estimates of heat flow derived from gas hydrates a gas hydrate phase boundary in marine sediments is often observed as an anomalous reflector in seismic profiling data from the depth of the reflectors heat flow values can be estimated using the phase relation of the gas hydrate system and inferred density and thermal conductivity of sediments this method is applied to the data in the nankai trough around central america and along the blake outer ridge the estimated values consistent with those measured by conventional means suggest that derivation of heat flow by this method is feasible on the landward slope of the nankai trough we find that heat flow increases downslope toward the floor of the trough

rate of outward growth of the mediterranean ridge accretionary complex abstract the position as a function time of the deformation front on the southwest flank of the mediterranean ridge accretionary complex is constrained as follows 1 a the deformation front is now active 2 b the site of core ban84 05gc was still near the abyssal plain when displaced shallow water benthic foraminifera of inferred african provenance were redeposited within an upper pliocene age unit 3 c the site of core ban84 05gc on the outer flank of the mediterranean ridge was already within the topographically rugged accretionary complex when a pliocene debris flow was emplaced 4 d dsdp site 125 had already been uplifted into a topographically elevated position when lower pliocene pelagic ooze was deposited 5 e a gypsum bearing breccia in dsdp site 125 requires that the site was either on the abyssal plain or within the tectonically active outer perimeter of the accretionary complex during the messinian salinity crisis 6 f dsdp site 377 had already been uplifted into a topographically elevated position when middle miocene age pelagic marl was deposited 7 g dsdp site 377 was still on or near the abyssal plain when early to lower middle miocene age smectite bearing turbidites of inferred african provenance were deposited and 8 h the mediterranean ridge began to grow by offscraping against a backstop formed by the alpine nappes of the hellenic arc at the time that subduction began 33 ma together these constraints define a range of potential growth curves for the mediterranean ridge with a rate of outward growth of approximately 0 5 to 2 cm yr this growth rate is faster than that inferred for most other modern accretionary prisms both as an absolute value and as a fraction of the subduction velocity an unusually thick incoming section and or an unusually weak evaporitic decollement may contribute to the rapid growth rate the inferred age of accretion does not increase linearly with distance from the deformation front rather there is an apparent acceleration of the rate of outward growth through time this observation may reflect a true acceleration of growth rate in response to the onset of back arc extension in the overriding plate aegean sea and or in response to the development of a weak basal decollement following the introduction of halite into the incoming stratigraphic section alternatively the non linearity of the age vs distance curve may indicate that shortening of the offscraped sediments continues across the breadth of the mediterranean ridge rather than being confined to the outer flank

strain distribution over the east mediterranean ridge a synthesis incorporating new sea beam data abstract we present the results of several multi narrow beam sea beam traverses of the mediterranean ridge on board r v jean charcot these results can be considered to be complementary in resolution and coverage to the previously published long range side scan sonar gloria results a characteristic of the mediterranean ridge is the presence over about half of its surface of an extensive fold system affecting the messinian and plio quaternary cover which is dissected by conjugate strike slip faults the resolution of sea beam was used to map the main characteristics of folding and faulting this analysis together with previously available data enables us to define the strain pattern over the mediterranean ridge we thus now have an overall view of the strain pattern from a marginal basin aegea through the hellenic trench and adjacent sea floor to the opposite continent the bottom of the hellenic trench is a boundary between an extensional domain to the north and a compressional one to the south the southern domain is strongly affected by the presence of the lybian promontary it is related to the formation of an accretionary ridge due to the inability of the upper 3 4 km of sediments to subduct this ridge is similar to the accretionary prism but it is much wider with respect to its height probably because of the existence of a decollement over a level of overpressure the hellenic trench then is in the situation of a fore arc basin its origin is due to the fact that the aegean continental slope is not vertical but rather has a small 4 dip as a result a sedimentary wedge symmetric but shorter than the main accretionary wedge develops over the buried portion of the slope it is characterized by landward thrusts such an origin probably applies to fore arc basins in general

cyclical behavior of thrust wedges insights from high basal friction sandbox experiments scaled sandbox experiments with high basal friction simulating the growth of accretionary wedges display cycles alternating between frontal imbricate thrusting and underthrusting of long undeformed sheets by contrast low basal friction experiments with otherwise similar and constant initial conditions produce a classic frontal imbricate fan through repeated failure along frontal thrusts the cyclical behavior observed in high basal friction experiments is expressed by three quantities 1 the average spacing between frontal thrusts 2 the advance and retreat of the deformation front and 3 the frontal slope alpha of the actively deforming wedge as a long sheet is underthrust the front is steepened through slumping until the maximum critical angle is reached then frontal thrusting resumes and the accretion of imbricate slices builds the wedge forward thereby lowering the taper to the minimum critical angle at shallow tapers a long unit is underthrust and subsequently uplifts shortens and steepens the overlying wedge through backthrust deformation thus completing the cycle underthrusting of long units offers a simple mechanism for underplating overlying units it also provides a possible explanation for temporally and spatially varying wedge geometries in nature when basal frictions attain 80 90 of the internal friction

structure of the makran subduction zone from wide angle and reflection seismic data abstract makran is one of the largest accretionary wedges on the globe formed by the convergence between the eurasian and the arabian plates it is characterised by an extremely high sediment input of 7 km and a shallow subduction angle we present seismic velocity models from four wide angle seismic lines that image the wedge sediments and the subducted oceanic crust three strike lines show complex but rather one dimensional structures where the observed seismic phases have been studied with the reflectivity method a 160 km long dip line was surveyed coincident with a mcs line collected by cambridge university in 1986 prestack depth migration of this data with the new wide angle velocities shows improved images compared to earlier results interpretation of the wide angle and the mcs data indicates that a decollement is developed along a bright reflector within the turbiditic sequence more than 3 km of sediment bypasses the first accretionary ridges underthrusting and is transported to greater depth this might help to explain the sparse earthquake activity associated with subduction here in the strike lines low velocity layers occur landward of the deformation front in zones where active thrusting or underplating takes place the oceanic crust is characterised by a strong velocity contrast between layer ii and layer iii

from foreland to forearc domains new multichannel seismic reflection survey of the mediterranean ridge accretionary complex eastern mediterranean abstract the mediterranean ridge of the eastern mediterranean sea is a huge accretionary wedge emplaced as a consequence of the africa aegean plate convergence that has a rate estimated at 40 mm yr processing of recent multichannel seismic data and integration with previous results seismic profiling swath bathymetry sonar have facilitated the study of the overall deformation pattern of the area which includes three distinct major structural domains facing various forelands 1 1 the outer domain is bounded southward by the present day wedge toe and towards the north by topographic slope breaks and clear changes in deformational style beneath the ionian and levantine mediterranean ridge branches the base of messinian evaporites seems to act as a major decollement level 2 2 the central or crestal domain the shallowest one shows evidence of mud diapiric and mud volcano activity and is overthrusting northward over the inner domain facing the libyan margin this area which displays spectacular mud volcano fields appears much wider and shallower 3 3 the less deformed inner domain is bounded northward by the hellenic trench system beneath its ionian inner area seismic velocities characteristic of continental crust have been recorded while within its levantine region it is made of a series of disconnected and uplifted basement blocks strabo seamounts along the matapan and pliny trenches structures which could be caused by transpressive deformation are locally observed we believe that both the north south structural arrangement and the clear west east lateral variations of the mediterranean ridge relate to its progressive collision against the libyan margin promontory this event which may have initiated 5 6 myr ago may have induced a a differentiated paleogeography of messinian evaporites later available for tectonic accretion leading to the further lateral structural variations of the outer domain and b strain partitioning between the central mediterranean ridge and the hellenic trench system

