differential stress strain rate and temperatures of mylonitization in the ruby mountains nevada implications for the rate and duration of uplift kno ledge of the m agnitude of the differential stress during the formation of mylonitic rocks provides constramts on mechanical and thermal models for the exhumation of the metamorphosed footwalls of major low angle detachment faults we have analyzed the differential flow stress during the mylonitization of quartzose rocks in the ruby mountains nevada using grain size piezometers and kinetic laws for grain g owth quartzites from mylonitic shear zones in lamoille canyon and secret creek gorge have grain sizes of 91 151 m and 42 64 m respectively the peak temperature during mylonitization was 630 i 50 c and analysis of grain growth kinetics indicates that mylonitization continued during cooling to temperatures s450 c quartz grain size piezometers suggest that the mylonitization occurred under differential stresses crl cr3 of 38 64 mpa or maximum shear stresses of 19 32 mpa extrapolation of quartzite flow laws indicates that the mylonitization occurred at strain rates between 10 10 and 10 13 1 arguments presented m the paper suggest that the likely range of strain rates is 10 11 to 10 12 1 these strain rates are compatible with displacement rates of the order of 23 mm yr l along a 1 5 km thick simple shear zone such a shear zone dipping 15 would produce an uplift rate of 5 8 km m y 1 and a horizontal extensioo rate of 22 km m y l this uplift rate indicates that midcrustal mylonitic rocks could have been lifted up along a 1 5 km thick simple shear zone dipping 15 in 2 6 m y

the hawasina basin a fragment of a starved passive continental margin thrust over the arabian platform during obduction of the sumail nappe abstract a complete revision of the lithostratigraphy of the hawasina nappes in the eastern and central oman mountains as compared to that of the previous stratigraphy leads to a new palaeogeographical reconstruction of the basin five very distinct zones have been determined representing five principal structural units that succeed one another from the external to the internal zones as follows 1 1 the hamrat duru basin 2 2 the baid platform 3 3 the al avidh trough 4 4 the misfah platform and 5 5 the umar basin the history of the hawasina basin began in the late permian with the formation on the northeastern edge of gondwana of a vast intracontinental basin the hamrat duru basin however this basin became a passive continental margin only during the middle late triassic formation of the neo tethys by crustal spreading and accretion began at this time in the internal part of the basin and in continuity with the umar basin the misfah platform and the al aridh trough which formed at the same time within the hawasina basin from this period onwards the main geomorphological features of the basin and of the margin were fixed and persisted up to the late cretaceous the phase of extension by which the basin was subsequently affected during the late tithonian berriasian was mainly manifested by generalized subsidence these main stages in the formation and evolution of the hawasina basin are marked by extensive development of in general alkaline but in places transitional and locally tholeiitic morb magmatism the sediments of the hamrat duru basin are dominated by turbidites composed of material mainly derived from the arabian platform whereas the al aridh trough and umar basin are characterized by pelagic sediments however the al aridh and umar grabens also contain layers of breccia of variable extent containing clasts and blocks of late permian and late triassic shallow water carbonates derived from the adjacent baid and misfah platforms sedimentation in the hawasina basin thus began locally in the late permian and by the triassic it was taking place in the remaining parts of the basin continuing up to the late turonian early senonian when the basin closed and overthrusting during obduction of the sumail ophiolite occurred sedimentation ended the overthrusting took place in three stages 1 1 a southerly directed compressive pre early obduction phase during which the hawasina basin was detached from its substratum to form an obduction prism 2 2 an obduction phase sensu stricto which by gravitational mechanisms induced major displacement of the principal hawasina units and their piling up on the arabian platform with the outermost units at the base and the innermost at the top and 3 3 a late obduction phase which caused local north northeastward backfolding possibly associated with the development of a fracture cleavage which in the basal structural levels may pass into true schistosity

