seismic risk in turkey the aegean and the eastern mediterranean the occurrence of large magnitude earthquakes summary the file of turkish seismicity developed by kandilli observatory istanbul for earthquakes to 1970 is extended here up to 1978 using 1sc and pde data entries into this file are maintained on the surface wave magnitude scale m and conversion of body wave magnitude mb to m has been carried out where necessary using a formula derived for turkish earthquakes completeness analysis suggests that magnitudes m 4 5 may be used for statistical evaluation of seismic risk this file is analysed by a range of methods to provide a suite of risk forecasts forecasting results from least squares and maximum likelihood estimates of the whole process gutenberg richter cumulative frequency law of earthquake magnitude occurrence and from the part process of gumbel s first extreme value distribution all show small systematic differences in forecasts but all three methods lead to magnitude forecasts which fall well within the range of standard deviation however these forecasts are obtained by subdividing turkish seismicity in a cellular manner and many of these cells show curvature of the earthquake frequency magnitude distribution curve concave with respect to the origin gumbel s third asymptotic distribution of extreme values is chosen as an appropriate statistical description approximate upper bounds o to earthquake surface wave magnitude occurrence are evaluated and estimates of largest magnitudes expected over an interval of 75yr are forecast with uncertainties values of w are asymptotic uncertain and theoretically correspond to infinite return periods strain energy release diagrams are then invoked to estimate empirically the large magnitude m which is equivalent to the total strain energy which may be accumulated in a region this equivalent magnitude m is consistently less than w and there is a finite waiting time typically ranging from about 15 to 70 yr during which the energy equivalent to m3 may be accumulated

estimation of seismic hazard parameters for the himalayas and its vicinity from complete data files the maximum likelihood estimation of earthquake hazard parameters has been made in the himalayas and its surrounding areas on the basis of a procedure which utilizes data containing complete files of the most recent earthquakes the entire earthquake catalogue used covers the period from 1900 1990 the maximum regional magnitude m max the activity rate of the seismic event the mean return period r of earthquakes with a certain lower magnitude m max m along with their probability of occurrence as well as the parameter b of of gutenberg richter magnitude frequency relationship have been determined for six different seismic zones of the himalayas and its vicinity it is shown that in general the hazard is higher in the zone nei and ban than the other four zones the high difference of the b parameter and the hazard level from zone to zone reflect the high seismotectonic complexity and crustal heterogeneity

kinematics and dynamics of the pamir central asia quantifying surface deformation and force balance in an intracontinental subduction zone kinematic and dynamic models quantify deformation and force balance in the pamir a region undergoing the rare and poorly understood process of intracontinental subduction we constrain a detailed kinematic model with 506 recent gps velocities and quaternary fault slip rates and show that the pamir is organized like the himalaya and tibet with regions of 1 localized strain rate 100e 9 yr along the pamir frontal thrust system the subduction interface similar to the himalaya and 2 distributed north south compression and east west extension similar to tibet through standard thin viscous sheet methods we demonstrate that the lithospheric force balance in the pamir is a combination of stresses caused by gravitational potential energy and india eurasia convergence accommodated at a subduction interface in this case the pamir frontal thrust system we find that strain rate and deviatoric stress patterns near the pamir frontal thrust system are characteristic of a mature subduction zone despite its initiation in continental lithosphere although the pamir and tibet are kinematically and dynamically similar the pamir is stiffer overall than tibet perhaps due to the presence of the highly arcuate geometrically stiffened continental slab at depth

evaporitic and biosiliceous cyclic sedimentation in the miocene of the gulf of suez depositional and diagenetic aspects abstract the middle to late miocene evaporite bearing formations which outcrop along the gebel zeit and gems highs in the southwestern gulf of suez egypt consist of thick calcium sulphate beds rhythmically interbedded with marlstones siltstones and claystones containing various amounts of biogenic silica as well as pure diatomites the calcium sulphate beds are composed mainly of gypsum and anhydrite subaqueous crustallisation in subpermanent brine ponds of marine origin predominated during deposition of the selenite and laminated gypsum early diagenetic interstitial growth of nodular anhydrite occurred episodically when the water level dropped in response to increasing salinity associated with the precipitation of halite in the deeper troughs nevertheless most of the nodular facies resulted from the burial conversion of gypsum to anhydrite which has been sometimes rehydrated into gypsum a last phase of gypsum dehydration has occurred at the surface related to the present hot and dry climatic conditions the biosiliceous deposits contain either well preserved opal a or diagenetically altered opal ct clinoptilolite quartz and even clays diatom frustules such diagenetic changes are commonly described in the oceanic biosiliceous deposits the fossil content of these diatomitic layers implies shallow marine conditions on the shelves of highs induced by block faulting the cyclic succession of evaporited and biosiliceous deposits recorded the alternation of high sea level stands associated with high organic productivity probably enhanced by seasonal input of nutrients and low sea level stands associated with hypersaline conditions and evaporite deposition locally the sharp contact between the diatomites and gypsum argues for a rapid evolution of salinity towards high concentration up to gypsum saturation and more the formation of finely laminated diatomitic sediments is known to require either high organic productivity or anoxic conditions in bottom intermediate waters and eventually both processes in the gebel zeit and gemsa areas a local association of the biosiliceous sediments with diagenetic carbonates and native sulphur resulting from processes of bacterial sulphate reduction confirms that anoxic conditions occurred in the deep water body or in the sediments large amounts of organic compounds were involved in these diagenetic processes immature organic matter was consumed during an early diagenetic stage of transformation it is assumed that this organic matter was supplied either by the biosilica rich interbeds or the organic rich laminated carbonates the initial organic content of the biogenic deposits was impoverished as early as the first stages of sedimentation and diagenesis although a large part of the organic matter could have been destroyed by these early diagenetic processes and further oxidation in outcrops these sediments can be considered as potential source rocks in nearby less exposed sequences

regional variation of the ω upper bound magnitude of giii distribution in hindukush pamir himalaya and the adjacent regions a perspective on earthquake hazard the upper bound magnitude or maximum magnitude in 28 seismogenic source zones in the hindukush pamir himalaya and the adjacent regions have been computed with the help of a complete and homogeneous earthquake catalogue during the period 1900 2010 to estimate the earthquake hazards in the region the gumbel s third asymptotic distribution giii of extreme value method is used to estimate this parameter in this study a comparison of maximum magnitude obtained by giii distribution is carried out with kijko sellevoll method it is observed that the maximum earthquake magnitudes estimated by kijko sellevoll and giii methods are comparable to each other and the average of differences of their values is only 0 13 the results also estimate the most probable earthquake magnitude that can be expected in next 100 years m100 in all 28 seismogenic source zones an effort is made to make regression relations between and maximum magnitude estimated by kijko sellevoll method mmax ks and and maximum observed magnitude mmaxobs the estimated values exceeded the value of 7 00 in 15 and 8 00 in 5 of the 28 seismogenic source zones the geographical distribution of and m100 in 28 seismogenic source zones of the study region is visualized to analyze the localized seismicity parameters it is observed that earthquake hazard level varies spatially from one zone to another which suggests that examined region have high crustal heterogeneity and seismotectonic complexity

event stratigraphy and paleoceanography in the fortuna basin southeast spain a scenario for the messinian salinity crisis the timing and duration of evaporitic deposition during the messinian in the mediterranean is poorly known to assist with this problem we have correlated sedimentological events at widely separated locations in the mediterranean and dated paleoceanographic events in the world ocean event stratigraphy our working hypothesis is as follows the messinian salinity crisis caused the formation of evaporites in a deep desiccated basin which implies that 1 the evaporites were deposited during a steady marine influx 2 multiple intervals of isolation led to drastic changes in the sediments and in the fauna of the mediterranean 3 the sea level of the mediterranean sea was mostly several hundreds of meters below that of the atlantic ocean and 4 the seaways betic and rif strait connecting the mediterranean with the atlantic during the late miocene were affected by eustatic sea level changes caused by antarctic glaciation we found supporting evidence in the sedimentological and paleoceanographic records in the betic strait fortuna and sorbas basins which was one of the sites of major inflow into the late miocene mediterranean sea drastic sea level changes and fluctuations during the late tortonian and messinian caused chronologically 1 the deposition of brackish sediments overlying marine deposits 2 the progradation of deltas 3 reef growth at lower levels and their destruction by erosion and coverage by conglomerates 4 evaporite deposition alternating with marine marls containing atlantic diatom assemblages and 5 five marine terrestrial cycles associated with coral reefs based on event stratigraphy the following scenario for the messinian salinity crisis is proposed the tripoli formation chron 6r1 5n2 5 93 5 7 ma the lower evaporites chron 5n2 5r 5 7 5 5 ma and the upper evaporites lowermost gilbert c 5 25 5 15 ma were deposited during influx of atlantic water into the mediterranean during this time a unidirectional circulation dominated from the atlantic through the betic strait only during the intra messinian inundation chron 5n1 into lowermost gilbert c 5 4 5 25 ma that occurred after a short intra messinian erosional event chron 5n1 5 5 5 4 ma was a normal bidirectional circulation pattern established this is reflected in the diatomites of the chicamo formation and the mediterranean site 124 as well as most likely in a reef on cyprus the marine record of the pliocene filling of the mediterranean is missing in these basins overall marine water inflow into the mediterranean through the betic strait dominated during the messinian the threefold marine terrestrial cyclicity with severe paleoceanographic changes in the fortuna and sorbas basins favor the deep desiccated basin model