geomorphological study of an area with mud diapirs south of crete mediterranean ridge abstract during meteor cruise 25 4 in 1993 a multibeam bathymetric hydrosweep survey was carried out on the mediterranean ridge in an area south of crete comprising the original olimpi and prometheus 2 mud diapir fields the morphology is characterized by subcircular domes and elongated ridges features the studied area is subdivided from west to east in three relief provinces with predominance of the domes in the middle one we suggest that all the domes are related to mud diapirism as is proved for some of them by sonographic surveys and already published sediment cores the orientation of the relief elements domes and ridges corresponds to that of the large morphotectonic features the nw se orientation hellenic trench prevails in the western province the ne sw orientation ptolemy pliny and strabo trenches is dominant in the eastern province the domes occur in the middle province where both orientations influence jointly the morphotectonic pattern on the mediterranean ridge the concentration of the mud diapirs is inferred to be due to crossing fault systems which may have led to an enhancement of conduits for ascending mud breccias and fluids the high resolving sonographic records of a w e running survey displayed a detailed anatomy of mud domes volcanoes with features of broad variation of backscatter intensities such as flow lobes and streams circular elevations mini domes and depressions pockmarks of small and smallest scale as a very characteristic feature not described before in the areas outside the mud domes an extensive network like pattern of small high backscatter lineations has been observed which is tentatively interpreted as a system of fractures joints of near surface mg calcite crusts their orientations were found to correspond to the regional morphotectonic pattern

experimental frictional heating of coal gouge at seismic slip rates evidence for devolatilization and thermal pressurization of gouge fluids high velocity 1 m s friction experiments on bituminous coal gouge display several earthquake related phenomena including devolatilization by frictional heating gas pressurization and slip weakening stage i is characterized by sample shortening and reduction in the coefficient of friction from 1 to 0 6 stage ii is characterized by high frequency 5 hz oscillations in stress and strain records and by gas emissions stage iii is marked by rapid weakening 0 1 to 0 35 and sample shortening together with continued gas emissions stage iv produces stable stress records and continued weakness 0 2 but without gas emission stage i shortening is due to compaction of the gouge and the weakening is attributed to mechanical or thermal effects stage ii behavior is interpreted as due to coal gasification and fluctuations in fluid pressure resulting in high frequency stick slip type behavior dramatic reduction in shear stress in stage iii is attributed to gas pressurization by pore collapse and corresponds to a frictional instability analogous to nucleation of an earthquake microstructural observations indicate the deformation was brittle during stages i and ii but ductile during stages iii and iv time dependent finite element frictional heat models indicate the center of the samples became hot 900 c during stage ii whereas the edge of samples remained relatively cold 300 c vitrinite reflectance of coal samples shows an increase in reflectance from 0 5 to 0 8 over the displacement interval 20 40 m 20 40 s indicating that the reflectance responds to frictional heating on a short time scale the energy expended per unit area in these low stress large displacement experiments is similar to that of higher stress 50 mpa short displacement 1 m earthquakes 107 j m2

calibrating the bulk rheology of active obliquely convergent thrust belts and forearc wedges from surface profiles and velocity distributions the geometrical profiles and surface velocity distributions in active forearc wedges depend on the bulk rheology and the surface tractions analytical and numerical solutions of the force balance and comer flow circulation equations indicate that the mechanical parameters should in principle all be directly determinable from the observational variables the analysis suggests the following preliminary conclusions 1 the surface profiles of forearc wedges are con sistent with either a bulk plastic rheology or a viscous rheol ogy with a transition to plastic or coulomb behavior in the upper few kilometers 2 comer flow circulation in the absence of accretion is diagnostic of wedges with a bulk viscous rheology the surface strike normal velocity and the surface profile both depend on the bulk viscosity and the viscous coupling at the base allowing these mechanical vari ables to be determined current estimates of this velocity in some wedges suggest the bulk viscosity is not less than 3x10 9 pa s 3 obliquely converging nonaccreting plastic and coulomb wedges show no distributed strike parallel shear if the obliquity is above a critical angle the wedge is separated from the upper plate by a strike slip fault defining a forearc sliver the critical angle of obliquity the velocity of the forearc sliver and the wedge geometry allow the boundary tractions to be determined in obliquely convergent viscous wedges the strike parallel velocity is distributed throughout the wedge decaying exponentially from the rear to the front its value at the rear and the length scale for the decay allow the bulk viscosity and the coupling at the rear to be determined slip vectors in some forearc wedges and thrust belts suggest complete partitioning of the strike parallel component of motion characteristic of a bulk viscous rheology others show partitioning above a limiting angle of obliquity suggesting bulk plastic behavior there are no documented cases of active margins where there is an absence of partition ing for obliquity 45 o which appears to role out coulomb behavior as a general description for the bulk rheology of forearc wedges

fluid inclusion technique for determining maximum temperature in calcite and its comparison to the vitrinite reflectance geothermometer theory laboratory experiments and empirical observation suggest that many aqueous fluid inclusions in calcite reequilibrate during overheating and therefore some homogenization temperatures t h record a temperature close to the maximum reached by the rock this characteristic suggests that aqueous fluid inclusions in calcite can be used to establish maximum temperature t peak to test this hypothesis we have compiled fluid inclusion t peak mean random vitrinite reflectance r m and present day t peak from 46 diverse geologic systems that have been at t peak from 10 4 to 10 6 yr present t peak ranged from 65 to 345 c t h modes and means ranged from 59 to 350 c and r m data ranged from 0 4 to 4 6 spanning the temperature and thermal maturity range associated with burial diagenesis hydrothermal alteration and low grade metamorphism plots of t h and t peak data for systems thought to be currently at maximum temperature demonstrate close agreement between t h and present t peak in sedimentary basins although caution should be applied the relation suggests that t h of aqueous fluid inclusions in calcite may be a useful measure of maximum temperature this study also compares t h to mean random vitrinite reflectance r m to offer further support for the use of t h as a measure of t peak and to provide a better understanding of r m t h correlates well with r m and results in a curve similar to r m vs t peak calibrations determined by other workers the strong correlation correlation coefficient r 0 93 between t peak and r m in these systems suggests that maximum temperature is the major control on thermal maturation

deformation of the western mediterranean ridge importance of messinian evaporitic formations abstract multichannel seismic data were acquired across several areas of the mediterranean ridge m r in the eastern mediterranean sea during the mcs prismed survey march 1993 the m r is a wide high standing and elongated swell emplaced between the southern aegean and africa as a consequence of the long term convergence and the ongoing collision between europe and africa sedimentary piling and associated deformation have built a thick deformed sediment wedge the internal geological structures of which remain poorly understood processing of two mcs lines pm 02 and pm 03 across the western branch of the mediterranean ridge reveals that messinian evaporite bearing formations which were deposited within the deep ionian oceanic basin 5 my ago play an important part in the present structural deformation of the m r accretionary wedge within the outer m r the messinian lower evaporites are believed to act as one of the main decollement levels while the top of these formations may operate as a disharmonic layer below thin plio quaternary sediments the present day m r toe appears to be affected by ductile flow deformation however most of the accreted outer wedge is believed to have resulted from the progressive piling up and associated thrusting of messinian and overlying plio quaternary sediments toward the northeast i e toward the central m r domain a major backthrust zone indicates a structural boundary between the outer wide and convex m r and its inner rather flat domain which is inferred to act as a backstop

