bioturbation influence on reservoir quality a case study from the cretaceous ben nevis formation jeanne d arc basin offshore newfoundland canada the delineation ben nevis l 55 well located in the hebron ben nevis field offshore newfoundland targeted the cretaceous ben nevis formation in the petroleum rich jeanne d arc basin this case study focuses on the bioturbated net pay horizons and assesses the importance of animal sediment interactions in controlling the porosity and permeability of sandstone reservoir intervals our data reveal that bioturbation can either reduce permeability and porosity by as much as approximately 33 or enhance it by up to 600 dependent on burrow type and behavior of the trace making organism the net pay interval in the cored interval of ben nevis l 55 is characterized by ophiomorpha dominated ichnofabrics the action of bioturbators can be classified in terms of sediment mixing sediment cleaning sediment packing and pipe work building strategies bioturbation has the potential to 1 increase isotropy or uniformity of grain size by destroying sedimentary laminae through burrow homogenization or 2 decrease isotropy by selectively sorting grains into burrow lining and fill by grain size and through creation of open burrow systems filled with later sediments of differing character to the host sediment the petrophysical characteristics of the reservoir facies are highly dependent on trace fossil morphology presence or absence of burrow linings nature of burrow fills burrow size and bioturbation intensity mudstone rich facies and ichnofabrics containing mudstone filled and or lined burrows e g ophiomorpha and clusters of chondrites have the net effect of permeability reduction in contrast permeability enhancement is documented from muddy sandstone facies with clean sand filled burrows e g thalassinoides and clean sandstones with burrow mottled or diffuse to massive textures

eclogite blueschist relationships as evidenced by mineral equilibria in the high pressure metabasic rocks of sifnos cycladic islands greece

high pressure eclogites in northern jiangsu southern shandong province eastern china abstract eclogites are distributed for more than 500 km along a major tectonic boundary between the sino korean and yangtze cratons in central and eastern china these eclogites usually have high p assemblages including omphacite kyanite and or coesite or its pseudomorph and form a high p eclogite terrane they occur as isolated lenses or blocks 10 cm to 300 m long in gneisses type i serpentinized garnet peridotites type ii and marbles type iii type i eclogites were formed by prograde metamorphism and their primary metamorphic mineral assemblage consists mainly of garnet pyrope prp 15 40 mol omphacite jadeite jd 34 64 mol pargasitic amphibole kyanite phengitic muscovite zoisite an sio2 phase apatite rutile and zircon type ii eclogites characteristically contain no sio2 phase and are divided into prograde eclogites and mantle derived eclogites the prograde eclogites of type ii are petrographically similar to type i eclogites the mantle derived eclogites have high mgo feo fe2o3 and cr2o3 compositions in bulk rock and minerals and consist mainly of pyrope rich garnet prp 48 60 mol sodic augite jd 10 27 mol and rutile type iii eclogites have an unusual mineral assemblage of grossular rich grs 57 mol garnet omphacite jd 30 34 mol pargasite rutile pargasitic and taramitic amphiboles calcic plagioclase an68 epidote zoisite k feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites this suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and or amphibolite facies rocks with near isothermal compression reflecting crustal thickening during continent continent collision of late proterozoic age equilibrium conditions of the prograde eclogites range from p 26 kbar and t 500 750 c in the western part to p 28 kbar and t 810 880 c in the eastern part of the high p eclogite terrane the prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part subsequent reactions manifested by 1 narrow rims of sodic plagioclase or paragonite on kyanite and 2 symplectites between omphacite and quartz are interpreted as an effect of near isothermal decompression during the retrograde stage the conditions at which symplectites re equilibrated tend to increase from west p 9 kbar and t 680 c equilibrium temperatures of type ii mantle derived eclogites and type iii eclogite are 730 750 c and 680 c respectively

coesite bearing eclogites from the dabie mountains central china petrogenesis p t paths and implications for regional tectonics coesite and kyanite bearing eclogites are abundant in the southern part of the dabie mountains southern dabie terrane sdt two types of eclogites from the sdt types iii and iv were selected for detailed paragenetic study type iii eclogites most abundant in the northern part of the sdt occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite garnet phengite epidote coesite kyanite carbonate rutile ilmenite these minerals exhibit weak compositional zoning and contain few mineral inclusions type iv eclogites mostly in the southern part of the sdt occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite garnet glaucophane kyanite epidote phengite quartz rutile ilmenite garnets of type iv eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite phengite epidote quartz and rutile in the core and omphacite barroisite and mg katophorite in the rim prograd

interpreting high pressure phengite 40 ar 39 ar laserprobe ages an example from saih hatat ne oman new single grain fusion and core rim 40ar 39ar laserprobe phengite data from the saih hatat high pressure terrane in ne oman show that individual samples yield a range of apparent ages which is similar to that previously reported from across the entire terrane the majority of the determined ages are older than the previously reported u pb zircon peak metamorphic age core to rim age variations within individual grains range from no discernible difference across the grain to grains with older cores or rarely older rims some samples manifest all three patterns numerical diffusion modelling shows that due to the peak temperature of ca 550 c the measured apparent ages cannot be explained by simple cooling or by partial retention of crystallisation or detrital ages in an open system the age variability is better explained by spatially and temporally variable open or closed system behaviour at the mm cm scale coupled with pervasive and heterogeneously distributed excess argon anomalously old eclogite phengite 40ar 39ar ages are due either to internally derived 40ar inherited from a k bearing precursor or externally derived 40ar distributed by grain boundary fluids mica rich schists within the eclogite boudins yield younger phengite ages suggesting excess argon was absent or diluted pelites hosting the eclogite appear to have been affected by later fluid ingress during deformation and greenschist facies overprint and yield very variable ages commonly with apparently older rims on younger cores the grain and sample scale age variations measured in saih hatat indicate that the grain boundary network in eclogite pods was not an efficient transfer pathway for argon transport whereas the grain boundary network in the surrounding pelites acted as a more efficient pathway on the timescale of the metamorphic cycle

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

lawsonite omphacite bearing metabasites of the pam peninsula ne new caledonia evidence for disrupted blueschist to eclogite facies conditions

prominence of ichnologically influenced macroporosity in the karst biscayne aquifer stratiform super k zones a combination of cyclostratigraphic ichnologic and borehole geophysical analyses of continuous core holes tracer test analyses and lattice boltzmann fl ow simulations was used to quantify biogenic macroporosity and permeability of the biscayne aquifer southeastern florida biogenic macroporosity largely manifests as 1 ichnogenic macroporosity primarily related to postdepositional burrowing activity by callianassid shrimp and fossilization of components of their complex burrow systems ophiomorpha and 2 biomoldic macroporosity originating from dissolution of fossil hard parts principally mollusk shells ophiomorpha dominated ichnofabric provides the greatest contribution to hydrologic characteristics in the biscayne aquifer in a 345 km 2 study area stratiform tabular shaped units of thalassinidean associated macroporosity are commonly confi ned to the lower part of upward shallowing highfrequency cycles throughout aggradational cycles and in one case they stack vertically within the lower part of a high frequency cycle set broad continuity of many of the macroporous units concentrates groundwater fl ow in extremely permeable passageways thus making the aquifer vulnerable to long distance transport of contaminants ichnogenic macroporosity represents an alternative pathway for concentrated groundwater fl ow that differs considerably from standard karst fl ow system paradigms which describe groundwater movement through fractures and cavernous dissolution features permeabilities were calculated using lattice boltzmann methods lbms applied to computer renderings assembled from x ray computed tomography scans of various biogenic macroporous limestone samples the highest simulated lbm permeabilities were about fi ve orders of magnitude greater than standard laboratory measurements using air permeability methods which are limited in their application to extremely permeable macroporous rock samples based on their close conformance to analytical solutions for pipe fl ow lbms offer a new means of obtaining accurate permeability values for such materials we suggest that the stratiform ichnogenic groundwater fl ow zones have permeabilities even more extreme 2 5 orders of magnitude higher than the jurassic super k zones of the giant ghawar oil fi eld the fl ow zones of the pleistocene biscayne aquifer provide examples of ichnogenic macroporosity for comparative analysis of origin and evolution in other carbonate aquifers as well as petroleum reservoirs

