crustal structure of the northern nova scotia rifted continental margin eastern canada 1 the nova scotia continental margin off eastern canada marks a transition from a volcanic to a nonvolcanic style of rifting the northern nonvolcanic segment of the margin was studied by a 490 km long refraction seismic line with dense air gun shots coincident with previous deep reflection profiles a p wave velocity model was developed from forward and inverse modeling of the wide angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles the continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5 7 6 9 km s crustal thinning down to 3 km occurs in a 180 km wide zone and the sediment cover in this area is up to 15 km thick farther seaward a 150 km wide transition zone is observed with a 5 km thick lower layer 7 2 7 6 km s interpreted as partially serpentinized mantle at the landward end this layer is overlain by highly altered continental crust 5 4 km s extending up to the seaward limit of the jurassic salt province farther seaward the upper layer is interpreted as exhumed and highly serpentinized mantle 5 1 km s separated from the lower layer by subhorizontal reflectivity which probably represents a serpentinization front oceanic crustal thickness is 4 km with layer 2 velocities of 4 6 5 0 km s layer 3 velocities of 6 4 6 55 km s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile this reduced magma production might be related to the proximity of the newfoundland transform margin

crystalline placer gold from the rio neuquen argentina implications for the gold budget in placer gold formation recent fluvial sediments of the rio neuquen argentina contain variable amounts of placer gold in addition to rounded detrital placer gold the sediments also contain pristine crystalline gold type 1 gold which occurs as overgrowths on detrital placer gold cores type 1 gold is locally rimmed by gold with a wormlike or crystalline texture type 2 gold the chemistry of the primary cores indicates variable intragrain and intergrain compositional heterogeneity this is interpreted to indicate a multisource origin type 1 gold is composed of virtually pure gold 98 at au and type 2 gold is an au hg amalgam 4 9 at hg both varieties are authigenic in origin the formation of the virtually pure type 1 gold is interpreted to be due to the addition of au from an external source rather than by the removal of ag and other metals the auhgag type 2 gold is interpreted to be due to hg contamination from historic mining type 1 gold is interpreted to be analogous to the ag poor rims present on many placer gold grains the evidence presented in this study supports the view that the ag poor rims on placer gold grains are the result of au addition and not ag removal this study also has implications for both quantitative morphological and paragenetic studies of placer gold grains

can biological toxicity drive the contrasting behavior of platinum and gold in surface environments abstract in spite of the similar chemical properties of gold au and platinum pt au is highly mobile in surface environments whereas pt appears to be far less so in this study we assess if geomicrobial processes are likely to cause these differences as the mobility of pt and au should differ little based on thermodynamic solubility alone to achieve an accurate comparison it is important that both metals occur in the same environment mineral and groundwater samples were obtained from the fifield pt au field in new south wales australia where pt and au nuggets occur in a series of tertiary eluvial and alluvial paleo placers in particular we studied the m scale dispersion of au and pt within an extraordinary 10 mm sized fragment of ferruginous paleochannel material which contained abundant native au and isoferroplatinum grains gold grains displayed complex secondary morphologies indicative of biogeochemical transformations whereas isoferroplatinum grains appeared smooth and well rounded and showed no signs of supergene transformation gold grains were surrounded by a dusting of highly pure metallic au particles 10 m in diameter whereas no metallic pt particles were detected a search for ionic pt was also unsuccessful a series of biotic and abiotic incubation experiments was conducted to investigate the hypothesis that these differences in mobility are driven by interactions with microbiota biofilms consisting of metallophilic bacteria formed on ultraflat au surfaces and caused significant surface transformations in contrast only subtle changes were observed on pt surfaces incubated under similar conditions minimal inhibitory concentrations for au complexes are more than an order of magnitude lower than those measured for pt complexes in cupriavidus metallidurans cells this higher cell toxicity of mobile au compared to pt complexes can lead to toxic levels of mobile au in the vicinity of au grain surfaces the elevated toxicity drives the formation of au detoxifying biofilms that catalyze the biomineralization of spheroidal nano particulate and bacteriomorphic au in contrast this does not occur on isoferroplatinum grains due to the lower toxicity of pt in conclusion these results are consistent with microbial adaptation to element toxicity driving the cycling of precious metals in surface environments

single bubble air gun array for deep exploration large tuned air gun arrays operated in off shore petroleum exploration are also used for deep penetration marine seismic reflection surveys conducted to define structures in the earth s crust because of the attenuation of higher frequencies the useful upper frequency limit of these records is usually about 50 60 hz the aim of this paper is to report on a method of seismic pulse generation that preferentially concentrates the air gun s energy in the low range of the seismic frequency band by centering the output on the first bubble pulse instead of the initial primary pulse experimental results show that due to the increased low frequency energy content of this single bubble pulse air gun arrays considerably reduced both in size and volume can generate the necessary acoustic energy for deep seismic exploration

tethys atlantic interaction along the iberia africa plate boundary the betic rif orogenic system abstract initial se dipping slow subduction of the ligurian tethys lithosphere beneath africa from late cretaceous to middle oligocene twisting to a later faster e dipping subduction of the subcrustal lithosphere is proposed as an efficient geodynamic mechanism to structure the arcuate betic rif orogenic system this new subduction related geodynamic scenario is supported by a kinematic model constrained by well dated plate reconstructions tectonic sedimentary and metamorphic data sets the slow initial se dipping subduction of the ligurian tethys realm beneath the malaguide upper plate unit is sufficient to subduct alpujarride and nevado filabride rocks to few tens of kilometers of depth in middle eocene times the shift from se to e dipping subduction during latest oligocene early miocene was possibly caused by both the inherited geometry of the highly segmented ligurian tethys domain and by the fast roll back of the subducted lithospheric slab the early miocene rather synchronous multiple crustal and subcrustal processes comprising the collision along the betic front the exhumation of the hp lt metamorphic complexes the opening of the alboran basin its flooring by hp alpujarride rocks and subsequent ht imprint can be explained by the fast nw and w directed roll back of the ligurian tethys subcrustal lithospheric slab the w retreat of the ligurian tethys lithosphere in middle late miocene times could partly explain the initiation of its lateral tear and consequent subcrustal processes from latest miocene onward the betic rif system evolved under both the northerly push of africa resulting in tightening at crustal and subcrustal levels and by the distinct current dynamics of the steep lithospheric slab the sw directed scape of the rif fold belt is one of the most striking evidences linked to the recent evolution of the squeezed betic rif system between africa and iberia

the in vitro growth of gold grains gold colloids octahedral platelets and foils directly and indirectly formed from the reduction of soluble au i thiosulfate and au iii chloride complexes by iron oxidizing bacteria cyanobacteria and sulfate reducing bacteria were combined in an experimental system this system represented simplified biogeochemical conditions occurring within a fluvial environment in which placer au could occur in this study biofilm formation and physical aggregation i e sedimentation processes were critical for the accumulation of nanometer to micrometer sized au particles into grains 4 5 mm in size characterization of grain surface textures by scanning electron microscopy in association with monitoring soluble au concentrations over time suggested that dissolution and reprecipitation processes were occurring at the au grain fluid interface this laboratory model demonstrates that the biogeochemical cycling of au can contribute to the formation of anomalous enrichments such as placer au deposits

deep structure of the continental margin and basin off greater kabylia algeria new insights from wide angle seismic data modeling and multichannel seismic interpretation abstract during the algerian french spiral survey aimed at investigating the deep structure of the algerian margin and basin two coincident wide angle and reflection seismic profiles were acquired in central algeria offshore greater kabylia together with gravimetric bathymetric and magnetic data this 260 km long offshore onshore profile spans the balearic basin the central algerian margin and the greater kabylia block up to the southward limit of the internal zones onshore results are obtained from modeling and interpretation of the combined data sets the algerian basin offshore greater kabylia is floored by a thin oceanic crust 4 km with p wave velocities ranging between 5 2 and 6 8 km s in the northern hannibal high region the atypical 3 layer crustal structure is interpreted as volcanic products stacked over a thin crust similar to that bordering the margin and related to miocene post accretion volcanism these results support a two step back arc opening of the west algerian basin comprising oceanic crust accretion during the first southward stage and a magmatic and probably tectonic reworking of this young oceanic basement during the second westward opening phase the structure of the central algerian margin is that of a narrow 70 km magma poor rifted margin with a wider zone of distal thinned continental crust than on the other margin segments there is no evidence for mantle exhumation in the sharp ocean continent transition but transcurrent movements during the second opening phase may have changed its initial geometry the plio quaternary inversion of the margin related to ongoing convergence between africa and eurasia is expressed by a blind thrust system under the margin rising toward the surface at the slope toe and by an isostatic disequilibrium resulting from opposite flexures of two plates decoupled at the continental slope this disequilibrium is likely responsible for the peculiar asymmetrical shape of the crustal neck that may thus be a characteristic feature of inverted rifted margins

shear margins continent ocean transform and fracture zone boundaries as exploration extends into deeper water it has become more important to understand the nature of the earth s crust beneath offshore sediments that is heat flow and related source rock potential usually require that sediments be deposited over continental crust hence delineating the boundary between oceanic and continental crust becomes important the material in this article was originally presented at the workshop the crust and its structure at seg s recent annual meeting in calgary these results part of a larger dissertation research project also have been presented to several oil and gas exploration companies in houston

flow splitting modifies the helical motion in submarine channels 1 intricately meandering channels of various scales constitute a major morphological feature of the submarine slope and fan systems these channels act as conduits of density driven gravity underflows and in turn are shaped by these underflows the relationship between channel curvature and the dynamics of sediment laden underflows commonly known as turbidity current has been an enigma and recently a subject of controversy this contribution unravels the flow field of turbidity current at submarine channel bends captured from large scale laboratory experiments the experimental results show that a mildly sloping channel bank greatly enhances the tilt of the turbidity current ambient water interface so much so that the current completely separates from the convex or the inside bank we also show that irrespective of the shape of the channel cross section two cells of helical flow appear in confined submarine bend flow the near bed cell has a circulation pattern similar to that observed in fluvial channels the other cell has an opposite sense of rotation if on the other hand a portion of the flow detaches from the body of the current and spills to the concave or outside overbank area the upper circulation cell becomes suppressed by the resulting lateral convection

segmentation of the farallon slab abstract recent tomography images reveal a complex 3d mantle structure beneath western united states with feature morphology varying rapidly with depth by assimilating plate motion history paleo age of sea floor and paleo geography of plate boundaries in a 3 d numerical model we simulate the farallon juan de fuca subduction during the past 40 ma we find that the highly segmented upper mantle structure of western u s is a direct result of the farallon subduction we show that the tilted horseshoe shaped fast seismic anomaly beneath nevada and utah at 300 600 km depth range is in fact a segment of curled slab subducted since 15 ma and the shallower linear slab beneath the cascades is younger than 5 ma the distinct morphology between these two parts of the subduction system indicates the strong influence of the fast trench rollback since 20 ma the northward migrating jf pa na triple junction and the toroidal flow around slab edges the observed mantle structures are used to constrain the rheology of the upper mantle through matching the shape depth and location of modeled subducted slab segments the inferred viscosity for the asthenosphere is 5 10 19 pa s and those for the transition zone and lower mantle are 1 5 10 21 pa s and 2 10 22 pa s respectively the slab is found to be about 2 orders of magnitude stronger than the ambient mantle above 410 km depth but of similar order of magnitude viscosity in the transition zone

