living on the edge evaluating the impact of stress factors on animal sediment interactions in subenvironments of a shelf margin delta the mayaro formation trinidad abstract integrating sedimentological and ichnological characteristics of the gelasian mayaro formation along the se coast of trinidad allows recognition of a river dominated and wave influenced shelf edge deltaic succession of the paleo orinoco river developed under strong slope instability four main sedimentary settings and twelve subenvironments developed on and beyond the outer shelf i e at the shelf margin have been identified extreme paleoenvironmental conditions make the succession rarely and sporadically colonized ichnologic evidence suggests that shelf edge deltas are among the most stressful marine environments due to a combination of physicochemical factors in response to the intrinsic sedimentary processes and the relative hierarchy of their influences specific to every subenvironment within any particular subenvironment the relative dominance of the fluvial feeder system the action of waves and rare influence of tides and slope instability determine the combinations and ranking of stress factors river dominated shelf edge delta subenvironments demonstrate the extreme influence of stress factors related to channel activity e g salinity changes high sedimentation and erosion rates flocculation of mud seasonal variations of fluvial flux whereas subenvironments dominated by waves storms and oceanic swells away from the axial feeder s are mostly influenced by barform morphodynamics and its effects on local physiography the paleo orinoco delta is represented not only by the delta lobe s developed at the shelf edge but by delta lobe s formed on the outer shelf as well outer shelf deltaic subenvironments manifest typical ichno sedimentological signatures of a normal wave influenced in this case inner shelf delta being susceptible to extensive gravitational instabilities the delta system suffered from canyonization and smaller gullying and subsequent filling n b hereafter we use the term canyon for both canyons and smaller gullies the shelf edge delta system was also associated with shelf attached mass transport systems such depositional subenvironments dominated by slope instability and sediment gravity flows appear to be the most unconducive for benthic colonization resulting in almost complete absence of bioturbation characterization of the stress factors as the functions of parameters arising from specific sedimentary and or oceanographic processes chemical conditions and preservational constraints specific to each subenvironment leads to the construction of a comprehensive ichnological and depositional analog model for shelf edge deltas in general and for the accommodation driven low latitude shelf edge deltas at an active oblique foreland setting in particular

comparison of smectite and illite rich gouge frictional properties application to the updip limit of the seismogenic zone along subduction megathrusts along plate boundary subduction thrusts the transformation of smectite to illite within fault gouge at temperatures of 150 c is one of the key mineralogical changes thought to control the updip limit of seismicity if correct this hypothesis requires illite rich gouges to exhibit frictionally unstable velocity weakening behavior here we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay rich gouges we sheared 5 mm thick layers of commercially obtained pure ca smectite a suite of smectite quartz mixtures and natural illite shale grain size ranging from 2 to 500 m in the double direct shear geometry to shear strains of 7 30 at room humidity and temperature xrd analyses show that the illite shale contains dominantly clay minerals and quartz within the clay sized fraction 40 mpa our data specifically the velocity strengthening behavior of illite shale under a wide range of conditions do not support the hypothesis that the smectite illite transition is responsible for the seismic aseismic transition in subduction zones we suggest that other depth and temperature dependent processes such as cementation consolidation and slip localization with increased shearing may play an important role in changing the frictional properties of subduction zone faults and that these processes in addition to clay mineralogy should be the focus of future investigation

spatangoid produced ichnofabrics bateig limestone miocene spain and the preservation of spatangoid trace fossils a spatangoid produced ichnofabric is described from the miocene bateig limestone se spain this ichnofabric is characterized by the dominant presence of large meniscate burrows bichordites produced by irregular echinoids this constitutes an unusual mode of occurrence for spatangoid bioturbation as their traces are most typically preserved in bases and tops of sandstone event beds in fact despite their important role as burrowers in modern settings that can be extended back to the early cretaceous based on their body fossil record spatangoid trace fossils scolicia and bichordites are comparatively rare several factors play an important role in their preservation mechanism of burrowing sediment characteristics early diagenesis and presence absence of deep tier burrowers spatangoid produced ichnofabrics such as those from the bateig limestone characterize depositional settings with intermittent deposition of event beds where there is an absence of deeper tier bioturbation

trace fossils biology and taphonomy 1990 special topics in palaeontology 3 280 pp e17 95 pbk e40 00 hbk unwin hyman london isbn 004445 6867 pbk 3035 hbk

crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

the influence of sediment blanketing on subduction zone seismicity abstract subduction of oceanic lithosphere is now occurring beneath the aegean and central caspian seas and beneath the indo burman ranges in ne india generating zones of earthquakes that reach depths of more than 150 km their existence is surprising in general where the temperature of the mantle part of the oceanic lithosphere can be estimated earthquakes are confined to material whose temperature is below 600 c in these three regions the oceanic lithosphere is overlain by sediments with low thermal conductivity whose thickness is probably 10 25 km the temperature of the oceanic crust and upper half of the lithosphere should be considerably increased by such a thick sedimentary cover yet there is no obvious difference between these subduction zones and those where there is little sediment on the subducting plate detailed thermal modelling of the temperature shows that for the first 40 ma the deposition of even a thick sedimentary layer has little effect on the thickness of the region of lithosphere whose temperature is below 600 c principally because little heat is conducted across the moho the modest effect of sediment blanketing over a period as long as 40 ma was quite unexpected over the next 200 ma the lithospheric temperature increases until the moho temperature reaches 600 c this behaviour has important implications for oil and gas generation in the sediments and for the existence of large earthquakes at depths of 150 km or more beneath the hindu kush pamirs and romania

organism substrate interactions and astrobiology potential models and methods abstract organism substrate interactions and their products biogenic structures are important biosignatures on earth this study discusses the application of ichnology the study of organism substrate interactions to the search for present and past life beyond earth three main questions are addressed 1 why to look for biogenic structures i e traces and ichnofabrics beyond earth 2 what biogenic structures to expect on other planets moons and asteroids 3 how to study extraterrestrial biogenic structures review of terrestrial evidence highlights a set of properties that make traces and ichnofabrics important for the search of potential extraterrestrial life trace fossils preserve the activity of soft bodied organisms biogenic structures are resilient to processes that obliterate other biosignatures i e mechanical and chemical degradation diagenesis tectonism and metamorphism traces are very visible biosignatures traces indicate environment and behaviour traces can be universal biosignatures i e biosignatures ideally suited for detecting any type of life a model of organism substrate interactions beyond earth is here proposed expected extraterrestrial traces are those that manifest behaviours that allow to maintain homeostasis excavations meandering traces and biodeposition structures most of the existing rovers and orbiters provide basic instruments for searching these traces it is here suggested that the search for extraterrestrial biogenic structures by rovers would also benefit from artificial adjustable lighting gpr lidar and drilling equipment with optical televiewer in this study open access databases of rover and orbiter imagery have been searched for traces and ichnofabrics but no unquestionable evidence of biogenic structures beyond earth has been found besides those produced by humans this sounds along the lines of the famous fermi paradox if the universe is teeming with aliens where are their traces results of this search show that habitable environments are not the only place to look for biogenic structures non habitable environments such as moons without atmosphere can favour the preservation of shallow tier traces the better preservation potential of traces compared to other biosignatures greatly widens the issue of planetary protection including the interaction between astronauts or vehicles and the substrate may produce disturbances although this study highlights a new direction of study with the tools and concepts of ichnology dialogue between the astrobiological and ichnological communities is needed to use its full potential and possibly answer one of the major questions of science does life exist beyond earth

microtectonics of low p low t carbonate fault rocks abstract with the aim of deducing some general microtectonic processes responsible for the development of carbonate fault cores rock samples were collected in ten of such structures which are different in size attitude kinematics displacement and tectonic environment samples were thin sectioned and analysed under an optical microscope microscopic evidence i e at the scale of tens to hundreds of microns shows that grain size reduction occurred mostly by cataclasis and occasionally by pressure solution cataclasis involved three main processes here named intragranular extension fracturing chipping and shear fracturing intragranular extension fracturing is more common in the early stages of cataclasis and produces a coarse breccia consisting of angular grains in a few cases pre existing weaknesses and flaws control the fracture pattern associated with intragranular extension fracturing chipping is more common in the advanced stages of cataclasis and produces a gouge consisting of a few survivor rounded grains within a fine matrix shear fracturing seems less frequent than the other two processes and usually occurs in the advanced stages of cataclasis by considering the microscopic and mesoscopic evidence and the dissimilar frequency of dissolution structures in the analysed fault cores and damage zones it is inferred that the studied fault zones probably acted as conduit barrier permeability systems