structure of the mediterranean ridge accretionary complex from seismic velocity information seismic velocities obtained from ocean bottom hydrophone expanding spread profile and multi channel seismic data were used to compile a velocity model for the mediterranean ridge along a 220 km long transect extending from the sirte abyssal plain to the cleft region near the hellenic trough a 200 300 m thin layer of plio quaternary sediments with velocities of 1800 2200 m s 1 covers the whole ridge the messinian evaporites 4000 4500 m s 1 occur in the southwest as a tectonically thickened layer and in a basin just northeast of the crest of the ridge in the intervening region however the evaporites appear absent and the seismic velocities are generally lower arched reflectors imaged in the depth migrated section suggest that the sediments beneath the ridge crest belong to a pre messinian accretionary wedge beneath the messinian evaporites a northeastward thinning layer of probable tertiary sediments shows laterally increasing velocities from 3300 m s 1 to 4600 m s 1 assuming that the layer thinning is caused by compaction due to increased overburden alone we have calculated a porosity reduction from 15 to 4 and an associated fluid expulsion of 10 km3 km 1 along the trench axis this corresponds to c 60 of the initial fluid volume of an undeformed sediment column from the abyssal plain the almost impermeable evaporitic cap over these sediments leads to high fluid pressures at the base of the evaporites likely to make this horizon the basal decollement of the modern accretionary system a 2 5 km thick unit of probable mesozoic carbonates with velocities of 4500 4600 m s 1 is inferred at c 8 km depth the top of the oceanic crust occurs at a depth of about 10 km the results from this study have widespread implications for the understanding of the regional geological history

brine pools associated with mediterranean ridge mud diapirs an interpretation of echo free patches in deep tow sidescan sonar data abstract mak 1m deep tow sidescan sonar surveys have revealed two echo free patches in an area of mud diapirs on the mediterranean ridge the unusual shape of these patches and absence of returned signal can be explained more easily by the presence of brine pools rather than as a topography effect the high acoustic impedance contrast at the interface between brine and seawater a result of the salinity contrast there causing a relatively high contrast in sound velocity and water density acts as a screen preventing sidescan sonar sound signal at low grazing angles from reaching the seafloor sound is reflected and refracted away from the sonar towfish without any backscatter the inferred brine pools appear to be associated with faults recognized on sonographs and subbottom profiler records this first observation of brine pools related to mud volcanoes and mud diapirs suggests that salt is present in areas of mud diapirism possibly playing a role in that process or at least that the brines and mud may share flow paths within the mediterranean ridge

age variation in the source of the diapiric mud breccia along and across the axis of the mediterranean ridge accretionary complex abstract pleistocene or younger mud diapirism is recorded along the arcuated crest of the mediterranean ridge accretionary complex over a distance of more than 450 km the extruded mud supported breccia named mud breccia is poorly fossiliferous but the matrix may be dated as late aptian to albian and late burdigalian to langhian in age based on the calcareous plankton content calcareous nannofossils and planktonic foraminifera diagnostic of other ages are sparse within the two above mentioned major assemblages they indicate the presence of sediments attributable 1 to the santonian and cenomanian which occur in association with the mid cretaceous assemblages toronto unnamed and aros domes and 2 to the serravallian which occur in association with the late burdigalian langhian assemblages in the gelendzhik dome aptian albian to late cretaceous sediments are pelagic in origin and are similar to the scisti a fucoidi and scaglia rossa formations well known from continental italy mixed dolomitic clasts and crystals also occur within the matrix indicating the presence of portions of the evaporitic sequence of messinian age in four cores from the western locations single very rare plankton specimens indicative of other ages different from those mentioned above also occur within the matrix at various places they are interpreted as reworked primarily within the late burdigalian langhian sediments the age of the emplacement of the breccia should be older than the middle pleistocene as supported by the occurrence of clasts of this age in the mud breccia and is still an ongoing process at least in the napoli dome

coal maturation by frictional heat during rapid fault slip 1 the detection of frictional heating effects along faults provides key insight into the dynamics of earthquakes and faulting thermal maturity of organic matter has been considered a possible fault thermometer that records the frictional heat signature of ancient earthquakes however whether or not organic matter can mature on the order of seconds typical earthquake rise time remains uncertain here we present the results of experiments aimed at revealing coal maturation by frictional heat generated at slip velocities representative of natural earthquakes of up to 1 3 m s our results show that coal can mature coseismically in 11 seconds at temperatures induced by frictional heat ranging from 26 to 266 c even with a temperature rise to only 28 7 c over 15 m displacement in 3 2 hours coal can slightly mature within a shear localized zone the commonly used kinetic model of vitrinite maturation cannot predict the experimental results a kinetic model involving the effect of flash temperature at grain contacts and mechanochemical effects on reaction kinetics is necessary to better estimate heat generation along a fault

paleothermal structure of the shimanto accretionary prism shikoku japan role of an out of sequence thrust vitrinite reflectance study on the cretaceous shimanto accretionary prism of southwest japan shows that the vitrinite reflectance varies from 0 6 to 3 0 corresponding to the estimated maximum paleotemperature variation of 90 210 c the thermal structure shows a sharp discontinuity across an out of sequence thrust separating it into two thermal blocks in each block vitrinite reflectance values increase systematically toward the prism younging direction and the thermal structure is clearly overprinted on the tectonic imbrication of turbidite and melange units this provided us with a set of data to calculate the geometry and displacement of the out of sequence thrust the fission track zircon partial annealing ages indicate that this thrust was active about 43 ma probably in response to the accretion of eocene rocks this study shows that initial thickening of the accretionary prism in an order of several kilometres was mainly controlled by out of sequence thrust

a fluid model for the shape of accretionary wedges in this paper the accretionary wedges landward of most oceanic trenches are modelled assuming a newtonian fluid rheology it is shown that lubrication theory is applicable and that the addition of material to the wedges can be neglected in determining the shape of the wedges the predicted shape of the wedges is in good agreement with bathymetric profiles across the kurile ryukyu and aleutian wedges from the shape of the above wedges it is deduced that accretionary sediments have viscosities between 1018 and 1019 poise

thermal regime of the southwest japan subduction zone effects of age history of the subducting plate abstract during a mid miocene at about 15 ma tectonic reorganization an ocean spreading ridge perpendicular to the strike of the nankai trough subduction zone stopped spreading since that time there has been subduction of the cooling fossil spreading ridge in the present study we have developed a time dependent thermal subduction model for this region using the finite element method there are good seismological and geological constraints for the plate geometry and for the subduction history of the past 15 ma boundary conditions are specified so that the oceanic plate subducting at the nankai trough becomes cooler as it gets older the model results agree with the present heat flow trend that decreases landward and explain the paleothermal regime of high mid miocene temperatures inferred from land geological studies the tectonics of the region prior to 15 ma has some uncertainties assuming subduction of an active spreading ridge or an 80 ma old lithosphere prior to 15 ma give the same results for the present thermal regime of the seaward portion of the forearc but different results for the most landward region 250 km the thermal history of the forearc since 15 ma can be summarized as a rapid warming period as a consequence of ridge subduction followed by a cooling trend to the present as a result of the aging of the subducting plate the results illustrate the thermal consequences of one type of ridge subduction they also demonstrate that the thermal regime of a subduction zone depends critically on the age history of the subducting oceanic lithosphere especially if it is young as well as such parameters as the subducting plate dip angle and thickness of insulating sediments on the incoming oceanic crust this dependence is especially important when the thermal regime is used to constrain the seismogenic behaviour of the subduction thrust fault