crustal structure of the southwest indian ridge at 66 e seismic constraints the southwest indian ridge represents a slow spreading end member of the global mid ocean ridge system and as such its structure places important constraints on models of melt supply and delivery from the mantle at ridges we present results from a wide angle seismic experiment conducted at the ridge axis at 66 e in a region that has comprehensive swath bathymetric gravity and magnetic data coverage and where the full spreading rate is 12 mm yr 1 based on these data the experiment traversed four spreading segments crustal thickness and velocity structure were determined along three intersecting profiles each 100 km long using shots from a 10 gun 71 l tuned airgun array towed at 15 m depth and fired at 40 s intervals recorded on three ocean bottom hydrophones on each profile obh data show high amplitude arrivals from oceanic layer 2 lower amplitude arrivals from layer 3 and wide angle reflections from the moho forward modelling and inversion of traveltime picks from these data show that the crust consists of a 1 5 2 5 km thick layer 2 with a high velocity gradient and a 0 5 3 0 km thick layer 3 with a low velocity gradient and a crustal thickness of 2 2 5 4 km additional constraints on the models come from 2 d modelling of gravity data along the profiles corrected for 3 d effects of off line bathymetry along axis the thickness of layer 2 varies little but layer 3 is thick at segment centres and very thin at segment boundaries along a flowline profile crustal thickness varies by up to 75 per cent from its minimum value in 3 myr the reduced crustal thickness is consistent with observations from very slow spreading ridge axes elsewhere and may be explained by conductive cooling of the upwelling mantle the large along axis variations in layer 3 thickness indicate that magmatic accretion is focused at segment centres and melt is delivered to segment ends perhaps only by lateral dyke propagation flowline variations in crustal thickness may result from episodicity of melt supply on timescales of 3 myr and by tectonic extension during amagmatic periods velocities at the top of layer 2 are poorly correlated with crustal age based on magnetic anomalies suggesting also that episodicity is decoupled between adjacent segments

seismic evidence of exhumed mantle rock basement at the gorringe bank and the adjacent horseshoe and tagus abyssal plains sw iberia the gorringe bank is a gigantic seamount that separates the horseshoe and tagus abyssal plains offshore sw iberia in a zone that hosts the convergent boundary between the africa and eurasia plates although the region has been the focus of numerous investigations since the early 1970s the lack of appropriate geophysical data makes the nature of the basement and thus the origin of the structures still debated in this work we present combined p wave seismic velocity and gravity models along a transect that crosses the gorringe bank from the tagus to the horseshoe abyssal plains the p wave velocity structure of the basement is similar in the tagus and horseshoe plains it shows a 2 5 3 0 km thick top layer with a velocity gradient twice stronger than oceanic layer 2 and an abrupt change to an underlying layer with a five fold weaker gradient velocity and density is lower beneath the gorringe bank probably due to enhanced fracturing that have led to rock disaggregation in the sediment starved northern flank in contrast to previous velocity models of this region there is no evidence of a sharp crust mantle boundary in any of the record sections the modelling results indicate that the sediment overlays directly serpentinite rock exhumed from the mantle with a degree of serpentinization decreasing from a maximum of 70 80 under the top of gorringe bank to less than 5 at a depth of 20 km we propose that the three domains were originally part of a single serpentine rock band of nature and possibly origin similar to the iberia abyssal plain ocean continent transition which was probably generated during the earliest phase of the north atlantic opening that followed continental crust breakup early cretaceous during the miocene the nw se trending eurasia africa convergence resulted in thrusting of the southeastern segment of the exhumed serpentinite band over the northwestern one forming the gorringe bank the local deformation associated to plate convergence and uplift could have promoted pervasive rock fracturing of the overriding plate leading eventually to rock disaggregation in the northern flank of the gb which could be now a potential source of rock avalanches and tsunamis

thermal evolution rate of exhumation and tectonic significance of metamorphic rocks from the floor of the alboran extensional basin western mediterranean high grade metamorphic rocks drilled at ocean drilling program site 976 in the alboran sea show a pt path characterized by decompression from about 1050 mpa 40 km depth to 350 mpa 13 km depth accompanied by an increase in temperature from about 550 50 c to 675 25 c the ar ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20 5 18 ma which coincides with the initiation of sedimentation in the alboran sea basin the alboran sea formed by miocene extension on the site of a late cretaceous to paleogene contractional orogen and extension coincided with thrusting in the peripheral parts of the betic rif arc which surrounds the basin on three sides thermal modeling of the pt path was carried out with the aim of constraining geodynamic models for the formation of the basin variables considered in the modeling included 1 the thickness and thermal gradient of the postorogenic lithosphere 2 the radiogenic heat production in the thickened crust 3 the time gap pause between the end of contractional tectonics and the start of extension 4 removal of lithospheric mantle below 125 75 or 62 5 km and 5 the rate of extension the only combinations of variables that produce modeled pt paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause to produce high temperatures in rocks initially at 40 km depth removal of lithosphere below 62 5 km to produce further heating during decompression extension by a factor of 3 in 6 m y to delay the attainment of the maximum temperature until the rocks reached shallow depths and final exhumation and cooling in about 3 3 m y to satisfy radiometric and petrological constraints this gives a maximum of about 9 m y for exhumation from 40 km depth to the surface lithospheric stretching in response to plate boundary forces such as trench rollback without removal of lithosphere cannot explain the late onset of heating and the high temperatures reached by these rocks removal of lithosphere at depths significantly greater than 62 5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature only a combination of significant postcollisional radiogenic heating then wholesale removal of lithospheric mantle below the orogenic crust followed by rapid stretching can explain the observed pt path these results appear to support some form of lithospheric delamination as the primary cause for the formation of the alboran sea basin

active thrust faulting offshore boumerdes algeria and its relations to the 2003 mw 6 9 earthquake 1 we investigate the active seismogenic fault system in the area of the 2003 mw 6 9 boumerdes earthquake algeria from a high resolution swath bathymetry and seismic survey a series of 5 main fault propagation folds 20 35 km long leave prominent cumulative escarpments on the steep slope and in the deep basin fault activity creates plio quaternary growth strata within uplifted areas such as a rollover basin on the slope and piggyback basins in the deep ocean most thrusts turn to fault propagation folds at the sub surface and depict ramp flat trajectories we find that the two main slip patches of the 2003 mw 6 9 boumerdes earthquake are spatially correlated to two segmented cumulative scarps recognized on the slope and at the foot of the margin the overall geometry indicates the predominance of back thrusts implying underthrusting of the neogene oceanic crust

e w opening of the algerian basin western mediterranean we present geophysical and geological evidence of fast 2 cm yr 1 in the east and 5 cm yr 1 in the west e w spreading between 16 and 8 ma in the region behind the gibraltar arc in the algerian basin

seismic structure of the central tyrrhenian basin geophysical constraints on the nature of the main crustal domains in this work we investigate the crustal and tectonic structures of the central tyrrhenian back arc basin combining refraction and wide angle reflection seismic was gravity and multichannel seismic mcs reflection data acquired during the medoc mediterraneo occidental 2010 survey along a transect crossing the entire basin from sardinia to campania at 40 n the results presented include a 450 km long 2 d p wave velocity model obtained by the traveltime inversion of the was data a coincident density model and a mcs poststack time migrated profile we interpret three basement domains with different petrological affinity along the transect based on the comparison of velocity and velocity derived density models with existing compilations for continental crust oceanic crust and exhumed mantle the first domain includes the continental crust of sardinia and the conjugate campania margin in the sardinia margin extension has thinned the crust from 20 km under the coastline to 13 km 60 km seaward similarly the campania margin is also affected by strong extensional deformation the second domain under the cornaglia terrace and its conjugate campania terrace appears to be oceanic in nature however it shows differences with respect to the reference atlantic oceanic crust and agrees with that generated in back arc oceanic settings the velocities depth relationships and lack of moho reflections in seismic records of the third domain i e the magnaghi and vavilov basins support a basement fundamentally made of mantle rocks the large seamounts of the third domain e g vavilov are underlain by 10 20 km wide relatively low velocity anomalies interpreted as magmatic bodies locally intruding the mantle

crustal structure of the southwest indian ridge at the atlantis ii fracture zone the southwest indian ridge is a slow spreading end member of the mid ocean ridge system the deepest borehole penetrating the lower oceanic crust ocean drilling program hole 735b lies on the eastern transverse ridge of the atlantis ii fracture zone at 57 e a wide angle seismic survey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous to the east of atlantis bank on which hole 735b is located the crust consists of a 2 2 5 km thick high velocity gradient oceanic layer 2 and a 1 2 km thick low velocity gradient layer 3 the transform valley has a 2 5 3 km thick crust with anomalously low velocities interpreted to consist largely of highly serpentinized mantle rocks the seismically defined crust is thickest beneath the borehole where layer 2 is thinner and the lower crust is inferred to contain 2 3 km of partially serpentinized mantle the seismic velocity models are consistent with gravity data which show weak residual mantle bouguer anomalies because the regions of thinner crust have lower crustal densities stress variations deduced from mass balances between the transform valley floor and the adjacent transverse ridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically the variation of crustal thickness with spreading rate defined by data from the southwest indian ridge and elsewhere is consistent with models of melt generation in which the upwelling mantle is cooled by conductive heat loss at very slow spreading rates resulting in reduced melt generation under the spreading axis large segment scale variations in crustal thickness suggest subcrustal along axis migration of melt toward segment centers

the application of electron microscopy and microanalysis in conjunction with organic petrology to further the understanding of organic mineral association examples from mount isa and mcarthur basins australia organic petrology supported by electron microscopical and micro analytical techniques was applied to organic matter in proterozoic sediments to better understand hydrothermal processes responsible for ore grade mineralisation it was shown that organic maturation was not only closely linked to the geological history of the sediments but also highlighted heat transfer by convection as differentiated from conduction solely through sediment burial and step wise subsidence water rock ratios effect organic maturation in hydrothermal systems and erratic reflectance profiles are indicators of convective heat transfer identification and characterisation of organic materials in terms of source rock and migrated hydrocarbons was shown to be a powerful tool in reconstructing the thermal history of sediments identifying hydrothermal episodes fluid pathways and heat source in the northern australian proterozoic basins higher reflectance of organic matter towards the central parts of the mount isa basin and some of the most northerly parts point to proximity to higher heat flow at times in contrast to relatively low temperatures prevailing in the western parts of the basin next to the murphy inlier a close correlation shown between peak organic reflectance values and super sequence boundaries farther highlighted the valuable information to be gained from organic petrology by allowing the separation of processes responsible for metal dissolution and transportation from those inducing precipitation c 2001 elsevier science b v all rights reserved

polyphase tectonic evolution of fore arc basin related to step fault as revealed by seismic reflection data from the alboran sea w mediterranean 25 pages 14 figures 1 table seismic reflection and bathymetric data collected during the marlboro and saras cruises are stored at sismer and seanoe sea scientific open data edition https doi org 10 17600 11480100 https doi org 10 17600 12450090 and https campagnes flotteoceanographique fr campagnes 12000010

numerical models of continental and submarine erosion application to the northern ligurian margin southern alps france italy we present a new numerical surface process model allowing us to take into account submarine erosion processes due either to submarine landslides or to hyperpycnal currents a first set of models show that the frequency of hyperpycnal flows influences the development of submarine canyons at the mouth of continental rivers further experiments show that an increase in submarine slope leads to faster regressive canyon erosion and a more dentritic canyon network whereas increasing the height of the unstable sediment pile located on the shelf break leads to wider and less dendritic canyons the models are then applied to the western segment of the north ligurian margin northwestern mediterranean which displays numerous submarine canyons with various sizes and morphologies from west to east canyon longitudinal profiles as well as margin perpendicular profiles progressively change from moderately steep concave up shapes to steeper linear to convex up shapes suggesting increasing eastward margin uplift moreover the foot of the margin is affected by a marked slope increase with evidences of mass transport due to landslides numerical models which reproduce well the north ligurian margin morphologic features indicate that the western part of the margin is submitted to rather low i e 0 4 mm yr 1 uplift and intense submarine erosion due to frequent hyperpycnal currents whereas the eastern part bears more rapid i e 0 7 mm yr 1 uplift and has little or no hyperpycnal currents copyright 2014 john wiley sons ltd

