imbricate flysch trough gibraltar arc an analogue modelling approach abstract the miocene aljibe thrust imbricate flysch trough in the external zone of the gibraltar arc shows large changes in structural style towards the foreland rheological variation of the decollement rocks triassic evaporitic rocks and mesozoic limestones appears to be the main factor controlling the contrasts in structural style that characterise adjacent wedge domains in order to test this hypothesis we carried out analogue experiments involving three dimensional variations of decollement material a plate of viscous material was placed in the central part of the models at the base of the sand multilayers different geometries of the viscous plate were used experimental results show a strong link between the rheological properties of the decollement material and the structure of the overlying thrust wedge these results compare well with the internal geometry and vergence distribution in the aljibe thrust imbricate indeed the tectonic style of the wedge can be explained as a consequence of variations of rock type along the basal decollement

reviewing the puzzling intracontinental termination of the aracuai west congo orogenic belt and its implications for orogenic development abstract palinspastic reconstructions suggest that the late proterozoic cambrian brasiliano pan african orogenic belt in southeast brazil and west congo terminated northwards into an embayment within the sao francisco congo cratonic unit the orogenic shortening that created the aracuai west congo orogen in this embayment has been explained by tightening of the horseshoe shaped sao francisco congo craton in a fashion referred to as nutcracker tectonics we show that this model is incompatible with the general orogenic evolution proposed in recent literature which involves 1 50 m y of subduction of oceanic crust and associated arc formation followed by 2 collisional orogeny and crustal thickening quantitative considerations show that the original nutcracker model is too rigid to explain even the second crustal thickening part let alone any long pre collisional history to soften the model we suggest that the so called sao francisco congo bridge was broken by a 150 km wide orogenic corridor along the current african atlantic margin this corridor adds sufficient mobility to the system to explain the orogenic thickening of the crust to 60 65 km however even with this additional softening the confined nature of this orogen is incompatible with prolonged arc development we therefore suggest that oceanic crust was nonexistent or very limited in the macaubas basin and reject the widely published model involving 50 m y of subduction of oceanic crust and related arc development instead we find strong support for a hot intracontinental orogen model in the currently available p t geochronologic petrographic and structural data in this model extensive melting and flow of the middle crust is likely to have caused spreading of the upper crust in an orogenic setting that was created by collisions along the n w and s margins of the sao francisco craton from 630 ma

multi stage crustal accretion by magmatic flare up and quiescence intervals in the western margin of the sao francisco craton u pb hf and geochemical constraints from the almas terrane abstract the global plate tectonic regime in early paleoproterozoic times is highly debated the interval 2 45 2 2 ga is known for a minima in juvenile magmatism but this is not a global phenomenon new results of whole rock geochemistry and u pb hf analysis in igneous and detrital zircons allied with existing isotopic and geophysical data allow to identify and constrain the duration of magmatic flare up and quiescence events in the western sao francisco paleoplate igneous samples yield ages indicating three accretionary magmatic events an older with ages ca 2476 4 9 ma to 2462 13 ma an intermediate at 2390 14 ma and a younger from 2235 26 ma to 2201 5 ma all presenting magmatic arc geochemical signatures xenoliths of quartzite and volcanic tuff from the upper greenstone sequence morro do carneiro fm are hosted in the 2211 9 ma tonalite and the maximum depositional age of the morro do carneiro basin is dated 2234 12 ma indicating a syn orogenic setting for this basin detrital zircon u pb age distribution for quartzites of the greenstone sequence shows peaks at 2 65 2 47 2 39 2 27 and 2 23 ga altogether the studied rocks record an accretionary orogeny with four distinct episodes episode s1 2 52 2 46 ga hf t values from 0 57 to 6 36 episode s2 2 43 2 37 ga hf t values from 0 10 to 4 30 episode r1 2 32 2 26 ga hf t values from 1 61 to 7 23 from detrital zircons episode r2 2 24 2 20 ga hf t values from 0 39 to 2 73 these early paleoproterozoic accretionary orogenies mark the onset of amalgamation of the sao francisco continental paleoplate that surrounds the craton with accretions of an exotic micro block and continental magmatic arcs indicating evolution from dominant siderian juvenile magmatism to rhyacian crustal magmatism these patterns show striking similarities to the orogenies in the mineiro belt and north china craton

moho depth and sediment thickness estimation beneath the red sea derived from satellite and terrestrial gravity data abstractwe sought to map the depth and density contrast of the mohorovicic discontinuity moho across the red sea area and to model sedimentary thickness from gravity data the gravity data that are used are a combination of satellite and terrestrial gravity data processed into a bouguer anomaly grid a 200 km low pass filter was used to separate this grid into regional and residual gravity grids we inverted the regional gravity grid to a moho depth map based on a density contrast map that is constrained by published seismic results the interpreted moho is shallowest 10 km along the axis of the central red sea 13 km along the axis to the south and 20 km in the northern red sea the depth increased to 20 25 km at the coast and 35 40 km in the adjacent continents the residual gravity data provided insights into the tectonic fabric along the whole rift and provided a good correlation with magnetic lineaments where these are available we used the complete bouguer anomaly to model sedimentary

inversion of combined gravity and bathymetry data for crustal structure a prescription for downward continuation abstract we propose a new and improved recipe for inverting for crustal thickness variations from combined gravity and bathymetry data the new ingredients with respect to previous studies are the posing of this problem as an inverse problem and the use of a downward continuation filter which can be tailored to find the crustal thickness solution which satisfies a minimum slope or maximal smoothness criteria or a weighted combination of these desired solution features we explore several suggestions for determining the optimum smoothing parameters for a given crustal thickness inversion then we compare this new method to that used previously in marine gravity studies we find it to be an improved recipe for dealing with gravity data with observation errors of 3 5 mgal but not by much this recipe will be most useful in extracting more geological information from marine gravity data where apparent observation errors are 1 mgal e g from future global positioning satellite navigated studies

the margin between senja and spitsbergen fracture zones implications from plate tectonics abstract analysis of multichannel seismic data from the continental margin off svalbard between the senja and spitsbergen fracture zones suggests that the transition between continental and oceanic crust is located at or close to the hornsund fault zone in the late paleocene early eoeene 57 m y the region between svalbard and northeast greenland was subjected to regional shear movements associated with a transform system between the young lofoten greenland basin and the arctic ocean approximately 50 m y ago the spreading axis migrated to the northeast creating a deep basin north of the greenland senja fracture zone forming the passive margin between bear island and 76 5 n north of 76 5 n the regional transform was maintained at the time of the main reorganization of relative plate motion 36 m y the northern margin evolved a continental fragment was possibly cut off from the svalbard margin forming a small microcontinent the microcontinent appears as the submarine ridge which has been associated with the hovgaard fracture zone it is suggested that the sediments west of the hornsund fault zone are not older than eocene in the south and mid oligocene in the north the position of the spreading axis has greatly influenced the margin sedimentation

a correspondence of altimetric gravity texture to abyssal hill morphology along the flanks of the southeast indian ridge 1 we examine the relationship between altimetric gravity and abyssal hill textures along the flanks of southeast indian ridge where previous studies observed a progressive west to east increase in abyssal hill roughness and scales such a relationship is often considered unlikely because abyssal hills are typically smaller than upward continuation filter scales however our results which exclude ridges fracture zones and large pseudofaults show that altimetric gravity roughness and characteristic scales in this region also increase from west to east and at the largest scales gravity texture exhibits ridge parallel lineaments synthetic profiles also demonstrate how the texture parameters of a filtered field are related to those of the unfiltered field particularly where the characteristic scale of the latter is smaller than the filter length

carbon and oxygen isotope profiles across meso neoproterozoic limestones from central brazil bambui and paranoa groups we present carbon and oxygen isotopic data on carbonates along three profiles of the neoproterozoic bambui group in central brazil this unit covers an area of more than 300 000 km2 and comprises carbonate silicliclastic sediments at the base that grade into siliciclastic sediments towards the top the bambui group overlies by unconformity the paranoa group which consists mostly of siltstone quartzite and minor limestone the data presented here improve the stratigraphic correlation within the bambui basin and show that it evolved in an environment significantly different from that of the paranoa basin our data show large fluctuations of 13cpdb in limestones from the bambui group from 0 8 to 13 5 in all the three studied areas some of these fluctuations represent stratigraphic markers that can be used as a chronostratigraphic tool within a basin scale this observation is relevant considering the lack of fossil record and other stratigraphic markers in neoproterozoic sequences we also present the first isotopic profiles along the paranoa bambui transition which shows that the 13cpdb values grade from 1 0 in the paranoa group to 2 6 in the lower portion of the bambui group increasing up to 12 in the upper part of this unit based on our carbon isotope data as well as other geological mineralogical and nd isotope studies we argue that the sediments of the paranoa group were deposited on an open platform that was fully connected to the ocean on the other hand the sediments of the bambui group were deposited in an epicontinental sea and during a tectonic inversion in a foreland basin at about 790 600 ma this unit displays an increased amount of clastic sediments upwards we argue that the high carbon isotope values observed in limestones and marlstones from the bambui group are correlated to worldwide high carbon isotope values reported for the neoproterozoic however we also point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic composition of the bambui carbonates

sequence stratigraphic framework of proterozoic successions in eastern brazil abstract first order regional unconformities separate proterozoic supracrustal rocks of the gas bearing sao francisco craton and surrounding thrust and fold belts eastern brazil into four genetic stratigraphic units which constitute 1st order basin fill sequences espinhaco canastra paranoa vazante macaubas and bambui sequences the espinhaco 1st order sequence ca 1 73 1 50 ga records a stage of aborted lithospheric stretching of the sao francisco congo continental mass the espinhaco basin displays a steer s head geometry where four 2nd order sequences are recognized prerift rift transitional and flexural third order rift propagation unconformities allowed the definition of three 3rd order synrift sequences within the rift 2nd order sequence the 2nd order flexural sequence of the espinhaco basin can be subdivided into three 3rd order transgressive regressive sequences the canastra paranoa vazante and macaubas 1st order sequences ca 1200 630 ma comprise rift to drift successions deposited during the breakup of the supercontinent rodinia and the opening of the brazilide and adamastor oceans respectively to the west and east of the sao francisco paleocontinent according to seismic data the passive margin canastra paranoa vazante 1st order sequence can be divided into two 2nd order sequences rift and passive margin the passive margin 2nd order sequence can be further subdivided into three 3rd order sequences the outcropping canastra paranoa vazante sequence consists mainly of shallow marine deposits which contain black shales marls with intervals showing high total organic carbon toc contents comprising one of the highest potential hydrocarbon source rocks of the sao francisco basin the bambui foreland 1st order sequence ca 750 550 ma formed as a consequence of thrust loading related to shortening in the brasilia fold belt on the western flank of the sao francisco craton during the closing of the brazilide ocean and the assembly of the west gondwana supercontinent the bambui sequence shows two distinct successions which reflect the main basin paleogeographic settings foredeep and flexural ramp three transgressive progradational 2nd order sequences can be recognized in the ramp succession of the bambui basin

