abyssal serpentinites as gigantic factories of marine salts and oil abstract serpentinites occur worldwide in various geological settings such as in magma starved oceanic basins abyssal serpentinites are long term storage for chlorine and carbon the latter forming dominantly reduced and subordinately oxidized compounds hence serpentinites salts and reduced carbon phases coexist in an abyssal association although huge reservoirs of salts and reduced carbonaceous matter in abyssal serpentinites are thought to be lacking this may be more a reflection of intrinsic limits in research methods than an indication of their absence on the other hand shallow seated hydrocarbon rich mud volcanoes and diapirs consisting of salts hydrocarbons oils and gases and clayey muds including mudded serpentines can provide evidence for such an oceanic association here we search for scientific data in support of the oceanic association and hence we propose a reappraisal of exploitable worldwide salt hydrocarbon reservoirs even gigantic in size abyssal serpentinites are well known source rocks of abiotic gaseous hydrocarbons via fischer tropsch type reaction ftt whereas macromolecular carbonaceous matter mostly bitumen found in serpentinites is postulated to be biogenic this inference is based mainly on three assumptions i ftt in nature does not yield oils ii hydrocarbons derive essentially from the thermal alteration of microbial molecules iii the presence of biomarkers traditionally considered evidence of the biological origin of oils nevertheless some lines of strong evidence disprove these accepted conventions i lab experiments demonstrated that ftt can synthesize significant quantities of liquid hydrocarbons oils under conditions compatible with the deeper and more reduced level of serpentinite hosted hydrothermal systems ii macromolecules generally thought to derive from microbial activity can be abiotically generated as documented in extraterrestrial bodies iii biomarkers are evidence for the proliferation of microbial communities which subsist on serpentinization and ftt products thus they are biological pollutants moreover seawater driven serpentinization consumes water producing huge amounts 11 kg halite m 3 of peridotite of salts that can be stored in the deeper zone of abyssal type hydrothermal systems thus the origin of shallow sited salt deposits even gigantic in size can be related to either the advective upwelling of hot hydrothermal brines due to the dehydration of abyssal serpentinites or to the passive upwelling of buoyant saline geobodies piercement structures are indeed seismically imaged in several localities such as south atlantic the gulf of mexico the serpentinite belt extending from cuba to hispaniola the north sea trench and the zagros orogenic belt similar lines of evidence at the microscopic scale have been found in some hyblean serpentinite xenoliths south eastern sicily being representative of the unmetamorfosed in situ relic of the palaeo tethys ocean upon which sicily and its off shore areas lie in spite of the accepted stereotype of hydrocarbons as fossil fuels and hence finite resources reduced carbon compounds abiotically produced in abyssal serpentinites may be copious protracted over time and thus renewable

the hellenic subduction beneath the peloponnesus first results of a microearthquake study abstract a preliminary examination of the 1070 earthquake locations determined from 6 weeks of recording in 1986 by 46 stations show that the seismicity is spread over a wide area of the peloponnesus and the western hellenic arc and throughout the whole crust no clear individual faults can be identified from the seismicity but clusters of activity are observed in some places seismicity is concentrated above 40 km and deeper earthquakes were not numerous only 28 of the 466 events with uncertainties in depth less than 5 km occurred deeper than 40 km seismicity deeper than 30 km defines a flat zone at a depth between 40 km and 70 km starting from the trench to about 200 km towards the northeast further northeast the dip of the seismic zone abruptly changes to 45 fault plane solutions for the deeper events generally indicate t axes plunging northeast within the subducted slab therefore we interpret the seismicity deeper than 30 km as due to the superposition of two different causes 1 the steep zone is due to the subduction of the african litospheric plate beneath the aegean and 2 the shallow flat zone located between the trench and the argolide is partly due to the loading of the overriding aegean plate which is deforming above the african plate

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

propagation of a strike slip plate boundary within an extensional environment the westward propagation of the north anatolian fault we document the establishment of the aegea anatolia eurasia plate boundary in pliocene pleistocene time before 2 ma no localized plate boundary existed north of the aegean portion of the anatolia plate and the shear produced by the motion of anatolia aegea with respect to eurasia was distributed over the whole width of the aegean west anatolian western portion in 4 5 ma a shear zone comparable to the gulf of corinth was formed in the present sea of marmara the initial extensional basins were cut by the strike slip main marmara fault system after 2 5 ma shortly after the plate boundary migrated west of the sea of marmara along the northern border of aegea from the north aegean trough to the gulf of corinth area and to the kefalonia fault there it finally linked with the northern tip of the aegean subduction zone completing the system of plate boundaries delimiting the anatolia aegea plate we have related the change in the distribution of shear from miocene to pliocene to the formation of a re

deformational styles of the eastern mediterranean ridge and surroundings from combined swath mapping and seismic reflection profiling abstract recent swath mapping and seismic reflection profiling across the eastern levantine branch of the mediterranean ridge mr in the eastern mediterranean sea illustrate a strong variability of the deformational styles that characterize this precollisional accretionary prism along a north south cross section of the mr a structural analysis based on surface mapping combined with vertical seismic reflection data reveals two main structural domains a southern outer domain consists of a series of three disconnected distinct fold belts folding affects a sedimentary cover which includes an approximately 2 km thick pliocene and quaternary wedge in this domain resting on fairly thick messinian evaporitic sequences that act as probable decollement layers the mr inner domain includes three regions showing evidences of strong internal deformations and of numerous probable mud cones and mud flows but no seismic evidence of significant underlying messinian evaporites can be detected the inner sub regions are thrusting northwards over an area made of faulted and locally uplifted acoustic basement blocks that constitute the southernmost extension of the crete continental margin acting as a continental buttress for the mr this innermost domain is itself structurally disconnected from the crete continental margin by the en echelon pliny troughs system that shows evidence of left lateral displacement altogether the different structural and sedimentary cover patterns reveal a strong contrast between both mr inner and outer domains and important lateral variations within the inner mr itself strike slip faulting seams to characterizes both areas and large scale mud accumulations potentially exist in the northern one our results support a model of imbricated accretionary prisms including at least two stages 1 a pre messinian stage during which the inner mr probably developed in response to northward subduction of the african lithosphere beneath southern europe and 2 a messinian to post messinian period during which the kinematics of the aegean anatolian microplates and the presence of thick messinian deposits became prevalent and which led to the creation of the outer mr folded wedge piling against the previous one in our interpretation the occurrence of thick messinian evaporites in this area induces important local and regional modifications of the structural pattern strike slip activities reflect partitioning related to oblique subduction and likely lateral escape of the sedimentary cover in the whole studied area