earthquakes gravity and the origin of the bali basin an example of a nascent continental fold and thrust belt we infer from the bathymetry and gravity field and from the source mechanisms and depths of the eight largest earthquakes in the bali region that the bali basin is a downwarp in the crust of the sunda shelf produced and maintained by thrusting along the flores back arc thrust zone earthquake source mechanisms and focal depths are inferred from the inversion of long period p and sh waves for all events and short period p waves for two of the events centroidal depths that give the best fit to the seismograms range from 10 to 18 km but uncertainties in depth allow a range from 7 to 24 km the p wave nodal planes that dip south at 13 to 35 7 strike roughly parallel to the volcanic arc and are consistent with thrusting of crust of the bali basin beneath it the positions of the earthquakes with respect to crustal features inferred from seismic and gravity data suggest that the earthquakes occur in the basement along the western end of the flores thrust zone the slip direction for the back arc thrust zone inferred from the orientation of the earthquake slip vectors indicates that the thrusting in the bali basin is probably part of the overall plate convergence as it roughly coincides with the convergence direction between the sunda arc and the indian ocean plate summation of seismic moments of earthquakes between 1960 and 1985 suggests a minimum rate of convergence across the thrust zone of 4 2 mm a the presence of back arc thrusting suggests that some coupling between the indian ocean plate and the sunda arc occurs but mechanisms such as continental collision or a shallow subduction of the indian ocean plate probably can be ruled out the present tectonic setting and structure of the bali basin is comparable to the early forelands of the andes or western north america in that a fold and thrust belt is forming on the continental side of an arc trench system at which oceanic lithosphere is being subducted the bali basin is flanked by the tertiary java basin to the west and the oceanic flores basin to the east and thus provides an actualistic setting for the development of a fold and thrust belt in which structure and timing of deformation can change significantly along strike on the scale a few hundred kilometers

miocene rise of the shillong plateau and the beginning of the end for the eastern himalaya abstract a common feature of convergent plate boundaries is the self organization of strain exhumation and topography along discrete arcuate boundaries deviations from this geometry can represent first order changes in stress applied at a plate boundary that must affect how strain is partitioned within the interior of an orogen the simplicity of the himalayan fold and thrust belt seen along its central portion breaks down along the eastern extremity of the arc where the 400 km long shillong plateau has developed this change in strain partitioning affects nearly 25 of the arc and has not previously been considered to be important to the orogen s development new low temperature thermochronometry data suggest this structure initiated in mid to late miocene time significantly earlier than was previously estimated from the sedimentary record alone development of the shillong plateau may be linked to a number of kinematic changes within the himalayan and burman collision zones that occur at the same time these events include the onset of e w extension in central tibet eastward expansion of high topography of the tibetan plateau onset of rotation of crustal fragments in southeastern tibet and re establishment of eastward subduction beneath the indo burman ranges we suggest that the coincidence of these tectonic events is related to the dismemberment of the eastern himalayan arc signifying a change in regional stress applied along the india eurasia burma plate boundaries discrepancies between vertical long term faulting rates and geodetically derived far field convergence rates suggest that the collisional boundary in the eastern himalayan system may be poorly coupled due to introduction of oceanic and transitional crust into the eastern plate boundary the introduction of dense material into the plate boundary late in the orogen s history may explain regional changes in the strain field that affect not only the himalaya but also the deformation field more than 1000 km into the tibetan plateau

focal depths and fault plane solutions of earthquakes and active tectonics of the himalaya synthetic seismograms were compared with long period body waves for nine earthquakes with epicenters in the himalayan arc to determine depths of foci and to improve fault plane solutions focal depths are shallow 10 20 km inferred slip vectors are locally perpendicular to the mountain range they plunge very gently about 10 deg in the eastern sections of the range and more steeply about 25 deg in western sections assuming india to be a rigid plate the radially oriented slip vectors imply that southern tibet extends at about half the rate of underthrusting in the himalaya and therefore probably at about 5 10 mm yr the shallow depths and gentle dips of the fault planes at least for the events in the eastern half of the range are consistent with coherent underthrusting of the indian plate beneath at least the lesser himalaya the steeper dips of fault planes in the western part of the arc might reflect deformation of the overriding thrust plate or simply a steepening of the main underthrusting zone beneath the greater himalaya

fault gouge evolution and its dependence on normal stress and rock strength results of discrete element simulations gouge zone micromechanics dem simulations of gouge zone evolution

numerical simulation of localisation phenomena in a fault zone a lattice solid model was developed to study the physics of rocks and the nonlinear dynamics of earthquakes and is applied here to the study of fault zone evolution the numerical experiments involve shearing a transform fault model initialised with a weak and heterogeneous fault zone during the experiments a fault gouge layer forms with features that are similar to those observed in recent laboratory experiments involving simulated fault gouge beeler et al 1996 this includes formation of reidel r 1 shears and localisation of shear into bands during the numerical experiments as in the laboratory experiments decreases of the gouge layer strength correlate with decreases in gouge layer thickness after a large displacement a re organisation of the model fault gouge is observed with slip becoming highly localised in a very narrow basal shear zone this zone is such that it enhances the rolling type micro physical mechanism that was responsible for the low heat and fault strength observed in previous numerical experiments mora and place 1998 1999 and proposed as an explanation of the heat flow paradox hfp the long time required for the self organisation process is a possible reason why the weak gouge layers predicted by the numerical experiments and which could explain the hfp have not yet been observed in the laboratory the energy balance of a typical rupture event is studied the seismic efficiency of ruptures of the gouge layer is found to be low approximately 4 substantially lower than previous estimates and compatible with typical field based estimates

controls on the size and geometry of landslides insights from discrete element numerical simulations abstract we use numerical two dimensional discrete element method dem to study the controls on the size and geometry of an individual landslide this method allows us to analyze the temporal and spatial evolution of slope failure and landsliding from the intact pre failure slope to the restabilized post failure slope the size of our simulated landslides reflects the amount of disintegrated material available for sliding which is controlled by the peak strength of the slope material and to a lesser degree by the pre failure slope angle the observed size of natural landslides therefore reflects mainly the local natural material strengths we hypothesize that the observed global characteristic landslide size is a result of the limited thickness generally up to a few meters of disintegrated and weathered material that exists on hillslopes poised and ready for sliding the primary controls on the landslide geometry quantified as the ratio of their thickness to length t l prove to be the residual friction angle of the slope material and the original slope angle the t l is observed to be unchanged for a given initial slope angle and independent of the slope material peak strength or the slope height we suggest that the narrow range of observed natural t l is a result of narrow ranges of mechanical properties and initial slope geometries involved in natural slope failure

crustal thickness and poisson s ratio variations in the northeast india asia collision zone insight into the tuting tidding suture zone eastern himalaya abstract crustal thickness and poisson s ratio were estimated at 11 broadband seismic stations established in a profile along the lohit river valley of arunachal himalaya passing across the tuting tidding suture zone ttsz the average crustal thickness h and poisson s ratio were estimated with the help of h k stacking analysis of receiver functions at each station the study reveals a thickening of crust from 46 km beneath the brahmaputra valley in the west to 55 km in the western part of the lohit plutonic complex lpc marginal uplifts of moho depths 50 52 km are observed at some stations to the eastern part of the profile ltl hwi and sdl stations the estimated poisson s ratio in the brahmaputra valley is low 0 23 suggesting the felsic composition of the crust the poisson s ratio is intermediate in the mishmi thrust zone 0 249 0 261 and in some parts of the lpc high poisson s ratio 0 277 0 293 is observed beneath the ttsz and western part of the lpc indicating the effect of aqueous fluid partial melt present in the crust the seismicity in the study area has been investigated based on data recorded by our network and data obtained from the reviewed catalogue of the international seismological center isc bulletin seismicity is observed to be distributed mainly at a shallow depth range 1 15 km however earthquakes of m 4 0 are primarily concentrated at 25 35 km depths this bimodal distribution of seismicity has a significant role in understanding subsurface configuration beneath the profile