the sedimentary evolution of the red sea rift a comparison of the northwest egyptian and northeast saudi arabian margins abstract comparison of the sedimentary sequences outcropping on opposite sides of the red sea although based on relatively limited data reveals many similarities the cretaceous and paleogene pre rift series are very similar throughout the northwestern parts of the red sea and in the gulf of suez syn rift sediments both in saudi arabia and in egypt may be readily subdivided into a basal continental unit a transgressive marine series an evaporitic formation and a final pliocene mixed clastic carbonate phase these marked variations express a comparable geodynamic evolution on opposite sides of the rift however these similarities are modified by important differences which are partly chronological and concern the precocity of initial syn rift sedimentation in the midyan region of saudi arabia there are also important lithological differences within each of the major units the chronological and lithological differences may be due either to the diachronous evolution of the various segments of the basin or to different environments of sedimentation whose localisation differed with respect to a common rift axis

mineral biosignatures in evaporites presence of rosickyite in an endoevaporitic microbial community from death valley california rosickyite is a rare form of sulfur sulfur monoclinic symmetry that is not thermodynamically predicted to be stable at earth s surface temperatures instead it reverts to the more common sulfur form orthorhombic symmetry here we show for the first time that rosickyite exists and is stably maintained within an endoevaporitic microbial community from the salt pan of death valley california we hypothesize that this mineral is formed by a cycle of microbial dissolution of gypsum caso4 2h2o to sulfide and reoxidation of the sulfide to elemental sulfur rosickyite within a stable oxygen sulfide gradient maintained by the organisms furthermore we report a microstratigraphic layering of mineral types that correlates with layering of the microbial community knowledge of how microbial communities can affect the mineral assemblages of evaporite deposits on earth can help us to identify potential markers of the past or present existence of life on extraterrestrial bodies bearing evidence of ancient seas or lakes

a probabilistic assessment of earthquake hazard parameters in nw himalaya and the adjoining regions the maximum likelihood estimation method is applied to study the geographical distribution of earthquake hazard parameters and seismicity in 28 seismogenic source zones of nw himalaya and the adjoining regions for this purpose we have prepared a reliable homogeneous and complete earthquake catalogue during the period 1500 2010 the technique used here allows the data to contain either historical or instrumental era or even a combination of the both in this study the earthquake hazard parameters which include maximum regional magnitude mmax mean seismic activity rate the parameter b or b log e of gutenberg richter g r frequency magnitude relationship the return periods of earthquakes with a certain threshold magnitude along with their probabilities of occurrences have been calculated using only instrumental earthquake data during the period 1900 2010 the uncertainties in magnitude have been also taken into consideration during the calculation of hazard parameters the earthquake hazard in the whole nw himalaya region has been calculated in 28 seismogenic source zones delineated on the basis of seismicity level tectonics and focal mechanism the annual probability of exceedance of earthquake activity rate of certain magnitude is also calculated for all seismogenic source zones the obtained earthquake hazard parameters were geographically distributed in all 28 seismogenic source zones to analyze the spatial variation of localized seismicity parameters it is observed that seismic hazard level is high in quetta kirthar sulaiman region in pakistan hindukush pamir himalaya region and uttarkashi chamoli region in himalayan frontal thrust belt the source zones that are expected to have maximum regional magnitude mmax of more than 8 0 are quetta southern pamir caucasus and kashmir himanchal pradesh which have experienced such magnitude of earthquakes in the past it is observed that seismic hazard level varies spatially from one zone to another which suggests that the examined regions have high crustal heterogeneity and seismotectonic complexity

seismic risk of circum pacific earthquakes ii extreme values using gumbel s third distribution and the relationship with strain energy release in a previous paper makropoulos andburton 1983 the seismic risk of the circum pacific belt was examined using a whole process technique reduced to three representative parameters related to the physical release of strain energy these are m1 the annual modal magnitude determined using the gutenberg richter relationship m2 the magnitude equivalent to the total strain energy release rate per annum andm3 the upper bound magnitude equivalent to the maximum strain energy release in a region

carbonate evaporite siliciclastic transitions in quaternary rift sediments of the northwestern red sea abstract the quaternary and recent coastal environments of the egyptian red sea and gulf of suez are characterized by rapid lateral variations carbonates evaporites and siliciclastic sediments exist in close proximity although the climate is exceptionally dry the numerous wadis draining high hinterland relief deliver important quantities of coarse clastics to coastal environments during the rare periods of flash flooding however because this efficient supply is sporadic coastal waters are contaminated for short catastrophic periods which are followed by a rapid return to clear water carbonate sedimentation the warm arid conditions on the other hand favour both carbonate reef production and evaporite mineral precipitation rapid lateral transitions between continental siliciclastic and marine carbonate facies are favoured by a particular morphology closely related to the active distensional tectonic setting within the red sea rift block faulting which began in the early miocene and continues today has resulted in numerous subparallel grabens and horsts situated between the onshore basement relief and the coast the structural depressions grabens tend to trap much of the episodic siliciclastic input favouring permanent carbonate production in the nearby littoral environments

silica diagenesis ii general mechanisms abstract amorphous silica phases opal a precipitate in nature due to the formation of dense colloids in supersaturated alkaline aqueous solutions with low relative concentrations of other ions opal a dissolves and yields solutions of still relatively high silica content in pore waters containing abundant cations open framework polymers form which flocculate to yield opal ct opal ct becomes increasingly ordered primarily due to preferential growth of cristobalite relative to tridymite and crystal size increase opal ct dissolves to yield pore waters of low silica concentration which allows slow growth of quartz crystals from monomeric solution the quartz crystals then slowly increase in size and crystallinity carbonates appear to enhance opal ct formation possibly due to the activity of positively charged hydroxyl complexes hence polymerization in relatively pure systems is involved in opal a formation and polymerization in impure systems is involved in opal ct formation and slow growth from monomeric low silica concentration solutions is involved in precipitation of quartz in sedimentary realms

probabilistic assessment of earthquake hazards in the north east indian peninsula and hindukush regions the himalayan region is one of the most seismic prone areas of the world the north east ne indian peninsula and the hindukush regions mark the zone of collision of the indian and eurasian plates the probability of the occurrence of great earthquakes with magnitude greater than 7 0 during a specified interval of time has been estimated on the basis of four probabilistic models namely weibull gamma lognormal and exponential for the ne indian peninsula and hindukush regions the model parameters have been estimated by the method of maximum likelihood estimates mle and the method of moments mom the cumulative probability is estimated for a period of 40 years from 1964 and is ranging between 0 881 to 0 995 by the year 1995 using all four models for the ne indian peninsula the conditional probability is also estimated and it is concluded that the ne indian peninsula would expect a great earthquake at any time in the remaining years of the present century for the hindukush region the cumulative probability has already crossed its highest value but no earthquake of magnitude greater than 7 0 has occurred after 1974 in this area it may attribute to the occurrence of frequent shocks of moderate size as seventeen earthquakes of magnitude greater than 6 0 including four greater than 6 4 have been reported until 1994 from this region

late neogene laminated and opal rich facies from the mediterranean region geochemical evidence for mechanisms of formation abstract the mechanisms for the formation of laminated and opal rich sediments from southern italy bianco and vrica and southern sicily gela are investigated using a geochemical approach the vrica laminates are relatively rich in organic carbon while the gela laminites are not carbon nitrogen ratios carbon isotopic composition and lipid content of the organic matter at vrica indicate that the laminates represent periods of increased input of terrestrial organic matter and enhanced preservation based on oxygen isotopic studies of planktonic foraminifera a basin stratification model is presented to explain the formation of laminites at both vrica and gela biogenic silica and carbonate content and the carbon isotopic composition of benthic foraminifera suggest that the bianco diatomites represent periods of increased productivity the isotopic composition of planktonic foraminifera from bianco indicates that the increased productivity was a result of basin eutrophication brought on by continental runoff the low organic carbon content of the bianco diatomites is attributed to continuous bottom water ventilation the results of laminites studies indicate that they are not always rich in organic matter as generally assumed moreover the formation of organic rich laminites can be the result of enhanced preservation not increased productivity results from bianco indicate that preservation plays a primary role in controlling the organic carbon content of sediments

tectonic and sedimentary evolution of the gulf of suez and the northwestern red sea abstract the structural pattern of the gulf of suez and red sea rift is inherited from the basement tectonics n140 faults clysmic direction parallel to the regional elongation of the rift are associated with submeridian aqaba direction and sub e w faults duwi direction that also play an important role in the rifting processes the pre rift sedimentary cover includes cretaceous and eocene platform deposits it was slightly deformed prior to rifting the syn rift sedimentary sequence comprises four major units groups a to d delimited by unconformities group a oligocene to early burdigalian continental to restricted subaquatic deposits group b burdigalian to middle miocene open marine sediments group c middle to upper miocene major evaporitic episode and group d plio pleistocene which returns to an open marine environment four majors stages of structuration are recorded an initial stage of compressive deformation direction of shortening close to nw se characterized by strike slip movements and associated drag folds early group a a tilted block stage late group a corresponding to the period of highest extensional rate a horst and graben stage group b resulting in the cutting out of the tilted blocks by numerous synthetic faults group b corresponds to the period of highest subsidence rate when a flexuration stage of the margin expressed by a centripetal evolution of the subsidence is associated with major periferal uplift