metamorphic evolution of the susunai metabasites in southern sakhalin russian republic the susunai complex of southeast sakhalin represents a subduction related accretionary complex of pelitic and basic rocks two stages of metamorphism are recognized 1 a local low p t event characterized by si poor calcic amphiboles 2 a regional high p t event characterized by pumpellyite actinolite epidote sodic amphibole sodic pyroxene stilpnomelane and aragonite the major mineral assemblages of the high p t susunai metabasites contain pumpellyite epidote actinolite chlorite epidote actinolite chlorite epidote na amphibole na pyroxene chlorite haematite the na amphibole is commonly magnesioriebeckite the na pyroxene is jadeite poor aegirine to aegirine augite application of empirically and experimentally based thermobarometers suggests peak conditions of t 250 300c p 4 7 6 kbar textural relationships in susunai metabasite samples and a petrogenetic grid calculated for the fe3 rich basaltic system suggest that pressure and temperature increased during prograde metamorphism

eye to eye with a mega sheath fold a case study from wadi mayh northern oman mountains sheath folds are highly curvilinear folds typically considered to develop by hinge rotation toward the transport direction during intense deformation most sheath folds are recognized on the centi meter to meter scale with very few detailed studies of larger kilometer scale structures we here present evidence for a megascale sheath fold superbly exposed in the northern oman mountains the saih hatat culmination consists of an exhumed highpressure subduction zone with deep level eclogites and garnet blueschists as sifah unit epidote blueschists hulw unit and carpholite bearing metasediments part of the upper plate sheath folds are present at a variety of scales throughout the as sifah hulw and upper plate tectonic packages curvilinear fold hinges along a 15 km long profi le outline the wadi mayh mega sheath fold which closes to the ssw and faces to the nne it consists of at least four secondary sheath folds with cross sectional y z eye folds measur ing 200 m stacked on top of one another within permian and triassic shelf car bonates these eye folds display highly elliptical geometries ryz 5 and cat s eye fold patterns consistent with simple general shear the sheath fold is enveloped by ordovician and older rocks that do not show equal amounts of internal deformation the upper bounding envelope is a detachment that has been subsequently sheath folded the lower boundary of the sheath fold is the upper plate lower plate detachment which forms a major high strain shear zone showing little vertical displacement but signifi cant horizontal displacement the sheath fold displays a ssw elongation x axis of at least 15 km and possibly up to 25 km coupled with signifi cant amounts of vertical fl attening and wnw ese shortening

interlayered eclogites blueschists and epidote amphibolites from ne oman a record of protolith compositional control and limited fluid infiltration abbreviations ab albite ac acmite act actinolite alm aleclogites blueschists clinopyroxene rich rocks and epidote ammandine aln allanite ap apatite aug augite bar barroisite bt biotite phibolites occur interlayered on a millimetre to decimetre scale in a cc calcite chl chlorite cpx clinopyroxene crs crossite czo clinozoisite mafic unit interbedded with calcareous and quartz mica schists in dol dolomite ep epidote fe ox fe oxides gln glaucophane gross ne oman these rocks constitute part of the continental margin grossularite gt garnet hm haematite jd jadeite kln kaolinite kspar of oman subducted during the cretaceous petrological data indicate k feldspar mgt magnetite oxychl oxychlorite pg paragonite ph that they evolved through a clockwise p t path and attained peak phengite qz quartz pyrp pyrope rt rutile spess spessartine tc talc conditions at t 560 c p 12 kbar where the mineral ttn titanite win winchite zrn zircon assemblages of the blueschists eclogites and clinopyroxene rich rocks were all stable whole rock major trace and rare earth element data indicate that the mineralogical differences between these rock types resulted from the metamorphism of interlayered basaltic flows introduction and tuffs of variable compositions possibly reflecting different degrees interlayered blueschists eclogites and or greenschists are of magmatic differentiation the interlayered epidote amphibolites common in several areas such as cazadero california are chemically similar to the blueschists and appear to have formed e g oh et al 1991 the shuksan suite washington from the eclogites and blueschists during unroofing aided by fluid state dungan et al 1983 owen 1989 the tauern infiltration oxygen isotope data for minerals and whole rocks window eastern alps e g selverstone et al 1992 and indicate low fluid rock ratios during metamorphism and unroofing several cycladic islands e g okrusch brocker 1990 the data also suggest that different layers may have interacted with this interlayering has been attributed to 1 differences fluids of different isotopic signatures throughout their history these in the p t conditions of equilibration in the different results are similar to data reported for other areas of continent layers e g reinsch 1979 ridley 1984 2 original continent collision or continental crust subduction differences in the chemical compositions of the layers before metamorphism e g dungan et al 1983 oh et al 1991 or 3 differences in the extent of retrograde overprinting as a result of variable fluid infiltration e g schliestedt matthews 1987 brocker 1990 barrientos selverstone 1993

petrology of fe mg carpholite bearing metasediments from ne oman abstract fe mg carpholite occurs in metasediments of tectonically disrupted basement shelf and foreland basin units that structurally underlie the semail ophiolite in ne oman in the lower grade structurally higher units fe rich carpholite coexists with paragonite quartz illite kaolinite and chlorite whereas in deeper units fe mg carpholite occurs with pyrophyllite sudoite phengite and or chloritoid mineral compositions in these units indicate that chlorite is more magnesian than coexisting fe mg carpholite at low temperatures and pressures but at higher metamorphic grades xmg decreases in the order sudoite carpholite chlorite chloritoid this suggests a reversal in fe mg partitioning between fe mg carpholite and chlorite at temperatures below or close to those of the breakdown of kaolinite quartz to pyrophyllite and at xmg 0 35 phase relations and mineral equilibria indicate that the p t conditions of formation of the fe mg carpholite bearing rocks of ne oman range from 280 315 c 3 6 kbar for the structurally highest units to 325 440 c 6 9 5 kbar for the deepest units indicating a systematic down section increase in metamorphic grade textural relations in these rocks interpreted in the context of pertinent equilibria are consistent with the clockwise p t paths previously constrained for these units from petrological studies of interlayered isofacial mafic rocks

sedimentary and tectonic origin of a transgressive surface of erosion viking formation alberta canada the upper albian viking formation in the subsurface of alberta contains at least four major erosional surfaces or bounding discontinuities the highest of these surfaces viking erosion surface 4 ve4 can be correlated basinwide there is an easily recognized well log marker in the shales 10 50 m above ve4 known as the base of fish scales bfs an isopach map of the bfs ve4 interval has been constructed using 9987 wells over an area of about 85 800 km 2 the depositional surface represented by bfs was probably smooth and almost flat during deposition and the isopach map can be interpreted as showing the topography of the ve4 surface the surface is stratigraphically deepest in the northeast and rises southwestward its topography is characterized by a broad east west trending arch and two linear steps that trend northwest southeast the ve4 surface is overlain by a coarse lag a few centimeters to about 5 m thick with clasts up to 15 cm proximally sw and 7 mm distally ne topographic relief on ve4 is at least 40 m ve4 is interpreted as having formed initially during a major lowering of relative sea level with modification of the subaerial topography and removal of all evidence of subaerial exposure during the ensuing southwestward marine transgression thus ve4 is a transgressive surface of erosion several well log markers onlap the ve4 surface they represent coarse to granule sandstones with extremely sharp bases these tongues of sandstone can be traced up to 20 km basinward from their points of onlap and are interpreted as thin lower shoreface sandstones that formed during minor forced regressions super imposed on the overall transgression the initial regression that immediately preceded ve4 implies a fall of relative sea level of at least 40 m and a northeastward movement of the shoreline of at least 250 km these two figures imply a basinward gradient of about 0 00016 structure contour maps of the top of the mannville group albian and the top of the wabamun upper devonian show smooth unbroken surfaces in the study area with strikes different from those of the arch and steps thus there appear to be no older structural elements that influence viking trends the broad east west trending arch cannot be identified at wabamun mannville or older viking horizons and had disappeared before the bfs horizon was deposited it is at least 170 km wide and 6 m high and formed shortly before the ve4 transgression thus there may have been structural control confined to the viking that influenced changes in relative sea level at the end of viking deposition

