a different perspective on glauconite as a chronometer for geologic time scale studies there are advocates of the use of glauconite as a chronometer for geologic time scale studies who insist that if the geohistory of the sample in question is known in detail one can ascertain before hand that the sample will yield a radiometric result that is accurate in the geological sense examples are presented indicating that while some glauconite samples may yield the correct age within their assigned analytical uncertainties when compared to high temperature minerals others clearly do not and are too young by varying degrees the outright acceptance of published results on glauconite can result in erroneous age estimates of various biostratigraphic levels and stage or epoch boundaries

determination of present day stress tensor and neotectonic interval in the spanish central system and madrid basin central spain a brittle deformation tectonic analysis was performed in central spain spanish central system and madrid basin in order to decipher and understand the deformation processes that take place in a typical intracontinental zone 1174 fault slickensides obtained in materials with ages between late cretaceous and quaternary have been analyzed by means of fault population analysis methods to reconstruct paleostress tensors nine earthquake focal mechanisms have been determined with magnitudes ranging between 3 and 4 1 with regard to regional structural features and sedimentary record data the characteristics of present day and neotectonic stress fields have been figured out which determine the neotectonic period for this region thus we have established that the intraplate zone represented by central spain has been subjected to a stress field from the middle miocene until the present day with a largest horizontal shortening direction shmax located between n130e and n160e finally three paleostress maps with the main active structures are presented for a middle miocene to late miocene the period when the spanish central system was mainly formed b late miocene to quaternary and c the present day stress field deduced from earthquake focal mechanisms

stratigraphy composition and form of the deccan basalts western ghats india in the western ghats between latitudes 18 20 n and 19 15 n 7000 km2 of deccan basalt have been mapped with the primary objective of establishing a flow stratigraphy as a guide to the volcanic history of the flood basalts using over 70 measured vertical sections major and trace element analyses of nearly 1200 samples and rare earth and87sr 86sr determinations for over 60 samples we divide the basalt into three subgroups and ten formations in this paper we describe the seven principal formations in the area and the most prominent individual flows

superposition of extensional detachments during the neogene in the internal zones of the betic cordilleras in the internal zones of the betic cordilleras extensional structures have developed from the upper oligocene to the present day they are contemporaneous with compressional structures folds and thrusts in the external zones from the upper oligocene to the aquitanian extension occurred in the malaguide alpujarride detachment and related structures show varying kinematics in different sectors younger deformations with a top to the n sense of movement have affected nevado filabride ductile shear zones alpujarride ductile and brittle shear zones and malaguide rocks normal faults at least from the late burdigalian up to the lower tortonian displacements have occurred in the alpujarride nevado filabride detachment deformations have been generally non coaxial with a top to the w sense of movement stretching lineation trends in the nevado filabride rocks curve from e to w suggesting a progressive variation of the ductile shear zone kinematics related to the alpujarride nevado filabride detachment between the aquitanian and lower tortonian stages deformations from the lower tortonian to the present day are normal faults formed in extensional settings in the upper part of the crust and folds and strike slip faults which indicate n s to nnw sse shortening directions and e w to ene wsw extension directions

a radiometric age for the cretaceous tertiary boundary based upon k ar rb sr and u pb ages of bentonites from alberta saskatchewan and montana bentonites associated with coal seams at the north american cretaceous tertiary k t boundary in the red deer valley of alberta in the frenchman valley of saskatchewan and near hell creek montana were dated by the k ar rb sr and u pb methods based upon palynological stratigraphic correlation and the presence of a sharp iridium anomaly the three sampled bentonite horizons all occur less than 1 m above the established k t boundary there is evidence of possible systematic errors giving dating results outside nominal statistical error values despite possible constant errors the weighted mean of nine average dates is calculated as 64 3 1 2 ma 2 standard deviation an unweighted averaging produces a grand mean age of 64 4 1 2 ma

global volcanism biological mass extinctions and the galactic vertical motion of the solar system summary the recent findings of nearly matching long term cyclicity in biological mass extinctions geomagnetic reversals impact cratering and other terrestrial processes have evoked a major controversy we report here a compilation and analysis of major global magmatic episodes showing a significant enhancement of volcanic activity with a periodicity of 33 million years for the last 250 million years these magmatic episodes match extremely well the best available astronomical estimates of the periodic 31 5 myr galactic disc crossing events during vertical motions of the solar system these events also have a close linkage with the marine biological mass extinctions and other geological rhythms it is argued that the prolonged volcanic activity instead of impact cratering may have been the more immediate primary cause for profound climatic and other environmental deterioration sufficient to create biological crises on a global scale the volcanic periodicity scheme is well supported by the evidence of recent increases in volcanic activity however the alternative hypothesis of the death star nemesis can be rejected since it does not explain the recent increase in volcanism and impact cratering during the last 4 3 myr in view of its quieter apehelion position lying beyond the oort cloud of comets

rift basins in western margin of india and their hydrocarbon prospects with special reference to kutch basin the western continental margin of india can be classed as a divergent or passive margin the western continental shelf is an extensive carbonate bank bombay offshore basin passing into clastic sediments on the north and south three craton margin embayed basins kutch cambay and narmada in the northern part of the shelf are filled predominantly with clastic sediments these basins occupy grabens bounded by faults diverging seaward the grabens were formed by three rift systems along major precambrian tectonic trends the rifting developed sequentially from north to south around the saurashtra horst kutch basin was formed in the early jurassic followed by cambay basin in early cretaceous time and the narmada in the late cretaceous it appears that these rifting ev nts occurred at successive stages during the northward migration of the indian plate after its break from gondwanaland in late triassic or early jurassic it is inferred that these rift basins opened up successively as a result of the counterclockwise drift of the indian craton bombay offshore and cambay are two major oil producing basins in the western margin these basins are characterized by high geothermal gradients attributed to the shallowness of the mantle in this region oil has not been found in kutch basin which is mainly an onshore mesozoic basin the basin depocenter shifted offshore at the northwestern part of the continental shelf where the shelf is wide

signals of degeneration of the sub crustal part of the indian lithosphere since the break up of gondwanaland abstract currently the continental fragments of the erstwhile gondwanaland supercontinent are widely separated a relative comparison of magsat anomalies of these fragments reveals a striking similarity across all the rifted continental margins except at the western and southern margins of india it reflects that magnetized crust has largely remained unchanged elsewhere since the break up of gondwanaland a similar analysis of satellite derived free air gravity anomalies also indicate a similar continuity across other rifted margins except those of india these anomalies characterise the existing differences in the crustal and upper mantle structures and causal dependence on the relative mobilities of the gondwanic continents it is evident that after the break up only the indian continental structure has suffered a dramatic change possibly due to its super northward drift from antarctica during late cretaceous early tertiary

striated and pitted pebbles as paleostress markers an example from the central transect of the betic cordillera se spain abstract striated and pitted pebbles provide scarce structures that preserve information on the stresses that their host rocks have undergone this information can be obtained by the measurement of a large number of microfaults with striae and solution marks within a small rock volume for non rotational deformation the statistical procedures for microfault analysis provide a valid tool for determining the overprinting of successive stress ellipsoids including their axial ratios and the orientations of the main axes the trends of compressions obtained from striae can be compared with the determinations from the pole of pebble solution pits however in complex tectonics settings the solution pits of several deformation phases are mixed and only striae analysis allows overprinted paleostresses to be accurately distinguished the analysis of several pebbles from the same outcrop including five from moderately complex settings allows determination of the homogeneity of the paleostresses at outcrop scale the detection of redeposited pebbles and supports the results of microtectonic analysis for large areas solution mark distributions on pebbles depend on the burial and tectonic stresses conglomerates from shallow levels such as those from quaternary fluvial terraces only record horizontal compressional solution marks because the minimum vertical stress needed to develop these structures are not reached by burial in the central betic cordillera striated and pitted pebbles are composed of carbonate surrounded by a matrix containing siliciclastic elements the study of several outcrops located across a transect of the cordillera shows a change in the recent stress field while conglomerates near the internal external zone boundary show extensional stresses that may be related to the uplift of the cordillera since tortonian times the outcrops located in the external zone and up to the mountain front indicate the existence of horizontal nw se and ne sw compressions related to prolate ellipsoids these two compression directions which affect conglomerates up to the quaternary in the same outcrop may be produced by a local permutation of stress axes which in general indicates nw se compression related to the eurasia africa plate boundary convergence but which locally may switch to an orthogonal compression