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

integrated 3d geophysical and geological modelling of the hercynian suture zone in the champtoceaux area south brittany france this paper combines geological knowledge and geophysical imagery at the crustal scale to model the 3d geometry of a segment of the hercynian suture zone of western europe in the champtoceaux area brittany france the champtoceaux complex consists of a stack of metamorphic nappes of gneisses and micaschists with eclogite bearing units the exhumation of the complex during early carboniferous times was accompanied by deformation during regional dextral strike slip associated with a major hercynian shear zone the south armorican shear zone sasz dextral shearing produced a km scale antiformal structure with a steeply dipping axial plane and a steeply eastward plunging axis armor 2 deep seismic profile shows that the regional structure was cut by a set of faults with northward thrusting components based on the seismic constraint direct 2d crustal scale modelling was performed throughout the champtoceaux fold on seven radial gravity profiles also using geological data and density measurements from field and drill hole samples the 3d integration of the cross sections the digitised geological map and the structural information foliation dips insure the geometrical and topological consistency of all sources of data the 2d information is interpolated to the whole 3d space using a geostatistical analysis finally the 3d gravity contribution of the resulting model is computed taking into account densities for each modelled geological body and compared to the bouguer anomaly the final 3d model is thus compatible with the seismic and gravity data as well as with geological data main geological results derived from the modelling are i the overall 3d geometry of the south dipping thrust system interpreted on the seismic profile emphasises northward thrusting and folding of the champtoceaux complex which was coeval with strike slip along the south armorican shear zone ii the gravity modelling suggests the presence of a relatively dense body below the champtoceaux complex that could be interpreted as a result of relative uplift of midcrustal material during thrusting along the e w trending wrench thrust system iii the northern limb of the champtoceaux anticline is a relatively shallow feature and iv vigneux synkinematic granitic body is a laccolith sheared and rooted along the southern branch of the sasz and spreads away from the strike slip zone within weak country rocks d 2004 elsevier b v all rights reserved

slab segmentation controls the interplate slip motion in the sw hellenic subduction new insight from the 2008 mw 6 8 methoni interplate earthquake we present an integrated approach of the seismic structure and activity along the offshore sw hellenic subduction from combined observations of marine and land seismic stations our imaging of the slab top topography from teleseismic receiver function analysis at ocean bottom seismometers supports a trenchward continuation of the along dip slab faults beneath the peloponnesus we further show that their morphostructural control accounts for the backstepping of the thrust contact of the mediterranean ridge accretionary wedge over the upper plate local seismic activity offshore sw peloponnesus constrained by ocean bottom seismometer observations reveals a correlation with specific features of the forearc the matapan troughs we study the mw6 8 14 02 2008 interplate earthquake offshore sw peloponnesus and show that its nucleation rupture zone and aftershocks sequence are confined to one slab panel between two adjacent along dip faults and are thus controlled by not only the offshore slab top segmentation but also the upper plate sea bottom morphology

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

crustal investigation of the hellenic subduction zone using wide aperture seismic data abstract we present the results of a wide aperture seismic onshore offshore study crete seismic experiment in the cretan region as part of the hellenic arc compressional system three seismic lines were carried out on and around the island of crete in order to investigate the crustal structure of the region up to 119 three component recording stations were deployed on each profile that observed seismic energy generated by a 48 l airgun array and eight 20 kg landshots a total of 6208 shots were fired upon completing the fieldwork the vertical components of all stations were evaluated 300 common receiver gather crg sections of the ocean bottom seismographs obs and land stations as well as 100 common source gather csg sections of the land shots and selected airgun shots were compiled and modeled in order to generate a 2d p wave velocity depth model for each profile the accuracy of the model depends on the depth and position along the profiles and does not exceed 5 for both depth and p wave velocity we identified strong lateral variations in crustal and sedimentary thickness mainly in a north south direction but also along strike east west the crust is continental and has a maximum thickness of 32 5 km below northern central crete its subdivision in an upper v p 5 8 6 3 km s locally up to 6 5 km s and a lower v p 6 4 6 9 km s part is justified by a first order discontinuity with v p velocity a contrast of up to 0 6 km s the eastern part of crete shows a significantly thinner crust of 24 to 26 km to the north the crustal thickness decreases to 15 km below the central cretan sea the prominent decrease of the moho depth north of central crete is interpreted to represent the northern end of a microcontinent that was subducted in oligocene times and later surfaced by buoyant escape stockhert et al 1999 thomson et al 1999 the p t t d history of the high pressure rocks of crete greece denudation by buoyant escape in exhumation processes normal faulting ductile flow and erosion ring u lister g willet s brandon m eds spec publ of the geol soc of london p 154 to the south and southwest of the island the continental crust gradually thins to a minimum of 17 km and at approximately 100 km off the southern coast of crete it is in contact with oceanic crust below the mediterranean ridge upper mantle velocities were determined to be 7 7 km s below the cretan sea and 8 0 km s south of crete below the continental cretan crust a 6 to 7 km thick layer with v p velocities between 6 6 and 7 1 km s was identified on each line and could be followed by reflections to a depth of 42 km it is decoupled from the overlying continental crust at central crete and is interpreted as oceanic crust presently under subduction towards the nne below the aegean sea

physical properties and state of the crust and upper mantle of the eastern mediterranean sea deduced from geophysical data abstract by evaluating the geophysical information available for the eastern mediterranean sea ems a crustal thickness map and some conclusions as to the nature of the crustal type were drawn it was found that the ionian abyssal plain is floored by an oceanic type of crust which is 14 km thick and covered by sediments of the order of 8 km thick the herodotus abyssal plain in the levantine sea is floored by a crust 22 km thick and covered by sediments ranging between 10 and 15 km thick the eastern mediterranean rise extending more or less continuously from the apulian plateau to rhodes and the south of the antalya basin is of a thickness ranging between 28 and 34 km the thickness of its sedimentary cover is of the order of 10 km the borders of the eastern mediterranean sea are built up of thick continental crust for example the hellenides of western greece range between 40 and 44 km thick and western turkey has a similar crustal thickness the crust of sicily also exceeds 40 km whereas the southern and eastern borders of the ems are approximately 30 32 km thick tectonically the northern border of the ems is built up of active continental margins defined by the calabrian hellenic and cyprus antalya arcs whereas the southern border represents a passive continental margin controlled mainly by subsidence and stretching to the east the dead sea megashear limits the area the deep basins of the eastern mediterranean sea are tectonically controlled by subsidence and partial stretching the mediterranean ridge or rise is still an area of dispute regarded by some authors as a subsiding continental fragment and separated from the hellenic arc by the hellenic trench whereas others consider it to be a zone of crustal shortening thickened mainly by thrusting both the western and southwestern borders of the mediterranean rise and the hellenides are controlled by compressional processes similar to those deforming the calabrian arc the stravo and pliny trenches to the east southeast border of the hellenic arc are partly affected by strike slip movements various plate tectonic models of the area have been proposed by several authors though the complexity of the deformation hardly permits reliable reconstructions the lack of reliable paleomagnetic information and the absence of linear magnetic anomalies do not favour the application of plate tectonic concepts vertical processes due to the uprising of lithothermal systems below the hellenic and calabrian arcs may also be regarded as the driving force controlling the present day deformation