a recalibration of the chlorite biotite muscovite geobarometer massonne 1981 reported experiments on the composition of muscovite in the assemblage phlogopite muscovite k feldspar quart the experimental data have been used to recalculate the equilibrium conditions for the chlorite muscovite biotite quartz assemblage which has been proposed as a new geobarometer by powell and evans 1983 the recalculated equilibrium shows much steeperdp dt slopes than those calculated by powell and evans 1983 nevertheless with the necessary precautions it is suggested that the barometer may provide the critical user with valuable pressure estimates

overview of tectonic settings related to the rifting and opening of mesozoic ocean basins in the eastern tethys oman himalayas and eastern mediterranean regions

experimental investigations of blueschist greenschist transition equilibria pressure dependence ofal2o3 contents in sodic amphiboles a new geobarometer

the stability of ca na pyroxene in low grade metabasites of high pressure intermediate facies series minor metamorphic ca na pyroxene occurs as minute crystals and as thin rims around primary augites in metabasites intermediate between the prehnite pumpellyite pp and pumpellyite actinolite pa facies in the sanbagawa belt shikoku japan their compositions vary systematically with mineral paragenesis from augite to acmite with a constant jadeite component of about 20 molvo the ca na pyroxenes in the pumpellyite bearing assemblages contain a very low acmite component acaaugssjd2s which increases through ac25aug55jd2s in the alkali amphibole bearing and ac66augzsjdrs in the hematite bearing assemblages finally attaining an aegirine composition ac65aug1sjd25 in the magnetite bearing assemblages they are distinguished from relict igneous augites chemically by very low opx and tio2 contents and morphologically by fibrous crystal habit the pyroxene bearing metabasites contain assemblages of chlorite pyroxene pumpellyite epidote actinolite alkali amphibole stilpnomelane hematite and magnetite which are incompatible with the higher grade and widely developed pumpellyiteactinolite facies assemblage epidote chlorite xr 0 50 actinolite pumpellyite the reaction augite chlorite pumpellyite actinolite h2o has been suggested for such a transition the stability relations of augite chlorite pumpellyite were analyzed from comparative mineral parageneses in low grade metabasites from new caledonia new zealand and the appalachians the assemblage may have a wedge shaped p t space bounded against the high tpumpellyite actinolite facies by the above reaction and against the low t prehnite pumpellyite facies by the reaction prehnite actinolite augite chlorite with probable negative slope from the mineral parageneses in the metabasites of the sanbagawa belt and elsewhere it is concluded that the ca na pyroxenes occur within a restricted p t space between the prehnite pumpellyite and pumpellyite actinolite facies

structural evolution of an a type subduction zone lofoten rombak area northern scandinavian caledonides the climactic phase of the caledonian orogeny in scandinavia involved collision of the baltic and greenland cratons in late ordovician to silurian time we infer that the collision resulted in large scale subduction of the leading edge of the baltic craton beneath an andean type greenland continental margin deep erosional levels in the lofoten rombak area norway sweden 69 n permit direct examination and evaluation of the structural evolution of a continental or a type subduction zone the early stages of collision involved en bloc underthrusting of the continental margin to depths of at least 30 km during this process deformational effects in the lower plate were restricted to its upper few hundred meters ultimately en bloc a type subduction should be limted by buoyancy effects as convergence continued the lower plate became involved in a series of imbricate thrusts with the same vergence as the intial subduction zone the temporal and spatial relationships between these basement involved thrusts are similar to those observed in foreland fold and thrust terrains field and geophysical data bearing on lower plate behavior elsewhere in the caledonldes and in other collisional orogens suggest that the structural features observed in the lofoten rombak area may be generally characteristic of a type subduction in continent continent collisional settings

constrictional extensional tectonics in the northern oman mountains its role in culmination development and the exhumation of the subducted arabian continental margin

foraminiferal zonation of the viking interval in the hasler shale albian northeastern british columbia a dominantly arenaceous foraminiferal fauna of over 33 genera and 65 species was recovered from silty shales in the mid portion of the hasler formation in the upper albian part of the fort st john group the assemblages lie stratigraphically below occurrences of neogastroplites and below the miliammina manitobensis zone the foraminifera come from above the occurrence of haplophragmoides gigas gigas although assignment of the beds is to the upper part of the h gigas zone the upper part of the haplophragmoides gigas zone may be divided into several subzones above the h gigas gigas sub zone in ascending order viz h gigas ssp a subzone reophax tundraensis subzone trochammina depressa subzone trochammina umiatensis subzone and reophax troyeri subzone the assemblages have close affinities with the faunas of both the overlying and underlying shales and are an expression of a late shallowing deepening cycle within the skull creek joli fou seaway they were penecontemporaneous with viking sandstone

tectonics of the arabian margin associated with the formation and exhumation of high pressure rocks sultanate of oman continental crustal rocks now structurally beneath the allochthonous samail ophiolite underwent blueschist to eclogite facies metamorphism prior to the emplacement of the oman ophiolite onto the arabian margin the recognition of a major low angle fault within this polydeformed and polymeta morphosed sequence of metacarbonates metabasites quartzites and quartz mica schists greatly simplifies the interpretation of the structure and metamorphic zonation within the saih hatat window ne oman carpholite bearing upper plate rocks consist of pre permian to jurassic rocks that have been folded into large recumbent nappe structures which exhibit a marked increase in deformation intensity toward the boundary with the lower plate the lower plate rocks have higher peak pressure and temperature assemblages they are exposed in two windows separated by jabal abu daud although more intensely deformed the lower plate rocks are recognizable as metamorphosed continental platform sediments fold axes are parallel to the regional nne ssw lineation sense of shear indicators yield a transport direction of south over north in the lower plate opposite to the sense of motion inferred for the emplacement of the ophiolite lower plate eclogite facies metabasalts are only preserved in kilometer scale megaboudins found in the easternmost window at as sifah these metamorphic assemblages along with their remnant east west fabrics define the existence of a short lived arabian platform directed nascent subduction zone the stretching lineation elsewhere within the upper and lower plates the hatat schist the basement and the metamorphic sole of the ophiolite is consistently nne ssw suggesting that the exhumation of high pressure metamorphic rocks of saih hatat is related to the ophiolite obduction exhumation of the high pressure rocks was accompanied by intense deformation involving regional scale fold nappes in a convergent margin setting the geotherm remained suppressed for a period 30 myr greater than the thermal relaxation time of the crust the geometric and thermal constraints from oman may be applicable to the general problem of the formation and preservation of high pressure low temperature rocks

sm nd and zircon u pb ages from garnet bearing eclogites ne oman constraints on high p metamorphism poly deformed and poly metamorphosed glaucophane eclogite mega boudins beneath the samail ophiolite oman record an early subduction related high p metamorphism as well as subsequent overprinting deformation and metamorphism related to exhumation previously published rb sr ages of 78 ma and 40ar 39ar ages of 82 79 ma record the major ne directed shearing event that partially exhumed the eclogites to a shallower crustal level new sm nd garnet garnet leachate whole rock isochron data from garnet bearing eclogite assemblages in the as sifah subwindow in ne oman are 110 9 ma dg02 87d 5 point isochron and 109 13 ma dg02 86e 3 point isochron on the basis of microfabric and field structural relationships these ages are interpreted to reflect the timing of prograde peak high p metamorphism in the rocks structurally beneath the samail ophiolite this metamorphism clearly predates the age of formation of the obducted samail oceanic lithosphere 97 94 ma as well as the subsequent obduction onto the margin 80 70 ma a u pb shrimp zircon age from small 200 m in length zircons with herring bone textured zoning from dg02 87d indicate that rapid zircon growth associated with high si phengites occurred at 82 1 ma zircon growth is possibly related to liberation of zr on garnet breakdown during decompression metamorphism under high p conditions with exhumation these data require that crustal stacking models attendant with ophiolite obduction are inappropriate to explain the oman high p metamorphism