sequence stratigraphy and chemostratigraphy of an ediacaran cambrian foreland related carbonate ramp bambui group brazil abstract in the terminal neoproterozoic drastic climate changes associated with biological innovations are coupled to isotope and elemental geochemical anomalies however lateral variability and local depositional controls may affect global geochemical signals which can only be tracked through a proper stratigraphic paleogeographic assessment here we investigate the sequence stratigraphy and chemostratigraphy of the basal units of the bambui group central east brazil this stratigraphic unit records a foreland basin system developed during the ediacaran cambrian west gondwana assembly and represents a 1st order sequence in which the two lowermost 2nd order sequences record major geochemical disturbances the first 2nd order sequence started with the deposition of a transgresive systems tract possibly in a postglacial scenario which accompanies a negative to positive 13ccarb excursion the early highstand systems tract represents the establishment of a marine carbonate ramp throughout the basin in terms of chemostratigraphy it corresponds to a 13ccarb plateau close to 0 and sr ca ratios around 0 001 the late highstand stage coincides with a remarkable increase in sr content and sr ca ratios at basinal scale occurrences of the cloudina sp late ediacaran index fossil were reported in this stage an erosional unconformity associated with a dolomitic interval locally including subaerial exposure features marks the top of the first 2nd order sequence this sequence boundary heralds an abrupt increase in 13ccarb values up to 14 these extremely high 13ccarb values and high sr ca ratios persist throughout the overlying sequence as a result of progressive and enhanced restriction of the foreland basin system basin restriction at this stage has implications for the paleontological and chemostratigraphic record of epicontinental basins of the west gondwana in the terminal ediacaran late ediacaran sr rich intervals in these basins show unusually non radiogenic 87sr 86sr ratios which may represent local depositional controls and deviations from the modern oceanographic models physiographic barriers and stressful conditions likely represented extreme environments for metazoan colonization

origin of the uinta recess sevier fold thrust belt utah influence of basin architecture on fold thrust belt geometry abstract structural trends in the sevier fold thrust belt define a pronounced concave to the foreland map view curve the uinta recess in north central utah this recess separates two convex to the foreland curves the provo salient on the south and the wyoming salient on the north the two limbs of the recess comprise transverse zones fault zones at a high angle to the regional trend of the orogen that border the flanks of the east west trending uinta cottonwood arch our structural analysis indicates that the transverse zones formed during the sevier orogeny and that they differ markedly from each other in structural style the charleston transverse zone ctz on the south side of the arch initiated as a complex sinistral strike slip fault system that defines the abrupt northern boundary of the provo salient the mount raymond transverse zone mrtz on the north side of the arch represents the region in which the southeast verging southern limb of the gently curving wyoming salient was tilted northwards during the laramide phase of uplift of the uinta cottonwood arch in effect the mrtz represents an oblique cross section through a thrust belt the contrasting architecture of these transverse zones demonstrates how pre deformation basin geometry influences the geometry of a fold thrust belt analysis of isopach maps indicates that at the time the sevier fold thrust belt formed the area just north of the present site of the uinta cottonwood arch was a basement high with a gently dipping north flank and a steeply dipping south flank thus predeformational sediment thickened abruptly to the south of the high and thickened gradually to the north of the high as illustrated by sandbox models the distance that a fold thrust belt propagates into the foreland depends on the thickness of the sedimentary layer being deformed so the shape of the salient mimics the longitudinal cross sectional shape of the sedimentary basin where basins taper gradually along strike the thrust belt curves gently but where basins taper abruptly along strike the thrust belt curves so tightly that it disarticulates and becomes bounded laterally by a strike slip accommodation zone the geometry of the uinta recess provides a field example of this concept differential movement of sevier thrusts led to formation of gradually curving thrusts on the north side of the high because of the gradual slope of the high s north flank but led to the along strike disarticulation of thrusts on the south side of the high because of the steep slope of the high s south flank in effect therefore thrust belt map view geometry provides insight into predeformational basin geometry

u pb and hf isotope study on detrital zircons from the paranoa group brasilia belt brazil constraints on depositional age at mesoproterozoic neoproterozoic transition and tectono magmatic events in the sao francisco craton abstract the eastern portion of the brasilia belt part of the neoproterozoic tocantins province in central brazil is mainly constituted by metasedimentary rocks that partially represent the evolution of a passive margin developed during the meso neoproterozoic in the western margin of the sao francisco paleocontinent new in situ u pb and lu hf isotope analyses on detrital zircons by la mc icp ms were obtained for clastic sedimentary rocks of the paranoa group considered to be the oldest unit of the this sedimentary sequence the u pb data define a maximum depositional age of 1 54 ga and a minimum depositional age of 1 04 ga the latter obtained on diagenetic xenotime overgrowth on zircon grains combined u pb and lu hf data permitted to reconstruct the tectonomagmatic evolution of the potential source areas identified in the sao francisco craton two zircon populations at 1 54 and 1 78 ga with hf t values from 5 1 to 5 5 are possibly related with the mesoproterozoic extensional magmatism registered in the arai and espinhaco rifts located in the brasilia belt and in the sao francisco craton respectively the most important paleoproterozoic zircon population at 2 0 2 25 ga shows positive hf t 0 to 8 and paleoproterozoic tdm values indicating a continuous long period of magmatism with generation of juvenile crust during the paleoproterozoic two minor zircon populations at 2 1 and 2 15 ga showing negative hf t values 0 to 10 1 and tdm values between 2 65 and 3 22 ga may represent arc related magmatism with reworking of archean crust during the evolution of an active margin of the sao francisco craton a third minor zircons population at 2 05 ga with strongly negative hf t values between 8 6 and 17 7 and tdm values between 2 93 and 3 46 ga may represent a collisional magmatism that characterized the final stage of an active margin evolution with reworking of early archean crust several minor zircon populations of archean ages 2 7 2 85 3 1 and 3 42 ga derived from the archean nuclei in the interior of the sao francisco craton suggest the presence of different magmatic pulses that repeatedly reworked a juvenile paleoarchean crust the oldest event of generation of juvenile crust is registered by zircons with u pb age of 3 42 ga positive hf t values of 3 0 and tdm of 3 54 ga

blueschist facies metamorphism in the ophiolitic melange of the late proterozoic delhi fold belt rajasthan india abstract the south delhi fold belt of late proterozoic age contains ophiolitic melange of contrasting ensialic rift and arc trench marginal basin tectonic domains the trench metagreywacke contains relict blueschist assemblages which have been overprinted by regional greenschist facies metamorphism

paleorift structure constrained by gravity and stratigraphic data the statherian arai rift case abstract gravimetric and stratigraphic data were used to investigate the paleoproterozoic arai paleorift a failed statherian continental rift located in the western margin of the sao francisco craton where basement and cover were affected by the neoproterozoic brasiliano orogeny euler deconvolution tilt total horizontal gradient amplitude and upward continuation technics were applied to terrestrial gravimetric data in order to investigate the rift s main faults location direction and depth allowing to identify its main horsts grabens volcanic and plutonic centers we found that rift faults occur to a maximum depth of ca 38 km but major fault throw occurs from 4 to 8 km deep and attenuates from 8 to 12 km probably the brittle ductile transition zone at the time of rifting practically disappearing at 20 km stratigraphic data and basement mapping were used in order to constraint gravimetric results we classify the arai rift as a passive three armed failed rift narrow to divergent type that produced preferably anorogenic rapakivi related magmas most of it still lodged in the crust from surface down to ca 19 km deep and subsidiary mafic magmatism the results indicate the deep occurrence of low density magmas beneath the rift s main axis detected up to 20 km deep correlation to other global statherian rifts show that the sao francisco craton was strongly affected by taphrogenesis during the statherian together with siberia north america and north china cratons finally by comparing our results to recent rifts we found that the ethiopian rift s morphology is quite similar to the arai surrounding the tanzanian craton the cenozoic east africa rift system morphology is compared to the arai espinhaco rift system which surrounds the sao francisco craton the major contribution of this paper is the recognition of arai paleorift surface and subsurface morphology up to now unknown over an area of ca 45 000 km2

carbonate chemostratigraphy of the vazante group brazil a probable tonian age abstract the thick carbonate siliciclastic succession of the vazante group was deposited on the western border of the sao francisco craton central brazil which was deformed during the brasiliano orogeny that ended at 540 ma in this research we studied five drill cores and outcrops to obtain integrated isotope data 13ccarb 18ocarb and 87sr 86sr as well as u pb ages of detrital zircons to understand the chronostratigraphic relationships between the tonian and cryogenian periods the vazante group forms a succession of more than 3 km thick of limestones dolostones shales and rare sandstones these rocks were deposited on a marine shelf that shows distinct 13c variations throughout the stratigraphic succession the lower limestone of this group lagamar formation has low positive 13c values 0 3 1 0 and 87sr 86sr ratios of 0 70646 0 70692 with 423 1 719 ppm sr and possible vase shaped microfossils vsms that may be equivalent to those found in huaibei group in northern china lower tonian or has stratigraphic correlations to the vsms bearing units of the upper tonian the serra do poco verde formation has a plateau of moderate 13c positive values 0 0 4 followed by dolostones of the morro do calcario formation which is a 50 m thick interval with negative values as low as 4 the upper unit of the vazante group serra da lapa formation begins with a sharp negative 13c excursion locally overlying a few layers of diamictite which may be the equivalent of the sturtian glacial age restrictions on establishing the depositional age of the vazante group continue however the combination of biostratigraphic and chemostratigraphic data and the detrital zircons spectra suggest a tonian age for the vazante group

neoproterozoic glacial deposits from the aracuai orogen brazil age provenance and correlations with the sao francisco craton and west congo belt article i nfo glacigenic diamictite successions of the macaubas group are widespread in the western domain of the aracuai orogen east of the sao francisco craton brazil diamictites also occur on this craton and in the african counterpart of the aracuai orogen the west congo belt detrital zircon grains from the matrix of diamictites and sandstones from the macaubas group were dated by the u pb shrimp technique the geochronological study sets the maximum depositional age of the glacial diamictites at 900 ma and indicates multiple sources for the macaubas basin with ages ranging from 900 to 2800 ma sm nd tdm model ages determined on whole rock samples range from 1 8 ga to 2 5 ga and get older up section comparison of our data with those from the cratonic area suggest that these glacial deposits can be correlated to the jequitai and carrancas diamictites in the sao francisco craton and to the lower mixtite formation of the west congolian group exposed in africa the 900 1000 ma source is most probably represented by the zadinian mayumbian volcanic rocks and related granites from the west congo belt however one of the most voluminous sources with ages in the 1 1 1 3 ga interval has not been detected in the sao francisco congo craton possible sources for these grains could occur elsewhere in africa or possibly from within the brasilia belt in western central brazil

early cenozoic crust at the norwegian continental margin and the conjugate jan mayen ridge multichannel seismic profiles at the voring plateau margin off norway and the northern jan mayen ridge have provided a framework for the early tertiary evolutionary history of these areas which were adjacent prior to the opening of the norwegian sea we propose that their paleogene evolution is quite similar and that the two acoustic basement reflectors ee and jo were formed in the early eocene by extrusion of flow basalts from the time of opening to about anomaly 23 time oceanic seafloor was generated by icelandic type spreading during that time the central jan mayen fracture was the main transform the seaward dipping reflector sequences are flow basalts generated by the icelandic spreading the existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation during the early eocene subaerial spreading episode which is related to the north atlantic volcanic province basalt flows also covered the adjacent thinned and heavily intruded continental crust it is proposed that the continent ocean boundary is located just landward of anomaly 24b a short distance seaward of the voring plateau escarpment reflector k is only recognized landward of this boundary in a region of transitional crust the escarpment experienced syndepositional faulting and the most elevated parts of the voring marginal high did not subside below shallow water depths before oligocene miocene time we suggest that the voring margin and the jan mayen ridge represent volcanic passive margins the evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust

tectono sedimentary evolution of sedimentary basins from late paleoproterozoic to late neoproterozoic in the sao francisco craton and aracuai fold belt eastern brazil abstract two first order regional unconformities separate proterozoic supracrustal rocks of the sao francisco craton and the aracuai fold belt eastern brazil into three genetic units these constitute three superimposed first order basin fill cycles the espinhaco macaubas salinas and bambui megasequences the paleo mesoproterozoic espinhaco megasequence ca 1730 1500 ma was deposited in an intracratonic rift sag basin in four stages pre rift rift transitional and flexural each of these stages is bounded by regional second order unconformities basin evolution was controlled primarily by tectonics although sea level variations were also important in the transitional and flexural stages after a hiatus of several hundred million years subsidence renewed during extension related to the early neoproterozoic breakup of the supercontinent rodinia with rift to passive margin sedimentation in the macaubas salinas basin ca 950 700 ma initial glaciogenic sedimentation possibly induced by uplift related to a mantle plume occurred in a rift setting subsequent a shelf slope deep sea passive margin deposits represent opening of a northern branch of the adamastor ocean tectonic magmatic climatic and eustatic processes all combined to control basin evolution the bambui megasequence ca 800 650 ma was deposited in a foreland basin during convergent and collisional tectonics related to closure of the brazilide ocean and generation of the brasilia fold belt west of the sao francisco paleocontinental region the evolution of the bambui basin whose fill is characterized by three transgressive progradational shallowing upward sequences was mainly controlled by tectonic processes

geochemistry and isotopic characteristics of mafic phulad ophiolite and related rocks in the delhi supergroup rajasthan india implications for rifting in the proterozoic abstract regionally metamorphosed mafic rocks phulad ophiolite in the delhi supergroup n w india have tholeiitic compositions the data indicate that there are two independent rock suites phulad barr jetgarh pbj meta basalts with element abundances and ratios resembling midocean ridge basalts morb and ranakpur desuri r d meta basalts and meta gabbros with relatively high al2o3 and low tio2 for similar mgo and high lil element rb ba sr coupled with low hfs zr nb and ree sm nd abundances the compositions of these rocks are characteristic of ocean arc basalts dioritic rocks at ranakpur exhibit low k tholeiitic differentiation trends and have close chemical affinity with the spatially associated meta basalts ranakpur diorites define a sm nd whole rock isochron corresponding to an age of 1012 78 ma with an initial nd isotopic composition that is similar to bulk earth sm nd mineral isochrons for a meta gabbro diorite and quartz diorite at ranakpur correspond to ages of 838 36 835 43 and 791 43 ma respectively these younger ages reflect metamorphic re equilibration of isotope compositions during widespread granite and granodiorite emplacement in this region during the late proterozoic model sm nd ages for the meta basalts calculated relative to undepleted mantle vary between 1300 1700 ma the chemistry of both mafic rock suites indicates derivation from source regions which had been depleted during previous melting episodes so the model ages are a maximum age constraint ranakpur desuri mafic rocks have negative initial nd 1 0 to 4 5 and low sm nd ratios suggesting crustal contamination or reworking with a few exceptions the pbj meta basalts have positive initial nd 1 5 to 4 8 indicating that they were derived from heterogeneously enriched although longterm depleted mantle the chemical data support the hypothesis that the pbj meta basalts are fragments of proterozoic ocean crust

asymmetric geophysical signatures in the greenland norwegian and southern labrador seas and the eurasia basin a thorough examination of geophysical data from the greenland norwegian sea eurasia basin and southern labrador sea shows significant asymmetry of several parameters basement topography adjusted for sediment loading free air gravity anomaly spreading half rate and seismicity with respect to crustal age 1 1 average zero age depth 0 57 m y b p depth of highest rift mountain summits and depth to magnetic basement 10 30 km from axis of mohns and knipovich ridges is less on the north american plate flanks the zero age depth asymmetry is 400 500 m for the eurasia basin 0 57 m y b p and for mohns ridge 57 22 m y b p and 150 200 m for younger mohns ridge crust 22 0 m y b p and for the extinct aegir ridge 57 27 m y b p there is little or no asymmetry in the labrador sea except near the extinct rift valley where the east flank is 150 300 m shallower magnetic depth to source computations provide an independent confirmation of basement asymmetry the belts 10 30 km from the axis of mohns and knipovich ridges are 100 150 m shallower on the west flank of these ridges the shallower ridge flank is topographically rougher so that average rift mountain summits are 300 m shallower on the west flanks of the mohns knipovich ridges a larger asymmetry than for average zero age depth the amount of topographic asymmetry is greatest near the mohns knipovich bend asymmetry appears to be greatest for ridges oriented normal to the spreading direction and less for oblique spreading 2 2 free air gravity anomaly asymmetries of 5 to 20 mgal sign indicates west flank is more positive are associated with topographic asymmetry at least within 10 15 m y of the axis of mohns and knipovich ridges gravity is reduced on the older flanks west of the extinct mid labrador ridge and east of mohns ridge asymmetric crustal layer thicknesses or densities provide one possible explanation although deep seated sources e g mantle convection unrelated to the crust cannot be excluded 3 3 spreading half rate was about 5 15 lower on the north american plate flanks of mohns ridge 57 35 m y and in the eurasia basin 0 57 m y thus the fast spreading flank tends to produce deeper smoother crust however topographic asymmetry cannot relate only to spreading rate asymmetry since for the young mohns ridge crust 9 m y b p faster spreading and higher topography are both associated with the west flank 4 4 mid plate seismicity is higher on the eurasia eastern flank of mohns and knipovich ridge but this effect may be unrelated to the other three the fluid dynamical model of stein et al correctly explains the sense of spreading rate asymmetry the north american plate moving faster over mantle is growing more slowly however the other asymmetries and their causal relationships remain theoretically unexplained

u pb geochronology of the southern brasilia belt se brazil sedimentary provenance neoproterozoic orogeny and assembly of west gondwana abstract the brasilia belt borders the western margin of the sao francisco craton and records the history of ocean opening and closing related to the formation of west gondwana this study reports new u pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the sao francisco craton and date the orogenic events leading to the amalgamation of west gondwana ages of detrital zircons by id tims and la mc icpms were obtained from metasedimentary units of the passive margin of the sao francisco craton from the main tectonic domains of the belt the internal allochthons araxa group in the araxa and passos nappes the external allochthons canastra group serra da boa esperanca metasedimentary sequence and andrel ndia group and the autochthonous or cratonic domain andrel ndia group the patterns of provenance ages for these units are uniform and are characterised as follows archean paleoproterozoic ages 3 4 3 3 3 1 2 7 and 2 5 2 4 ga paleoproterozoic ages attributed to the transamazonian event 2 3 1 9 ga with a peak at ca 2 15 ga and to the ca 1 75 ga espinhaco rifting of the sao francisco craton ages between 1 6 and 1 2 ga with a peak at 1 3 ga revealing an unexpected variety of mesoproterozoic sources still undetected in the sao francisco craton and ages between 0 9 and 1 0 ga related to the rifting event that led to the individualisation of the sao francisco paleo continent and formation of its passive margins an amphibolite intercalation in the araxa group yields a rutile age of ca 0 9 ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin detrital zircons from the autochthonous and parautochthonous andrel ndia group deposited on the southern margin of the sao francisco craton yielded a provenance pattern similar to that of the allochthonous units this result implies that 1 6 1 2 ga source rocks must be present in the sao francisco craton they could be located either in the cratonic area which is mostly covered by the neoproterozoic epicontinental deposits of the bambui group or in the outer paleo continental margin buried under the allochthonous units of the brasilia belt crustal melting and generation of syntectonic crustal granites and migmatisation at ca 630 ma mark the orogenic event that started with westward subduction of the sao francisco plate and ended with continental collision against the parana block and goias terrane continuing collision led to the exhumation and cooling of the araxa and passos metamorphic nappes as indicated by monazite ages of ca 605 ma and mark the final stages of tectonometamorphic activity in the southern brasilia belt whilst continent continent collision was proceeding on the western margin of the sao francisco craton along the southern brasilia belt eastward subduction in the east was generating the 634 599 ma rio negro magmatic arc which collided with the eastern sao francisco margin at 595 560 ma much later than in the brasilia belt thus the tectonic effects of the ribeira belt reached the southernmost sector of the brasilia belt creating a zone of superposition the thermal front of this event affected the proximal andrel ndia group at ca 588 ma as indicated by monazite age the participation of the amazonian craton in the assembly of western gondwana occurred at 545 500 ma in the paraguay belt and ca 500 ma in the araguaia belt this together with the results presented in this work lead to the conclusion that the collision between the parana block and goias terrane with the sao francisco craton along the brasilia belt preceded the accretion of the amazonian craton by 50 100 million years

remagnetization in the tennessee salient southern appalachians usa constraints on the timing of deformation article i nfo the appalachian fold thrust belt is characterized by a sinuous trace in map view creating a series of salients and recesses the kinematic evolution of these arcuate features remains a controversial topic in orogenesis primary magnetizations from clastic red beds in the pennsylvania salient show pennsylvanian rotations that account for about half of the curvature while kiaman aged permian remagnetizations display no relative rotation between the limbs the more southern tennessee salient shows a maximum change in regional strike from 65 in virginia to 10 in northern georgia paleomagnetic results from thirty two sites in the middle to upper ordovician chickamauga group limestones and twenty sites from the middle cambrian rome formation red beds were analyzed to constrain the relative age of magnetization as well as the nature of curvature in the tennessee salient results from three sites of the silurian red mountain formation were added to an existing dataset in order to determine whether the southern limb had rotated after thermal demagnetization all three sample suites display a down and southeasterly direction albeit carried by different magnetic minerals the syn tilting direction of the chickamauga limestones lies on the pennsylvanian segment of the north american apparent polar wander path apwp indicating that deformation was about half completed by the late pennsylvanian the rome and red mountain formations were also remagnetized during the pennsylvanian both the chickamauga limestones and rome red beds fail to show a correlation between strike and declination along the salient suggesting either that the salient was a primary non rotational feature or that secondary curvature occurred prior to remagnetization as it did in pennsylvania moreover remagnetized directions from the red mountain sites show no statistical difference between the southern limb of the salient and the more northeasterly trending portion of the fold thrust belt in alabama thus all of the studied units in the tennessee salient are remagnetized and show no evidence for rotation this confirms that remagnetization was widespread in the southern appalachians and that any

a sedimentary overlap assemblage links australia to northwestern laurentia at 1 6 ga abstract the columbia nuna supercontinent existed from approximately 1 9 ga to 1 3 ga laurentia was part of columbia and the western edge of laurentia current coordinates was likely proximal to a large landmass during parts of this interval here we present detrital zircon ages of a paleoproterozoic sedimentary succession in northern yukon canada that bear on the evolution of columbia the sedimentary succession is preserved as clasts within 1 60 ga hydrothermal megabreccias analyses of detrital zircon reveal abundant 1 78 1 68 ga zircon with evolved hf isotope values 16 1

