crustal contamination in palaeogene east greenland flood basalts plumbing system evolution during continental rifting abstract palaeogene central east greenland flood basalts have sr nd and pb isotope characteristics consistent with crustal contamination of icelandic type basalts in the lower basalts of central east greenland amphibolite facies gneiss is the dominant contaminant at low stratigraphic levels whereas contamination with granulite facies gneiss prevails in the upper part of the lava succession the contamination probably occurred by partial melting of the crust and can be modelled by assimilation fractionation crystallisation processes similar contaminants have been detected in lavas from odp leg 152 off the se greenland coast although in a reverse sequence contamination in lower basalts and odp leg 152 lavas which erupted through an archaean proterozoic substrate differs from lavas in ne greenland which overly the caledonian fold belt and large mesozoic sediment deposits in the ne greenland lavas the dominant contaminant appears to be a dacitic component formed by partial melting of mesozoic sediments changes in magma supply rates from the mantle may be the determining factor for the sequence of contaminants and may also control the depth of ponding in crustal chambers the compositional variation of the least contaminated east greenland basalts suggests that melts were produced from mantle sources similar to present day iceland and north atlantic morb at the time of continental breakup but distinction between mantle sources is very complicated ree systematics indicate a maximum in initial depth of mantle melting at kangerlussuaq during formation of the east greenland flood basalts consistent with a plume being centered in that region

compressive and tensile failure of inclined well bores and determination of in situ stress and rock strength in this paper we investigate the occurrence of compressive and tensile failures of arbitrarily inclined well bores under a wide variety of stress conditions the principal assumptions in this analysis are that the rock is isotropic and that it deforms elastically to the point of failure as has been shown by previous investigators for a given stress state and well bore orientation it is straightforward to predict the orientation of the failures around the well bore as well as whether failure is likely to occur depending on such parameters as rock strength and borehole fluid pressure however as the stress state is almost never known in situ we demonstrate how observations of compressive and tensile wall failures in inclined holes can be used to constrain in situ stress orientations and magnitudes if there are independent data on the magnitude of the least principal stress from either leak off or microfrac tests and on the formation pore pressure we further demonstrate how once the stress state is determined it is possible to assess both an upper bound on the effective in situ rock strength and the degree to which increasing the borehole fluid pressure or mud weight can reduce the likelihood of borehole failure through application of this methodology to an inclined well bore in an area of complex faulting in the gulf of mexico we illustrate how it is possible to utilize observations of borehole failures to determine the magnitude and orientation of the stress tensor in areas such as offshore sedimentary basins where drilling inclined well bores is quite common

in situ stress measurements by hydraulic fracturing for a rock mass with many planes of weakness when hydraulic fracturing is performed on a rock mass which contains many natural cracks and planes of weakness most of the induced cracks are inclined transverse cracks we propose a method which can be used under such a circumstance to determine the stress state the method consists of two stages the first stage determines the magnitude and orientation of principal stresses by using shut in pressures of inclined transverse cracks and the circumferential position of longitudinal cracks on the wellbore surface the second stage estimates the ranges of errors of the stress state determined in the first stage by taking into account that the shut in pressures and orientations of the cracks are the factors which give rise to errors of the determined stress state we have applied this method to data obtained by the hydraulic fracturing performed at two wellbores drilled from a drift way at the kamaishi mine it was found that the ranges of errors of the principal stress magnitude were small the orientation of the maximum principal axis of stress had a relatively high reliability in all directions although the ranges of errors of the directions of the minimum and intermediate principal stresses showed a tendency to spread more widely in the n s direction

inversion of borehole breakout orientation data well bore breakouts are zones of spalling and fracture that form on opposite sides of a well bore and tend to change the cross sectional shape of the borehole from circular to roughly elliptical in vertical holes drilled in areas where one principal stress is vertical breakout orientations tend to parallel the direction of the regional minimum principal stress compressive such breakout orientations are often valuable indicators of the direction of the principal stresses we will show in this paper that the variation of the breakout direction as the borehole deviates from vertical gives further information about the relative magnitudes of all the three principal stresses based upon the variation of breakout directions as a function of deviations and azimuths of the borehole along its depth an inverse problem is formulated to estimate both the direction of the minimum horizontal stress and the relative magnitudes of the horizontal stresses to the vertical stress tests with synthetic data sets show that the power of the method is dependent on the stress regime strike slip normal and thrust generally the inverse problem in the case of thrust faulting is less constrained whereas in the case of normal faulting it allows for a determination of all three parameters sh sv sk sv and with a reasonable certainty interpretation of the data from the siljan deep drilling project in sweden shows that the azimuth of the minimum principal horizontal stress is very well constrained to n 18 4 0 3 e while the ratio of the maximum principal horizontal stress to the vertical stress is constrained to 1 1 0 1 the ratio between minimum principal horizontal stress and principal vertical stress is poorly constrained to the interval 0 0 0 95 information of drilling mud leakage and borehole stability analysis provides a value of 0 61 stephansson et al 1990 these results are in fair agreement with the results from earthquake fault plane solutions in south central fennoscandia with dominant strike slip faulting and an inferred minimum horizontal stress direction of n45 e slunga 1981

sedimentary challenge to snowball earth evidence from the magnetic field fossilized in sedimentary rocks suggests that more than 600 million years ago ice occupied tropical latitudes a popular explanation for these findings the snowball earth concept envisages a fully frozen earth for millions of years caused by a runaway ice albedo feedback a rapid catastrophic meltback at very high levels of atmospheric carbon dioxide is thought to have ended this extreme climatic state however sedimentary rocks deposited during these cold intervals indicate that dynamic glaciers and ice streams continued to deliver large amounts of sediment to open oceans throughout the glacial cycle the sedimentary evidence therefore indicates that despite the severity of glaciation some oceans must have remained ice free significant areas of open ocean have important implications for the survival and diversification of life and for the workings of the global carbon cycle the snowball earth concept envisages a fully frozen earth for millions of years several times during the neoproterzoic era between 1 000 and 542 million years ago however the sedimentary evidence suggests that despite the severity of glaciation some oceans must have remained ice free

stress directions in eastern north america determined to 4 5 km from borehole elongation measurements stress directions have been determined to depths of 4 5 km in eastern north america from borehole elongation measured by dipmeter calipers in 47 wells the average maximum horizontal stress directions are eastern canada n54 e 7 appalachian basin n58 e 8 and the illinois basin n89 w 5 stress directions determined for the illinois and appalachian basins are in agreement with published interpretations from hydraulic fractures and earthquake fault plane solutions new results from eastern canada and the appalachian basin show that stress directions are everywhere consistent with the midcontinent stress province these data suggest that the maximum horizontal stress does not rotate from the ene midcontinent trend to nw near the atlantic coast a directional analysis of hydraulic fractures petal centerline fractures and natural fractures from the appalachian basin shows that borehole elongation is systematically aligned perpendicular to the directions of hydraulic fractures centerline fractures and northeast striking natural fractures but is not systematically aligned with northwest striking natural fractures these observations indicate that borehole elongations provide reliable stress orientations

relating shear sonic anisotropy directions to stress in deviated wells we develop a model that relates the polarization direction of a dipole sonic fast shear wave propagating along the borehole hereafter called fsa to the relative deviatoric stress tensor for arbitrary well orientations we first define stress quantities or subsidiary principal stress relevant for sonic dipole shear characterized sufficiently far away into the formation to be unaffected by borehole stress concentration next we show analytically for wells oriented within principal stress planes and numerically for wells outside principal stress planes that the stress induced fsa coincides with the maximum normal stress direction orthogonal to the borehole maximum subsidiary principal stress using a nonlinear elastic model for isotropic unstressed background media and plane wave solutions this model is independent of stress sensitivity parameters this result is a consequence of first the direct relationship between the stressellipsoid factor and the shear stiffness difference ratio r 2

isotopic evidence for the age and origin of pitchstones and felsites isle of eigg nw scotland an 8 point whole rock and mineral rb sr isochron for the sgurr of eigg yields an age of 52 1 1 0 ma 2 this is the youngest accurate date yet reported for igneous activity in the british tertiary province and indicates that sub acid rocks of the small isles represent one of the latest episodes of hebridean magmatism pb isotope compositions of these rocks yield an apparent age of 2660 75 ma 2 interpreted as a mixing line between lewisian torridonian and mantle derived pb and indicating the presence of lewisian basement under eigg pb sr and nd isotopic evidence points to contamination of basaltic differentiates and granulite facies lewisian melts by torridonian sandstone but precludes significant involvement of amphibolite facies lewisian which may have been largely removed by erosion before deposition of the torridonian sediments in the small isles

scales of stress heterogeneity near active faults in the santa barbara channel southern california the santa barbara channel represents the offshore portion of the ventura basin in southern california ongoing transpression related to a regional left step in the san andreas fault has led to the formation of e w trending en echelon fault systems that accommodate localized shortening across the basin recent studies have suggested that faults within the channel could be capable of a multisegment rupture and producing a m w 7 7 8 1 tsunamigenic earthquake however dynamic rupture models producing these results do not account for stress heterogeneity with only sparse information available on the stress field in this region further borehole derived stress constraints are essential for obtaining a more comprehensive understanding of the hazards related to the complex fault systems we used caliper logs from 19 wells obtained from industry to identify stress induced borehole breakouts beneath the holly and gail oil platforms in the channel our newly developed forward modeling technique provides constraints on the orientations and relative magnitudes of the three principal stresses at gail we determine a reverse faulting stress regime s hmax 1 7 s hmin 1 6 sv 1 0 and an s hmax azimuth of n45 e our results are consistent with local structures which reflect deeper regional scale trends and with similar studies onshore nearby at holly an s hmax rotation from n36 w to n57 e occurs across 100 m depth in a single well and differs from nearby results indicating that short length scale 10 km laterally and 1 km in depth stress heterogeneity is associated with complex changes in fault geometry