reaction pathways during the progressive deformation of a blueschist metabasite the role of chemical disequilibrium and restricted range equilibrium there are discrete masses of un deformed metabasite within the blueschist series of the island of syros greece around the margins of these masses are zonal sequences through rocks showing intracrystalline deformation but without a geometric fabric to rocks with discrete and anastomosing shear zones and finally to penetratively foliated rocks with isolated relics of the original undeformed texture textural relics suggest that this spatial sequence is at least qualitatively also a temporal sequence this progressive shear zone deformation took place concurrently with a glaucophane epidote to eclogite reaction the reaction pathways in the rocks that underwent the shear zone deformation can be compared with those in rocks of a similar composition that suffered a longer deformation history and show no relics of an undeformed parent although the final assemblages are in both cases the same the pathways are different these differences are in part related to reactions promoted by the change from local to bulk equilibrium on the onset of deformation in the rocks they are also related to the crystallization and later breakdown during the sequence of progressive equilibration of a metastable phase in this case an impure glaucophane

formation and exhumation of blueschists and eclogites from ne oman new perspectives from rb sr and 40ar 39ar dating seven eclogite facies samples from lithologically different units which structurally underlie the semail ophiolite were dated by the 40 ar 39 ar and rb sr methods despite extensive efforts phengite dated by the 40 ar 39 ar method yielded saddle hump or irregularly shaped spectra with uninterpretable isochrons the total gas ages for the phengite ranged from 136 to 85 ma clinopyroxene phengite epidote phengite and whole rock phengite rb sr isochrons for the same samples yielded ages of 78i2 ma we therefore conclude that the eclogite facies rocks cooled through 500 u ca tc 7 8i2 ma and that the 40 ar 39 ar dates can only constrain maximum ages due to the occurrence of excess ar inhomogeneously distributed in different sites our new results lead us to conclude that high pressure metamorphism of the oman margin took place in the late cretaceous contemporaneous with ophiolite emplacement previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a ne dipping zone choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c 50 km to 10 15 km within c 10 ma and led to their juxtaposition against overlying lower grade rocks final exhumation of all high pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate

compaction driven evolution of porosity and permeability in natural mudstones an experimental study this paper describes a series of experiments designed to investigate the influence of lithology on the compactional loss of porosity and permeability in mudstones two intact samples of london clay with clay fractions of 40 and 67 were compacted to 33 mpa effective stress clay fraction permeability porosity pore size distribution and specific surface area were measured and their evolution was monitored throughout the compaction process electron microscopy was combined with mercury porosimetry to trace the collapse of the pore structure with increasing effective stress in both cases porosity loss occurred primarily by the collapse of large pores this process is more obvious in the coarser grained sample because throughout the compaction process it has a much broader range of pore radii and a much greater mean pore radius consistent with the pore size distributions the permeability of the coarser sample ranges from 10 10 m s 1 to 10 12 m s 1 while that of the finer grained sample ranges from 4 10 12 m s 1 to 5 10 14 m s 1 during progressive compaction from 2 to 33 mpa the compressibility of the finer grained sample is greater than that of the coarser grained sample 0 15 as opposed to 0 07 however in both cases the compressibility is much lower than that inferred for lithologically similar samples compacted over geological timescales the demonstration that both porosity and lithology clay fraction influence the permeability of mudstones should allow the development of more realistic porosity permeability relationships which take into account lithological variations exhibited by mudstones

structure petrology and metamorphic history of the nome group blueschist terrane salmon lake area seward peninsula alaska an 80 mi 2 area adjacent to salmon lake mapped at a scale of 1 36 680 includes a section of the nome group schist that is at least 2 000 m thick it is relatively intact with relict primary lithologic layering and not melange like or highly disrupted by faults the nome group metasediments include pelitic schist graphitic quartzite pure and impure marble calcareous schist chlorite albite schist chloritic quartz albite and mafic schists metaigneous rocks include fe ti rich metabasite and granitic to tonalitic orthogneiss the protoliths are precambrian to paleozoic in age and radiometric ages indicate that the high pressure metamorphism of the nome group is late jurassic blueschist facies assemblages occur in nome group metabasites and in most metasedimentary schists the blueschists are synkinematic they formed during an episode of widely distributed penetrative deformation and mesoscopic intrafolial isoclinal folding of consistent orientation the nome group of the salmon lake area underwent a prograde monocyclic polyfacial high p t metamorphism initial metamorphism was in the lower t regime of the lawsonite glaucophane field 300 c 7 kb and prograded with increasing t and p into the epidote almandine glaucophane field through an apparent p t maximum as indicated by the assemblage glaucophane omphacite garnet 420 c 9 10 kb the later stages of the metamorphism were characterized by increased t p likely during initial stages of decompression and the return of the normal geothermal gradient to the transition between the blueschist facies and the high p albite epiclote barroisite field 450 to 525 c 7 8 kb this high p t cycle was followed by an incipient static retrogressive metamorphism at low greenschist facies conditions during the later uplift of the nome group in the late cretaceous and early tertiary

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

subduction zone polarity in the oman mountains implications for ophiolite emplacement two end member models have been proposed to account for the structure and metamorphism of rocks beneath the semail ophiolite in the oman mountains model a involves a single continuous ne directed subduction away from the continental margin during the late cretaceous the ophiolite and underlying thrust sheets of distal to proximal oceanic sediments were emplaced a minimum of 250 km sw onto the continental margin subduction of triassic jurassic oceanic basalts to c 10 kbar c 39 km depth led to the accretion of amphibolite facies rocks to the base of the ophiolite thrusting propagated towards the continental margin and ended with subduction of the thinned continental crust to c 20 kbar c 78 km depth choking the subduction zone buoyancy forces caused the rapid exhumation of eclogites blueschists and carpholite grade hp rocks along the ne margin of the continental plate during the later phase of foreland propagating thin skinned thrusting in the sw ne facing backfolding and backthrusting occurred in the hinterland with the final exhumation of the hp rocks model b follows recent suggestions that a nascent sw dipping subduction zone dipping beneath the continental margin existed between 130 and 95 ma prior to formation and emplacement of the ophiolite a major ne facing fold nappe structure in the pre permian basement rocks of saih hatat is interpreted as reflecting subduction beneath the margin two high pressure metamorphic events have been suggested the first predating ophiolite emplacement the second caused by ophiolite loading this model is untenable being based on a misinterpretation of the ne facing structures in northern saih hatat and on some dubious older 40ar 39ar cooling ages from the eclogite facies rocks of as sifah we conclude that all structures in northern oman and all the reliable geochronology point to a single emplacement obduction event lasting from cenomanian turonian time c 95 ma when amphibolites were accreted along the metamorphic sole of the ophiolite to campanian time when the continental margin was subducted to the ne producing blueschists and eclogites to the final thin skinned emplacement of all thrust sheets which ended before the late maastrichtian at c 68 ma

biostratigraphy of the albian paddy member lower cretaceous peace river formation goodfare alberta thirty seven taxa of arenaceous foraminifera recovered from cores of the upper part of the paddy member peace river formation and the superjacent lower shaftesbury formation in the goodfare area of northwestern alberta are assigned to the lower part of the verneuilina canadensis subzone of the miliammina manitobensis zone of the lower cretaceous upper albian a stratigraphic hiatus of eight foraminiferal subzones is implied between the paddy member and subjacent cadotte member progressive transgression of the m manitobensis sea is reflected by a succession of four faunules of foraminifera and trace fossils shallow water facies and reduced salinity are indicated for the paddy environment and a return to normal salinity during deposition of the overlying shaftesbury formation a distinctive miliammina ammotium faunule is figured from the upper part of the paddy member

comment on structural evolution metamorphism and restoration of the arabian continental margin saih hatat region oman mountains by m p searle et al