basin evolution and fault reactivation in the western cameros basin northern spain mesozoic cenozoic sedimentation in the cameros basin area strongly reflects tectonic factors the tectonic history is dominated by two principal extensional phases triassic late jurassic early cretaceous and strong pyrenean compression oligocene late variscan nw se and ne sw fault sets were repeatedly reactivated as dip slip and wrench faults respectively throughout extension and compression e w variscan thrust planes showed relatively minor reactivation in this area mesozoic fault control on sedimentation resulted in significant lateral variations in stratigraphic and sedimentological development variation in deformation style and thrust architecture reflects the differing responses of the laterally heterogeneous sequence to oligocene compression

constraints imposed by rift inheritance on the compressional reactivation of a hyperextended margin mapping rift domains in the north iberian margin and in the cantabrian mountains the alpine pyrenean cantabrian orogen developed along the plate boundary between iberia and europe involving the inversion of mesozoic hyperextended basins along the southern biscay margin thus this margin represents a natural laboratory to analyze the control of structural rift inheritance on the compressional reactivation of a continental margin with the aim to identify former rift domains and investigate their role during the subsequent compression we performed a structural analysis of the central and western north iberian margin based on the interpretation of seismic reflection profiles and local constraints from drill hole data seismic interpretations and published seismic velocity models enabled the development of crustal thickness maps that helped to constrain further the offshore and onshore segmentation based on all these constraints we present a rift domain map across the central and western north iberian margin as far as the adjacent western cantabrian mountains furthermore we provide a first order description of the margin segmentation resulting from its polyphase tectonic evolution the most striking result is the presence of a hyperthinned domain e g asturian basin along the central continental platform that is bounded to the north by the le danois high interpreted as a rift related continental block separating two distinctive hyperextended domains from the analysis of the rift domain map and the distribution of reactivation structures we conclude that the landward limit of the necking domain and the hyperextended domains respectively guide and localize the compressional overprint the le danois block acted as a local buttress conditioning the inversion of the asturian basin

magnetovariational study over a seismically active area in the deccan trap province of western india this paper presents the results of a magnetovariational study undertaken over a basalt covered region of western india which has been experiencing a swarm of seismic activity since early 1986 the most striking feature of the transient variations is the anomalous enhancement of the east west y component at stations bounding the seismically active belt from the nature of the horizontal transfer functions and the anomalous induction arrows it is inferred that this behaviour is related to the presence of a resistive body embedded in a north south oriented conductive belt consistent with an evolutionary model of deccan volcanism and other geophysical data it is suggested that the central resistive block coincident with the concentrated belt of earthquake epicentres represents either a dormant volcanic plug or a plutonic body rooted in upwarped conducting mantle to account for the phase relationship between the anomalous y variations and the normal north south x variations it is proposed that the upwarped asthenosphere with its associated high geothermal anomaly acts as a conducting path to channel induced current from the region of the cambay triple junction located towards the northwest of the array the existence of the strong induced currents in the region of the triple junction conforms with the induction arrow pattern observed at the array stations

analysis of a gradual change in stress regime example from the eastern iberian chain spain abstract the compression extension passage in the eastern iberian chain during the miocene is analyzed structural data suggest that it is a gradual change in stress regime which does not involve rotation but interchange of 1 and 2 axes one of them remaining always vertical the gradual stress evolution is interpreted by a numerical graphic method y r diagram which optimizes combinations of y azimuth of the maximum horizontal axis and r stress ratio explaining groups of fault movements the y r diagrams of some fault populations measured in the castellon region provide optimum tensors appearing within a continuous cloud of solutions from the compressional field to the extensional one movement sequences obtained from superimposed striations indicate a systematic stress change between these two regimes

the effects of basement ramps on deformation of the prebetics spain a combined field and analogue modelling study abstract in this study we test whether pre existing basement faults control deformation in the prebetic zones of the betic cordillera we focus on an area in the northern part of the prebetics which is dominated by a transpressional fault system consisting of a nne ssw striking fault zone and e w striking small scale thrusts field observations show that both sets of faults formed in a regional nw se to nnw sse directed compressional stress field during the miocene we propose that the faults formed on top of a basement containing basement ramps overlain by a decollement of late triassic evaporites and clays these basement ramps have either an e w or nne ssw orientation and result from extensional faulting prior to the compressional phase the style of faulting in the overlying sediments is controlled by the variation in orientation of the pre existing basement ramps in a uniform stress field where the ramps have an e w orientation small scale thrusting and folding occurs where the ramps are oriented nne ssw the fault zone acts as an oblique slip fault the model was tested by analogue tectonic experiments incorporating a ductile layer representing the decollement overlain by a brittle layer representing the overlying sediments the pre existing basement ramps were represented by rigid material the close similarity between the structures produced during the analogue experiments and those observed in the field suggests that pre existing basement faults exert a major role on deformation of the prebetics

the alpujarride nevado fibabride extensional shear zone betic cordillera se spain in the betic cordillera the initial thickening of the alboran domain produced an alpine high pressure low temperature hp lt metamorphism in the alpujarride and nevado filabride complexes after 19 ma this thickening process evolved to an extensional stage with uplift rates of between 1 and 2 mm year 1 the extensional stage generated structures in a heterogeneous shear regime with a top to the west sense of movement the present day alpujarride nevado filabride contact is a detachment fault generated in this shearing regime in the footwall the first structure developed in this extensional stage is a planar linear fabric characterized by constrictive strain that is axial planar to kilometric recumbent tight to isoclinal folds the planar linear fabric is folded by inclined close to open folds with axes parallel to the stretching lineation the planar linear fabric and folds are deformed by a late extensional crenulation cleavage near the detachment surface finally brittle deformation was generated within the same kinematic framework as that of the ductile structures in the hanging wall meanwhile deformation had a brittle character producing faults and joints

seismotectonics of the sierra albarrana area southern spain constraints for a regional model of the sierra morena guadalquivir basin limit abstract a new approach to the present day stress field of the sierra albarrana area southern spain has been inferred from the application of fault population analysis fpa to a set of fault striae pairs and a short sample of earthquakes and from the simultaneous determination of stress tensor and fault plane solutions for this last set the first population of data concerned 226 fault and striae pairs obtained in 8 field outcrops in their turn seismic data correspond to 45 polarities of p wave arrival and take off angles for a set of source station pairs these data were obtained in a microseismicity survey of 30 months carried out in an area of 50 40 km 2 four fpa methods have been chosen the slip model the right dihedral method a 3d random search of the stress tensor and the stress inversion method in addition fault planes have been selected by using the symmetries established in the slip model and the methodology of fpa has also been applied to seismic data in this way it has been possible to take advantage of two sources of information too small to be considered independently the use of complementary methods based on the fpa has allowed the possible solutions for the fault planes to be constrained and has yielded a picture of the tectonic evolution of the area the analysis of the seismic information and the comparison of the results obtained independently with fpa methods provide a reliable interpretation of the present day stress distribution in the zone the regional stress tensor shows a compression close to uniaxial with its compressive axis y or hmax oriented n132e and plunging 0 19 focal mechanisms correspond to either reverse or normal faults some of them with a noticeable strike slip component the reverse faults show a nw dip and are deeper than the normal ones a regional deformation model including a crustal flexure with axis parallel to guadalquivir river has also been sketched

diagenetic crystallization and oxidation of siderite in red bed buntsandstein sediments from the central iberian chain spain abstract compactional deformation facilitated replacement of dolomite and calcite by siderite and its subsequent oxidation in carbonate cemented red beds of the triassic buntsandstein in the iberian chain locally the sedimentary clasts were cemented by carbonate that was derived from dissolution of locally exposed dolomite in the basement microstructures indicate that during sedimentation of the rocks oxidizing conditions prevailed in the sediments and the basement was reddened by impregnation of hematite reducing conditions prevailed during deformation of the sediments ferric iron was reduced to fe 2 that reacted with deformed dolomite and calcite cement to produce fine grained siderite at a later stage siderite crystallites were partly oxidized to form a secondary phase of brown ferric oxide goethite locally goethite transformed to fine grained hematite that caused secondary reddening of the sediments the reactions are associated with a combined volume loss of the solid phases of c 50 per reaction mol this was accommodated by the formation of pores oxidation of siderite was associated with release of co 2 localized dissolution took place of feldspar and concurrently growth of kaolinite occurred by acidifying condition during release of co 2 the relation of redox reactions and deformation is comparable to those in red bed conglomerates in the region reductive dissolution occurred at sites of stress concentration particularly at contact points of pebbles late stage precipitation of ferric oxides and pyrolusite took place at oxidizing conditions in association with uplift

