seismicity of the koyna region and regional tectonomagmatism of the western margin india recent findings on the meso cenozoic tectonomagmatism and deep seated anomalous geophysical structures suggest a close linkage between the seismicity of the koyna region the westernghat uplift wg u and associated thermomechanical and fluid activities the wg u seems to be the result of late cretaceous thermal mobilization erosion of the deccan trap cover and superposition of compressional stress the association of seismicity with uplift seems to result from movement of deep seated heat and fluids volatiles along the edges or boundary faults of the uplift because the force required for crustal deformation depends on the relief observed gradients in relief may be attributed to the differential erosion rates and heat inputs due to the time gap of 50 ma in the break ups and plume activities on the eastern and western sides and consequence magmatism further the geology and tectonics strongly indicate that the western margin wm is a relic of a mobile arm ma that included madagascar and which formed a part of the proterozoic mobile belt of greater india fort 85 ma the mobile nature of the wm facilitates mantle upwellings and transient elevation of isotherms at depth raising the possibility of intermittent metamorphism and greater deformation superposition of the ongoing compression and uplift induced forces make local permeability and pore fluid pressure vital in triggering the seismic slip over the peninsular shield certain representative model calculations have been carried out to estimate change in the e m induction characteristics caused by an intermittent hydraulic connectivity the results show a drop in the resistivity which could be a useful monitoring index the close connection of uplift and fluid activity as discussed here seems applicable for other active parts of the south indian shield sis also

crustal accretion beneath the koyna coastal region india and late cretaceous geodynamics abstract in 1967 a major earthquake in the koyna region attracted attention to the hitherto considered stable indian shield the region is covered by a thick pile of deccan lava flows and characterized by several hidden tectonic features and complex geophysical signatures although deep seismic sounding studies have provided vital information regarding the crustal structure of the koyna region much remains unknown the two available dss profiles in the region have been combined along the trend of bouguer gravity anomalies unified 2 d density modelling of the koyna crust mantle suggests a ca 3 km thick and 40 km wide high velocity high density anomalous layer at the base of the crust along the coastline the thickness of this anomalous layer decreases gradually towards the east and ahead of the koyna gravity low the layer ceases to be visible based on the seismic and gravity data interpretation in the geodynamical rheological boundary conditions the anomalous layer is attributed to igneous crustal accretion at the base of the crust it is suggested that the underplated layer is the imprint of the magmatism caused by the deep mantle plume when the northward migrating indian plate passed over the reunion hotspot

early hettangian benthic planktonic coupling at doniford sw england palaeoenvironmental implications for the aftermath of the end triassic crisis abstract this paper presents a high resolution micropalaeontological study integrated with geochemical and sedimentological analyses of the triassic jurassic tr j transition at the doniford section sw england our analyses indicate the renewal of surface carbonate production in the blue lias formation early hettangian after the end triassic mass extinction based on the high abundance of surface carbonate producers schizosphaerellids which occurred in phase with development at sea floor of calcareous hyaline benthic foraminifers we deduce that a benthic pelagic coupling resulted in both the initial restoration of the biological carbonate pump and also a more efficient export production the presence of monospecific phytoplankton assemblages and benthic foraminiferal assemblages typical of poorly oxygenated environments suggest that the environmental stress continued after the end triassic maximal perturbation complete environmental recovery was prevented by a succession of alternating dysaerobic and anoxic phases that favoured the deposition of black shale sediments the hypothesis of a methane release mechanism as a trigger for the global carbon cycle perturbation and greenhouse conditions during the early jurassic is discounted based on evidence from the decoupled carbon isotope records of carbonate and organic matter apart from local control of a high amplitude sea level change across the tr j boundary the succession and nature of events at doniford are consistent with a scenario of global environmental and biotic changes triggered by the physical and chemical effects of the central atlantic magmatic province camp volcanism a super greenhouse climate in the aftermath of the end triassic mass extinction appears to be the results of the combination of the long term co2 accumulation in the atmosphere system due to the camp volcanic activity as well as the inefficiency of the biological pump due to reduced export production

traces of carnian volcanic activity in the transdanubian range hungary the south alpine dinaridic realm was affected by igneous activity in the middle triassic the marine carbonate platforms and the adjacent basins contain highly variable intrusive volcanic assemblages we studied the petrography and determined the zircon u pb ages of the triassic volcanic products in the transdanubian range the geochemical features and thus the geodynamic context of the magmatism are badly known as the rocks experienced variable chemical alteration the exact duration of the igneous activity is also poorly constrained as the geochronological data of the former studies were obtained mostly by the weathering sensitive k ar and rb sr methods and thus some data even being younger than the age of the stratigraphic cover the presence of andesite dikes and of pebbles and cobbles 20 cm of basalt andesite rhyolite and of rhyolitic tuff in the triassic carbonate platform deposits indicates that within the transdanubian range formed a volcanic complex in triassic the major mineralogical and geochemical features of the transdanubian igneous suite are similar to the triassic formations in the southern alps however dissimilar zircon composition excludes the immediate relationship of the zircon bearing silicic formations in the two tectonic units new u pb ages show that the beginning of the volcanic activity is probably coeval with the eruption of the widespread pietra verde trachytic tuffs in the upper anisian ladinian successions but the majority of the ages are younger than those ash layers the new age constraints give a bench mark for the termination of the volcanic activity in carnian time in the transdanubian range

lithosphere and flood basalts so called hotspot magmas including ocean island basalts and continental flood basalts differ isotopically from mid ocean ridge basalts morb and require a reservoir or a component that has been isolated from the morb source for more than 2 billion years it has been widely assumed that a suitable isolated reservoir might be the continent lithosphere