structural and stratigraphic controls on the origin and tectonic history of a subducted continental margin oman eclogites and blueschists exposed in saih hatat oman record the subduction and exhumation of continental crustal material beneath the cretaceous semail ophiolite during ophiolite obduction the eclogite bearing lower plate originally part of oman s distal continental margin is exposed in two tectonic windows through the less metamorphosed upper plate by a previously mapped low angle high strain decollement structure a major tectonic break currently poorly exposed records the juxtaposition of the highest pressure eclogites and garnet blueschists against lower pressure epidote blueschists the subsequent exhumation of the entire lower plate to mid crustal levels is marked by a pervasive shearing event associated with a regional greenschist facies overprint the decollement truncates structures and the metamorphic field gradient in the lower plate but does not significantly truncate structures or stratigraphy in the upper plate it is not responsible for the exhumation of the high pressure rocks to mid crustal levels most of the displacement across this structure was accommodated during continuing convergence after the subduction system had ceased to be active and post ophiolite emplacement onto the platform carbonate sequences a revised tectonic model is presented which accounts for the structural geochronological and metamorphic observations

petrology of lawsonite pumpellyite and sodic amphibole bearing metabasites from north east oman the assemblages chlorite pumpellyite lawsonite albite quartz chlorite lawsonite quartz epidote and chlorite epidote albite quartz occur in metabasaltic blocks and veins in a metamorphosed tectonic melange in the structurally highest unit of the autochthonous and parautochthonous section underlying the semail ophiolite in saih hatat north east oman the pre permian basement of this section contains mafic units characterized by the assemblage crossite epidote chlorite quartz albite el actinolite these assemblages indicate a down section increase in metamorphic grade from lawsonite albite facies conditions in the melange to epidote blueschist facies conditions in the basement application of empirically and experimentally based thermobarometers as well as petrogenetic grids calculated for a model basaltic system indicates that the p t conditions of metamorphism ranged from 3 to 6 kbar and 250 to 300d c for the melange and p 6 8 kbar t 310d c for the basement units textural relations interpreted in the context of petrogenetic grids indicate that these units followed clockwise p t paths of evolution the estimated p t conditions and down section increase in metamorphic grade in central western and northern saih hatat are consistent with the hypothesis relating metamorphism to the late cretaceous tectonic loading of the continental margin by an ophiolite slab 18 km in thickness these results contrast with field and petrological observations documented for blueschists and eclogites exposed along the eastern coast of saih hatat which may have formed at an earlier stage in response to an early cretaceous collisional event

glaucophane chloritoid bearing assemblages from ne oman petrologic significance and a petrogenetic grid for high p metapelites pelitic layers and lenses interbedded with blueschists and eclogites in saih hatat ne oman contain chloritoid and sodic amphibole bearing mineral assemblages that are useful for reconstructing the p t history of the area textural and mineral chemical relations suggest that coexisting glaucophane gln and chloritoid ctd formed at the expense of chlorite chl paragonite pg and later broke down to garnet gt pg during prograde metamorphism according to the reaction gln ctd qz gt pg h2o during retrogression gln and chl first formed at the expense of gt and pg followed by the breakdown of ctd and gt to chl the final stages of retrogression are marked by the breakdown of gln to an aggregate of chl albite ab a projection from quartz qz h2o and phengite ph on the al2o3 fe2o3 feo mgo na2o plane in the system nfmash is best suited for the representation of the phase relations in high p metapelites petrogenetic grids for the model systems nmash and nfash were calculated using program geo calc berman et al 1987 and its database berman 1988 after the retrieval of s i o and h f o for gln and ctd by mathematical programming and calculating all possible reactions among gln ctd chl jadeite jd ab gt pg talc tc pyrophyllite prl and kyanite ky the calculated petrogenetic grid for the system nfash shows that fe ctd and ferroglaucophane coexist at p 6 5 kbar and t 6 kbar this grid is consistent with the p t estimates for high p metapelites from oman new caledonia seward peninsula ile de groix sifnos and peloponnese where gln ctd bearing units are interbedded with cofacial mafic blueschists and eclogites the grid also explains the observed textural relations in the metapelites of oman and is consistent with the clockwise p t path proposed for this area but differs from the grids of guiraud et al 1990 in showing a larger stability field for gln ctd that overlaps with the stability fields of jd qz as well as ab the petrogenetic grid calculated for the system nmash shows that mg ctd gln bearing assemblages require unusually low geothermal gradients to form in metapelites it also shows that mg rich ctd tc coexist at high pressures over a wider p t range than predicted by guiraud et al 1990 this grid can therefore be applied to high p metamorphic assemblages from the eastern and western alps

obduction related planar and linear fabrics in oman under the oman ophiolites three different types of alpine synmetamorphic fabrics are seen each of them is taken as the signature of a particular stage of the obduction process the complex high grade fabric in the basal metamorphic sheet is linked to the intraoceanic detachment stage pre obduction sensu stricto the l s fabric of the muscat nappes and hatat internal border associated with low temperature high pressure metamorphism is regarded as the effect of an early obduction that is the epicontinental thrusting of the matrah peridotites the much simpler fabric of the anchizonal hawasina and autochthonous cover resulted from the gravity sliding of the semail ophiolites during the final obduction phase

femg partitioning between coexisting garnet and phengite at high pressure and comments on a garnet phengite geothermometer abstract the fe mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite mg 67 in the presence of quartz and water at pressures of 20 35 kb and temperatures of 800 1000 c compositions of coexisting garnet and mica indicate a linear relation between both the inkd fe mg garnet fe mg phengite and temperature and inkd and pressure in the above p t range this fe mg exchange reaction between garnet and phengite is shown to be dependent on the ca content of the garnet and on the mg number of the bulk composition these two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46 and by using a synthetic basaltic composition the overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems respectively however because of non ideality in the exchange reaction this geothermometer should not be used in any practical application outside the composition ranges studied also careful consideration of the presence of fe3 in phengite must be made if the fe3 content of the natural phengite is unknown then the temperatures obtained will be maximum temperatures only

transition from subduction to exhumation related fabrics in glaucophane bearing eclogites oman evidence from relative fabric chronology and 40ar 39ar ages abstract controversy over the age of peak metamorphism and therefore the tectonic evolution of the arabian margin relates to the polydeformed and polymetamorphosed nature of glaucophane bearing eclogites from the saih hatat window beneath the allochthonous samail ophiolite in ne oman these eclogites contain relicts of earlier fabrics structures and metamorphic assemblages and provide a record of change from subduction to exhumation the eclogites are part of a mafic layer that was disrupted into boudins up to 0 5 km in length within a lower plate shear zone as sifah shear zone the megaboudins not only preserve the relicts of the highest grade of metamorphism but also an early ene trending lineation and sheathlike isoclines enveloped by the flat lying schistosity the boudin bearing layer is isoclinally folded with calc schist mafic schist and quartz mica schist where the regional folds have axes parallel to the ne trending stretching lineation a type folds textural evidence suggests multiple growth events for garnet and clinopyroxene requiring polymetamorphism of the mafic layers that formed the eclogite megaboudins the surrounding calc schist and quartz mica schist are both intensely deformed with transposition foliation containing an ne trending lineation in phengite and asymmetric shear indicators such as c type shear bands and asymmetric pressure shadows around garnets that give top to the ne sense of shear although consistent ene trending lineations in all the boudins suggest that they have largely acted as passive nonrotating rigid bodies the presence of ne vergent asymmetric mesofolds extensive dynamic recrystallisation multiple generations of phengites and a range of 40ar 39ar apparent ages within the megaboudins suggest however that they have not acted entirely passively during the later deformation phengites isolated from the high p low t fabrics show groupings in 40ar 39ar apparent ages interpreted as distinct metamorphic cooling intervals at 140 135 120 98 and 92 80 ma microstructural relations suggest that age groupings younger than 100 ma reflect phengite growth during exhumation with the top to the ne shearing the older ages 120 110 ma from fabrics that give top to the s shear sense may reflect growth during the subduction phase the combination of groupings of apparent argon ages older than the crystallisation age of the samail ophiolite the suggestion of different geothermal gradients and superposed metamorphism suggest that the eclogites and garnet blueschists formed as a result of underthrusting along a break that was not directly related to the metamorphic sole of the ophiolite the glaucophane eclogites are interpreted as having formed at different times under varying pressure temperature conditions during underthrusting with variations in the rate of underthrusting allowing thermal equilibration and or rapid cooling at different crustal levels