geology and genesis of sulfur deposits at ras gemsa area red sea coast egypt field petrographic and geochemical analyses indicate that the native sulfur at ras gemsa is off diagenetic origin the sulfur formed together with megacrystalline dolomite biogenic dolomite and biogenic calcite by bacterial action on sulfate in the presence of organic matter hydrocarbons the sulfur occurs in an uplifted horst block consisting of evaporite carbonate rocks overlying the middle miocene oil and gas bearing strata the evaporite deposits are interpreted to be a composite sequence that includes deposits of shallow lagoonal subaqueous and supratidal sabkha environments two types of dolomite microcrystalline and megacrystalline are recognized in the studied sequence stable isotope data showed that the microcrystalline dolomite has high 18 o values mean 1 05 pdb and relatively low 13 c values mean 3 2 pdb these results indicate that the microcrystalline dolomite was deposited mainly under marine evaporitic conditions the megacrystalline dolomite is characterized by negative 18 o values mean 6 8 pdb and strongly negative 13 c values mean 24 05 pdb the negative 18 o values are probably due to the effect of hot conditions or depleted meteoric water and or biological effects during dolomite crystallization the negative 13 c values signify the important role played by the sulfate reducing bacteria in the presence of organic matter

age of the badenian salinity crisis impact of miocene climate variability on the circum mediterranean region massive evaporites were deposited in the central european paratethys sea during the badenian salinity crisis bsc the scarcity of absolute age data has hampered a thorough understanding of these salt deposits here we present a robust chronology for this catastrophic event by 40 ar 39 ar dating of volcanic tuffs below and within the badenian salts in southern poland the onset of bsc evaporite deposition is dated at 13 81 0 08 ma and the entire event is estimated to have lasted 200 600 k y correlation to oxygen isotope records shows that the bsc evaporites were just preceded by glacial event mi 3b suggesting a causal relationship the corresponding sea level fall most likely restricted the open marine connection to the mediterranean thereby trapping the salt in the deep paratethys basins

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

authigenic zeolites in late pleistocene sediments of the south atlantic angola basin two types of authigenic zeolites from late pleistocene hemipelagic sediments of the south atlantic were examined by xrd and sem analysis the xrd characteristics and the thermal behaviour of the zeolites suggest that type 1 is composed of clinoptilolite type 2 was identified as mordenite based on morphological features and qualitative elemental composition their authigenic nature is suggested from the shape of the crystals and their growth on and within foraminifera tests volcanic ash material acting as precursor minerals to zeolite formation could not be found nor were the zeolites directly related to siliceous remains an authigenesis from pore waters must be assumed mineral stabilities for amorphous sio2 and zeolites were calculated with the computer program solmineq88 interstitial waters proved to be oversaturated throughout the core in respect to all zeolites considered biogenic opal represents the limiting solid phase for si concentrations in pore waters therefore the dissolution of opal is regarded as the main source of si for interstitial waters below 9 m core depth where opal is only present in traces detrital feldspars and the zeolites themselves are affected by dissolution this may indicate the metastable nature of the zeolites also corroborated by the presence of two almost isochemical members of the same zeolite group

a deterministic seismic hazard map of india and adjacent areas a seismic hazard map of the territory of india and adjacent areas has been prepared using a deterministic approach based on the computation of synthetic seismograms complete with all main phases the input data set consists of structural models seismogenic zones focal mechanisms and earthquake catalogues there are few probabilistic hazard maps available for the indian subcontinent however this is the first study aimed at producing a deterministic seismic hazard map for the indian region using realistic strong ground motion modelling with the knowledge of the physical process of earthquake generation the level of seismicity and wave propagation in anelastic media synthetic seismograms at a frequency of 1 hz have been generated at a regular grid of 0 2 x 0 2 by the modal summation technique the seismic hazard expressed in terms of maximum displacement d m a x maximum velocity v m a x and design ground acceleration dga has been extracted from the synthetic signals and mapped on a regular grid over the studied territory the estimated values of the peak ground acceleration are compared with the observed data available for the himalayan region and are found to be in agreement many parts of the himalayan region have dga values exceeding 0 6 g the epicentral areas of the great assam earthquakes of 1897 and 1950 in northeast india represent the maximum hazard with dga values reaching 1 2 1 3 g the peak velocity and displacement in the same region is estimated as 120 177 cm s 1 and 60 90 cm respectively

seismic hazard evaluation for specific seismic regions of the world tsapanos t m and burton p w 1991 seismic hazard evaluation for specific seismic regions of the world tectonophysics 194 153 169 seismic hazard is estimated for fifty of the most seismically active countries of the world using the technique of gumbel s third asymptotic distribution of extreme values and hazard maps are produced for this purpose a catalogue of 9700 earthquakes with magnitudes m 2 5 5 has been compiled and earthquakes in the time period 1898 1985 are examined here an equal area grid point mesh of about 5 o latitude x 5 o longitude is chosen to represent the local seismicity this cell size corresponds approximately to the area of romania which is the smallest country examined the results estimate the most probable maximum magnitude for a time period of 85 years which exceeds magnitude 7 for all fifty of the cellular seismic regions and the contour maps and tables show its spatial variation and rank the countries in terms of overall relative seismic hazard the maps effectively produce a brief seismic hazard atlas

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

an enhanced image of the pamir hindu kush seismic zone from relocated earthquake hypocentres summary we determine the shape of the seismic zone in the pamir hindu kush region de ned by 30 42 0 n 68 78 0 e by obtaining improved hypocentral locations with 90 per cent con dence limits of less than 30 km the depth error bar for most of the earthquakes of about 6000 shallow and intermediate depth earthquakes available s and depth phase arrival times are also used together with the p wave arrival times in the joint hypocentre determination technique to obtain the best possible hypocentral locations the study region is divided into three depth ranges 0 60 60 160 and 160 km the 0 60 km depth zone is then subdivided laterally into 19 blocks with the deeper regions divided into two blocks each the improved delineation of the seismic zone obtained by using the relocated hypocentres implies that the intermediate depth seismicity in the pamir hindu kush region is most simply explained by a single s shaped seismic zone 700 km long and no more than 30 km wide and with most activity concentrated at 100 300 km depth the main features observed are 1 the eastward steepening of the north dipping hindu kush seismic zone through to its overturning at its eastern end beneath the pamirs where it dips to the southeast 2 the curvature and forking of the subducting slab at depths greater than 200 km within the eastern part of the hindu kush seismic zone 3 the very abrupt cut oi in intermediate depth seismicity at 90 110 km depth with no extension to shallower depths under the pamirs and with a persistent gap between the intermediate and shallow seismicity in the northern pamirs and 4 the unusual horizontal t axes for intermediate depth earthquakes of the pamir seismic zone which align with its curvature this study shows that the seismic zone under the hindu kush has stress axes which follow the classical pattern for subducting slabs controlled by gravity whereas the pamir region has horizontal t axes that follow the trend of the contorted seismic zone this suggests that the pamir seismic zone is a slab deformed due to ow in the upper mantle

probabilistic appraisal of earthquake hazard parameters deduced from a bayesian approach in the northwest frontier of the himalayas a straightforward bayesian statistic is applied in five broad seismogenic source zones of the northwest frontier of the himalayas to estimate the earthquake hazard parameters maximum regional magnitude mmax value of g r relationship and seismic activity rate or intensity for this purpose a reliable earthquake catalogue which is homogeneous for mw 5 0 and complete during the period 1900 to 2010 is compiled the hindukush pamir himalaya zone has been further divided into two seismic zones of shallow h 70 km and intermediate depth h 70 km according to the variation of seismicity with depth in the subduction zone the estimated earthquake hazard parameters by bayesian approach are more stable and reliable with low standard deviations than other approaches but the technique is more time consuming in this study quantiles of functions of distributions of true and apparent magnitudes for future time intervals of 5 10 20 50 and 100 years are calculated with confidence limits for probability levels of 50 70 and 90 in all seismogenic source zones the zones of estimated mmax greater than 8 0 are related to the sulaiman kirthar ranges hindukush pamir himalaya and himalayan frontal thrusts belt suggesting more seismically hazardous regions in the examined area the lowest value of mmax 6 44 has been calculated in northern pakistan and hazara syntaxis zone which have estimated lowest activity rate 0 0023 events day as compared to other zones the himalayan frontal thrusts belt exhibits higher earthquake magnitude 8 01 in next 100 years with 90 probability level as compared to other zones which reveals that this zone is more vulnerable to occurrence of a great earthquake the obtained results in this study are directly useful for the probabilistic seismic hazard assessment in the examined region of himalaya

the upper miocene diatomaceous sediments of the northernmost mediterranean region a lamina scale investigation of an overlooked palaeoceanographic archive