middle upper triassic carbonate platforms in minorca balearic islands implications for western tethys correlations abstract in order to progress in the knowledge of the middle to upper triassic evolution of the western tethys realm an integrated approach which includes new sedimentological sequence stratigraphic and biostratigraphic data has been accomplished in the carbonate marine successions muschelkalk facies of that age in minorca balearic islands spain the new biostratigraphy which includes six successive ammonoid biozones allowed to assign these carbonate successions to the uppermost anisian lower carnian interval the integration of the new chronostratigraphic framework with the sedimentological analysis allowed to recognize five main successive stages of carbonate platform evolution 1 initial marine transgression and shallow ramp development uppermost anisian 2 carbonate ramp drowning anisian ladinian boundary 3 prevalence of open sea conditions ladinian early carnian 4 abrupt sea level drop intra lower carnian and 5 shallow carbonate ramp and transition to sabkha systems keuper facies furthermore the sequence stratigraphic analysis allowed to divide some of these stages into depositional sequences minorca was located in the westernmost tethys area during the triassic in a key paleogeographic location close to the present day iberia sardinia and the cottian and southern alps the new data have allowed an interregional comparative analysis among these areas with recognition of major suprarregional events and episodes in the framework of the western tethys evolution

long term cycles of triassic climate change a new δ18o record from conodont apatite jat and isw gratefully acknowledge financial support from the australian research council dp1096252 mr is grateful for financial support from progetto d ateneo 2009 cpda090175 09 and 2012 cpda 121100 and prin 2008 2008bef5z7 005

palaeomagnetism and magnetic fabrics of the late palaeozoic volcanism in the castejon laspaules basin central pyrenees implications for palaeoflow directions and basin configuration the authors thank cristina garcia and teresa roman for her help during field and laboratory work and manuel tricas for the preparation of thin sections this work forms part of the research projects cgl2009 08969 and cgl2006 05817 spanish ministry of education and is included in the objectives of the pre doctoral grant ap 2009 0554 esther izquierdo programa de formacion de profesorado universitario spanish ministry of education funding from the instituto de estudios altoaragoneses is also acknowledged bou acknowledges the jaedoc postdoctoral programme financed with esf the authors acknowledge the careful and constructive revisions by b henry and an anonymous reviewer

u pb zircon geochronology and tempo of the end permian mass extinction the mass extinction at the end of the permian was the most profound in the history of life fundamental to understanding its cause is determining the tempo and duration of the extinction uranium lead zircon data from late permian and early triassic rocks from south china place the permian triassic boundary at 251 4 0 3 million years ago biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the changhsingian pulse of the end permian extinction corresponding to the disappearance of about 85 percent of marine species lasted less than 1 million years at meishan a negative excursion in 13 c at the boundary had a duration of 165 000 years or less suggesting a catastrophic addition of light carbon

stream networks and long term surface uplift in the new madrid seismic zone stream networks are sensitive to low rates of surface uplift and can be used to decipher the history of large earthquakes even where faults do not rupture the surface as in intraplate seismic zones statistical analysis of alluvial network data from topographic maps in the new madrid seismic zone in the central united states shows that stream segment gradients deviate the most from an estimated natural stream profile where surface uplift is greatest evidence of cumulative deformation distilled from stream network patterns represents at least several meters of differential surface uplift during holocene time which suggests that more than one cycle of surface deformation occurred

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

re os geochronology of arctic black shales to evaluate the anisian ladinian boundary and global faunal correlations abstract boreal and tethyan faunas differ throughout the triassic limiting the use of biostratigraphy for global correlation rhenium osmium re os dating of organic rich black shales calibrated with existing biostratigraphy provides a new means to establish correlations of global fauna using absolute time here we present re os radiometric ages for middle triassic organic rich shales from two biostratigraphically defined sections at svalbard and the svalis dome in the barents sea mature black shales from svalbard inferred to be middle to late anisian define a 241 2 2 2 ma isochron with an initial 187 os 188 os ratio of 0 831 0 025 the highest seawater os isotope ratio yet recorded between the cambrian and lower jurassic svalis dome shales in the uppermost anisian yield a 239 3 2 7 ma age and initial 187 os 188 os ratio of 0 679 0 020 the higher initial 187 os 188 os ratio for svalbard shales most likely reflects global seawater at the time of deposition whereas the lower initial 187 os 188 os ratio for the svalis dome may represent seawater with limited communication to the open ocean as suggested by the ratio of trace metal abundances and total organic carbon contents these age data indicate a maximum age of 239 3 2 7 ma for the anisian ladinian boundary in the arctic boreal realm this age agrees with the proposed boundary in the tethyan realm 240 to 243 ma and thus confirms the correlations between boreal and tethys based on biostratigraphy and magnetostratigraphy our study affirms that re os geochronology of black shale is a reliable method for obtaining depositional ages and establishing biostratigraphic correlations between paleogeographically separated regions

growth mode of middle triassic carbonate platforms in the western dolomites southern alps italy abstract the correlation of carbonate platform and slope sediments with biostratigraphically and radiometrically dated basinal successions in the western dolomites allows the reconstruction of the growth of a middle triassic carbonate platform schlern rosengarten over a time interval of approximately five ammonoid zones reitzi secedensis curionii gredleri archelaus zones the whole platform evolution had a total duration of

magnetotelluric studies across the kurduwadi gravity feature abstract the deccan traps have a predominantly negative bougner anomaly with two prominent regions of gravity low along the nasik koyna and nasik kurduwadi lines on the basis of these a rift zone was suggested along the nasik kurduwadi region this rift theory was not supported by later deep seismic sounding dss studies subsequent gravity modelling studies indicated that the observed anomalies could be explained in terms of undulations in the crust mantle boundary and the conrad discontinuity it was thus felt that the region should be reinvestigated to check for the rift valley using magnetotelluric mt techniques a linear profile about 100 km long was identified for these studies magnetotelluric data were collected at 11 stations with an average station spacing of 8 15 km the data at ten of these stations were found to be good for the purpose of analysis the rotation of the impedance tensor showed a regional rotation angle of 60 east of north with this rotation the direction of the major resistivity is approximately perpendicular to the direction of gravity low as well as the west coast of the indian peninsula one dimensional 1d analysis at all stations showed a five layered structure a regional geoelectric cross section is proposed using the 1d data the influence of the west coast was studied using two dimensional 2d modelling techniques the deccan basalts were observed to be about 500 m thick overlying the granitic upper crust a mid crustal conductor with a conductance of about 200 s was delineated at a depth of 12 18 km this shows good agreement with the conrad discontinuity reported from the dss and gravity studies the moho discontinuity was not seen in the data presumably because it is not accompanied by any detectable change of electrical resistivity the lithosphere asthenosphere boundary is shallow at a depth of about 80 100 km the asthenosphere is not well formed as is evident from the large resistivities 50 100 m signature of sediments below the traps could not be found in the survey area