amphibole plagioclase equilibria an empirical model for the relation albite tremolite edenite 4 quartz equilibria between plagioclase calcic amphibole and quartz can be described in part by the relation among mineral components naalsi3o8 ca2mg5si8o22 oh 2 naca2mg5alsi7o22 oh 2 4sio2 this relation governs the partitioning of na between plagioclase and the a site of coexisting amphibole data from natural amphibolites reveal that this partitioning is systematic and sensitive to metamorphic grade the ideal portion of the equilibrium constant kid xna a x a xab derived from natural samples is sensitive to bulk composition inasmuch as both plagioclase and amphibole are highly non ideal samples from a single outcrop have values ranging from 0 5 xab 0 74 to 4 1 xab 0 10 the continuous reaction naalsi3o8 ca2mg5si8o22 oh 2 naca2mg5alsi7o22 oh 2 4sio2 proceeds to the right with increasing grade of metamorphism and for a given bulk composition kid increases with increasing temperature two related discontinuous reactions actinolite albite hornblende oligoclase quartz and actinolite oligoclase hornblende anorthite quartz also proceed to the right with increasing metamorphic grade and result in changes in the topology of a phase diagram that describes the partitioning of na between plagioclase and amphibole a site a schreinemakers net is presented that is consistent with natural occurrences the results of this study should aid in the delineation of metamorphic facies within amphibolites

development of concepts concerning the genesis and emplacement of tethyan ophiolites in the eastern mediterranean and oman regions this article uses evidence mainly from the eastern mediterranean and oman to help understand the origin and emplacement of ophiolites field studies in these regions emphasize the diversity of ophiolite related sedimentary and tectonic processes topics to be discussed here include the sedimentary covers of ophiolites their metamorphic soles related melanges and emplaced continental margin slope rise units modes of ophiolite emplacement will also be considered and tectonic reconstructions that have a direct bearing on the origin and emplacement of ophiolites the writer argues that there is compelling evidence that many ophiolites formed by spreading above intraoceanic subduction zones evidence from their intact sedimentary covers however indicates that such subduction was only incipient in most cases as directly associated volcanic arcs or back arc basins are rare metamorphic soles record fragments of oceanic lithosphere derived from settings ranging from open ocean to continental slope rise settings underlying melanges record the subduction of most of the ocean lithosphere formed at spreading ridges continental margin units beneath ophiolites demonstrate that these ophiolites were formed in ocean basins that were mainly initiated by continental rifting in the eastern mediterranean and oman regions ophiolite emplacement was commonly achieved by trench margin collision although strike slip transpression and large scale accretion along subducting margins played a role in some cases taking account of the mesozoic palaeotectonic setting of the eastern mediterranean region it is widely believed that ophiolites of two main ages mid jurassic and late cretaceous were formed in several small oceanic basins separated by rifted microcontinents the history of tethys in pre mesozoic time remains more open to question lastly giant subduction related ophiolites as in oman cyprus or the balkans may not be forming anywhere in the world ocean today but may instead relate to rare events in earth history when regional to global plate boundaries were drastically reorganised

new high precision p t estimates for oman blueschists implications for obduction nappe stacking and exhumation processes oman blueschists and eclogites lie below the obduction nappe of the semail ophiolite in one of the key areas on earth for the study of plate convergence here new metamorphic and tectonic constraints are provided for the central yet poorly constrained hulw unit sandwiched between the low grade units 10 kbar 300 c and the as sifah eclogites pmax 23 kbar tmax 600 c tweequ multi equilibrium thermobarometry using both compositional mapping and spot analyses and raman spectroscopy of carbonaceous material yield a high precision p t path for the hulw and as sheikh units and reveal that they shared a common p t history in four stages i a pressure decrease from 10 12 kbar 250 300 c to 7 9 kbar 300 350 c ii almost isobaric heating at 8 10 kbar from 300 350 c to 450 500 c iii a pressure decrease at moderate temperatures 450 500 c and iv isobaric cooling at 5 6 kbar from 450 500 to 300 c no significant pressure or temperature gap is observed across the upper plate lower plate discontinuity to the north and west of the hulw unit the combination of tectonic and p t data constrains the stacking chronology of the three main metamorphic units comprising the saih hatat window i e the ruwi quryat the hulw as sheikh and the diqdah as sifah units these results strengthen the view that the tectonic and metamorphic data are conveniently accounted for by a simple n vergent continental subduction of the passive arabian margin below the obduction nappe along a cold p t gradient

retrograde evolution of eclogite facies rocks from ne oman evidence from fluid inclusions and petrological data eclogite facies rocks from eastern saih hatat oman metamorphosed under peak conditions of p 12 kbar and t of 550 580 c have several types of fluid inclusions in quartz clinopyroxene and garnet most of these inclusions are two phase at room t and contain an aqueous fluid belonging to the system nacl mgcl2 h2o final melting temperatures for most inclusions range from 4 to 0 c but values as low as 13 c have been recorded indicating salinities of 490 c inclusions with three phases at room t also contain a predominantly low salinity aqueous fluid with xco2 12 kbar to 4 kbar or less at t 500 550 c this conclusion is supported by the scatter in homogenization temperatures for these inclusions and their large variety of shapes and sizes which range from irregular with hair like fractures surrounded by satellite inclusions to negative crystals the replacement of ferroan dolomite ankerite and quartz by calcite talc and of rutile calcite by titanite or rutile glaucophane by ilmenite albite in these eclogites are also consistent with a stage of isothermal decompression following near isothermal decompression to depths of 10 15 km brittle deformation prevailed leading to the formation of layer parallel and crosscutting fractures that were filled by quartz calcite this stage was followed by near isobaric cooling recorded by the isochores for fluid inclusions in vein quartz and by the occurrence of dendritic networks of decrepitated and imploded fluid inclusions in these veins

structural evolution metamorphism and restoration of the arabian continental margin saih hatat region oman mountains abstract 170 m y of relatively stable passive margin sedimentation along the northern continental margin of arabia was abruptly terminated during the cenomanian turonian 95 ma when the oman continental margin collapsed and subsided rapidly aruma basin to accommodate obduction of the semail ophiolite complex and underlying thrust sheets haybi and hawasina complexes in the oman mountains the ophiolite was emplaced at least 200 km over the passive margin probably over 450 km in total from ne to sw during the later stages of obduction the leading edge of the continental margin was subducted to depths where carpholite bearing rocks 6 8 kbar blueschist 12 15 kbar and eclogite ca 20 kbar facies metamorphism formed in a ductile deforming ne dipping subduction zone along the northeastern margin of the saih hatat culmination in ne oman the mesozoic shelf carbonates are deformed by recumbent folding on all scales the present day structure is dominated by four sw vergent thrust slices in carpholite grade rocks at higher structural levels overlying lower units of crossite bearing blueschist hulw unit garnet glaucophane chloritoid blueschists and garnet clinopyroxene phengite glaucophane eclogites 15 20 kbar the largest shear zone separates 7 8 kbar hulw unit blueschists above from 12 15 kbar retrogressed eclogites and ca 20 kbar eclogites of the as sifah unit below further inboard sw a major shear zone separates the lower plate hulw blueschist facies rocks from the upper plate sedimentary rocks upper plate lower plate discontinuity which also show carpholite grade hp metamorphism towards the ne restoration of the entire saih hatat continental margin shows that the upper plate lower plate discontinuity cuts down section to the sw as far as the proterozoic hatat schists ne directed extensional crenulation schistosity and nne oriented stretching lineations in the eclogite and blueschist facies rocks are consistent with sw directed exhumation of footwall hp rocks ne facing folds and spectacular sheath folds with greatly attenuated limbs in the upper plate sediments are interpreted as antithetic backfolds with shortening in the upper plate balanced by the subduction of the lower plate consistent with a ne directed subduction of the continental margin rocks beneath the sw obducting ophiolite haybi and hawasina thrust sheets recent suggestions of a nascent sw directed subduction beneath the oman margin are not consistent with the sedimentary evolution of the shelf and slope carbonates or the geological structure of saih hatat two main phases of tectonic uplift of saih hatat and jebel akhdar are firstly the latest cretaceous ramp culmination associated with foreland propagating thrusts in the sw and backfolds in the ne and secondly post late eocene oligocene uplift by ca 2000 m