structural evolution of a major appalachian salient recess junction consequences of oblique collisional convergence across a continental margin transform fault most orogenic belts exhibit broadly arcuate regional map view trends of structural patterns commonly the inflection between opposing curves recesses and salients is a transverse zone marking a linear array of along strike contrasts in regional structure and stratigraphy analysis of such transverse zones can provide insight into the processes affecting orogenic curvature curvature is especially distinctive along the west flank of the appalachians where the tightest salient recess juncture is a thinned skinned transverse zone in georgia separating the alabama recess and tennessee salient and extending entirely across the width of deformed laurentian cover rocks this zone s most notable feature is a large oblique hanging wall ramp within the frontal metamorphic allochthon formed during alleghanian collisional events the basal decollement steps several kilometers stratigraphically upward across this ramp from proterozoic basement and its overlying late proterozoic rift sequence tennessee salient southwestward into basal cambrian and younger rocks alabama recess this ramp resulted from oblique sinistral displacement of the allochthon against an earlier east facing dextral continental margin transform fault that initially separated adjacent rifted margins of opposite polarity differences in the distribution thickness and facies of units indicate that important variations in basin geometry existed across the curvature inflection before paleozoic deformation suggesting that the orogenic curvature in part results from an inherited basin architecture translation of the allochthon over the ramp caused deflection of the tectonic transport trajectory major cross folding and rotation of earlier structures associated out of plane deformation propagated outward across the foreland producing the array of thinned skinned transverse structures aligned into the transverse zone

prediction of sediment thickness in the norwegian greenland sea from gravity inversion 1 we explore the feasibility of inverting freely available altimetry derived gravity grids for sediment thickness on oceanic crust a four layer earth model is adopted assuming that the gravity signal from the base of the sedimentary layer can be fully isolated from the observed free air gravity the following additional data are required 1 shipboard bathymetry for reliable bouguer correction 2 oceanic crustal ages to correct for gravity anomalies related to the thermal history of lithospheric breakup and seafloor spreading and 3 some wide angle seismic observations of sediment thickness a fixed relationship between the base of the sedimentary layer and the moho is also required because of limited gravity resolution at large depths the final inversion setup searches for three control parameter values density of uppermost sediments density of oceanic crystalline crust and thermal diffusivity of mantle rock that minimize misfit between the earth model and observations applied to the norwegian greenland sea the inversion successfully predicts locations of 20 25 km wide depocenters and crystalline crustal highs yet 1 km thickness errors are common even in well constrained areas the largest errors are associated with underplated crust active mid ocean ridges and the continent ocean boundary indicating locally incorrect assumptions of mantle thermal structure and crystalline crustal thickness the thicknesses of 4 km thick submarine fans are underpredicted probably because of too low sedimentary densities in the earth model if these limitations are acceptable or do not apply because of simple crustal structure gravity inversion may be used for mapping regional sedimentary patterns e g in the arctic ocean

experimental insights on the geometry and kinematics of fold and thrust belts above weak viscous evaporitic decollement abstract this paper discusses the differences in mechanical properties and kinematics between fold and thrust belts detaching on evaporitic decollements and those detaching on stronger detachments physical experiments are described that model shortening of a thick brittle cover overlying a weak viscous decollement to gain a better understanding of these differences we tested the influence of 1 the decollement layer thickness and 2 the presence of a deformable backstop on the hinterland side and of a decollement pinch out on the foreland side because of the very low shear strength of the viscous decollement folds and thrusts did not propagate according to a piggy back sequence but by centripetal back and forth propagation additional shortening was accommodated by growth of all existing structures fold symmetry and thrust vergence varied between experiments models confined between two rigid vertical endwalls always deformed by symmetric folding and thrusting in models having a deformable backstop and a foreland decollement pinch out forethrusting initially dominated folds were asymmetric and fault blocks rotated in models having a thick decollement layer folds kept growing asymmetrically diapirism also depended on the initial decollement geometry diapirs formed only in models having a deformable backstop and were restricted to the hanging wall of a fault related fold finally the implications of such kinematics on the thermal history are discussed as well as the likelihood of hydrocarbon maturation and preservation

neoproterozoic oceanic crust remnants in northeast brazil the borborema province of northeast brazil occupies a strategic position in the central portion of west gondwana linking three of its major cratonic constituents the sao francisco congo amazon and west africa cratons the southern portion of the province the riacho do pontal fold belt comprises an association of exhalative rocks with metabasalts having transitional mid oceanic ridge basalt geochemistry monte orebe complex that suggests the preservation of remnants of neoproterozoic oceanic crust this view is supported by geophysical data the riacho do pontal fold belt corresponds to the inflexion of a paired positive negative bouguer anomaly similar to other precambrian suture zones sm nd isotope data for the metabasalts yield a whole rock isochron age of 819 120 ma with an initial e nd t 4 4 indicating derivation from a depleted mantle source the onset of subduction within the borborema province ca 630 ma caused the inversion of basins and obduction of slices of oceanic crust in this scenario the riacho do pontal fold belt represents a complete late neoproterozoic plate tectonics cycle involving the collision of the sao francisco craton lower plate with the pernambuco alagoas block upper plate this interpretation challenges current views that the borborema province has acted as a coherent block since paleoproterozoic time part of the atlantica supercontinent suggesting instead a dynamic setting where multiple plates interacted during the proterozoic the monte orebe ophiolite provides a link with other cryogenian oceanic crust occurrences in central brazil and west africa indicating the preservation of a transcontinental neoproterozoic suture zone in the heart of west gondwana

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

present status of the geochronology of the precambrian rocks of rajasthan abstract age determinations mostly by rb sr analyses of the precambrian rocks of rajasthan by us and by others are summarized and discussed broad periods of acid magmatism at 1 3000 2900 m y 2 2600 2500 m y 3 2000 1900 m y 4 1700 1500 m y and 5 850 750 m y were identified the oldest rocks in the area are the yet undated banded gneisses bgc east of udaipur intruded by the untala granite dated at 2950 m y and hence of mid archean age the basal status hitherto attributed to the berach granite dated at 2600 m y is no longer tenable the radiometric control on the beginning and duration of the overlying aravalli supergroup is not yet satisfactory though a lower limit at 2000 m y is indicated heron s original delhi rocks have recorded two magmatic events widely separate in space and time while the earliest granitic activity at 1600 m y is recorded only in the alwar basin in the northeast the younger activity between 850 750 m y is widespread as shown by the nearly concordant ages of erinpura type granites along the aravalli mountain range and the malani rhyolites in the western plains of the aravalli range

marinoan glaciation in east central brazil abstract remnants of a neoproterozoic glaciation in east central brazil are represented by thin diamictite layers jequitai formation and correlative units locally overlying striated pavements on the sao francisco craton the diamictites are covered by the sete lagoas formation of the basal bambui group which is generally accepted to be a typical cap carbonate sequence although most authors have preferred a mid cryogenian post sturtian age for it based mainly on pb pb whole rock data the sete lagoas formation bears lithostratigraphic and isotopic characteristics that are identical to early ediacaran cap carbonates worldwide including a basal thin 0 10 m pale and flinty cap dolostone preserving a drop in 13c values from around 3 2 to 4 5 with associated 18o around 5 and crystal fan facies interpreted as aragonite pseudomorphs ediacaran zircons have been recovered from the middle of the sete lagoas formation constraining the deposition of its upper half to be younger than 610 ma rodrigues 2008 although there is an unconformity below the point where the zircons were collected it is short lived as suggested by the identical typically ediacaran 87sr 86sr values above and below 0 7074 0 7076 carbonate clasts from the jequitai formation and correlative diamictite bearing units in the fold belts that surround the sao francisco craton canabravinha and serra do catuni formations display similar ranges in 13c 6 7 to 2 6 suggesting the erosion of a pre glacial carbonate platform with negative 13c values i e the islay and or trezona anomalies the cratonic carrancas formation on the other hand yielded pale dolostone clasts with 13c in a small range between 4 2 and 3 4 and 18o values around 6 5 these clasts could be derived from the cap dolostone unit itself in which case the carrancas formation would represent resedimented basal sete lagoas formation and imply that sections of the sete lagoas formation sitting atop the carrancas formation are incomplete the base truncated sections have confused previous attempts to correlate the sete lagoas formation with other cap carbonate successions in light of the available lithostratigraphic isotopic and u pb zircon data we propose that the sete lagoas formation represents a basal ediacaran cap carbonate sequence 635 610 ma deposited after the marinoan glaciation in east central brazil

new gravity data in the arctic ocean comparison of airborne and ers gravity new gravity fields from airborne gravimetry and from ers 1 and 2 satellite altimetry cover extensive portions of the arctic ocean these two data sets may constitute as much as 60 of the data contributions to the arctic gravity project compilation here we evaluate the accuracy and resolution of these data and quantify their impact on the compilation both gravity determinations compare favorably with geological survey of canada surface measurements in the beaufort sea airborne 1 86 2 09 mgal rms ers 2 64 3 11 mgal rms comparisons between the airborne and ers data over the chukchi borderlands reveal a 4 38 mgal rms difference over the smoother region of the field and 7 36 mgal rms over the rugose field generated by the shallow ridges and deep troughs coherency between the two data sets in the chukchi region implies a resolution of 19 km comparison with science ice expedition submarine measurements over chukchi plateau suggests that the ers field resolves even shorter wavelength signal than the airborne data whereas in the beaufort sea the airborne data showed better coherence to ground truth data long wavelength differences exist between the two data sets expressed as a 2 3 mgal offset over the chukchi region this study highlights the respective strengths of the two data sets the ers gravity field has the advantage of ubiquitous coverage of the ocean south of 81 5 degreesn a denser sampling of the gravity field and a recovery of signal down to similar to 15 km the airborne data cover a significant portion of the polar hole in the satellite coverage have lower measurement noise and recover somewhat higher anomaly amplitudes in the 25 100 km wavelength range

tectonic evolution of the paranoa basin new evidence from gravimetric and stratigraphic data abstract field gravimetric and stratigraphic surveys were conducted with the aim to constraint the mechanisms responsible for the initiation of the stenian tonian paranoa basin central brazil a subject not yet studied in detail the paranoa group crops out in the external zone of the brasilia belt a neoproterozoic orogen in the western margin of the sao francisco craton detailed geological mapping confirmed the existence of a regional scale fault that controlled sedimentation of the paranoa group during the deposition of its basal formations revealing important details about basin initiation and early evolution gravimetric modeling indicates the existence of paleorift structures beneath the paranoa sequence in the study area results from both stratigraphic and gravimetric surveys show strong evidence of mechanical subsidence by faulting during basin initiation unsorted angular clasts cut by quartz veins and brecciated boulders present in the basal conglomerate support this hypothesis basin initiation faults coincide with deeper paleorift faults and are thus interpreted as reactivations of the older statherian arai rift the reactivations favored an initial regime of mechanical subsidence dominated by the development of epirogenic arches subsiding at different rates apart from faulting activity the post basal sequence presents no evidence of rift environment in the strict sense besides the great lateral continuity and relatively constant thickness of facies indicate that an initial mechanic subsidence rapidly gave way to flexural subsidence during subsequent stages of basin evolution the paranoa group do not present reliable characteristics that would allow its strict classification as a passive margin its main stratigraphic characteristics tectonic location and basement architecture indicate that the paranoa group was deposited in a cratonic margin basin and may have been either connected to a passive margin basin at times of sea level rise or evolved to a passive margin later in time