the relationship between placer gold particle shape rimming and distance of fluvial transport as exemplified by gold from the klondike district yukon territory canada a semiquantitative study of various physical features of approximately 2 700 gold particles from 21 lode deposits and 36 placer deposits in the klondike district yukon territory canada was undertaken the shape parameters of roundness and flatness were determined using a classification method there appears to be a systematic variation between these parameters and between the parameters and the distance of transport of the particle these relationships were confirmed by quantitative measurements on critical samples rim characteristics were also estimated they show a systematic relationship between one another with the particle shape and less clearly with the distance of transport gold particle shape data shows a smooth well defined relationship to distance of transport from the lode source of the gold both roundness and flatness show a rapid increase within the first 5 km from the lode source and a slower more linear increase beyond this relationship seems to apply worldwide the relationship between the shape of a gold particle and the rim characteristic supports the conclusion that rims are formed by the removal of ag hg and cu and not by the precipitation of au thus rim thickness and the associated high fineness is probably the result of the dynamic process of ag removal in stagnant sediments to create the rim and abrasion in active sediments to remove the rim the composition of the rims represents the stable composition of gold in normal surficial environments together these observations place limits on the occurrence of new gold hammering is the main cause of shape change in fluvially transported gold particles its principal effect is to flatten particles abrasion has its most marked effect on surface texture hammering and abrasion both decrease the mass size and increase the roundness of the gold particles it is therefore possible to make a connection between hydraulic conditions during gold particle transport and its shape change

identification and tectonic implications of a tear in the south american plate at the southern end of the lesser antilles 1 in the southeast corner of the caribbean westward subduction of atlantic oceanic south america beneath the lesser antilles transitions to east west transform motion between continental south america and the caribbean plate this geometry requires negatively buoyant subducting oceanic south american lithosphere to progressively detach from positively buoyant continental south american lithosphere the most widely accepted model is slab break off with oblique arc continent collision and northwest dipping continental subduction precipitating narrow rifting in the subducting slab in contrast the subduction transform edge propagator step model conceptualizes progressive detachment along a vertical dip slip tear through the lithosphere with stress focused at the edge of the propagating transform boundary we present four types of seismic data to resolve the ongoing lithospheric detachment local seismicity receiver functions wide angle seismic velocity inversion and a regional balanced cross section constrained by petroleum industry data these four data sets image a near vertical tear extending through the entire lithosphere revealing a key mechanism for the structural evolution of venezuela

history of subduction and back arc extension in the central mediterranean summary geological and geophysical constraints to reconstruct the evolution of the central mediterranean subduction zone are presented geological observations such as upper plate stratigraphy hp lt metamorphic assemblages foredeep trench stratigraphy arc volcanism and the back arc extension process are used to define the infant stage of the subduction zone and its latest back arc phase based on this data set the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross sections from the northern apennines and from calabria to the gulf of lyon can be derived further the reconstruction is used to unravel the main evolutionary trends of the subduction process results of this analysis indicate that 1 subduction in the central mediterranean is as old as 80 myr 2 the slab descended slowly into the mantle during the first 20 30 myr subduction speeds were probably less than 1 cm year x1 3 subduction accelerated afterwards producing arc volcanism and back arc extension and 4 the slab reached the 660 km transition zone after 60 70 myr this time dependent scenario where a slow initiation is followed by a roughly exponential increase in the subduction speed can be modelled by equating the viscous dissipation per unit length due to the bending of oceanic lithosphere to the rate of change of potential energy by slab pull finally the third stage is controlled by the interaction between the slab and the 660 km transition zone in the southern region this results in an important re shaping of the slab and intermittent pulses of back arc extension in the northern region the decrease in the trench retreat can be explained by the entrance of light continental material at the trench

morphogenesis of the sw balearic continental slope and adjacent abyssal plain western mediterranean sea we present the seafloor morphology and shallow seismic structure of the continental slope south east of the balearic promontory and of the adjacent algero balearic abyssal plain from multibeam and chirp sonar data the main purpose of this research was to identify the sediment pathways from the balearic promontory to the algero balearic deep basin from the early pliocene to the present the morphology of the southern balearic margin is controlled by a sw ne structural trend whose main expressions are the emile baudot escarpment transform fault and a newly discovered wsw ene trend that affects the sw end of the escarpment and the abyssal plain we relate the two structural trends to right lateral simple shear as a consequence of the miocene westward migration of the gibraltar arc newly discovered steep and narrow volcanic ridges were probably enabled to grow by local transtension along the transform margin abyssal plain knolls and seahills relate to the subsurface deformation of early stage halokinetic structures such as salt rollers salt anticlines and salt pillows the limited thickness of the overburden and the limited amount of deformation in the deep basin prevent the formation of more mature halokinetic structures such as diapirs salt walls bulbs and salt extrusions the uppermost sediment cover is affected by a dense pattern of sub vertical small throw normal faults resulting from extensional stress induced in the overburden by subsurface salt deformation structures shallow gas seismic character and the possible presence of an active polygonal fault system suggest upward fluid migration and fluid and sediment expulsion at the seafloor through a probable mud volcano and other piercement structures one large debris flow deposit named formentera debris flow has been identified on the lower slope and rise of the south formentera margin based on current observations we hypothesize that the landslide originating the formentera debris flow occurred in the holocene perhaps in historical times

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

demerara plateau the structure and evolution of a transform passive margin summary the amazon cone experiment acquired two transects profile sd a across the demerara plateau part of the french guiana northeast brazil continental margin to better understand rift and transform style margin evolution profile a images an intermediate type margin formed as a result of trans tensional extension in this paper we describe the modelling of wide angle and multichannel seismic and gravity data from profile d to reveal whole crustal structure and features exhibiting transform characteristics combining these results with other studies in the region and comparing our results with young rift analogues we develop a model of along margin segmentation that explains the evolution of the west equatorial atlantic interpretation of the velocity depth model for profile d shows a 35 37 km thick continental crust which thins to 10 11 km over a distance of 320 km this thinning is accommodated in two regions the narrowest region associated with the ocean continent transition is interpreted to have formed by dextral shearing of the margin along major transform zones that accommodated the initial break up geometry of the central atlantic given the orientation of the margin relative to local fracture zone traces it is likely that the second region of thinning located 162 km landward of the ocean continent transition is a result of rifting suborthogonal to the profile orientation there is no evidence of rotated faulted blocks half graben structures or riftrelated magmatism manifest as either seaward dipping reflectors or as a high velocity region within the lower crust the demerara plateau is therefore interpreted as a margin segment comprising thinned continental crust bound to the north and south by transform type zones in which trans tensional extension is accommodated in contrast to profile a modelling suggests that the eastern margin exhibits a relatively broad region of crustal thinning associated with extension consistent with a rift type setting offshore unusually thin oceanic crust of 3 3 5 7 km thickness is identified which is consistent with similar observations further south in the absence of identifiable magnetic anomalies best estimates of the initial half spreading rate of 20 mm yr 1 suggest that the thin crust throughout the region is unlikely to be a result of ultra slow spreading but instead it is most likely due to a reduced magma supply at numerous long lived transform faults reflected by those presently offsetting the mid atlantic ridge in this equatorial setting

iridium and other platinum group elements as geochemical markers in sedimentary environments iridium anomalies are commonly used as indicators of extraterrestrial processes which caused global extinctions however platinum group element pge anomalies may result from several different processes both syn and postdepositional acting separately or jointly the sources and processes of enrichment involved are extraterrestrial asteroid comet cosmic dust impact ejecta volcanogenic pge rich condensates precipitation from seawater low sedimentation rate anoxic conditions microbial concentration dissolution reprecipitation exhalative hydrothermal leaching transport and precipitation at a redox boundary geochemical characteristics of pge and accompanying elements in sediments can be used to identify the contributions of some specific sources and processes 1 extraterrestrial chondritic ru ir and os ir ratios low 187os 186os ratio high ir compared to pt pd and au 2 volcanogenic high iridium and se as sb and in contents 3 exhalative hydrothermal high au pd and pt relatively low ir contents 4 precipitation from seawater high pt and low pd contents also high cd and ag contents under anoxic conditions and 5 reduction from intermediate and low temperature solutions high pge and cu or mo u ni contents

weathering profile of the cerro de maimon vms deposit dominican republic textures mineralogy gossan evolution and mobility of gold and silver abstract cerro de maimon in the central dominican republic is currently the only vms deposit under production in the caribbean region it is hosted in the maimon formation of early cretaceous age which is part of the oldest and chemically most primitive island arc in the caribbean from bottom to top this deposit can be divided into i a primary sulfide zone ii a supergene enrichment zone and iii an oxidized zone this study reports new data on the textural and mineralogical characteristics of the oxidized zone gossan leached capping zone with emphasis in the au ag bearing phases the mineral paragenesis of the oxidized zone is essentially composed of goethite hematite quartz and barite botryoidal cellular and brecciated textures can be distinguished botryoidal and brecciated textures dominate in the upper parts of the oxidized zone whereas cellular textures are more common in the intermediate and lower parts however the weathering profile is very heterogeneous the leached capping profile shows evidence of both transported and indigenous gossans gold in the oxide paragenesis is extremely pure 99 au suggesting that chemical refining took place silver occurs mainly as iodargyrite agi and minor agbr agcl in botryoidal aggregates particles of au ag alloy in the primary mineralization exposed to a weathering environment can be leached and transported by various agents chemical and biochemical that may exist simultaneously in the presence of halides gold and silver can be leached and transported in a wide range of ph eh conditions especially if iodine is present silver is leached more rapidly and over a broader range of ph eh conditions and is preferentially transported as iodine complexes than other halides the presence of iodargyrite in the cerro de maimon gossan fully developed under tropical conditions suggests that this mineral cannot be considered an indicator of extremely arid environments as typically claimed by many authors iodargyrite occurrence would rather point to the presence of saline halide rich groundwater in oxide deposits

structural styles and neogene petroleum system around the yusuf habibas ridge alboran basin mediterranean sea the algerian offshore is part of the southern margin of the western mediterranean sea the western part of this offshore area represents the transitional margin between the south algero balearic basin and the alboran basin the study area includes the southern and eastern parts of the alboran basin and the northwestern part of the algerian margin and is in the western part of the plate boundary between eurasia and africa figure 1 the yusuf habibas ridge is a major ew striking structure of this complex plate boundary separating the eastern and southern parts of the alboran basin from the south algero balearic basin martinez garcia et al 2011 and references therein the ridge played an important role during the neogene alboran westward block migration between the africa and iberia plates while the kabylies blocks migrated southward and accreted to africa furthermore the ongoing nw se convergence between africa and iberia has induced a new stress field since 7 ma ago replacing an earlier stress

crustal structure of the eastern algerian continental margin and adjacent deep basin implications for late cenozoic geodynamic evolution of the western mediterranean the karukera spur is an easternmost submerged rise of the lesser antilles fore arc in the guadeloupe archipelago culminating about 4000 m above the fore arc basin 150 km west of the deformation front the analysis of 3500 km 2d high resolution multichannel seismic reflection and multibeam bathymetric data and the study of 14 core samples allow for the first time to reconstruct its sedimentary anatomy and tectonic evolution seven seismic units us1 to us7 are evidenced organised into four major depositional sequences separated by erosional surfaces units us1 to us4 thicken to the south southeast on the spur the sedimentary deposits rest upon a deeply eroded metamorphic basement of the caribbean plate as in la desirade island sequence 1 units us1 and us2 remains unknown in its lower part because it does not outcrop its upper part comprises early miocene shallow water carbonates sequence s2 unit us3 only comprises an upper miocene transgressive systems tract deposited into basinal environment it is bounded by major erosional surfaces sb2 and sb3 sequence 3 us4 to us6 is zanclean to pleistocene it is dominantly composed of shallow water reefal deposits sequence 4 corresponds to recent deposits the karukera spur experienced three main extensional episodes with alternations between uplift and subsidence periods uplifts are responsible for emersions and erosions subsidence severely drowned shallow water deposits the first extensional episode led to the formation of horsts and grabens in the basement prior to early miocene deposits the second extensional episode occurred during upper miocene in accordance with data in surrounding islands the third tectonic episode developed since calabrian noticeable is the constant reactivation of n130 n150 n150 n180 and n40 trending faults since early miocene and the neoformation of e w trending faults since calabrian our results suggest that at the latitude of guadeloupean archipelago the lesser antilles subduction zone is an erosional margin because of the successive sweepings of the santa lucia then tiburon ridges