biogenically enhanced reservoir properties in the medicine hat gas field alberta canada abstract spot minipermeametry and micro ct analytical techniques were employed to evaluate the effect s of bioturbation on porosity and permeability distributions in reservoir facies of the highly prolific medicine hat gas field of alberta canada this study focuses on five cm scale samples that occur at different stratigraphic positions and intersect discrete bioturbated horizons within the medicine hat member spot minipermeability results demonstrate that permeability in bioturbated sandstones is up to two orders of magnitude greater than those of the muddy matrix thus corresponding to a dual porosity fluid flow system graphing bioturbation intensity versus measured permeability indicates that burrows are well connected horizontally and provide flow conduits micro ct models reveal that burrowed sandstones are isolated and planiform in character rare interpenetrations from vertically oriented trace fossils serve to interconnect these hydraulically isolated strata results show that bioturbated sandstones possess reservoir properties comparable to those associated with laminated sandstones yielding an anisotropic porous medium understanding the impact of trace fossils on porosity permeability distributions in the medicine hat gas field can be used to identify potential reservoir from previously interpreted non reservoir rock and ultimately improve reserve estimations

coseismic radiation of the 2008 m w 7 9 wenchuan earthquake and its relationship to fault complexities the long rupture length and complex fault geometries of the 2008 mw 7 9 wenchuan earthquake offer a great opportunity to investigate the relationship between coseismic radiation and fault complexities in this study we first improved the compressive sensing back projection method cs bp with window time adjustment and parallel computation to enhance its spatial temporal resolution and computational efficiency then we applied the cs bp to invert for the coseismic radiation of the wenchuan earthquake in multiple frequency bands our results show that the coseismic radiation of the wenchuan earthquake is frequency dependent and strongly associated with the fault geometries low frequency radiation sources are concentrated on smooth fault segments with large coseismic slip while higher frequency sources are mainly related to fault complexities e g xiaoyudong fault gaochuan fault bend leigu fault bend and nanba fault step over an acceleration of rupture speed from 1 5 to 3 0 km s is also observed the near xiaoyudong fault where strong high frequency energies were radiated finally we discuss the role of rupture speed in mediating the relationship between coseismic radiation and fault complexities on one hand sudden changes of rupture speeds contribute to strong high frequency radiation on the other hand fault complexities can also strongly affect the rupture speed a sudden break of a large asperity is able to propel the acceleration while rupture front may decelerate when it jumps across geometric barriers

review of characteristics of low permeability gas reservoirs in western united states tight gas reservoirs generally are defined as gas bearing rocks with an in situ permeability to gas of less than 0 1 md in 1980 the national petroleum council estimated that undiscovered recoverable gas resources in low permeability tight rht reservoirs in the united states excluding eastern gas shales are about 574 tcf 16 1012 m3 the possible recovery of this volume of gas was based on 9 mcf 1979 dollars and a level of recovery technology presently unknown advanced technology most of this resource is in the western united states the two main types of tight rocks are shallow high porosity reservoirs and deep low porosity reservoirs the high porosity rocks are tight because of their fine grain size the low porosity rocks are tight because diagenetic alteration has resulted in them having small microvugs porosity poorly connected by sheet or cracklike very thin capillaries the low porosity rocks are very stress sensitive and may be overpressured and all tight rocks are susceptible to formation damage to varying degrees during drilling and treatment the western tight gas reservoirs can be placed in two major geologic and engineering categories blanket and lenticular blanket reservoirs generally respond to artificial stimulation in a predictable manner whereas the response of lenticular reservoirs is mostly unpredictable many obstacles complicate the identification and recovery of gas from these unconventional reservoirs accurate borehole geophysical log analysis is generally difficult and economic stimulation commonly artificial fracturing of the tightest rocks is difficult or impossible with present technology geologists and engineers must better understand the characteristics of these reservoirs work together to improve present recovery technology and develop new innovative drilling and stimulation techniques

subducted and obducted ophiolites of the central alps paleotectonic implications deduced by their distribution and metamorphic overprint abstract the scenario of the closure of the tethyan realm in the central alps as reconstructed from the distribution and intensity of high pressure overprint can be interpreted as follows in the western part of the alps consecutive subduction and collision occurred in two well separated oceanic domains first in the piemontais domain zermatt saas zone and then in the valais domain versoyen further to the east where the two oceanic domains had merged ino a single ocean closure of the piemontais ocean led to obduction of oceanic crust onto the northern continental margin the ophiolites so produced were later involved in a greenschist facies stage which is related to the second collision and definitive closure of the alpine realm the change in tectono metamorphic evolution is spatially related to the eastern extremity of the brianconnais rise

statistical modeling of biogenically enhanced permeability in tight reservoir rock abstract bioturbation is generally perceived to be detrimental to bulk permeability by reducing primary grain sorting homogenizing sediment and introducing mud as burrow linings and feces recent studies show however that some ichnogenera and biogenic fabrics serves to increase porosity and permeability in tight hydrocarbon reservoirs subtle changes in sand and silt distributions such as may be generated by bioturbation can greatly affect the resulting porosity and permeability distribution despite this permeability across unfractured sedimentary reservoirs is commonly assessed solely on the basis of average grain size this study of the lower cretaceous viking fm integrates sedimentary and ichnologic features to define recurring hydrofacies that possess distinct permeability grades grain size lithology bioturbation index and trace fossil suites were described from a cored section of well 14 30 22 16w4 the k max values from small plugs and full diameter core samples were used to represent each hydrofacies hydrofacies were qualitatively defined at the bed bedset scale based on sedimentary ichnological and permeability attributes all of which affect flow pathways in heterolithic facies the markov chain method was employed to compare the vertical transitions of permeability k max within a borehole against grain size and hydrofacies at the bed to bedset scale this provided an intuitive framework for interpreting facies relationships such as coarsening upwards successions the results show that in the studied core grain size only correlates to permeability in homogeneous rock units the transiograms show that the volumetric proportions of different k max classes show a 15 correlation with grain size compared to a 97 correlation with the hydrofacies indicating that variations in permeability down the well are strongly related to variations in the hydrofacies the hydrofacies approach potentially can be used as a conceptual framework for the spatial modeling of permeability in tight hydrocarbon reservoirs where grain size may not be the primary factor on permeability distributions

structure and metamorphism of blueschist eclogite facies rocks from the northeastern oman mountains the northern part of the saih hatat window oman shows high pressure metamorphic rocks derived from shelf sediments and pre premian continental basement and is atypical of sub ophiolite metamorphism elsewhere the high pressure rocks are divided into structural units originally bounded by foreland propagating thrusts formed during ophiolite obduction although now many contacts are backthrusts normal faults or extensional shear zones metamorphic breaks exist across many unit boundaries the deepest unit as sifah has eclogite facies assemblages in metabasites and metapelites which record evolution of p t conditions along a clockwise path culminating al 23 2 5kbar 540 75 c contrasting markedly with overlying units 5 10 kbar 200 500 c although separated by 40 ar 39 ar apparent ages our tectonic model relates all the high p units to a single convergent event in the late cretaceous the as sifah eclogites were exhumed in two stages i tectonic emplacement against other units at c 20 25 km depth and ii exhumation of the entire high p zone by culmination collapse after obduction

coexisting amphiboles in an eclogite from the western alps new constraints on the miscibility gap between sodic and calcic amphiboles crystal chemical relationships between coexisting sodic and calcic amphiboles have been studied in eclogitic metagabbros from the aosta valley western alps textural analysis gives evidence of three successive high pressure parageneses 1 pre kinematic high grade blueschist assemblages preserved as polymineralic inclusions in garnet cores and made of glaucophane and actinolite stage a 2 synkinematic eclogite assemblages composed of garnet omphacite glaucophane actinolite white mica clinozoisite quartz rutile stage b 3 post kinematic epitactic overgrowths of barroisitic amphibole on glaucophane and actinolite stage c p t conditions of the eclogitic metamorphism have been estimated at around 500 550 c 16 kbar glaucophane and actinolite coexist as discrete grains in stage a and b assemblages this texture and the chemistry of the amphiboles unambiguously denotes the existence of a miscibility gap between sodic and calcic amphiboles from nam4 0 80 in actinolite to nam4 1 70 in glaucophane at t 500 550 c a comparison with published analyses allows a new solvus along the glaucophane actinolite join to be drawn the later barroisitic amphibole stage c exhibits strong chemical zonation indicating disequilibrium growth this amphibole cannot either be used to define a miscibility gap with glaucophane or actinolite or be considered as an intermediate stage between these two end members