electromagnetic constraints on strike slip fault geometry the fraser river fault system magnetotelluric data from four profiles crossing the eocene strike slip fraser river fault in southwestern british columbia suggest that it penetrates the entire crust this conclusion is supported by seismic reflection observations of a 2 3 km step in the moho slightly to the east of the surface expression of the fault but is at variance with an interpretation of the seismic data in which the fault soles into mid crustal reflectors that seem to be continuous across the fault trace a crustal penetrating geometry supports the proposal that the fraser river fault forms part of a 2500 km long intracontinental transform fault system in northwestern north america modeling studies resolve a thin highly conducting mid crustal zone that is connected electrically to the conducting lower crust beneath the coast belt low 13 c values close to the fraser fault system suggest that the electromagnetic signature of this zone may be due to the presence of organic carbon

timing of the permian triassic biotic crisis implications from new zircon u pb age data and their limitations abstract the most profound biotic crisis in the earth s history causing the near extinction of both terrestrial and marine life occurred at the end of the permian period about 253 myr ago and marks the paleozoic mesozoic era boundary the cause of this event is still a matter of vigorous debate with both brief and catastrophic as well as gradual mechanisms having been proposed similar to a recent landmark study this study uses the u pb method on zircons from the uppermost permian lowermost triassic ash fall deposits at meishan zhejiang province se china in order to examine time and rate constraints for these events the results of both this study and previous work show that for these ash layers the effects of pb loss are combined with varying amounts and sources of inheritance resulting in an age scatter which prohibits the extraction of a statistically robust age in many cases though the effects of pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid as opposed to commonly applied air abrasion prior to analysis the presence within a single ash layer of multiple generations of older xenocrysts in many cases only slightly older than the depositional age has made quantitative interpretation even more difficult when these combined phenomena bias individual zircon ages by less than a percent they are extremely difficult to deconvolute and if multi grain analyses are used can become impossible to recognize because of the resulting age averaging monte carlo simulations using actual measurements of individual zircon crystals show that age excursions due to pb loss and xenocrystic contamination for the meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi grain zircon samples are compared thus this study uses only high precision analyses of single crystals whether from our work or that of previous studies three main conclusions have emerged first our data require a significant increase in the age of the permian triassic boundary by more than 2 myr compared to the previous study which shifts the age to a value older than 253 ma second neither our data nor those from previous work can confirm or negate the possibility of a very abrupt biotic crisis third even large suites of very high quality single zircon u pb analyses for these tuffs cannot in most cases yield objective reliable and robust dates with accuracies at the sub myr level though the temptation to perform arbitrary selection of subsets of the analyses for that purpose is almost irresistible the last conclusion is not an indictment of zircon u pb dating in general other rocks and other zircon populations can and do behave very differently and further technical advances will likely improve our ability to prepare grains or sub grains of adequately enhanced quality for analysis consequently the results of the present study strongly suggest that for problems requiring time scale accuracy inferences from zircon u pb dating must be based on sufficiently large suites of single crystal or crystal domain high precision analyses

lower palaeozoic and precambrian igneous rocks from eastern england and their bearing on late ordovician closure of the tornquist sea constraints from u pb and nd isotopes the u pb isotope ages and nd isotope characteristics of asuite of igneous rocks from the basement of eastern england show that ordovician calc alkaline igneous rocks are tectonically interleaved with late precambrian volcanic rocks distinct from precambrian rocks exposed in southern britain new u pb ages for the north creake tuff zircon 449 13 ma moorby microgranite zircon 457 20 ma and the nuneaton lamprophyre zircon and baddeleyite 442 3 ma confirm the presence ofan ordovician magmatic arc tectonically interleaved precambrian volcanic rocks within this arc are verified by new u pb zircon ages for tuffs at glinton 612 21 ma and orton 616 6 ma initial end values for these basement rocks range from 4 to 6 consistent with generation of both c 615 ma and c 450 ma groups of rocksin continental arc settings the u pb and sm nd isotope data support arguments for an ordovician fold thrust belt extending from england to belgium and that the ordovician calc alkaline rocks formed in response to subductionof tornquist sea oceanic crust beneath avalonia

mid triassic integrated u pb geochronology and ammonoid biochronology from the balaton highland hungary ladinian middle triassic strata of the balaton highland west central hungary comprise interbedded marine carbonate and volcaniclastic rocks the sediments are noted for their rich ammonoid faunas which allow detailed biostratigraphic subdivision and correlation for the first time isotopic dating of the tuff layers was carried out to calibrate the age of ammonoid zones and subzones four successive horizons were dated from the felsouu ors section a candidate stratotype for the base of the ladinian stage within the reitzi zone which is interpreted here as the basal ladinian unit the following biostratigraphically tightly constrained u pb zircon ages were obtained felsoeoersensis subzone 241 1 0 5 ma liepoldti subzone 241 2 0 4 ma reitzi subzone 240 5 0 5 and 240 4 0 4 ma a redeposited tuff from the gredleri zone at liter yielded an additional u pb age of 238 7 0 6 ma the new isotopic ages are in agreement with published u pb dates from the southern alps together they allow estimation of the start of the ladinian at 241 5 ma and of the end of the stage at 237 ma a duration of