lateral velocity resolution from three dimensional seismic refraction data summary a method for estimating the lateral velocity resolution from 3 d seismic refraction traveltimes is presented the method is non linear in that synthetic data calculated from checkerboard models are inverted using an iterative tomographic approach incorporating smoothness constraints and updated ray paths at each iteration two applications are presented 1 real data from the faeroe basin experiment using the ray coverage and noise level of approximately 50 000 picks and 2 ideal data from the same experiment corresponding to 100 per cent data recovery equivalent to about three times as many ray paths the refraction data constrain the sedimentary and basement structure to approximately 12 km depth a comparison of the results from the two data sets provides a qualitative check on the resolution method illustrates the pitfalls of using ray coverage as an indicator of resolution and demonstrates the potential resolution of currently feasible 3 d refraction experiments 32 checkerboard models consisting of alternating positive and negative velocity anomalies superimposed on the preferred final model of the basin were tested each with a diverent cell size position or orientation the problem of ray bending associated with the 5 per cent checkerboard anomalies is shown to be small using models with four cell sizes allowed for resolution estimates from 3 to 15 km this range being imposed by the frequency content of the data and the areal dimensions of the study area 44 26 km an operator centred on each model node measures the local semblance between the known and recovered checkerboard models a determination of the smallest well resolved cell size at each model node is then made using a 0 7 semblance threshold to provide a spatially dependent lateral resolution estimate resolution for the real data set varies from an average of just over 3 km at 2 km depth to 10 km at 10 km depth where there is a local maximum in ray coverage due to a concentration of ray bottoming points for the ideal data set the average lateral resolution drops ov roughly linearly from better than 3 km at 2 km depth to 6 3 km at 10 km depth the largest diverence between the resolution associated with the ideal and real data occurs at 7 2 km depth where it averages 4 6 km for the former and 10 8 km for the latter due to the near absence of ray bottoming points between 7 and 9 km the results show that the final model for the faeroe basin obtained by regularized inversion with smoothness constraints is consistent with the resolution estimates in the sense that the model does not contain structure with a wavelength smaller than the resolution estimate at any point the results for the ideal data show that good quality 3 d refraction experiments can provide a lateral resolution of no worse than the receiver spacing shot spacing for land data throughout most of the sampled volume

formation of high grade ignimbrites part ii a pyroclastic suspension current model with implications also for low grade ignimbrites analogue experiments in part i led to the conclusion that pyroclastic flows depositing very high grade ignimbrite move as dilute suspension currents in the thermo fluid dynamical model developed the degree of cooling of expanded turbulent pyroclastic flows dynamically evolves in response to entrainment of air and mass loss to sedimentation initial conditions of the currents are derived from column collapse modeling for magmas with an initial h2o content of 1 3 wt erupting through circular vents and caldera ring fissures the flows spread either longitudinally or radially from source up to a runout distance that increases with higher mass flux but decreases with higher gas content temperature bottom slope and coarser initial grain size progressive dilution by entrainment and sedimentation causes pyroclastic currents to transform into buoyant ash plumes at the runout distance the ash plumes reach stratospheric heights and distribute 30 80 of the erupted material as widespread co ignimbrite ash pyroclastic suspension currents with initial mass fluxes of 107 1012 kg s can spread for tens of kilometers with only limited cooling although they move as supercritical strongly entraining currents for the eruption conditions considered here with increasing eruption mass flux cooling during passage through the fountain diminishes while cooling during flow transport increases the net effect is that eruption temperature exerts the prime control on emplacement temperature pyroclastic suspension currents can form welded ignimbrite across their entire extent if eruption temperature is to 1 3 tmw the minimum welding temperature high eruption rates a large fraction of fine ash and a ring fissure vent favor the formation of extensive high grade ignimbrite for very hot eruptions producing sticky partially molten pyroclasts analysis of particle aggregation systematics shows that factors favoring longer runout also favor more efficient aggregation which reduces runout as a result very high grade ignimbrites cannot spread more than a few tens of kilometers from their source in cooler pyroclastic currents particles do not aggregate and the sedimentation process may involve re entrainment of particles which potentially leads to more extensive cooling and longer runout such effects however are only significant when net erosion of substrate occurs model results can be employed to estimate mass flux and duration of ignimbrite eruptions from measured ignimbrite masses and aspect ratios the model also provides an alternative explanation of the observed decrease in h lratios with ignimbrite mass

sediment driven downslope flow in submarine canyons and channels three dimensional numerical experiments abstract the role of submarine canyons and channels in sediment driven downslope flow sediment plumes is examined using a three dimensional rotational numerical model that couples the hydrodynamics and sediment transport the model domain consists of a bottom ocean layer of constant height coupled with an essentially inert upper ocean the model equations are cast in a rotated bottom following coordinate system in which vertical grid spacing is independent of the ocean depth and bathymetry can be resolved accurately this allows for tracing bottom attached sediment plumes decameters in height from shallow water into great depths of the ocean the calculations reproduce morphologic features related to the occurrance of sediment plumes such as the formations of 1 localized deposition areas of sediment off the mouth of submarine canyons and 2 levees at both sides of submarine channels furthermore it is demonstrated that sediment plumes are not only important for the transport of littoral sedimen

moment tensor solutions for small and moderate earthquakes in the ibero maghreb region 1 we applied time domain moment tensor inversion of local and regional waveforms to small and moderate mw 3 5 5 7 shallow earthquakes from the iberian peninsula northern morocco and northern algeria for the 6 years period from november 1995 to march 2002 and the previous network of autonomously recording seismograms nars experiment moment tensor solutions were obtained for 58 events considerably increasing the total number of available solutions in the study area for each event we performed a moment tensor inversion and a double couple grid search for simple faulting events the grid search is valuable as a quality test for its ability to reveal potential ambiguities of the solutions and to assess confidence limits of fault plane parameters or principal axes orientation the computed mechanisms show regional consistency a large part of the iberian peninsula is characterized by normal faulting mechanisms with sw ne oriented t axes thrusting and se nw compression is dominant in algeria in the alboran sea the westernmost part of the mediterranean and the transition between both regimes strike slip mechanisms dominate with approximately n s oriented p axes this pattern suggests a regional anomaly characterized by clockwise rotation of the principal horizontal stress orientations

preliminary evidence for the involvement of budding bacteria in the origin of alaskan placer gold lacelike networks of micrometre size filiform gold associated with alaskan placer gold particles are interpreted as low temperature pseudomorphs of a pedomicrobium like budding bacterium submicron reproductive structures hyphae and other morphological features similar to those of pedomicrobium manganicum occur as detailed three dimensional facsimiles in high purity gold in and on placer gold particles from lillian creek alaska in a scanning electron microscope survey the majority of gold particles at nine alaskan placer deposits appear to include gold that has accumulated chemically at low temperatures in and on the cells of p manganicum similar bacterioform gold from a paleozoic deposit in china and from the precambrian witwatersrand deposit in south africa may indicate that bacterioform gold is widespread

traces in the dark sedimentary processes and facies gradients in the upper shale member of the upper devonian lower mississippian bakken formation williston basin north dakota u s a abstract black organic rich rocks of the upper shale member of the upper devonian lower mississippian bakken formation a world class petroleum source rock in the williston basin of the united states and canada contain a diverse suite of mudstone lithofacies that were deposited in distinct facies belts the succession consists of three discrete facies associations fas these comprise 1 siliceous mudstones 2 quartz and carbonate bearing laminated mudstones and 3 macrofossil debris bearing massive mudstones these fas were deposited in three facies belts that reflect proximal to distal relationships in this mudstone system the macrofossil debris bearing massive mudstones fa 3 occur in the proximal facies belt and contain erosion surfaces some with overlying conodont and phosphate lithoclast lag deposits mudstones with abundant millimeter scale siltstone laminae showing irregular lateral thickness changes and shell debris in the medial facies belt quartz and carbonate bearing laminated mudstones dominate exhibiting sub millimeter thick siltstone layers with variable lateral thicknesses and localized mudstone ripples in the distal siliceous mudstone facies belt radiolarites radiolarian bearing mudstones and quartz and carbonate bearing laminated mudstones dominate overall total organic carbon toc contents range between about 3 and 10 wt with a general proximal to distal decrease in toc content abundant evidence of bioturbation exists in all fas and the lithological and toc variations are paralleled by changes in burrowing style and trace fossil abundance while two horizontal traces and two types of fecal strings are recognized in the proximal facies belt only a single horizontal trace fossil and one type of fecal string characterize mudstones in the distal facies belt radiolarites intercalated into the most distal mudstones are devoid of traces and fecal strings bedload transport processes likely caused by storm induced turbidity currents were active across all facies belts suspended sediment settling from near the ocean surface however most likely played a role in the deposition of some of the mudstones and was probably responsible for deposition of the radiolarites the distribution pattern of high toc sediments in proximal and lower toc deposits in some distal facies is interpreted as a function of higher accumulation rates during radiolarian depositional events leading to a decrease in suspension derived organic carbon in radiolarite laminae the presence of burrows in all fas and nearly all facies in the upper bakken shale member indicates that dysoxic conditions prevailed during its deposition this study shows that in intracratonic high toc mudstone successions such as the upper bakken shale member bed load processes most likely dominated sedimentation and conditions promoted a thriving infaunal benthic community as such deposition of the upper bakken shale member through dynamic processes in an overall dysoxic environment represents an alternative to conventional anoxic depositional models for world class source rocks

the deposition of elemental gold from gold i thiosulfate complexes mediated by sulfate reducing bacterial conditions the role of sulfate reducing bacteria in the precipitation of elemental gold through a geologic stratum was investigated using column experiments the mixed culture dominated by sulfate reducing bacteria was isolated from the driefontein consolidated gold mine witwatersrand basin republic of south africa bacterially mediated gold precipitation from the gold i thiosulfate complex was more efficient than the corresponding abiotic experiments in the bacterial systems sulfate reducing bacteria i e desulfovibrio sp deposited gold inside the cells as spherical nanoparticles over time these nanoparticles of gold were released from the cells and deposited at cell surfaces and in the bulk solutions ultimately these nanoparticles of gold contributed to the formation of micrometer scale octahedral gold crystals occasional framboidlike structures 1 5 m diam and millimeter scale gold foils typically surrounding the silicate grains spherical gold particles 1 m diam were precipitated in the abiotic experiments and the formation of octahedral gold framboidlike structures and gold foils were not observed in these experiments the reduction and enrichment of elemental gold by sulfate reducing bacteria occurred without the concomitant death of the bacteria suggesting that in natural systems this process could go on over geologic time scales as long as nutrients and gold i thiosulfate were provided to the system

the crustal structure across the transform continental margin off ghana eastern equatorial atlantic forward modeling of a suite of wide angle seismic lines across the transform continental margin at the eastern end of the romanche fracture zone off ghana has shown the transition from continental to oceanic crust to be confined to a narrow 6 to 11 km wide zone located at the foot of the steep continental slope the structure of the adjacent oceanic and continental crusts has also been resolved these results are confirmed and enhanced by gravity and magnetic models the crust of the ocean continent transition zone is characterized by high velocities 5 8 7 3 km s 1 a high density 3 10 mg m 3 and high magnetizations 1 1 1 25 a m 1 this rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin the oceanic crust within 70 km of the margin is abnormally thin compared to normal atlantic oceanic crust and shows an average thickness of just 4 4 km we suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded on at least three sides by cold continental lithosphere the preferred seismic model also shows a new fracture zone 30 km southeast of the ocean continent transition which is characterized by low velocities in the upper crust a lens shaped layer with velocities of 7 2 7 4 km s 1 at the base of the crust and a crustal thickness of just 3 4 km the continental crust appears largely unaffected by the proximity of the adjacent oceanic crust there is no evidence for underplating of the continental crust adjacent to the ocean continent transition zone