the baffin bay lavas and the value of picrites as analogues of primary magmas the baffin bay picrites have been the focal point of a controversy concerning the mgo content of primary magmas derived from the upper mantle a sample population of 48 lava chilled margins collected across the baffin bay volcanic succession at the northeastern tip of padloping island exhibits a prominent compositional mode between 14 and 16 weight percent mgo 19 22 mg cation units mg 100 cations the petrography of these samples however requires that the padloping magmas were mixtures of olivine crystals and liquid at their eruption olivine phenocrysts constituted 15 to 30 volume percent of these magmas and retain compositions requiring coexisting liquid compositions with only 10 to 13 5 weight percent mgo 14 18 5 mg however highly magnesian olivine xenocrysts up to fo 93 found in the most magnesian lavas require the former existence of liquids with at least 18 weight percent mgo 24 mg if these xenocrysts represent early cumulates then the primary liquids of the padloping suite must have been at least this mgo rich with temperatures greater than 1 425 c such primary liquids could have evolved by olivine crystallization to a steady state equilibrated crystal liquid mixtures in a shallow reservoir system prior to eruption the compositions of the liquids of these mixtures appear to have been perched at the point of plagioclase saturation at approximately 1 275 c

crustal structure and magmatism of north atlantic continental margins enormous variations exist along rifted margins of the north atlantic in the magmatic activity that accompanied continental breakup the north atlantic margins exhibit a full range of magmatic behaviour one end of the spectrum commonly termed volcanic margins is found in the northern north atlantic off greenland norway and northwest britain where huge volumes of igneous rock were added to the continental crust as it rifted and the adjacent oceanic crust is considerably thicker than normal the other end of the spectrum so called non volcanic margins occurs further south off france and spain where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced the volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere large quantities of igneous rock were produced as the continents broke up to form the north atlantic where the newly initiated iceland plume brought abnormally hot mantle beneath the rift much smaller quantities of melt were produced further south where mantle temperatures were normal during rifting the distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years as is common during the initial stages of rifting then the rising asthenospheric mantle has time to cool by conduction heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region as a consequence of the reduced mantle temperatures considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts this explains the abnormally thin oceanic crust found adjacent to the non volcanic rifted continental margins in the north atlantic as the new ocean widens the spreading centre becomes isolated from the continental margins on either side by newly formed warm oceanic lithosphere and the rate of rifting generally increases the oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces and all the melt formed by decompression of the mantle solidifies close to the spreading axis basaltic melts can intrude continental crust far more easily than oceanic crust basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts this redistributes melt laterally towards the continent in both volcanic and to a lesser extent in non volcanic margins and may be a contributory factor in leaving unusually thin crust in the first formed ocean immediately adjacent to some rifted margins it also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the rockall area northwest of britain

lithospheric thinning during the evolution of continental large igneous provinces a case study from the north atlantic tertiary province lithospheric thickness can act as an important physical control on the composition of asthenosphere derived melts by restricting the minimum depth at which decompression melting can occur a case study of plume related tertiary lavas from the isle of mull scotland has revealed the presence of three successive magma types which become progressively more depleted in the incompatible trace elements geochemical modeling suggests that melting initially took place within the garnet iherzolite stability field but subsequently involved more extensive melting 10 of spinel iherzolite at shallower mantle depths these results can be explained in terms of a progressively thinning lithosphere beneath the region during the evolution of the volcanic center the observed degree of lithospheric thinning cannot be explained by extension alone and it appears that some relatively rapid lower lithospheric erosion by the plume head is also required theoretical modeling suggests that lithospheric erosion can be a comparatively rapid process and this has important implications for the various starting plume models

evidence from the rare earth element record of mantle melting for cooling of the tertiary iceland plume widespread flood basalt volcanism and continental rifting in the northeast atlantic in the early tertiary period 55 myr ago have been linked to the mantle plume now residing beneath iceland1 2 3 4 5 although much is known about the present day iceland plume6 7 8 9 its thermal structure composition and position in the early tertiary period remain unresolved estimates of its temperature for example range from 1 600 c in some plume models3 to 1 500 c based on the volume and composition of basaltic crust10 11 12 several recent studies4 have emphasized similarities in the thermal and chemical structure of the tertiary and present day plumes to argue for stability of the mantle anomaly whereas others12 13 relate variations in basalt volumes and compositions to changes in plume flux moreover some authors1 2 13 have assumed that the plume was rift centred for its entire history whereas others argue that it became ridge centred only after plate separation14 15 here we report compositional data for 6 000 metres of flood basalts erupted in east greenland close to the inferred plume axis that we use to constrain the tertiary plume structure rare earth element systematics place limits on the pressures and extents of mantle melting and show that the mantle was initially moderately hot 1 500 c but that its temperature declined during flood volcanism these observations are difficult to reconcile with current plume head models and call for important lithospheric control5 10 16 17 18 on actively upwelling mantle along the rifted margin

snowline depression in the tropics during the last glaciation five primary methods have been used to reconstruct pleistocene snowlines or equilibrium line altitudes elas in the tropics 23 53n 23 53s during the last glaciation but each has inherent errors that limit the accuracy of the results additional potential errorsin determiningela depression involveestimates of modernsnowline altitude datingresolution topographicreconstructionof former glaciers orographic e ects the presence of rockfall debris on glaciers and calculation of regional ela gradients eustatic sea level lowering during the last glaciation is an additional factor in uencing estimates of ela depression ela in cases where modern snowline lies above a mountain summit only a minimum value for ela can be obtained at 12 tropical sites in africa the americas to 103s latitude and paci c islands estimates of average ela range from 440 to 1400m but most fall in the range of 800 1000m mean 1 900 135m in a regional study of ela depression in the southern tropical andes 8 223s an average ela of ca 920 250m has been reported based on the assumption that glacier mass balance was controlled solely by ablation season temperature and assuming a full glacial temperature lapse rate of 63c km depression of mean annual temperature in glaciated alpine areas was ca 5 4 0 83c if adjusted for a sea level fall of 120m at the glacial maximum this value is reduced to 4 7 0 83c the gure is based on the unlikely assumption that accumulation on alpine glaciers has been invariant nevertheless it is similar to values of temperaturedepresson 5 6 43c for the last glaciationobtainedfrom variousterrestrial sites but contrasts with tropical sea surface temperature estimates that are only 1 33c cooler than present 2001 elsevier science ltd all rights reserved

controls on permo carboniferous precipitation over tropical pangaea a gcm sensitivity study abstract a series of late paleozoic climate model simulations was developed using the genesis atmospheric general circulation model to investigate the independent roles of gondwanan deglaciation atmospheric co 2 rise and regional tectonism on the long term 10 6 yr evolution of continental precipitation over tropical pangaea the model results indicate that either deglaciation or a co 2 increase from 1 to 8 present atmospheric levels could have caused substantial drying and warming at low latitudes a result that is consistent with late paleozoic proxy records yet the atmospheric processes that led to drying differ between these cases the deglaciation of gondwana reduces low latitude temperature gradients in the southern hemisphere decreasing by as much as 60 the meridional overturning in the southern hemisphere winter cell of the hadley circulation the decline in overturning reduces convective precipitation over equatorial pangaea in contrast high levels of co 2 have little effect on the large scale meridional overturning circulation but increase continental temperatures leading to high evaporation rates and reduced available soil moisture the source of convective precipitation on land in comparison to deglaciation and p co 2 regional uplift erosion of the central pangaean mountains cpms has a secondary effect on tropical precipitation in genesis and cannot explain the long term aridification and warming of western pangaea we suggest that the deglaciation of gondwana and a concomitant rise in atmospheric p co 2 could explain much of the late paleozoic climate change over low latitude pangaea

tethyan oceanic currents and climate gradients 300 m y ago we reconstruct the oceanic circulation pattern of the tethys ocean 300 m y ago by placing late carboniferous early permian climate sensitive biotic associations from gondwana and laurasia on a pangea paleogeography constrained by selected paleomagnetic data warm climate fossils and facies from iran located at that time along the gondwanan margin of arabia are compatible with the existence in the tethys ocean of a warm subtropical surface current gyre whereas cold surface currents swept the glaciated gondwanan margin at higher southern latitudes redistributing cold biota toward the tropics this tethyan surface current system and the associated narrow zonal barrier show similarities to recent glacial climate patterns when placed on a large scale paleogeographic reconstruction of pangea of the b type it neatly explains the otherwise problematic observation that the carboniferous permian biota of iran and northern arabia is dominated by warm euramerican and or russian taxa that are strikingly different from typical cold gondwanan associations

significance of loessite in the maroon formation middle pennsylvanian to lower permian eagle basin northwest colorado abstract quaternary loess deposits are widespread on the earth s surface yet pre quaternary loess deposits have rarely been reported the maroon formation middle pennsylvanian to lower permian of the eagle basin northwest colorado includes a siltstone dominated facies interpreted as loessite lithified loess along its downwind basin margin the section of inferred loessite in the maroon formation is locally at least 490 m thick and consists in large part of structureless and nearly structureless beds of homogeneous sandy siltstone bed contacts are generally planar to undulatory and are either horizontal or are characterized by gentle relief loessite beds are separated by common claystone drapes and weakly developed paleosols and by rare pond deposits channel deposits and eolian ripp e laminated deposits the loess interpretation is based on 1 the homogeneity and dominance of the sandy silt grain size 2 the relative lack of primary sedimentary structures 3 the gentle character of most bedding contacts and the common mantling of irregular depositional topography 4 the inferred paleogeographic setting and 5 the absence of suitable alternative interpretations the loessite grades laterally into mixed fluvial eolian deposits of the maroon formation in the main part of eagle basin which served as the loessite sediment source deposition of the maroon formation was probably strongly affected by cyclic climatic changes synchronous with fluctuations in late paleozoic continental ice sheets the paleogeography and paleoclimatology of the maroon formation depositional system are not unique suggesting that there are probably many other ancient loessites that have gone unrecognized

compression directions north of the san fernando valley determined from borehole breakouts borehole breakouts in 4 nearly vertical oil wells and several other deviated holes in the san fernando valley and santa susana mountains suggest a maximum horizontal compressive stress direction s h of n49 w these wells provide information about the stress field from 1974 to 1983 in the uppermost crust 3 km depth near the aftershock zones of the january 1994 northridge earthquake and the february 1971 sylmar earthquake this direction of s h is anomalous with respect to the n to nne directions of s h seen in other regional data but is consistent with the structural complexity of this zone including local changes in strike of major thrust fault zones and the presence of lateral ramps in both the santa susana and san fernando faults