u pb zircon geochronology of precambrian tin bearing continental type acid magmatism in central brazil abstract u pb geochronological data for alkali rich granitic and associated rhyolitic rocks from two different tin bearing sub provinces of the goias tin province in central brazil reveal two distinct episodes of mid proterozoic continental acid magmatism at ca 1770 ma and 1600 ma ago u pb zircon data for the sucuri and soledade granites of the eastern rio parana sub province rps define upper intercept ages of 1767 10 ma and 1769 2 ma respectively these ages are indistinguishable from a u pb age of a rhyolite from the basal portion of the arai group 1771 2 ma these results attest to the contemporaneity of the intrusion of the granites and the crystallization of the rhyolitic lavas in the rio parana sub province u pb isotopic results from zircons from the serra da mesa granite in the western rio tocantins sub province rts indicate that the granitic igneous activity in that area is ca 150 ma younger than that in the rps the inheritance pattern observed in the zircons from the arai rhyolite suggests that the sialic basement in the rps region may include an early proterozoic ca 2 2 ga component inheritance of possibly archaean age was also found in zircons from the sucuri granite lower intercept age of 658 ma for the soledade granite zircons indicate pb loss during the late proterozoic brasiliano pan african thermo tectonic event 650 550 ma similar associations of tin bearing alkali rich granites with rhyolites and succeeding continental sediments are known in other parts of the precambrian shield of brazil this indicates that extensive areas of this shield experienced extensional tectonics in intracontinental settings between ca 1700 and 1900 ma ago

basement controlled faulting of paleozoic strata in southern ontario canada new evidence from geophysical lineament mapping abstract basement fault reactivation is now recognized as an important control on sedimentation and fault propagation in intracratonic basins in southern ontario the basement consists of complexly structured mid proterozoic ca 1 2 ga crystalline rocks and metasedimentary rocks that are overlain by up to 1500 m of paleozoic sedimentary strata reactivation of basement structures is suspected to control the location of paleozoic fault and fracture systems but evaluation has been hindered by a limited understanding of the regional structural characteristics of the buried basement new aeromagnetic and gravimetric lineament mapping presented in this paper better resolves the location of basement discontinuities and provides further evidence for basement controls on the distribution of paleozoic fault and fracture systems lineament mapping was facilitated by reprocessing and digital image enhancement micro levelling regional residual separation derivative filtering of existing regional gravity and aeromagnetic datasets reprocessed images identify new details of the structural fabric of the basement below southern ontario and delineate several previously unrecognized aeromagnetic and gravity lineaments and linear zones linear zones parallel the projected trends of mid proterozoic terrane boundaries identified by field mapping on the exposed shield to the north of the study area and are interpreted as zones of shearing and basement faulting mapped aeromagnetic and gravity lineaments show similar trends to paleozoic faults and fracture networks and broad zones of seismicity in southern ontario these new data support an inheritance model for paleozoic faulting involving repeated reactivation and upward propagation of basement faults and fractures into overlying cover strata

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

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

rare earth elements in the terminal ediacaran bambui group carbonate rocks brazil evidence for high seawater alkalinity during rise of early animals abstract rare earth elements plus yttrium rey mass fractions of ancient carbonate rocks are used to track changes in chemistry of past seawater here we investigate rey patterns in two carbonate sections from the ediacaran bambui group sao francisco basin brazil which comprise its two lowermost transgressive regressive second order sedimentary sequences shale normalised distributions vary with the sequence stratigraphy framework in the basal 2nd order sequence carbonate samples from the basal sequence transgressive systems tract display light rey lrey distributions slightly depleted to enriched that reflect input of freshwater possibly in a post glacial episode upwards carbonate rocks from the early highstand systems tract ehst yielded lrey enriched distributions which progressively turns into lrey shale normalized depleted distributions on samples from the late highstand systems tract lhst this portion of the sequence also displays y positive anomaly in some cases carbonate samples from the upper second order sequence do not display coherent patterns ce ce values 1 in most samples throughout the two sections suggest permanent anoxia of seawater the rey change from the ehst to lhst in the basal sequence marks an important paleoenvironmental overturn in the basin with increasing alkalinity in seawater driving rey fractionation and lrey depletion confinement of the basin in the inner areas of west gondwana due the uplift of marginal neoproterozoic orogens probably changed the weathering style of source areas to more congruent thus delivering a higher ionic influx to a restricted setting increasing alkalinity during lhst cloudina sp fragments were reported in this stage of the bambui group and in carbonate rocks with high sr mass fractions in other west gondwana basins supporting the hypothesis that the high alkalinity of seawater during late ediacaran may have driven the appearance of the first biomineralizing organisms

petrology of ocellar lamprophyres in south delhi fold belt danva sirohi district rajasthan india abstract ocellar lamprophyres are reported from danva in sirohi district rajasthan which lies in the pindwara watera sector of the south delhi fold belt sdfb they intrude mafic metavolcanics and garnetiferous biotite schist of the ajabgarh group of the delhi supergroup these lamprophyres are unaltered show porphyritic and panidiomorphic textures with olivine clinopyroxene amphibole biotite and spinels constituting the phenocryst phase in a ground mass of clinopyroxene amphibole mica and analcime petrochemically these lamprophyres can be classified as analcime monchiquite alkaline lamprophyre though they have some affinity towards ultramafic lamprophyres and lamproites chemistry of the minerals suggests that the clinopyroxenes have a compositional range of diopside and salite amphiboles are typically kaersutites biotites are titanium rich and spinels are al rich ulvospinels these minerals plot in discriminant fields of minerals from alkaline and ultramafic lamprophyres the danva lamprophyres are characterized by a variety of ocellar features that include i porphyritic ocellus with microphenocrysts of kaersutites and biotite in ground mass of analcime ii zoned ocellus with concentric zones of carbonates and analcime and iii composite ocellus in which type i ocellus enclose the type ii ocellus these ocellar features are interpreted to represent late stage magmatic segregation and magmatic crystallization involving two immiscible magmatic liquids the danva lamprophyres are in strike continuity of the pipela lamprophyres and therefore confirm alkaline magmatism for over 20 kms in sdfb during end neoproterozoic tectono thermal event the close spatial association of the lamprophyres with cu zn au deposits of danva and pipela area and the first report of monchiquite with affinity towards ultramafic lamprophyres and lamproites may be significant in gold and possibly diamond exploration in the sdfb

global correlation of the vazante group sao francisco basin brazil re os and u pb radiometric age constraints abstract the vazante group consists of precambrian carbonate dominated platform deposits that extend along more than 300 km in the external zone of the brasilia fold belt of the sao francisco basin in east central brazil the sequence is about 4 8 km thick and contains a preserved glaciomarine diamictite unit containing dropstone at the top and a lower diamictite unit at the bottom previous c and sr isotope profiles suggested the correlation of the upper diamictite unit with the sturtian glacial event ca 750 643 ma however new re os isotope data from the shales associated with the upper diamictites yield radiometric age estimates between 993 46 and 1100 77 ma u pb measurements on a suite of clear euhedral zircon crystals that were separated from the same shales associated with the upper diamictite and from the arkosic sandstone above the lower diamictite yield ages as young as 988 15 and 1000 25 ma respectively based on the re os and u pb ages the best age estimate of the vazante group is constrained to be 1000 1100 ma and thus the two diamictite units are not correlative with the sturtian glaciation s but most likely are records of glacial events that occurred during the late mesoproterozoic

influence of synkinematic sedimentation in a thrust system with two decollement levels analogue modelling abstract compressive systems in foreland domains are characterised by fold and thrust belts linked to the presence of one or several ductile layers in depth acting as a decollement level the main parameters controlling the structural evolution are the presence of a decollement level the amount and rate of shortening and the amount of synkinematic sedimentation the effect of these parameters has only been studied on a thrust belt scale furthermore only the effect of synkinematic sedimentation on a simple system with one decollement level has been studied at the scale of a single structure the aim of this study was to use analogue modelling to test the effect of shortening rate velocity and the localization of sedimentation on a single system characterised by the presence of two prekinematic decollement levels the main results showed variations in the structural vergence folding geometry symmetric or asymmetric the evolution of the deformation horizontal propagation versus vertical uplift and the decoupling of the lower and upper brittle structures in relation with the main parameters shortening rate and mass transfer the results of the experiments were then compared to natural examples from the sub andean thrust belt

controls on the geometry of fold thrust belt salients salients convex to the foreland curves in the map traces of orogens typically involve fold thrust belts formed along the foreland margin of orogens an examination of the map view shape degree of asymmetry dimensions and structural trend line patterns i e the variation in fold trend and fault strike along the length of the belt of 20 fold thrust belt salients worldwide illustrates relationships between salient geometry and the geologic setting in which the salient formed in order to understand these relationships we used simple sandbox models to simulate the formation of three of the several types of salients specifically we examined indenter controlled salients the geometry of which reflects interaction of a fold thrust belt with a hinterland indenter basin controlled salients the geometry of which reflects lateral variations in the thickness of predeformational basin fill and detachment controlled salients the geometry of which reflects lateral variations in the strength of a detachment surface our models demonstrate that thrusts in these three types of salients initiate with a curved shape and that rocks within thrust sheets on the limbs of salients can undergo a vertical axis rotation in response to simple shear without requiring major rotation of the fault trace furthermore trend line patterns distinguish basin controlled from indenter controlled salients specifically structural trend lines in basin controlled salients converge at the end points of the curve while in indenter controlled salients they diverge at the end points of the curve the asymmetry of a salient correlates with the asymmetry of the predeformational basin and or of the indenter and with the convergence direction thus our results explain geometric characteristics of individual salients and address controversies concerning paleomagnetic data in salients they also may help prioritize petroleum exploration targets in salients foreland basin oil fields concentrate at the apex of basin controlled salients because the geologic factors that generate the salient also contribute to localizing oil fields

effect of overburden thickness on thrust belt geometry and development laboratory sand models simulating development of thrust belts indicate that 1 distance between the backstop and the first formed thrust is a linear function of initial sand layer thickness 2 spacing between the first two thrusts is most likely a nonlinear function of initial sand layer thickness 3 initial dip of successive ramps decreases progressively toward the foreland because of variations in principal stress directions with depth and or changes of mechanical properties of sand with increasing overburden 4 faults to the hinterland steepen and become less active as hinterland thrust sheets undergo penetrative shear during sand wedge growth and 5 new ramp initiation occurs to the hinterland of the basal detachment tip line these observations can be interpreted both in context of area balance considerations and soil mechanics theory if initial sand layer thicknesses in our models represent syntectonic overburden in natural thrust belts i e distance between the ground surface and the detachment then our results suggest that locations of some thrust belt salients are controlled by along strike variations in stratigraphic thicknesses and that initial thrust spacing decreases progressively to the foreland if the thrust belt involves a foreland tapering basin furthermore our sand model observations suggest that the concept of a fixed rigid backstop in the hinterland of thrust belts may be misleading as a consequence of thickening the hinterland portion of the thrust belt becomes stabilized and effectively becomes the backstop itself