coupled thermo hydro chemical models of compacted bentonite after febex in situ test abstract febex f ull scale e ngineered b arrier ex periment is a demonstration and research project for the engineered barrier of a high level radioactive waste hlw repository febex is based on the spanish reference concept for radioactive waste disposal in crystalline rock according to which canisters are emplaced in horizontal drifts and surrounded by a compacted bentonite clay barrier the project includes two main large scale tests which started in february 1997 an in situ full scale test performed at grimsel switzerland and a mock up test operating at ciemat facilities in madrid spain coupled thermal hydrodynamic and chemical thc models for the engineered barrier have been developed from laboratory experiments and the mock up test dismantling of section 1 of in situ test which took place in the summer of 2002 provides a unique opportunity to test thc model predictions here a numerical thc model of febex in situ test is presented predictions performed with three conceptual geochemical models are compared to measured temperature water content and geochemical data obtained from aqueous extract tests of samples collected after dismantling of heater 1 these data require the use of inverse hydrogeochemical models for their interpretation in order to account for the geochemical processes suffered by bentonite samples during aqueous extraction model predictions reproduce temperatures and capture the trends of water content and most chemical species however there are some discrepancies near the heater which are especially significant for so 4 2 and hco 3 the thc model could be improved by taking into account different types of water in bentonite bentonite swelling protonation deprotonation by surface complexation and co2 degassing and dissolution processes

thermal effects of metamorphic fluids in subduction zones in subduction zones large volumes of fluid are released from the subducting slab during prograde metamorphism of hydrated oceanic crust once released metamorphic fluids tend to migrate upward owing to buoyancy fluid migration can affect the thermal structure of a subduction zone by 1 advecting heat and 2 causing retrograde hydration carbonation reactions in the hanging wall under most p t conditions the volumetric heat capacities of h 2 o and rocks are similar therefore the thermal effects of fluid migration and uplift are comparable calculations suggest that advective heat transport by fluid flow causes relatively minor thermal effects pervasive upward fluid flow results in thermal effects analogous to regional uplift rates on the order of 0 2 mm yr fluid flow channelized along the subduction shear zone translates the local thermal structure of the shear zone updip at 1 mm yr a more important thermal effect results from exothermic retrograde reactions in the hanging wall that release 30 kj mol of h 2 o consumed retrograde reactions significantly retard cooling of the hanging wall just as prograde reactions retard warming of the subducting slab

swaley cross stratification produced by unidirectional flows bencliff grit upper jurassic dorset uk unusual bedforms occur in the bencliff grit at osmington mills dorset the geometry of cross strata in stacked predominantly fine grained sandstone units is dominated by low angle troughs with locally perfectly preserved antiforms which wedge out onto climbing low angle erosion surfaces many of the antiforms hummocks and troughs swales appear to have grown at similar rates of sediment fall out others show draped antiforms and thickening into swales in both cases laminae are concordant with the lower bounding surfaces of the cross sets demonstrating that vertical accretion was the dominant process this geometry together with the conspicuous absence of angle of repose cross strata give the bedforms a resemblance to amalgamated hummocky cross stratification or swaley cross stratification many workers consider amalgamated hummocky cross stratification and swaley cross stratification to be identified with storm sedimentation on the shelf to shoreface thereby bracketting the structures both in terms of process and environment however the structures in the bencliff grit suggest that such a precise interpretation of amalgamated hummocky cross stratification or swaley cross stratification is inadvisable the geometry of the cross stratification suggests deposition under a highly sediment charged but essentially unidirectional flow this study therefore supports recent views that hummocky and swaley styles of cross stratification are in reality a form of low angle trough cross stratification produced primarily by unidirectional flows clearly since undulatory forms of stratification are substrate and process dependent and not environmentally specific great care must be taken in the interpretation of palaeoenvironmental settings especially where exposures are limited as for instance in borehole cores it is unlikely that core data can be used to identify bedforms analogous to hummocky cross stratification or swaley cross stratification unless boreholes are highly deviated