thermal constraints on the seismogenic portion of the southwestern japan subduction thrust for coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault in this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature we have developed a transient thermal model for the nankai subduction zone of southwest japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from 1 seismicity and tsunami data for two great subduction earthquakes 2 the coseismic faulting extent of these events estimated from geodetic deformation data and 3 the interseismic locked zone determined from interseismic geodetic data the nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures it has earthquake tsunami and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain on the nankaido margin off shikoku island the thermal model indicates that a temperature of 350 c taken to be the limit for seismic initiation from laboratory and field data is reached on the subduction thrust fault 150 km from the trench a transition zone into which rupture may extend with decreasing offset taken to be 450 c extends an additional 45 km downdip the thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 nankaido ms 8 2 earthquake off shikoku island and with the downdip extent of the present locked zone in the region of the 1944 tonankai ms 8 2 earthquake to the northeast the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent the seismogenic locked zone from earthquake and geodetic data is also narrower thus our analysis of the southwest japan margin indicates that all three constraints on the downdip extent of the seismogenic zone thermal coseismic and interseismic geodetic data are in general agreement the study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature the thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip the subducting plate age and thus heat flow and the thickness of the insulating sediments on the incoming plate are also very important to the thermal regime and thus to the seismogenic width the relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as cascadia where there have been no great historical events

the role of salt in fold and thrust belts davis d m and engelder t 1985 the role of salt in fold and thrust belts in n l carter and s uycda editors collision tectonics deformation of continental lithosphere tectonophysics 119 67 88 the style of deformation in thin skinned fold and thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported fold and thrust belts that form atop a salt layer such as the appalachian plateau the franklin mountains in northwestern canada and the jura of the alps among others share several readily observable characteristics as predicted by a simple mechanical model for fold and thrust belts a detachment in salt permits a thrust belt to have an extremely narrow cross sectional taper in addition predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt other common attributes of salt basal thin skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin

radioactive heat generation and its thermal effects in the alps apennines boundary zone abstract radioactive heat production data of rocks cropping out at the alps apennines boundary zone nw italy are presented samples were analysed in the laboratory by means of a gamma ray spectrometer they are representative of the savona unit and the ophiolitic sequences with metasediments and sediments of the voltri massif the sestri voltaggio zone and the lavagna nappe the heat production rate of sedimentary rocks ranges from 1 05 limestones to 2 52 w m 3 shales in ophiolites it varies from 0 04 serpentinites to 0 24 w m 3 metabasalts orthogneisses of the savona unit show the highest value 2 92 w m 3 potassium contributes to the heat generation on average by 17 in the metasedimentary and sedimentary rocks thorium contribution accounts on average by 43 except for dolomites in which 97 of the radioactive heat is produced by uranium calc schists shales radiolarites and phyllites give a th u ratio ranging from 3 to 4 that is very close to that of orthogneisses this ratio is lower in limestones almost all ophiolites and dolomites most rocks show a k th ratio ranging between 2 10 3 and 4 10 3 these data together with available thermal conductivity information are used in a finite element simulation to analyse the effects on the heat flux and temperature distribution due to lateral variation of thermal parameters changes in structure and composition of the uppermost part of the crust result in small wavelength variation of surface heat flux the low radioactive heat production of ophiolites produces a decrease of about 10 mw m 2 in the surface heat flux on the voltri massif

influence of kinetics on the smectite to illite transition in the barbados accretionary prism low chloride pore fluids observed along faults in clay rich accretionary complexes are commonly attributed to the release of interlayer water during the smectite to illite transformation however to date there has been no thorough analysis of the location and quantity of fluids that may be generated by this mechanism to address this problem a temperature and time dependent rate expression describing the dehydration reaction was coupled to a kinematic model of the northern barbados accretionary complex temperatures in the complex were estimated by modeling heat flow through the prism sediments as they thicken arcward the sediments temperature time histories were computed using a model for the velocities of sediment motion through the complex in this model the prism sediments follow uniformly diverging paths from the toe while the underthrust sediments undergo uniaxial strain the model predictions are validated against clay mineralogy data from barbados island mudstones our results show that the location of the peak dehydration rate is 20 km farther arcward in the underthrust sediments than in the prism complex the fresh water produced by the reaction results in pore waters that are 10 30 fresher at a distance of 50 70 km from the prism toe this constraint indicates a probable fluid migration path of more than 50 km from the reaction zone to the sites where freshened pore fluids have been observed the peak rate of fluid production when expressed as the volume of fluid per volume of sediment per second is 2 l0 15

mud volcanoes and gas hydrates in the anaximander mountains eastern mediterranean sea abstract detailed multibeam sedimentological and geophysical surveys provide ample new data to confirm that the anaximander mountains eastern mediterranean are an important area for active mud volcanism and gas hydrate formation more than 3000 km of multibeam track length was acquired during two recent missions and 80 gravity and box cores were recovered morphology and backscatter data of the study area have better resolution than previous surveys and very detailed morphology maps have been made of the known targeted mud volcanoes amsterdam kazan and kula especially the amsterdam crater and the related mud breccia flows gas hydrates collected repeatedly from a large area of amsterdam mud volcano at a sub bottom depth of around 0 3 1 5 m resemble compacted snow and have a rather flaky form new gas hydrate sites were found at amsterdam mud volcano including the mud flow sloping off to the south gas hydrates sampled for the first time at kazan mud volcano are dispersed throughout the core samples deeper than 0 3 m and display a rice like appearance relative chronology and ams dating of interbedded pelagic sediments late holocene hemipelagic sapropel layer s1 and ash layers within the mud flows indicate that successive eruptions of kula mud volcano have a periodicity of about 5 10 kyrs new mud volcanoes identified on the basis of multibeam backscatter intensity were sampled documented as active and named athina and thessaloniki gas hydrates were sampled also in thessaloniki mud volcano the shallowest 1264 m among all the active mediterranean sites at the boundary of the gas hydrate stability zone biostratigraphical analyses of mud breccia clasts indicated that the source of the subsurface sedimentary sequences consists of late cretaceous limestones paleocene siliciclastic rocks eocene biogenic limestones and miocene mudstones rough estimations of the total capacity of the anaximander mud volcanoes in methane gas are 2 56 6 40 km3

distribution of radioactive heat production around an active fault and in accretionary prisms of southwest japan abstract contents of radioactive heat producing elements u th and k were measured on core samples from a borehole drilled into the shear zone of the active nojima fault southwest japan the heat production rate is rather uniform in a granite layer below 560 m with an average of 1 6 w m 3 in the lower part of the granite layer which is close to the fault zone the u th and u k ratios seem to be lower than those in the upper part a possible cause of this anomaly is redistribution of u by groundwater flows in highly fractured layers through monitoring of the temperature profile in the borehole a local anomaly which may be associated with groundwater flow along a fracture zone was detected in the uppermost part of the hole the conductive heat flow was found to increase with depth which is possibly due to downward fluid flow in the surrounding formations measurements of heat production were also made on rock samples from permian to cretaceous accretionary prisms in southwest japan the heat production of sandstone and mudstone major components of the accretionary prisms is about 1 5 w m 3 on average which is similar to the average value for granitic rocks in southwest japan these values are significantly higher than those reported in accreted terranes in the cascadia subduction zone

organic carbon provenance and maturity in the mud breccia from the napoli mud volcano indicators of origin and burial depth abstract mud volcanism on the mediterranean ridge is caused by extrusion of overpressured sediments with consequent formation of spectacular dome shaped features composed of mud breccias at the seafloor the organic material in the mud breccia of the napoli mud volcano is a mixture of different facies stratigraphic origin and thermal maturities one portion is synsedimentary organic material with only minor diagenetic alterations and represents sedimenting material that was embedded into the mud volcano during its extrusion the mud breccia also contains thermally mature organic material of mainly terrestrial provenance with algae of fresh and brackish water origin vitrinite reflectance data of this maturity generation range from 0 65 to 0 90 roil and thus characterize thermally mature source rocks a rank which is corroborated by fluorescence and molecular characteristics the predominance of vitrinite in the maceral assemblages and the occurrence of biomarkers of terrigenous origin suggest that the major part of the mud matrix derives from a lacustrine or riverine sedimentary unit in the subsurface possibly from the messinian stage a third generation of organic material includes inertinites and vitrinites of high reflectance which represent recycled organic matter present in any marine sediment by use of the lopatin method for modelling the thermal maturation of hydrocarbon source rocks from the vitrinite reflectance data we calculated that the depth of mobilization ranges from 4900 m to 7500 m depending upon the temperature gradient used