interaction of two successive alpine deformation fronts constraints from low temperature thermochronology and structural mapping nw iberian peninsula the lateral termination of the alpine pyrenean orogen relief onshore is located in the nw iberian peninsula it overlies a variscan basement iberian massif where the sedimentary record of the alpine tectonic is very scarce thus the characterisation of the tectonic evolution of the lateral termination is difficult and timing and geometries of the main tectonic structures remain unclear combining the tectonothermal histories obtained by modelling of the apatite fission track data aft with structural mapping allows for a comparative study of the different tectonic scenarios and deformation transfer in the lateral termination of an orogen aft ages for the studied area vary from 53 5 12 9 and 222 12 ma from late triassic to early eocene the beginning of the cenozoic cooling episodes is in agreement with the infilling of the tertiary basins late eocene or oligocene calculated uplift for the alpine orogeny is around 2 400 m the cantabrian mountains were uplifted and emplaced southwards and the main period of exhumation began in the palaeogene at rates of 0 02 mm a and continued during the neogene at rates of 0 06 mm a however the galaico leoneses mountains located to the south of the studied area were uplifted and emplaced northwards during the neogene showing more rapid uplift rates of 0 08 mm a suggesting that the western termination of the alpine pyrenean orogen relief is the result of the successive interaction of two alpine deformation fronts

a search for iridium in the deccan traps and inter traps it has been suggested that flood basalts in the deccan india might be associated with events at the cretaceous tertiary boundary ktb a search for iridium in 47 samples from lava flows and inter trap sediments in the deccan yields negative results concentrations are not statistically different from zero with a minimum detection level on the order of 0 1 ng g 1 ppb this does not help to constrain the debate on the internal vs external origin of ktb boundary events since for instance ir could have been outgassed from the magma and original concentrations of only 0 05 ng g 1 prior to outgassing are sufficient to account for the worldwide ir excess mass at the ktb

inversion of moment tensor focal mechanisms for active stresses around the microcontinent iberia tectonic implications the iberian microcontinent and its connected oceanic crust are affected by deformations related to the eurasian african plate boundary active stress inversions from populations of moment tensor focal mechanisms have been performed around and inside the iberian peninsula using a total of 213 moment tensor estimates main results are 1 the tensorial solutions show better consistency and lower misfits compared to those obtained previously from first p arrival focal mechanisms 2 along the eurasia africa western boundary the type of active stresses progressively changes easternwards from triaxial extension to uniaxial compression along the terceira ridge the gloria fault zone and the gulf of cadiz 3 in the betics alboran rif zone uniaxial extension predominates with shmax n155oe trending 4 in n algeria uniaxial compression reappears 5 the iberian foreland is currently under strike slip to uniaxial extension tensorial conditions

mesozoic extensional tectonics in the southeast iberian chain the desert de les palmes area in the southeast iberian chain belongs to a mesozoic ne sw high which separated the early cretaceous basins of the maestrat and aliaga penyagolosa from the little orpesa basin its structure is characterized by the development of a system of ne sw to ene wsw extensional listric faults detached in a shallow upper crustal level 1 7 2 2 km mostly affecting the pre upper cretaceous rocks these faults record two well differentiated rifting periods 1 a first late triassic early jurassic rifting period that divided the desert de les palmes high in several blocks 2 a second early cretaceous rifting period only developed in the eastern margin of the desert de les palmes high which was related to the opening of the maestrat aliaga penyagolosa and orpesa basins based on the comparison of the main features of this mesozoic structure with an analysis of the structural and subsidence data already known in the neighbouring mesozoic basins maestrat aliaga penyagolosa and columbrets a geodynamic scenario for the crustal evolution of the eastern iberian chain is also suggested this involves four evolutionary stages 1 triassic rift late permian hettangian 2 early and middle jurassic postrift sinemurian oxfordian 3 late jurassic and early cretaceous rift kimmeridgian middle albian which includes a short hauterivian postrift period and 4 late cretaceous postrift late albian maastrichtian

stress and palaeostress in the betic rif cordilleras miocene to the present palaeostress orientation in the betic rif cordilleras can be determined for the miocene to the quaternary by analysis of brittle microstructures faults joints striated pebbles and stylolites in the central and northern south iberian domain 1 was subhorizontal and trended nw se from the early miocene to the present in the northern alboran domain from the burdigalian to the serravallian palaeostress ellipsoids were oblate with 3 subhorizontal and trending ene wsw in the southern part of the south iberian domain and in the northern part of the alboran domain betic cordillera from the tortonian to the quaternary palaeostress ellipsoids were variable in the granada region they were prolate with 1 subvertical but in the almeria region they were triaxial with 2 subvertical and 3 subhorizontal and trending ne sw to e w in the rif the alboran and african maghrebian domains were affected by palaeostresses with 1 subhorizontal varying from ne sw in tortonian times to n s in the plio quaternary a study of earthquake focal mechanisms by the right dihedra method indicates that the present day general stress field around the betic rif cordilleras is compressional with 1 subhorizontal and trending nw se in the betic rif cordilleras this general stress field is modified and the present day orientations of 1 2 and 3 are nearly the same as those of the palaeostresses in the pliocene and quaternary the distribution and evolution of stresses in the betic rif cordilleras from the miocene to the present can be correlated with the main neogene deformations extensional structures in the central and northern alboran domain and the southern part of the south iberian domain and compressional structures in the south iberian and african maghrebian domains and in the southern part of the alboran domain

insights into the tectonic topography of the present day landscape of the central iberian peninsula spain abstract the landscape of today s central iberian peninsula has been shaped by ongoing tectonic activity since the tertiary this landscape comprises a mountain ridge trending e w to nw se the central system separating two regions of smooth topography the basins of the rivers duero and tajo in this study we explore interrelationships between topography and tectonics in the central iberian peninsula regional landscape features were analysed using a digital elevation model dem slope gradients and slope orientations derived from the dem were combined to describe topographic surface roughness topography trend surfaces inferred from harmonic analysis were used to define regional topographic features low roughness emphasizes the smooth nature of the basins topography where surfaces of homogeneous slope gradient and orientation dominate high roughness was associated with abrupt changes in gradient and slope orientation such as those affecting crests valley bottoms and scarp edges present in the mountain chain and in some deep incised valleys in the basins one of the applications of roughness mapping was its capacity to isolate incised valley segments the area distribution of incised rivers shows their prevalence in the east on a regional scale the topographic surface can be described as a train of ne sw undulations or waves of 20 km wavelength these undulations undergo changes in direction and interruptions limited by n s trending breaks e w and ne sw troughs and ridges clearly mark structural uplifts and depressions within the central system these structures are transverse to the compressive nw se stress field that controlled the deformation of the central iberian peninsula from the neogene to the present they represent the upper crustal folding that accommodates alpine shortening n s breaks coincide with late miocene faults that control the basins sedimentation further associated palaeoseismic structures suggest the recent tectonic activity of n s faults in the eastern part of the tajo basin apatite fission track analysis data for this area suggest the occurrence of a significant uplift episode from 7 to 10 ma which induced the river incisions appearing in the roughness map n s and ne sw faults could be seismogenic sources for the current moderate to low seismic activity of the east tajo basin and southeast central system although n s fault activity has already been established we propose its significant contribution to shaping the landscape