contemporary kinematics of the southern aegean and the mediterranean ridge summary this study focuses on the kinematics of the southern aegean and the mediterranean ridge mr a quantification of the deformation of the mr is essential for both evaluating physical models of accretionary wedges in general and for obtaining a self consistent model of the surface deformation over the entire nubia eurasia nu eu plate boundary zone in the eastern mediterranean previous kinematic studies have not properly considered the deformation field south of the hellenic arc although this study focuses on the deformation field of the mr we also discuss the kinematics of the southern aegean because the geometry and movement of the hellenic arc determine to a large extent the kinematic boundary conditions for kinematic studies of the mr we calculate a continuous velocity and strain rate field by interpolating model velocities that are fitted in a least squares sense to published global positioning system gps velocities in the interpolation we use information from a detailed data set of onshore and offshore active faulting to place constraints on the expected style and direction of the model strain rate field in addition we use the orientations of tracks left by seamounts travelling into the wedge to further constrain the offshore deformation pattern our model results highlight the presence of active shear partitioning within the mediterranean ridge high compressional strain rates between the ridge crest and the deformation front accommodate approximately 60 70 per cent of the total motion over the wedge and the outward growth rate of the frontal thrust is 4m m yr 1 strain partitioning within the wedge leads to 19 23 mm yr 1 of dextral motion at the wedge backstop contact of the western mr whereas the pliny and strabo trenches in the eastern mr accommodate 21 23 mm yr 1 of sinistral motion the backstop of the western mr is kinematically part of the southern aegean which moves as a single block the aegean block ae at 33 34 mm yr 1 in the direction of s24 w 1 towards stable nubia nu our model confirms that there is a clear divergence between the western and eastern hellenic arc and we argue for a causal relation between the outward motion of the arc and the gradient in the regional geoid anomaly our results suggest that a significant driving source of the surface velocity field lies south of the hellenic arc and only for the southeastern aegean could there be some effect as a result of gravitational collapse associated with density differences within the overriding plate

late quaternary environmental changes in southernmost south america reflected in marine calcareous macro and microfossils abstract late quaternary environmental changes especially sea level fluctuations and glacial advances and retreats affected the distribution of the benthic marine fauna which inhabited the beagle channel in southernmost south america to evaluate these changes calcareous macro and microfossils obtained from holocene marine sediments along a sector of the western side of the beagle channel have been used for a multi proxy approach in total 119 species were recovered 65 mollusks 24 bivalves 35 gastropods and 6 chitons 33 foraminifera and 21 ostracods other preserved calcareous groups which appeared in minor proportions were brachiopods and cirripeds the calcareous fossil assemblages from the early holocene represent an ecosystem transition which started with vacant niches first occupied by opportunistic species and or eurytopic taxa these were then replaced by more diverse fauna assemblages associated with the proliferation of habitats which occurred under truly marine conditions during the mid holocene when holocene calcareous fauna is compared with its pleistocene counterparts from the same region some differences in the proportion of taxonomic groups and taxa composition are noted these partly follow the heterogeneity of habitats in the beagle channel and water masses of different origin which have either greater or less influence throughout the late quaternary but they can also be interpreted in association with environmental changes after the last glacial maximum in conclusion the data provide insights into both spatial and temporal changes and show evidence of a gradual transformation from a brackish to a marine environment caused by the holocene sea transgression it also documents late quaternary faunistic changes associated with topographic and oceanographic local conditions in this microbasin

bhp develops airborne gravity gradiometer for mineral exploration bhp s project falcon a research effort specifically aimed at building the world s first airborne gravity gradiometer agg system has developed two fully operational systems that are capable of sensitive high resolution detection of small variations in the earth s gravity at the level required in exploration for mineral and hydrocarbon resources

geometric shapes derived from airborne gravity gradiometry data new tools for the explorationist we display airborne gravity gradient data using curvature attributes of the equipotential surface to derive geometric shapes that can be associated with structural and geologic features concepts used in differential geometry that describe all geometric aspects of a surface are applied to airborne gravity gradiometry data gravity gradient components are ideal for this application because they are related to curvature of the gravitational potential we also emphasize that with gravity methods the surfaces for which the curvature attributes are derived represent the succession of equipotential surfaces which under favorable circumstances correspond to one or more geologic surfaces that dominate the signature of the airborne gravity gradiometry data

gps derived strain rate field within the boundary zones of the eurasian african and arabian plates we use the gps velocity field 1988 1998 for eastern mediterranean and asia minor to determine the crustal deformation strain rate field in an area bounded by 35 n and 43 n and 20 e and 48 e we calculate the normal and shear strain rate components associated with the major faults and compare these qualitatively with seismological data uncertainties in the calculation of the strain rates reach 50 nstrain yr 1 in sparsely observed parts of anatolia whereas we estimate errors 20 nstrain yr 1 in the aegean and marmara regions the largest compressional strain rate components in the eastern part of the study area occur along the greater caucasus mountain front reaching 70 nstrain yr 1 1 nstrain yr 1 0 0317 10 15 s 1 the north anatolian fault zone is the clearest feature in the shear strain rate field it is expressed as a pronounced dextral strike slip fault zone reaching rates of up to 170 nstrain yr 1 this holds true also for the izmit area where the august 17 1999 earthquake occurred central anatolia is almost strain free whereas extension prevails in western anatolia the principal axes of extension vary around the n s direction with strain rates of up to 85 nstrain yr 1 these extensional areas coincide with graben features and normal faulting earthquakes the central and southwestern aegean sea is strain free with values far below 40 nstrain yr 1 the seismic cluster around the dodekanissa islands southeastern aegean sea coincides with nw se oriented extension attaining strain rates of up to 90 nstrain yr 1 this area of extension also exhibits recent active volcanism the entire hellenic arc shows compressional strain rates perpendicular to the arc the pliny strabo troughs along the eastern segment of the arc show left lateral shear strain rates reaching 80 nstrain yr 1 significant extension is found in central greece with a nne ssw oriented maximum of 120 nstrain yr 1 centered around the gulf of corinthos the kephalonia fault zone in nw greece is a distinct dextral fault zone separating apulia from the rapidly moving aegean microplate right lateral shear strain rates reach 150 nstrain yr 1