underpinning tectonic reconstructions of the western mediterranean region with dynamic slab evolution from 3 d numerical modeling no consensus exists on the tectonic evolution of the western mediterranean since 35 ma three disparate tectonic evolution scenarios are identified each portraying slab rollback as the driving mechanism but with rollback starting from strongly different subduction geometries as a critical test for the validity of each tectonic scenario we employ thermomechanical modeling of the 3 d subduction evolution from each tectonic scenario we configure an initial condition for numerical modeling that mimics the perceived subduction geometry at 35 ma we seek to optimize the fit between observed and predicted slab morphology by varying the nonlinear viscoplastic rheology for mantle slab and continental margins from a wide range of experiments we conclude that a tectonic scenario that starts from nw dipping subduction confined to the balearic margin at 35 ma is successful in predicting present day slab morphology the other two scenarios initial subduction from gibraltar to the baleares and initial subduction under the african margin lead to mantle structure much different from what is tomographically imaged the preferred model predicts slab rotation by more than 180 east west lithosphere tearing along the north african margin and a resulting steep east dipping slab under the gibraltar strait the preferred subduction model also meets the first order temporal constraints corresponding to mid miocene 16 ma thrusting of the kabylides onto the african margin and nearly stalled subduction under the rif gibraltar betic arc since the tortonian 8 ma our modeling also provides constraints on the rheological properties of the mantle and slab and of continental margins in the region

characteristics and origin of coarse gold in late pleistocene sediments of the cariboo placer mining district british columbia canada the cariboo placer mining district 1000 km2 sited in the interior plateau of central british columbia canada is the premier placer gold mining district of the province gold is recovered from three late pleistocene sedimentary facies postglacial fluvial gravels 25 ka this study reports the morphology size roundness sphericity of 1636 gold grains ranging in size from 0 25 to 17 mm recovered from 19 placer mines older gravels contain the smallest gold grains mean grani size 1 53 mm grains of intermediate size occur in till 2 23 mm and the coarsest gold occurs in postglacial gravels 2 34 mm with a mean of 1 93 mm for the mining district as a whole the most common grain shapes are sub rounded discoidal 14 73 of the grain population sub angular discoidal 10 88 and sub rounded sub discoidal 9 59 the most angular grains occur in postglacial gravels in situ growth of coarse angular grains is indicated by a composite grain structure consisting of aggregates of gold particles welded together by high grade ag 2 filamentous gold in situ coarsening may be reliant on organic complexing agents produced below a dense forest cover an evolutionary sequence of grain form from angular aggregates to rounded pumpkin seed grains is suggested rounded grains commonly show a crystalline structure which may result from the cold hammering of gold during transport fracturing along crystal boundaries is common gold grains may undergo cycles of coarsening rounding diagenesis and breakup in response to repeated recycling through pleistocene sedimentary environments

seismotectonics of the lower cheliff basin structural background of the el asnam algeria earthquake the 1980 el asnam earthquake should be understood within its tectonic environment a seismotectonic map of the lower cheliff basin scale 1 500 000 has been synthesized from known geologic tectonic and seismological work on the region and from satellite photos the data set was integrated by a computer in order to handle different scales and projections several important tectonic features divide the region into 5 domains active faults and recent folds are consistent with a nnw sse direction of compression the cheliff basin is limited to the south by the relizane fault the basin is also crossed by three broken folds the eastern one being the one associated with the el asnam fault historical and instrumental seisinicity seems to be located at both ends of relizane fault it is suggested that the sites corresponding to folds and reverse faulting crossing the basin are relevant to seismic risk a simple tectonic model is proposed

mineralogical and geochemical evidence for coupled bacterial uranium mineralization and hydrocarbon oxidation in the shashagetai deposit nw china unusual mineral structures have recently been found in a sandstone hosted roll type uranium deposit in the middle jurassic zhiluo formation in the shashagetai deposit the northern ordos basin nw china the structures possess a chemical composition and crystal structure characteristic of mineral coffinite usio4 1 x oh 4x which occurs as nanoparticles with size ranging from 5 to 25 nm these structures are interpreted to be fossilized microorganisms based on mineralogical biosignatures including morphology size occurrence of biogenic coffinite as nano crystals and biological elements such as p the intimate intergrowth of coffinite with secondary pyrite of bacterial origin as defined by low 34 s values and calcite cements with petroleum derived carbon supports its biogenic origin oil inclusions in the host sandstone are characterized by abundant n alkanes slightly increased pr nc17 and ph nc18 ratios significant amounts of demethylated hopanes and tricyclic terpanes and the existence of unresolved complex mixtures these characteristics are interpreted to be a result of mixing of an earlier heavily degraded oil with a later charged fresh oil subsequently the oils were slightly degraded these lines of evidence lead to the proposal that the reduction of sulfate and oxidization of petroleum are likely synchronous with reduction of hexavalent u vi to tetravalent u iv uranium by sulfate reducing bacteria srb the discovery of a natural association of microorganism like structures a uranium mineral and biodegraded petroleum has implications for uranium biomineralization and fossil fuel exploration

tectonic history of the western tethys since the late triassic the tectonic history of the western tethys since the late triassic is illustrated through a set of computer generated plate reconstructions which are based on a rigorous plate motions model of this region the model is constrained by the atlantic plate kinematics and on land geologic evidence and defines 13 tectonic phases spanning the time interval from the late ladinian 230 ma to the present the kinematics associated with the late triassic western tethyan rifts produced the detachment of a large composite fragment from the northern margin of gondwana it can be considered as the eastern propagation of the central pangea breakup during the early jurassic these rift zones became inactive while new zones of extension formed along the southern margin of eurasia the eastern margin of iberia and within the rifted northern gondwana fragment itself plate motions associated with the first two extensional centers can still be considered as an eastern branch of the central atlantic plate kinematics conversely the kinematic parameters of the latter rift result from the composition of the euler rotation describing the central pangea breakup and the euler pole of closure of the paleo tethys ocean the late triassic early jurassic rifting phases determined the formation of a number of independent microplates at the interface between africa and eurasia starting from the early cretaceous convergence between africa and eurasia triggered further deformation within the dispersed continental fragments and the formation of backarc basins at the active margins ultimately leading to an increase in the number of tectonic elements that were moving independently in the western tethyan region during the late cretaceous and the cenozoic the proposed tectonic evolution of the western tethys area is compatible with both global scale plate kinematics and geological constraints from on land data observed across the present day mosaic of displaced terranes surrounding the mediterranean region

sedimentology and gravel bar morphology in an archaean braided river sequence the witpan conglomerate member witwatersrand supergroup in the welkom goldfield south africa

geodynamics of the emile baudot escarpment and the balearic promontory western mediterranean abstract the 348 km long 105 155 km wide and 1000 2000 m high northeast trending balearic promontory separates the valencia trough to the northwest from the south balearic algerian basin to the southeast it consists of three blocks ibiza at the southwest end mallorca in the center and menorca at the northeast end of the promontory the ibiza and mallorca blocks were thrust faulted and folded in the miocene the menorca block appears to be undisturbed but strata within the transform separating the mallorca and menorca blocks are folded and faulted miocene and younger volcanic pinnacles also are extensive along the transform separating the ibiza and mallorca blocks we infer that the tectonic features on the balearic promontory are the creations of the late oligocene miocene southeast migration of the blocks making up the promontory way from the spanish margin leading to the opening of the valencia trough behind them the westward displacement of the alboran microplate along the emile baudot escarpment on the southeast side of the promontory and the opening of the south baleric algeria basin behind it and the clockwise rotation of the ibiza and mallorca blocks produced by the passing alboran microplate

undercrusting by serpentinite beneath rifted margins the west galicia passive margin spain is bounded by a continuous belt of serpentinized peridotite here we present the results of petrological studies of the peridotite 39ar 40ar dating of high temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift ductile shearing and pervasive serpentinization of mantle rocks occurred at early cretaceous time during the rifting stage of the margin the serpentinite layer resting on fresh peridotite is probably several kilometres thick at present seismic data and gravity data cannot distinguish serpentinite from crustal material but our results from the galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn rift hydrothermal activity in some cases this undercrusting process might explain the young age and mobility of the moho beneath stretched continental basement

early miocene continental subduction and rapid exhumation in the western mediterranean new lu hf garnet growth ages from high pressure rocks from the structurally lowest terrane nevado filabride complex in the betic cordillera of southern spain indicate early miocene subduction of continental crust requiring significant revision of the timing and processes of orogeny in the western part of the alpine orogenic system mafic eclogite and pelitic schist give lu hf ages in the range 18 14 ma and fission track data show that the terrane was exhumed by 12 8 ma the overlying terrane alpujarride complex shows early tertiary high pressure temperature metamorphism this was followed by a thermal event that accompanied rapid crustal extension at 22 18 ma leading to the formation of the alboran sea backarc basin the evidence therefore suggests two phases of continental subduction separated by a phase of large scale intraorogenic extension the two terranes are now separated by a mid late miocene low angle extensional fault the timing and paleogeographic setting suggest that the nevado filabride terrane originally was close to the rifted south iberian margin and was subducted during westward motion of the extending upper terrane between the converging african and iberian plates

surface seismic and electrical methods to detect fluids related to faulting in the absence of drilling surface based geophysical methods are necessary to observe fault zones and fault zone physical properties at seismogenic depths these in situ physical properties can then be used to infer the presence and distribution of fluids along faults although such observations are by nature indirect and become less exact with greater depth multiple observations of a range of such geophysical properties as compressional and shear seismic velocity vp and vs vp v5 ratio related to poisson s ratio resistivity and attenuation in and adjacent to fault zones offer the greatest hope of making inferences of the fault zone geometry fluids in the fault zone and fluid reservoirs in the surrounding crust for simple geometries fault zone guided waves can provide information on fault zone width and velocities for faults of the order of 200 m wide to address the question of whether a narrow fault zone can be imaged well enough at depths of seismic rupture to infer the presence of anomalously high fluid rock ratios we present synthetic seismic tomography and magnetotelluric examples for an ideal case of a narrow fault zone with a simple geometry large changes in material properties and numerous earthquakes within the fault zone a synthetic 0 5 km wide fault zone with 20 velocity reduction is well imaged using local earthquake tomography when sequential velocity inversions are done the true fault width is found even to 9 km depth although the calculated amplitude of the velocity reduction is lower than the actual amplitude vp vs is as well determined as vp magnetotelluric imaging of a synthetic fault zone shows that a conductive fault zone can be well imaged within the upper 10 km further a narrow 1 km very low resistivity 3 ohm m fault core can be imaged within a broad 5 km low resistivity 10 ohm m fault zone illustrating that regions of a fault containing large quantities of interconnected fluids within a broader conductive fault zone should be detectable thus variations in fluid content and fluid pressure can be inferred from electrical and seismic methods but there will always be uncertainty in these inferences due to the trade off with other factors such as intrinsic variations in porosity mineralogy and pore geometry the best approach is combined modeling of varied seismic and electrical data

seismic evidence for the presence of jurassic oceanic crust in the central gulf of cadiz sw iberian margin abstract we investigate the crustal structure of the sw iberian margin along a 340 km long refraction and wide angle reflection seismic profile crossing from the central gulf of cadiz to the variscan continental margin in the algarve southern portugal the seismic velocity and crustal geometry model obtained by joint refraction and reflection travel time inversion reveal three distinct crustal domains the 28 30 km thick variscan crust in the north a 60 km wide transition zone offshore where the crust abruptly thins 20 km and finally a 7 km thick and 150 km wide crustal section that appears to be oceanic in nature the oceanic crust is overlain by a 1 3 km thick section of mesozoic to eocene sediments with an additional 3 4 km of low velocity unconsolidated sediments on top belonging to the miocene age gulf of cadiz imbricated wedge the sharp transition between continental and oceanic crust is best explained by an initial rifting setting as a transform margin during the early jurassic that followed the continental break up in the central atlantic the narrow oceanic basin would have formed during an oblique rifting and seafloor spreading episode between iberia and africa that started shortly thereafter bajocian and lasted up to the initiation of oceanic spreading in the north atlantic at the tithonian late jurassic earliest cretaceous the velocity model displays four wide prominent south dipping low velocity anomalies which seem to be related with the presence of crustal scale faults previously identified in the area some of which could well be extensional faults generated during this rifting episode we propose that this oceanic plate segment is the last remnant of an oceanic corridor that once connected the alpine tethys with the atlantic ocean so it is in turn one of the oldest oceanic crustal fragments currently preserved on earth the presence of oceanic crust in the central gulf of cadiz is consistent with geodynamic models suggesting the existence of a narrow westward retreating oceanic slab beneath the gibraltar arc alboran basin system