compression directions in southern california from santa barbara to los angeles basin obtained from borehole breakouts borehole elongation in 71 drill holes was used to infer breakout orientation and directions of maximum horizontal principal stress s h for six areas west of the san andreas fault in southern california santa barbara ojai central ventura basin east ventura basin west los angeles basin and east los angeles basin breakouts were determined from analysis of oriented four arm caliper data the breakouts form at the position of the maximum compressive stress on the borehole wall if the borehole is vertical and parallel to one of the principal stress directions the breakouts will form parallel to the minimum horizontal principal stress s h orthogonal to the maximum horizontal principal stress s h observations from deviated boreholes permit some constraints on the relative magnitudes of the principal stresses in most cases the data permit either a thrust faulting s v s h s h or strike slip faulting s h s v s h stress regime with ne to nw directions of s h these results are broadly consistent with results from focal mechanism studies hauksson 1990 li 1996 and with breakout and focal mechanism data present in the world stress map database zoback 1992 however we find systematic variations in s h directions suggestive of strong heterogeneity in the stress field at shallow depths similar to that present in the cajon pass borehole shamir and zoback 1992 anomalous nw directions of s h in the san fernando valley region and near the whittier fault may be related to structural complexities and or lateral ramps in nearby fault systems

stress orientations from oil well fractures in alberta and texas many oil wells in alberta exhibit spalling of the walls known as breakouts which elongates the holes with the longer axis aligned northwest southeast this alignment is observed over an area in excess of 4 105 km2 in siltstones sandstones carbonate sediments and one shale formation through the stratigraphic column from devonian to cretaceous it is unrelated to dip of the beds we have elsewhere suggested that these breakouts are produced through stress concentration near the hole walls in a stress field having large unequal horizontal principal stresses and with the larger compression oriented northeast southwest it is probable that this northeast southwest principal stress is 1 this paper adds new data from oil wells in alberta and northern british columbia and shows that the breakouts and by inference the stress orientations are consistent through much of the western canadian sedimentary basin it also uses evidence from hydraulic fracturing in west central alberta and from steam injec

sr and pb isotope micro analysis of plagioclase crystals from skye lavas an insight into open system processes in a flood basalt province crystals in many magmatic rocks are heterogeneous in terms of their isotope composition detailed crystal isotope stratigraphy cis studies have shown that crystals act as reliable recorders of their magma source composition and of the pathways of magma interaction and successfully identify the end members involved in magmatic systems in this paper we have analysed cores and rims and single plagioclase crystals from samples of the skye volcanic centre part of the british palaeogene igneous province the isotope analyses reveal that plagioclase crystals from different volcanic samples through thelava succession in skye skye main lava series preshal more basalts and big feldspar lavas have larger sr and pb isotopic ranges than the isotopic ranges found in hundreds of whole rock analyses the large sr and pb isotopic variation in plagioclase crystals in skye is due to variable degrees of crustal contamination of the parental magmas during fractional crystallization and during the ascent of such magmas to the surface the isotope variation of plagioclase crystals in the early lavas skye main lava series reflects assimilation fractional crystallization processes dominated by the lower crust whereas plagioclase crystals from later erupted magmas big feldspar lavas and preshal more basalts show assimilation of upper crust superimposed on previous lower crustal contamination processes the variability in sr and pb isotopes documented in different crystals within the same rocks shows that the crystals have been aggregated in magmas from different sites of storage and differentiation during the ascent of the magmas to the surface

an exhumed late paleozoic canyon in the rocky mountains abstract landscapes are thought to be youthful particularly those of active orogenic belts unaweep canyon in the colorado rocky mountains a large gorge drained by two opposite flowing creeks is an exception its origin has long been enigmatic but new data indicate that it is an exhumed late paleozoic landform its survival within a region of profound late paleozoic orogenesis demands a reassessment of tectonic models for the ancestral rocky mountains and its form and genesis have significant implications for understanding late paleozoic equatorial climate this discovery highlights the utility of paleogeomorphology as a tectonic and climatic indicator

major element records of variable plume involvement in the north atlantic province tertiary flood basalts major element variations in north atlantic tertiary province primitive early erupted alkaline transitional tholeiite basalts recalculated to a restricted value of mgo give insights into the process of plume related magmatism basalts primitive enough to be crystallizing only olivine were recalculated to a proposed primary magma composition of 15 wt mgo the recalculated dataset shows clear inter element correlations including a strong significant negative correlation between fe and si indicating polybaric melt segregation overlap between basalt compositions and experimental melts from a fertile fe rich low mg number 85 5 peridotite suggests that relative to normal peridotite with mg number 89 the north atlantic basalt source was fe rich linear regression of the experimental data gives apparent pressures of magma segregation of 17 5 37 kbar with intra region variability in the depth derivation from the melt column for each sample thus suggesting that lithospheric thickness lid effect control on magma generation may have been overemphasized in recent studies comparable source composition magma segregation depth and calculated mantle potential temperature 1440 1460 c throughout the province supports the previously suggested plume impact model arriving below east greenland derived from a variably enriched and depleted lower mantle source given the good agreement between conclusions drawn from major element data and previously published results we suggest that restricted mgo recalculated datasets may be usefully applied to study other large igneous provinces

stress induced borehole elongation a comparison between the four arm dipmeter and the borehole televiewer in the auburn geothermal well the nature and origin of borehole elongation recorded by the four arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in auburn new york a preferred orientation n10 w s10 e 10 and a less prominant e w orientation of borehole elongation was observed on two runs of the dipmeter comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures obtained from hydrofracturing tests and televiewer data show that the n10 w s10 e borehole elongations 1 are axisymmetric 2 are aligned with the minimum horizontal stress sh and 3 are not associated with natural fractures intersecting the well these elongations are interpreted as stress induced well bore breakouts the e w elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts this asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter this study demonstrates that the dipmeter can be used to determine the orientation of sh by mapping breakouts confirming the results of earlier less detailed studies and provides a firm basis for mapping regional stress patterns using existing dipmeter data

constraints on pennsylvanian glacioeustatic sea level changes using oxygen isotopes of conodont apatite conodonts from u s midcontinent cyclothems were studied for oxygen isotopes in order to constrain pennsylvanian glacioeustatic sea level fluctuations pennsylvanian deposits of the midcontinent united states are composed of cyclic alternations of thin transgressive limestones offshore gray to black phosphatic shales and thick regressive limestones a sequence that is underlain and overlain by nearshore to terrestrial shales with paleosols and coal beds glacioeustatic sea level fluctuations are considered the primary cause for the formation of these cyclothems oxygen isotope analyses of conodont apatite from the black 20 1 0 5 vienna standard mean ocean water vsmow and gray shale units 20 5 0 5 vsmow show lowest average 18 o values whereas conodont elements from the regressive 21 0 0 3 vsmow and transgressive limestone units 21 1 0 6 vsmow are enriched in 18 o the maximum change in 18 o of conodonts from the black shale and carbonate units from individual cyclothems is 1 7 the 1 7 c difference in 18 o compares relatively well to pleistocene interglacial glacial changes in 18 o of equatorial surface dwelling foraminifers and suggests that pennsylvanian glacioeustatic sea level changes may have been of comparable amplitude however since the pennsylvanian glacial maxima are represented by terrestrial sediments and are not documented in the conodont oxygen isotope record pennsylvanian glacioeustatic sea level changes were probably larger than the 120 m fluctuations recorded for the pleistocene glaciations

sr isotope data for the tertiary lavas of northern ireland evidence for open system petrogenesis the lavas of ne ireland represent the largest remnant of the british and irish tertiary igneous province in this paper we present new sr isotope data which by virtue of extreme variation provide evidence of extensive open system behaviour in the petrogenesis of these flood basalts to some extent the initial 87 sr 86 sr ratios 87 sr 86 sr 60ma 0 703 0 715 record the effects of late stage hydrothermal processes however leaching experiments allow primary magmatic signatures to be inferred with confidence coherent increases in initial 87 sr 86 sr ratio with differentiation indices and incompatible trace element ratios imply a role for crustal contamination very significant differences in the style of crustal interaction are recorded by discrete units within the basalt pile further there are also marked contrasts with tertiary basalts elsewhere in the united kingdom in both cases such variations appear to respond to changes in the basement geology reflecting the important control of crustal wall rocks on the geochemistry of these basalts

lithospheric thickness as a control on basalt geochemistry variations in the thickness of the lithosphere are likely to influence the trace element compositions of basaits by controlling the distribution of melt and the minerals in the melting assemblage in the asthenosphere examples of systematic trace element variations that appear to be related to the thickness of the lithosphere are found in both oceanic and continental basalts in the oceans lithospheric thickness is closely related to age and its control is manifested by a relation between trace element composition and age of basement ocean crust in atlantic ocean island basalts beneath continents lithospheric thickness is less age sensitive but it varies widely in response to extensional tectonics contemporary hot spots which have been implicated in the generation of ancient flood basalts show variations in the rare earth element content of their associated basalts which apparently reflect changes in lithospheric thickness between the time of flood basalt eruption and the present

genesis of flood basalts from eclogite bearing mantle plumes on the basis of the geochemistry of flood basalts we investigate the hypothesis that the source material for these magmas includes a significant eclogite component derived from ancient subducted oceanic crust entrained into mantle plumes at the core mantle boundary we present finite element models of the melting in starting plumes plume heads in both continental and oceanic regions starting plume models in which the plume heads are composed solely of pyrolite mantle are incapable of generating sufficient volumes of melt or any melt at all for reasonable plume head temperatures 1400 1600 c or lithosphere ages 6 25 100 m y this result is in contrast to the results of farnetani and richards 1994 whose models require higher plume head temperatures 1600 1700 c and a weak mobile lithosphere in order to generate volumes of melt similar to flood basalts plumes composed of 15 wt eclogite yield volumes of melt comparable to flood basalts in both oceanic and continental settings the maximum extent of melting in these models is about 10 30 this agrees with the model of hofmann and white 1982 who argued that ocean island basalts are derived from subducted oceanic crust if the embedded eclogite thermally reequilibrates with the surrounding plume material then melt volumes and total extents of melting increase by a additional factor of 2 3 a simple two dimensional rifting model for a slow spreading ocean basin underlain by a lens of hot plume material shows that plume related melting occurs over 50 60 m y a much longer period than for flood basalts