crustal structure along mehmadabad billimora profile in the cambay basin india from deep seismic soundings abstract deep seismic sounding dss investigations were carried out along a 220 km long n s profile from mehmadabad 22 50 n 72 46 e to billimora 20 46 n 72 58 e in the cambay basin india for studying the detailed crustal structure in the region seismic refraction as well as narrow and wide angle reflection travel time data were obtained for various crustal boundaries starting from the sedimentary section down to the moho by continuous profiling analysis of reversed reflection and refraction travel time data has yielded crustal velocity depth functions for the mehmadabad broach and broach navsari sections along the profile which are significantly different from each other the average velocities at depths in the magmadabad broach section have been found to be considerably lower than those in the broach navsari section by about 25 at 5 km 17 at 10 km 7 at 20 km and 2 at the moho depth and even deeper down in the upper mantle the presence of a relatively thicker sedimentary section especially in the jambusar broach area as revealed by the reversed refraction data analysis is responsible for this gross difference in the average velocity functions for the two parts of the profile and the same have therefore been used for obtaining the detailed crustal depth along the profile the cambay basin which is well known to be bounded by step faults on the eastern and western margins is also dissected into seven major crustal blocks in the n s direction from mehmadabad to billimora the various crustal blocks identified are from the north 1 i mehmadabad block 2 ii tarapur block 3 iii petlad katana block 4 iv jambusar broach block 5 v kosamba block 6 vi surat block and 7 vii navsari billimora block these blocks are displaced up or down relative to the adjacent blocks due to movements along the faults bounding them most of the faults have been found to extend quite deep and to affect the moho boundary considerably as is quite evident from the relief of the moho found along the profile further the seismic activity observed in the region during historical as well as modern times suggests that these deep faults may even be currently active the jambusar broach block which is well known as a broad regional syncline with the maximum thickness of cenozoic sediments has been found to be a major graben revealing a maximum crustal thickness of 38 40 km the granitic basement in this graben is also the deepest at 6 0 6 5 km depth towards the north and south of this major graben the moho as well as the granitic basement are found to be at relatively shallower depths in various crustal blocks the boundary velocity at the moho has been found to be 8 1 8 2 km sec whereas at the granitic basement level there seems to be a lateral velocity variation from 5 7 to 6 3 km sec the crustal depth section obtained in the present study reveals a close relationship between the deep and shallow structural features in this region the deccan traps which form the basement of the overlying tertiary sediments over most part of the profile vary in thickness from 1 0 to 1 8 km however the trap surface rises to very shallow depths on the southern part of the profile the boundary velocity in the deccan traps has been found to vary between 4 8 and 5 2 km sec in the anklesvar kosamba area there is also evidence for the presence of a 1 2 km thick layer of mesozoic sediments with a velocity of about 4 0 km sec underlying the 1 1 km thick deccan traps layer

a new theory of the origin and evolution of the deccan traps india abstract the deccan traps india represent one of the largest continental flood basalts and its origin and phases of evolution have remained puzzling for the earth science community an integrated analysis of available geochronological geochemical and geophysical data of post mesozoic volcanism clearly rejects the dominating theory of origin from a hot spot e g la reunion at about 23 s 55 e in the indian ocean the deccan traps show imprints of three events belonging to a worldwide peak magmatism series matching with a galactic catastrophism cyclicity of 33 m y study further reveals that the deccan magmas were not directly derived by the partial melting of the deep mantle but appear to have their origin at a depth of less than 40 km as a consequence of crustal rifting and secondary melting caused by the rise of geoisotherms due to frictional heating generated at the lithospheric asthenospheric boundary during india s fast south north journey from antarctica

topo europe studying continental topography and deep earth surface processes in 4d abstract topo europe is a multidisciplinary international research program that addresses the interaction of processes inherent to the deep earth lithosphere mantle with surface processes erosion climate sea level which together shaped the topography of europe the objective of this programme is to assess neotectonic deformation rates and to quantify related geo risks such as earthquakes flooding landslides rock falls and volcanism research incorporating data interactive modeling focuses on the lithosphere memory and neotectonics with special attention on the thermo mechanical structure of the lithosphere mechanisms controlling large scale plate boundary and intraplate deformations anomalous subsidence and uplift and links with surface processes and topography evolution the topo europe natural laboratories in which these processes are analyzed cover a wide range of geodynamic settings these include the post collisional alpine carpathian pannonian basin system the very active aegean anatolian and apennines tyrrhenian orogens and back arc basins the caucasus levant area in the arabia europe collision zone the iberian peninsula caught up between alpine orogens the meta stable west and central european platform the stable east european platform and the seismically active and elevated scandinavian continental margin the topo europe project is a component of the international lithosphere program a european science foundation eurocores project provides funding for part of the research for a total amount of 13 5 m euro creating new research positions for 60 young researchers

stress pattern in portugal mainland and the adjacent atlantic region west iberia the portuguese mainland territory is located close to the azores gibraltar plate boundary in a tectonic setting responsible for significant neotectonic and seismic activities however few data concerning the present regional lithospheric stress field were available as testified by recently published maps of stress indicators for the europe and mediterranean regions one of the authors already presented a synthesis on this subject cabral 1993 where geological and geophysical stress indicators were considered in this paper we introduce new information mainly a considerable amount of borehole breakout data the updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145 standard deviation 21 for the maximum horizontal stress direction shmax on the average the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth while the borehole elongation results are consistent with the mean shmax trend these differences in stress trend suggest a regional progressive rotation of the shmax direction from nnw sse to wnw ese since the upper pliocene to estimate stress trajectories new and published stress indicators in the adjacent atlantic area and northern africa were also investigated showing a very uniform nw se shmax trend in west iberia a high level of horizontal compressive stress acting oblique to the western portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model of activation of this passive margin with the nucleation of a subduction zone in the atlantic sw of iberia at the gorringe submarine bank which is propagating northward along the base of the continental slope at the transition between thinned and normal continental crust

origin of the permian triassic iberian basin central eastern spain abstract the iberian basin was an intracratonic rift basin in central eastern spain developed since early permian times the basin boundary faults were normal listric faults controlling an asymmetric extension propagating northeast with time hercynian or older lineaments controlled the orientation of the iberian basin and extension was accommodated basically in the hanging wall block by the formation of secondary grabens and a central high the basin was related with the coeval ebro catalan and cuenca mancha basins and their connections are discussed subsidence curves show that the early permian early jurassic period of extension can be subdivided into three rifting episodes and a flexural one extension factor increases from 1 17 in the northwest to 1 29 near the mediterranean coast the increasing extension rate was accommodated by transfer faults trending nne ssw more important in the levante area the rift evolution is intermittent and seems to reflect distinct stress fields the collapse of the late hercynian orogen and related increased heat flux extension and rifting is the most probable origin of the iberian basin and related basins the origin of the catalan and the valencia prebetic basins is related to the southwards migration of the hesse burgundy rift along the eastern margin of the iberian microplate

a review of alpine tectonics in portugal foreland detachment in basement and cover rocks abstract the alpine foreland in portugal was deformed by compressional tectonism during the miocene in the nne ssw oriented lusitanian basin most folds and thrusts in the meso cenozoic cover are oriented ene wsw parallel to the alpine front in the betic cordillera and verge towards the north northwest and south southeast the thrusts are connected by lateral ramps most of these are oriented nne ssw to n s and show sinistral movement and some are transpressional the lateral ramps result from reactivation of older extensional faults related to crustal thinning of the continental margin in the e w oriented algarve basin a simpler basin inversion occurred with older e w normal faults reactivated as essentially pure thrusts in both basins alpine structures formed above decollements in the hettangian evaporite clastic complex variscan basement was also deformed by ene wsw reverse faults during miocene time the similarity in orientation and style of the basement structures to those in the cover suggests that they also occurred by detachment but their larger scale indicates that the detachment is deep and involves much of the crust thus we interpret the central cordillera in which basement rocks are thrust over miocene sediments on both sides as a pop up of crustal scale elevated above downward flattening faults that dip towards each other and merge into a single deep detachment alpine structures in the iberian foreland are therefore similar in structural style to those of the appalachian and laramide forelands of north america and the alpine foreland of northwest europe

palaeostress evolution of the iberian peninsula late carboniferous to present day abstract three main stages can be differentiated in iberia from the upper carboniferous to the present the end of the variscan orogeny late carboniferous the permian early cretaceous rifting and the alpine orogeny late cretaceous to present in this contribution the main features of the stress fields determined in each of these stages are compiled and revised from the analysis of the brittle deformations mainly microfault population stresses are related to the development of the principal structures of each stage in the late variscan fracturing stage sinistral strike slip faults prevailed in a stress field with ne sw subhorizontal compression in the permian to early cretaceous rifting stage the extensional trends varied in each sector during the alpine orogeny primarily the margins of the iberian massif the betic and pyrenee mountain chains were deformed however numerous deformations are also identified in its interior where large mountain chains such as the central system and the iberian chain developed these deformations occurred in a complex stress field where zones of nw se to nne ssw horizontal compression prevailed albeit predominantly extensional zones existed as well at present the stress field in most of iberia has a nw se compressive trend associated with active deformations in the betic cordillera where a continental subduction developed however simultaneously there are zones in which extensional settings predominate such as the internal zones of the betic cordillera and the gulf of valencia region making for quite a complex current stress field