the origin of intraplate deformation in the atlas system of western and central algeria from jurassic rifting to cenozoic quaternary inversion abstract analysis of petroleum exploration data supplemented by paleostress data enabled discussion of the origin of the deformation in the western and central saharan atlas algeria this intraplate area has recorded the breakup of pangea late triassic the opening of the maghrebian tethys since the dogger and subsequently its closure oligocene to present however the two periods of strong coupling between europe and africa late lutetian and pleistocene which correspond to rapid uplifts of the atlas system and important deformations are not collision related they can be correlated to the beginning and the end of the development of the western mediterranean sea

characterizing the caribbean south american plate boundary at 64 w using wide angle seismic data 1 we present wide angle velocity modeling results from profile 64 w of the broadband ocean land investigation of venezuela and the antilles arc region bolivar project line 64w is a 460 km long approximately north south onshore offshore reflection refraction transect located approximately at 64 w longitude the profile extends across the transform plate boundary between the southeastern caribbean car and south american sa plates east of the profile the plate boundary bends to the north and sa subducts beneath car we utilize first arrival tomography to resolve a smooth velocity field for the sediments and upper middle crust and then use a layered approach to resolve a sharp velocity contrast for the moho simultaneously inverting refracted pn and reflected pmp arrivals we image crustal and upper mantle structure across the plate boundary zone we interpret that the strike slip system that accommodates relative motion between car and sa extends near vertically through the entire crust and offsets the moho we see no evidence supporting a major component of convergence and rather than a wide boundary zone of overlapping lithospheric plates we interpret the plate boundary to be confined to the 33 km wide near vertical strike slip system previously interpreted thrust faults flanking the strike slip system appear to be confined to the upper middle crust and may be related to the detachment of subducting south american lithosphere at the southern terminus of the lesser antilles subduction zone east of 64 w

a ligurian western mediterranean sea geophysical transect revisited summary data from a new deep penetration seismic reflection line lisa cruise 1995 onland seismic recording lisa and marine esps expanding spread profiles croc ii cruise 1981 are integrated to study the crustal structure of the north ligurian basin the correlation of these different seismic data located along an antibes ile rousse transect provides accurate information on the nature of the crust on the provencal margin the multichannel seismic mcs profile lisa01 shows a major step in the basement this structure is associated with a crustal change corresponding to the ocean boundary on the corsica margin we do not observe such a structure and the ocean boundary is constrained by magnetic anomalies these anomalies are identical to those observed on the provencal margin above the main basement structure on the mcs line the messinian salt and the bottom water multiple obscure the crustal image below 7 s two way traveltime twtt nevertheless the esps provide information about the crust below 7 s twtt these esps were reprocessed and analysed by matching traveltime and amplitude variations in the x t and t p domains the main result is the identification of a transitional zone on the continental margin provencal and corsica margin charac terized by a layer with velocities of 7 2 7 3 km s x1 in the lower crust recordings from land stations on the corsica margin define the geometry of this layer in the centre of the basin the oceanic crust is about 6 km thick from the corsica margin to the centre of the basin the moho depth decreases from 18 to 13 km the velocity model deduced from offshore data is consistent with the esp velocity model and mcs line lisa01 the combined analysis of these data allows us to propose a new boundary of the oceanic crust in the north ligurian basin

overriding plate structure of the nicaragua convergent margin relationship to the seismogenic zone of the 1992 tsunami earthquake we present 2 d seismic velocity models and coincident multichannel seismic reflection images of the overriding plate and the inter plate boundary of the nicaragua convergent margin along two wide angle seismic profiles parallel and normal to the trench acquired in the rupture area of the 1992 tsunami earthquake the trench perpendicular profile runs over a seamount subducting under the margin slope at the location where seismological observations predict large coseismic slip along this profile the igneous basement shows increasing velocity both with depth and away from the trench reflecting a progressive decrease in upper plate rock degree of fracturing upper mantle like velocities are obtained at approximate to 10 km depth beneath the fore arc sandino basin indicating a shallow mantle wedge a mismatch of the inter plate reflector in the velocity models and along coincident multichannel seismic profiles under the slope is best explained by approximate to 15 velocity anisotropy probably caused by subvertical open fractures that may be related to fluid paths feeding known seafloor seepage sites the presence of a shallow partially serpentinized mantle wedge and the fracture related anisotropy are supported by gravity analysis of velocity derived density models the downdip limit of inter plate seismicity occurs near the tip of the inferred mantle wedge suggesting that seismicity could be controlled by the presence of serpentinite group minerals at the fault gouge near the trench the inferred local increase of normal stress produced by the subducting seamount in the plate boundary may have made this fault segment unstable during earthquake rupture which could explain its tsunamigenic character

nanoparticle factories biofilms hold the key to gold dispersion and nugget formation biofilms living on gold au grains play a key role in the biogeochemical cycle of au by promoting the dispersion of au via the formation of au nanoparticles as well as the formation of secondary biomorphic au gold grains from queensland australia are covered by a polymorphic organic inorganic layer that is up to 40 m thick it consists of a bacterial biofilm containing au nanoparticles associated with extracellular polymeric substances as well as bacterioform au focused ion beam fib sectioning through the biofilm revealed that aggregates of nanoparticulate au line open spaces beneath the active biofilm layer these aggregates bacterioform au type 1 resulted from the reprecipitation of dissolved au and their internal growth structures provide direct evidence for coarsening of the au grains at the contact between the polymorphic layer and the primary au bacterioform au type 2 is present it consists of solid rounded forms into which crystal boundaries of underlying primary au extend and is the result of dealloying and ag dissolution from the primary au this study demonstrates that 1 microbially driven dissolution precipitation and aggregation lead to the formation of bacterioform au and contribute to the growth of au grains under supergene conditions and 2 the microbially driven mobilization of coarse au into nanoparticles plays a key role in mediating the mobility of au in surface environments because the release of nanoparticulate au upon biofilm disintegration greatly enhances environmental mobility compared to au complexes only

from mantle to crust stretching the mediterranean abstract the origin of forces driving the deformation of the continental crust near subduction zones and especially in backarc regions is debated this work is based on a compilation of sks fast splitting directions that give an image of flow lines in the mantle around the mediterranean subduction zones and a comparison with stretching and shear directions in metamorphic core complexes that show the pattern of deformation at the scale of the middle and lower crusts we find that 1 the two sets of directions are parallel in the three main backarc regions namely the alboran sea the tyrrhenian sea and the aegean sea showing that the lithosphere deforms with the same direction of stretching in the crust and the mantle suggesting that 2 crustal deformation is mainly driven from below by slab retreat and 3 the lithospheric fabric is reset within a few millions of years in backarc environments

erosion and flexural uplift along transform faults summary in all geodynamic settings intracontinental continent ocean intraoceanic the main morphological feature of transform plate boundaries is a narrow and elongated valley bounded on at least one side by an uplifted shoulder several mechanisms have been proposed to explain this uplift differential thermal subsidence lateral heat transfer extension perpendicular to the transform but none can be used in all settings we propose that the erosion of a lithospheric plate along the transform boundary may be an alternative mechanism that can be active in all geodynamic settings erosional unloading produces flexural uplift along the transform border which does not exceed 35 40 per cent of the difference in height between the lithospheric plate and the transform valley our model perfectly fits the morphology of several examples in the continental lithosphere the uplifted shoulder of the dead sea transform as well as marginal ridges in continental transform margins along an oceanic transform fault the erosion of the transform border only partially explains the uplift of the highest transverse ridges but may amplify other mechanisms

active tectonics of the algerian atlas mountains evidence from aftershocks of the 1980 el asnam earthquake summary the el asnam algeria earthquake of 1980 october is the largest earthquake to have occurred in north africa since instrumental records began ms 7 3 it was caused by movement on a segmented reverse fault which is part of the fault zone that separates the chelif alluvial plains from the coastal range of the atlas mountains the coastal range is the actively deforming plate boundary between africa and eurasia in this paper the results of an aftershock study carried out using a 28 station portable network in the epicentral area in the 5 weeks after the main shock are presented a total of 4517 aftershocks have been located using both p and s phases of these 4517 events 1279 pass a set of quality criteria and are used to discuss the detailed tectonics of the aftershock zone the southwestern half of the aftershock zone has a relatively restricted band of seismicity running parallel to the main fault in cross section this seismicity is observed to lie almost exclusively in the footwall of the main fault and indicates both antithetic reverse faulting and extension parallel to the fault zone the only exception to this pattern occurs at a junction of the main fault segments where a distinct cluster of events with very variable focal mechanisms is located in the hangingwall this cluster may be equated with a barrier that impeded the rupture of the main shock beyond the southwest end of the main fault a mixture of strike slip and reverse faulting occurred and suggests a mechanism whereby this end of the fault can act as an asperity for earthquake generation aftershocks in the northeastern half of the study area indicate movement on a stack of listric reverse faults the focal mechanisms of these events show that the n dipping nodal planes assumed to be the fault planes flatten with depth we suggest that the faults sole into a low angle 20 decollement at about 8 10 km depth at the base of the aftershock zone we argue that the northeastern part of the 1980 aftershock zone is typical of the actively deforming coastal belt of northwest algeria subduction of the western mediterranean basin beneath the north african margin does not seem to occur instead the margin is absorbing the motion with old normal faults being reactivated as thrusts

streamer tomography velocity models for the gulf of corinth and gulf of itea greece summary the gulf of corinth goc greece is a rapidly extending active continental rift with a record of large damaging earthquakes an extensive multichannel seismic mcs survey of the goc conducted in 2001 provided in addition to the processed mcs images the opportunity to constrain velocity structure using refracted arrivals recorded along a 6 km long streamer we use first arrival traveltimes to derive tomographic p wave velocity models for several profiles collected in the central portion of the goc eight of the profiles are closely spaced north south lines crossing the goc and extending into the gulf of itea goi a ninth profile is an east west oriented tie line the n s profiles image the relatively simple velocity structure of the deep corinth rift basin and more complicated structure of the northern margin of the currently active rift integration of the velocity models with migrated mcs sections shows that south of the goi the basement which comprises mesozoic nappes occurs at a velocity of 4 5 km s 1 in the velocity models although the actual velocity at or just below the top of basement is probably closer to 5 5 5 km s 1 the maximum sediment thickness in the corinth basin is 2 2 km the basement shallows to the north into a fault bounded terrace in the central region between the two gulfs sediment cover in this central region decreases in thickness from west to east beneath the goi low average velocities beneath the rift onset reflector indicate the presence of pre rift sediments the pre rift velocity structure in the goi is complex with significant lateral variation from west to east the e w line shows that high velocity basement is shallow 1 km depth and flat to the west of the goi but dips 20 east down to 1 5 km beneath the pre rift sediments of the goi

the gibraltar subduction a decade of new geophysical data the gibraltar arc spans a complex portion of the africa eurasia plate boundary marked by slow oblique convergence and intermediate and deep focus seismicity the seemingly contradictory observations of a young extensional marine basin surrounded by an arcuate fold and thrust belt have led to competing geodynamic models delamination and subduction geophysical data acquired in the past decade provide a test for these models and support a narrow east dipping subduction zone seismic refraction studies indicate oceanic crust below the western gulf of cadiz tomography of the upper mantle reveals a steep east dipping high p wave velocity body beneath gibraltar the anisotropic mantle fabric from sks splitting shows arc parallel fast directions consistent with toroidal flow around a narrow westward retreating subducting slab the accompanying wsw advance of the rif betic mountain belt has constructed a thick pile of deformed sediments an accretionary wedge characterized by west vergent thrust anticlines bathymetric swath mapping images an asymmetric embayment at the deformation front where a 2 km high basement ridge has collided subduction has slowed significantly since 5 ma but deformation of recent sediments and abundant mud volcanoes suggest ongoing activity in the accretionary wedge three possible origins for this deformation are discussed gravitational spreading overall nw se convergence between africa and iberia and finally a wsw tectonic push from slow but ongoing roll back subduction in the absence of arc volcanism and shallow dipping thrust type earthquakes evidence in favor of present day subduction can only be indirect and remains the object of debate continued activity of the subduction offers a possible explanation for great m 8 5 earthquakes known to affect the area like the famous 1755 great lisbon earthquake recent gps studies show sw motion of stations in n morocco at velocities of 3 6 mm yr indicating the presence of an independent block a rif betic alboran microplate situated between iberia and africa