evolution of mauna kea volcano inferences from lava compositions recovered in the hawaii scientific drilling project the lower 776 m of core recovered during the initial phase of the hawaii scientific drilling project hsdp contains lavas erupted from mauna kea volcano tholeiitic and alkalic basalts including an fe ti rich flow are intercalated in the upper 58 m of mauna kea lavas similar basaltic sections are subaerially exposed on the lower east flank of mauna kea the fe ti rich lavas reflect large amounts of clinopyroxene plagioclase and olivine fractionation within the crust and upper mantle but the range from tholeiitic to alkalic compositions reflects variable extents of melting of a garnet bearing source based on abundances of incompatible elements the extent of melting for a basanitoid was a factor of 2 less than that for nearly coeval tholeiitic lavas all flow units in the lower 718 m of the hsdp core are tholeiitic lavas their variability in major element compositions reflect variable accumulation of olivine incompatible element abundance ratios in these lavas reflect a complex temporal variation in extent of melting within the tholeiitic part of the core lavas from 800 m to 950 m formed by the largest extent of melting whereas tholeiitic lavas from the bottom of the core and from just below the tholeiitic to alkalic transition formed by lower degrees of melting inferred melt compositions at 16 mgo show that the 200 to 400 ka mauna kea lavas from the hsdp core and the 250 ka subaerial exposures define an inverse correlation between sio2 and feo contents based on experimental studies this correlation is caused by differing pressures of melt segregation furthermore abundances of nb and sio2 are also inversely correlated in these calculated melts in general the younger lavas are relatively enriched in feo and incompatible elements but are depleted in sio2 these trends are interpreted to reflect an overall trend of increasing pressure of melt segregation and decreasing extent of melting with decreasing eruption age there are however geochemical variations which indicate short term reversals in this long term trend previously the geochemical trends accompanying the transition from tholeiitic to alkalic volcanism at hawaiian volcanoes have been interpreted as reflecting the effects of increasing distance from the plume axis the long term geochemical trends of tholeiitic lavas in the hsdp core also reflect migration of mauna kea away from the hawaiian plume

a comparative sem study on the micromorphology of glacial and nonglacial clasts with varying age and lithology preliminary research on striated clasts from a variety of depositional environments suggests that scanning electron microscopy sem of striated clasts of varying lithology in diamictons may prove useful in interpreting a glacial origin evaluating whether sem analysis of clasts from diamictons is an applicable technique to define a glacial origin requires a better understanding of the microfeatures occurring on glacial and nonglacial clasts we describe microtextures and surface characteristics for samples of quartzite granite limestone basalt chert pillow basalt and quartz pebbles collected from a variety of depositional environments our study suggests that it is possible to differentiate between glacial and nonglacial deposits based on frequency and morphology of diagnostic surface microfeatures observed on entrained clasts these microfeatures are best preserved and identified on competent and monomineralic samples e g quartzite chert limestone and poorly preserved on less resistant and p

palaeomagnetism and radiometric dating of the british tertiary igneous province muck and eigg summary the lavas dykes and other intrusive bodies of muck and eigg have been sampled for palaeomagnetic purposes all the bodies are reversely magnetized except for four dykes of muck which are normally magnetized the lavas show variations of direction of magnetism believed to be a record of secular variation the radiometric dates support the geological evidence that the lavas of eigg and muck predate parts of the rhum central igneous complex the lavas of rhum and canna and some of those of skye the combined evidence shows that most of the activity took place during an r n r n sequence between 63 and 58 ma the younger r n sequence is the same as the first part of the r n r sequence found in mull but the sgurr pitchstone of eigg and parts of the eastern redhills of skye belong to later reversed intervals and correspond to chron 26r and chron 26 the igneous activity occurred over a 10 myr period from about 63 ma and can probably be associated with chrons 27r to 23r of the harland et al time scale

precise 40ar 39ar age for the initiation of palaeogene volcanism in the inner hebrides and its regional significance sanidine crystals from two tuff layers at the base of the palaeocene eigg lava formation on the isle of muck inner hebrides yield step wise ar release plateau ages of 62 8 0 6 2 and 62 4 0 6 ma using the laser 40 ar 39 ar dating technique these determinations which are in accord with but significantly more precise than earlier results provide a definitive age for the inception of igneous activity in the small isles from stratigraphical inferences the muck ages provide precise maximum constraints on the possible age of skye magmatism and possibly most of the british tertiary volcanic province the new ages significantly predate the oldest ages determined so far for tertiary magmatism in ne greenland but are similar to ages obtained from w greenland and offshore se greenland and support the concept that the initiation of regional volcanism was primarily controlled by lithospheric thin spots the muck sanidine age appears to precisely date the first manifestation of the impingement of the iceland mantle plume on the lithosphere beneath nw scotland

generation and polybaric differentiation of east greenland early tertiary flood basalts major element trace element isotope and petrographic studies of magma diverentiation mantle melting the basal volcanics in the kangerdlugssuaq fjord region of the east greenland margin provide constraints on the generation and subsequent diverentiation and contamination histories of magmas during the early stage of continental rifting creating the north atlantic ocean basin most primitive lavas of the succession are introduction shown to have accumulated olivine corrections for crystal ac the distinctive geochemical features of continental flood cumulation yield estimates for erupted primitive liquids with mgo basalts cfbs are strongly controlled by the composition contents between 9 and 13 wt the occurrence of suspended and structure of the continental lithosphere and underfo88 olivine phenocrysts further requires the existence of parental lying mantle significant controversy remains concerning magmas with mgo contents up to 17 wt and feo contents up the balance of contributions from the lithosphere and to 14 wt quantitative modeling of fractionation indicates the mantle and the mechanisms by which the various that kangerdlugssuaq magmas primarily diverentiated at moderate source components contribute to basalt petrogenesis pressures 8 kbar to form dunitic and wehrlitic cumulates at how are basalt compositions influenced by generation depths of 25 km evolved east greenland flood basalts from the in anomalous regions in the deep mantle plumes by scoresby sund region record a more complex fractionation history interaction between rising magmas and the continental involving olivine clinopyroxene fractionation in the lower crust lithospheric mantle and by contamination with conas found for the lower basalts followed by olivine plagioclase tinental crust how are the diverentiation con clinopyroxene fractionation at shallow crustal levels cor tamination magma mixing and storage histories of cfbs relations between sio2 incompatible elements and sr nd isotopic controlled by the structure of the continental lithosphere ratios for the lower basalts are consistent with bulk assimilation and how do the thermal and chemical features of the of up to 15 20 archean leucogneiss with unradiogenic sr and mantle and the extensional history of the continental nd isotopic ratios characteristic of lewisian type lower crust lithosphere avect mantle melting and compositions of comparisons between inferred magma types for lower basalts and primary liquids estimated primary liquids from elsewhere in the north atlantic these questions are best addressed in the context of show complex relationships among feo mgo tio2 contents a geologically and geochemically well constrained cfb and cao al2o3 ratios that relate to regional and temporal changes province such as the north atlantic tertiary province in mantle temperatures lithospheric thickness and involvement of separation of east greenland from northern europe in at least two end member mantle sources we postulate that the the early tertiary at 60 myr ago was accompanied by lower basalts of east greenland were derived from iron poor voluminous volcanic activity the blosseville coast of mantle depleted by prior melt extraction and distinct from morb east greenland extending from kangerdlugssuaq fjord source mantle that was enriched in tio2 and lrees by a to scoresby sund fig 1 contains spectacularly exposed metasomatic event shortly before early tertiary magmatism we sections of lavas these sections begin at the basal contact associate this depleted mantle source with the ancestral iceland of the lavas with underlying cretaceous tertiary sediments and archean basement and continue to the plume anomaly

magma sources and plumbing systems during break up of the se greenland margin preliminary results from odp leg 152 abstract odp leg 152 drilled a transect across the se greenland seaward dipping reflector sequence the earliest and most landward lava flows hole 917a lower and middle series represent a pre break up continental sequence showing an overall variation from olivine basalt to dacite evolution in deep crustal magma reservoirs with a dwindling supply of primitive magma and an increasing role of crustal contamination can account for the variation eruption of picrite and olivine basalt magmas 917a upper series marked a dramatic change in the style of magmatism to one of unrestrained passage of primitive magma from the mantle to the surface during the final phase of break up the younger parts of the seaward dipping reflector sequence 915a and 918d comprise compositionally uniform basalt 7 6 0 8 mgo suggesting that an effective magmatic filtering system was established soon after break up in magma chambers associated with a spreading axis the continental to oceanic magmatic transition is marked by clear changes in composition the continental magmas had a garnet lherzolite source indicated by low y zr and were contaminated with continental lithospheric mantle and crust high ba zr the majority of the break up and oceanic volcanic rocks show no evidence for lithospheric contamination low ba zr and appear to have had a source in progressively shallower depleted morb source asthenosphere increasing y zr low nb zr

climate model comparison of gondwanan and laurentide glaciations geologic studies indicate that the carboniferous glaciation on gondwanaland was approximately as extensive as the ice sheets during the pleistocene however there is one major difference between the climate boundary conditions for the two ice sheets the gondwanan ice sheet was located on a supercontinent three different levels of sensitivity experiments were conducted to examine the effect of the large landmass on the magnitude of summer warming over the ice sheets 1 simulations with present solar luminosity and present orbital forcing resulted in summer temperatures over the gondwanan ice sheet 12 17 c greater than over the pleistocene laurentide ice sheet 2 lowering the solar constant 3 or modifying the seasonal insolation cycle to a cold summer orbit reduced the warming but still led to significant differences between gondwanan and pleistocene simulations and 3 the combined effect of lowering the solar constant and modifying the seasonal insolation cycle to a cold summer orbit resulted in temperature patterns over the gondwanan ice sheet similar to the pleistocene model simulations also predict tropical sea surface temperatures about 4 c less than at present as a result of reduced solar luminosity these results suggest that conditions necessary to explain gondwanan ice sheet stability may be known but required boundary conditions would be more extreme than in the pleistocene although a number of uncertainties remain in these calculations they help to better define critical conditions for glaciation for one of the most prolonged periods of continuous glaciation in earth history