geomagnetic reversals from impacts on the earth the impact of a large extraterrestrial object on the earth can produce a geomagnetic reversal through the following mechanism dust from the impact crater and soot from fires trigger a climate change and the beginning of a little ice age the redistribution of water near the equator to ice at high latitudes alters the rotation rate of the crust and mantle of the earth if the sea level change is sufficiently large 10 meters and rapid in a few hundred years then the velocity shear in the liquid core disrupts the convective cells that drive the dynamo the new convective cells that subsequently form distort and tangle the previous field reducing the dipole component near to zero while increasing the energy in multipole components eventually a dipole is rebuilt by dynamo action and the event is seen either as a geomagnetic reversal or as an excursion sudden climate changes from other causes such as volcanic eruptions could also trigger reversals this mechanism may not be the sole cause of geomagnetic reversals but it can account for the rapid drop of the dipole component preceding a reversal the predominance of multipole components during a transition the associations of microtektites temperature drops and extinctions with reversals and the possible correlation between peaks in the geomagnetic reversal rate and the times of mass extinctions the model may also account for the long term changes in the average rate of reversals we make several testable predictions

super mobility of hot indian lithosphere negi j g pandey o p and agrawal p k 1986 super mobility of hot indian lithosphere tectonoph vsics 131 147 156 the great mobility of the indian subcontinent during the last 180 million years 9000 km of south north motion with an anticlockwise rotation of about 60 o has long been a major puzzle for theorists of plate tectonics a relative analysis of the available heat flow data the estimated temperature depth regime of india and the gravity anomalies reveals that the indian lithosphere does not have the characteristics of a typical shield zone and that it has higher radioactivity smaller viscous drag decidedly low viscosity of 10 p lower density and a well defined low velocity zone as compared to other shield areas of the world these factors appear to contribute significantly to the faster northward movement of the thin indian lithosphere as compared to the almost immobile african plate

bombay gravity high and eruption of deccan flood basalts india from a shallow secondary plume abstract the western margin of india especially the area surrounding the gravity high of bombay is covered by a thick pile of deccan flood basalts the massive eruption is one of the largest continental volcanic events at the cretaceous tertiary biological mass extinction boundary 65 ma ago the area is characterized by several hidden tectonic features and their origin and evolution have remained controversial in the search for magma source structure attention was focused on an unusual oval shaped positive gravity anomaly over the bombay coastal zone the gravity analysis indicates the presence of two subsurface structures at depths of 4 5 0 5 km and 18 2 km these depths correspond to the top of the crustal basaltic layer and an upwarped moho discontinuity respectively the positive gravity anomaly in three dimensional analysis reveals a possible magma conduit structure and the near absence of the granitic layer the unusually shallow moho layer coincides with the region of a broad high heat flow anomaly average heat flow more than 80 mw m2 the geophysical signatures from the neighborhood show that the asthenosphere has welled up considerably and is located at a depth of about 40 km high heat flow and asthenospheric depth estimates suggest melting conditions at very shallow depths it is suggested that the highly differentiated magma associated with the major eruptive phase of deccan traps did not originate at the d layer it is hypothesized that magma was brought to the surface by a major secondary plume at a much shallower depth within the lithosphere and possibly at the crust mantle boundary as a consequence of subcrustal erosion and secondary melting caused by the extreme velocity of india during the late cretaceous and early tertiary substantial melting of the base of the lithosphere due to frictional heating might have resulted in the accumulation of a large quantity of magma below the thinned lithosphere at the pre existing fracture zone near the continental margin the presence of a magmatic conduit structure above the moho derived from the inversion of gravity data and other tectonic conditions suggests a possible triggering of the magma reservoir by a large bolide impact 10 km diameter at the cretaceous tertiary boundary near bombay

a subsurface stress analysis and its possible relation with seismicity near the itoiz reservoir navarra northern spain abstract we study the effects of surface water loads and moderate magnitude earthquakes on the subsurface state of stress and its relation with seismicity near the newly constructed itoiz reservoir in the western pyrenees northern spain to do this we initially compute the evolution of the stress changes in the subsoil due to the water load distribution during the impounding of the reservoir and relate it with the main seismicity occurred after the beginning of impoundment after this we compute the coseismic and postseismic stress changes produced by the main events of the seismic series and study its influence on the triggering of the aftershocks stress changes due to the water loads are computed by means of the boussinesq solution for a homogeneous elastic half space whereas the coseismic stress changes due to earthquakes are obtained using the equations for an internal elastic dislocation in a half space the effects over the possible seismogenic faults are in both cases accounted by means of the coulomb failure stress criterion cfs we considered different threshold values and apparent friction coefficients of cfs for the effective static triggering then we calculate the pore pressure changes produced by the time evolution of the surface water loads as also the pore pressure changes generated by the elastic dislocations due to the largest earthquake of the series mainshock the fault parameters for the computation of the cfs of the perturbing seismic events were selected from published focal solutions to define the target fault parameters we took into account the focal solutions of the earthquakes and aftershocks themselves the parameters of mapped faults based on the tectonic setting and those of optimally oriented faults due to the absence of reliable estimations for the slip distributions of the main earthquakes we assumed for stress computations a semi elliptical distribution of slips over the fault results show the existence of a mainly positive stress influence of the surface water loads on the main earthquake of the series in two different senses the elastic coulomb failure stress and the change in pore pressure however due to different uncertainties on the possible values of the apparent coefficient of friction for the zone there is not a clear indication of an effective earthquake triggering of the mainshock due to the elastic stress changes by the surface water loads on the other hand a more clear effect is found on the possible triggering of aftershocks due to the largest earthquakes of the series it is also found a positive pore pressure influence of the two largest events on most of the aftershocks

evaluation of reservoir temperatures and local utilization of geothermal waters of the konkan coast india abstract geochemical studies on fifteen geothermal manifestations 38 70 c from the konkan coast geothermal province of india have been used to evaluate the reservoir temperatures activity studies of the minerals and the waters present in the reservoirs suggest that the thermal waters are in equilibrium with montmorillonite kaolinite and quartz at about 100 c reservoir temperatures of these geothermal systems as estimated by geochemical thermometers are 88 to 128 c and thus too low for economic electricity production

crustal structure of the north iberian continental margin from seismic refraction wide angle reflection profiles abstract the structure and geodynamics of the southern margin of the bay of biscay have been investigated from a set of 11 multichannel seismic reflection profiles recorded also at wide angle offsets in an onshore offshore network of 24 obs obh and 46 land sites this contribution focuses on the analysis of the wide angle reflection refraction data along representative profiles the results document strong lateral variations of the crustal structure along the margin and provide an extensive test of the crustal models previously proposed for the northern part of the iberian peninsula offshore the crust has a typical continental structure in the eastern tip of the bay which disappears smoothly towards the nw to reach crustal thickness close to 10 km at the edge of the studied area 45 n 6 w the analysis of the velocity depth profiles altogether with additional information provided by the multichannel seismic data and magnetic surveys led to the conclusion that the crust in this part of the bay should be interpreted as transitional from continental to oceanic typical oceanic crust has not been imaged in the investigated area onshore the new results are in good agreement with previous results and document the indentation of the bay of biscay crust into the iberian crust forcing its subduction to the north the interpreted profiles show that the extent of the southward indentation is not uniform with an alpine root less developed in the central and western sector of the basque cantabrian basin n s to ne sw transfer structures seem to control those variations in the indentation degree

terrestrial maria the origins of large basalt plateaus hotspot tracks and spreading ridges large lava plateaus which form abruptly within plates without apparent tectonic cause appear to be the terrestrial equivalents of lunar maria deep erosion of lava plateaus reveals that their magma chambers are gabbro and granophyre complexes the classic lopoliths the close association of the deccan plateau with the timing of the terminal cretaceous boundary clay suggests that it is an impact crater large enough to cause pressure relief melting in the asthenosphere basalt then flooded the crater to form a lava lake the terrestrial equivalent of a lunar mare part of the crater rim survives in the amirante ridge the carlsberg oceanic ridge and chagos laccadive hotspot track formed simultaneously with the deccan plateau several other mesozoic and cenozoic lava plateaus are parts of similar arrays of simultaneous features one of those is the southern oregon part of the columbia plateau with its associated continental rifting in the northern basin and range and the yellowstone hotspot track along t