proterozoic first order sedimentary sequences of the sao francisco craton eastern brazil abstract the sao francisco craton in eastern brazil hosts sedimentary sequences deposited between the paleoarchean 3300 ma and late neoproterozoic 580 ma proterozoic successions occurring in this region comprise five 1st order sedimentary sequences which besides episodes of global significance record major basin forming events the ca 8000 m thick minas itacolomi 1st order sequence exposed in the brazilian mining district of the quadrilatero ferrifero and containing as marker bed the lake superior type caue banded iron formation tracks the operation of a wilson cycle in the paleoproterozoic era the quartz arenite dominated espinhaco i and ii sequences record at least two major rift sag basin forming events which affected the host continent of the sao francisco craton at around 1 75 ga and 1 57 ga the macaubas sequence and its correlatives in the extracratonic domains witness the individualization of a sao francisco congo plate in synchronicity with the break up of rodinia in the cryogenian period the sao francisco congo plate together with various fragments derived from rodinia reassembled to form gondwana in the ediacaran period in the course of the gondwana amalgamation orogenic belts developed along the margins of the craton its interior converted into foreland basins received the shallow water carbonates and pelites of the bambui 1st order sequence and its correlatives

structure and origin of the fleurieu and nackara arcs in the adelaide fold thrust belt south australia salient and recess development in the delamerian orogen abstract the regional map pattern of structural trends in the early paleozoic adelaide fold thrust belt of south australia resembles an upright letter s in general structures in the fold thrust belt verge toward the gawler craton to the west so the northern curve nackara arc of the s is a salient whereas the southern curve fleurieu arc is a recess the map view axial trace of the nackara arc coincides with the thickest portion of the adelaidean late proterozoic basin and the map view axial trace of the fleurieu arc coincides with the intersection between the adelaidean and kanmantoo cambrian basins our structural analysis of the adelaide fold thrust belt demonstrates that the cross sectional structural geometry of the nackara arc contrasts markedly with that of the fleurieu arc in the nackara arc the belt consists of open detachment folds that probably formed above an evaporite hosted decollement whereas in the fleurieu arc the belt consists of an imbricate fan of basement involved thrusts further the foreland edge of the nackara arc coincides with a blind ramp whose strike parallels fold axes of the arc whereas the foreland edge of the fleurieu arc coincides with emergent thrusts that cut obliquely across earlier formed folds of the arc in the transition region between the two arcs the fold thrust belt consists of an intensely sheared basal duplex overlain by open folds structural contrasts between the nackara and fleurieu arcs indicate that the two curves did not form in the same way we present a model for development of these curves based on a comparison of their structural features to those of curves in other fold thrust belts and based on previous studies of curve formation using sandbox analogs we suggest that curvature of the nackara arc reflects the control of basin stratigraphy on the width of a fold thrust belt while curvature of the fleurieu arc reflects oroclinal bending of the fold thrust belt when it impinged on the southeastern corner of the gawler craton our model explains the structural contrast between the two arcs and illustrates correlations between fold thrust belt curve formation and pre deformational basin geometry

the puzzle assembled ediacaran guide fossil cloudina reveals an old proto gondwana seaway abstractduring the ediacaran the clymene ocean separated the lau rentia amazonia and rio apa cratons from several landmasses to the west forming the proto gondwana supercontinent however no clear evidence about the existence of ediacaran epeiric seas over those landmasses has been found here we report and discuss the discovery of the ediacaran guide fossil cloudina sp associated with other metazoan body and trace fossils in the bambui group central eastern brazil the ediacaran age of the bambui group and the paleogeographic position of cloudina bearing successions in brazil antarctica namibia and argentina suggest a scenario of ocean con nectivity among coeval intracratonic basins of south america africa and antarctica at the end of neoproterozoic time the new nding epitomizes one of the most important paleontological discoveries ever made in south america helping to solve an old paleogeographic puz zle of the gondwana supercontinent introduction the bambui group crops out in central eastern brazil and is one of the most studied neoproterozoic sedimentary successions in south america it comprises an extensive at lying cratonic cover in erosional contact with paleoproterozoic and archean basement rocks of the sao francisco craton in its eastern and westernmost occurrences the bam bui group is deformed and metamorphosed by the brasilia and aracuai mobile belts respectively fig 1a the regional tectonostratigraphy has been interpreted as re ecting deposition in a foreland basin related to gondwanan collisions involving the sao francisco craton pimentel et al 2011 the bambui group bg overlies rocks of the macaubas group and jequitai formation and the carrancas conglomerate units supposedly de posited during sturtian glaciation the unit is 700 1000 m thick misi et al 2007 and comprises limestones and dolomites of the sete lagoas for mation at the base that grade upward to shales and siltstones of the serra de santa helena formation followed by marls siltstones limestones and sandstones of the lagoa do jacare serra da saudade and tres marias formations fig 1b the precise age of the bg is not well established precluding accurate correlation with other supposedly coeval successions such as the corumba group brazil itapucumi paraguay arroyo del soldado uruguay si erras bayas argentina and nama group namibia the presence of

the palaeobiology and geochemistry of precambrian hydrocarbon source rocks abstract organic carbon productivity and formation of hydrocarbon source rocks during the early precambrian was almost exclusively the product of the growth of microbial mats indirect evidence of microbial mats can be traced back to at least 2 6 2 7 ga neoarchaean with the earliest evidence of mat development in siliciclastic sediments coming from the 2 9 ga mesoarchaean predominantly marine sedimentary rocks of the mozaan group in south africa the earliest direct evidence for terrestrial microbial mats in siliciclastic sediments comes from the 2 75 ga palaeoproterozoic fluviolacustrine sediments of the hardey formation of the pilbara craton in western australia well preserved proterozoic hydrocarbons provide valuable information about the early evolution of the biosphere eukaryotic steranes biomarker for eukaryotic cells and therefore evolved forms of life are present in the geological record from about 2 7 ga but they are not abundant or diverse within archaean communities which tend to be dominated by archaea isoprenoids some hydrocarbons have been generated and migrated from archaean organic rich shales but they were probably not of sufficient volume to be of commercial interest the world s oldest significant hydrocarbon source rocks are palaeoproterozoic in age and include the shungite deposits 2 0 ga in the lake onega region of arctic russia there is strong evidence of a global biospheric oxygenation event at c 1300 1250 ma mesoproterozoic in conjunction with a first order positive shift in the marine carbon isotope record this is supported by the appearance of the oldest bedded marine gypsum deposits and of the earliest unambiguously multicellular eukaryotes at this time this oxygenation event probably played a significant role in supporting the more diverse eukaryotic communities preserved in the neoproterozoic molecular record and provided the volume of organic material required to generate commercial volumes of hydrocarbons hydrocarbon source rocks of late mesoproterozoic and early neoproterozoic age are widespread and include highly organic rich shales deposited in restricted basinal settings adjacent to stromatolitic carbonate banks by c 850 ma the neoproterozoic molecular record is dominated by hopanes from cyanobacteria with a significant abundance and diversity of eukaryotic steranes including those of multicellular eukaryotes red and green algae as well as molecular evidence for heterotrophic protists the most widespread hydrocarbon source rocks of mid to late neoproterozoic age are commonly transgressive organic rich black shales associated with inter glacial and post glacial phases of the neoproterozoic global scale glaciations the relative dominance of microbial mats in the contribution of organic matter as a source for hydrocarbon generation probably decreased significantly during the late neoproterozoic and earliest cambrian perhaps as a result of the rapid growth of grazing metazoan communities or possibly as a result of changes in seawater chemistry and or nutrient supply precambrian and infracambrian petroleum systems are relatively abundant and widespread the oldest live oil recovered to date is sourced from mesoproterozoic rocks within the velkerri formation roper group of the mcarthur basin of northern australia dated at 1361 21 ma and 1417 29 ma re os dates with at least the initial phase of oil generation and migration having taken place before 1280 ma however the geologically oldest commercial production is currently from the somewhat younger mid to late neoproterozoic cryogenian ediacaran petroleum systems of the lena tunguska province in east siberia and in southern china from the latest neoproterozoic to early cambrian huqf supergroup in oman and potentially in the near future from the age equivalent mawar supergroup in western india

hot versus cold orogenic behavior comparing the aracuai west congo and the caledonian orogens observations and modeling show that temperature controls crustal rheology and therefore also the orogenic evolution of continent continent collision zones and the associated tectonic style in order to explore the effect of temperature in a natural environment we compare eroded sections through the unusually cold lower paleozoic north atlantic caledonian scandian collision zone and the very hot brasiliano pan african aracuai west congo orogen a cold and stiff subducting caledonian continental margin was able to subduct as a rather coherent unit to ultrahigh pressure conditions twice as deep as the pan african brasiliano crust that was quickly heated and softened and got involved in partial melting furthermore the caledonian collision developed large coherent thrust sheets that were transported hundreds of kilometers toward the foreland this was never achieved in the hot aracuai west congo orogen where much of the tectonic stress was absorbed by the partially molten central part of the orogen through magmatic state deformation major mylonite zones thrusts such as those seen in the caledonides are therefore less common in the aracuai west congo orogen further the deep continental subduction in the caledonides developed a strongly asymmetric collision zone with rapid variations in pressure and temperature in contrast the aracuai west congo orogen soon developed into a more symmetric geometry due to its easily flowing hot crust with a relatively flat base and a corresponding plateau in its upper part deformation of the cold caledonian crust was controlled by plate tectonic stress while gravitational forces more strongly influenced the hot brasiliano pan african example

salinization processes in the unconfined aquifer of bou areg ne morocco a geostatistical geochemical and tomographic study hydrogeological and geochemical data in conjunction with the results of an electrical imaging tomographic survey were examined to determine the main factors and mechanisms controlling the groundwater chemistry and salinity of the unconfined aquifer of bou areg on the mediterranean coast of ne morocco in addition statistical and geochemical interpretation methods were used to identify the distribution of the salinity multivariate statistical analysis cluster and principal component factors revealed the main sources of contamination groups a b and c in the cluster analysis and factors 1 3 factor 1 ce cl k so42 and mg2 factor 2 ca2 hco3 and ph factor 3 no3 represent the signature of seawater intrusion in the coastal zone the influence of marly gypsum outcrops in the upstream zone and anthropogenic sources respectively the ionic delta the ionic ratio the saturation index and stuyfzand s method were applied to evaluate geochemical processes the results obtained indicate on the one hand the phenomenon of salinization in both the coastal and the upstream zones and on the other the dilution of groundwater by recharge cation exchange is shown to modify the concentration of ions in groundwater locally with respect to salinization processes in the coastal zone the results of electrical imaging tomography show that salinity increases both with depth and laterally inland from the coastline due to seawater intrusion

late proterozoic glaciation in central eastern brazil synthesis and model abstract in the surroundings of the middle proterozoic sao francisco craton of central eastern brazil numerous traces of an ancient glaciation can be found almost all criterua necessary to prove an ancient glaciation occur including scratched and facetted pebbles a striated surface quarrying outwash plains eskers varvites dropstones decollement structures etc poorly sorted tillites with varying pebble composition are widely distributed continental tillites and glaciomarine sequences interfinger in the east and southeast of the craton traces of ice movement suggest flow directions of the ice towards the east and the southeast of the craton at the coast the wet base glaciers passed over to floating shelf ice in the northeast glaciers also came from the serrinha craton in northeastern bahia the glaciation covered an area of at least 1300 700 km the sequences containing traces of glaciation discordantly overlie archean to middle proterozoic rocks they in turn are overlain by late proterozoic calcareous and psammopelitic sediments of the bambui group present data indicate that glaciation took place around 1000 ma ago