possible mechanism of mud volcanism at the prism backstop contact in the western mediterranean ridge accretionary complex abstract the eastern mediterranean seafloor has numerous mud volcanoes most of which form a well defined belt within the mediterranean ridge medridge accretionary complex however mud volcano fields in the western medridge are less well known as those in the central and eastern medridge this study investigates material cycling and fluid migration within the western medridge we propose a possible ascent style of the ejecta forming the medee hakuho mud volcano mhmv in the western medridge by applying the vitrinite reflectance technique to ejecta samples first we model the 2 d thermal structure in the western medridge taking into account frictional heating on the plate interface to help estimate the source depth of the mhmv ejecta the result suggests an effective coefficient of friction of around 0 01 and a temperature of about 160 15 c along the plate interface at a distance of 180 km from the deformation front the location of the seaward toe of the aegean backstop second we evaluate the source depth of the mhmv ejecta using vitrinite reflectance in conjunction with the modeled thermal structure the results suggest that the ejecta matrix showing vitrinite reflectance values of 0 6 was subjected to a temperature of around 85 c corresponding to a depth of approximately 5 km below the seafloor kmbsf whereas older clasts of aptian or earlier age with vitrinite reflectance values of 0 6 1 0 are derived from much deeper depths most of the clasts are considered to have been lifted to the depth of 5 kmbsf as a result of underplating at the toe of a rigid backstop that had developed below mhmv after underthrusting related to plate subduction at that depth fluid pressures that are dramatically increased because of underplating promote the ascent of fluid rich sediments and entrain clasts along an existing fault in the accretionary wedge

structural setting and tectonic control of mud volcanoes from the central mediterranean ridge eastern mediterranean based on a recent marine geophysical data set including swath bathymetry acoustic imagery and six channel seismics recorded over a large area of the mediterranean ridge mr in early 1998 during the prismed 2 survey this paper presents a study of the various relationships observed between tectonic features cutting across the central mediterranean ridge accretionary wedge and massive mud expulsions known as mud volcanoes identified over large areas of the ridge regional mapping of two of the mud volcano fields previously only partly investigated olimpi and united nations rise revealed the presence of many new mud expulsion centres and a third new mud volcano field has been identified all the mud features show great variability in morphology size and backscatter strength of their surrounding mud flows based on their contrasting morpho acoustic characteristics we propose a classification into three main groups of mud construction 1 mud volcanoes these consist of subcircular and prominent reliefs often associated with high backscatter mud flows 2 mud domes similar to mud volcano but smaller these occurrences correspond to weakly reflective mud constructions 3 mud plateaus represent a third category which appears as wide often highly reflective and rather flat mud extrusions from all available data an attempt to explain the different mud ascent processes and driving forces is discussed in relation to the initial collision structural setting of the central mediterranean ridge within this area most of the mud constructions have been observed to be associated with tectonic features and in particular with strike slip faulting for the first time as a hypothesis we propose in this paper two different ascent processes to explain the contrasting mud constructions 1 extrusion for the mud volcanoes and plateaus and 2 intrusion for the mud domes connected to two different mud levels controlled by the crustal geometry of this pre collision area and especially the southwards extension of the cretan continental crust below the mediterranean ridge accreted sediments

lightning leader stepping k changes and other observations near an intracloud flash 1 an intracloud lightning flash in central new mexico began with the initiation of a negative stepped leader at an altitude of 8 2 km above sea level as this leader propagated eastward and upward at 9 1 km above sea level it passed about 200 m to the north of a balloon borne electric field change instrument esonde after the first leader stopped a second negative stepped leader began near the point of origin of the first leader but it propagated away from the esonde from the changes in the electric vectors and the locations of impulsive radio frequency sources detected by a lightning mapping array lma we conclude the following 1 the first negative stepped leader was not preceded by any significant charge rearrangement due to positive leaders 2 each step of the first negative leader had both a forward going wave and a step recoil wave that propagated simultaneously backwards away from the leader tip along the existing channel the presence of a step recoil wave during each step leads to an explanation for the existence of stepping 3 after the first nearby leader stopped step recoil waves from the second distant leader may have found their way onto the channel formed by the first leader 4 after the second leader stopped waves carrying negative charge propagated along the channel of the first leader producing strong k changes in the electric field at the esonde and providing a good record of the wavefront shapes

the structure and evolution of the western mediterranean ridge the mediterranean ridge is a unique accretionary complex consisting of five key elements the frontal slope the upper slope the crest of the ridge the cleft area and the inner plateau the imerse data show that these correspond roughly to the locus of frontal accretion of underplating of a pre messinian wedge of complex faulting and possible strike slip tectonics and of a backstop of hellenic nappes covered by a messinian forearc basin the frontal portion of the complex is a post messinian accretionary wedge composed of messinian evaporites and overlying tightly folded plio quaternary sediments underlain by pre messinian sequences attached to the african plate the basal detachment at the front of the wedge occurs at the base of the evaporites moving further to the northeast the upper slope the basal detachment cuts to deeper levels leading to the development of duplex structures where pre messinian units are subcreted beneath the messinian evaporites just behind the subcreted units the evaporites thin and may be absent on the crest of the ridge this region we interpret as the site of a pre messinian accretionary wedge we suggest that following the deposition of thick evaporites in the messinian the pre messinian accretionary tectonics continued as subsurface accretion subcretion beneath the evaporites although the crest of the ridge is largely devoid of evaporites local deep evaporite basins observed here formed as local closed basins on top of the pre messinian wedge we infer that the messinian sealevel was at about the level of the ridge crest that is 3000 m below present allowing for isostatic adjustment to the removal of the water load this would imply a sealevel drop of at least 2000 m the cleft basins mark the northeast limit of the accretionary complex thick evaporite deposits to the northeast beneath the inner plateau may have been deposited in a messinian forearc basin the evaporites of the inner plateau are underlain by a thin pre messinian sequence and by crystalline basement of the hellenic nappes this basement forms the backstop to the accretionary complex

eastern mediterranean tectonics and tsunami hazard inferred from the ad 365 earthquake historical accounts describe an earthquake and tsunami on 21 july ad 365 that destroyed cities and drowned thousands of people in coastal regions from the nile delta to modern day dubrovnik the location and tectonic setting of this earthquake have been uncertain until now here we present evidence from radiocarbon data and field observations that western crete was lifted above sea level by up to 10 m synchronously with the ad 365 earthquake the distribution of uplift combined with observations of present day seismicity suggest that this earthquake occurred not on the subduction interface beneath crete but on a fault dipping at about 30 within the overriding plate calculations of tsunami propagation show that the uplift of the sea floor associated with such an earthquake would have generated a damaging tsunami through much of the eastern mediterranean measurement of the present rate of crustal shortening near crete yields an estimate of 5 000 yr for the repeat time of tsunamigenic events on this single fault in western crete but if the same process takes place along the entire hellenic subduction zone such events may occur approximately once every 800 yr in the year ad 365 an earthquake and tsunami destroyed much of the eastern mediterranean coastal regions the distribution of uplift at the time suggests that the earthquake occurred on a fault within the overriding plate at the subduction zone beneath crete and not on the subduction interface itself