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

stable isotopic constraints on the nature of the syntectonic fluid regime of the canadian cordillera oxygen hydrogen and carbon isotopic analyses of rocks from the canadian cordillera document regional variations in the origins and evolution of crustal fluids high temperature systems in unmetamorphosed felsic volcanic rocks are characterized by low delta sup 18 o values veins in regional metamorphic rocks are characterized by delta sup 18 o values that reflect approximate oxygen isotopic equilibration of the fluids with regional rock units the delta d studies indicate that both systems were dominated by meteoric water the delta sup 13 c values for vein carbonate indicate a regional lithologic control on delta sup 13 c values the data indicate that convection of surface fluids to depth of 10 or more kilometers during orogenesis was common the existence of deep convection cells involving meteoric water further indicates that fluid pressures to these depths must have been approximately hydrostatic regional structural style appears to exert an important control on the depth of penetration of surface fluids

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

biostratigraphic and radiometric age data question the milankovitch characteristics of the latemar cycle southern alps italy comment and reply

direct radiometric dating of the devonian mississippian time scale boundary using the re os black shale geochronometer many phanerozoic time scale boundaries are characterized by oceanic anoxia and mass extinction events with the deposition of black shale the re os isotope system in black shale can be used to provide depositional ages for these rocks thus yielding direct radiometric ages for time scale boundaries we demonstrate that the re os black shale geochronometer can yield precise ages useful for time scale research through study of the devonian mississippian boundary within the exshaw formation canada the re os date determined places the devonian mississippian boundary at 361 3 2 4 ma 2 including uncertainty model 1 mean square of weighted deviates 1 2 in accord with recent calibration interpolated from u pb zircon dates 360 7 0 7 ma for the hasselbachtal section germany

a seismic hazard map of india and adjacent areas abstract we have produced a probabilistic seismic hazard map showing peak ground accelerations in rock for india and neighboring areas having a 10 probability of being exceeded in 50 years seismogenic zones were identified on the basis of historical seismicity seismotectonics and geology of the region procedures for reducing the incompleteness of earthquake catalogs were followed before estimating recurrence parameters an eastern united states acceleration attenuation relationship was employed after it was found that intensity attenuation for the indian region and the eastern united states was similar the largest probabilistic accelerations are obtained in the seismotectonic belts of kirthar hindukush himalaya arakan yoma and the shillong massif where values of over 70 g have been calculated

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

the age of the permian triassic boundary abstract the 5 cm boundary clay bed in the chinese stratotype section through the permian triassic boundary has been recognised as a bentonite shrimp ion microprobe dating of zircons in the bentonite indicates a magmatic age of 251 2 3 4 ma 2 this is the first direct constraint on the numerical age of the permian triassic boundary future refinements of ages at this important but poorly constrained level of the phanerozoic timescale may depend on re analysis of this uniquely placed volcanic horizon and other bentonites in the fossiliferous chinese upper permian and lower triassic the utility of defining the permian triassic boundary in the chinese stratotype section in the vicinity of known dateable horizons should be considered

δ13c and δ18o values of triassic brachiopods and carbonate rocks as proxies for coeval seawater and palaeotemperature abstract a dataset of 160 isotope 13c and 18o values from anisian ladinian carnian and rhaetian articulate brachiopod shells complemented by 158 carbon and oxygen isotope values from whole rock carbonates define the first continuous stable isotope baseline trends for the triassic seawater the carbon isotope data suggest the existence of several short term high amplitude excursions in the early triassic followed by a predominance of values around 0 5 1 during the middle triassic a rise to 3 5 during the carnian plateau at this level during the late carnian to early norian and a 1 5 decline in the middle norian to values around 2 during the late norian rhaetian interval the causation scenarios for these rapid oscillations are at present equivocal but may in part reflect a biological instability of the carbon cycle following the recovery from the end permian extinction event and or an input of mantle derived co2 from enhanced volcanic activity the 18o values from well preserved brachiopods from the tethyan realm range from 3 9 to 0 6 v pdb these values require open marine triassic seawater 18o values close to 0 v smow for the calculated temperatures to be within the range of tolerance of the coexisting reef building corals a 2 18o increase within the uppermost cordevolian and early julian suggests either a distinct temperature decline in the southern alps during that time interval an increase in seawater salinity or their combination oxygen isotope values for the muschelkalk brachiopods range between 6 2 and 2 0 and likely reflect a strong influx of meteoric waters into the germanic basin that shifted the oxygen isotopes to more negative values

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

improving the upper triassic numerical time scale from cross correlation between tethyan marine sections and the continental newark basin sequence abstract magnetostratigraphic cross correlations between biochronologically well dated tethyan marine sections and the astronomically tuned sedimentary sequence from the continental newark basin eastern north america help to improve the upper triassic time scale we show here that the most recent radiometric results constraining the age of the base and top of the upper triassic 237 and 200 ma respectively strengthen the chronostratigraphic calibration of the newark sedimentary sequence proposed by krystyn et al earth planet sci lett 203 2002 343 351 these results allow one to reconcile the milankovitch based cyclostratigraphy from the newark sequence the magnetostratigraphic data from both marine and continental environments and upper triassic biostratigraphy including data on tethyan marine faunas and on terrestrial vertebrates according to this calibration the successive upper triassic stages would have very different durations 10 myr for the carnian 237 227 ma 25 myr for the norian 227 202 ma and 2 myr for the rhaetian 202 200 ma but only if the entire rhaetian is recorded in the newark sequence we further show that the biozone durations in the upper triassic would have been highly variable and as a whole would have lasted much longer than in the lower and middle triassic

grain orientation and depositional ramps as flow direction indicators of a large scale pyroclastic flow deposit in japan flow lineation was measured using the orientation of the elongated crystals and lithic fragments in the large scale welded ata pyroclastic flow deposit in southwestern japan in order to analyze the effect of preflow topographic relief three different types of lineations were obtained flow lineations radially away from the source were obtained only for samples collected from the surface of the pyroclastic flow plateau samples collected along the floor of narrow valleys have lineations parallel to the axis of valley samples collected from valley walls tend to be parallel to the slope of the valley this evidence suggests the following process earlier flow material moves along the valley axis in the narrow valley bottom when the flow is trapped within a valley it moves toward the bottom of the valley just prior to the final settlement when the basement relief is filled up with the earlier flow units the flow maintains its original radial movement until final settlement asymmetrical depositional ramps on the original surface of the pyroclastic flow provide an independent indicator of the flow direction