palynological calibration of devonian events at near polar palaeolatitudes in the falkland islands south atlantic in the devonian the falkland islands were part of the high latitude cape basin west falkland palynological assemblages are at a low thermal maturity level but also very low in diversity and dominated by simple spores with rare chitinozoans and acritarchs the south harbour member contains rare verrucate and sculptured trilete spores and is early lochkovian the fish creek member palynofloras are late lochkovian the best correlative datum is the transgressive fox bay formation where the palynological assemblage includes ramochitina magnifica and is equivalent to the sequence b transgression in brazil dated as late pragian earliest emsian the early eifelian upper fox bay formation spore assemblage is comparable to the sequence c transgression in brazil and the chote event the facies and position of the base port philomel formation prasinophyte rich black shale suggests the late eifelian transgression ka ak event above this level is the inception of geminospora lemurata base givetian the upper port stanley formation contains a late but not latest famennian assemblage comparisons with both south africa and south america indicate a number of correlative transgressive and regressive events that match to euramerica these correlations are at a continental scale and confirm eustatic control of these devonian events

creation and preservation of subduction related inverted metamorphic gradients peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost u s cordillera inverted metamorphic gradients are preserved in the pelona orocopia schist southern california the catalina schist california borderland the central metamorphic belt klamath province the south fork mountain schist northern california coast ranges and possibly the shuksan suite northern washington with the exception of the south fork mountain schist peak metamorphic temperatures increase structurally upward from 400 c to 650 c over 1 2 km indicating metamorphic gradients in excess of 100 c km 1 estimates of metamorphic gradients in some terranes such as the catalina schist depend critically on the effects of postmetamorphic faulting the tectonic settings and high metamorphic pressures 800 mpa suggest that the inverted metamorphic gradients formed in paleosubduction zones four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls whereas the pelona orocopia schist formed beneath granitic crystalline rocks the high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones two separate two dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved rapid subduction 10 cm yr 1 beneath young oceanic lithosphere 10 ma results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of 100 c km 1 in the top portion of the downgoing slab in order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and or greenschist facies devolatilization reactions that weaken the descending slab alternatively inverted metamorphic gradients may represent the continuous accretion of material at rates of 1 10 mm yr 1 under conditions of declining temperature continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 ma continued subduction of 1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient synsubduction uplift and or moderately fast postsubduction uplift rates of 1 mm yr 1 are required to preserve the inverted metamorphic gradient the models predict that 1 the high grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction and 2 inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km

kar and 40ar39ar study of metamorphic rocks associated with the oman ophiolite tectonic implications abstract k ar analyses on extracted minerals are reported for a variety of metamorphic rocks associated with the sumail ophiolite amphibolites lying at the sole of the ophiolite yield ages of 95 100 ma which are viewed as reflecting times of crystallization high pressure metamorphics of the saih hatat reveal complex results white micas range in age from 80 to 131 ma whereas blue amphiboles indicate ages that are systematically lower than those of coexisting white micas investigation of a few white micas by the 40 ar 39 ar step heating method yields rather intricate age spectra featuring low apparent ages in the first and the last stages of gas release and high apparent ages in between two explanations can be equally envisaged for these convex upward age spectra the first is the mixing of two generations of micas corresponding to two main metamorphisms the first one m1 is a low to medium temperature high pressure event which conceivably occurred between 130 and 114 m y ago the second m2 overprints m1 and has produced rocks typical of the greenschist facies it took place 80 m y ago and also affected the sole of the ophiolites the second is the presence of excess argon in mica mixtures with complex degassing properties thus the two metamorphic phases identified by microscopic inspection are not significantly different in age they occurred in the 70 80 ma interval moreover k ar dates on amphibole from gabbroic dikes intersecting the peridotites suggest that they are genetically linked to the mafic part of the ophiolites assuming that metamorphism is a tracer of tectonic events we view the infraophiolitic amphibolites as the result of an intraoceanic thrusting which took place near a spreading center nevertheless the uncertainty as to the age of the blueschist metamorphism precludes the possibility of indicating a timetable based on metamorphic ages for the motion shift of africa relative to eurasia during the late cretaceous a tentative model of the obduction of the sumail ophiolite is proposed assuming an age of about 130 ma for m1 we stress differences with respect to earlier reconstructions which advocated an 80 m y old blueschist metamorphism