distribution of methane hydrate bsrs and its implication for the prism growth in the nankai trough abstract detailed mapping of a bottom simulating reflector bsr which marks the phase transition from the methane hydrate layer above the reflector to free gas below was conducted in the nankai accretionary prism off shikoku and tokai bsrs are widely distributed in the prism slope from the toe region to the forearc basin bsr positions provide us with the regional heat flow variations using pressure temperature conditions for methane hydrate stability estimated heat flows generally show constant values about 50 mw m2 shallower than the middle slope of the prism and gradually increase seaward in the lower prism slope occurrences of bsrs are regarded as accumulation of free gas beneath the base of a gas hydrate stability field bghs and or concentration of methane hydrate above the bghs these conditions can be accomplished by updip migration of methane gas because it is unlikely that such methane concentration is completed by in situ biogenic methanogenesis within sediments including low total organic carbon moreover sedimentation uplifting and sediment stacking by thrust faulting cause upward migration of the bghs and migration of methane from the dissociated hydrate to new bghs such recycling of methane gas may have actively occurred in accretionary prisms in contrast there are five regions of no bsrs the nankai trough floor prism toe slope basin steep slope and deep sea canyon the trough floor the prism toe and the slope basin are characterized by young sediments with low production of methane gas and sub horizontal strata unsuitable for migration of gases and fluids erosion at the steep slope and the canyon causes removal of hydrated sediments and downward movement of the bghs bsr distribution and thermal structure estimated from bsr positions offer information about active processes occurring in accretionary prisms

the mediterranean ridge and related mud diapirism a background abstract the mediterranean ridge stretching from the calabrian rise to the florence rise is the largest structural unit of the eastern mediterranean sea it is directly related to ongoing convergence and collision of the african and eurasian plates starting in the oligocene and is considered to be a giant accretionary complex consisting of intensively folded and faulted rocks of the african margin since its origin in the late miocene the ridge continued to grow up and outward experiencing more deformation because of the developing collision the mud diapirism and mud volcanism are usual and wide spread phenomena for the mediterranean ridge that developed as a result of an intensive tectonic overburden due to stacking of rock units by thrusting and strong lateral compressional stress pressing up and squeezing plastic sedimentary series out onto the seafloor

rise of deep diapirs diapiric ascent is often assumed to occur in a mantle with a viscosity of order 10 21 p the value estimated from glacial rebound studies segregation of melt from residual crystals en route is then a necessary consequence of the slow ascent velocity the temperatures and stresses associated with diapirs however are much greater than those involved in glacier unloading and the controlling viscosity is therefore relatively low high stresses may also result from the volume change associated with the partial melting process itself stress and temperature feedback considerably increase the velocity of ascent of a deep diapir and this may allow the melt and the residual crystals to rise together without separating the material delivered to shallow mantle magma chambers may therefore be similar in composition to the deeper source region this reopens the possibility that a deep eclogite layer 200 km depth may be the source region for mid ocean ridge basalts

spatial transition from compression to extension in the western mediterranean ridge accretionary complex the mediterranean ridge has been described since 1982 as an accretionary prism related to the hellenic subduction zone however little attention has been paid to the actual mechanical boundary between the sedimentary wedge and the crustal backstop of the outer hellenic arc using seismic velocity structures from recent esp s pasiphae cruise we propose a new interpretation of existing geological and geophysical data around the western hellenic trenches the velocity structure of the inner unit of the mediterranean ridge suggests that the late neogene to quaternary sequences were deposited above a high velocity rigid body we propose to correlate the boundary of this rigid body backstop of the accretionary wedge with a series of oblique reflectors dipping to the southwest near dsdp sites 126 and 377 the backstop extends further south than previously thought and decreases by one third the total width of the compressional wedge it has thus major implications on the volume of accreted material the short wavelength folding of plio quaternary sediments observed on the outer wall of the south matapan trench is probably a consequence of gravity gliding layers above the high velocity body gently dipping 2 to the northeast using the messinian evaporite as a decollement on the other hand the hellenic trenches appear to be narrow half grabens affected by the widespread extensional deformation active on the outer hellenic arc they are situated on the landward boundary of a wider and poorly sedimented fore arc basin we conclude that extensional deformation extends at least over the width of the hellenic trenches on the continental side of the backstop whereas compressional deformation stops near the edge of the backstop itself 100 km to the southwest

the crust of the western mediterranean ridge from deep seismic data and gravity modelling summary we present a crustal cross section of the western mediterranean ridge from the african margin to the aegean sea constrained by the morphology of the ridge new seismic reflection and refraction results obtained during a two ship cruise performed in december 1988 pasiphae cruise the distribution of seismicity and gravity modelling morphologically the western mediterranean ridge can be divided into three main units from the sirte abyssal plain to the hellenic trench a broad outer unit a narrow central unit and an inner unit the seismic front orgeaward extent of the interplate seismicity between the african lithosphere and the cretan arc is located 50 km seaward of the hellenic trench seismicity is thus distributed widely over the innermost unit of the ridge suggesting that brittle material is present at shallow depth beneath the inner portion of the mediterranean ridge the refraction results of the pasiphae cruise show that the crust in the ionian basin and beneath the outer and central units of the western mediterranean ridge is oceanic and that the moho discontinuity progressively deepens towards the hellenic arc in agreement with the accretionary prism model high velocity material occurs at shallow depth beneath the innermost unit of the ridge gravity modelling also indicates a significant increase in density beneath the inner unit we consequently interpret the inner unit of the ridge as the backstop of the western mediterranean ridge prism possibly composed of peloponnesus thrust sheets

thermal maturity of a fold thrust belt based on vitrinite reflectance analysis in the western foothills complex western taiwan abstract burial depth cumulative displacement and peak temperature of frictional heat of a fault system are estimated by thermal analysis in the fold thrust belt of the western foothills complex western taiwan based on the vitrinite reflectance technique the regional thermal structure across the complex reveals that the rocks were exposed to maximum temperatures ranging from 100 c to 180 c which corresponds to a burial depth of 3 7 6 7 km a large thermal difference of 90 c were observed at the shuilikeng fault which make the eastern boundary of the fold thrust belt where it is in contact with metamorphic rock of hsuehshan range the large thermal difference corresponds to cumulative displacements on the shuilikeng fault estimated to be in the range of 5 2 6 9 km however thermal differences in across the shuangtung and chelungpu faults cannot be determined apparently due to small vertical offsets the large displacement observed across the shuilikeng fault is absent at the other faults which are interpreted to be younger faults within the piggyback thrust system localized high temperatures adjacent to fault zones were observed in core samples penetrating the chelungpu fault three major fracture zones were observed at core lengths of 225 m 330 m and 405 m and the two lower zones which comprise dark gray narrow shear zones a value of vitrinite reflectance of 1 8 higher than the background value of 0 8 is limited at a narrow shear zone of 1 cm thickness at the fracture zone at 330 m the estimated peak temperature in the range of 550 680 c in the shear zone is far higher than the background temperature of 130 c and it is interpreted as due to frictional heating during seismic faulting