phase petrology and p t conditions of mafic blueschists from the meliata unit west carpathians slovakia blueschists occurring as layers in calcite marbles of the meliata unit occur along the so called roznava tectonic line situated in the southern part of the gemericum slovakia mineral assemblages and compositions from seven blueschists localities and one occurrence of amphibolite facies rocks overprinted by blueschist metamorphism were investigated the most common minerals in the blueschists are blue amphibole epidote and albite some fe2 and al rich rocks also contain garnet and chloritoid respectively na pyroxene with a maximum 50 jadeite component was also found the blue amphiboles correspond mostly to crossite and also to glaucophane and ferroglaucophane in some samples almandine and spessartine rich garnet has very low mgo content 3 wt the si content in phengite ranges between 3 3 and 3 5 pfu calculated on the basis of 11 oxygens the zoning patterns of blue amphibole garnet and chloritoid suggest their formation during a prograde stage of metamorphism the p t conditions of metamorphism are estimated to be about 380 460 c and 10 13 kbar pressures of 7 5 8 5 kbar and temperatures of 350 370 c were obtained for some actinolite and aegirine rich rocks apart from chlorite other mafic minerals formed during retrograde metamorphism are biotite and occasionally also actinolite

oscillatory zoning in eclogitic garnet and amphibole northern serpentinite melange cuba a record of tectonic instability during subduction exotic blocks of eclogite from distant localities along the northern serpentinite melange of cuba have comparable p t histories that include high pressure prograde sections 450 600 c 15 kbar associ ated with subduction of oceanic lithosphere and retrograde sections within the albite epidote amphibolite facies 500 c 10 kbar related to melange uplift 40 ar 39 ar and rb sr cooling ages 118 103 ma of one of the blocks indicate pre aptian subduction and aptian albian uplift detailed x ray imaging and profiling further reveals that minerals in these eclogite blocks notably garnet and amphibole display subtle but well defined oscillatory zoning that developed along the prograde trajectory of the rocks previous to attainment of peak eclogitic conditions the chemistry e g coupled changes of mg and mn in garnet and of si ti al and na in amphibole and geometry euhedral to anhedral shapes of the oscillations can be interpreted in terms of subtle fluctuations in p t during the general prograde subduction related metamorphic path a near equilibrium model is presented for the formation of oscillations at near peak conditions by means of recurrent dissolution growth reaction processes this model for near peak conditions and the chemical signatures of earlier oscillations notably in amphibole suggest that episodes of retrogression upward movement affected parts of the subducting slab it is proposed that these retrograde episodes record the tectonic rupture of the subducting slab and probably of the upper plate mantle either due to the intrinsic dynamic behaviour of subduction systems or to the effects of the plate tectonic rearrangement of the caribbean region during the early cretaceous

a new geobarometer for the assemblage biotite muscovite chlorite quartz the equilibrium thermodynamics of the reaction 4kmga1si40 o oh mg5a1 si30 o oh muscovlte chlorite muscovite biotite ka13si 0 oh 3kmg3aisi 0 0 oh 7si0 4h o quartz fluid can be formulated as a geobarometer thermo dynamic data for the mg a1 celadonite end member of muscovite is derived from the reversed mineral equilibria experiments of velde i 965 using the calculation method of powell 1978 giving afh 1298 5823 5 kj s 1298 0 3060kjk v 1298 13 8kjkbar

cretaceous high pressure metamorphism in ne oman and its relationship to subduction and ophiolite nappe emplacement an occurrence of glaucophane bearing schists is reported from near muscat in ne oman in rocks which underlie the upper cretaceous semail ophiolite nappe these blueschists formed by the metamorphism of quartz sandstones shales and basic igneous rocks belong to a pre permian basement in the area metamorphic micas phengites from these rocks have been dated at between 101 and 80 ma k ar ages indicating a mid to late cretaceous age for the high pressure event a syn to late f 1 assemblage of almandine glaucophane phengite in the pelites shows that the peak of metamorphism reached about 400 450 c and 4 8 kbars pressure this was followed by a greenschist facies overprint with the formation of epidote and albite porphyroblasts in the meta igneous rocks and a widespread retrogression to chlorite the high pressure metamorphism is ascribed to the effects of partial subduction of the ne oman continental margin during the initial stages of ophiolite obduction and the subsequent greenschist facies event to uplift of the area during the later stages of emplacement of the nappes

bioturbation and bioirrigation extend the open exchange regions in permeable sediments large scale experimental exclusion of lugworms arenicola marina from 400 m2 intertidal fine sand revealed significant effects of their bioturbation and bioirrigation on sediment characteristics benthic infauna composition and the dominant mineralization and benthic pelagic exchange processes in the sediment absence of lugworms resulted in sediment clogging with organic rich fine particles an eightfold decrease in sediment permeability low oxygen penetration depths and accumulation of reduced mineralization products in the pore water the sand flat inhabited by lugworms had low fine particle and chlorophyll contents and low sulfide and nutrient concentrations in the pore water the effects were not limited to the vicinity of lugworm burrows but extended throughout the entire sediment down to 20 cm depth sediments with the lugworm shared the characteristic of low organic advection driven permeable sand rather than of muddy diffusion dominated sediments in the absence of lugworms areal oxygen uptake rates depended strongly on hydrodynamic conditions under calm conditions sedimentary oxygen uptake was slightly higher at the exclusion site experimental flushing using controlled hydrodynamic conditions showed that oxygen uptake at the lugworm site would be higher during more dynamic conditions e g storms due to significantly deeper oxygen penetration permitted by higher sediment permeability our results indicate an interactive effect of bioturbating organisms and hydrodynamics on water sediment exchange processes and highlight the importance of benthic infauna for sedimentary processes even in physically dominated systems

geometry and significance of internal windows and regional isoclinal folds in northeast saih hatat sultanate of oman saih hatat contains tiered sets of low angle structural breaks that reflect a complex stacking of sheets which requires multiple upward movement of units within an unstable orogenic wedge in a convergent setting early recumbent folds within the saih hatat window are subparallel to but cut by a major low angle contractional fault decollement that separates lower plate high pressure rocks some eclogites exposed in two windows hulw and as sifah windows from high pressure upper plate rocks containing carpholite and lawsonite parts of this decollement had been mapped or speculated upon to explain the variation in metamorphic grade across saih hatat upper plate fold nappes are controlled by the mapped decollement surface below the decollement sheath like lower plate closures formed independently in an intense zone of top to the northeast shearing eclogites occur within mafic boudins at the structurally lowest level of the retrograde lower plate shear zone this zone was responsible for the partial exhumation of the lower plate very early in the observed structural history and has a normal movement sense the two lower plate windows are essentially foliation domes reflecting fold interference between regional scale sheath like recumbent isoclines and more open upright northwest north and northeast trending fold sets the early upper plate fold nappes reflect a major top to the north or northeast movement of the rocks away from the oman margin at the same time the ophiolite was being emplaced over the top in the opposite direction top to the southwest or south south directed faulting and shearing is superimposed on the early recumbent folds in the mountain range mapped relationships and structural observations agree with bureau de recherches geologiques et minieres mapping that recognised fold nappes in this part of the oman mountains all of the mapped structures are associated with exhumation of the high pressure rocks making reconstruction of the margin during high pressure metamorphism problematic

stress state permeability relations for overconsolidated clays analysis of experimental data of soils in both the normally consolidated and overconsolidated states reveal that the proposed relationships are tenable preconsolidation stress level influences the level of permeability