palaeostresses associated with thrust sheet emplacement and related folding in the southern central pyrenees huesca spain brittle mesostructures have been studied in the detached mesozoic and lower tertiary cover rocks of the south central pyrenees huesca province spain in order to reconstruct the state of stress within the thrust sheets during tertiary compression the structure of the area studied consists of nw se to e w striking south verging thrust sheets located west of the south pyrenean central unit the palaeostresses obtained show dominant n s to ene wsw compression directions for stress tensors with horizontal 1 axes and e w to nw se extension directions for stress tensors with vertical l axes the values of the stress ellipsoid ratios for stress tensors with horizontal 1 are predominantly between 0 and 0 5 indicating uniaxial compression to wrench regime regarding their relationship with macro structures the palaeostresses show transport parallel compression in most sites although thrust parallel compression is also found there are no differences between palaeostresses found in the hanging walls and the footwalls of thrusts stress tensors with vertical 1 do not show systematic relations with macro structures but transport parallel extension and thrust parallel extension are found in some sites the analysis of palaeostresses in areas of fold and thrust belts shows a high degree of complexity and the results are not easy to interpret the continuous variation from n s to ene wsw in the orientation of the compression axes is here interpreted in two ways 1 as the result of the clockwise rotation of the mesozoic and tertiary cover which was pushed in the east by the south pyrenean central unit with a constant n to nne regional compression and 2 due to ne sw compression resulting from a sw directed local transport of minor thrusts palaeostress analysis can therefore reveal important information about thrust kinematics at high structural levels where ductile deformation is absent

effects of the pyrenean collision on the paleocene stratigraphic evolution of the southern iberian margin southeast spain rapid sea level changes are generally interpreted in terms of glacio eustasy but they can also be explained by the lithospheric motions associated with changes in intra plate stress in eastern spain latest senonian paleocene time was characterized by a wide spread regression which culminated in the emergence of large parts of the carbonate platform new data indicate that the platform emergence was accompanied by slope faulting and a sudden appearance of olistostromes and sandy turbidites at the southern iberian margin of southeast spain these changes in basin geometry are the result of a tectonic event which is dated as middle paleocene ca 60 ma this tectonic event can be correlated with the onset of collision in the pyrenees regional geologic data and paleostress measurements provide evidence for far field transmission of the pyrenean compressive stresses through the iberian lithosphere it is concluded that the platform emergence and slope steepening at the southern iberian margin reflect the vertical lithospheric motions of a basin flank subjected to compression

active continental subduction beneath the betic cordillera and the alboran sea p and s wave seismic tomography detect a low velocity anomaly in the upper mantle beneath the betic cordillera and the alboran sea region the anomaly is associated with the intermediate depth seismicity h

seismic activity at the western pyrenean edge abstract the present day seismicity at the westernmost part of the pyrenean domain reported from permanent networks is of low to moderate magnitude however it is poorly constrained due to the scarce station coverage of the area we present new seismic data collected from a temporary network deployed there for 17 months that provides an enhanced image of the seismic activity and its tectonic implications our results delineate the westward continuity of the e w pyrenean band of seismicity through the variscan basque massifs along the leiza fault ending up at the hendaya fault this seismicity belt is distributed on a crustal scale dipping northward to almost 30 km depth other relevant seismic events located in the area can be related to the central segment of the pamplona fault and to different e w thrust structures

late cretaceous and paroxysmal cretaceous tertiary extinctions the various geological signatures at cretaceous tertiary time including iridium and other associated elements microspherules and shock deformation features are compatible with the suggestion that the transition is marked by a period of intense volcanism the volatile emissions from this volcanism would lead to acid rain reduction in the alkalinity and ph of the surface ocean global atmospheric temperature changes and ozone layer depletion these environmental effects coupled with those related to the major sea level regression of the late cretaceous provide the framework for an explanation of the selective nature of the observed extinction record

crustal deformation and submarine canyon incision in a meso cenozoic first order transfer zone sw iberia north atlantic ocean using a dense grid of high quality 2d seismic profiles dredge and outcrop data the offshore prolongation of a first order transfer zone the messejana plasencia fault zone mpfz is explained within the context of oblique rifting between southwest iberia newfoundland and west tethys the offshore mpfz is shown to comprise a 5 10 km wide region of deformation oblique to the continental margin reactivated in the mesozoic as part of a wider transcurrent domain the sao vicente sub basin here the geometry of faults and strata denotes the generation of a pull apart basin during the jurassic early cretaceous in contrast its late cretaceous to cenozoic evolution favours left lateral transpression during the counter clockwise rotation and eastward migration of iberia towards its present position erosion was subsequently enhanced in the sao vicente sub basin due to the reactivation of syn rift structures by documenting the evolution of the sao vicente sub basin and adjacent mpfz this work demonstrates 1 the temporal and spatial scales in which first order transfer zones accommodate crustal movements during continental rifting and subsequent inversion episodes 2 the generation of an extensive region of strain accommodation in the vicinity of the mpfz an observation with profound implications to future palaeogeographic reconstructions of the north atlantic ocean and 3 that the sao vicente canyon the physiographic expression of the mpfz incised the margin as early as the latest cretaceous paleocene synchronously with the onset of tectonic uplift in southwest iberia in such a setting the sao vicente sub basin and mpfz formed important by pass corridors for sediment sourced from proximal areas of the margin at present the mpfz comprises a complex releasing restraining bend accommodating important vertical and horizontal movements in southwest iberia based on earthquake data from similar transfer zones the mpfz should be able to generate large magnitude earthquakes and potentially destructive tsunamis

palaeogene evolution of deformation in the northeastern iberian peninsula during the palaeogene alpine compression three units were differentiated in the studied area south of the ebro basin the catalan coastal range dominated by ne sw major basement faults with a sinistral movement the iberian range where the important basement faults have a nw se direction and a reverse movement often with a dextral component the linking zone between these two ranges where an e w dominant structural direction is marked by an array of folds and thrusts with a northward vergence in the mesozoic cover from the analysis of both these major structures and the small scale structures it can be deduced that the compression in the studied area has evolved from a nw se direction lower middle eocene to a n s direction and to a ne sw one uppermost oligocene the major structures are due to the n s compression later the stress field progressively changed to a distensive regime we suggest a relation between the compressive phases and the displacement direction of the iberian plate with reference to the european plate

stress field and thrust kinematics a model for the tectonic inversion of the cameros massif spain abstract alpine tectonics of the cameros thrust has been studied from several points of view geometry surface and sub surface structure three dimensional outline of the thrust kinematics transport direction and dynamics characterization of the regional compressional stress field all three aspects are related by means of a model based on bott s equation under a n to nne horizontal compression with an average stress ratio r z x y x 0 2 the eastern ramp of the cameros thrust oriented 155 30 w underwent a strike slip right lateral movement with a small reverse component this gave rise to rigid displacement of the cameros block towards the nnw and produced a guided movement on the western frontal ramp oriented 060 13 s

stress deflection in a tectonic compressional field a model for the northwestern iberian chain spain by means of fault population analysis using the right dihedra y r diagram and etchecopar s methods the states of stress in a number of sites of the cameras massif northwestern iberian chain spain and nearby areas of the ebro basin have been determined these states of stress have been used as a framework in order to infer a model of the actual stress field in that area during the main stage of the alpine compression oligocene lower miocene the three main compression directions obtained nw se nne ne sw can be explained by a single compression field with horizontal 1 trending nne and vertical 2 whose primary trajectories are deflected due to 1 large scale strike slip faults or lateral ramps of thrusts or 2 frontal ramps of thrusts oblique to the main compressive axis the reliability of this scheme has been checked by means of a two dimensional mathematical model based upon the finite element method and applied on a sheet whose rheological properties are similar to those of a standard limestone this model simulates the main features of the regional stress field and it was constructed after determining the average parameters of the possible external or primary field and simulating the major faults that could deflect the stress trajectories