geophysical evidence for a transform margin offshore western algeria a witness of a subduction transform edge propagator for the first time a deep seismic data set acquired in the frame of the algerian french spiral program provides new insights regarding the origin of the westernmost algerian margin and basin we performed a tomographic inversion of traveltimes along a 100 km long wide angle seismic profile shot over 40 ocean bottom seismometers offshore mostaganem northwestern algeria the resulting velocity model and multichannel seismic reflection profiles show a thin 3 4 km thick oceanic crust the narrow ocean continent transition less than 10 km wide is bounded by vertical faults and surmounted by a narrow almost continuous basin filled with miocene to quaternary sediments this fault system as well as the faults organized in a negative flower structure on the continent side marks a major strike slip fault system the extremely sharp variation of the moho depth up to 45 3 degrees beneath the continental border underscores the absence of continental extension in this area all these features support the hypothesis that this part of the margin from oran to tenes trending n65 n70 degrees e is a fossil subduction transform edge propagator fault vestige of the propagation of the edge of the gibraltar subduction zone during the westward migration of the alboran domain

propagation of a lithospheric tear fault step through the western boundary of the calabrian accretionary wedge offshore eastern sicily southern italy the rollback of a segmented slab of oceanic lithosphere is typically accompanied by vertical lithospheric tear fault s along the lateral slab edge s and by strike slip movement in the upper plate defined as a step fault subduction tear edge propagator the neogene evolution of the central mediterranean is dominated by the interaction between the slow africa eurasia convergence and the se ward rollback of the ionian slab that leads to the back arc opening of the tyrrhenian sea here we present post stack time migrated and pre stack depth migrated archimede 1997 multichannel seismic lines that were acquired offshore eastern sicily at the foot of the malta escarpment first we identify the recent deformation along the lateral ramp of the calabrian accretionary wedge towards the east the calabrian wedge is formed by the accretion of the post evaporitic sediments above a decollement at the base of the messinian evaporites at the latitude of syracuse 50 km east of the malta escarpment a major n150 degrees e trending crustal scale and vertical fault slices through the entire accretionary wedge this fault cuts by several kilometers through the pre evaporitic messinian sediments and into the basement the vertical offset along this vertical fault decreases from north to south and the fault is no longer observed on the seismic lines 50 km se of the alfeo seamount a previously published moho depth isocontour map offshore sicily and the recent gps data combined with the presence of strike slip movements ne onshore sicily allow us to identify this 200 km long crustal scale fault as the surface expression of a step fault the presence of syntectonic pleistocene sediments on top this crustal scale fault suggests a recent lithospheric vertical movement of the step fault in response to the rollback of the ionian slab and to the se ward advance of the calabria peloritan block

the southernmost margin of the tethys realm during the mesozoic and cenozoic initial geometry and timing of the inversion processes 1 along the northern border of africa pangea breakup has been diachronic during the jurassic the alpine tethys propagated northeastward from the atlantic to the alps during the permian the neo tethys propagated westward from oman to northwestern arabia then a secondary and late branch of neo tethys gave birth to the east mediterranean basin finally the two oceans connected at end of jurassic times achieving the development of africa northern plate boundary by the late cretaceous convergence between africa and eurasia led to the progressive closure of the tethys realm the continental collision is not completely achieved and the different segments of the confrontation zone maghreb central and east mediterranean zagros and oman expose different stages of the process however we emphasize the existence of synchronous geodynamic events from one end of the system to the other although they do not have the same meaning two of them are particularly important the campanian santonian c s event corresponds to 1 obduction and exhumation of high pressure low temperature metamorphic rocks around the arabian promontory 2 inversion along the margins of the east mediterranean basins and 3 lithosphere buckling in the atlas system maghreb and adjacent sahara platform the middle late eocene mle event corresponds to 1 the onset of collision at the northern corner of arabia 2 the onset of slab retreat in the mediterranean and 3 inversion along the margin of the east mediterranean as well as in the atlas the c s event coincides with a change in plate kinematics resulting in an abrupt increase of convergence velocity the mle event coincides with a period of strong coupling between the africa and eurasia plates and an abrupt decrease of convergence velocity in the middle of the system the central mediterranean seems to escape to the effects of convergence and is the site of quite permanent extensional movements since the triassic

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

evidence for active subduction beneath gibraltar comment and reply comment the new seismic reflection and tomographic data from the gibraltar region presented by gutscher et al 2002 1 will require a reassessment of the recent tectonics of this part of the africa eurasia plate boundary we argue here however that the data do not support their interpretation in terms

morphogenesis and composition of native gold in the chelopech volcanic hosted au cu epithermal deposit srednogorie zone bulgaria native gold is an important economic component of the complex ores of the chelopech high sulphidation volcanic hosted epithermal au cu deposit bulgaria the ore consists of pyrite chalcedonic silica chalcopyrite enargite luzonite tennantite bornite sphalerite galena and numerous other sulphide arsenide and telluride minerals gold is paragenetically associated with most of the arsenic bearing and base metal sulphide minerals the chemistry and morphology of gold grains which were separated from disintegrated ores were systematically studied by scanning electron microscopy sem and electron probe microanalysis epma the gold is characterised by high fineness 950 and a mean composition of 94 1 wt au 5 27 wt ag 0 53 wt cu and 0 10 wt fe gold grains show variable morphology subhedral flakes irregular grains euhedral isometric crystals elongated rods wires and fine fibrous crystals 111 twins fine dendrite like formations spongy gold and polycrystalline grains the densest faces 111 are morphologically the most important morphometric measurements show a pronounced flatness of the gold particles the mean value of the corey factor a measure of flatness from 0 to 1 spherical is 0 14 and the mean length to width value is 1 64 it is suggested that crystal growth of gold took place in small voids fine cracks and intergranular space the geometry of which have controlled the highly variable grain shapes no direct correlation between the size and composition of gold grains exists but larger grains tend to be of higher fineness

contrasting crustal production and rapid mantle transitions beneath back arc ridges the eastern lau spreading centre located on the lau back arc basin hydrothermal vent system between fiji and tonga is a focus site of the ridge 2000 research programme of the us national science foundation the first large scale seismic study of the area reveals large and abrupt across and along strike changes in crustal properties implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes the authors conclude that stable broad triangular upwelling regions as inferred for mid ocean ridges cannot form near the mantle wedge corner instead the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water rich low viscosity mantle when it is near the arc as the ridge moves away from the arc a tipping point is reached at which the material is rapidly released resulting in rapid changes in the character of the crust formed at the ridge this study shows that across strike and along strike changes in crustal properties at the eastern lau spreading centre are large and abrupt implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes it is concluded that stable broad triangular upwelling regions as inferred for mid ocean ridges cannot form near the mantle wedge corner instead the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water rich low viscosity mantle when it is near the arc as the ridge moves away from the arc a tipping point is reached at which that material is rapidly released resulting in rapid changes in the character of the crust formed at the ridge the opening of back arc basins behind subduction zones progresses from initial rifting near the volcanic arc to seafloor spreading1 during this process the spreading ridge and the volcanic arc separate and lavas erupted at the ridge are predicted to evolve away from being heavily subduction influenced with high volatile contents derived from the subducting plate 2 3 4 5 current models4 6 7 8 predict gradational rather than abrupt changes in the crust formed along the ridge as the inferred broad melting region beneath it migrates away from heavily subduction influenced mantle in contrast here we show that across strike and along strike changes in crustal properties at the eastern lau spreading centre are large and abrupt implying correspondingly large discontinuities in the nature of the mantle supplying melt to the ridge axes with incremental separation of the ridge axis from the volcanic front of as little as 5 km seafloor morphology changes from shallower complex volcanic landforms to deeper flat sea floor dominated by linear abyssal hills upper crustal seismic velocities abruptly increase by over 20 and gravity anomalies and isostasy indicate crustal thinning of more than 1 9 km we infer that the abrupt changes in crustal properties reflect rapid evolution of the mantle entrained by the ridge such that stable broad triangular upwelling regions as inferred for mid ocean ridges9 10 cannot form near the mantle wedge corner instead the observations imply a dynamic process in which the ridge upwelling zone preferentially captures water rich low viscosity mantle when it is near the arc as the ridge moves away from the arc a tipping point is reached at which that material is rapidly released from the upwelling zone resulting in rapid changes in the character of the crust formed at the ridge

crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin implications for thermal exchanges across the palaeotransform boundary summary crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin is determined by 2 d modelling of wide angle seismic data and gravity data the resulting models together with geological data recently collected by drilling on the margin are compared with available thermomechanical models of transform margins in particular we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere these effects include i ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it from the continent towards the ocean and ii thermal uplift of the transform margin in the region of the cote d ivoire ghana transform margin the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent ocean boundary is approached the interpreted models lack a transitional domain between continental and oceanic crusts and abnormally thin oceanic crust is found along the margin these results exclude large scale ductile flow of lower continental crust during the margin formation the geological data are moreover not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere moderate heating of the cote d ivoire ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models two main factors may have contributed to reduce thermal exchanges across the continent ocean boundary the first factor is that the oceanic edge may have been colder than assumed by thermal models an abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the cote d ivoire ghana transform margin the oceanic crustal structure is similar to that found along oceanic fracture zones where oceanic crust is also emplaced near a ridge transform intersection in both cases abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of i a cold edge effect of the adjacent continental lithosphere ii accretion at the end of a ridge segment away from the main magma supply point along the spreading axis and iii an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere the second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults which reduces the lateral thermal conduction in the lithosphere since water is much less conductive than rock during the margin formation the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre we propose that this thermal buffer hypothesis may be generalized to most transform margins implying a lower amount of continental heating than assumed by the available thermomechanical models

the alboran sea western mediterranean revisited with a view from the moroccan margin this paper presents a new depth to acoustic basement map based on a grid of seismic profiles in the western alboran basin the basement is more than 10 km deep an examination of a seismic profile of this region suggests that late oligocene early miocene 5 km thick synrift formation has been accumulated in this basin this west alboran basin has been formed in an extensional setting but in a fore arc position south of the balearic islands then westwards transported behind a subduction zone

noble gases k u th and pb in native gold we present determinations of the noble gas and pb isotopic abundances and of k th and u concentrations of native gold our results demonstrate that gold is an excellent carrier for crustal volatiles but direct dating of gold using the u th 4he 40k 40ar and u fission xe methods was not successful for various reasons the main significance of this work is the great sensitivity of gold for trapped gases as well as for gases that were produced in situ which gives the prospects of using gold and its fluid and solid inclusions for the study of paleogas composition numerous nuclear effects characterize the noble gas inventory of placer gold from switzerland and italy vein gold from italy south africa and venezuela and lode gold from south africa the degassing patterns obtained by mass spectrometry show a low temperature release of volatiles around 500 c from fluid inclusions mainly in vein gold and a high temperature release from solid inclusions and the gold itself the low temperature volatiles represent species that were trapped when the gold crystallized we investigated the following trapped species the isotopes of he ne ar kr xe and pb and the abundances of k u th h2o and co2 the crustal gases trapped by gold comprise 3he from 6li n 3h 3he 4he and 40ar from the u th and k decay and xe from 238u fission we observe 4he 40ar 3 9 for the radiogenic trapped gases of tertiary gold and a ratio of 1 4 for archean gold these ratios are consistent with the production ratios from u and k at the respective times and demonstrate that gold can be used as a sampler of ancient atmospheric gases the concentrations of u and th range from a few parts per billion to a few parts per million and those of k and pb range up to some tens of parts per million the antiquity of trapped pb is indicated by the pb pb model age of about 3000 ma for the lead extracted from vein gold and quartz of the lily gold mine south africa gold also contains noble gases produced in situ by u th and k decay we obtained 4he up to the extremely high concentration of 6 8 10 2 cm3 stp g stp atmospheric pressure and 0 c for gold separated from a specimen of the witwatersrand west rand gold fields indicating that this gold has a large u content