proximal distal ichnofabric changes in a siliciclastic shelf early pliocene guadalquivir basin southwest spain abstract the lower pliocene deposits cropping out along the atlantic coast of cadiz in the conil cabo roche area formed on a shelf gently deepening towards the w nw up to five types of facies can be recognized distributed in belts from proximal to distal parts of the shelf 1 coarse grained sandstone with trough cross stratification facies on the most proximal parts of the shelf inner shelf deposits 2 medium grained sandstone with low angle planar cross stratification facies deposited in a more distal position due to storm currents transporting sediment offshore this facies intercalates with the previous trough cross bedded sandstone 3 massive fine grained sandstone and siltstone facies laterally and vertically related with the previous facies type and representing sediments formed in the middle shelf close to the storm wave base 4 tabular sandstone bed facies corresponding to storm bed deposits intercalated within the two previous facies 5 greenish bluish shale facies accumulated on the outer shelf below storm wave base three ichnofabrics associated with these facies are recognized 1 the two cross bedded facies are dominated by a macaronichnus ichnofabric characterized by macaronichnus and bichordites traces that represent colonization of a shifting sandy substrate by polychaetes macaronichnus tracemakers and by sea urchins bichordites producers 2 a rosselia ichnofabric consisting of vertical traces of rosselia cross cutting macaronichnus characterizes the interval of massive fine grained sandstone and silt facies and interbedded tabular sandstones this ichnofabric formed under high sedimentation rates with rosselia traces exhibiting stacked patterns that indicate vertical readjustment following deposition 3 massive fine grained sandstone and silt facies are distinguished by a cylindrichnus ichnofabric this fabric was produced by the activity of a moderately diverse community dominated by sessile burrow dwelling probably superficial detritus feeding polychaetes rosselia and cylindrichnus tracemakers and vagile deposit feeding polychaetes planolites producers in addition infaunal deposit detritus feeders producing spreiten structures teichichnus are found in distal areas pervasive indistinct mottling of the sediment of this facies was probably due to the activity of very shallow burrowers in the uppermost sediment tiers this proximal distal replacement of ichnofabrics reflects the change of benthic communities across the shelf this distribution does not fit the traditional seilacherian ichnofacies model for soft ground shallow marine settings in which proximal vertical dwelling burrows of the skolithos ichnofacies dominate high energy environments in the pliocene of cadiz shallow tier burrows of deposit detritus feeders characterize the high energy facies instead which fit better with the cruziana ichnofacies this deviation can be related with the particular palaeoenvironmental conditions thus vagile opportunistic deposit feeders colonized the dune foreset when the dunes were stable and the progradation of the structure favored the preservation of the traces

advancement in source estimation techniques using broadband regional seismograms one important constraint on source retrieval from regional seismograms comes from the amplitude difference between various phases such as pnl surface wave sv sh because the misfit errors used in some waveform inversions are normalized by the data and synthetics the amplitude information in the data has not been fully utilized in this article we modify the cut and paste source estimation technique zhao and helmberger 1994 by removing this type of normalization it is shown that the modified method increases the stability and resolution of inversion when multiple stations at different distance ranges are used a distance scaling factor is introduced to compensate for the amplitude decay with distance by applying the technique to the terrascope data we have determined source mechanisms and depths of 335 southern californian events with m l 3 5 the amplitude decays with distance are r 1 13 for pnl r 0 55 for love waves and r 0 74 for rayleigh waves in contrast to generally shallow source depths reported by the southern california short period network the depth distribution from waveform inversion shows a strong peak around 12 km with few earthquakes occurring above 5 km and below 20 km

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

clockwise rotation of the brahmaputra valley relative to india tectonic convergence in the eastern himalaya naga hills and shillong plateau gps data reveal that the brahmaputra valley has broken from the indian plate and rotates clockwise relative to india about a point a few hundred kilometers west of the shillong plateau the gps velocity vectors define two distinct blocks separated by the kopili fault upon which 2 3 mm yr of dextral slip is observed the shillong block between longitudes 89 and 93 e rotating clockwise at 1 15 myr and the assam block from 93 5 e to 97 e rotating at 1 13 myr these two blocks are more than 120 km wide in a north south sense but they extend locally a similar distance beneath the himalaya and tibet a result of these rotations is that convergence across the himalaya east of sikkim decreases in velocity eastward from 18 to 12 mm yr and convergence between the shillong plateau and bangladesh across the dauki fault increases from 3 mm yr in the west to 8 mm yr in the east this fast convergence rate is inconsistent with inferred geological uplift rates on the plateau if a 45 n dip is assumed for the dauki fault unless clockwise rotation of the shillong block has increased substantially in the past 4 8 myr such acceleration is consistent with the reported recent slowing in the convergence rate across the bhutan himalaya the current slip potential near bhutan based on present day convergence rates and assuming no great earthquake since 1713 a d is now 5 4 m similar to the slip reported from alluvial terraces that offsets across the main himalayan thrust and sufficient to sustain a mw 8 0 earthquake in this area

the weakness of earthquake faults numerical experiments using the particle based lattice solid model produce simulated earthquakes model faults with a thin gouge layer are sufficiently weak relative to those without gouge to explain the heat flow paradox hfp stress drop statistics are in agreement with field estimates models with a thick granular fault zone exhibit a strong evolution effect results are initially similar to those of laboratory experiments but after a sufficient time the system self organizes into a weak state the long time required for self organization could explain why weak gouge has not been observed in the laboratory the new results suggest an hfp explanation without the so called fatal flaws of previously proposed solutions they demonstrate that fault friction potentially undergoes a strong evolution effect and could be dependent on gouge microstructure this raises questions about the extent to which laboratory derived friction laws can be used in macroscopic domain earthquake simulation studies

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

the 2015 april 25 gorkha nepal earthquake and its aftershocks implications for lateral heterogeneity on the main himalayan thrust the mw 7 8 gorkha nepal earthquake on 2015 april 25 initiated 80 km northwest of the capital city of katmandu and ruptured 150 km of the frictionally locked downdip segment of the main himalayan thrust mht beneath the central nepal himalaya avouac et al 2015 the earthquake resulted in 4 m of average slip of the himalayan mountains over the indian plate in the ssw direction mitra et al 2015 the main shock fault spanned between the meisoseismal zone of the 1505 mw 8 5 earthquake to its west kumar et al 2006 and the rupture zone of the 1934 mw 8 2 8 4 nepal earthquake to its east bilham wallace 2005 sapkota et al 2013 the last known great earthquake in this region of nepal occurred in 1833 m 7 5 ambraseys douglas 2004 and has a significant overlap with the rupture area of the gorkha main shock adhikari et al 2015 the main shock was followed by a series of moderate to strong aftershocks the largest one mw 7 3 occurred 18 d after the main shock on 2015 may 12 fig 1 albeit the loss of life and property inflicted by this damaging earthquake and its aftershocks it has provided an unprecedented opportunity to study the source properties of himalayan mega thrust earthquake and its relationship to the geometry of the mht which so far is poorly understood

earthquake processes of the himalayan collision zone in eastern nepal and the southern tibetan plateau summary focal mechanisms determined from moment tensor inversion and first motion polarities of the himalayan nepal tibet seismic experiment himnt coupled with previously published solutions show the himalayan continental collision zone near eastern nepal is deforming by a variety of styles of deformation these styles include strike slip thrust and normal faulting in the upper and lower crust but mostly strike slip faulting near or below the crust mantle boundary moho one normal faulting earthquake from this experiment accommodates east west extension beneath the main himalayan thrust of the lesser himalaya while three upper crustal normal events on the southern tibetan plateau are consistent with east west extension of the tibetan crust strike slip earthquakes near the himalayan moho at depths 60 km also absorb this continental collision shallow plunging p axes and shallow plunging ew trending t axes proxies for the predominant strain orientations show active shearing at focal depths 60 90 km beneath the high himalaya and southern tibetan plateau beneath the southern tibetan plateau the plunge of the p axes shift from vertical in the upper crust to mostly horizontal near the crust mantle boundary indicating that body forces may play larger role at shallower depths than at deeper depths where plate boundary forces may dominate