areal balancing and estimate of areal reduction in a thin skinned fold and thrust belt cantabrian zone nw spain constraints on its emplacement mechanism abstract the cantabrian zone is a curved thrust and fold belt forming the frontal part of the hercynian orogen in northwestern spain the present day structure is the result of interference between different kinds of structures describing the arc and a set of cross folds showing a radial pattern some of the structures can be traced all around the arc while some others are of limited lateral extent so that the displacement they produce is transferred to some other kind of structure taking into account geometrical data and field evidence on the direction of thrust motion an area balanced model has been obtained this model permits the restoration of the cantabrian zone to its pre deformed stage and shows that the areal reduction has been about 50 globally considered the structure of the cantabrian zone corresponds to a detached and deformed wedge of sediments which was being thrusted and folded as it was being subjected to translation the wedge before deformation was flat topped and had a basal backslope but as it was being deformed a surface foreslope was generated slope and thickness evaluations fulfil the required theoretical conditions for the decollement to be possible under compressional conditions lack of tectonic denudation at the back persistence of the basal backslope and the general structure of the belt indicate compression and crustal shortening only in the most frontal part of the belt gravitational gliding of rock slabs took place and chaotic mixtures were formed contributing to the infilling of the carboniferous basin in the very core of the arc

the betic segment of the lithospheric trans alboran shear zone during the late miocene abstract the betic sinistral transcurrent shear zone betic cordilleras crosses the southeastern part of spain the study of the geodynamics of this area is approached from tectonic sedimentary magmatic and geophysical data large strike slip movements tectono sedimentary processes and magmatic events are seen at the boundary between two different lithospheric slabs they are located in a thinned compartment with a high geothermal gradient the large scale movements vertical and horizontal and the intensity of magmatic activity are the expression of the mechanical behaviour of this hot and thin crust all the geological events from the late miocene to recent are integrated in a crustal scale cross section which shows that the betic sinistral transcurrent shear zone widely affects the lithosphere it appears that subduction zones are not the only geodynamic context in which calc alkaline magmatism occurs

fold trends and stress deviation in the alicante fold belt southeastern spain abstract palaeostress orientations derived from analysis of stylolites and small scale faults can be correlated with plate movements however stress deflections up to the kilometre scale occur at large scale faults the regional fold trend in the eastern pre betic alicante region is east northeast which is characteristic of the betic cordilleras in the southern part of the alicante region an anomalous n s fold trend dominates biostratigraphic data and regional structural analysis show no clear difference in the age of the two fold trends although dome and basin structures suggest overprinting the n s folds are interpreted as en echelon folds in a right lateral wrench zone palaeostress analysis supports this interpretation the e w compression which caused the n s folds is a local fault related deviation of the regional betic nw se compression

intraplate paleostresses reconstructed with calcite twinning and faulting improved method and application to the eastern paris basin lorraine france this study deals with the reconstruction of intraplate paleostresses calcite twinning inversion provides paleostress directions and differential values even in poorly outcropping areas however existing methods are not well adapted to multiphase deformation cases and the calculation of stress values provides unreliable estimates modifications were brought to a calcite twinning inverse method and to the processing of results in a new twin data separation process the whole set of data is used to calculate each tensor the variability of yield stress value for twinning with strain hardening is taken into consideration a first order yield stress calcite grain size dependence function has been established with a sample showing various grain sizes this modified method of calcite twinning inversion was applied in the eastern paris basin lorraine france in this platform area measured fractures are preferentially trending ne sw and nw se at all scales the meso cenozoic paleostress succession reconstructed with fault slips is complex jurassic sedimentation occurred during the tethys rifting a mainly strike slip phase with r1 successively striking nnw and nne is attributed to the tertiary pyrenean orogenesis and is followed by wnw extension in the oligocene after local ene compression wnw compression was probably due to the alpine late miocene collision the calcite twinning inversion has confirmed these tectonic events and their succession order and the directions of stress axes were specified using grain size and strain rate sensitive yield stress values the differential stresses can be estimated with an uncertainty of few mpa they range between 20 and 30 mpa depending on the tectonic event the oligocene paleodepths of the sampled calcite estimated using differential stress values are structurally sensible thus the original calcite twinning inverse method has been improved complex paleostress successions such as in platform areas may be deciphered by combination with fault slip inversion even if additional improvement is possible this calcite twinning inverse method opens new ways to determine stress distributions in tectonic plates d 2004 published by elsevier b v

nature vs culture present day spatial distribution and preservation of open air rock art in the coa and douro river valleys portugal abstract late glacial and iron age open air rock art of the coa river valley shows a similar spatial distribution with several clusters along the coa and douro river tributaries that are mostly exposed to the southeast in this report we try to determine whether the artists of both periods deliberately chose the same natural panels for rock art or its present day spatial distribution is imposed by formation and weathering processes previous or subsequent to the engraving geological structural analysis from regional to field scales shows a nne ssw sinistral strike slip fault system that crosses the study area together with a set of fracture joints with the same orientation and formed by the same tectonic stress direct field measurement and the description of 713 natural panels engraved and un engraved reveal that the preserved rock art panels correspond to the most common tectonic fracture joint systems nne ssw of the study area locally the hydrographic network is conditioned by the same structural control differential weathering exists between the panels exposed on opposite margins of watercourses with preferential degradation of the rock art panel surfaces exposed to the nw we propose that on the scale of the valley the surface weathering of the rock art panels results from differential solar radiation humidity lichen and bryophyte colonisations interpretation of field observations a frequency probabilistic procedure pair wise comparison matrix and geographic information system analysis were combined to evaluate a coa panel formation and preservation predictive model using archaeological topographical and hydrological data four variables were extracted and weighted from the collected data including topographic slope and aspect solar radiation and cost weighted distance to watercourses which were used as environmental input data the archaeological input data rock art occurrences were used to calculate the variable ratings and to evaluate both the coa panel formation and preservation predictive model and external validation maps with the results showing an agreement of 80 and 70 respectively field verification revealed unknown rock art panels in areas with high and very high values the coa panel formation and preservation predictive model provides a useful framework to guide survey and heritage management

a possible k t boundary bolide impact site offshore near bombay and triggering of rapid deccan volcanism abstract the temporal coincidence of a major biological mass extinction including dinosaurs the well known iridium excess anomaly at the cretaceous tertiary k t boundary and the eruption of deccan flood basalts at about 65 ma has aroused global interest among geologists and biologists it is widely debated whether the mass extinction and iridium anomaly are due to an asteroid impact or the massive outpouring of extensive deccan volcanism an oval shaped unusual positive gravity anomaly 10 000 km 2 in area near bombay has attracted our attention during a search for an impact site near deccan basalts a detailed gravity interpretation indicates the presence of a fossil conduit structure of 12 km height extending from a shallow crust mantle boundary at 18 km to an approximate depth of 6 km from the surface the conduit structure with a maximum diameter of about 35 km at its base may originate from cracking of a weak pre deccan trap shallow upwarped mantle the structure may have been caused by a bolide impact which triggered the eruption of massive flood basalts deccan traps on the western margin of the fast moving indian plate an impact in this locality can explain the sudden detachment of the arcuate seychelles block from india as well as the large scale reorganisation of plate boundaries in the indian ocean our hypothesis of impact triggered volcanism at 65 ma advocates a bimodal cause for the mass extinction at the k t boundary extraordinary geothermal and structural conditions of the nearby region are also discussed as circumstantial evidence to support the twin cause mechanism by weakened features and the presence of partial melt at subcrustal depth