the survival recovery and diversification of metazoan reef ecosystems following the end permian mass extinction event abstract the triassic period records important ecological transitions in the aftermath of the end permian mass extinction and is a key interval in the evolution of modern coral reefs there have been several critical developments in our understanding of triassic reef evolution over the past decade the timing of events and duration of stages have changed dramatically the discovery of metazoan reefs in the early triassic details about the environmental perturbations that drove the extinction the relationship between tectonic activity and platform margin reef proliferation and additional proxy evidence for the co evolution of coral reef builders and their photosymbionts here we provide an up to date synthesis of reef collapse and recovery dynamics following the end permian extinction specifically integrating recent discoveries the evolution of reef ecosystems can be divided into five phases based on their composition 1 microbial metazoan reefs represent survival communities that characterize the immediate extinction aftermath 2 the re establishment of reefs built by metazoans small sponge biostromes and bivalve buildups is observed in oxygenated settings in the olenekian early triassic 3 towards the end of the olenekian and into the anisian middle triassic low diversity tubiphytes dominated reefs formed which represent the first triassic platform margin reefs platform margin reefs however are not widespread until the late anisian 4 late anisian reefs also record a composition change and increase in species richness with sponges and tubiphytes as the main reef builders 5 the first scleractinian corals which are the main reef builder in modern marine reef ecosystems evolved during the anisian but are not reported as dominant reef builders until the late triassic the radiation of coral reefs is posited to be coupled to the acquisition of photosymbionts e g zooxanthellae there is clearly a stepwise evolution of reef types during the triassic however once each reef type appears it persists throughout the remainder of the triassic the survival recovery and diversification of reef ecosystems is therefore more complex than previously outlined particularly with respect to the earliest post extinction ecosystems these recent advances highlight the need to thoroughly document the faunal compositions of understudied reef systems as well as to continue the exploration of triassic ecosystems in underrepresented regions

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

magnetic fabric investigations of pyroclastic deposits from phlegrean fields southern italy magnetic fabric investigations of pyroclastic deposits from phlegrean fields southern italy

cause of upper triassic climate crisis revealed by re os geochemistry of boreal black shales abstract the triassic period is bracketed by two of the big five phanerozoic mass extinctions though long viewed as a period of climatic stability emerging data suggest multiple climatic swings and at least one severe ecological crisis linking these climatic instabilities with probable causes is hampered by poor age control within the triassic time scale here we present new re os ages for shale sections straddling middle upper triassic stage boundaries nominal re os isochron ages of 236 6 and 239 3 ma for the top and base of the ladinian upper middle triassic bring absolute time into the contentious triassic time scale and place the beginning of the late triassic about 12 m y earlier than previously assigned a marked decrease in initial 187os 188os in upper ladinian shale records input from wrangellian flood basalts an instigator in the carnian late triassic pluvial event and accompanying radiation of key fossil groups e g dinosaurs and calcareous nanoplankton an absolute time scale is proposed for the anisian ladinian carnian boundaries based on re os geochronology

biostratigraphic and radiometric age data question the milankovitch characteristics of the latemar cycles southern alps italy the combination of age diagnostic fossils in platform carbonates geometrical relationships with coeval basinal successions and single zircon u pb ages from volcaniclastic layers in basinal deposits constrain the age of a cyclic succession in the interior of the latemar platform to a narrow time span in the middle triassic with a duration shorter than 5 m y precession induced milankovitch band sea level oscillations are therefore excluded as controls for most of the 600 platform cycles reported from latemar statistical evaluation and graphic space time analyses of cycle stacking patterns alone may therefore not allow an unambiguous recognition of milankovitch frequencies in ancient platform carbonates

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

hydrogeochemistry of the koyna river basin india hydrogeochemistry of the koyna river basin famous for the koyna earthquake magnitude 7 of 1967 has been studied basalt is the primary aquifer laterites alluvium and talus deposits form aquifers of secondary importance groundwater generally occurs under water table conditions in shallow aquifers deeper aquifers are associated only with basalts one hundred and 87 water samples were collected from various sources such as dugwells borewells springs and surface water including 40 samples for analysis of iron only major constituents were analyzed analyses show that the concentrations of ca2 exceed that of mg2 in almost all water samples the concentrations of na are generally next to ca2 and are always higher than that of k and co32 and so42 are very low and are often negligible groundwater in borewells tapping deeper aquifers has higher mineralization compared to that in dugwells representing shallow aquifers majority of the water samples are dominated by alkaline earths ca2 mg2 and weak acids hco3 co32 groundwater from shallow aquifers is generally calcium bicarbonate type 53 and calcium magnesium bicarbonate type 27 in case of deeper aquifer it is mostly calcium magnesium bicarbonate type 29 sodium bicarbonate type 24 calcium bicarbonate type 19 calcium magnesium sodium bicarbonate type 19 and sodium calcium bicarbonate type 9 groundwater water is generally fit for drinking and irrigation purposes except in the lower reaches of the koyna river basin which is affected by near water logging conditions

high moho temperature in the indian shield abstract moho temperatures in the indian shield region are estimated using step exponential and power law models of the depth variation of radiogenic heat with temperature dependent thermal conductivity the moho temperatures and heat flows are around 550 600 c and 0 8 hfu 33 mwm 2 in the southern part of the indian shield and 850 900 c and 0 95 hfu 40 mwm 2 in its northern part it is argued that such high temperatures and their lateral variations may contain signatures of deccan trap volcanism and himalayan orogeny the implications of the temperature estimates on some related crustal parameters viz p n wave velocity curie isotherm depth and melting point temperature at moho depth are also indicated