seismic faults in the aegean area abstract reliable fault plane solutions of shallow earthquakes and information on surface fault traces in combination with other seismic geomorphological and geological information have been used to determine the orientation and other properties of the seismic faults in the aegean and surrounding area thrust faults having an about nw se strike occur in the outer seismic zone along western albania westernmost part of mainland of greece ionian sea south of crete south of rhodes the inner part of the area is dominated by strike slip and normal faulting strike slip with an about ne sw slip direction occurs in the inner part of the hellenic arc along the line peloponnesus cyclades dodecanese southwest turkey as well as along a zone which is associated with the northern aegean trough and the northwesternmost part of anatolia all other regions in the inner part of the area are characterized by normal faulting the slip direction of the normal faults has an about sw ne direction in crete n38 e and an about e w direction n81 e in a zone which trends n s in eastern albania and its extension to western mainland of greece in all other regions central greece southern yugoslavia and bulgaria western turkey the slip of the normal faults has an about n s direction

evolution of the calabrian accretionary wedge central mediterranean 1 the neogene tectonics of the central mediterranean are related to the subduction and trench rollback of the ionian basin under eurasia causing the opening of the liguro provencal and tyrrhenian back arc basins and the formation of the calabrian accretionary wedge the calabrian accretionary wedge is a partially submerged accretionary complex located in the ionian offshore and laterally bounded by the apulia and malta escarpments while the history of the back arc extension process is fairly well defined the structure and evolution of the wedge are still poorly known we have analyzed and interpreted the available reflection multichannel and single channel seismic profiles in the ionian offshore and integrated them with other geological and geophysical data acquired in the last 40 years here we present unpublished seismic profiles to provide a new map of the tectonic structure of the ionian offshore and to define the structure of the wedge and its evolution during the last 15 myr our reconstruction points out that the messinian salinity crisis represents an important break in the evolution of the wedge as the basal decollement ramps up onto the messinian salt deposits producing a dramatic and fast forward propagation of the frontal thrust and resulting underplating of the underlying crustal ionian sequence during progressive trench rollback our results provide new insight into the style of accretion in a weakly converging setting which is typical for the mediterranean region

moho topography beneath the corinth rift area greece from inversion of gravity data summary our aim is to understand better the rifting process by imaging the moho depth variation beneath corinth and evvia we present here the results of a gravity inversion analysis in the region of the corinth and evvia rift system and compare them to those obtained independently from teleseismic tomography and receiver function analyses the results of these different studies appear to be consistent and show 1 a 10 km crustal thickening in the western part of the area beneath the hellenides mountains 2 nw se trending periodic crustal thinning and 3 a maximum crustal thinning north of the gulf of corinth this 4 km thinning is unlikely to be the result of the rifting alone which seems to have been reactivated since only 1 ma we propose here a geodynamical scenario in two major steps to explain the evolution of corinth area aegean miocene extension involving boudinage resulted in periodic crustal thinning consistent with observations these lithospheric instabilities could have favoured rupture initiation in particular areas especially near the city of corinth then the reactivation of the corinth rift extension 1 myr ago led to westward rift propagation the offset observed between the maximum crustal thinning and the gulf of corinth could be accommodated by a lowangle normal fault at about 10 15 km depth the corinth rift is thus asymmetrical and was initiated in places of crustal weakness due to miocene lithospheric instabilities

seismicity of the hellenic subduction zone in the area of western and central crete observed by temporary local seismic networks temporary local seismic networks were installed in western crete in central crete and on the island gavdos south of western crete respectively in order to image shallow seismically active zones of the hellenic subduction zone more than 4000 events in the magnitude range between 0 5 and 4 8 were detected and localized the resulting three dimensional hypocenter distribution allows the localization of seismically active zones in the area of western and central crete from the mediterranean ridge to the cretan sea furthermore a three dimensional structural model of the studied region was compiled based on results of wide angle seismics surface wave analysis and receiver function studies the comparison of the hypocenter distribution and the structure has allowed intraplate and interplate seismicity to be distinguished high interplate seismicity along the interface between the subducting african lithosphere and the aegean lithosphere was found south of western crete where the interface is located at about 20 to 40 km depth an offset between the southern border of the aegean lithosphere and the southern border of active interplate seismicity is observed in the area of crete the offset varies laterally along the hellenic arc between about 50 and 70 km a southwards dipping zone of high seismicity within the aegean lithosphere is found south of central crete in the region of the ptolemy trench it reaches from the interface between the plates at about 30 km depth towards the surface in comparison the aegean lithosphere south of western crete is seismically much less active including the region of the ionian trench intraplate seismicity within the aegean plate beneath crete and north of crete is confined to the upper about 20 km between 20 and 40 km depth beneath crete the aegean lithosphere appears to be seismically inactive in western crete the southern and western borders of this aseismic zone correlate strongly with the coastline of crete

incipient continental collision and plate boundary curvature late pliocene holocene transtensional hellenic forearc crete greece u ring t c brachert write in their recent paper ten veen kleinspehn 2003 proposed that the western messara graben of central crete underwent transtensional deformation since about 3 4 ma transtension is supposed to be dominantly resolved along 070 striking sinistral faults and that the 070 striking faults have strike slip normal slip ratios of 10 100 1 ten veen kleinspehn 2003 then use their results from the western messara graben to propose that sinistral transtension should represent tectonic escape of central and eastern crete during incipient collision of the libyan passive margin the interpretation of the kinematic data on which ten veen kleinspehn 2003 base their tectonic model is in contrast to earlier studies which have attributed the young i e pliocene to recent kinematics of crete largely to radial extension angelier et al 1982 fassoulas 2001 ring et al 2001 ten veen kleinspehn 2003 p 167 argued that onshore structural assemblages is critical to deciphering forearc kinematics and therefore we present our kinematic data from onshore faults in the western and central messara graben and compare our data together with the fault slip data of fassoulas 2001 from the heraklion graben and microseismicity data hatzfeld et al 1993 delibasis et al 1999 with the fault slip data of ten veen kleinspehn 2003 fig 9 for discussing the kinematics of the young faults we believe that fault slip data directly reflect the kinematics of the faults from which they were collected and therefore allow testing the proposition often veen kleinspehn that sinistral transtension is supposed to be mainly resolved at the 070 striking faults the latter of which are known to be an integral part of the youngest set of faults on crete fassoulas 2001 and are in part seismically active delibasis et al 1999 in general

consistency investigation vertical gravity estimation and inversion of airborne gravity gradient data a case study from northern sweden for airborne gravity gradient data it is a challenge to distinguish between high frequency intrinsic and dynamically produced noise caused by the aircraft and small scale effects from shallow dens