the hydrological setting occurrence and significance of gypsum in late quaternary salt lakes in south australia well exposed often laminated gypsum sequences occur in many quarternary salt lakes in southern south australia the gypsum in the salt lakes is classified by increasing grain size into gypsite gypsarenite and selenite the salt lakes are classified by age and hydrological setting into coastal salinas which are holocene sea water fed groundwater lakes and continental playas which are late pleistocene endorheic basins a study of the relationships between coastal salina hydrology and the associated gypsum deposition has shown the different types of gypsum form under distinct hydrological regimes as the hydrology above a coastal salina depositional surface changes through time so does the type of gypsum deposited application of a gypsum depositional model derived from a study of the coastal salina gypsum to those portions of a continental playa gypsum unit where deposition is no longer occurring confirms the applicability of the model to non salina gypsum deposits

marine planktic diatom correlation of lower messinian deposits in the western mediterranean abstract samples from late neogene mediterranean outcrops were analysed for diatom content the presence of certain species in the tripoli formation caltanissetta basin sicily and in the beida formation cheliff basin nw algeria permits correlation between these sections and the paleomagnetic reversal record the occurrence ofthalassiosira praeconvexa thalassiosira miocenica thalassiosira convexa var aspinosa andasterolampra acutiloba indicates that the tripoli and beida formations were deposited over an interval ranging from the upper half of paleomagnetic epoch 6 into the lower half of epoch 5 5 6 to 5 9 ma previous diatom correlations were too young the occurrence ofnitzschia reinholdii andasterolampra acutiloba in pre evaporitic diatomites from the upper turre formation se spanish sorbas basin is used to place deposits at this site tentatively in paleomagnetic epoch 5 or 6

probabilistic assessment of earthquake hazards in the indian subcontinent the indian subcontinent is one of the most seismic prone areas of the world the himalayan mountains in the north mid oceanic ridges in the south and earthquake belts surrounding the indian plate all show that the subcontinent has undergone extensive geological and tectonic processes in the past the probability of the occurrence of earthquakes with magnitude 6 m b 7 during a specified interval of time has been estimated on the basis of four probabilistic models namely lognormal weibull gamma and exponential distribution for the indian subcontinent the seismicity map has been prepared using the earthquake catalogue from the period 1963 1994 and six different zones have been identified on the basis of clustering of events the model parameters have been estimated by the method of maximum likelihood estimates mle and method of moments mom a computer program package has been developed for all four models which represents the distributions of time intervals fairly well the logarithmic of likelihood ln l is estimated for testing the models and different models have been found to be plausible the probability of different magnitude thresholds has been evaluated using the gutenberg richter formula log n a bm for magnitude distribution the constants a and b have been computed for each region and found to be varying between 5 46 8 53 and 0 87 1 34 respectively

regional variation of the ω values in the circum pacific belt a data set of earthquakes which occurred in the circum pacific regions has been used to compute the values this parameter is obtained through gumbel s third asymptotic distribution of extreme values and is well known as upper bound magnitude the results show relatively higher values in south america central america and mexico than in the western part of the circum pacific belt the results are interpreted in terms of the different mechanical structure heterogeneity of the materials in the eastern and western parts of the examined area

authigenic natroalunite in middle miocene evaporites from the gulf of suez gemsa egypt this paper deals with a new occurrence of sedimentary natroalunite na k al3 so4 2 oh 6 in the caprock of a diapir of middle miocene evaporites from the gemsa peninsula located on the southwestern coast of the gulf of suez field observations petrographic examinations and stable isotope 18o 34s measurements on the associated authigenic phases of anhydrite and native sulphur argue for the genesis of natroalunite at a rather high temperature i e higher than 75 c by the reaction of clay minerals with sulphuric acid the sulphuric acid was produced by the concatenation of the following reactions which are thought to increase the diagenetic temperature bacterial reduction of sulphate evaporites and oxidation of hydrogen sulphide to native sulphur and sulphate where aerated conditions prevailed in the pore fluids these changes through time from reducing to oxidizing diagenetic conditions were controlled by the progressive uplift of the diapiric system toward the land surface so that the draining ground waters became progressively oxygenated

a probabilistic estimate of the most perceptible earthquake magnitudes in the nw himalaya and adjoining regions nw himalaya and its neighboring region 25 40 n and 65 85 e is one of the most seismically hazardous regions in the indian subcontinent a region that has historically experienced large to great damaging earthquakes in the present study the most perceptible earthquake magnitudes mp are estimated for intensity i vii horizontal peak ground acceleration a 300 cm s2 and horizontal peak ground velocity v 10 cm s in 28 seismogenic zones using the two earthquake recurrence models of kijko and sellevoll bulletin of the seismological society of america 82 1 120 134 1992 and gumbel s third asymptotic distribution of extremes giii both methods deal with maximum magnitudes the earthquake perceptibility is calculated by combining earthquake recurrence models with ground motion attenuation relations at a particular level of intensity acceleration and velocity the estimated results reveal that the values of mp for velocity v 10 cm s show higher estimates than corresponding values for intensity i vii and acceleration a 300 cm s2 it is also observed that differences in perceptible magnitudes calculated by the kijko sellevoll method and giii statistics show significantly high values up to 0 7 0 6 and 1 7 for intensity acceleration and velocity respectively revealing the importance of earthquake recurrence model selection the estimated most perceptible earthquake magnitudes mp in the present study vary from mw 5 1 to 7 7 in the entire zone of the study area results of perceptible magnitudes are also represented in the form of spatial maps in 28 seismogenic zones for the aforementioned threshold levels of intensity acceleration and velocity estimated from two recurrence models the spatial maps show that the quetta of pakistan the hindukush pamir himalaya the caucasus mountain belt and the himalayan frontal thrust belt kashmir kangra uttarkashi chamoli regions exhibit higher values of the most perceptible earthquake magnitudes mw 6 0 these spatially distributed values indicate good agreement with high seismic hazard zones in previously published hazard maps based on seismicity maximum expected earthquake magnitudes during specific time intervals and maximum expected acceleration these maps have useful implications in preparing earthquake selection criteria for the anti seismic design of non critical structures in the study region

travel times and attenuation relations for regional phases in the barents sea region a database containing 45 events in the barents sea region has been compiled and analyzed with the aim of evaluating crustal models travel times and attenuation relations in the context of performing regional detection threshold monitoring of this region the 45 events are mostly located around the circumference of the study area due to the virtually aseismic nature of the barents sea itself regional p n and s n phases were observable for most events in the database while p g and l g phases were only observable for events with raypaths that do not cross the tectonic structures in the barents sea this corroborates a number of previous observations of l g wave blockage within the barents sea three existing velocity models were evaluated with a model having slightly lower s velocities than earlier assumed in the upper mantle giving the overall best fit to the observed arrivals in order to estimate magnitudes short term average sta and spectral amplitude values were calculated in several frequency bands for all phase arrivals in the database there were no significant differences between spectral and sta amplitudes so the latter were used as this parameter is more efficient to calculate in real time processing an inversion was performed in order to determine an attenuation relation specific for this region the resulting magnitudes based on p n p g s n and l g phases gave an internally consistent reasonably stable set of values which can be calibrated towards any existing global or regional scale

mixed carbonate siliciclastic sedimentation on a miocene fault block gulf of suez egypt abstract the miocene sediments at abu shaar red sea egypt include both carbonates and detrital silicates whose distribution is closely related to a pre miocene basement fault block as such they demonstrate the relationships between local distensional tectonics and sedimentation because of the vertical bathymetric relief of the basement block the carbonates include both a spectacular talus and massive platform facies in spite of the presence of scattered corals sedimentation is not truly reefal as deposits consist essentially of muddy bioclastic and silicate sands the most typical ecological elements are numerous spectacular stromatolites rigidity of the sedimentary mass was mainly induced by early marine cementation carbonate siliciclastic transitions on and around the abu shaar platform were determined mainly by basement morphology and related pre miocene tectonics

regional variation of the ω upper bound magnitude of giii distribution in and around turkey tectonic implications for earthquake hazards complete data set of earthquakes in turkey and the adjacent areas has been used in order to compute the values in 24 seismic regions of turkey the parameter is obtained through gumbel s third asymptotic distribution of extreme values and is well known as upper bound magnitude this is an interpretation that no earthquake magnitude greater than can occur in a region the results also estimate the most probable magnitude for a time period of 100 years the estimates of exceed the value of 7 00 in 20 of the 24 seismic regions an effort is also made to find a relation between the magnitude and the length of a fault in the complicated tectonics of turkey and the surrounding area earthquake hazard revealed as tables and maps are also considered for turkey and the surrounding area