analogue modelling of a reactivated basement controlled strike slip zone sierra de albarracin spain application of sandbox modelling to polyphase deformation this paper presents the results of an analogue modelling study on the reactivation of riedel shears generated by basement induced sinistral strike slip faulting it is based on a natural example in the sierra de albarracin iberian range spain the area has a polyphase deformation history defined by the variscan and alpine orogenies late variscan deformation was concentrated in a wide nw se shear zone with accompanying kilometre scale e w riedel shears which divided the palaeozoic basement into large fault blocks alpine reactivation resulted in differential movements on the riedel shears as evidenced by a nw se chain of palaeozoic inliers surrounded by a mesozoic cover that generally shows minor deformations except near the e w riedel shears where strata locally appear in near vertical to overturned position sandbox analogue modelling was applied to improve insight into the structural history it focused on the kinematics of spontaneously developed en echelon riedel shears reactivated in a rotated stress field sand with a controlled added strength was used to form riedel shears in a first deformation phase to act as weak zones for a second phase the modelling showed that in the first deformation phase large pop up structures developed between the riedel shears in a basement induced sinistral strike slip zone later reactivation in the n060 e and n135 e shortening directions was taken up respectively by sinistral reverse and dextral reverse shear along the pre existing riedel shears but only if the sand on one side of the fault zone was allowed to move freely along the other scissor faulting along the riedel shears with their complex 3d geometry increased the height of the up squeezed blocks for experiments with fixed boundaries and no oil water emulsion layer between the base plate and sand pack thrusting at the backstop occurred rather than reactivation of the riedel shears this approach provided robust insights on the 4d development of the sierra de albarracin area the geological society of london 2006

active faulting and neotectonics in the baelo claudia area campo de gibraltar southern spain abstract the strait of gibraltar area is part of the western eurasian african convergence zone characterized by a complex pattern of deformation including thrusting and folding and active normal faulting generally the area is of low seismicity only some minor earthquakes have been recorded in the last hundred years archaeoseismological data evidences earthquake destruction occurring twice during roman times a better neotectonic framework and knowledge on the paleostress evolution of the strait of gibraltar area is necessary to find the local sources for those events and to establish an understanding of the recent deformation paleoseismic evidence for one moderate earthquake event around 6000 5000 bp along the normal carrizales fault is described in this paper off shore high resolution seismic investigations structural and paleostress data high resolution gpr and geoelectrical resistivity measurements outcrop investigations and trenching studies are discussed the data reveal that active faulting takes place along n s trending normal faults hence n s directed normal faults in the area are claimed as local candidates for moderate earthquake activity return periods of moderate earthquakes in the order of at least 2000 2500 years in the study area may have to be taken into account structural data such a paleostress data and joints are presented and a deformation history for the strait of gibraltar area in southern spain is developed in this study

late quaternary paleoseismic evidence on the munebrega half graben fault iberian range spain the munebrega plio quaternary half graben is a nw se trending neotectonic depression located in the central sector of the intraplate iberian range ne spain the master fault of the half graben offsets an upper pleistocene pediment deposit forming an upslope facing scarp a trench dug across the fault scarp exposed a 25 m wide deformation zone consisting of graben and horst fault blocks with fissures in the upper part of the scarp and a monoclinal flexure affected by normal and reverse faults in the lower part of the scarp we infer a minimum of three faulting events over the past 72 ka yielding an average maximum recurrence interval of 24 ka the oldest event 72 41 ka produced an antislope scarp on the relict pediment surface confining deposition to the downthrown block cross cutting faults affecting sedimentary units deposited in the sediment trap produced by the first event provide evidence for at least two younger events 33 19 ka the measured cumulative vertical displacement 7 4 m yield a minimum vertical slip rate of 0 10 0 01 mm year 2 error for the past 72 ka if the paleoearthquakes ruptured the whole mappable length of the fault ca 20 km they probably had moment magnitudes ca 6 9 stirling et al bull seismol soc am 2002 such earthquakes would have been more than a magnitude unit larger than the largest ones recorded historically in the iberian range these results suggest that the official seismic hazard assessments based solely on the historic and instrumental record may underestimate the seismic hazard in the area

geophysics going with the mantle flow measurements of directional travel speed of seismic waves constrain flow in the upper mantle laboratory experiments suggest that high pressure can change the mantle s mineral alignment leading to a 90 offset in the direction of the fastest seismic waves

regional patterns of tectonic stress in europe nearly 1500 stress orientation determinations are now available for europe the data come from earthquake focal mechanisms overcoring measurements well bore breakouts hydraulic fracturing measurements and young fault slip studies and sample the stress field from the surface to seismogenic depths three distinct regional patterns of maximum compressive horizontal stress shmax orientation can be defined from these data a consistent nw to nnw shmax stress orientation in western europe a wnw ese shmax orientation in scandinavia similar to western europe but with a larger variability of shmax orientations and a consistent e w shmax orientation and n s extension in the aegean sea and western anatolia the different stress fields can be attributed to plate driving forces acting on the boundaries of the eurasian plate locally modified by lithospheric properties in different regions on average the orientation of maximum stress in western europe is subparallel to the direction of relative plate motion between africa and europe and is rotated 17 clockwise from the direction of absolute plate motion the uniformly oriented stress field in western europe coincides with thin to medium lithospheric thickness approximately 50 90 km and high heat flow values 80 m w m2 in western europe a predominance of strike slip focal mechanisms implies that the intermediate principal stress is vertical the more irregular horizontal stress orientations in scandinavia coincide with thick continental lithosphere 110 170 km and low heat flow 50 m w m2 the cold thick lithosphere in this region may result in lower mean stresses associated with far field tectonic forces and allow the stress field to be more easily perturbed by local effects such as deglaciation flexure and topography the stress field of the aegean sea and western anatolia is consistent with n s extension in a back arc setting behind the hellenic trench subduction zone the stress field is influenced in places by regional geologic structures e g in the western alps where shmax directions show a slight tendency toward a radial stress pattern not all major geologic structures however appear to affect the shmax orientation e g in the vicinity of the rhine rift system horizontal stress orientations are continuous

structure and evolution of the western continental margin of india deduced from gravity seismic geomagnetic and geochronological studies abstract available geochronological gravity seismic and geomagnetic data on the western continental margin of india between 8 20 n are presented based on the data three models describing the evolution of the crustal structure are developed model 1 describes the evolution of the crustal structure along 19 n the main basic igneous activity that accompanied the tectonic events since the late cretaceous resulted in the formation of rifts along the coast thus giving rise to the present day structure the acid volcanic event that preceded the above activity at about 93 ma ago is responsible for the observed crustal structure along 13 30 n shown in model 2 this event resulted in the formation of the st mary group of islands along this latitude counterclockwise rotation of the indian plate during the tertiary period caused rifting of the continental blocks giving rise to the laccadive kerala graben bounded by the chagos laccadive ridge on the west and the west coast fault on the east vertical movements of the rifted blocks promoted the deposition of both continental and marine sediments on the coast west of the west coast fault this tectonic event was accompanied by upwarping of the mantle crust thus giving rise to the present day structure shown in model 3

late cenozoic stress field and fracturing in the iberian chain and ebro basin spain detailed study of mesoscopic faults and joints affecting neogene and pleistocene deposits in the iberian chain and the ebro basin involving statistical study of fracture directions and palaeostress analysis from striated fault planes together with analysis of macrostructures suggest a complex late cenozoic stress field in which both primary and secondary stress systems are superposed 1 regional n s compression was caused by convergence between the iberian and african plates 2 multidirectional extension was caused by crustal doming in the iberian chain 3 stress trajectories were deflected near major faults 4 small scale swapping of the horizontal tension axes 2 and 3 was caused by development of fractures at right angles to either the primary or the deflected 3 axis

major circular structure beneath southern lake huron defined from potential field data newly compiled and merged canadian and united states potential field data for the area of southern lake huron reveal a large circular structure that has a diameter that may extend to 100 km the aeromagnetic image shows a circular central high with a diameter of 6 km located at fat 43 14 21 n long 82 19 88 w surrounded by a principal ring with a diameter of 50 km and indications of conformable arcuate structure extending to a diameter of about 100 km the prevailing northeast striking aeromagnetic signature of the grenville front and adjacent grenville front tectonic zone is interrupted and may be overprinted by the circular feature the bouguer gravity field shows a strong positive anomaly that extends southeast from the grenville front and terminates in an arcuate complex of positive anomalies that correlate with the aeromagnetic field the gravity field illustrates the circular structure with an annular low and central high as well as positive extensions that follow the foliate pattern of magnetic anomalies marking the outer area of the feature there is no strong evidence of the feature in the bathymetry data of southern lake huron although the general coastline of the southern end of lake huron closely follows the outline of the main circular feature the interruption of the grenville trends the clear regular circular signature of the feature and the interpreted magnetic source depths at the precambrian surface suggest an event possibly as young as early paleozoic in the absence of basement samples the origin of the feature is unconfirmed however the scale and potential field anomaly pattern of the feature are comparable to several of the larger impact structures exposed on the canadian shield

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