the mediterranean ridge backstop and the hellenic nappes the core of the mediterranean ridge backstop consists of a pile of hellenic nappes that migrated outward from the aegean continent within the adjacent mediterranean basins during late middle miocene about 15 ma and the present mediterranean ridge has developed since that time by accretion of a new wedge the arguments we use are 1 a similarity in thickness seismic velocity and structure between the backstop and the hellenic nappes in southern peloponnesus 2 the geometry of the westward limit of the backstop that is the one expected from the amount of relative displacement of the nappes on land 3 an agreement between the estimated ages of the present accretionary wedge and the period of activity of two major faults connecting the backstop to the adjacent aegean continent the seaward limit of the backstop is situated about 170 km southwest of western crete where the displacement of the nappes is maximal and it is close to the margin near the ionian islands where their displacement is small we assume that the hellenic subduction zone migrated southwestward relative to eurasia during middle miocene and that the corresponding gravity collapse of the aegean continent was maximum at this time because the westward extrusion of anatolia had not yet started

accretion structural style and syn contractional sedimentation in the eastern mediterranean sea we analyse four multichannel seismic profiles collected by ogs from 1971 to 1979 along the outer part of the mediterranean ridge accretionary complex in order to understand the nature of the deformation affecting african plate sediments as they approach the africa europe plate boundary tectonic style as revealed by our data and previous studies varies considerably along the plate boundary and this variation suggests the division of the margin in three provinces characterised by different structural patterns and deformation rates 1 near orthogonal terminal subduction initial collision ionian and sirte foredeeps 2 incipient continental collision collision of continental promontories of the african margin off the cyrenaica peninsula 3 oblique terminal subduction initial collision herodotus foredeep this work aims to relate the tectonic features revealed by the seismic profiles to the geodynamic and stratigraphic parameters of the convergent margin these are essentially related to the geometry of subduction rate of convergence and obliquity and to the lithology of the incoming sedimentary section as well as the accumulation rate in the foredeep in the sirte province accumulation rate in the foredeep is relatively low and deformation is related to thrusting and folding which accounts for sediment shortening and uplift at the outer deformation front in the provinces of oblique terminal subduction the rate of normal convergence is lower the sedimentary section is thicker and the foredeep fed by the nile river has an higher syn tectonic sedimentation rate here deformation produces a broad band of large wavelength and high amplitude folds folding in this area is produced both by plate convergence related shortening and by movement of salt triggered by differential loading of the prograding nile deep sea fan in the area of incipient continental collision the foredeep is a v shaped trough as the abyssal plain has been completely subducted and the folded sediments of the accretionary complex rest directly on the african continental slope such differences in structural style are explained by great differences in strain rate that in the herodotus foredeep province has been calculated as at least four times weaker than in the western sector two factors contribute to decrease strain rate syn contractional accumulation and normal component of the effective convergence rate the former increases from west to east and possibly causes overpressuring on the decollement level salt mobilisation landward and seaward vergent folding and a very low surface taper in the herodotus foredeep the latter decreases from west to east and favours thrusting in the western sector

frontal accretion along the western mediterranean ridge the effect of messinian evaporites on wedge mechanics and structural style in the context of the imerse project several crossings of the deformation front of the western mediterranean ridge were made in the region of the sirte abyssal plain the messina abyssal plain and the intervening region in this paper we present seismic images and interpretations across the deformation front with particular emphasis on the role the messinian evaporites have played in controlling the accretionary tectonics of the thin frontal portion of the wedge the seismic images show that the basal detachment generally is located at the base of the evaporites from a consideration of the mechanics of the wedge for both coulomb and plastic rheologies we show that the low wedge taper c 2 requires that the detachment is characterised by extreme fluid overpressuring within 2 of lithostatic in places and that the basal yield stress less than 1 mpa is lower than that of a wet salt decollement zone this supports the seismic interpretation that the detachment occurs in overpressured sediments beneath the impermeable evaporites lateral variations in the accretionary style can be related to lateral variations in evaporite thickness the effectiveness of the evaporite as an impermeable seal and to local relief on the subducting plate these factors control the escape of fluids from beneath the evaporites and hence fluid pressure and basal yield stresses

geochemistry and diagenesis of stratabound calcite cement layers within the rannoch formation of the brent group murchison field north viking graben northern north sea abstract up to four calcite cemented horizons doggers form impermeable barriers to fluid flow within the middle jurassic rannoch formation and are correlatable across the murchison field calcite precipitated during early diagenesis within high porosity permeability sandstones at the top of coarsening shoaling upward shoreface cycles calcite 13c and 18o compositions range from 4 1 to 13 4 pdb and 6 6 to 16 7 pdb respectively sr concentrations of up to 1334 ppm are consistent with marine carbonate sources probably shell fragments but no viable intraformational carbonate source has been identified in the murchison field area initial 87sr 86sr compositions 0 71109 0 71266 are higher than middle jurassic seawater 0 7073 and consistent with precipitation from modified porewaters containing significant proportions of continentally derived meteoric fluids enriched in 87sr as a result of basement weathering or percolation through hinterland soils unconsolidated detritus an internal source of 87sr is not considered viable in view of the high proportion up to 25 clastic constituents of unaltered detrital alkali feldspar and mica within the rannoch formation geochemical and isotope data indicate correlations between increasing 18o composition and increasing iron and magnesium content within calcite calcium concentrations decrease with increasing 18o for calcites geochemical data trends can be interpreted differently in terms of either static or evolving 18o porewater models static 18o porewater models using jurassic early cretaceous meteoric water 18 o 6 smow predict cement precipitation temperatures of 17 77 c however 13c compositions are more depleted than those typical of carbon derived from shell debris and correlation between decreasing 18o and decreasing 13c suggests modification of a meteoric derived porewater system during burial via contribution of low 13c carbon derived from a deep basinal source however it is difficult to constrain accurately the degree to which porewater 18o may have evolved during burial the degree to which porewater 18o could have evolved from that of a jurassic early cretaceous meteoric water can be loosely constrained as from 18 o 6 smow to 18 o 2 smow if the temperature at which the most 18o depleted cements precipitated is limited by present day bottom hole temperatures 110 c for wells currently at maximum burial depth

global positioning system constraints on plate kinematics and dynamics in the eastern mediterranean and caucasus we present and interpret global positioning system gps measurements of crustal motions for the period 1988 1997 at 189 sites extending east west from the caucasus mountains to the adriatic sea and north south from the southern edge of the eurasian plate to the northern edge of the african plate sites on the northern arabian platform move 18 2 mm yr at n25 5 w relative to eurasia less than the nuvel 1a circuit closure rate 25 1 mm yr at n21 7 w preliminary motion estimates 1994 1997 for stations located in egypt on the northeastern part of africa show northward motion at 5 6 2 mm yr also slower than nuvel ia estimates 10 1 mm yr at n2 4 e eastern turkey is characterized by distributed deformation while central turkey is characterized by coherent plate motion internal deformation of 2 mm yr involving westward displacement and counterclockwise rotation of the anatolian plate the anatolian plate is de coupled from eurasia along the right lateral strike slip north anatolian fault naf we derive a best fitting euler vector for anatolia eurasia motion of 30 7 0 8 n 32 6 0 4 e 1 2 0 1 myr the euler vector gives an upper bound for naf slip rate of 24 1 mm yr we determine a preliminary gps arabia anatolia euler vector of 32 9 1 2 n 40 3 1 1 e 0 8 0 2 myr and an upper bound on left lateral slip on the east anatolian fault eaf of 9 1 mm yr the central and southern aegean is characterized by coherent motion internal deformation of 2 mm yr toward the sw at 30 1 mm yr relative to eurasia stations in the se aegean deviate significantly from the overall motion of the southern aegean showing increasing velocities toward the trench and reaching 10 1 mm yr relative to the southern aegean as a whole

extreme efficiency of mud volcanism in dewatering accretionary prisms drilling results from two mud volcanoes on the mediterranean ridge accretionary complex as well as bottom sampling and the wealth of geophysical data acquired recently have provided fundamental knowledge of the 3d geometry of mud extrusions mud volcanism is generally related to buoyancy density inversion and is triggered by the collision of the african and eurasian blocks forcing undercompacted clayey sediments to extrude along faults in the central and hinterlandward parts of the prism volumetric estimates of extruded mud in several well studied areas were based on pre stack depth migrated seismic profiles across the entire up to 150 km wide prism the resulting volumes of mud were combined with ages from mud dome drilling so that rates of mud extrusion were obtained subtracting the solid rock mass from the bulk mud volume using physical property data fluid flux as a function of mud volcanism alone has been quantified for the first time the volume of fluid extruding with the mud is found to be variable but reaches up to 15 km3 fluid per km trench length and ma along cross sections with abundant mud volcanoes such large fluid quantities in a region some 50 150 km behind the deformation front exceed estimates from those elsewhere where undoubtedly the majority of the interstitial fluid is lost due to compaction such fluids near the backstop are likely to result predominantly from mineral dehydration and diagenetic reactions at depth and consequently provide a window to understand deeper processes along the deep decollement more importantly the enormous rates with which such fluids and liquified mud escape along the out of sequence faults alter fluid budget calculations in subduction zones drastically