the red sea 50 years of geological and geophysical research abstract the red sea displays lithosphere that is in transition from rifting to drifting and therefore provides enormous opportunity to understand incipient seafloor spreading development despite more than 50 years of extensive geological and geophysical research there remains significant conjecture concerning the timing of sea floor spreading initiation the extent of spreading along the axis of the red sea and the geodynamic processes responsible for the onset of crustal extension red sea tectonic models based on geological data are dominated by single stage rift models involving protracted stretching of continental crust followed by sea floor spreading at ca 5 ma and include both asymmetric and symmetrical extension models geophysical data suggest that an oceanic crust occurs beneath the red sea shelves although exactly how this crust has evolved remains undecided equally arguments have been made in favor of the sedimentary shelves forming on extended continental crust and that oceanic crust is restricted to the present day red sea axial valley however our synthesis shows that none of the models proposed so far is applicable to the entire red sea basin the distribution and timing of the red sea crustal extension and sea floor spreading may have been influenced by subduction related far field forces due to complex plate interactions along the northern edge of the arabian plate during early miocene our reassessment suggests that following the arrival of the afar plume the southern red sea underwent rapid extension culminating in the transition from rifting to seafloor spreading which stalled during the early miocene in the central red sea due to either slowing of the convergence between arabia and eurasia or the onset of the arabian plate passive margin collision with eurasia off axis magmatism is focused on the arabian margin and afar depression and has been episodically active since the miocene we consider a formation of the red sea as a hybrid process involving both active and passive rifting

structural control on along strike variation in the seismicity of the northwest himalaya an overview of seismicity along the northwest himalaya exhibit along strike segmentation intricately controlled by the subsurface topographic ridges on the underthrusting indian plate as well by rift and nappe structures in the overriding wedge of the himalaya the segmentation exists for thrust dominated large magnitude earthquakes m 6 seated on the active detachment beneath the outer and lesser himalaya segmentation also prevails for moderate and small magnitude earthquakes concentrated in a narrow himalaya seismic belt hsb straddling northern lesser himalaya and southern higher himalaya numerical calculations of stress distribution favour that the degree of seismicity in the hsb is a good proxy to the presence of mid crustal ramp connecting the locked section of active detachment to aseismically slipping detachment beneath higher himalaya further gap or diffused pattern in the concentrated seismicity in the hsb in agreement with mapped high electrical conductive structure suggests ramp structure may be absent where underthrusting delhi hardwar ridge interacts with the himalaya arc in the nappe dominated tectonics the accommodation of the accumulating strains on the listric thrust faults produces increased frequency of moderate magnitude earthquakes and thereby possibly influences stress level on the detachment this may explain relatively less frequent occurrences of large earthquake in the kangra chamba region compared to that in the garhwal himalaya the low level seismicity in the simla region may also be influenced by the active kaurik chango rift in the higher himalaya as the former casts a stress shadow on the latter

lithofacies and water body record of messinian evaporites in nijar basin se spain abstract the depositional lithofacies and fabrics of messinian evaporites have been studied to evaluate the palaeodepth of water body in nijar basin se spain the nijar messinian strata consist of three formations abad marls yesares evaporites and feos rocks gypsum deposits dominate the yesares formation and the basal feos formation six gypsum lithofacies have been identified including coarse twinned selenite grass like selenite hemiradial to radial selenite turbidite selenite in the yesares formation and gypsum conglomerate laminated gypsum and hemiradial to radial selenite in the feos formation the size of yesares selenites decreases upward while carbonate content and trace element abundance increase in each selenite bed and the whole formation suggesting a shallowing up sequence these gypsum lithofacies were deposited in subaqueous environments commonly at 10 30 m and occasionally up to 100 m or deeper for the turbiditic selenites based on the interpretations of gypsum fabrics marl intervals basin configuration and geochemistry the sea level of nijar basin experienced two major drops with a magnitude of 40 m before and after the deposition of yesares evaporites however the basin was never desiccated during the evaporite deposition and the change of sea level for the alternative deposition of gypsum beds and marl intervals was commonly around 50 80 m probably resulting from the glacio eustacy fluctuations feos gypsum reflects another evaporative event and is correlated to the upper evaporite while the yesares selenite is correlated to the lower evaporite based on the interpretations of lithofacies geochemistry and sealevel change as seawater in nijar basin was continuously supplied by the central mediterranean basins and remained in subaqueous conditions during the messinian salinity crisis therefore the desiccation theory needs to be reexamined

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

probabilistic assessment of earthquake recurrence in northeast india and adjoining regions northeast india and adjoining regions 20 32 n and 87 100 e are highly vulnerable to earthquake hazard in the indian sub continent which fall under seismic zones v iv and iii in the seismic zoning map of india with magnitudes m exceeding 8 7 and 6 respectively it has experienced two devastating earthquakes namely the shillong plateau earthquake of june 12 1897 mw 8 1 and the assam earthquake of august 15 1950 mw 8 5 that caused huge loss of lives and property in the indian sub continent in the present study the probabilities of the occurrences of earthquakes with magnitude m 7 0 during a specified interval of time has been estimated on the basis of three probabilistic models namely weibull gamma and lognormal with the help of the earthquake catalogue spanning the period 1846 to 1995 the method of maximum likelihood has been used to estimate the earthquake hazard parameters the logarithmic probability of likelihood function ln l is estimated and used to compare the suitability of models and it was found that the gamma model fits best with the actual data the sample mean interval of occurrence of such earthquakes is estimated as 7 82 years in the northeast india region and the expected mean values for weibull gamma and lognormal distributions are estimated as 7 837 7 820 and 8 269 years respectively the estimated cumulative probability for an earthquake m 7 0 reaches 0 8 after about 15 16 2010 2011 years and 0 9 after about 18 20 2013 2015 years from the occurrence of the last earthquake 1995 in the region the estimated conditional probability also reaches 0 8 to 0 9 after about 13 17 2008 2012 years in the considered region for an earthquake m 7 0 when the elapsed time is zero years however the conditional probability reaches 0 8 to 0 9 after about 9 13 2018 2022 years for earthquake m 7 0 when the elapsed time is 14 years i e 2009

aerodynamics of stratovolcanoes during multiphase processes 1 pyroclastic density currents pdcs are gas particle flows generated during explosive eruptions which are often erupted over the flanks of stratovolcanoes these volcanoes may have different shapes which can affect the flow aerodynamics and hence the depositional processes here multiphase numerical simulations are carried out in order to define semiquantitative relationships among the pdc behavior particle response and deposit formation three stratovolcano shapes are used straight convex and concave and by means of numerical simulations their effects both on the flow structure and depositional processes are highlighted the current starts moving as a homogeneous flow and then it rapidly evolves to a turbulent boundary layer moving in contact with the ground overlaid by a companion wake region results show that thin boundary layers produce thick deposits of massive layers whereas thick boundary layers produce thin laminated deposits moreover concave wake regions would produce thick massive deposits of fine ash whereas convex wake regions would produce thin ash deposits

tectonic control for evaporite formation in the eastern betics tortonian spain tectonic uplift and sea level lowering are two proposed mechanisms to explain the development of thresholds in front of the widespread evaporite basins of the circum mediterrranean region during the miocene here we apply a multi disciplinary approach to the continuously marine sequences of the murcia cartagena basin to investigate which mechanism is responsible for the tortonian evaporites of the eastern betics first we develop a high resolution chronology for the venta de la virgen section by integration of biostratigraphic magnetostratigraphic and isotopic dating results next we construct palaeobathymetry and geohistory curves for this section and the abad composite of the sorbas basin we show that the apparent differential vertical motions between the two sections cannot be explained by sea level change but only by local tectonics the murcia cartagena basin shows significant tectonic uplift during the late tortonian and early messinian while the neighboring fortuna basin was rapidly subsiding we conclude that tectonic activity on the alhama de murcia fault was responsible for the emergence of a threshold that finally led to evaporite formation in the fortuna basin

late miocene paleoenvironmental changes in north africa and the mediterranean recorded by geochemical proxies monte gibliscemi section sicily abstract the astronomically tuned marls of the monte gibliscemi section sicily constitute an archive to trace the late miocene palaeoenvironmental conditions 9 7 7 0 ma in north africa here we have utilised carbonate content and al normalised geochemical proxies to trace changes in terrigenous source area and bottom water ventilation the terrigenous input into the section is dominated by north african river systems draining into the eastern mediterranean the proxy parameters indicate that the palaeoenvironmental conditions in north africa were humid from 9 5 ma onward with high fluvial input to the mediterranean increases in the si al and mg al ratios occurred from 8 4 to 8 2 ma and from 8 05 to 7 75 ma with maximum values similar to those of messinian diatomites these peaks indicate conditions of enhanced biosiliceous productivity and the presence of authigenic clay formation sluggish water circulation in the mediterranean during those times is inferred from the mn al and v al behaviour late miocene changes of the betic southern spain and rifian morocco mediterranean atlantic gateways are interpreted as the driving force for the changes in water circulation and the transgression associated with the opening of the rifian corridor is interpreted to occur at 7 8 ma

regional time predictable modeling in the hindukush pamir himalayas region abstract the seismic characteristic of hindukush pamir himalaya hph and its vicinity is very peculiar and has experienced many widely distributed large earthquakes recent work on the time dependent seismicity in the hindukush pamir himalayas is mainly based on the so called regional time predictable model which is expressed by the relation log t cm p a where t is the inter event time between two successive main shocks of a region and m p is the magnitude of the preceded main shock parameter a is a function of the magnitude of the minimum earthquake considered and of the tectonic loading and c is positive 0 3 constant in 90 of the cases with sufficient data parameter c was found to be positive which strongly supports the validity of the model in the present study a different approach which assumes no prior regionalization of the area is attempted to check the validity of the model nine seismic sources were defined within the considered region and the inter event time of strong shallow main shock were determined and used for each source in an attempt at long term prediction which show the clustering and occurrence of at least three earthquakes of magnitude 5 5 m s 7 5 giving two repeat times satisfying the necessary and sufficient conditions of time predictable model tp model further using the global applicability of the regional time and magnitude predictable model the following relations have been obtained log t t 0 19 m min 0 52 m p 0 29 log m 0 10 63 and m f 1 31 m min 0 60 m p 0 72 log m 0 21 01 where t t is the inter event time measured in years m min the surface wave magnitude of the smallest main shock considered m p the magnitude of preceding main shock m f the magnitude of the following main shock and m 0 the moment rate in each source per year these relations may be used for seismic hazard assessment in the region based on these relations and taking into account the time of occurrence and the magnitude of the last main shock in each seismogenic source time dependent conditional probabilities for the occurrence of the next large m s 5 5 shallow main shocks during the next 20 years as well as the magnitudes of the expected main shocks are determined