reconstructing early permian tropical climates from chemical weathering indices this study was financially supported by the national natural science foundation of china grants 41302083 41572078 the national basic research program of china grant 2011cb808800 and the fundamental research funds for the central universities grant cugl140402 china university of geosciences wuhan p a cawood acknowledges support from nerc grant ne j021822 1

mauna loan and kilauean tholeiites with low ferromagnesian fractionated 100 mg mg fe2 ratios primary liquids from the upper mantle

pollen percolation rates in euroamerican era cultural deposits in the northeastern united states abstract dimbleby s the palynology of archaeological sites new york academic press 1985 p 3 data from forested sites indicate that pollen percolates downward in soils at the rate of 1 cm in c 30 years data from archaeological sites and former agricultural fields in the northeastern united states suggest that pollen percolates downward at the faster rate of c 1 cm in 4 2 years in soils not sheltered by woodland this in turn suggests that the pollen sequences of former non forest soils preserved by rapid natural and cultural burial record much shorter periods of time than previously believed

co2 forced climate and vegetation instability during late paleozoic deglaciation the late paleozoic deglaciation is the vegetated earth9s only recorded icehouse to greenhouse transition yet the climate dynamics remain enigmatic by using the stable isotopic compositions of soil formed minerals fossil plant matter and shallow water brachiopods we estimated atmospheric partial pressure of carbon dioxide pco2 and tropical marine surface temperatures during this climate transition comparison to southern gondwanan glacial records documents covariance between inferred shifts in pco2 temperature and ice volume consistent with greenhouse gas forcing of climate major restructuring of paleotropical flora in western euramerica occurred in step with climate and pco2 shifts illustrating the biotic impact associated with past co2 forced turnover to a permanent ice free world

a record of glacial aridity and milankovitch scale fluctuations in atmospheric dust from the pennsylvanian tropics abstract atmospheric dust is an important climatic agent as well as a valuable climatic archive this study documents systematic variations in a single upper pennsylvanian shallow water carbonate buildup of the midland basin west texas that was isolated from fluvio deltaic input and consists almost entirely of carbonate bounded by sequence boundaries thin 0 02 to 1 m intervals of dark pyritic siliciclastic mudrock with subtle pedogenic features occur commonly at the sequence boundaries and are interpreted as loessite deposited during lowstands primarily on the basis of paleogeographic isolation of the buildup the silicate mineral fraction smf extracted from the underlying carbonate which includes quartz clay and trace feldspars and heavy minerals varies in concentration weight from 0 015 to 6 69 in the carbonate this smf contains both fine grained detrital and authigenic silica interpreted to be eolian on the basis of geochemical petrographic and grain size characteristics as well as paleogeography hence the smf is considered a proxy for atmospheric dust whole rock geochemistry suggests that the dust was at least partly derived from uplifts of the ancestral rocky mountains with a possible minor contribution from active volcanic centers located to the present day south and southwest the estimated mean accumulation rate mar of dust in highstand carbonate facies full interglacial relative to carbonate facies that formed during relative fall in sea level suggest that dust flux increased 4 to 10 times accompanying the transition to glacial conditions mars in the siliciclastic mudrock which accumulated at lowstand full glacial to incipient transgression suggest glacial phase increases in dust flux of nearly 400 to 4000 times relative to full interglacial conditions spectral analyses of dust abundance through the study section show variations consistent with milankovitch forcing

constraints on the mantle sources of the deccan traps from the petrology and geochemistry of the basalts of gujarat state western india the late cretaceous early tertiary flood basalts in the gujarat area of the northwestern deccan traps kathiawar peninsula pavagadh hills and rajpipla exhibit a wide range of compositions from picrite basalts to rhyolites moreover the basaltic rocks have clearly distinct tio2 contents at any given degree of differentiation and strongly resemble the low titanium and hightitanium basalts found in most of the gondwana continental flood basalt cfb suites four magma groups are petrologically and geochemically distinguished 1 a low ti group characterized by rocks with varying sio2 saturation and with tio2 1 8 wt in picrites nb 19 p p m zr av 6 5 and tt v av 47 3 an intermediate ti group with tio2 contents slightly lower than the high ti rocks at the same degree of evolution and with correspondingly lower incompatible trace element contents and ratios in particular k2o nb ba and zr y av 5 2 4 a potassium rich group kt broadly similar in geochemical character to the high ti group but showing more extreme k rb and ba enrichment av k20 na20 l ba y 20 the most primitive low ti and high ti picrites when corrected for low pressure olivine fractionation show distinct major and trace element geochemistry in particular for cao ai2o3 cao tio2 and al2o3 tio2 and moderate but significant variations in their sio2 and fe2ost contents these characteristics strongly suggest the involvement of different mantle sources more depleted for the low ti picrites and richer in cpxfor the high ti picrites but with broadly the same pressures of equilibration 27 14 kbar this in turn suggests a strong lateral heterogeneity in the gujarat trap mantle low ti picrites and related differentiates in kathiawar are reported systematically for the first time here and suggest the existence of hfse depleted mantle in the northwestern deccan traps with extension at least to the seychelles islands and to the area of the bushe formation near bombay in the pre drift position before the development of the carlsberg ridge the absence of correlations between lile hfse ratios and sio2 argues against crustal contamination processes acting on the low ti picrites possibly owing to their probably rapid uprise to the surface consequently the mantle region of this rock group was probably re enriched by small amounts of ule rich materials the substantially higher trace element enrichment of the least differentiated high ti picrites relative to the basalts of the ambe noli and mahableshwar formations of the western ghats testifies also to the presence of more incompatible element rich oib4ike mantle sources in northern and northwestern gujarat these sources were geochemicaily similar to the present day reunion mantle sources

40ar 39ar step heating ages of the tertiary igneous rocks of mull scotland twenty one samples from throughout the mull tertiary igneous succession were dated of these eight gave a satisfactory age plateau and the dates show a systematic decrease throughout the succession extrusion of the lavas began c 60 ma ago contemporaneous with the bulk of activity in the british tertiary igneous province igneous activity continued until c 57 ma probably the lavas were extruded during the reversely magnetized chron 26r development of the central igneous complex continued into the normal chron 26 while the late dykes were intruded during 25r if this is so then according to the polarity timescale of harland et al 1982 the duration of igneous activity lies between 0 7 and 4 7 ma consistent with the dates the usefulness of the age plateau selection criteria of lanphere dalrymple 1978 is supported by the results

new evidence on the structure of the labrador sea greenland continental margin a 275 km section of a regional seismic reflection profile from the greenland side of the labrador sea has been reprocessed and interpreted three zones of different structural style have been identified the innermost of which is block faulted magnetic modelling based on the seismic line shows that sea floor spreading anomalies are confined to the outermost zone and the oldest identified anomaly is 27n all attempts to model the area landward of anomaly 27n as a series of remanent magnetizations of alternating polarity failed however a model in which the innermost zone is interpreted as extended and block faulted continental crust and the intermediate zone as continental crust intruded and overlain by reversely magnetized igneous material fits the magnetic and seismic data it is concluded that at least in the northern labrador sea sea floor spreading started in palaeocene times chron 27 and large areas formerly thought to be underlain by oceanic crust should now be considered to be continental one plate kinematic consequence may be a reduced need to postulate large strike slip movements along nares strait

coal derived rates of atmospheric dust deposition during the permian despite widespread evidence for atmospheric dust deposition prior to the quaternary quantitative rate data remains sparse as dust influences both climate and biological productivity the absence of quantitative dust data limits the comprehensiveness of models of pre quaternary climate and biogeochemical cycles here we propose that inorganic matter contained in coal primarily records atmospheric dust deposition to test this we use the average concentration of inorganic matter in permian coal to map global patterns and deposition rates of atmospheric dust over pangea the dust accumulation rate is calculated assuming permian peat carbon accumulation rates in temperate climates were similar to holocene rates and accounting for the loss of carbon during coalification coal derived rates vary from 0 02 to 25 g m 2 year 1 values that fall within the present day global range a well constrained east west pattern of dust deposition corresponding to expected palaeoclimate gradients extends across gondwana with maximum dust deposition rates occurring close to arid regions a similar pattern is partially defined over the northern hemisphere patterns are consistent with the presence of two large global dust plumes centred on the tropics the spatial patterns of dust deposition were also compared to dust cycle simulations for the permian made with the community climate system model version 3 ccsm3 key differences between the simulations and the coal data are the lack of evidence for an antarctic dust source higher than expected dust deposition over n and s china and greater dust deposition rates over western gondwana this new coal based dust accumulation rate data expands the pre neogene quantitative record of atmospheric dust and can help to inform and validate models of global circulation and biogeochemical cycles over the past 350 myr

basalts generated by decompressive adiabatic melting of a mantle plume a case study from the isle of skye nw scotland