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

modern shoreface and inner shelf storm deposits off the east texas coast gulf of mexico abstract the modern shoreface and inner shelf sediments off the east texas coast on the gulf of mexico were examined using sediment cores and high resolution seismic data low sediment input and low effective accommodation space resulted in the overall thinness of the modern marine sediment cover in the region erosion on the inner shelf during historic 100 yr times produced patches of sand in a predominantly mud dominated environment storm beds are scarce on the inner shelf the shoreface succession off galveston island is characterized by offshore directed crosscutting storm channels of varying sizes the existence of these channels and the relatively high frequency of storms and hurricanes that have affected the region during historical imes are inconsistent with the paucity of storm deposits on the inner shelf the offshore directed channelized flows may have been prevented from going beyond the shoreface by strong along shelf storm currents that impinged on the shoreface in contrast to the storm record on the east texas shelf the central texas shelf shows a greater frequency of and more laterally persistent storm sand beds due to greater effective accommodation space and higher sediment supply there

the sedimentological significance of concentrically laminated burrows from lower cretaceous ca bentonites oxfordshire concentrically laminated burrows are common at outcrop and in core in mesozoic and younger shallow marine siliciclastic sedimentary rocks they are generally referred to cylindrichnus concentricus howard an imperfectly understood ichnotaxon here considered to be a nomen dubium concentric lamination can be formed in several ways either actively contrived by the producer or by passive filling draught filling of a burrow with constricted apertures in the latter case the fill cannot be regarded as an ichnotaxobase passive infilling of concentrically laminated bow form deep tier burrows is described from lower cretaceous aptian sedimentary rocks associated with fuller9s earth ca bentonite at baulking oxfordshire the distribution and taphonomy provide information on the sedimentological setting of the tephra and associated sediments the lower seam was already compacted and of clay grade before it was repeatedly eroded and colonized the latter has significantly reduced seam quality subsequent sedimentation was essentially aggradational including the uniquely preserved upper bentonite which had a complex recovery history among other ichnotaxa concentrically laminated thalassinoides and possibly diplocraterion may be misidentified as cylindrichnus since thalassinoides and diplocraterion are important indicators of bounding surfaces information of significance in sequence stratigraphy may be lost as a consequence citations of cylindrichnus in the literature are briefly discussed many must be assigned to other ichnotaxa

paleomagnetic results from oman ophiolites related to their emplacement abstract the paleomagnetic results from the cretaceous oman ophiolites suggest large rotations of the ophiolitic nappe during the intraoceanic thrusting and continental obduction which preceded its last emplacement onto the arabian margin and they confirm that the ophiolites were formed and thrust at equatorial latitudes furthermore they may indicate the occurrence of short reversals during the cretaceous long normal polarity period our paleomagnetic study was mainly conducted on metalliferous sediments interbedded within the volcanics of the ophiolites ninety nine cores from 31 samples from fifteen outcrops distributed along the nappe were analysed alternating field and stepwise demagnetisation revealed complex multicomponent magnetisations carried by hematite and magnetite among the various components a stable component was commonly found in each hand specimen from the same outcrop with varying unblocking temperature or field spectra because of varying composition from one sample to another after tilt correction the inclinations of these stable components were all found to be low and the declinations grouped around nw se and nne ssw axes each group containing roughly antipodal directions the occurrence of antipodal directions is inconsistent with the ages of the sediments situated in the middle of the cretaceous long normal polarity period but is consistent with previously published findings of dual polarity in gabbros and dikes the two nw se and nne ssw paleomagnetic groups may correspond to two successive episodes of magnetisation acquired during diagenesis or during high or low temperature hydrothermal activity the large rotations of the nappe that these magnetisations suggest would be consistent with the transport directions recorded within the metamorphic soles in further studies the magnetic mineralogy and the paleomagnetic results will have to be related to hydrothermal activity and the particular magnetic properties of our manganese rich sediments will need to be better understood