middle triassic orbital signature recorded in the shallow marine latemar carbonate buildup dolomites italy a new time frequency analysis of sea level controlled carbonate platform cycles in the middle triassic latemar massif dolomites italy reveals a strong depositional signature with characteristics of dominant forcing by climatic precession modes corresponding to long and short precession components at 1 21 7 k y and 1 17 6 k y underwent amplitude modulations matching earth9s orbital eccentricity with major frequency components at 1 400 k y 1 125 k y and 1 98 k y obliquity appears as a minor component at 1 35 4 k y the latemar signature thus constitutes the oldest pristine milankovitch signature yet observed in the geologic record its fidelity rivals that of the pliocene pleistocene record originally used to confirm the theory of orbitally forced climates this evidence deepens a widely noted disagreement between radiometric and cyclostratigraphic time scales for the latemar buildup the latemar cycles indicate that orbitally forced sea level oscillations were operative in the ice free middle triassic hothouse world

a u pb and 40ar 39ar time scale for the jurassic published time scales provide discrepant age estimates for jurassic stage boundaries and carry large uncer tainties the u pb or 40 ar 39 ar dating of volcaniclastic rocks with precisely known stratigraphic age is the preferred method to improve the calibration a radiometric age database consisting of fifty u pb and 40 ar 39 ar ages was com piled to construct a revised jurassic time scale accepted ages have a precision of 5 ma 2 or better and are con fined to no more than two adjacent stages the majority of these calibration points result from integrated bio and geochronologic dating in the western north american cordillera and have not been previously used in time scales di rect dates are available only for the triassic jurassic boundary and the initial boundary of the crassicosta chron and the callovian stage the chronogram method was used to estimate all early and early middle jurassic zone boundaries attempted here for the first time late middle jurassic substage boundaries and late jurassic stage boundaries signif icant improvement is achieved for the pliensbachian and toarcian where six consecutive zone boundaries are deter mined the derived zonal durations are disparate varying between 0 4 and 1 6 ma the latest jurassic isotopic database remains too sparse therefore chronogram estimates are improved using interpolation based on magnetochronology the initial boundaries of jurassic stages are proposed as follows berriasian jurassic cretaceous 1418 18

tracing multiple resedimentation on an isolated karstified plateau the bauxite bearing miocene red clay of the southern bakony mountains hungary abstract the vorosto red lake formation located in the southern bakony mountains and the balaton highlands hungary is a red clayey continental assemblage containing hard up to fist sized bauxite pebbles occurring in a large apparent stratigraphic gap between underlying karstified triassic carbonates and mid miocene to quaternary cover the origin and the exact stratigraphic position of the assemblage have been controversial for a long time in this study petrographic observations on the bauxite pebbles revealed common features with cretaceous bauxite deposits known in the region whereas heavy mineral composition of the red clay matrix is similar to those known from the eocene bauxite horizon of the region single grain zircon u pb ages obtained from the bauxite pebbles and their red clayey matrix show similar late archean to jurassic age components additionally cenozoic u pb ages are well represented in the mostly euhedral zircon crystals separated from the red clays whereas this volcanogenic contribution completely missing from the bauxite pebbles sem morphology and related edx chemical analysis of clay minerals indicate polygenetic detrital origin for the red clays the main source material of the bauxite bearing vorosto formation is most probably local cretaceous and eocene bauxite deposits exposed during the middle late miocene these tropical weathering products were partly decomposed and degraded but dilution by siliciclastic contribution is negligible we suggest a transport mechanism dominated by local redeposition of mostly pelitic and allitic material through seasonal muddy debris flows within a karstic landscape

hot spots magma underplating and modification of continental crust most common mantle derived magmas have densities at moho pressures that exceed the density of average continental crust thus unless the crust is tightly coupled with the mantle many such magmas should penetrate at or near the moho interface the scale of such a phenomenon can be impressive recent hot spots may have perturbed more than 5 of the planet s surface the details of the process will depend on the ease of melting of the base of continents underplating can explain continental construction from below moho structures or lack thereof delamination of the continental basement partial subduction and density cleaning of crust the formation of massive anorthosite complexes and major changes of elevation of continental blocks in the thermally turbulent archean the very dense and hot komatiite magmas must have had even more profound influences modern studies of the subduction process show that the mantle must be heterogeneous and that volatiles can be carried to great depths in significant volumes

geological contacts thermal springs and earthquakes in peninsular india abstract the occurrence of thermal springs in peninsular india shows a distinct pattern that is related to geological contacts tectonic units and earthquakes it is observed that most of these thermal springs occur either near the contact of two geological units or along prominent tectonic units in the peninsular shield the seismicity map of peninsular india from historical times up to 1990 for earthquakes of magnitude 3 5 indicates a striking correlation between seismicity and thermal springs the b values which characterize the thermal spring areas of the western coast are higher 0 8 1 1 than those obtained in similar areas in the rest of the peninsular shield 0 47 0 6 this indicates that the stresses are released by frequent micro to moderate earthquakes in the thermal springs area along the west coast of india whereas in peninsular india the stress release is through infrequent moderate earthquakes

komatiites and picrites evidence that the plume source is depleted high temperature magmas such as picrites and komatiites are candidates for the material in the hot axial region of mantle plumes in current plume theories these magmas should have an enriched ocean island basalt oib type chemistry high mgo magmas however are often more like depleted mid ocean ridge basalts morb in trace element and isotopic chemistry this is a paradox for deep mantle plume or primitive mantle theories but is expected if hotspots and morb share a common depleted source and the transition to oib chemistry involves shallow level cooling crystal fractionation and contamination ancient komatiites appear to have come from the same deep depleted reservoir as modern picrites and morb

reply to discussion of magnetostratigraphic confirmation of a much faster tempo for sea level change for the middle triassic latemar platform carbonates by d v kent g muttoni and p brack earth planet sci lett 228 2004 369 377 by l hinnov

paleoclimatic significance of phanerozoic reefs a database of pre quaternary phanerozoic reefs is used to test the significance of ancient reefs as paleoclimatic tracers the compilation of reef paleolatitudes through time and comparison with published paleoclimate curves shows that neither the width of the tropical reef zone nor the total latitudinal range of reefs is correlated with published estimates of paleotemperature however reefs trace paleoclimate indirectly algal reefs tend to prevail during icehouse climatic intervals and distinct high latitude reefs are only developed in cold intervals whereas during greenhouse episodes the reef zone usually ended abruptly at a particular latitudinal boundary in the subtropics additionally different biotic reef types tended to be concentrated in different latitudes the lowest latitudes have usually been occupied by coralline sponge or microbial reefs coral reefs tended to grow in intermediate latitudes and bryozoan reefs constantly occupied the highest latitudinal position