new views on the structure and rheology of the lithosphere over the last 10 years a series of developments have led to a new understanding of what controls the variations in lithosphere strength structure and evolution that produce dramatic contrasts between the geological histories of oceans ancient shields and young orogenic belts those developments involve a wide range of observations from a great diversity of geological geophysical and geochemical disciplines that none the less provide a mutually consistent and coherent overall picture this paper summarizes in one place the essential stages in the evolution of the relevant ideas and observations that have led to this situation

a multidisciplinary study of ecosystem evolution through early pleistocene climate change from the marine arda river section italy the arda river marine succession italy is an excellent site to apply an integrated approach to paleoenvironmental reconstructions combining the results of sedimentology body fossil paleontology and ichnology to unravel the sedimentary evolution of a complex marine setting in the frame of early pleistocene climate change and tectonic activity the succession represents a subaqueous extension of a fluvial system originated during phases of advance of fan deltas affected by high density flows triggered by river floods and overlain by continental conglomerates indicating a relative sea level fall and the establishment of a continental environment an overall regressive trend is observed through the section from prodelta to delta front and intertidal settings the hydrodynamic energy and the sedimentation rate are not constant through the section but they are influenced by hyperpycnal flows whose sediments were mainly supplied by an increase in apennine uplift and erosion especially after 1 80 ma the arda section documents the same evolutionary history of coeval successions in the paleo adriatic region as well as the climatic changes of the early pleistocene the different approaches used complement quite well one another giving strength and robustness to the obtained results

particle dynamics simulations of rate and state dependent frictional sliding of granular fault gouge recent simulations using the particle dynamics method pdm have successfully captured many features of natural faults zones as illuminated in laboratory studies however 2 d simulations conducted on idealized assemblages of particles using simple elastic frictional contact laws yield friction values considerably lower than natural materials and lack time and velocity dependent changes in strength that influence dynamic fault slip here preliminary results of new pdm simulations are described in which particle motions are restricted as a proxy for particle interlocking and out of plane contacts and time dependent contact healing is introduced to capture temporal strengthening of granular assemblages frictional strength is increased and in the absence of interparticle rolling can attain values observed in the laboratory the resulting mechanical behavior is qualitatively similar to that described by empirically based rate state friction laws providing new physical insight into the discrete mechanics of natural faults

large and great earthquakes in the shillong plateau assam valley area of northeast india region pop up and transverse tectonics abstract the tectonic model of the shillong plateau and assam valley in the northeast india region the source area for the 1897 great earthquake ms 8 7 and for the four 1869 1923 1930 and 1943 large earthquakes m 7 0 is examined using the high precision data of a 20 station broadband seismic network about 300 selected earthquakes m 3 0 recorded during 2001 2009 are analysed to study the seismicity and fault plane solutions the dominating thrust reverse faulting earthquakes in the western plateau may be explained by the proposed pop up tectonics between two active boundary faults the oldham brahmaputra fault to the north and the dapsi dauki thrust to the south though the northern boundary fault is debated the more intense normal and strike slip faulting earthquakes in the eastern plateau mikir massif and in the assam valley on the other hand are well explained by transverse tectonics at the long and deep rooted kopili fault that cuts across the himalaya and caused the 2009 bhutan earthquake mw 6 3 it is conjectured that the complex tectonics of the shillong plateau and transverse tectonics at the kopili fault make the region vulnerable for impending large earthquake s

sedimentary evidence of the neogene evolution of se spain the neogene sediments of se spain are dominated by gravity flow deposits ranging from millimetre scale dilute turbidites to kilometre scale olistholiths details of the neogene sequences at finestrat alcoy and fontanars provide examples of responses to periodic tectonic events in a variety of background settings the main control on sedimentation in each area was the episodic rise of triassic gypsiferous mudstone diapirs this probably resulted from periodic regional extension and compression due to contraction and spreading of the alboran sea marginal basin

plateau pop up in the great 1897 assam earthquake the great assam earthquake of 12 june 1897 reduced to rubble all masonry buildings within a region of northeastern india roughly the size of england and was felt over an area exceeding that of the great 1755 lisbon earthquake1 hitherto it was believed that rupture occurred on a north dipping himalayan thrust fault propagating south of bhutan2 3 4 5 but here we show that the northern edge of the shillong plateau rose violently by at least 11 m during the assam earthquake and that this was due to the rupture of a buried reverse fault approximately 110 km in length and dipping steeply away from the himalaya the stress drop implied by the rupture geometry and the prodigious fault slip of 18 7 m explains epicentral accelerations observed to exceed 1g vertically and surface velocities exceeding 3 m s 1 ref 1 this quantitative observation of active deformation of a pop up structure confirms that faults bounding such structures can penetrate the whole crust plateau uplift in the past 2 5 million years has caused the indian plate to contract locally by 4 2 mm yr 1 reducing seismic risk in bhutan but increasing the risk in northern bangladesh

kimberlite wall rock fragmentation processes venetia k08 pipe development current kimberlite pipe development models strongly advocate a downward growth process with the pipe cutting down onto its feeder dyke by means of volcanic explosions evidence is presented from the k08 kimberlite pipe in venetia mine south africa which suggests that some pipes or sub components of pipes develop upwards the k08 pipe in pit exposure comprises 90 vol chaotic mega breccia of country rock clasts gneiss and schist and 10 vol coherent kimberlite sub horizontal breccia layers tens of metres thick are defined by lithic clast size variations and contain zones of shearing and secondary fragmentation textural studies of the breccias and fractal statistics on clast size distributions are used to characterize sheared and non sheared breccia zones and to deduce a fragmentation mechanism breccia statistics are compared directly with the statistics of fragmented rock produced from mining processes in order to support interpretations results are consistent with an initial stage of brecciation formed by upward moving collapse of an explosively pre conditioned hanging wall into a sub terranean volcanic excavation our analysis suggests that the pre conditioning is most likely to have been caused by explosions either phreatic or phreatomagmatic in nature with a total energy output of 2 7 109 kj 656 t of tnt a second stage of fragmentation is interpreted as shearing of the breccia caused by multiple late kimberlite intrusions and possible bulk movement of material in the pipe conduit related to adjacent volcanism in the k02 pipe

grain fracture in 3d numerical simulations of granular shear we present a new method to implement realistic grain fracture in 3d numerical simulations of granular shear we use a particle based model that includes breakable bonds between individual particles allowing the simulation of fracture of large aggregate grains during shear grain fracture simulations produce a comminuted granular material that is texturally comparable to natural and laboratory produced fault gouge our model is initially characterized by monodisperse large aggregate grains and gradually evolves toward a fractal distribution of grain sizes with accumulated strain comminution rate and survival of large grains is sensitive to applied normal stress the fractal dimension of the resultant grain size distributions 2 3 0 3 and 2 9 0 5 agree well with observations of natural gouges and theoretical results that predict a fractal dimension of 2 58 copyright 2005 by the american geophysical union

an ichnofabric approach to the depositional interpretation of the intensely burrowed bateig limestone miocene se spain abstract the foraminiferal rich pelagic bateig limestone forms several varieties of the important building stones quarried at bateig hill in southeastern spain three principal ichnofabrics bichordites mottled palaeophycus and mottled ophiomorpha are recognized which are present in at least two possibly up to four repeated successions cycles each succession begins with an erosional event the bichordites ichnofabric represents a new type of facies formed as thin turbidity grain flow stratiform units derived from sediment slips off a fault into deep water each slipped unit became almost completely bioturbated by infaunal echinoids colonizing by lateral migration because of the thinness of the units successive colonizations tended to truncate the underlying burrows giving rise to a pseudo stratification as the bichordites ichnofabric accumulated on the fault apron thus reducing the effective height of the fault scarp the substrate gradually came under the influence of currents traversing the shelf this led to a change in hydraulic regime and to the mottled palaeophycus and mottled ophiomorpha ichnofabrics in sediment deposited under bed load transport and associated with laminar and cross stratified beds and local muddy intervals reactivation of the fault triggered erosion and channeling and a return to grain flow sedimentation and to the bichordites ichnofabric of the succeeding cycle the highest unit of the bateig limestone is formed entirely of cross stratified calcarenites with occasional ophiomorpha ophiomorpha primary lamination ichnofabric and is similar to many shallow marine facies but they still bear a significant content of pelagic foraminifera the sedimentary setting bears resemblance with that described for the pleistocene monte torre paleostrait and the modern strait of messina italy where the narrow morphology of the depositional area enhanced tidal currents and allowed for high energy sandy deposition in relatively deep areas more data on the miocene paleogeography of the bateig area should provide further testing for this hypothesis the ichnofacies and stacking of the bateig limestone differ from the classic seilacherian model in that they reflect changes in hydraulic process and are associated with faulting and subsidence and changes in sediment supply recognition of the unusual ichnofabrics and their relationships provides a clear indication of the overall dynamic setting