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

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

thermal structure of the ionian slab we propose a thermal model of the subducting ionian microplate the slab sinks in an isothermal mantle and for the boundary conditions we take into account the relation between the maximum depth of seismicity and the thermal parameter lth of the slab which is a product of the age of the subducted lithosphere and the vertical component of the convergence rate the surface heat flux dataset of the ionian sea is reviewed and a convective geotherm is calculated in its undeformed part for a surface heat flux of 42 mw m 2 an adiabatic gradient of 0 6 mk m 1 a mantle kinematic viscosity of 1017 m2 s 1 and an asthenosphere potential temperature of 1300 c the calculated temperature depth distribution compared to the mantle melting temperature indicates the decoupling limit between lithosphere and asthenosphere occurs at a depth of 105 km and a temperature of 1260 c a 70 km thick mechanical boundary layer is found by considering that the maximum depth of the seismic events within the slab is 600 km a lth of 4725 km is inferred for a subduction rate equal to the spreading rate the corresponding assimilation and cooling times of the microplate are about 7 and 90 myr respectively the thermal model assumes that the mantle flow above the slab is parallel and equal to the subducting plate velocity of 6 cm yr 1 and ignores the heat conduction down the slab dip the critical temperature above which the subduced lithosphere cannot sustain the stress necessary to produce seismicity is determined from the thermal conditions governing the rheology of the plate the minimum potential temperature at the depth of the deepest earthquake in the slab is 730 c

extension and exhumation of the hp lt rocks in the hellenic forearc ridge localized intensive ductile and ductile brittle extension of along the arc orientation is prominent in phyllite quartzite unit pqu rocks in kythera and the adjacent southeastern peloponnese but declines in prominence to the north and to the south along the cretan peloponnese ridge using zircon and apatite fission track dating we find that this structural characteristic is correlated with the youngest zircon fission track cooling ages of 14 to 9 ma and with the highest pressure temperature condition recorded in the metamorphic rocks below the structural detachment the cooling ages of the high pressure phyllite quartzite unit pqu rocks along the hellenic forearc ridge show that exhumation migrated from both crete and from the peloponnese to the area with the youngest ages in kythera and the southeastern peloponnese starting in the early miocene and continuing to the present trench rollback and slab retreat expanded the hellenic arc and this bending of the arc from an initial more rectilinear geometry increased areas of oblique convergence and we suggest this localized the subsequent arc parallel extension and local enhanced exhumation of hp rocks the zircon fit exhumation ages from the arc parallel stretching episode restrict the ductile part of this episode in exposed rocks to between about 14 and 9 ma apatite ft ages suggest brittle along arc extension continued to about 7 ma this episode is proposed to be the result of a temporary higher rate of rollback and slab retreat younger normal faults including those which define the margins of the present hellenic arc show return to arc normal extension

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

a subducting seamount beneath the mediterranean ridge abstract two transverse morphological tracks across the front of the mediterranean ridge were swath mapped with a hydrosweep instrument system along the tracks ridges and troughs at the front of the accretionary prism are disrupted and bent in the direction of relative plate convergence the longer of the two tracks extends to bannock basin a brine filled depression with a closed outer moat around a central mound seismic reflection data and a gravity anomaly show a buried circular seamout beneath bannock basin whose summit is the mound this indicates an origin of the larger transverse track and bannock basin from a subducting seamount beneath bannock basin the seamount has displaced about 340 km 3 of the messinian section and yet the seafloor has not been uplifted therefore considerable evaporitic material is inferred to have been displaced by the seamount and bannock basin is a likely vent for large quantities of dissolved materials from the disrupted messinian evaporitic sequence

metalliferous mining geophysics state of the art in the last decade of the 20th century and the beginning of the new millennium the downturn in mining activity experienced during the 1990s did not preclude significant new developments in various areas of mining geophysics the methodology for acquiring and compiling data has kept pace with the latest technological developments from global positioning system navigation to raster displays and parallel computing wavelet transforms principal component analysis and fractals have begun to find successful applications in both processing and interpretation of geophysical data methods of quantitatively interpreting inverting anomalies in terms of 2 d and 3 d models of causative bodies are becoming common for the first time it is possible to make airborne gravity gradient measurements suitable for use in mineral exploration lower transmitter frequencies for airborne time domain electromagnetic em systems have enabled surveys in areas where conductive cover previously screened basement conductors the use of approximation algorithms has allowed the transformation of either time domai

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

s velocity structure and radial anisotropy in the aegean region from surface wave dispersion summary the lithospheric structure of the aegean region is investigated by analysis of rayleigh wave fundamental mode dispersion measurements isotropic 1 d models for almost 100 two station ray paths across the region display distinct variations in the moho depth and crustal s wave velocities the descending slab of the subducting african plate can be resolved down to 120 km depth beneath the volcanic arc three different regions are distinguished in terms of moho depth 1 the forearc with large crustal thicknesses between 38 and 48 km and an average of 43 km 2 the northern aegean with an average moho depth of 28 km and 3 the southern aegean central volcanic arc i e cyclades and sea of crete with an even thinner crust of around 25 km lateral variations in structure between 25 and 55 km depth indicate a marked difference between the western and eastern forearc collocated with pronounced changes in trench and slab geometry as well as published deformation rates s velocities between 25 and 55 km depth are low everywhere beneath the forearc but increase from the peleponnesus to crete an abrupt change occurs between western and central crete in terms of the visibility of the aegean moho and the seismic structure of the lithospheric mantle wedge an aegean mantle wedge with s velocities above 4 4 km s 1 is only observed to the east of central crete whereas to the west velocities of less than 4 0 km s 1 occur down to the plate contact these low velocities above the slab may indicate the presence of a melange of metamorphic rocks at the depths a low velocity asthenospheric layer is observed beneath the sea of crete and the cyclades below 40 km depth between the thinned lithosphere above and the slab below the observed radial anisotropy in the northern part of the aegean is likely to be due to preferred orientation of anisotropic minerals within the lower crust possibly caused by lateral ductile flow associated with recent lithospheric extension

three dimensional regularized focusing inversion of gravity gradient tensor component data we develop a new method for interpretation of tensor gravity field component data based on regularized focusing inversion the focusing inversion makes its possible to reconstruct a sharper image of the geological target than conventional maximum smoothness inversion this new technique can be efficiently applied for the interpretation of gravity gradiometer data which are sensitive to local density anomalies the numerical modeling and inversion results show that the resolution of the gravity method can be improved significantly if we use tensor gravity data for interpretation we also apply our method for inversion of the gradient gravity data collected by bhp billiton over the cannington ag pb zn orebody in queensland australia the comparison with the drilling results demonstrates a remarkable correlation between the density anomaly reconstructed by the gravity gradient data and the true structure of the orebody this result indicates that the emerging new geophysical technology of the airborne gravity gradient observations can improve significantly the practical effectiveness of the gravity method in mineral exploration