carbonate replacements after sulfate evaporites in the middle miocene of egypt abstract massive zones of diagenetic carbonate which replace sulfate deposits are observed in the middle miocene evaporites in several outcrops along the egyptian coast of the gulf of suez these carbonates display marked similarities of morphology lithofacies and stable isotope composition to diagenetic carbonates described in some portions of messinian gessoso solfifera formation of sicily and in the castiles of the upper permian castile formation of the delaware basin the mechanism of mineral transformation is basically bacterial sulfate reduction because the low 13c values of these carbonates argue for the organic origin of carbon the 18o values fall i to two distinct groupings indicating that the carbonates were crystallized during two different diagenetic stages the early diagenetic stage occurred soon after deposition in pore waters of marine origin and at different levels of evaporation organic matter used by bacteria was produced in situ the late diagenetic stage occurred when the tectonic or halokinetic uplift of the sequence caused flushing of the sediments by groundwaters of the phreatic aquifer and hydrocarbon migration furthermore to explain the very low 18o values of these carbonates it is inferred that locally ambient temperatures reached more than 35 c due to the large quantities of energy released during the bacterial sulfate reduction

pressure evolution during explosive caldera forming eruptions abstract caldera forming eruptions of silicic magmas result from a complex coupling of the mechanics and fluid dynamics of the associated magma chamber field studies of caldera forming eruption products suggest that great pressure variations occur inside the magma chamber and associated conduits during these eruptions pressure evolution during explosive caldera forming eruptions is investigated through a simple model that describes the first order quantitative behaviour of the chamber we consider a piston like model that assumes a coherent block subsiding along circular sub vertical ring faults into the magma chamber this subsidence occurs after significant decompression of the chamber by an initial central vent eruption we assume that the initial pressure distribution in the chamber is magmastatic once collapse has begun the chamber roof is supported by the magma so that magma pressure at the chamber roof increases to lithostatic we suggest that pressure variations during caldera forming eruptions are mainly controlled by variations in magma volatile content regardless of what induces the formation of ring faults the model suggests that the occurrence of explosive caldera forming events depends on the strength of the chamber walls and the depth water content and aspect ratio of the magma chamber no significant differences exist between model results for a cylindrical or a more realistic elliptical magma chamber geometry of comparable aspect ratio assuming a constant strength of the host rock the mass fraction of magma that must be erupted during the central vent phase in order to trigger caldera collapse ranges for deep gas poor chambers from a few percent up to 40 for shallow gas rich chambers the model suggests that zoned chambers tend to collapse earlier than homogeneous chambers dike shaped chambers will erupt less magma than sill like chambers before caldera collapse initiates although dike like geometries are not associated with stress fields appropriate to create ring faults the model suggests that once initiated caldera collapse will tend to force out most or all of the volatile rich magma from the chamber for volatile rich magma chambers the total volume of erupted magma during caldera forming event is of the same order as the chamber volume the model also explains the variation in the erupted mass during the different phases of explosive caldera forming eruptions and is in good agreement with natural examples

spatial mapping of earthquake hazard parameters in the hindukush pamir himalaya and adjacent regions implication for future seismic hazard abstract the study deals spatial mapping of earthquake hazard parameters like annual and 100 years mode along with their 90 probability of not being exceeded nbe in the hindukush pamir himalaya and adjoining regions for this purpose we applied a straightforward and most robust method known as gumbel s third asymptotic distribution of extreme values giii a homogeneous and complete earthquake catalogue during the period 1900 2010 with magnitude mw 4 0 is utilized to estimate these earthquake hazard parameters an equal grid point mesh of 1 longitude x 1 latitude is chosen to produce detailed earthquake hazard maps this performance allows analysis of the localized seismicity parameters and representation of their regional variations as contour maps the estimated result of annual mode with 90 probability of nbe is expected to exceed the values of mw 6 0 in the sulaiman kirthar ranges of pakistan and northwestern part of the nepal and surroundings in the examined region the 100 years mode with 90 probability of nbe is expected to exceed the value of mw 8 0 in the hindukush pamir himalaya with caucasus mountain belt the sulaiman kirthar ranges of pakistan northwestern part of the nepal and surroundings the kangra himanchal pradesh and kashmir of india the estimated high values of earthquake hazard parameters are mostly correlated with the main tectonic regimes of the examined region the spatial variations of earthquake hazard parameters reveal that the examined region exhibits more complexity and has high crustal heterogeneity the spatial maps provide a brief atlas of the earthquake hazard in the region

authigenic zeolites and their relation to silica diagenesis in odp site 661 sediments leg 108 eastern equatorial atlantic at odp site 661 from the continental margin of northwest africa authigenic clinoptilolites were investigated in detail based on chemical data crystal size and shape and their intergrowth with associated mineral phases two types of clinoptilolite type a and b occur representing different diagenetic conditions under which the formation of zeolites took place clinoptilolite type a was found in a sediment section comprises numerous hiatuses and stratigraphically condensed sediments and could have been formed during early diagenesis in contrast clinoptilolite type b was precipitated in campanian sediments during later diagenesis after the transformation from opal a to opal ct had taken place at site 661 the silica source for the authigenesis of both clinoptilolite types is biogenic opal volcanic ash material occurs but in low amounts and thus is not a prerequisite for the formation of clinoptilolite at site 661

miocene benthic foraminiferal isotope records a synthesis abstract 18 o 16 o and 13 c 12 c ratios of miocene benthic foraminifera from a number of atlantic pacific and indian ocean dsdp sites 71 77b 206 208 238 279 289 296 329 357 and 366a have been compiled these provide a rather detailed history of miocene deep water especially in the pacific ocean bottom water temperatures rose during the early miocene and then declined rapidly during the middle miocene this decline was accompanied by an increase in antarctic glaciation late miocene bottom temperatures and antarctic ice volumes are inferred to be similar to today s but exhibited some fluctuation the early miocene ocean was less thermally stratified at intermediate and abyssal depths while the late miocene deep ocean had a thermal structure generally similar to the modern ocean foraminiferal carbon isotope ratios at most of the sites varied quasi sympathetically throughout the miocene these variations must reflect comparable variations in the mean 13 c 12 c of marine hco 3 however the causes of such variations are not yet clear

possible influence of subducting ridges on the himalayan arc and on the ruptures of great and major himalayan earthquakes abstract subduction of bathymetric features such as ridges seamounts fractures etc on the subducting plate influences the arc morphology and earthquake ruptures we analyse their effect on the development of the arcuate shape of the himalayan arc and on the ruptures of great and major himalayan earthquakes besides the two most prominent ridges in the indian ocean namely the chagos laccadive deccan ridge and the 90 e ridge which are assumed to extend up to the himalayan arc at least three major subsurface ridges have been mapped on the underthrusting indian plate under the indo gangetic plains it appears that the subduction of the two most prominent ridges contributed to the development of the arcuate shape of the himalayan arc the interaction and subduction of the other subsurface ridges probably influenced the himalayan arc morphology by causing a localised cusp in the frontal topography also these ridges probably acted as barriers to the ruptures of the major and great himalayan earthquakes

a mutually consistent seismic hazard source model for southern california abstract a previous attempt to integrate geological geodetic and observed seismicity data into a probabilistic hazard source model predicted a rate of magnitude 6 to 7 earthquakes significantly greater than that observed historically one explanation was that the discrepancy or apparent earthquake deficit is an artifact of the upper magnitude limit built into the model this was controversial however because removing the discrepancy required earthquakes larger than are seen in the geological record and larger than implied from empirical relationships between fault dimension and magnitude although several articles have addressed this issue an alternative integrated source model without an apparent deficit has not yet appeared we present a simple geologically based approach for constructing such a model that agrees well with the historical record and does not invoke any unsubstantiated phenomena the following factors are found to be influential the b value and minimum magnitude applied to gutenberg richter seismicity the percentage of moment released in characteristic earthquakes a round off error in the moment magnitude definition bias due to historical catalog incompleteness careful adherence to the conservation of seismic moment rate uncertainty in magnitude estimates obtained from empirical regressions allowing multi segment ruptures cascades and the time dependence of recurrence rates the previous apparent deficit is shown to have resulted from a combination of these factors none alone caused the problem nor solves it the model presented here is relatively robust with respect to these factors