use of chloride mass balance and environmental isotopes for evaluation of groundwater recharge in the alluvial aquifer wadi tharad western saudi arabia groundwater recharge estimation has become a priority issue for humid and arid regions especially in regions like saudi arabia where the precipitation varies over space and time as a result of topography and seasonality wadi tharad is a typical arid area in western saudi arabia within its drainage area of 400 km 2 the groundwater system shows a graded hydrochemical zonation from the hydrocarbonate in the upper reach to the chloride zone in the lower reach the saturation index si varies depending on the concentrations of carbonate minerals the mean for calcite and dolomite is about in equilibrium e g zero value as halite and gypsum indices are negative it is undersaturated isotopic compositions of h and o in the groundwater show that the groundwater recharge resources are mainly from meteoric water the chloride mass balance method was refined to estimate the amount of recharge which is probably 11 of the effective annual rainfall these results can be used to improve the accuracy of future groundwater management and development schemes

kinematic evolution of the aracuai west congo orogen in brazil and africa nutcracker tectonics during the neoproterozoic assembly of gondwana abstract the neoproterozoic aracuai west congo a wc orogen is one of many brasiliano pan african orogens that developed during the assembly of west gondwana this orogen was split apart in mesozoic time due to opening of the south atlantic the aracuai orogen now underlies eastern brazil whereas the west congo belt fringes central africa s atlantic coast significantly at the time it formed the a wc orogen was bounded on the west north and east by the sao francisco congo craton a crustal block that had the shape of a lopsided upside down u thus the orogen was partially confined during tectonism in that it occupied an enclave surrounded on three sides by cratonic crust formation of the a wc orogen resulted in kinematically complex deformation substantial crustal shortening and production of a large volume of magma how such features could develop in this particular setting has long been a mystery our field studies in the aracuai orogen together with published data on the west congo belt characterize the kinematic picture of the a wc orogen and lead to a tectonic model that addresses its evolution in our model the a wc orogen formed in response to closure of the macaubas basin this basin was underlain by oceanic crust in the south but tapered northward into a continental rift which terminated against the cratonic bridge linking the eastern and western arms of the sao francisco congo craton closure occurred when the western arm now the sao francisco craton rotated counterclockwise towards the eastern arm now the congo craton this closure may have been driven by collision of the paranapanema amazonian and kalahari cratons against the external margins of the sao francisco congo craton rather than by slab pull associated with subduction of the macaubas basin s floor thus the process of forming the a wc orogen resembled the process of crushing of a nut between two arms of a nutcracker such nutcracker tectonics led to a series of kinematically distinct deformation stages initially internal portions of the orogen flowed northwards then substantial crustal thickening occurred in the orogen s interior and the deformation front migrated outwards producing thrust belts that overlapped the internal margins of the craton with continued closure space in the enclave became restricted and the orogen s interior underwent lateral escape to the south late stage extensional collapse triggered both production of late to post collisional granites and exhumation of high grade rocks from mid crustal levels

early to late ediacaran conglomeratic wedges from a complete foreland basin cycle in the southwest sao francisco craton bambui group brazil abstract stratigraphic isotopic and geochronological data from two late neoproterozoic aged conglomerate wedges in the southwest sao francisco craton support the interpretation that the ediacaran bambui group in east central brazil was deposited in a foreland basin the sambura formation forms the base of the bambui group in the southwestern part of the bambui basin and was deposited synchronously with the brasilia orogeny it is interpreted to be a sedimentary product of a retrogradational coastal alluvial fan system deposited in an underfilled flexural foredeep during the early stages of foreland basin development sometime between 630 and 560 ma the basal sete lagoas formation carbonates were deposited towards the cratonic margin on the forebulge which provided an ideal environment for carbonate production the lateral relationship between the sambura and sete lagoas formations further implies that an unconformity was generated by foreland flexure and that this unconformity separates an early ediacaran phase of the foreland basin from a late ediacaran phase the lagoa formosa formation was deposited in the latter phase after peak orogenesis with a provenance that includes post orogenic granites and zircons as young as 560 ma it records a prograding turbidite fan system in the lagoa formosa formation that was deposited during orogenic unroofing and basin wide shallowing in a filled stage of the foreland basin a shift from highly enriched 13 c values towards global like carbon isotopes values in carbonates within the lagoa formation in conjunction with the occurrence of banded iron formation may suggest deposition in a basin with anoxic and ferruginous deep waters in the bambui basin in the latest ediacaran

do lamprophyres carry gold as well as diamonds diamonds are now known from kimberlites lamproites and alkaline and ultramafic lamprophyres here we point out that these rocks are also significantly enriched in gold as are calc alkaline lamprophyres the average gold abundance in lamprophyres seems to be at least an order of magnitude higher than in common igneous rocks and many individual values are 100 1 000 times higher this high gold content may reflect two factors 1 lamprophyres tap exceptionally deep regions of the earth regions not only where diamond is stable but also where gold may be more enriched than in the source regions for other igneous rocks 2 lamprophyre magmas are suitable carriers of gold from depth because in their high co2 h2o f k rb and ba but moderate s contents they closely mirror fluids actually known to deposit gold veins in the crust most types of lamprophyre rise rapidly enough from great depths to retain any diamond and gold along with rich mantle xenolith suites calc alkaline lamprophyres on the other hand undergo extensive crustal interactions which cause them to pick up crustal xenoliths and to lose any diamond and mantle xenoliths along with at least some of their gold this last may account for their association with mesothermal for example archaean gold deposits

sediment thickness in the southern canada basin abstract multichannel seismic reflection data are used in conjunction with deep crustal seismic refraction data to estimate the thickness of sediments in the southern canada basin of the arctic ocean north of alaska the sediments are interpreted to be of hauterivian mid early cretaceous to holocene age comparison of the seismic reflection character of seismic reflections in the study area with that in other basins indicates that a base of sediment top of oceanic layer 2 reflection is not present above the depth at which the water bottom multiple obscures all deeper arrivals which is in conflict with the conclusions drawn from aeromagnetic refraction and other reflection studies seismic velocity structure determined from the reflection data indicates that the reflections above the multiple are from sedimentary strata in the absence of seismic reflection evidence for the top of layer 2 above the multiple we estimate total sediment thickness by using the layer 3 refractions and subtracting an average assumed layer 2 thickness from the top of layer 3 assuming that an average thickness of oceanic layer 2 1 4 km overlies layer 3 in the southern canada basin sediment thickness in the study area is estimated to range between 6 5 km where water depth is 3 8 km to greater than 11 km where the water depth is 2 km this is nearly double that of any previous estimates and should have a significant effect on calculations such as the age of canada basin regional heat flow and long term sedimentation rates

direct dating of the sete lagoas cap carbonate bambui group brazil and implications for the neoproterozoic glacial events the end of the neoproterozoic is punctuated by glacial deposition but the chronology of these deposits is hindered presently by the paucity of geochronological data here we present new radiometric dating for the basal sete lagoas cap carbonate deposits that overlie glacial units in the sao francisco craton six samples from aragonite pseudomorph crystal rich facies showing pristine textures and constant 87sr 86sr ratios around 0 7075 yielded a pb pb isochron age of 740 22 ma which is interpreted as the depositional age for these remarkably preserved rocks this age can be used to infer a low to moderate palaeolatitude of 20 30 for carbonate and glacial deposition in addition as it overlaps the ages obtained for the oldest neoproterozoic glacial successions our result reinforces the idea of a long standing sturtian interval suggesting that this event represents either different discrete glaciations or a protracted event encompassing almost 80 ma

anatomy of a basin controlled foreland fold thrust belt curve the tres marias salient sao francisco basin brazil abstract salients as common features of fold thrust belts correspond to convex to foreland map view curves hosting sedimentary sequences younger than 1 8 ga the intracratonic sao francisco basin of southeastern brazil encompasses two foreland fold thrust belts of opposite vergences the brasilia on the west and the aracuai on the east these belts culminate in the central portion of the basin in form of two almost touching antitaxial curves developed in response to diachronic orogenic events affecting the basin fill units during the ediacaran brasiliano pan african assembly of west gondwana located in the external sector of the thin skinned brasilia foreland fold thrust belt the ca 130 km long and 80 km wide tres marias salient involves the up to 2500 m thick ediacaran bambui sequence in order to characterize the overall architecture and processes involved in the generation of the tres marias salient we conducted a detailed structural investigation of the southern segment of the brasilia foreland fold thrust belt based on surface seismic well and aerogeophysical data interpretation the salient is frankly dominated by monotonous trains of upright chevron folds bounded by a small number west dipping thrust faults the structural traces of thrusts and fold hinges on maps trend preferentially ns in the central portion of the salient curving progressively toward ne and nw in its southern and northern limbs respectively the entire system is linked to a spoon shaped detachment zone located near the base of the ediacaran bambui strata whose maximum thickness roughly coincides with the salient culmination our analysis led to the conclusion that the tres marias salient corresponds to a basin controlled curve generated during a single and progressive deformational episode its fold dominated character and the high symmetry of its internal fabrics can be explained by the low friction of the associated detachment located along a lower bambui sequence black shale and the mechanical behavior of the pelite dominated deposits incorporated in the fold thrust wedge the occurrence of seepages and recent gas discoveries in the tres marias salient area point toward its potential for unconventional hydrocarbon resources our investigation suggests that the main basin controls that led to the development of the salient might have controlled the quality and distribution of proterozoic petroleum system elements

crustal structure beneath the sub himalayan fold thrust belt kangra recess northwest india from seismic reflection profiling implications for late paleoproterozoic orogenesis and modern earthquake hazard abstract most compressional orogens include salients and recesses along their strike the age and origin of which can be hard to ascertain in the kangra recess in the trace of the main boundary thrust mbt the largest such recess within the active himalayan orogen the sub himalayan sedimentary fold thrust belt increases in width to as much as 90 km the kangra basin but narrows to as little as 10 km in the adjoining nahan salient of the mbt the subathu basin to the southeast new seismic reflection profiling places the himalayan decollement at 6 8 km depth above thin but reflective meso to neoproterozoic vindhyan lesser himalayan series equivalent strata these data show that the vindhyan sedimentary rocks are thinner in the kangra recess than further southeast allowing the hypothesis that the width of the lesser himalayan thrust belt and the existence of the kangra recess could be related to the pre deformation basin thickness this hypothesis obviates the need for control of the kangra recess by a lateral ramp in the main himalayan thrust so making it more likely that the kangra segment could rupture as part of an earthquake far larger than the devastating 1905 m 7 8 kangra earthquake below the proterozoic sedimentary rocks our reflection data show a west southwest dipping reflective fabric spanning a 30 km crustal thickness which we infer corresponds to a widespread ulleri wangtu orogenic event at c 1850 ma affecting a pre tethyan indian continental margin thickening the basement by c 20 the deepest 10 km of this 50 km thick crust shows a more horizontal arguably younger reflectivity though the moho is not clearly marked by strong reflectors

liquid immiscibility and melt segregation in alkaline lamprophyres from labrador abstract sannaites alkaline lamprophyres with ti rich mica and or amphibole and k feldspar forming part of the aillik bay alkaline dyke swarm contain two varieties of leucocratic globular structures type i are interpreted to represent immiscible silicate liquids whereas type ii are caused by segregation of late stage melt type i globules are characterised by enrichment in al 2 o 3 sio 2 na 2 o and k 2 o and depletion in cao mgo feo total fe tio 2 and p 2 o 5 relative to the sannaite groundmass mafic minerals inside and outside the type i globules have cores of similar composition but rims which follow different trends type ii globular structures are commonly zoned they have a carbonate analcite inner zone interpreted as the site of a former gas bubble and an outer zone representing late stage liquid minerals in the outer zones show extreme zonation along chemical trends similar to those in the groundmass but contrasting with those in type i globules boundary nucleation is common in type ii but rare in type i the aillik bay sannaites are believed to lie at the alkali alumina rich extremity of the miscibility gap rocks with type ii structures frequently have a two stage groundmass which may be caused by metastable immiscibility compositionally adjacent to the stable miscibility gap