marine event beds and recolonization surfaces as revealed by trace fossil analysis turbidites and tempestites exhibit characteristic distributions of trace fossils to gain maximum advantage of the sedimentological and palaeoecological significance of these traces an attempt should be made to relate each trace to the associated colonization surface this approach allows traces initiated at the normal top of an event bed to be distinguished from traces that subsequently passed into the bed from a higher colonization level or were initiated on the eroded surface of the bed

ichnofauna record cryptic marine incursions onto a coastal floodplain at a key lower mississippian tetrapod site ichnofossils from a lower mississippian tournaisian coastal plain succession with a rich vertebrate fauna including early tetrapods record repeated short lived marine interactions that influenced floodplain lake development and ecosystems the ichnofauna contrasts with the more typical scoyenia ichnofacies of other devonian carboniferous tetrapod sites in the tournaisian ballagan formation of the scottish borders chondrites is identified in 128 horizons within a 500 m succession and is associated with lesser occurrences of phycosiphoniform burrows diplocraterion rhizocorallium and zoophycos chondrites and phycosiphoniform burrows commonly occur within dolostones interpreted as part of a saline hypersaline lake facies association containing a non marine to marginal marine macrofauna of bivalves ostracods spirorbis serpula and sarcopterygian fish marine scolecodonts are reported from 18 horizons of diverse lithology four of which co occur with chondrites the single colonisation single tier high ichnofabric index and thin beds mean 10 cm thick that characterise chondrites horizons indicate 1 adverse environmental conditions 2 rapid colonisation of the sediment and 3 short lived bioturbation episodes other ichnotaxa identified are monocraterion and asterosoma which occur exclusively within the overbank or fluvial facies associations and are here interpreted to have lived in freshwater conditions the chondrites trace makers are interpreted as having originated in shallow marine waters and were transported into floodplain lakes by storm surge events although a few examples suggest more significant transgressions ichnofossils are much more common than marine body fossils or scolecodonts and thus record cryptic marine incursions that might otherwise remain unidentified this study contributes to our understanding of the complex mosaic of environments present when terrestrial tetrapods first appeared

laminaset geometries in fine grained shallow marine sequences core data from the rannoch formation north sea and outcrop data from the kennilworth member utah usa and the bencliff grit dorset uk sandbodies from storm dominated marine and marginal marine environments commonly contain intervals of laminated fine sandstones a characteristic of such lamination is the presence of low angle cross lamination in order to model correctly the effects of such lamination on a waterflood of an oil bearing shoreface sequence it was necessary to quantify the geometry of the laminaset elements this challenge has been greatly complicated by the lack of outcrop of the formation of interest the middle jurassic rannoch formation of the north sea only occurs in the subsurface where it is not possible in core to measure the aspect ratio of laminasets directly in this study the laminaset geometry data that can be obtained from core e g apparent set thicknesses were collected for the rannoch formation these data were compared with similar data from potential outcrop analogues in 1 the cretaceous kennilworth member of the blackhawk formation in utah usa and 2 the upper jurassic bencliff grit from the dorset coast uk a quantitative analysis of laminaset geometries has been used to compare subsurface core with potential outcrop analogues the rannoch formation core is characterized by numerous low angle truncations we have measured these features in two wells means of 7 2 and 12 1 mean apparent set thicknesses were 0 24 and 0 19 m in the outcrop sections studied truncation angles ranged from 9 6 to 13 4 and mean set thicknesses from 0 24 to 0 34 m mean bounding surface dips of 5 8 and 8 6 and mean laminaset lengths of 2 3 and 4 1 m were also measured directly in the field and by using photomosaics on the basis of this comparison the kennilworth member in utah was found to be the most suitable and the geometries i e aspect ratios measured there were used to generate an appropriate geometry of rannoch laminaset geometries for use in engineering studies laminaset length 2 0 m laminaset thickness 0 2 m