earthquake hazard parameters in crete island and its surrounding area inferred from bayes statistics an integration of morphology of the seismically active structures and seismological data the study addresses the evaluation of earthquake hazard parameters such as maximum regional magnitude mmax and the slope of gutenberg richter law where b log e for the hellenic wadati benioff zone and the overriding lithospheric plate in the area of crete and its surroundings the seismicity of the area is divided in a cellular 1 0 1 0 manner allowing analysis of the localized earthquake hazard parameters and graphical representation of their spatial variation our approach incorporates the recently updated earthquake catalogue for greece and the adjacent areas the consideration of the morphology of the deep seismically active structures in the studied area and use of a probabilistic procedure for estimating the earthquake hazard parameters one of the main inconsistencies in the earthquake hazard assessment is the estimation of the maximum magnitude and the related uncertaint y the bayesian approach applied in the present is a straightforward technique for evaluating the earthquake hazard parameters and is based on the following assumptions poissonian character of seismic events flow a frequency magnitude law of gutenberg richter s type with cutoff maximal value for estimated parameter and a seismic catalogue having a rather sizeable number of events i e 50 events at least per cell for five cells in which the number of events is less than 50 an effort is made to produce synthetic data the re assessed parameters obtained from the synthetic data show no significant difference and the real data of the five cells are finally taken into account although the estimated uncertainty is high for four random cells we constructed hazard curves showing the probabilities that a certain magnitude m will be exceeded in one year and the return periods in years that are expected for a given magnitude these are particularly useful for the mapping of earthquake hazard in regions of either low or high seismic activity as is crete and the adjacent area the obtained results show that the w and e parts of both subducting and overriding plates differ in the spatial distribution of all the estimated earthquake hazard parameters the mmax distribution indicates strong coupling between the western portions of the interacting plates mmax 6 3 to the south of 36 n the smaller values of mmax mmax 1 0 are found to the south and east of crete for the wadati benioff zone and over the central part of the island and the area to the northeast of it cell 11 for the continental wedge which suggests nonuniform stress field and or heterogeneous material

a tomographic image of indian lithosphere break off beneath the pamir hindukush region summary the p and s seismic anomalies in the upper mantle beneath the pamir hindukush collision zone and surrounding areas are investigated using more than 4 10 5 traveltimes from 5775 events located in the study region recorded by the stations of the worldwide seismic network and reported in the isc catalogue all available epicentral distance ranges of seismic rays are considered all sources are relocated using an algorithm which includes the double difference method in addition source parameters are corrected in the main inversion step simultaneously with the velocity models the high reliability of the resulting p velocity images is confirmed by various tests the s model provides much smoother patterns which are however generally consistent with p anomalies our model shows a complex image of lithospheric plate under the study region under sw hindukush the northwards subducting indian plate is imaged throughout the upper mantle and transition zone further to the ne within a 300 km wide zone the upper part of the plate overturns and then the slab breaks off after it narrows at 250 km depth the tomographic model also provides evidence for the southward subduction of the lithosphere beneath the southern tien shan down to at least 250 km depth to the north of tarim we observe a northward dipping of the tarim plate down to 500 km depth at the crustal depth our model shows alternating belts of high and low velocities parallel to the main thrusting faults which indicate complex crustal structure in this region

intraplate deformation of the indian subcontinent 1 while deformation at the earth s surface primarily occurs along tectonic plate boundaries major earthquakes have shaken regions deep within continental interiors three of the largest m 7 5 historic intraplate earthquakes occurred within the indian subcontinent suggesting the possibility of significant intraplate deformation we consider surface velocities determined from new gps data collected at 29 continuous gps stations and 41 survey mode gps stations in india between 1995 and 2007 to find a north south shortening rate of 0 3 0 05 nanostrain yr 1 which may be accommodated by 2 1 mm yr of more localized convergence across central india southward motions at 4 7 mm yr of sites on the shillong plateau in northeast india reflect rapid shortening and high earthquake hazard associated with active thrust faults bounding the plateau the width and magnitude of the elastic strain accumulation field across the himalaya varies little from 76 90 longitude but the strain is more broadly distributed and convergence rates are higher along the eastern 200 km of the range

source rock potential of evaporitic environment examination of modern saline lakes solar evaporation ponds and lagoons shows that the evaporitic environment can be very productive of organic matter few species survive in the brines but those that do commonly exist in great profusion in a marine evaporitic embayment the flow of surface currents is persistently toward regions of highest salinity so that there is a continual supply and concentration of nutrients prolific growth of phytoplankton may be similar to that in areas of upwelling in modern oceans only carbonates precipitate in the mesosaline part 4 to 12 salinity of such an evaporitic environment and no great dilution of organic matter by clastic or biogenic sediments occurs because stratification of brine may occur and reducing conditions may be a sociated with the bottom waters much of the organic matter produced can be preserved upon maturation the result may be a rich carbonate source rock frequently unrecognized in the geologic column in the middle east mesosaline conditions are known to have occurred many times from the triassic to cretaceous and may be responsible for the vast reserves of petroleum in the area evaporitic conditions may also have played a part in the petroleum productivity of many other areas including the michigan and paradox basins

silicification of sulphate evaporites and their carbonate replacements in eocene marine sediments tunisia two diagenetic trends samples of chert nodules diagenetic carbonates and evaporites gypsum anhydrite collected from the gypsiferous limestones of the kef eddour member ypressian priabonian near metlaoui and sehib tunisia show selective silicification with great variety in the silicified by products based on d 13 c values which support an organic origin for the carbon carbonates replaced evaporites microbially through bacterial sulphate reduction observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites anhydrite gypsum in the absence of early diagenetic carbonate phases silica with d 18 o values from 25 to 28ae6 standard mean ocean water smow replaced the outer parts of anhydrite nodules at ph 9 in the innermost parts of the anhydrite nodules prevented silica precipitation the record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus the microbial diagenetic replacement of evaporites bacterial sulphate reduction by carbonates calcite aragonite and dolomite favoured silica replacement of carbonates rather than evaporites silica with d 18 o signature of 21 to 26 smow replaced carbonates on a volume for volume basis yielding a more siliceous groundmass and accounting for 90 95 of the nodules the relatively higher d 18 o values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed meteoric marine to marine water the lower d 18 o values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage the silica supply for chert formation could be derived from the reworked bio siliceous deposits diatomites to the west of the basin vestiges of an opal ct precursor undetectable by x ray diffraction xrd were revealed by d 29 si magic angle spinning nuclear magnetic resonance investigations diagenesis of the extraformational and overlying clay rich beds the host limestones are clay poor as shown by xrd measurements and minor volcanogenic and hydrothermal contributions during early diagenetic stages

seismic hazard in greece ii ground acceleration abstract in a previous paper makropoulos and burton 1985 the seismic hazard in greece was examined in terms of magnitude recurrence using gumbel s third asymptotic distribution of extreme values and concepts of the physical process of strain energy release the present study extends the seismic hazard methods beyond magnitude to the estimation of expectations of levels of peak ground acceleration exceedance thus allowing for a direct comparison between these two methodologies as well as establishing information relevant to design and planning criteria the limited number of strong motion records do not permit regional study of attenuation of ground vibration in greece an average formula is derived from eight well known formulae which resulted from worldwide studies this is a 2164 e0 70m r 20 1 80 cm s 2 where a is peak ground acceleration m is earthquake magnitude and r is hypocentral distance in kilometres this formula agrees with the observed values of peak ground acceleration values recorded in greece acceleration seismic hazard is calculated at each of six chosen cities values of maximum acceleration with probability 70 of not been exceeded in the next 25 50 100 and 200 years are obtained along with corresponding values of velocity and displacement the same detailed acceleration evaluation is then applied to the whole area of greece by dividing it into cells of 0 5 lat 0 5 long and the results are illustrated through isoacceleration maps differences in magnitude and acceleration hazard maps reflect the fact that in acceleration hazard assessment the focal distance from a particular place in an important factor the cities of heraklion and rodhos have the lowest acceleration hazard although the expected earthquakes may have large magnitude intermediate depth earthquakes characterise these two cities acceleration estimates unlike magnitude hazard parameters refer to a particular place and not to an area around it hence even if two places have similar earthquake depth distributions the hazards may differ significantly because of the different spatial distribution of the foci this is observed in the case of athens and corinth these cities have almost the same magnitude hazard but the acceleration hazard is much lower for athens where the hazard is mainly due to more distant earthquakes the isoacceleration maps for greece as a whole also define areas of high seismic hazard these are the areas around cephalonia and leukas islands in the ionian sea and the eastern sporadhes lesbos islands and chalkidiki in the northern aegean sea at the 70 probability level the maximum acceleration is expected to be around 0 2g within the next 50 years the areas where the maximum acceleration at the 70 probability level is expected to reach a value of 0 3g in the next 200 years are around cephalonia and leukas islands and near the dardanelles