unravelling a proterozoic basin history through detrital zircon geochronology the case of the espinhaco supergroup minas gerais brazil abstract new u pb in situ zircon dating provides a new approach for the study of the espinhaco supergroup units exposed in the sao francisco craton and aracuai orogen located in southern espinhaco minas gerais the following two basins were formed in intraplate conditions i the lower espinhaco basin is marked by a volcano sedimentary sequence with alluvial fluvial and eolian deposits that were formed from 1 68 ga to 1 80 ga ii the overlying sequence called the upper espinhaco basin is a rift sag basin that presents basal diamond bearing continental deposits the sopa brumadinho formation with a younger zircon peak at 1192 ma which is the maximum age for deposition of the upper units the obtained u pb detrital zircon data of the studied units show age groupings related to the orogenic cycles of jequie neoarchean and transamazonian paleoproterozoic which are associated with the stability period from 1 8 ga to 0 91 ga in the sao franscico craton and record deposition during at least the three basinal cycles the stenian tonian ages of the upper espinhaco basin units 906 ma to 1192 ma suggest a direct link with the evolution of the rodinia supercontinent and represent a marker for mesoproterozoic to early neoproterozoic evolution in the sao franscico craton and adjacent areas

mixed carbonate siliciclastic sedimentation in forebulge grabens an example from the ediacaran bambui group sao francisco basin brazil abstract mixed carbonate siliciclastic successions have been described in multiple phanerozoic sedimentary settings recording the dynamic interplay of tectonics eustasy climate in situ carbonate production and variations in siliciclastic sediment supply the ediacaran bambui 1st order sequence i e bambui group covers most of the intracratonic sao francisco basin southeast brazil and encompasses thick packages of carbonate and fine to coarse grained siliciclastic strata recording a marine foreland basin stage that developed in the sao francisco plate during the neoproterozoic early paleozoic west gondwana assembly the ediacaran deposits unconformably overlie archean paleoproterozoic metamorphic assemblages of the sete lagoas basement high and fill a series of kilometer long grabens in the southern sao francisco basin seismic data reveal that these troughs developed through the extensional reactivation of ancient basement structures along with the tectonically driven forebulge uplift of the sete lagoas high in the early evolutionary stages of the bambui basin cycle based on the detailed description of continuous drill cores of a well recently drilled during hydrocarbon exploration campaigns we recognized two transgressive regressive 2nd order sequences preserved within one of the focused grabens i sequence 1 includes the glaciogenic deposits of the basal carrancas formation that grade upward into the carbonate ramp successions of the sete lagoas formation ii sequence 2 contains the siliciclastic dominated and deep water to deltaic strata of the serra de santa helena formation and passes upward into peritidal carbonates of the lagoa do jacare formation these sedimentary successions encompass suites of retrogradational aggradational and progradational lower rank cycles and are bounded by erosional surfaces regional seismic interpretation well data and the available literature indicate that most of these deposits and their correlatives are continuous over most of the sete lagoas basement high showing remarkable variations within the studied forebulge grabens our analysis demonstrates that the depositional architecture of sequence 1 and sequence 2 marks the interplay of successive episodes of tectonically driven forebulge uplift and regional eustatic and climatic changes the latter are mainly recorded by the lower second order sequence sequence 1 which contains typical sedimentary features deposited in the aftermath of an important glaciation affecting lower latitudes during the late neoproterozoic an additional local accommodation control is restricted to the forebulge grabens and is associated with episodic and local subsidence pulses as indicated by lower to higher rank cycles it has been responsible for the deposition of thicker transgressive to early regressive successions a few gravitational flow deposits and the development of the deltaic to shallow marine carbonate cycles of sequence 2 the overall stratigraphic architecture of the studied successions suggests that several basin mechanisms operating during the phanerozoic were also present in the late neoproterozoic the close association between organic rich post glacial shales potential reservoir facies and rift related migration paths and hydrocarbon traps indicate that forebulge grabens represent important elements to consider in hydrocarbon exploration campaigns

meso neoproterozoic isotope stratigraphy on carbonates platforms in the brasilia belt of brazil abstract carbonate platforms were present worldwide during the proterozoic eon and variations in their c and sr isotope ratios are commonly used as a correlation tool particularly through the neoproterozoic when rapid secular change in marine c and sr isotope values permit distinction between cryogenian glacial events in central brazil the late mesoproterozoic and neoproterozoic eras are represented by a thick succession of sedimentary rocks that were deposited and later deformed along the eastern margin of the sao francisco craton these strata are divided into the three major groups 1 the paranoa group which consists of a succession of sandstone siltstone rhythmite and selected intervals of carbonate 2 the macaubas group which consists of a glacial diamictite jequitai formation and 3 the bambui group with an important carbonate bearing succession that includes characteristic cap carbonate facies in its lower strata carbonate facies of paranoa and bambui groups typically occur in unconformable contact and when the diamictite of jequitai formation is absent it can be difficult to determine the stratigraphic position of these lithological similar groups furthermore uncertainties in the age of the bambui group has lead to several distinct interpretations regarding the age of the jequitai glacial diamictites we investigated the c o and sr isotopes and chemical composition of carbonate rocks in five measured sections including both pre and post glaciation carbonate successions the 13 c pdb values in the upper paranoa group occur in a narrow interval between 0 6 and 3 6 whereas the post glacial bambui group begins with substantially negative values as low as 5 7 in cap dolomite facies and rises to values up to 11 permil in limestone of the upper sete lagoas formation similarly carbonate rocks of the paranoa and bambui groups are distinct in terms of their 87 sr 86 sr ratio generally non radiogenic ratios between 0 7056 and 0 7068 are recorded in the upper paranoa group and ratios between 0 7074 and 0 7080 occurring within the bambui group the stratigraphic pattern of the c and sr ratios indicates distinctive isotopic characteristics for these two carbonate successions the isotopic data for the cabonates in the paranoa group are consistent with a sedimentation age in upper mesoproterozoic or lowermost neoproterozoic preceding the first cryogenian glaciation carbonate facies and the isotopic data for the lower bambui group suggest a relationship with the second cryogenian glaciation

orocline timing through joint analysis insights from the ibero armorican arc abstract the timing and kinematics of oroclinal bending in the core of the ibero armorican arc iaa has recently been constrained using paleomagnetic data from the cantabrian zone in northern iberia this study analyzes the joint patterns present in rock units deposited pre syn and post oroclinal bending systematic changes in the orientations of tensional joint sets in superimposed stratigraphic units are interpreted to record the progressive stages of oroclinal bending in the core of the iaa time constrains for joint set development are constrained by the known ages of the bounding unconformities that limit the studied stratigraphic units joint azimuth variability in the pre orocline rocks neoproterozoic to pre upper carboniferous is comparable to the present arc curvature of the orocline about 180 the joints in the syn orocline rocks upper pennsylvanian or stephanian 304 to 299 ma show a lower azimuthal variability that is comparable to about 50 70 of the total curvature seen in pre orocline rocks finally post orocline rocks permian contain joints that have uniform azimuths for each set across the entirety of the present day arc together these spatially and temporally distinct joint sets suggest that rotations in the cantabrian zone took place in the upper pennsylvanian during a ca 10 ma time period which agrees well with previous paleomagnetic arguments the data also provides supporting evidence for oroclinal bending by rotation around vertical axes of an initially linear or nearly linear orogenic belt and finally these data highlight the potential power in using tectonic joint sets for constraining thrust belt kinematics in curved orogenic systems when unconformity bounded stratigraphic sequences are present that are coeval with orocline development

reconstructing the kinematic evolution of curved mountain belts a paleomagnetic study of triassic red beds from the wyoming salient sevier thrust belt u s a determining the kinematic history and mechanics of curved fold and thrust belts is fundamental to understanding the tectonic evolution of mountain systems to better understand the development of a classic curved fold and thrust belt we completed an integrated paleomagnetic and strain study of the wyoming salient paleomagnetic data are reported here from 154 sites collected from red beds of the triassic ankareh formation in the salient and nine sites collected from the relatively stable foreland red beds display three components with distinctly different magnetic behaviors 1 a near primary triassic magnetization carried by hematite that is stable up to 680 c tr component 91 sites 2 a cretaceous chemical remagnetization carried partly by magnetite k component 32 sites and 3 a recent viscous magnetization that is mostly removed by 350 c site mean vectors for the tr and k components show a high degree of scatter from expected triassic and cretaceous reference directions suggesting significant tilt and rotation subsequent to magnetization acquisition restoration of tilt and folding for individual site means results in well clustered shallow and moderate inclinations for the tr and k components respectively and in variable declinations related to systematic vertical axis rotations statistical analysis of declinations for both components indicates that 75 of present day salient curvature resulted from secondary rotation and 25 of primary curvature was likely related to sedimentary basin architecture analysis of individual thrust systems indicates a slightly greater component of rotation in more internal sheets 80 compared to the frontal thrust sheets 65 suggesting that rotations were concentrated near the leading edge of the propagating fold and thrust wedge with only minor additional rotation of internal sheets transfer zones oblique ramps and more deformed overturned fold limbs display locally more complex patterns which can be understood through careful structural analysis when combined with internal strain data and regional structural relations paleomagnetic data support a kinematic model of a progressive arc with curved thrust slip paths and differential shortening that rotated early layer parallel shortening fabrics and produced minor strike parallel extension this kinematic history likely reflects a combination of processes including greater initial stratigraphic thickness and subsequent shortening and wedge propagation in the central part of the salient presence of a weak basal detachment and fault zone weakening that favored lower taper and buttressing by laramide foreland uplifts that formed along basement promontories at the north and south ends of the salient

mesoproterozoic erg and sand sheet system architecture and controlling factors galho do miguel formation se brazil abstract aeolian systems of the precambrian time were established in different palaeoclimatic and palaeogeographic conditions than the present different morphologies of dunes draas as linear oblique and star forms are widespread in the recent aeolian systems however the ancient aeolian deposits are rarely interpreted as these types of bedforms and usually referred to as transverse dunes the nature and complexity of bedform morphology in precambrian aeolian systems as well as the factors that allow the construction accumulation and preservation of these environments are still not well defined extensive exposures of galho do miguel formation in southeastern brazil represent a mesoproterozoic erg succession and exhibit a distinct facies architecture which contributed to the understanding of these deposits three architectural elements were identified i tangential cross stratified sandstone with superimposed trough cross bedded sandstone ii planar laminated sandstone and iii extensive flat bedded sandstone dominated by wrinkly laminae these elements are organized into three succession groups representative of simple transverse dune oblique compound draa and sand sheet systems the simple to compound dune draa system corresponds to the most central parts of the erg associated with a higher sediment availability and lower water table level conversely the sand sheet system occurs under low sediment availability and higher water table level in the marginal portions of the erg precambrian climatic oscillations may have changed the aeolian system morphology allowing the alternation of simple and complex aeolian dunes resulting in high frequency changes in sand sheet system and the remobilization of previously accumulated sediments over large geographic areas