effects of cohesion on the structural and mechanical evolution of fold and thrust belts and contractional wedges discrete element simulations particle based numerical simulations of cohesive contractional wedges can yield important perspectives on the formation and evolution of fold and thrust belts offering particular insights into the mechanical evolution of the systems results of several discrete element method simulations are presented here demonstrating the stress and strain evolution of systems with different initial cohesive strengths particle assemblages consolidated under gravity and bonded to impart cohesion are pushed from the left at a constant velocity above a weak unbonded decollement surface internal thrusting causes horizontal shortening and vertical thickening forming wedge geometries the mean wedge taper is similar for all simulations consistent with their similar residual and basal sliding friction values in all examples presented here both forethrusts and back thrusts occur but forethrusts accommodate most of the shortening fault spacing and offset increase with increasing cohesion significant tectonic volume strain also occurs with the greatest incremental volume strain occurring just outboard of the deformation front this diffuse shortening serves to strengthen the unfaulted domain in front of the deformed wedge preconditioning these materials for brittle dilative failure the reach of this volumetric strain and extent of decollement slip increase with cohesive strength defining the extent of stress transmission stress paths for elements tracked through the simulations demonstrate systematic variations in shear stress in response to episodes of both decollement slip and thrust fault activity providing a direct explanation for stress fluctuations during convergence

laboratory results indicating complex and potentially unstable frictional behavior of smectite clay a central problem in explaining the apparent weakness of the san andreas and other plate boundary faults has been identifying candidate fault zone materials that are both weak and capable of hosting earthquake like unstable rupture our results demonstrate that smectite clay can be both weak and velocity weakening at low normal stress 30 mpa our data are consistent with previous work which has focused on higher normal stress conditions 50 mpa and greater and found only velocity strengthening if natural fault zones contain significant smectite one key implication of our results is that localized zones of high pore pressure which reduce effective normal stress could be important in controlling potential sites of earthquake nucleation our experiments indicate that friction of smectite is complex and depends upon both sliding velocity and normal stress this complexity highlights the need for detailed experiments that reflect in situ conditions for fault gouges

active faulting in apparently stable peninsular india rift inversion and a holocene age great earthquake on the tapti fault we present observations of active faulting within peninsular india far from the surrounding plate boundaries offset alluvial fan surfaces indicate one or more magnitude 7 6 8 4 thrust faulting earthquakes on the tapti fault maharashtra western india during the holocene the high ratio of fault displacement to length on the alluvial fan offsets implies high stress drop faulting as has been observed elsewhere in the peninsula the along strike extent of the fan offsets is similar to the thickness of the seismogenic layer suggesting a roughly equidimensional fault rupture the subsiding footwall of the fault is likely to have been responsible for altering the continental scale drainage pattern in central india and creating the large west flowing catchment of the tapti river a preexisting sedimentary basin in the uplifting hanging wall implies that the tapti fault was active as a normal fault during the mesozoic and has been reactivated as a thrust highlighting the role of preexisting structures in determining the rheology and deformation of the lithosphere the slip sense of faults and earthquakes in india suggests that deformation south of the ganges foreland basin is driven by the compressive force transmitted between india and the tibetan plateau the along strike continuation of faulting to the east of the holocene ruptures we have studied represents a significant seismic hazard in central india

himalayan seismic hazard the devastating earthquake in bhuj on 26 january 2001 highlighted the high earthquake risk in the indian region of gujarat in their perspective bilham et al warn that major earthquakes may be expected in another part of india the himalayan arc an analysis of historic earthquakes suggests that several major earthquakes are overdue in this region but that recent low activity has focused attention away from the himalayan arc projections from earlier earthquakes suggest that just one of the possibly overdue himalayan earthquakes may lead to 200 000 predictable fatalities

impact of fluid pressure on failure mode in shear zones numerical simulation of en echelon tensile fracturing and transition to shear abstract en echelon veins are a set of tensile fractures that develop during the initiation of shear zones under certain conditions but factors that control fracture patterns remain unclear in this study we performed numerical simulations using the two dimensional distinct element method to understand the influence of effective normal stress on the development of fractures during shear deformation at low effective normal stress 10 mpa dilation takes place at an angle of 40 45 to the shear boundary and en echelon arrays of tensile fractures develop in contrast at high effective normal stress 150 mpa en echelon veins are not developed but a distinct shear plane forms a transient behavior characterizes intermediate effective normal stress 50 mpa early tensile cracks are generated and then propagate to form larger brittle faults in this case the larger strain energy released in the later stage is due to the greater shear strength under crustal conditions effective normal stress varies due to the change in fluid pressure our results suggest that en echelon veins are indicators of high fluid pressure close to lithostatic conditions i e effective normal stress close to zero and that a transition of the failure mode from tensile to shear could occur in response to a dynamic change of fluid pressure during fracturing

seismotectonics of bhutan evidence for segmentation of the eastern himalayas and link to foreland deformation abstract the instrumental record of bhutan is characterized by a lower seismicity compared to other parts of the himalayan arc to understand this low activity and its impact on the seismic hazard a seismic network was installed in bhutan for 22 months between 2013 and 2014 recorded seismicity earthquake moment tensors and local earthquake tomography reveal along strike variations in structure and crustal deformation regime a thickened crust imaged in western bhutan suggests lateral differences in stresses on the main himalayan thrust mht potentially affecting the interseismic coupling and deformation regime sikkim western bhutan and its foreland are characterized by strike slip faulting in the indian basement strain is particularly localized along a nw se striking mid crustal fault zone reaching from chungthang in northeast sikkim to dhubri at the northwestern edge of the shillong plateau in the foreland the dextral dhubri chungthang fault zone dcf causes segmentation of the indian basement and the mht between eastern nepal and western bhutan and connects the deformation front of the himalaya with the shillong plateau by forming the western boundary of the shillong block the kopili fault the proposed eastern boundary of this block appears to be a diffuse zone of mid crustal seismicity in the foreland in eastern bhutan we image a seismogenic flat portion of the mht which might be either related to a partially creeping segment or to increased background seismicity originating from the 2009 m w 6 1 earthquake in western central bhutan clusters of micro earthquakes at the front of the high himalayas indicate the presence of a mid crustal ramp and stress buildup on a fully coupled mht the area bounded by the dcf in the west and the seismogenic mht in the east has the potential for m7 8 earthquakes in bhutan similarly the dcf has the potential to host m7 earthquakes as documented by the 2011 sikkim and the 1930 dhubri earthquakes which were potentially associated with this structure

the oldest deep sea ophiomorpha and scolicia and associated trace fossils from the upper jurassic lower cretaceous deep water turbidite deposits of sw bulgaria deep sea trace fossils of the nereites ichnofacies occur in the uppermost kimmeridgian berriasian deep water turbidite deposits of the kostel formation in sw bulgaria the trace fossil assemblage contains tithonian ophiomorpha isp and ophiomorpha annulata which are the oldest deep sea representatives of this ichnogenus they indicate much older invasion of ophiomorpha into deep sea which was hitherto dated as to mid cretaceous it is not clear whether the invasion failed or ophiomorpha survived in deep sea refuge and became widespread after improving oxygenation of the deep seas after the cenomanian turonian anoxic event tithonian scolicia produced by irregular echinoids exhibits a similar distribution pattern

source estimation from broadband regional seismograms recently developed source inversion techniques do not take full advantage of the broadband nature of regional seismograms the reason is that green s functions generated from flat layered models are not correct for the complicated propagational phenomena associated with realistic crustal structure the inversion is usually performed by removing short periods and inverting only selected portions of the records in this article we introduce a source estimation technique that allows for better use of the entire broadband record when only imperfect green s function are available the procedure desensitizes the timing between the principal crustal arrivals by fitting portions of the green s functions independently the preferred source is the orientation that minimizes the average l1 and l2 norms in term of the four parameters m 0 strike dip and rake the l2 error emphasizes the longer period surface waves while the l1 error weights the shorter period body waves we obtain estimates of duration by modeling long period and broadband signals the depth determination can be obtained directly from the surface reflected phases sp mp or ss ms or by cycling through the depth dependent green s functions in addition to the source parameters we obtain t phase alignments shifts that can be used as green s function corrections for relocating other events or as a guide to deriving new crustal models