diagenetic clinoptilolite and opal ct from the middle eocene wittering formation isle of wight u k abstract thinly interbedded estuarine sandstones and mudstones in the middle eocene wittering formation bracklesham group isle of wight contain authigenic clinoptilolite opal ct and possibly authigenic smectite over a thickness of 8 5 m the clinoptilolite and opal ct cements have resulted in these beds being the only ones tightly cemented in the wittering formation the rest being soft sands and muds the unit contains abundant siliceous sponge spicules many of which are preserved as voids and were the likely source of the opal ct the presence of an organic rich rooted paleosol histosol immediately overlying the unit is believed to have contributed significantly to the early diagenetic history of the estuarine sediments in particular the formation of zeolite although the ph and dissolved silica of unmodified seawater are too low for zeolite formation high concentrations of dissolved silica are believed to have resulted from dissolution of siliceous sponge spicules opal a and formation of silica organic acid complexes organic acid dissolution of feldspars absent from the zeolite bearing interval is the most probable source of alumina and cations for zeolite formation ree data show that volcanic ash was never present and cannot therefore have been the source of the zeolite this investigation therefore demonstrates a novel route by which the pore fluid chemistry required for zeolite formation can be achieved

variable anelastic attenuation and site effect in estimating source parameters of various major earthquakes including m w 7 8 nepal and m w 7 5 hindu kush earthquake by using far field strong motion data strong motion records of recent gorkha nepal earthquake m w 7 8 its strong aftershocks and seismic events of hindu kush region have been analysed for estimation of source parameters the m w 7 8 gorkha nepal earthquake of 25 april 2015 and its six aftershocks of magnitude range 5 3 7 3 are recorded at multi parametric geophysical observatory ghuttu garhwal himalaya india 600 km west from the epicentre of main shock of gorkha earthquake the acceleration data of eight earthquakes occurred in the hindu kush region also recorded at this observatory which is located 1000 km east from the epicentre of m w 7 5 hindu kush earthquake on 26 october 2015 the shear wave spectra of acceleration record are corrected for the possible effects of anelastic attenuation at both source and recording site as well as for site amplification the strong motion data of six local earthquakes are used to estimate the site amplification and the shear wave quality factor q at recording site the frequency dependent q f 124f 0 98 is computed at ghuttu station by using inversion technique the corrected spectrum is compared with theoretical spectrum obtained from brune s circular model for the horizontal components using grid search algorithm computed seismic moment stress drop and source radius of the earthquakes used in this work range 8 20 1016 5 72 1020 nm 7 1 50 6 bars and 3 55 36 70 km respectively the results match with the available values obtained by other agencies

magnetostratigraphy and sedimentary evolution of the late miocene to early pleistocene sediments quseir region egyptian red sea an integrated sedimentological and magnetostratigraphic study has allowed a detailed understanding of the late miocene to early pleistocene evolution of the sediments in the quseir region of the egyptian red sea coast palaeomagnetic samples were collected from sections in six wadis covering the shagara formation the gabir and samh members of the wardan formation and the abu dabbab formation evaporites remanence properties are carried by magnetite haematite and goethite the characteristic remanence is typically carried by detrital magnetite and haematite with more recent overprints predominantly associated with haematite and goethite produced by the weathering of diagenetic pyrite the magnetostratigraphy has allowed the following detailed age assignments for the lithostratigraphic units the shagara formation ranges in age from late pliocene late piacenzian to middle pleistocene 0 6 2 5 ma the gabir member is latest messinian to earliest piacenzian in age 3 5 5 5 ma and the samh member late tortonian to mid messinian 6 0 7 5 ma the age of the top of the abu dabbab formation is probably mid tortonian 8 ma disconformities occur between all the lithostratigraphic units with a local angular unconformity between the shagara and wardan formations lowstands in global sea level appear to have a strong influence on the timing of these disconformities characteristic mixed alluvial and reef facies of the shagara formation are a response to the ephemeral wetter climate following the initiation of northern hemisphere glaciation at 2 4 ma enhanced by rift margin uplift of basement complexes to the west this tectonic activity was concentrated in the early piacenzian the marine gabir member was deposited during the early pliocene and latest messinian high sea level stands the late tortonian early messinian age and sedimentological character of the samh member indicates this unit was affected by marine flooding events which ultimately produced during drawdown phases the off shore zeit formation evaporites in half graben depocentres

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

m 6 earthquakes in southern california during the twentieth century no evidence for a seismicity or moment deficit abstract a broadly based report on seismic hazards in southern california wgcep 1995 concluded that the predicted seismicity exceeds that observed since 1850 a subsequent independent analysis argued that infrequent huge m 8 earthquakes are needed to explain the low rate of large earthquakes jackson 1996 frequency magnitude relationships and earthquake reporting suggest that the 1903 to 1997 catalog we present here with a b value of 1 0 and a rate of m 6 shocks of 0 42 to 0 49 yr 1 is nearly complete in contrast the 1850 to 1994 catalog used by wgcep is incomplete before the turn of the century and thus its reported seismicity rate of 0 32 m 6 shocks yr 1 is too low principally because the wgcep 1995 model results in b values of up to 4 0 for regions of lesser and blind faults the rate of m 6 shocks off the san andreas system predicted by the wgcep 1995 model is three times greater than that observed in this century because they obtained b 0 4 for m b 2 2 for m 7 3 on major faults their expected rate of m 7 san andreas shocks is twice as high as observed thus part of the seismicity and moment discrepancy identified by wgcep was caused by use of an incomplete catalog and part was caused by inappropriate b values we obtain a southern california moment release rate of 8 to 12 10 18 n m yr 1 which cannot be distinguished from the moment release estimated by fault slip or the moment accumulation inferred from plate motions or geodetically measured shear strain we thus find no evidence for a moment deficit significant aseismic moment release or for rare m 8 earthquakes off the san andreas fault system finally the number of m 6 earthquakes per decade does not depart significantly from a poisson process during this century and thus we find no evidence that the rate of seismicity is increasing now or at any other time since 1900

tectonic control on carbonate platform facies distribution and sequence development miocene gulf of suez abstract mid miocene deposition in the southwestern gulf of suez was governed by an extensional fault block topography shallow marine carbonates and clastics were deposited around the exposed block crests and in marginal half grabens of the rift while central troughs were filled by slope aprons and turbiditic and pelagic mudstones fan deltas developed at points along the fault block escarpments syn rift platform carbonates from one block crest esh el mellaha are described here the platform evolved in three stages generating three sedimentary sequences sequences 1 and 2 were separated by boundary fault reactivation sequences 2 and 3 by sea level drawdown exposure and karstification the sediments comprise six lithofacies associations la 1 6 reef la 1 fore reef la 2 and back reef embayment la 3 sediments form a small reef rimmed platform in sequence 1 at the southern end of the block fringing reefs and reef lagoon systems also developed along the footwall escarpment and on fan deltas which bridged the block bounding fault fringing reefs in sequence 2 generated a carbonate talus veneer la 4 along the whole 80 km length of the esh el mellaha fault scarp carbonate slope and reefal sediments preserved on the footwall exhibit abundant evidence of syndepositional fault movements with truncation of escarpment fringing sequences and repeated generation of reefal wedges minor drowning at the sequence 1 2 boundary caused deepening of lagoons progradation of leeward reefal margins with lagoonal patch reef development and back stepping and vertical accretion of the reefal rim platform development was followed by algal laminated dolomitic sediments la 5 in the area of the old platform interior sequence 3 much of the esh el mellaha ridge remained exposed during platform growth and shed coarse alluvial clastic sediments la 6 into the peripheral marine environments the relationships demonstrated between miocene sedimentation and structural setting have parallels in the modern environments of the zeit bay area

biogenic carbonates are they a criterion for underlying hydrocarbon accumulations an example from the gulf of suez region egypt the gulf of suez petroliferous basin is characterized by the occurrence of a middle upper miocene evaporite cap field and subsurface investigations in the southwest gulf of suez region indicate that the evaporites usually contain two types of biogenic carbonates in addition to sulfur the first type of carbonate is biogenic syngenetic in origin and was formed in low temperature euxinic aquatic environments by the reduction of dissolved sulfates and the oxidation of organic matter by sulfate reducing bacteria the resulting carbonate and sulfur are stratigraphically restricted within the evaporite sequence and occur as interlaminated gypsum calcite sulfur or as sulfur ooids sulfur globules and stromatolitic sulfur that are enclosed in gypsum or calcite the biogenic syngenetic carbonate and sulfur did not originate from petroleum and have a significance of euxinic aquatic settings the second type of carbonate is a biogenic epigenetic carbonate that formed under low temperatures by biochemical reactions between seeping hydrocarbons and the overlying miocene evaporites the carbonate is vuggy is included within or caps the evaporites and usually shows hydrocarbon stains or emits a hydrocarbon smell the sulfur is usually yellow but may be brown to black due to bitumen staining carbonates replace gypsum or anhydrite and occur as aragonite calcite or dolomite the d13c mean values for carbonate minerals range from 7 8 to 27 2 pdb being relatively heavier than the d13c values of the nearby crude oil 28 1 to 29 3 pdb carbon isotope values indicate that these carbonates were formed from the heavier organic carbon dioxide under reducing conditions where the lighter methane is stable the occurrence of biogenic epigenetic carbonates and sulfur in the evaporite deposits of the gulf of suez petroliferous basin is similar to that recorded in other basins such as the gulf coast basins of texas louisiana and southern mexico the fergana and amudarya depression of central asia and the mesopotamian depression in iraq the biogenic epigenetic carbonates or sulfur in some regions is considered as a criterion for petroleum microseepage that may indicate a potential hydrocarbon bearing reservoir at depth