styles of interaction between aeolian fluvial and shallow marine environments in the pennsylvanian to permian lower cutler beds south east utah usa the pennsylvanian to permian lower cutler beds comprise a 200 m thick mixed continental and shallow marine succession that forms part of the paradox foreland basin fill exposed in and around the canyonlands region of south east utah aeolian facies comprise i sets and compound cosets of trough cross bedded dune sandstone dominated by grain flow and translatent wind ripple strata ii interdune strata characterized by sandstone siltstone and mudstone interbeds with wind ripple wavy and horizontal planar laminated strata resulting from accumulation on a range of dry damp or wet substrate types in the flats and hollows between migrating dunes and iii extensive near flat lying wind rippled sandsheet strata fluvial facies comprise channel fill sandstones lag conglomerates and finer grained overbank sheet flood deposits shallow marine facies comprise carbonate ramp limestones tidal sand ridges and bioturbated marine mudstones during episodes of sand sea construction and accumulation compound transverse dunes migrated primarily to the south and south east whereas south westerly flowing fluvial systems periodically punctuated the dune fields from the north east several vertically stacked aeolian sequences are each truncated at their top by regionally extensive surfaces that are associated with abundant calcified rhizoliths and bleaching of the underlying beds these surfaces record the periodic shutdown and deflation of the dune fields to the level of the palaeo water table during episodes of aeolian quiescence fluvial systems became more widespread forming unconfined braid plains that fed sediment to a coastline that lay to the south west and which ran approximately north west to south east for at least 200 km shallow marine systems repeatedly transgressed across the broad low relief coastal plain on at least 10 separate occasions resulting in the systematic preservation of units of marine limestone and calcarenite between units of non marine aeolian and fluvial strata to form a series of depositional cycles the top of the lower cutler beds is defined by a prominent and laterally extensive marine limestone that represents the last major north eastward directed marine transgression into the basin prior to the onset of exclusively non marine sedimentation of the overlying cedar mesa sandstone styles of interaction between aeolian fluvial and marine facies associations occur on two distinct scales and represent the preserved expression of both small scale autocyclic behaviour of competing coeval depositional systems and larger scale allocyclic changes that record system response to longer term interdependent variations in climatic and eustatic controlling mechanisms the architectural relationships and system interactions observed in the lower cutler beds demonstrate that the succession was generated by several cyclical changes in both climate and relative sea level and that these two external controls probably underwent cyclical change in harmony with each other in the paradox basin during late pennsylvanian and permian times this observation supports the hypothesis that both climate and eustasy were interdependent at this time and were probably responding to a glacio eustatic driving mechanism

geochemical evolution of the tertiary mull volcano western scotland the early tertiary mull volcano western scotland is one of the most dissected and best exposed igneous complexes of the north atlantic province the new and published geochemical data enable us to chart the magmatic evolution of the mull volcano from the oldest lavas through the intrusive rocks of three overlapping igneous centres to the youngest dykes in this study we identify four successive magma types within the remnant volcano the earliest type the mull plateau group comprises mildly alkaline basaltic rocks with steep chondrite normalized rare earth element ree patterns this type is succeeded within the lava succession and dyke swarm by the coire gorm magma type with essentially flat chondrite normalized ree patterns a third magma type represented within the lava and dykes the central mull tholeiites is more depleted in incompatible trace elements than the preceding types and has flat to lree depleted chondrite normalized patterns the major intrusions and cone sheets of mull centre 1 and early centre 2 belong to this magma type midway through the igneous activity associated with centre 2 the magma type changed to become more alkalic and more enriched in incompatible trace elements this magma type the late mull type is found to persist through the cone sheets and major intrusions of centre 3 to the youngest dykes these changes in magma composition were related to variations in the mantle source and depth of partial melting beneath mull and or differences in the efficiency of melt pooling before ascent through the lithosphere with the exception of the early staffa magma sub type part of the mull plateau group the location of magma chambers in which the bulk of contamination occurred changed with time from deep lower crustal lewisian gneiss to shallow upper crustal moine schist intermediate members of the plateau group and the late mull magma type are enriched in fe ti and p relative to the central mull tholeiites we attribute this difference to the more alkalic nature of these suites lower fo2 and the formation of fe3 p complexes in the magma the intermediate rocks were important in magma mixing processes with two types of mixing identified on mull 1 cryptic mixing between basalts and low fe intermediate magmas typified by lavas and early basic cone sheets of the central mull tholeiite magma type 2 observable mingling between rhyolitic magmas and high fe intermediate magmas of the late mull type shown by the mixed magma bodies of the glen more and loch ba ring dykes the main factor in determining which type of mixing occurred appears to have been the density contrast between the various magmas it may safely be maintained that mull includes the most complicated igneous centre as yet accorded detailed examination anywhere in the world bailey et al 1924 memoir of the geological survey of great britain scotland hmso edinburgh 1924

magnesian basalts from the hebrides scotland chemical composition and relationship to the iceland plume abstract the hebridean tertiary igneous province lay 700 900 km south of a presumed plume centre and 500 km landward of a rifted continental margin prior to opening of the northeast atlantic the province includes a small number of magnesian basalts containing olivine phenocrysts with core compositions equal to or in excess of fo 89 olivine liquid equilibrium calculations suggest that these phenocrysts formed from liquids with mgo contents of 14 15 wt and in rare instances 18 20 wt the liquidus temperatures of these magmas imply mantle potential temperatures of 1350 1460 c at distances up to 900 km from the axis of the iceland plume their emplacement at c 60 ma implies either a long established hotspot or active channeling of plume material along the base of the plate towards a region undergoing considerable lower crustal extension

intershield geochemical differences among hawaiian volcanoes implications for source compositions melting process and magma ascent paths as hawaiian volcanoes develop their lavas systematically change in composition and isotopic ratios of sr nd and pb these trends provide important constraints for understanding plume related volcanism as a volcano migrates away from the hotspot there are also geochemical differences between hawaiian shields in particular lavas from adjacent shields such as kilauea and mauna loa on hawaii and koolau and waianae on oahu have significant differences in abundances of some major and incompatible elements and isotopic ratios of sr nd and pb some incompatible element abundance ratios such as zr nb and sr nb are correlated with intershield differences in sr and nd isotope ratios but these isotopic ratios are not correlated with intershield differences in major element composition or even parent daughter abundance ratios such as rb sr and sm nd moreover at kilauea and mauna loa the intershield differences have apparently persisted for a relatively long time perhaps 100 ka these intershield geochemical differences provide important constraints on plume volcanism specifically i each volcano must have distinct magma ascent paths from the region of melt segregation ii the 25 50 km distance between adjacent but geochemically distinct shields requires that the sources vary on a similar scale and that the melt production region is similarly restricted the absence of correlations between lava compositions and radiogenic isotope ratios provides evidence for significant differences in melting process such as each shield forming by a different mean extent of melting with melt segregation at different mean pressures two types of models are consistent with the intershield geochemical differences i a relatively large radius ca 40 km plume conduit with a systematic spatial distribution of geochemical heterogeneities or ii a small radius less than 20 km plume conduit composed of geochemically distinct diapirs because relatively small radius diapirs of limited vertical extent are too small to create the large hawaiian shields a possible alternative is a continuous conduit containing solitary waves which transport geochemically distinct packets of material

anomalous cold in the pangaean tropics comment comment soreghan et al 2008 1 present the hypothesis that glaciation existed to near sea level in the tropical region of pangaea in late paleozoic time their most important evidence for this comes from unaweep canyon a wind gap that crosses the uncompahgre plateau colorado united states the

the dynamical and thermal structure of deep mantle plumes abstract if the interpretation of the d layer at the base of the mantle as a thermal boundary layer with a temperature increment in the order of 800 k is correct then the formation of deep mantle plumes to vent material from it appears inevitable we demonstrate quantitatively that the strong temperature dependence of viscosity guides the upward flow into long lived chimneys that are 20 km in diameter near the base of the mantle and decrease in width with progressive upward softening and partial melting of plume material the speed of flow up the axis of the plume is correspondingly fast 1 6 m y 1 at the base and 4 8 m y 1 at 670 km depth thermal diffusive spreading of a heated plume is compensated by a slow horizontal convergence of mantle material toward the chimney in response to the lower pressure there this convergence which contributes only a small increment to the flux of material up the plume also serves to throttle the flow in the chimney the global plume mass flux necessary to transport 1 6 10 12 w of core heat upward through the mantle is 1 8 10 6 kg s 1 at its base plume material is probably still significantly below its solidus or eutectic temperature but substantial partial melting is very likely as it rises we speculate that a small fraction of this fluid component eventually emerges at the surface in hot spots with the fate of the remainder being unknown the behaviour and properties of d and of plumes are closely coupled not only are plumes a necessary consequence of a thermal boundary layer but their existence is impossible without that layer

anomalous cold in the pangaean tropics reply we thank w hood et al 2009 1 for their comment on our geology paper soreghan et al 2008 2 we recognize that our hypothesis of episodic cold in the pangaean tropics which relies in part on our proposal of a late paleozoic age and glacial origin for unaweep canyon is radical hence

the pennsylvanian pewamo formation and associated haybridge strata toward the resolution of the jurassic ionia red bed problem in the michigan basin u s a abstract two distinct and newly recognized formation scale stratigraphic units predominantly red bed sandstones and shales are found on the surface and in the shallow subsurface of the central michigan region here we use core field and laboratory observations to document these strata which we propose as one new formation the pewamo formation the pewamo formation consists of eolian quartz sandstones with red coloration due to iron oxide diagenesis and interbedded interdune lacustrine siliciclastic mudstones we also describe for the first time the haybridge strata which contains red sandstones red and gray shales and coal and has plant fossils and paleosol features macroscopic plant fossils and palynomorphs suggest a pennsylvanian age for both units these newly described continental deposits provide data about past environments life and climate for the late carboniferous of michigan in addition this study is an evaluation of michigan s controversial jurassic red beds and ionia sandstone

anomalous cold in the pangaean tropics the late paleozoic archives the greatest glaciation of the phanerozoic whereas high latitude gondwanan strata preserve widespread evidence for continental ice the permo carboniferous tropics have long been considered analogous to today9s warm and shielded from the high latitude cold here we report on glacial and periglacial indicators that record episodes of freezing continental temperatures in western equatorial pangaea an exhumed glacial valley and associated deposits record direct evidence for glaciation that extended to low paleoelevations in the ancestral rocky mountains furthermore the permo carboniferous archives the only known occurrence of widespread tropical loess in earth9s history the volume chemistry and provenance of this loess ite is most consistent with glacial derivation together with emerging indicators for cold elsewhere in low latitude pangaea these results suggest that tropical climate was not buffered from the high latitudes and may record glacial interglacial climate shifts of very large magnitude coupled climate ice sheet model simulations demonstrate that low atmospheric co 2 and solar luminosity alone cannot account for such cold and that other factors must be considered in attempting to explain this best known analogue to our present earth