the active tectonics of the eastern himalayan syntaxis and surrounding regions source parameters of 53 moderate sized earthquakes obtained from the joint inversion of regional and teleseismic distance long period body waves provide the data set for an analysis of the style of deformation and kinematics in the region of the eastern himalayan syntaxis focal mechanisms of eastern himalayan events show oblique thrust consistent with the n ne directed movement of the indian plate as it underthrusts a boundary that strikes at an oblique angle to the direction of convergence earthquakes near the sagaing fault show strike slip mechanisms with right lateral slip earthquakes on its northern splays however indicate predominant thrusting evidence that the dextral motion on the sagaing fault which accommodates a portion of the lateral motion between india and southeast asia terminates in a zone of thrust faulting at the eastern himalayan syntaxis remaining motion between india and southeast asia is accommodated in a zone of distributed shear in east burma and yunnan manifested by strike slip and oblique normal faulting east west extension crustal thinning and clockwise rotation of crustal blocks we determined strain rates throughout the region with a moment tensor summation using 25 years modern and 85 years modern and historic of earthquake data we matched the observed strains with a fifth order polynomial function and from this we determined both the velocity field and rotations with respect to a specified region velocities calculated relative to south china stationary show that the entire area extending from 20 n 36 n within deforming asia yunnan western sichuan and east tibet constitutes a distributed dextral shear zone with clockwise rotations up to 1 7 m y maximum in the region of the eastern syntaxis proper integrated strains across this zone relative to south china stationary show 38 mm yr 12mm yr of north directed motion at the himalaya remaining plate motion relative to south china fixed must be taken up by the underthrusting of india beneath the lesser himalaya strike slip motion on the sagaing fault and intraplate ne directed shortening within ne india as well as ne directed shortening within the eastern syntaxis proper 10 mm yr 2 mm yr of relative right lateral motion between india and southeast asia is absorbed in the region between the sagaing and red river faults 94 e 100 e it is the clockwise vorticity relative to south china associated with the deformation in yunnan east tibet and western sichuan that provides the relative north directed motion of 38 12 mm yr at the himalaya not all of the deformation is accommodated in right lateral shear between india and south china and between east tibet and south china velocity gradients exist that are parallel to the trend of the shear zone relative to a point within western sichuan 32 n 100 e the velocity field shows that the yunnan crust is moving s se at rates of 8 10 mm yr relative to south china there is no eastward expulsion of crustal material beyond the eastern margin of the tibetan plateau

subduction initiation and the rise of the shillong plateau abstract discrepancies between geodetically and geologically estimated thrust fault slip rates are generally viewed as a methodological problem even when slip rate is steady over geological time a discrepancy may exist because each method is sensitive to different deformation processes however this offers a tool to estimate the partitioning of convergence between footwall and hanging wall deformation and therefore a way to discriminate among orogenic styles here we investigate one such discrepancy for the shillong plateau a basement cored contractional orogen within the himalayan foreland basin using a regional block model to explain the modern geodetic velocity field and explicit uncertainty analysis of the geologic rates we show that this discrepancy cannot be reconciled simply by invoking uncertainties in individual methods our results indicate that the shillong plateau is not an ongoing forward break of the bhutan himalayas as was believed until recently instead the observed inter plate convergence and plateau uplift in this region may be driven primarily by an attempt of the negatively buoyant indian passive margin lithosphere the surma basin south of the plateau to initiate subduction as a result the uplift history of the plateau which constrains the geologic rate is significantly lower than expected given the geodetic convergence rate we propose that this convergence is largely accommodated by the transport of the footwall into the mantle this geodynamic scenario has important regional seismotectonic implications 1 the cold and brittle sinking passive margin may have enabled the deep extent 30 km and therefore large magnitude of the mw 8 shillong earthquake of 1897 2 the collapse of the indian lithosphere into the mantle may have created the anomalously deep 20 km surma basin and 3 this subsidence may also drive accelerated post miocene westward propagation of the indo burman wedge we propose that the shillong plateau is the only modern example of passive margin collapse and can serve as a natural laboratory to study the earliest phase of subduction

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

crustal structure of the western bengal basin from joint analysis of teleseismic receiver functions and rayleigh wave dispersion teleseismic data recorded at the broadband seismological observatory of the indian institute of technology iit kharagpur are analyzed to determine the seismic characteristics of the crust beneath the western bengal basin receiver functions calculated from the teleseismic p waveform for a range of back azimuths show little variation in the moho ps arrival time indicating a nearly one dimensional crustal structure beneath kharagpur transforming the time domain receiver functions into the h v p v s domain for a stack of all receiver functions reveals a 38 2 km thick crust with a v p v s of 1 73 0 01 receiver functions stacked in narrow bins of back azimuth and great circle arc distance were inverted jointly with group velocity dispersion data for periods from 15 to 45 sec joint inversion results reveal a near surface sedimentary layer of 1 5 km thickness a 13 km thick upper crust and a 23 km thick lower crust with the moho at a depth of 37 5 km the average crustal s wave velocity is 3 7 km sec the top 0 5 km of the sedimentary layer has very low s wave velocity and a large impedance contrast with the underlying layer this represents the thin layer of unconsolidated sediments and weathered laterites near the surface as seen from deep seismic sounding dss studies across the region the s wave velocity structure of the crust beneath kharagpur is the first such result for the western bengal basin and is broadly similar to that of the indian shield crust a comparison of the crustal thickness and lower crustal velocities between kharagpur and agartala seperated by the eocene hinge zone shows remarkable differences crustal models beneath agartala reveal 36 km thick crust with the presence of a high velocity lower crust 6 km thick possibly oceanic in nature which is absent beneath kharagpur the hinge zone therefore separates the continental crust of the western bengal basin from the eastern deep basinal oceanic crust both being overlain by sediments derived from the himalayas

composite ichnofabrics and tiering of burrows infaunal communities in marine environments typically are tiered that is different taxa live at different depths within the sediment tiered suites of biogenic structures yield complex biogenic sedimentary fabrics ichnofabrics with the traces of deep burrowing organisms overprinted on those of shallow burrowing organisms careful analysis of crosscutting relationships of burrows in such composite ichnofabrics allows reconstruction of the tiered nature of fossil endobenthic communities it is important to recognize that the best preserved and most prominently displayed trace fossils in most assemblages usually represent the deepest tier thus they were farther removed from the sea floor and therefore less indicative of actual sea floor conditions than the more poorly preserved traces of the shallower tiers on which the deeper traces are juxtaposed

a new trace fossil assemblage from the middle permian broughton formation southern sydney basin southeastern australia ichnology and palaeoenvironmental significance abstract the permian triassic sedimentary succession of the southern sydney basin in southeastern australia contains a wealth of well preserved trace fossils that are important for systematic ichnological palaeoecological and palaeoenvironmental interpretations in this study a new ichnofossil assemblage comprising macaronichnus palaeophycus psammichnites protovirgularia rosselia and teichichnus is documented from the tide influenced shoreface deposit of the middle permian jamberoo sandstone member of the broughton formation two distinct ichnofabrics are recognized i the psammichnites ichnofabric characteristic of the upper middle shoreface deposit and ii the crowded rosselia ichnofabric cri representing the lower shoreface the psammichnites dominated ichnofabric is interpreted to have resulted from opportunistic behaviours of unknown trace makers whereas the cri is interpreted to represent possible strategic behaviours of stress tolerant polychaetes e g terebellid spionid and cirratulid polychaetes in a distal shoreface with high sedimentation rate thus the psammichnites crowded rosselia ichnofabric succession can be considered to represent a response to an environmental shift from proximal to distal shoreface as such this study highlights the utility of these two ichnofabrics as stressed environmental indicators