gravity modelling of the hellenic subduction zone a regional study abstract the hellenic subduction zone is clearly expressed in the arc shaped distribution of earthquake epicenters and gravity anomalies which connect the peloponnesos with crete and anatolia in this region oceanic crust of the african plate collides northward with continental crust of the aegean microplate which itself is pushed apart to the south west by the anatolian plate and at the same time is characterised by crustal extension the result is an overall collision rate of up to 4 cm year and a retreating subduction process recent passive and active seismic studies on and around crete gave first but not in all details consistent structural results useful for supporting gravity modelling this was undertaken with the aim of presenting the first 3d density structure of the entire subduction zone gravity interpretation was based on a bouguer map newly compiled using data from land marine and satellite sources the anomalies range from 170 mgal cretan sea to 10 mgal mediterranean ridge 3d gravity modelling was done applying the modelling software igmas the computed bouguer map fits the low frequency part of the observed one which is controlled by variations in moho depth less than 20 km below the cretan sea and extending 30 km below crete and the extremely thick sedimentary cover partly up to 18 km of the mediterranean ridge the southernmost edge of the eurasian plate with its more triangular shaped backstop area was traced south off crete only 50 to 100 km further to the south the edge of the african continent was traced as well in between these boundaries there is african oceanic crust which has a clear arc shaped detachment line situated at the eurasian continental edge the subduction arc is open towards the north its slab separates hotter mantle material lower density below the updoming moho of the cretan sea from colder one higher density in the south subjacent to the upper continental crust of crete is a thickened layer of lower crust followed by the subducted oceanic crust with some mantle material as intermediate layer the depth of the oceanic moho below crete is 50 km the presence and structure of subducted or underplated sediments remains uncertain

new constraints on aegean subduction from seasat altimetry crustal structure profiles relative to the aegean fore arc area are modeled according to geoid undulations and free air gravity anomalies taking into account the subducted slab the models involve three layers with fixed density but a variable geometry for their boundaries the first profile drawn across the western convergent branch of the arc exhibits a large amount of subducted crustal material and a local trough in the moho under the so called mediterranean ridge superimposed on a long wavelength flexure increasing toward the trench the moho topography suggests a lithosphere with no rigidity below the mediterranean ridge while the flexure associated with the subduction implies an elastic thickness of 80 km this discrepancy leads us to assume the presence of an additional load acting downward below the mediterranean ridge on the second profile drawn across the eastern strike slip branch there is a deep trough in the moho far from the topographic trench this moho deepening represents a large isostatic unbalance and requires an active compensation mechanism thus both profiles and especially the eastern one where the strike slip dominant motion does not allow a proper subduction require a downward force acting seaward of the arc on the latter branch we propose that gravitational instability induces the growth of a cold bulb at the base of the old and dense african lithosphere it is noted that through viscous coupling with the top of the lithosphere a 115 km bulb can produce an important collapse of the moho and that the resulting gravitational anomalies are comparable to the observations the growth of such a cold lithospheric bulb may be the first stage of a southward jump of the aegean arc

on the shape of the subducting slab beneath the peloponnese greece the mediterranean sea floor is subducted beneath the western hellenic arc at a gentle angle for about 200 km and then steepens beneath the gulf of corinth the seismic zone extends to a depth of 150 km but the subducted intact lithosphere could reach 600 km the present relative motion between africa and aegea can be considered as the sum of 2 motions 1 slow 1 cm y relative motion between africa and europe which describes the rate at which the deep slab penetrates the asthenosphere and which started during the upper cretaceous time when africa started to converge toward europe and 2 fast 5 cm y relative motion between the southern aegea and europe manifest by internal deformation within the aegean which may have been active at present rates only since pliocene time i suggest that the increase in the total convergent rates which began about 5 my ago is responsible for the wide flat slab beneath the peloponnese

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

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

crustal structure of the ionian margin of sicily etna volcano in the frame of regional evolution abstract crustal imaging could be achieved on normal incidence reflection profiles offshore eastern sicily by using industrial grade reflection seismic with improved marine sources thick recent sediments a reflective pile including the mesozoic deposits a transparent upper crust and a band of low frequency reflections attributed to the lower crust are imaged in the seismic sections the structure of the crust and its thickness show features inherited from the mesozoic evolution as a passive margin by which ionian basin crust was formed around the hyblean continental promontory of africa to constitute the southern plate in the later convergence with europe the seismic images are also marked by the lithospheric deformation due to the neogene overriding of the northern part of this paleomargin by the calabro peloritan block of european continental crust this transpressive motion may have been guided along the northern part of this paleomargin where the seismic profiles evidence a hinge line between the northward upslope of the moho of that old passive margin and its downslope to the present slab under the tyrrhenian sea etna volcano is located at the intersection of this mantle upwarp by a zone of active sea bottom normal faults which cut across the formerly constructed compressional belt the onset of its volcanic activity is roughly coeval with that of the cessation of interplate thrusting and could hence be related to a change of the coupling of the ionian slab this slab is now probably disconnected from the overriding plate and rolled back in front of the expanding hot tyrrhenian asthenospheric dome with the mobilisation of a viscous mantle material at depth an active lithospheric fault is here imaged which cuts over more than 100 km into the ionian basin the fault runs from the tyrrhenian margin in the sse direction of etna updip of the southwestern lateral edge of this slab leaving north eastward the extruded calabrian block and on its south westward edge the uplifted crust and mantle structures of etna along it the crust including the mesozoic and deeper layers has sagged vertically in the segment in front of the slab with a finite throw across the fault increasing from the basin towards etna

collision related break up of a carbonate platform eratosthenes seamount and mud volcanism on the mediterranean ridge preliminary synthesis and implications of tectonic results of odp leg 160 in the eastern mediterranean sea abstract drilling of the eratosthenes seamount south of cyprus documented incipient collision of the african and eurasian plates the oldest sediments recovered mid cretaceous shallow water limestones are overlain by upper cretaceous to lower oligocene pelagic carbonates with several hiatuses following uplift a carbonate platform was established in the miocene eratosthenes was then below eustatic sea level during the messinian desiccation crisis the platform subsided to bathyal depths during the lower pliocene associated with localized breccia deposition further subsidence occurred in late pliocene early quaternary coeval with strong surface uplift of southern cyprus subsidence and break up of eratosthenes was achieved by a combination of flexural loading and normal faulting in addition the milano and napoli mud volcanoes were drilled on the northern flank of the mediterranean ridge accretionary complex south of crete a mainly extrusive sedimentary origin is indicated multiple debris flows include clasts of sandstone and limestone of at least partly miocene age both mud volcanoes are dated as 1 ma old and have been active episodically hydrocarbon gas is associated with both mud volcanoes while methane hydrates clathrates exist locally at milano the driving force of mud volcanism is overpressuring caused by incipient plate collision messinian evaporites may have acted as a localized seal material escaped through a zone of backthrusting against rigid cretan crust to the north