quantitative analysis of submarine flow deposit shape in the marnoso arenacea formation what is the signature of hindered settling from dense near bed layers abstract submarine sediment density flows are one of the volumetrically most important processes for sediment transport across earth the sediment concentration of flows that reach the deep ocean has never been measured directly and understanding these long runout flows remains a major challenge the miocene marnoso arenacea formation in the italian apennines is the only ancient sequence where individual submarine sediment density flow deposits single beds have been mapped out for more than 100 km down flow here we document the external shape and internal architecture of thirty two individual beds that record flow evolution and can be compared to deposit shapes in mathematical or experimental models the large number of beds allows modes of flow behavior to be identified larger volume turbidites are typically dominated by massive ta or planar laminated tb sandstone intervals that have a broad thickness maximum this shape is important because it suggests that massive and planar laminated sandstones record hindered settling from dense near bed layers which have high 10 by volume sediment concentrations previously some authors have inferred that planar laminated sandstones tb are deposited mainly by dilute flows the position of the broad thickness maximum moves basinward as the volume of sand in the flow increases this is consistent with mathematical modeling that suggests the position of the thickness maximum depends on flow thickness flow speed and sediment settling velocity as well as sediment concentration variations in seawater entrainment rate and local changes in seafloor gradient smaller volume turbidite sandstone intervals are finer grained and dominated by ripple cross lamination tc and have a near exponential decay in thickness that is consistent with deposition from a dilute sediment suspension the rate of near exponential thinning is controlled by sandstone volume in contrast turbidite mudstone intervals show an approximately linear increase in thickness with distance flows that entered the basin in opposite directions produced turbidite mudstone intervals that thicken towards the same location indicating that muddy turbidity currents can drain back over long distances to basinal lows

nanoforms of gold from abandoned placer deposits of wądroze wielkie lower silesia poland the evidence of authigenic gold mineralization abstract being the most recently exposed gold grains from historical tailings of underground and surface workings offer the opportunity to study supergene transformations occurring over historic periods in this study gold grains were collected from the abandoned historically mined deposits of w droze wielkie at lower silesia all three distinct gold sub types of detrital gold display surface morphotypes and internal textures of au and ag dissolution indicative of supergene gold modification as well as authigenic au nanoparticle formation and aggregation the indicative morphotypes include nano particulate semispherical or bubble like gold and aggregates of bacteriomorphic gold that could be evidence for microbial gold biomineralization sem investigation of the microtopography and internal texture of gold grains revealed indications of post depositional dissolution of au ag alloys and redistribution of these components within the sediment some of seed and bubble like growths are also embedded in clayey kaolinite masses within crevices suggesting that dissolution re precipitation processes occur at the gold grain interface authigenic gold occurs as overgrowths and aggregates of nearly pure gold on a detrital au ag alloy that are readily removed during transport but are replaced by re precipitation occurring over historic rather than geological periods

crustal structure of a young margin pair new results across the liguro provencal basin from wide angle seismic tomography tomographic inversion of two wide angle seismic profiles acquired during the sardinia cruise 2006 on the conjugate gulf of lions west sardinia margins pair offers insight into the deep structure of this young basin it is the first combined conjugate margins study in this area based on deep seismic data modelling of the two conjugate profiles reveals that the conjugate margins are symmetric in term of crustal velocity structure with a partitioning in three similar regions 1 a zone of low crustal vertical velocity gradients and a crustal thickness of 18 to 10 km interpreted to be thinned continental crust 2 a transitional zone characterised by high lower crustal velocities non typical of continental or normal oceanic crust and 3 a 4 to 5 km thick crust showing vertical velocity gradients and relative velocity ranges typical of oceanic crust this latter region is interpreted to be a thin oceanic crust resulting from a tectonic heritage and the possible influence of a cool slab back arc basin although the three regions are found on both sides of the margin they are asymmetric in terms of the width of these domains wide and smooth along the gulf of lions margin and narrow and abrupt along its conjugate sardinia side the width of region 2 is larger on the gulf of lions side 90 km than on the sardinia side 40 km crustal thickness in region 3 is about 1 km thinner on the sardinia side than on its conjugate in region 1 the crustal thickness is lower on the sardinia side at a given distance from the shelf break the differences in width of region 2 and in crustal thickness might be due to the origin as a back arc basin of the nw mediterranean basin with the final breakup located closer to the western sardinia margin also probably linked to the complex tectonic history of the region our results do not support a simple shear mechanism of the opening of the basin along a lithospheric detachment fault

crustal types and tertiary tectonic evolution of the alboran sea western mediterranean multichannel seismic reflection images across the transition between the east alboran and the algero balearic basins show how crustal thickness decreases from about 5 s two way traveltime twtt 15 km thick in the west east alboran basin to 2 s twtt typical of oceanic crust 6 km thick in the east algero balearic basin we have differentiated three different crustal domains in this transition mainly on the basis of crustal thickness and seismic signature boundaries between the three crustal domains are transitional and lack evidence for major faults tilted blocks related to extension are very scarce and all sampled basement outcrops are volcanic suggesting a strong relationship between magmatism and crustal structure stratigraphic correlation of lithoseismic units with sedimentary units of southeastern betic basins indicates that sediments onlap igneous basement approximately at 12 ma in the eastern area and at 8 ma in the western area linking seismic crustal structure with magmatic geochemical evidence suggests that the three differentiated crustal domains may represent from west to east thin continental crust modified by arc magmatism magmatic arc crust and oceanic crust middle to late miocene arc and oceanic crust formation in the east alboran and algero balearic basins respectively occurred during westward migration of the gibraltar accretionary wedge and shortening in the betic rif foreland basins arc magmatism and associated backarc oceanic crust formation were related to early to middle miocene subduction and rollback of the flysch trough oceanic basement subduction of this narrow slab beneath the alboran basin was coeval with collision of the alboran domain with the iberian and african passive margins and subsequent subcontinental lithosphere edge delamination along the betic rif margins

magmatic evolution of the alboran region the role of subduction in forming the western mediterranean and causing the messinian salinity crisis the magmatic evolution of the alboran region westernmost mediterranean contains important clues for improving our understanding of the origin of mediterranean style back arc basins and the desiccation of the mediterranean sea in the messinian we use new laser 40ar 39ar age and geochemical major and trace element and o sr nd pb isotope data from igneous rocks from southern spain the alboran sea and northern morocco to reconstruct the magmatic evolution of the westernmost mediterranean since the eocene lower oligocene dikes near malaga 33 6 0 6 ma and middle to upper miocene volcanic rocks from the alboran sea area 6 57 0 04 to 11 8 0 4 ma can be subdivided into two groups 1 lree depleted relative to n morb primarily tholeiitic series and 2 lree enriched primarily calc alkaline series volcanic rocks both groups are generally enriched in fluid mobile elements e g rb th u k and pb relative to fluid immobile elements e g nb ta lree the lree depleted group has 143nd 144nd 0 5128 0 5130 isotope ratios similar to atlantic morb but higher 87sr 86sr 0 7046 0 7100 in contrast the lree enriched group has less radiogenic nd 0 5121 0 5126 and tend to more radiogenic sr 0 7066 0 7205 isotopic composition pb isotope ratios are surprisingly uniform and have compositions similar to marine sediments analyses of mineral separates show that mafic melts with relatively low 18o 5 6 7 2 had high 87sr 86sr 0 7048 0 7088 7 4 10 6 14 1 and 8 4 40 0 49 3 modeling of the trace elements and sr nd pb o isotopic compositions provides compelling evidence for the contamination of the mantle source with hydrous fluids melts which can be explained through subduction of oceanic lithosphere beneath the alboran basin but not through detachment delamination of lithospheric mantle we present a geodynamic model that reconstructs the late eocene to quaternary evolution of the western mediterranean through westward roll back of subducted tethys oceanic lithosphere slab roll back resulted in a large scale reorganization of the western mediterranean paleogeography causing the closure of marine gateways linking the atlantic ocean to the mediterranean sea isolation of the mediterranean sea led to its desiccation causing the messinian salinity crisis

gold nugget growth during tectonically induced sedimentary recycling otago new zealand abstract an alluvial fan 5 km wide and 3 5 km long has been derived over the past 65 000 years from an actively rising schist mountain range dunstan range in central otago new zealand the fan can be locally subdivided into four distinctive consecutively formed gravel sequences the earliest of which has been deformed along the range margin these gravels are pre dated by miocene mature fluvial sediments manuherikia group which have been recycled into the alluvial fan gravels during uplift all these sedimentary sequences contain gold which has been mined sporadically manuherikia gold grains up to 13 wt ag are well rounded and typically 300 m across typical grain size of gold becomes progressively coarser with each gravel sequence in the alluvial fan and nuggets up to 2 cm occur in younger gravel sequences gold grain size increase is due to addition of authigenic gold on to pre existing grains authigenic gold occurs as a cement between earlier detrital grains 1 4 wt ag as amorphous coatings and protrusions 0 1 wt ag and as overgrowths of lattice intergrowths 0 1 wt ag gold nuggets formed in the gravels have overgrown detrital minerals and authigenic clay minerals chemical accretion of nuggets may be still continuing today in parts of the alluvial fan

multiphased tectonic evolution of the central algerian margin from combined wide angle and reflection seismic data off tipaza algeria 1 the origin of the algerian margin remains one of the key questions still discussed in the western mediterranean sea due to the imprecise nature and kinematics of the associated basin during the neogene for the first time the deep structure of the maghrebian margin was explored during the spiral seismic survey in this work we present a n s transect off tipaza west of algiers a place where the margin broadens due to a topographic high khayr al din bank new deep penetration seismic profiles allow us to image the sedimentary sequence in the algerian basin and the crustal structure at the continent ocean boundary modeling of the wide angle data shows thinning of the basement from more than 15km in the continental upper margin to only 5 6km of oceanic type basement in the algerian basin and reveals a very narrow or absent transitional zone analysis of the deep structure of the margin indicates features inherited from its complex evolution 1 an oceanic type crust in the deep basin 2 similarities with margins formed in a transform type setting 3 a progressive deepening of the whole sedimentary cover and the thickening of the plio quaternary sediments at the margin foot coeval with 4 a downward flexure of the basement in the basin these features argue for a multiphased evolution of the margin including 1 an early stage of rifting and or spreading 2 a late transcurrent episode related to the westward migration of the alboran domain and 3 a diffuse plio quaternary compressional reactivation of the margin

a miocene tectonic inversion in the ionian sea central mediterranean evidence from multichannel seismic data it is widely accepted that the central and eastern mediterranean are remnants of the neo tethys however the orientation and timing of spreading of this domain remain controversial here we present time migrated and pre stack depth migrated nw se oriented archimede 1997 lines together with the prismed01 1993 profile to constrain the evolution of the ionian basin our interpretation allows us to identify a large scale set of sw ne striking reverse faults beneath the ionian abyssal plain these primarily nw vergent faults are characterized by a spacing comprised between 10 to 20 km and a dip ranging from 60 to 65 degree sign following very recent paleogeographic reconstructions we propose that the set of n degree sign 55 features initially formed as normal faults during the nw se trending seafloor spreading of the ionian basin after its late triassic early jurassic rifting based on geometric comparisons with the intraplate deformation observed beneath the central indian ocean we show that the inherited oceanic normal faults were reactivated under compression as reverse faults well developed tortonian syntectonic basins developed nw of the major faults and the base of the messinian evaporites mobile unit is slightly folded by the activity of the faults we show that 3 4 km of total shortening occurs over a 80 km wide area beneath the ionian abyssal plain resulting in a bulk shortening of 3 5 5 we propose a link between the tortonian early messinian inversion of the fault pattern and a plate tectonic reorganization prior to the main phase of back arc opening of the tyrrhenian domain