deep seismic sounding studies in the north cambay and sanchor basins india summary deep seismic sounding dss studies have been carried out in the north cambay and sanchor sedimentary basins in western india along three lines covering about 350km seismic refraction and wide angle reflection data pertinent to the sedimentary basin as well as the deep crustal section have been recorded from 41 shot points using a 60 channel dfs v digital recording system with 200 m geophone spacing and 4 ms data sampling extensive modelling and interpretation of a large number of seismic record sections reveal four sub basins in the sedimentary section along these lines maximum depth to the granitic proterozoic basement p wave velocity 5 9 6 0 km s 1 is about 5000 m in the north sanchor and the patan sub basins and about 5600 m in the south sanchor sub basin the deepest part of the sedimentary basin is delineated within the gandhinagar sub basin where the basement depth reaches 7700 m the deccan traps p wave velocity 4 3 4 8 km s 1 form the base of the tertiary sediments almost in the entire study area except the extreme northern part there is also some indication of the presence of sub trappean mesozoic sediments along this profile within the sedimentary basin two horst features one near diyodar the diyodar ridge and the other northwest of mehsana the unhawa ridge are indicated by the seismic data consistent with the tectonics of the region the thickness of the upper crust in this region does not exceed 15 km p wave velocity reaching 6 3km s 1 a prominent low velocity zone velocity 5 5 km s 1 occurs in the depth range from 10 5 to 12 5 km the lower crust consists of two layers of velocities 6 6 6 9 km s 1 and 7 3 7 4 km s 1 the discontinuity between them occurring at 23 25 km depth the moho discontinuity pm velocity 8 0 km s 1 lies at a depth of 31 33 km the high velocity 7 3 7 4 km s 1 lower crustal layer represents underplating of the crust due to mantle upwelling and rifting with large scale extrusion of the deccan volcanics the large thickness of the tertiary sediments in the cambay basin and a relatively thin crust in the region suggest further rifting during the tertiary

significance of late permian triassic facies zones in terrane reconstructions in the alpine north pannonian domain abstract the present day setting of the tectonic units terranes making up the pre neogene basement of the pannonian basin and the west carpathians is the result of large scale displacements during the alpine collisional phases to explain this setting based on various concepts a number of models have been proposed in the last decade most of them agree in an eastward escape of the north pannonian terrane from the alpine domain towards the pannonian during the palaeogene early miocene period in the present paper a reconstruction of the pre escape setting of the displaced terranes is presented based on facies zones in selected time slices from the late permian to the late triassic according to our reconstruction the inner west carpathian units were located east of the austroalpine units forming the northern shelf of the euhallstatt meliatic basin of the vardar oceanic branch the drauzug should have been located in a position much more external than its present day setting the transdanubian range was situated in the northern neighbourhood of the southern alps whereas the bukk may have been located adjacent to the nw dinarides the mid transdanubian zone is a major shear zone containing elements of the julian alps and sava folds nappe system and the inner dinaric ophiolitic melange although the nappe tectonism may have played an important role in the structural evolution of the north pannonian terrane this mechanism alone can hardly produce hundreds of kilometres of facies offset on the opposite sides of strike slip zones

spectral analysis of the middle triassic latemar limestone abstract the middle triassic latemar limestone in the dolomites region northern italy consists of hundreds of vertically stacked meter scale platform carbonate cycles each cycle is composed of a shallow marine subtidal carbonate unit with a thin centimeter scale vadose diagenetic dolomite cap these latemar couplets are the products of repeated episodes of submergence emergence with durations estimated at 20 000 yr the couplets are arranged in upward thinning bundles of five indicating the presence of a 100 000 year modulating component we examined this high frequency cycling in the platform buildup using a variety of classical and advanced spectral analysis techniques we found si nificant evidence for the presence of a milankovitch forcing signal in this cycling we also present evidence that the adjacent coeval basinal facies the livinallongo formation exhibits a similar bundling pattern in its cm scale hemipelagic carbonate shale alternations we conclude that milankovitch forced small amplitude sea level oscillations were the most likely cause of these middle triassic carbonate rhythms

timing the end triassic mass extinction first on land then in the sea the end triassic marks one of the five biggest mass extinctions but current geologic time scales are inadequate for understanding its dynamics a tuff layer in marine sedimentary rocks encompassing the triassic jurassic transition yielded a u pb zircon age of 199 6 0 3 ma the dated level is immediately below a prominent change in radiolarian faunas and the last occurrence of conodonts additional recently obtained u pb ages integrated with ammonoid biochronology confirm that the triassic period ended ca 200 ma several million years later than suggested by previous time scales published dating of continental sections suggests that the extinction peak of terrestrial plants and vertebrates occurred before 200 6 ma the end triassic biotic crisis on land therefore appears to have preceded that in the sea by at least several hundred thousand years

synchrony and causal relations between permian triassic boundary crises and siberian flood volcanism the permian triassic boundary records the most severe mass extinctions in earth9s history siberian flood volcanism the most profuse known such subaerial event produced 2 million to 3 million cubic kilometers of volcanic ejecta in approximately 1 million years or less analysis of 40 ar 39 ar data from two tuffs in southern china yielded a date of 250 0 0 2 million years ago for the permian triassic boundary which is comparable to the inception of main stage siberian flood volcanism at 250 0 0 3 million years ago volcanogenic sulfate aerosols and the dynamic effects of the siberian plume likely contributed to environmental extrema that led to the mass extinctions