lithological control on the geomorphic evolution of the shillong plateau in northeast india abstract the shillong plateau in northeast india is a block of raised topography in the himalayan foreland which consists of crystalline basement rocks partially covered by a cretaceous to miocene sedimentary succession it is dominated by a mature low relief landscape surrounded by high relief fluvially dissected margins particularly along its southern flank which is bounded by the dauki thrust fault we use river profiles and geological relationships to show that the low relief plateau is a topographic expression of a re exposed basement palaeosurface following the stripping of sedimentary cover by scarp retreat we show that initiation of the wave of incision does not require surface rupture on the dauki fault or an increase in fault slip rate at the end of the miocene as suggested by previous studies we propose that incision has been spatially controlled by the slope of the basement palaeosurface likely moderated by an incision threshold river profiles in the shillong plateau cannot be interpreted as simple records of surface uplift the observed heterogeneous spatial pattern of steepness is a function of a dynamic landscape response to the erosion of layered lithology with contrasting erodibility such dynamics have implications for fluvial geomorphology highlighting that near horizontal lithological contacts can strongly influence river profiles and topography even when no longer physically preserved the topography of the northern shillong plateau is controlled by the structure of basement rocks and is reminiscent of stable cratonic interior landscapes consistent with its surface exposure during late cretaceous times

aseismic plate boundary in the indo burmese wedge northwest sunda arc plate motion crustal deformation and earthquake occurrence processes in the northwest sunda arc which includes the indo burmese wedge ibw in the forearc and the sagaing fault in the backarc are very poorly constrained plate reconstruction models and geological structures in the region suggest that subduction in the ibw occurred in the geological past but whether it is still active and how the plate motion between the india and sunda plates is partitioned between motion in the ibw and sagaing fault is largely unknown recent gps measurements of crustal deformation and available long term rates of motion across the sagaing fault suggest that 20 3 mm yr of the relative plate motion of 36 mm yr between the india and sunda plates is accommodated at the sagaing fault through dextral strike slip motion we report results from a dense gps network in the ibw that has operated since 2004 our analysis of these measurements and the seismicity of the ibw suggest that the steeply dipping churachandpur mao fault in the ibw accommodates the remaining motion of 18 2 mm yr between the india and sunda plates through dextral strike slip motion and this motion occurs predominantly through velocity strengthening frictional behavior i e aseismic slip the aseismic motion on this plate boundary fault significantly lowers the seismic hazard due to major and great interplate earthquakes along this plate boundary

stress transfer and connectivity between the bhutan himalaya and the shillong plateau abstract within the northern indian plate the shillong plateau is a peculiar geodynamic terrane hosting significant seismic activity outboard the himalayan belt this activity is often used as an argument to explain apparent reduced seismicity in the bhutan himalayas although current geophysical and geodetic data indicate that the bhutan himalayas accommodate more deformation than the shillong plateau we aim to quantify the extent to which the two geodynamic regimes are connected and potentially interact through stress transfers we compiled a map of major faults and earthquakes in the two regions and computed co seismic stress transfer amplitudes our results indicate that the bhutan himalayas and the shillong plateau are less connected than previously suggested major earthquakes in either of the two regions mainly affect transverse faults connecting them causing up to 40 bar coulomb stress change however this effect is clearly less on thrust faults of the either region up to 1 bar only the mw 8 25 1897 assam earthquake that affected the shillong plateau did not cause a stress shadow on the main himalayan thrust in bhutan as previously suggested similarly the mw 8 0 5 1714 bhutan earthquake had negligible impact on stress accumulation on thrust faults bounding the shillong plateau furthermore the main process shaping the regional stress patterns continues to be interseismic loading with complex boundary conditions in a diffuse deformation field involving the bengal basin and indo burman ranges while both the bhutan himalayas and the shillong plateau exhibit a compressional regime their stress evolutions are more weakly connected than hypothesized although our modelling suggests lateral increase in stress interactions from west less to east more in the bhutan himalayas a clearer picture will only emerge with better constrained fault geometries slip rates crustal structure and seismicity catalogues in the entire region of distributed deformation

sequential colonization of muddy turbidites in the eocene beloveža formation carpathians poland abstract a 100 350 m thick succession of centimeter thick fine grained turbidites form the lower mud dominated part of a prograding deep sea fan in the eocene belove a formation in the carpathian mountains of poland these strata contain over 50 ichnotaxa only six of which are common phycosiphon nereites chondrites scolicia halopoa ophiomorpha polished slabs and examination of cross cutting relationships show that post event colonization was sequential using modern analogues it can be shown that the colonization of different depth levels in the sediment at different times reflects changing geochemical conditions especially the re adjustment and re establishment of the redox boundary modern analogues allow an assessment of the timing of each colonization phase the phycosiphon producers entered first and occupied the well oxygenated muds 5 40 mm below the sediment surface as the redox boundary reformed nereites producers entered and probably utilized microbial matter just above the redox boundary about 15 30 mm below the sediment surface subsequently chondrites producers penetrated down to the oxygen deficient layers of the event bed or even deeper levels the halopoa producers entered with phycosiphon but reworked coarser layers contemporaneously with this sequential colonization deep permanent bioturbators which were unaffected by the event cross cut the post depositional suite the variability of the resultant ichnofabrics can be attributed in part to a patchy distribution of the benthos and the relative frequency of turbidite events five ichnofabrics which reflect the successive colonization episodes are recognized each was modified by elements of the permanent infauna

collision of the ganges brahmaputra delta with the burma arc implications for earthquake hazard abstract we take a fresh look at the topography structure and seismicity of the ganges brahmaputra delta gbd burma arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential the gbd the world s largest delta has been built from sediments eroded from the himalayan collision these sediments prograded the continental margin of the indian subcontinent by 400 km forming a huge sediment pile that is now entering the burma arc subduction zone subduction of oceanic lithosphere with 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land the prism starts at an apex south of the gbd shelf edge at 18 n and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit the front of the prism is blind buried by the gbd sediments thus the deformation front extends 100 km west of the surface fold belt beneath the comilla tract which is uplifted by 3 4 m relative to the delta this accretionary prism has the lowest surface slope of any active subduction zone the gradient of the prism is only 0 1 rising to 0 5 in the forearc region to the east this low slope is consistent with the high level of overpressure found in the subsurface and indicates a very weak detachment since its onset the collision of the gbd and burma arc has expanded westward at 2 cm yr and propagated southwards at 5 cm yr seismic hazard in the gbd is largely unknown intermediate size earthquakes are associated with surface ruptures and fold growth in the external part of the prism however the possibility of large subduction ruptures has not been accounted for and may be higher than generally believed although sediment clogged systems are thought to not be able to sustain the stresses and strain weakening behavior required for great earthquakes some of the largest known earthquakes have occurred in heavily sedimented subduction zones a large earthquake in 1762 ruptured 250 km of the southern part of the gbd suggesting large earthquakes are possible there a large but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism and is the only known candidate for a rupture of the plate boundary along the subaerial part of the gbd burma arc collision zone

active faulting and tectonics of burma and surrounding regions this study of the active tectonics of burma and surrounding regions is based mostly on an interpretation of landsat imagery and on fault plane solutions for shallow and intermediate earthquakes fold axes in the indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the indoburman ranges fault plane solutions of shallow earthquakes however do not show underthrusting at the present time instead p axes are oriented roughly north south parallel not perpendicular to the fold axes in the ranges p axes for fault plane solutions of intermediate shocks also trend north south parallel to the strike of the subducted slab and may indicate that the hanging slab is being dragged north by india through the surrounding asthenosphere the sagaing fault probably accommodates most of the right lateral slip of india past indochina but india s northward movement also causes internal deformation within burma thailand and yunnan differences between the tectonic fabric seen on the landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion in the orientations of strain and in the style of deformation in the late cenozoic presumably these changes reflect the transition from subduction to collision and strike slip motion as india penetrated into eurasia past burma large parts of burma and thailand may have rotated several tens of degrees clockwise in this process