heli borne gravity gradiometry in rugged terrain abstract for airborne gravity gradiometry in rugged terrain helicopters offer a significant advantage over fixed wing aircraft their ability to maintain much lower ground clearances crucially this provides both better signal to noise and better spatial resolution than is possible with a fixed wing survey in the same terrain comparing surveys over gentle terrain at margaret lake canada and over rugged terrain at mount aso japan demonstrates that there is some loss of spatial resolution in the more rugged terrain the slightly higher altitudes forced by rugged terrain make the requirements for terrain correction easier than for gentle terrain transforming the curvature gradients measured by the falcon gravity gradiometer into gravity and the complete set of tensor components is done by a fourier method over gentle terrain and an equivalent source method for rugged terrain the fourier method is perfectly stable and uses iterative padding to improve the accuracy of the longer wavelengths the equivalent source method relies on a smooth model inversion and the source distribution must be designed to suit the survey design

application of curvatures to airborne gravity gradient data in oil exploration abstractwe apply equipotential surface curvatures to airborne gravity gradient data the mean and differential curvature of the equipotential surface the curvature of the gravity field line the zero contour of the gaussian curvature and the shape index improve the understanding and geologic interpretation of gravity gradient data their use is illustrated in model data and applied to falcon airborne gravity gradiometer data from the canning basin australia

microearthquake seismicity and fault plane solutions in the southern aegean and its geodynamic implications summary seismicity mi 3 5 in the southern aegean located using data collected during seven weeks of recording by a temporary network of seismological stations largely follows the hellenic arc the sea of crete is nearly aseismic and only little activity is located south of the hellenic trench within the african plate focal mechanisms exhibit reverse faulting in the external part of the arc and normal faulting inside it this normal faulting indicates n s extension in the northern aegean the gulf of corinth the cyclades and dodecanese islands but nw se extension in southern peloponnese and western crete and e w extension in eastern crete this non uniform strain pattern suggests that the aegean region not only extends in a n s sense with the hellenic arc moving south westward relative to the eurasian plate but also by e w extension of its southern margin so that there is a net divergence of material

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

earthquake mechanisms in the hellenic trench near crete summary in the hellenic trench south of crete convergence between the southern aegean sea and africa occurs at a rate of at least 60 mm yr 1 previously published first motion fault plane solutions show a variety of different fault orientations and types but are not well constrained furthermore the lack of reliable focal depths for these earthquakes has obscured any simple pattern of deformation that might exist nonetheless the mechanisms of these earthquakes have strongly influenced views of the tectonics in the hellenic trench we have improved estimates of the fault parameters and focal depths for 14 of these earthquakes using long period p and sh waveforms the earthquake mechanisms fall into four groups a normal faults with a n s strike in the over riding material above the subduction zone b low angle thrusts with an e w strike at a depth of about 40 km c high angle reverse faults with the same strike but shallower focal depths than b d events within the suducting lithosphere with approximately e w p axes the thrusting in groups b and c is probably the mechanism by which the sediments of the mediterranean sea underplate and uplift crete these events have slip vectors in the direction 025 12 which represents the convergence direction between crete and africa along the sw facing boundary of the hellenic trench one of the events in group d occurred beneath the mediterranean ridge and involved high angle reverse faulting with a wnw ese p axis almost perpendicular to the direction of shortening deduced from folds at the surface the mediterranean sea floor in this region appears to be in a state of e w compression for reasons that are not clear

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

two ship deep seismic soundings in the basins of the eastern mediterranean sea pasiphae cruise summary new results from a two ship refraction and oblique reflection deep seismic cruise are presented to discuss the nature of the crust in the ionian sirte and herodotus abyssal plains these expanding spread profiles were processed and analysed in both the x t and p domains arrival times of reflected and refracted branches are matched by ray tracing in both domains in spite of a shallow evaporitic sequence messinian evaporites deposited on top of a thick sedimentary pile responsible for velocity inversions on many profiles we obtain excellent velocity control down to moho depth pmp and pn 8 4 8 6 km s 1 arrivals are observed the three basins have a relatively thin crust 8 to 11 km overlain by a thick sedimentary cover up to 10km in the herodotus abyssal plain the moho boundary and main crustal units identified in the basins can be followed beneath the calabrian prism to the west and beneath the mediterranean ridge to the east the crustal structure is of oceanic type for both the ionian and sirte basins where typical oceanic layer 2 and 3 are recognized the thin crust of the herodotus basin may be interpreted either as oceanic or thinned continental crust about 10 km thick the top of the crust of the herodotus basin is much deeper therefore the herodotus basin is probably significantly older than the ionian basin triassic versus early cretaceous in age

geological evidence for mud diapirism on the mediterranean ridge accretionary complex abstract a mud breccia has been repeatedly cored from four mud diapir fields located on the crest of the mediterranean ridge this breccia is composed of subrounded clasts supported by a clay rich matrix and ranges in age from cretaceous to early miocene sedimentological analysis allows identification of a primitive intrusive and a reworked mud breccia the latter being formed by mud extrusion and gravitational reworking of the former homogeneous mud sorted by grain size which occurs at the top of the diapirs has been interpreted as a lake deposit based on composition and carbon isotopic ratios gases present within the diapiric material suggest a partial thermogenic origin and gas escape structures have been identified in the mud breccia and host sediments carbonate crusts occurring in the host sediments are not related to bacterial oxidation of methane and consequent authigenic calcite precipitation their carbon and oxygen isotopic ratios reveal that they are characteristic of early deep sea lithification in the mediterranean sea the presence of a marker bed composed of mn nodules and bacterial colonies the mud lake deposits showing lower carbonate content than primitive mud breccia and the gas escape structures suggest the presence of active fluid vents in the vicinity of the diapiric structures evidence of active mud diapirism can be found from about 300 000 yrs b p to the present