oxygen and hydrogen isotope compositions of paleosol phyllosilicates differential burial histories and determination of middle late pennsylvanian low latitude terrestrial paleotemperatures abstract the clay mineralogy chemistry and stable hydrogen and oxygen isotope compositions were measured from 20 phyllosilicate samples representing 11 pennsylvanian age paleosol profiles taken from three cores in the illinois basin in order to assess their utility as proxies of low latitude terrestrial paleotemperatures the majority of the samples are mineralogical mixtures of illite smectite i s kaolinite and rarely discrete illite samples from a shallowly buried locality in the northern part of the basin are dominantly composed of smectite rich i s with variable amounts of kaolinite and no discrete illite samples from deeply buried interior parts of the basin are composed of illite rich i s variable amounts of kaolinite and discrete illite these phyllosilicate mixtures have 18 o v smow and d v smow values that range from 17 2 to 23 0 and 56 to 27 respectively assuming that the phyllosilicates preserve a record of isotopic equilibrium with pennsylvanian meteoric waters these oxygen and hydrogen isotope values correspond to crystallization temperatures ranging from 22 3 c to 55 3 c the clay mineralogy phyllosilicate 18 o and d values and calculated crystallization temperatures of 44 c to 55 c from deeply buried localities in the interior of the basin are not consistent with a pedogenic origin instead these trends are considered to be the result of diagenetic recrystallization of pedogenic minerals in response to greater depths of burial by 1 5 to 3 km in the southerly basin center localities as well as an interval of middle permian elevated heat flow associated with magmatic intrusions in the southern part of the basin phyllosilicate mineralogy 18 o and d values and calculated phyllosilicate crystallization temperatures from a shallowly buried northern locality in the illinois basin are consistent with a pedogenic origin and reveal a long term warming trend from an average temperature of 23 3 c in the lower desmoinesian to an average temperature of 32 3 c in the missourian this temperature change is coincident with a significant change in the composition of wetland vegetation in euramerica which has been attributed to a shift in low latitude pennsylvanian climate towards warmer and drier conditions in the late pennsylvanian this study reveals the presence of a dynamic late paleozoic paleoequatorial icehouse climate characterized by significant low latitude temperature variability unprecedented on modern earth

processes controlling the distribution of ti and al in weathering profiles siliciclastic sediments and sedimentary rocks abstract the ratio of ti to al varies greatly in primary igneous source rocks these elements are considered to be relatively immobile in most weathering regimes so that tio2 al2o3 ratios have been used to investigate the provenance of sediments and sedimentary rocks in modern weathering profiles that have undergone moderate weathering ti al ratios remain fairly constant so that siliciclastic sediments derived from them should have ti al ratios like that of the source materials under extreme weathering conditions however the upper most weathered parts of profiles commonly show a marked increase in ti al ratio that is thought to be due to preferential translocation of al rich phases under steady state weathering conditions one particular level of the profile may act as the main source of detrital material and may thus control the ti al ratio of sediments produced from it in a glacio fluvial setting minimum weathering moderate sorting there is considerable variation in ti content but al is nearly constant this is due to concentration of ti bearing mafic minerals such as biotite in the fine fraction in modern sediments derived from moderately weathered and sorted material the ti al distribution reflects mixing of al and ti enriched fine grained materials with sands that are depleted in these elements when a plot of tio2 vs al2o3 is constructed for a suite of such sands and muds the resultant trend differs significantly from that shown by a similar suite of unweathered glacio fluvial sediments comparison of these results with data from sedimentary rock suites suggests that trends shown by plots of tio2 vs al2o3 may provide insight into weathering and depositional history in addition to their use as provenance indicators

a model based evaluation of tropical climate in pangaea during the late palaeozoic icehouse abstract the late palaeozoic ice age lpia is the earth s penultimate icehouse climate geological proxies for tropical pangaean climate during the lpia are significantly modulated on 100 400 kyr and sub 100 kyr scales in addition some geological proxies suggest that equatorial continental areas may have been colder and more arid during lpia glacial intervals than during late cenozoic glacial intervals furthermore the relationship between polar and tropical climate variability remains controversial the climate dynamics underlying these phenomena are only partly understood nevertheless past modeling of lpia climate has suggested that precipitation near the equator was modulated by monsoonal circulation that itself was modulated by orbital variability here the earth s climate during asselian sakmarian time 299 284 ma was simulated with the community climate system model version 3 ccsm3 these simulations address model climate sensitivity to direct and indirect effects of glaciation as well as variability in the earth s orbit the results of these simulations suggest that sea level and orbitally forced monsoon variability were the primary controls on tropical precipitation when ice sheets were confined to polar latitudes any impact of orbital forcing on glaciation therefore could have led to aliasing between glacial and monsoonal effects in particular regions perhaps explaining contrasting geologically based interpretations of how tropical climate responded to glaciation glaciation of the central pangaean mountains would have led to widespread cold and aridity in equatorial pangaea but forming these glaciers would have required an unknown additional climate forcing with an unknown impact on tropical precipitation

was a short lived baffin bay plume active prior to initiation of the present icelandic plume clues from the high mg picrites of west greenland abstract the tertiary volcanism of the north atlantic region provides one of the most extensive and best described magmatic records of interaction between a starting plume and continental lithosphere and the role of plumes in continental break up many authors consider the entire volcanic province from baffin island in the north west to scotland in the south east to be the product of a single proto icelandic starting plume head initially centred beneath east greenland whose initial eruptions were strongly influenced by the pre break up structure of the continental lithosphere we demonstrate that the presence of mesozoic palaeocene extensional sedimentary basins that developed within older n s trending mobile belts influenced the location of the earliest on shore tertiary basalts even taking this into account however the prevalence of high magnesium picrites among volcanics bordering the davis strait in the extreme north west of the province and their much lower abundance in east greenland directly above the supposed plume axis contradict the single plume model best estimates of the most refractory liquid compositions erupted in the disko area imply potential temperatures beneath west greenland of 1540 1600 c such high temperatures are more consistent with the axial region of a separate hot plume beneath west greenland than with the periphery of the icelandic plume head west greenland picrites cannot be attributed to an expanding hot doughnut as they are not present in other peripheral areas likely to be swept by such a doughnut uniformly high feo contents and comparison with mckenzie bickle melting parameterisations suggest the west greenland picrites are the product of 25 melting constrained to depths of 60 90 by a lithospheric lid geographical zonation in incompatible element and radiogenic isotope enrichment holm et al 1993 provides support for a separate somewhat earlier plume centred beneath baffin bay picrites from svartenhuk halvo north are more enriched than equivalent picrites on disko and baffin island south this pattern reminiscent of trends along the reykjanes ridge would be radial to a plume axis beneath baffin bay but not to one centred beneath east greenland

effect of borehole deviation on breakout orientations well bore breakouts are zones of spalling and fracture that form on opposite sides of a well bore and tend to change the cross sectional shape of the borehole from circular to roughly elliptical in vertical holes drilled in areas where one principal stress sv is vertical breakouts tend to form at opposite ends of the borehole diameter parallel to the least compressive horizontal principal stress direction sh this paper uses an analytical elastic solution for stress at the wall of a borehole to analyze the rotation of breakout orientations away from the direction of sh as the borehole deviates from the direction of the vertical principal stress the calculated orientations of breakouts in deviated boreholes depend on the type of faulting regime in which the well was drilled i e normal strike slip or thrust on the deviation angle o of the borehole axis from vertical on the angle between the horizontal projection of the borehole axis and the direction of sh and on the relative magnitudes of the three principal stresses sh sh the greatest compressive horizontal stress and sv in the strike slip faulting regime regardless of the values of sv sh and sh a borehole must deviate at least 35 from vertical before the horizontal projection of the breakout orientation differs by more than 10 from sh in the normal and thrust faulting regimes however the borehole deviation angle ocrit required to rotate projected breakout orientations by 10 from sh approaches zero as the value of sh approaches that of sh in the thrust faulting regime only about 3 of all combinations of sv sh and sh will produce ocrit values less than 10 while about 12 of all such combinations will yield ocrit 10 in the normal faulting regime about 12 and 33 of such combinations will yield ocrit 20 in the thrust and normal faulting regimes respectively careful study of changes in breakout orientation as a function of borehole deviation may improve the resolution of inferred stress directions when studying breakouts in deviated boreholes in these two faulting regimes

comment on using borehole breakouts to constrain the complete stress tensor results from the sijan deep drilling project and offshore santa maria basin california by blair j zajac and joann m stock comment on using borehole breakouts to constrain the complete stress tensor results from the sijan deep drilling project and offshore santa maria basin california by blair j zajac and joann m stock

cool water carbonates in a paleoequatorial shallow water environment the paradox of the auernig cyclic sediments upper pennsylvanian carnic alps austria italy and its implications distinct parts of limestones within the upper paleozoic auernig group of the carnic alps austria and italy are characteristic of cool water carbonates the carnic alps were between 5 n and 10 s paleolatitude during the late carboniferous a position confirmed by dasyclad algae and fossil plants the floral association occurrence of coal seams and absence of evaporites indicate a humid tropical environment the entire section lacks abiotic components of typical warm water limestones no ooids and no aggregates occur within the auernig group parts of the limestones show surprisingly a cool water association of high diversity bryozoans brachiopods crinoids red algae sponge spicules and entomozoan ostracodes the genesis of these limestones atypical for a paleoequatorial setting cannot be explained by changes in salinity bathymetry or terrigenous input the water temperature possibly linked with upwelling nutrient supply and paleoceanographic currents is the most convincing cause of this unusual association paleoceanographic changes are interpreted as linked to contemporaneous glaciation deglaciation cycles in gondwana this paper shows that cool water carbonates in shallow water environments are not necessarily nontropical as generally interpreted future studies should consider water temperature oceanic circulation and availability of nutrients supplementary to bathymetry salinity and latitude related climate in interpretation of carbonate components