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

gps constraints on continental deformation in the africa arabia eurasia continental collision zone and implications for the dynamics of plate interactions 1 the gps derived velocity field 1988 2005 for the zone of interaction of the arabian african nubian somalian and eurasian plates indicates counterclockwise rotation of a broad area of the earth s surface including the arabian plate adjacent parts of the zagros and central iran turkey and the aegean peloponnesus relative to eurasia at rates in the range of 20 30 mm yr this relatively rapid motion occurs within the framework of the slow moving 5 mm yr relative motions eurasian nubian and somalian plates the circulatory pattern of motion increases in rate toward the hellenic trench system we develop an elastic block model to constrain present day plate motions relative euler vectors regional deformation within the interplate zone and slip rates for major faults substantial areas of continental lithosphere within the region of plate interaction show coherent motion with internal deformations below 1 2 mm yr including central and eastern anatolia turkey the southwestern aegean peloponnesus the lesser caucasus and central iran geodetic slip rates for major block bounding structures are mostly comparable to geologic rates estimated for the most recent geological period 3 5 myr we find that the convergence of arabia with eurasia is accommodated in large part by lateral transport within the interior part of the collision zone and lithospheric shortening along the caucasus and zagros mountain belts around the periphery of the collision zone in addition we find that the principal boundary between the westerly moving anatolian plate and arabia east anatolian fault is presently characterized by pure left lateral strike slip with no fault normal convergence this implies that extrusion is not presently inducing westward motion of anatolia on the basis of the observed kinematics we hypothesize that deformation in the africa arabia eurasia collision zone is driven in large part by rollback of the subducting african lithosphere beneath the hellenic and cyprus trenches aided by slab pull on the southeastern side of the subducting arabian plate along the makran subduction zone we further suggest that the separation of arabia from africa is a response to plate motions induced by active subduction

material transfer in accretionary wedges from analysis of a systematic series of analog experiments the structural evolution of accretionary wedges was modeled in a series of analog experiments with systematically varying initial boundary conditions the two principle parameters investigated were basal friction and thickness of sediment input relative to subducted sediment output material transfer in terms of degree and location of accretion and erosion is quantified as a function of these two parameters low basal friction produces long thin wedges by continuous frontal accretion of imbricate thrusts simultaneously substantial basal erosion may occur with little influence on the frontal dynamics the formation of an outer arc high with an accompanying forearc basin depends upon the relative amount of basal erosion the opening of the subduction window spacing of thrust faults is proportional to the layer thickness confirming the principle of self similar wedge growth from critical wedge theory high basal friction typically generates steep wedges marked by an erosional mass wasting frontal slope in the cases i o this is due to underthrusting and or subduction of the entire sedimentary section the case i o also features the late development of normal faulting and an extensional forearc basin for both low and high basal friction the material balance affects the wedge slope erosive conditions i o for low basal friction the primary modes of material transfer are frontal accretion and basal erosion for high basal friction the primary modes of material transfer are underthrusting underplating and frontal erosion applying the experimental results to active convergent margins parallels may be drawn thus the barbados ridge complex is interpreted to be a good example of a low basal friction wedge with an accretionary material balance while the japan and mariana margins provide examples of an erosive material balance

thermal history of deep sea sediments as a record of recent changes in the deep circulation of the eastern mediterranean 1 during three cruises of the medriff project in 1993 and 1994 154 geothermal heat flow measurements and seven ctd profiles in the water column have been collected along a 300 km sw ne oriented transect traversing the mediterranean ridge accretionary complex the original goal of the measurements was to identify areas of anomalous heat flow that could be interpreted as possible sites for fluid outflow contrary to expectations the upper few meters of the temperature profiles in the sediments showed decreasing temperature from the seafloor down to 3 to 6 m depth indicating consistently transient temperature regimes the only exception positive heat flow was found in the sirte abyssal plain measurements collected in the same area at different times indicated that the thermal structure in the bottom water and sediments had changed significantly at weekly monthly and interannual timescales one dimensional forward modeling of the conductive heat propagation into the sediment explains the observed thermal anomalies assuming up to 0 5 k warming of the bottom waters that propagated south westward from the matapan trench to the crestal area of the mediterranean ridge analysis of nonsteady state thermal profiles in the upper sediment provided time information on the onset of bottom water warming by what is now called the eastern mediterranean transient emt the sediment warming started in the matapan trench between spring 1992 and spring 1993 and reached the mediterranean ridge crest in spring summer 1993 the average propagation velocity of the thermal perturbation along the profile is about 0 5 1 6 km d 1 0 6 1 8 cm s 1

brine formation and gypsum precipitation in the bannock basin eastern mediterranean abstract sixty kilograms of gypsum crystals were collected at two dredging stations within a small anoxic hypersaline basin named the bannock basin close to the sirte abyssal plain the basin was investigated in some detail by means of ten precision positioned piston and gravity cores one nansen bottle hydrocast and three ctd stations the sediments of the basin floor are dark h 2 s rich and commonly laminated and contain centimetric crystals of gypsum the water column above the basin has normal salinity down to a depth of 3150 m below this depth the basin is filled with hypersaline brines formed from the dissolution of messinian evaporites outcropping on the steep eastern wall when the brines reach the necessary concentration in ca and in so 4 gypsum may precipitate along the steep eastern wall and within the sediments on the basin floor the very recent age of the gypsum is proved by the inclusions of quaternary planktonic fauna formainifera and pteropods and by radiometric techniques

the mediterranean ridge a mass balance across the fastest growing accretionary complex on earth depth migration of seismic reflection profiles across the mediterranean ridge accretionary complex between the african and eurasian blocks illustrates profound variations in the geometry and internal structure along strike structural interpretations of four cross sections together with bathymetric and acoustic surface information and drilling data are used to volumetrically balance the amount of subduction versus accretion with time results suggest the existence of three distinct scenarios with a jump in decollement in the west intense backthrusting in the central part between libya and crete and transcurrent tectonism in the east the onset of accretion coincides with exhumation of thrust sheets 19 ma followed by rapid sediment accretion with thick evaporite bearing incoming successions facilitating outward growth of the wedge the minimum rate of accretion 20 25 of the total sediment supply is observed in the central portion where the ridge suffers maximum deformation here the indenting leading edge of the african plate apparently forces the sediment into subduction or local underplating in contrast an estimated 40 60 of the available sedimentary input was accreted in the western domain where collision is less accentuated the results support the hypothesis that highly destructive forearc collisional events like slab break off and exhumation of thrust sheets can be followed by periods of accretion and continuous growth of accretionary wedges