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

low velocity layers in the subcrustal lithosphere beneath the deccan traps region of western india abstract travel times and relative amplitude modeling of a seismic record section from the koyna dss profile in the deccan traps region has yielded a velocity model for the indian continental subcrustal lithosphere the observed record section assembled with a reduction velocity of 6 km s 1 and using digitized data from analog deep seismic sounding dss recorders reveals a prominent wave group with an apparent velocity similar to but about 3 s after the reflected p phase from the moho pmp at a reduced time of 7 5 to 6 5 s and recording distance range from 70 to 95 km we interpret this strong amplitude phase as the reflection from an interface in the subcrustal lithosphere comparison of synthetic seismograms for a range of velocity models with the observed record section shows that the observed phase under investigation cannot be explained either by a reflected phase from a subcrustal velocity discontinuity with a single velocity increase or by the p to s converted phase pms from the moho subcrustal velocity models with a single low velocity layer lvl do not seem to give a satisfactory fit to the amplitude observations on the field record section we prefer a subcrustal velocity model with alternating lvl at least two with a velocity of 7 0 7 4 km s 1 separated by a thin high velocity layer in this model the synthetic amplitudes of the reflected phase from the top of the deeper lvl at 56 km depth well match those of the observed reflection phase it is inferred that the continental subcrustal lithosphere in this region of the indian shield has a lamellar structure with significant vertical variation of structural and mechanical properties the alternating lvl occurring at relatively shallow depths below the moho velocity decreasing from 8 3 to 7 4 km s 1 may be associated with zones of weakness and lower viscosity the lvl in the subcrustal lithosphere as well as the lvl inferred in the upper and the lower crust krishna et al 1989 together with the observed depth distribution of seismicity in this region suggest a well defined rheological stratification of the continental lithosphere with varying material properties

discussion of magnetostratigraphic confirmation of a much faster tempo for sea level change for the middle triassic latemar platform carbonates by d v kent g muttoni and p brack earth planet sci lett 228 2004 369 377 abstract kent et al report on new magnetostratigraphic data obtained from the middle triassic latemar carbonate platform dolomites italy the result is important because it addresses the so called latemar controversy and appears to corroborate radioisotope dated ash beds in the latemar platform indicating that the buildup must have taken place in 2 4 million years but not the 9 12 million years of milankovitch forcing inferred from cyclostratigraphic analyses unfortunately kent et al omit basic information that runs contrary to the conclusion that the latemar carbonates have yielded a primary paleomagnetic signal here the missing details are supplied by zooming in on the chronostratigraphic interval that was investigated in sum kent et al s results do not confirm a faster tempo for sea level change for the latemar as much as raise questions about the magnetization of these carbonate rocks there are also shortcomings in kent et al s reappraisal of the cyclic content of the cimon del latemar cdl series that need clarification finally the latemar controversy is examined in the context of the distribution of time in middle triassic stratigraphy of the dolomites

the triassic timescale new constraints and a review of geochronological data a review of geochronological data underlying the geological time scale for the triassic yields a significantly different time scale calibration than that published in the most recent compi lation geologic time scale 2004 this is partly due to the availability of new radio isotopic data but mostly because strict selection criteria are applied and complications arising from biases both systematic and random are accounted for in this contribution the ages for the base and the top of the triassic are constrained by u pb ages to 252 3 and 201 5 ma respectively these dates also constrain the ages of major extinction events at the permian triassic and triassic jurassic bound aries and are statistically indistinguishable from ages obtained for the siberian traps and volcanic products from the central atlantic magmatic province respectively suggesting a causal link ages for these continental volcanics however are mostly from the k ar 40 ar 39 ar system which requires accounting and correcting for a systematic bias of c 1 between u pb and 40 ar 39 ar isotopic ages the 40 ar 39 ar ages being younger robust age constraints also exist for the induan olenekian boundary 251 2 ma and the early middle triassic olenekian anisian boundary 247 2 ma resulting in a surprisingly short dur ation of the early triassic which has implications for the timing of biotic recovery and major changes in ocean chemistry during this time furthermore the anisian ladinian boundary is con strained to 242 0 ma by new u pb and 40 ar 39 ar ages radio isotopic ages for the late triassic are scarce and the only reliable and biostratigraphically controlled age is from an upper carnian tuff dated to 230 9 ma yielding a duration of more than 35 ma for the late triassic all of these ages are from u pb analyses applied to zircons with uncertainties at the permil level or better the resulting compilation can only serve as a guideline and must be considered a snapshot resolving some of the issues mainly associated with inaccurate and misinterpreted data in previous publi cations however further advances will require revision of some of the data presented here

high resolution upb dating of middle triassic volcaniclastics time scale calibration and verification of tuning parameters for carbonate sedimentation abstract we report high resolution single zircon u pb age data for middle triassic volcaniclastic intercalations in biostratigraphically calibrated pelagic successions of the southern alps the results require a redefinition of the chronometric scale for the middle triassic moreover they do not support current models relating cyclic sedimentation in platform carbonates of the dolomites to orbital tuning tight concordant age clusters were obtained for five volcaniclastic layers in three ammonoid biozones of late anisian to early ladinian age two layers in the nevadites secedensis zone yielded identical mean 206 pb 238 u ages of 241 2 0 8 0 8 ma and 241 2 0 8 0 6 ma errors given at the 95 confidence level a layer in the gredleri zone is dated at 238 8 0 5 0 2 ma and two horizons in the archelaus zone yield similar ages of 237 9 1 0 0 7 ma and 238 0 0 4 0 7 ma these results are significantly older than the age values of 233 235 ma assigned to the anisian ladinian boundary by several current time scales 1 4 moreover our estimate of 240 7 241 3 ma depending on biostratigraphic collocation for the anisian ladinian boundary casts doubts on the reliability of age values of 245 250 ma proposed by most time scales for the permian triassic boundary the occurrence of pelagic fossils in basinal sediments as well as in age equivalent shallow marine middle triassic platform carbonates in the dolomites allows the sedimentary sequences of both environments to be correlated the 800 m thick latemar platform western dolomites is characterized by cyclic stacking patterns which have been interpreted as results of milankovitch type high frequency low amplitude sea level fluctuations the 12 m y interval of platform growth postulated from the assignment of orbital periodicities to the platform carbonate cycles 5 6 is in conflict with a maximum time span of 4 7 m y allowed by the present zircon data