an exhumed late paleozoic canyon in the rocky mountains a reply we thank william hood for his discussion of soreghan et al 2007 we recognize that our interpretation of the events leading to the formation of unaweep canyon particularly our proposed paleozoic age of the ancestral canyon represents a significant departure from established models validation of our hypotheses regarding its age and origin would force revision of several longaccepted models ranging from the cenozoic tectonic and geomorphic evolution of this region to the climatic and perhaps tectonic framework of the permo pennsylvanian tropics represented by this system accordingly our work deserves close scrutiny and hood s discussion provides such an opportunity hood 2009 begins his discussion by stating that soreghan et al 2007 presented the hypothesis that unaweep canyon is a permian glacial valley that was filled by paleozoic sediment and subsequently exhumed by cenozoic rivers to clarify the focus of soreghan et al 2007 is the hypothesized paleozoic age of the canyon although we posed the question of a possible glacial origin in the final sentences of the article a more complete analysis of the evidence for a glacial origin however appears in soreghan et al 2008 although only abstracts of this aspect e g soreghan et al 2004 were published at the time that hood submitted his discussion nevertheless here we address all of the points raised by hood we treat each of his points using the subheadings he provides reexamination of the field evidence and paleomagnetic data

quantifying the heterogeneity of the tectonic stress field using borehole data the heterogeneity of the tectonic stress field is a fundamental property which influences earthquake dynamics and subsurface engineering self similar scaling of stress heterogeneities is frequently assumed to explain characteristics of earthquakes such as the magnitude frequency relation however observational evidence for such scaling of the stress field heterogeneity is scarce we analyze the local stress orientations using image logs of two closely spaced boreholes in the coso geothermal field with sub vertical and deviated trajectories respectively each spanning about 2 km in depth both the mean and the standard deviation of stress orientation indicators borehole breakouts drilling induced fractures and petal centerline fractures determined from each borehole agree to the limit of the resolution of our method although measurements at specific depths may not we find that the standard deviation in these boreholes strongly depends on the interval length analyzed generally increasing up to a wellbore log length of about 600 m and constant for longer intervals we find the same behavior in global data from the world stress map this suggests that the standard deviation of stress indicators characterizes the heterogeneity of the tectonic stress field rather than the quality of the stress measurement a large standard deviation of a stress measurement might be an expression of strong crustal heterogeneity rather than of an unreliable stress determination robust characterization of stress heterogeneity requires logs that sample stress indicators along a representative sample volume of at least 1 km

intrusion history of the portrush sill county antrim northern ireland evidence for rapid emplacement and high temperature contact metamorphism the gabbroic portrush sill in northern ireland part of the north atlantic igneous province intruded lower jurassic mudstones and siltstones about 55 ma ago we used petrologic observations and geochemical analyses to study how the sill interacted with the sedimentary rocks field relationships show that an upper sill and numerous associated minor intrusions were emplaced in the sedimentary host rocks before intrusion of the main sill some 10 m above its upper contact geochemical analyses reveal two magma contamination processes nb and ta anomalies coupled with incompatible element enrichment record contamination by deep crustal rocks whereas li pb and ba anomalies reveal a superficial contamination through fluid circulation at the contact between magmatic and sedimentary rocks analysis of mineral assemblages and geochemical data from the contact aureole demonstrate uniform metamorphic conditions between the two main intrusions and an absence of a thermal gradient the identification of pyrrhotite by magnetization analyses and of orthopyroxene by microprobe analyses indicates very high temperatures up to 660 c thermal modelling explains these temperatures as the coupled effects of the main sill and the earlier intruded upper sill and minor intrusions even though the chemical composition of the main sill suggests another type of parental liquid all three units were emplaced in a very short time certainly less than five years

a hot spot model for early tertiary volcanism in the n atlantic s u m m a r y the initial stages of rifting of the n atlantic were accompanied by massive extrusion of volcanic rocks and the accretion to the lower crust of even greater volumes of igneous rock emplacement of the bulk of the igneous rocks along the 2000 km long continental rift occurred in as little as 2 3 my the pattern and timing of magmatic activity can be explained by partial melting of anomalously hot asthenosphere as it welled up to fill the space left by the stretched and thinned continental lithosphere asthenosphere temperatures were elevated regionally by 100 150 over a 2000 km diameter region by the mushroom shaped head of hot material fed by a mantle plume which at present is centred beneath iceland

using borehole breakouts to constrain the complete stress tensor results from the sijan deep drilling project and offshore santa maria basin california we use borehole breakouts in nonvertical drill holes to constrain the directions of the three principal stresses and their relative magnitudes in this paper we start by modifying previously published breakout selection criteria to work with highly deviated borehole data we present a forward modeling technique using genetic algorithms and a nongradient n dimensional optimizer to find the best fitting stress state for a set of breakout data the stress state is parameterized by three euler angles and the stress state ratio o a technique is developed to determine the 95 confidence weighted misfit between a model and the data we then map out the 95 misfit confidence limits on the best fitting stress state this technique is applied to data published by qian and pedersen 1991 removing their constraint of a vertical principal stress direction reduces the misfit between the stress state and the breakout data we find that the best fitting stress state they report with a vertical principal stress direction lies outside our 95 confidence limits we also invert breakouts in the offshore santa maria basin california these data show a thrust faulting stress state with the maximum principal stress s 1 at n148 5 e plunging 31 5 the 95 confidence range for the azimuth of s 1 ranges from n143 0 e to n198 1 e the stress ratio o was found to be 0 82l 1 0 584

hercynian high altitude phenomena in the french massif central tectonic implications abstract stratified slope deposits diamictite and sedimentary structures such as water escape craters ghosts of ice crystals and solifluction flowage in the stephanian autunian of the french massif central are related to high altitude periglacial effects the clay mineral assemblage dominated by the association illite chlorite irregular interlayed clays and the slight weathering of feldspathic clasts and granite gneiss pebbles favour the prevalence of a cold to temperate climate at this time rather than a hydrolyzing climate of humid equatorial or tropical type the flora and entomofauna also indicate a seasonally varying climate with cold winters favouring the development of periglacial phenomena at high altitude in view of the equatorial paleogeographic position of the hercynian belt this type of climate could exist only at altitudes higher than 4500 m the tectonic implications of these altitudes are examined through a comparison with the present day himalayas and tibetan plateau

utilization of observations of well bore failure to constrain the orientation and magnitude of crustal stresses application to continental deep sea drilling project and ocean drilling program boreholes

late paleozoic tropical climate response to gondwanan deglaciation coupled climate biome model simulations of the late paleozoic were developed to determine the response of pangean tropical climate to gondwanan deglaciation the model simulations predict substantial changes over equatorial pangea including continental drying a reversal of equatorial winds warming heavier 18 o values of meteoric precipitation and the expansion of deserts and the contraction of forests the magnitude of these tropical responses is sensi tive to the extent of gondwana continental ice and the deglacial rise in atmospheric pco 2 boundary conditions that are not well known for the late paleozoic nonetheless the model predictions are consistent with climatic and environmental trends determined from terrestrial proxy data implying that the deglaciation of gondwana was a transformational climate event in tropical pangea

stress orientations from borehole wall fractures with examples from colorado east texas and northern canada azimuthally aligned breakouts in oil wells are explained as shear fractures in the zone of amplified stress difference near the borehole in a stress field having unequal horizontal principal stres

low latitude ordovician to triassic brachiopod habitat temperatures bhts determined from δ18o brachiopod calcite a cold hard look at ice house tropical oceans abstract some researchers have suggested that phanerozoic land based ice sheets extended occasionally into the tropical realm if true the tropical ocean at those times must have been distinctly colder than at the last glacial maximum lgm when northern hemisphere ice sheets did not extend below 38 latitude low latitude ocean temperatures derived from oxygen isotopes of brachiopod shells test this hypothesis by comparing the temperature regime for paleozoic and early mesozoic low latitude settings to the tropical temperature regime of the modern interstadial ocean and to mean temperatures estimated for the tropical ocean at lgm a running mean of ph adjusted brachiopod habitat temperatures bhts shows that paleozoic low latitude oceans were on average cool to cold relative to the modern interstadial tropical ocean at times during pennsylvanian serpukhovian tournaisian and ordovician silurian glaciations these tropical seas were indeed significantly colder on average than at the lgm ice sheets within tropical latitudes can be reasonably predicted at those times abundant and diverse paleozoic brachiopod communities reflect these cool tropical oceans consistent with modern brachiopod ecological preference for colder waters amplified paleozoic temperature oscillations suggest recurring global warming events which episodically drove these cold tropical oceans to temperatures significantly higher than the warmest modern tropical ocean

the relationship between e and n type magmas in the baffin bay lavas the picritic lavas of baffin island represent one of the most primitive phanerozoic volcanic suites in the world with mgo contents ranging from 22 wt 29 mg cation unit mg 100 cations for olivine rich lavas to 11 wt 16 mg for olivine poor lavas two magma types can be recognized on the basis of trace element and isotopic geochemistry n type magma which dominates the high mgo lavas has depleted lree patterns la sm n 0 6 0 7 typical of n morb k ti 0 5 and 87sr 86sr ranging between 0 7032 0 7039 these two magma types are however virtually indistinguishable in terms of major clements and many other trace elements the e and n type samples are intermixed throughout the volcanic succession indicating that both types of magma erupted contemporaneously although the compositional spectrum observed for major and highly incompatible elements is consistent with olivine fractionation crystal fractionation cannot account for the difference in the lree between e type and n type lavas crustal contamination involving a lower crust composition cannot reproduce the more magnesian e type lavas and can only repoduce the lowest la sm n ratios of the e type lavas if high degrees of assimilation 50 have occurred partial melting models can reasonably account for the distinct la sm n ratios and the similar zr y values of the two magma types but fail to reproduce the observed abundances or ree sr y and zr compositionally different mantle sources are required to explain the two distinct magma types observed in baffin island a model in which the mantle source is a mixture of enriched plume material and depleted entrained mantle in the head of a mantle plume may explain the contemporaneous eruption of n and e type magmas in the baffin bay picritic suite the baffin island e type lavas are less enriched in sr y and zr contents and have lower zr y for similar la sm n than the other e type lavas of the northern north atlantic region