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

a 3 d density velocity model between the cretan sea and libya abstract a 3 d density model for the cretan and libyan seas and crete was developed by gravity modelling constrained by five 2 d seismic lines velocity values of these cross sections were used to obtain the initial densities using the nafe drake and birch empirical functions for the sediments the crust and the upper mantle the crust outside the cretan arc is 18 to 24 km thick including 10 to 14 km thick sediments the crust below central crete at its thickest section has values between 32 and 34 km consisting of continental crust of the aegean microplate which is thickened by the subducted oceanic plate below the cretan arc the oceanic lithosphere is decoupled from the continental along a nw se striking front between eastern crete and the island of kythera south of peloponnese it plunges steeply below the southern aegean sea and is probably associated with the present volcanic activity of the southern aegean sea in agreement with published seismological observations of intermediate seismicity low density and velocity upper mantle below the cretan sea with 3 25 103 kg m3 and vp velocity of compressional waves around 7 7 km s which are also in agreement with observed high heat flow density values point out at the mobilization of the upper mantle material here outside the hellenic arc the upper mantle density and velocity are 3 32 103 kg m3 and vp 8 0 km s respectively the crust below the cretan sea is thin continental of 15 to 20 km thickness including 3 to 4 km of sediments thick accumulations of sediments located to the ssw and sse of crete are separated by a block of continental crust extended for more than 100 km south of central crete these deep sedimentary basins are located on the oceanic crust backstopped by the continental crust of the aegean microplate the stretched continental margin of africa north of cyrenaica and the abruptly terminated continental aegean microplate south of crete are separated by oceanic lithosphere of only 60 to 80 km width at their closest proximity to the east and west the areas are floored by oceanic lithosphere which rapidly widens towards the herodotus abyssal plain and the deep ionian basin of the central mediterranean sea crustal shortening between the continental margins of the aegean microplate and cyrenaica of north africa influence the deformation of the sediments of the mediterranean ridge that has been divided in an internal and external zone the continental margin of cyrenaica extends for more than 80 km to the north of the african coast in form of a huge ramp while that of the aegean microplate is abruptly truncated by very steep fractures towards the mediterranean ridge changes in the deformation style of the sediments express differences of the tectonic processes that control them that is subduction to the northeast and crustal subsidence to the south of crete strike slip movement between crete and libya is required by seismological observations

using combinations of spatial gradients to improve the detectability of buried conductors below or within conductive material the detection of conductive bodies is an important capability when exploring for massive sulfide deposits or looking for unexploded ordnance when these bodies are buried below conductive overburden or embedded in conductive material the use of an electromagnetic system to identify the bodies becomes problematic because the response of the overlying conductive material can be much greater that the response of the buried conductor i calculated the response of five models representing different conductivity distributions a buried conductor a uniform overburden with changes in the system altitude a paleochannel a thicker overburden and a thinner overburden the subtle response of the buried conductor was difficult to identify because it looked very similar to the responses of other structures that are not necessarily of interest the spatial gradients for the same five models showed that the greatest improvement in the relative size of the anomalous gradient response compared with the background gradient came for the cases in which the material closest to the surface changes in particular the paleochannel and thickening overburden models however identification of the deeper buried conductor was still problematic because of the large background gradients in theory the cylindrical symmetry of a dipole transmitter over a layered earth ensured that there were exact relations between the spatial derivatives hence it was possible to define two specific combinations that should be zero over a layered earth calculating these combinations for the five models showed that the anomalous zones stood out with significantly greater anomaly to background ratios the measurement of the gradients and the calculation of these combinations therefore provided a means of identifying anomalous zones in and below a conductive earth different relative sizes and shapes of the two combinations for different models provided a way of discriminating between the vertical conductor model and the four other models

earthquake locations in the western hellenic arc relative to the plate boundary four vertical component seismograph stations were operated in western crete southeastern peloponnesus the island of milos and south of corinth on mount didimon from 1981 to 1984 combining arrival times from this network the greek national network and volnet we relocated hypocenters within greece and the surrounding area the difference between the relocations and the locations published in the national observatory of athens bulletin in most instances were minor except for earthquakes located in the vicinity of the western hellenic arc in this area we found that epicenters thought to lie south of the plate boundary within the african plate relocated an average of approximately 60 km to the n30 e which places them at or north of the plate boundary we suggest that this difference is a minimum estimate of the location error and the true locations may lie still further to the nne we propose that anomalously slow seismic velocities in the aegean back arc upper mantle cause travel time delays to the seismograph stations located to the ne of the source volumes resulting in location errors in a direction away from these stations extrapolating from our observations to other segments of the hellenic plate boundary and to periods not covered by our operation we hypothesize that most or all earthquakes thought to have originated on the african plate within about 100 km of the hellenic arc were actually located at the plate boundary or on the aegean plate

seismic investigation of thick evaporite deposits on the central and inner unit of the mediterranean ridge accretionary complex abstract seismic reflection observations from the imerse cruise have confirmed the presence of at least two localised thick evaporite basins on the crest and inner plateau of the mediterranean ridge accretionary complex the discovery is supported by interval velocity results from both near normal incidence reflection data and wide angle seismic data these evaporite basins are located north of a deep trough known as the cleft and at a currently active thrust zone south of the cleft on the crest of the mediterranean ridge the polarity of the seismic reflections associated with the base of the evaporite shows a phase reversal relative to the seafloor this high amplitude phase reversed reflection indicates a decrease in velocity between the evaporite and the underlying sediment the existence of these thick evaporite deposits suggests that the distribution of messinian salt is much thicker 1 8 2 km on parts of the ridge crest than initially perceived by previous investigations we propose that these thick evaporite deposits were deposited in localised basins on the ridge formed in response to pre and syn messinian tectonics the evaporites on the inner plateau basin most likely represent the infill of a messinian forearc basin the existence of local deep evaporite basins on the crest of the ridge at depths of 2 7 3 3 km below sea level at the present day supports the hypothesis that the mediterranean sea level must have dropped by around 3 km below its present sea level during the messinian salinity crisis

geodetic determination of the kinematics of central greece with respect to europe implications for eastern mediterranean tectonics we use a new satellite laser ranging global positioning system slr gps solution at seven sites in anatolia and aegea to obtain a better definition of the extrusion motion of the anatolian aegean block with respect to europe we show that this motion can be described in a first approximation by a counterclockwise rotation which transfers most of the motion of arabia to anatolia we combine 78 displacement vectors obtained at common points of two triangulation nets measured in central greece in 1895 and 1975 with the slr gps measurements to compute the velocity field over greece with respect to europe these measurements indicate that central greece is a zone of extension between the anatolian aegean counterclockwise rotation to the south and the northern greece clockwise rotation to the north this extension is principally localized within the gulf of corinth to the east but is distributed to the west we then extrapolate this velocity field to the whole aegea and western anatolia using recently published gps results as well as the slr results the narrow dextral north anatolian fault which limits the velocity field to the north progressively gives way to a much wider boundary zone where extension becomes dominant we show that the collision between the mediterranean ridge and africa began 3 6 ma and we describe the modifications that this collision has produced on the kinematic pattern both in aegea and on the mediterranean ridge