bedform and stratification characteristics of some modern small scale washover sand bodies washover sand bodies commonly develop along microtidal coastlines in beach barrier island or spit settings wave runup usually in conjunction with an abnormally high water level may overtop the most landward berm of the beach and the foredune crest if one exists to produce overwash and subsequent runoff across the more landward subaerial surface two main elements of the resulting deposit are the washover fan and runoff channel newly formed small scale washover deposits were examined along the outer banks north carolina near pt mugu california and at presque isle lake erie pennsylvania the fans were formed in response to unidirectional landward transport and the runoff channels in response to unidirectional flow usually in a landward direction but sometimes in shore parallel then seaward direction where overwash carried across the fan surface and entered a pond or lagoon a small scale delta microdelta developed in this case the washover fan consisted of two subfacies the wetted but subaerial part of the fan and the subaqueous washover delta flow associated with the development of the fan and runoff channel produced distinctive sets of bedforms and internal stratification high velocity discontinuous surges moving across the fan surface resulted in the development of a plane bed and subhorizontal to low angle landward dipping planar stratification which comprised the major part of the fan similarly rhomboid forms were produced by high velocity sheet flow across the fan surface where flow carried into a standing body of water delta type foreset strata developed for this case the lateral structural sequence was subhorizontal planar stratification merging landward into landward dipping delta tabular foreset strata in the runoff setting where flow became channelized and continuous both upper flow and lower flow regime currents were typical upper flow regime bedforms included antidunes standing waves and plane beds the most commonly observed lower flow regime bedforms included microdelta like bars low amplitude bars linguoid ripples and sinuous crested current ripple trains the sets of sedimentary structures comprising modern washover sand bodies provide criteria for the identification of similar deposits in ancient sediments and for more specific interpretation of the environment

the low temperature brittle ductile transition in a quartzite and the occurrence of cataclastic flow in nature the paper describes the mechanical and microstructural characteristics associated with the brittle to cataclastic flow transition in an orthoquarzite oughtibridge ganister and compares its microstructural development with features of cataclastic deformation of rocks in nature

simulation of the metamorphic and deformational history of the metamorphic sole of the oman ophiolite emplacement of ophiolites onto less dense continental crust is an enigma the rates of emplacement the mechanisms of emplacement and the stress and temperature histories of the rocks are all incompletely understood the oman ophiolite is well exposed and has been studied extensively by field geologists an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during the early stages of the emplacement of the oman ophiolite is developed here to explain field relations and provide insight into the development of the metamorphic rocks at the base of the ophiolite the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating and heat advection by rock the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship in simulations involving young hot subducted lithosphere a wider portion of the subduction zone deforms by power law creep and deformation occurs at lower stresses and slower strain rates minimizing the deformational heating compared to colder older subducted lithosphere dehydration occurs at an accelerated rate in younger hotter lithosphere removing heat from the subducted plate at an earlier stage and causing the zone of brittle deformation to propagate downward more rapidly power law creep of subducted basaltic rocks is limited to temperatures greater than 500 c and occurs at differential stresses of the order of 100 mpa the simulations predict that metamorphic field gradients and the spatial distribution of rock types in the metamorphic sole of tethyan ophiolites might be used to infer the emplacement direction of the ophiolite

evidence of sea level fall in sequence stratigraphy examples from the jurassic comment and reply hallam 1999 1 emphasized the importance of distinguishing stratigraphic condensation from significant erosion of consolidated sediment in order to infer past relative sea level change he then embellished his previously published accounts of classic british jurassic exposures hallam 1988 2

lateral variability in shallow marine ichnofabrics implications for the ichnofabric analysis method ichnological analysis commonly undertaken both in the field and in core studies is generally carried out in vertical logged successions this work documents the results of a detailed ichnofabric study of two highly bioturbated sandstone beds separated by a mudstone within the thick lower jurassic succession of the yorkshire coast uk the two sandstone beds contain relicts of hummocky cross stratification and rare bioclasts of bivalves and belemnites bioturbation of the beds is intense and the diversity of trace fossils is considerable trace fossils present include teichichnus rectus teichichnus isp a thalassinoides isp palaeophycus heberti diplocraterion parallelum rhizocorallium rosselia socialis schaubcylindrichnus isp chondrites isp planolites isp and phycosiphon incertum ichnofabrics were studied at 5 m intervals parallel to the modern cliff line which is parallel to the inferred jurassic palaeo coastline the results demonstrate that there is considerable patchiness to ichnofabric even in a comparatively simple depositional setting such as a storm influenced shelf this observation is in line with studies of the modern sea floor the implications of patchiness in ichnofabric are broad ranging but most importantly suggest that caution is needed in the use of ichnofabric analysis for intra regional correlations

big bedding planes outcrop size and spatial heterogeneity influence trace fossil analyses abstract roadcuts core cliffs and even mountainside exposures often limit the lateral extent to which we can assess bioturbation on bedding planes simply because exposures of such surfaces are generally rare or small such constraints may hamper paleoenvironmental and paleoecological interpretations drawn from facies where most bioturbation is bed parallel or patchily distributed to explore how acute such constraints are we conducted a pilot study of bioturbation in three exceptionally large 5 m2 well exposed paleoenvironmentally similar bedding planes from the cambrian of wisconsin the goal was to test the hypothesis that trace fossils and biogenic sediment disruption can be distributed heterogeneously at the scale of meters to decameters mapping the size variation and distribution of trace fossils across these bedding planes reveals variable patchiness individual bioturbated patches tend to be large m scale but the density of biogenic sediment disruption within these patches is quite heterogeneous in addition bioturbated areas are separated by zones of undisturbed sediment that vary in size from 3 m2 these observations suggest that ancient bedding planes like modern substrate surfaces can exhibit considerable lateral bioturbation heterogeneity at a range of spatial scales caution is thus urged when extrapolating the distribution and diversity of trace fossils and other biogenic sedimentary structures across broad sedimentary horizons based on small 1 m2 bedding plane exposures

genesis of the batinah melange above the semail ophiolite oman abstract the batinah melange which overlies the late cretaceous semail ophiolite in the northern oman mountains comprises mostly sedimentary rocks of deep water facies alkalic lavas and intrusives all of continental margin affinities together with smaller volumes of semail ophiolitic and metamorphic rocks four intergradational textural types of melange can be recognized sheet melange has large 1 km intact sheets either with little intervening matrix or set in other melange types and with an organised sheet orientation fabric slab melange is finer textured 100 m and more disrupted block melange has smaller m blocks with some matrix and a weak to random block fabric clast melange is matrix supported rudite with a weak depositional clast fabric structural relationships particularly the absence of tectonic fabrics the decreasing strength of fragment fabrics with increasing fragmentation and the abundance of brittle fragmentation suggest that these melange types formed by either gravity driven sedimentary processes or superficial sliding or thrusting of individual rock slabs in the slab melange long sequences can be pieced together passing up from upper triassic mafic sub marine extrusives and sediments into radiolarian cherts hemipelagic and redeposited limestones and terminating in non calcareous radiolarities with mn deposits of early cretaceous age mafic sills are numerous these sequences can be matched with sub ophiolite rocks now exposed in fault corridors through the semail these sequences become progressively disrupted upwards in the corridors and can be traced continuously into overlying melange which then thins away from the corridors we argue that during late cretaceous emplacement over the arabian margin active fault corridors split the semail slab and acted as conduits up which sub ophiolite rocks were supplied to the ophiolite surface there the rocks were redisributed by superficial processes

the upper batinah complex oman allochthonous sediment sheets above the semail ophiolite the upper batinah complex consists of allochthonous sedimentary sheets that structurally overlie both the semail ophiolite and its cover of melange lower batinah complex sequences represent a former sedimentary transition across the outer part of a passive mesozoic continental margin from proximal to distal units include redeposited quartzitic sandstones and shallow water limestones large carbonate buildups floored by triassic mafic lavas and pelagic radiolarites or hemi pelagic calcilutites also underlain by triassic mafic lavas sheets of these upper batinah complex sequences were emplaced onto the upper surface of the semail ophiolite in late cretaceous time during or soon after the advance of the semail nappe onto the arabian margin emplacement was along subhorizontal fault zones with sheets of distal facies followed by successively more proximal sheets structural style suggests a low overburden during deformation with ductile flow of argillaceous material but more brittle behavior of othe