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

moment tensor solutions and triggering environment for earthquakes in koyna warna water reservoirs region india we consider nine earthquakes in the koyna warna reservoir region on the western side of the peninsular india the deviatoric moment tensors of these earthquakes have been evaluated by minimizing the least squares misfit between observed and synthetic seismograms we use broadband seismograms of observatories at kard and pune which are at distances of nearly 50 and 150 km respectively from the epicenters both surface wave inversion and the difference between the arrival times of sh and sv show the presence of an anisotropic crust however we have obtained an equivalent isotropic structure by improving the published crustal structures of this area through inversion of surface wave group velocity data the deviatoric moment tensors of the earthquakes are decomposed into two components double couple and compensated linear vector dipoles clvd the double couple components of all the nine earthquakes show normal faulting with minor strike slip the t axis is consistently subhorizontal with an average azimuth of 260 6 and the p axis is nearly vertical the fault planes of six events give average strike direction and dip respectively as 194 0 and 51 8 and are associated with the main fault of the area the other three events lie in the southern part of this area and have strike direction between sse and se which is parallel to the tectonic features in this part the clvd component is generally within 20 percent of the total moment tensor recent studies show that anisotropy can produce source mechanism with clvd up to 30 percent and can also cause high pore fluid pressure leading to fault instability more rapidly compared to conventional mechanism in an isotropic medium it appears that the anisotropic crust noted in the present work is generating the clvd component and also gives the proper environment to trigger earthquakes by reservoirs through pore fluid pressure

storm related sedimentation influenced by coastal configuration in the stratigraphic record of a tectonically active shelf upper pleistocene le castella terrace italy abstract analysis of patterns of coastal circulation and sediment dispersal is an essential step for the study of controlling factors influencing the long term dynamics of coastal systems modern settings offer the possibility to monitor relevant parameters over relatively short time spans however geological examples complement this perspective by providing a time averaged record where longer trends and stratigraphically significant processes can be evaluated this study investigates the shallow marine deposits of le castella terrace upper pleistocene southern italy to document how patterns of circulation influenced by coastline configuration can affect the preserved millennial scale depositional record of a progradational shoreline system the regressive portion of the le castella terrace deposits developed during a relative sea level highstand and falling stage consists of a progradational wedge mainly composed of redistributed skeletal particles of a coeval shallow water carbonate factory preservation of the morphology of the paleocoastline and abundant current related sedimentary structures allow reconstruction of the predominant sediment dispersal dynamics responsible for the formation of this sedimentary wedge facies and paleocurrent analysis indicate offshore and alongshore sediment transport modes consistent with coastal circulation driven by storms normally incident to the shoreline and a sharp change in coastline orientation this coastal inflection influenced circulation patterns causing flow separation and eddy formation in the lee of the curved coastline syndepositional tectonic deformation also affected the architecture of the preserved deposits controlling the nucleation and development of a clinostratified body and determining localized lateral stratigraphic variability this study illustrates how transient but recurrent circulation patterns associated with changes in coastal orientation and related to high energy storm events can leave a predominant signature in the stratigraphic record of microtidal shallow marine successions

distribution ecology and taphonomy of the organism trace scolicia in northeast atlantic deep sea sediments abstract the organism burrow scolicia was studied in deep sea sediment cores from several regions of the northeast atlantic including the iceland faeroe ridge rockall plateau and reykjanes ridge the data comprised sedimentological and physical measurements and x ray radiographs scolicia generally can be observed only in radiographs grain size and sorting of sediments control the distribution of the echinoids that produce scolicia if developed under optimum ecological conditions in well sorted coarse silts to fine sands scolicia dominates the biogenic structures with its very characteristic ichnofabrics its distribution pattern is influenced mainly by bottom currents sedimentation rate and the faunal association indirectly a dependence on water depth also seems to exist the preservation and recognition of scolicia in lithified sediments needs special conditions these include restricted competition by organisms of the deeper burrowing tiers its generation within the optimum sediment grain size or its production at the sand silt mud interface

active transverse faulting within underthrust indian crust beneath the sikkim himalaya summary deep focus earthquakes within the underthrust indian lower crust beneath the himalaya occur in very specific regions and have distinct source characteristics the study of the source mechanisms of these earthquakes provides valuable constraints on the kinematics of deformation of the underthrust indian plate and its influence on the active deformation of the overlying himalayan wedge one of the most significant regions of these deep focus earthquakes is beneath the sikkim and bhutan himalaya we study the source characteristics of the 2011 september 18 mw 6 9 deep focus sikkim main shock and its major aftershocks using global regional and local waveform data we determined the focal mechanism of the main shock using moment tensor inversion of global p and sh waveforms and ascertained the earthquake fault plane using rupture directivity from regional p wave spectra the main shock originated at 53 4 km depth and ruptured at least 20 km thickness of the underthrust indian lower crust faulting occurred on a near vertical dextral strike slip fault oriented nw se strike 127 d ip 81 and rake 167 oblique to the local strike of the himalayan arc the rupture initiated from the se end of the fault and propagated to the northwest the main shock was followed by 20 small to moderate aftershocks mb 3 0 which we relocated using phase arrival times we computed the focal mechanisms of the larger ones mb 3 5 using local waveform inversion we find that all aftershocks originated se of the main shock between depths of 12 and 50 km and have dominantly strike slip mechanisms our results combined with the source mechanisms of earthquakes from previous studies reveals that the entire underthrust indian crust is seismogenic and deforms by dextral strikeslip motion on oblique structures beneath the sikkim and bhutan himalaya these active oblique structures with transverse motion possibly mark the western boundary of the clockwise rotating microplates in northeast india observed from gps geodesy

sedimentary facies during early stages of flooding in an extensional basin the breche rouge de carboneras late miocene almeria se spain the intramontane basins of the betic cordil leras se spain formed subsequent to the main phase of orogenic deformation during the middle miocene in a close genetic relation to the trans alboran shear zone left lateral movements along a local branch carboneras fault zone cf strike ne sw of this zone played a major role in controlling the formation and dynamics of the nijar carboneras basin to the south of the fault a major phase of strike slip faulting is recorded during the late tortonian the expression of this event is the brche rouge de carboneras brc which seals a deep denudational surface on top of dislocated fault blocks formed by volcanics of the cabo de gata complex and early tortonian shallow marine calcarenite the sedi mentary facies of this widely distributed unit in the carboneras subbasin mirror the submarine topography and the distribution of the fault zones along strike slip fault zones autoclastic breccias and neptunian dikes preferentially oriented nw se and ne sw occur which are interpreted to represent the near surface expression of the faults red limestone forms the groundmass of the autoclastic breccia and infills of neptunian dikes which exhibit multiple phases of opening of fissures gravita tional sedimentary infill lithification and renewed cre ation of cracks steep relief probably along fault scarps was mantled by epiclastic volcanic conglomerate with a red carbonate matrix well lithified coarse skeletal lime stone rich in planktonic foraminifera formed pavements along sediment starved rocky surfaces in deep water laterally within topographic depressions the pavement limestone grades into thick accumulations of skeletal rudstone composed of fragmented azooxanthellate corals and stylasterid hydrozoans which were concentrated by powerful bottom currents and gravitiy flows within the shallow water zone of dip slope ramps cross bedded calcarenite and calcirudite formed based on textures fabrics and biota rocks of the brc were grouped into nine genetic lithofacies which document cryptic deep aphotic and shallow photic environments typical of a sediment starved extensional basin

numerical simulation of earthquake faults with gouge toward a comprehensive explanation for the heat flow paradox the particle based lattice solid model is used to simulate transform faults with and without fault gouge stick slip frictional behavior is observed in two dimensional numerical experiments of model faults both with and without gouge when no gouge is present the fault is strong and the heat generation and stress drops are correspondingly high in disaccord with observations surrounding the heat flow paradox in contrast when a gouge is specified the fault is weak and the heat generation as well as stress drops are low in quantitative agreement with observational constraints the heat flow is low on average and during short periods of aseismic creep seismic efficiencies are compatible with observationally based bounds counter intuitively the fault strength decreases as the intrinsic friction between particles is increased beyond a given threshold the mechanism for low fault strength and heat is rolling and jostling of fault gouge grains during slip this allows macroscopic movement of the fault with only minimal slip between surfaces of the gouge grains as this dynamical mechanism operates during seismic and aseismic slip it provides an explanation for the lack of a heat flow anomaly in both the seismic and creeping parts of the san andreas fault the simulation results provide the first comprehensive and quantitative possible explanation of the heat flow paradox and suggest that fault gouge plays a fundamental role on the dynamics of earthquake faults whether rolling and jostling of fault gouge particles provides the explanation for the heat flow paradox in nature remains to be validated by observation evidence

pliocene pleistocene sequences bounded by subaerial unconformities within foramol ramp calcarenites and mixed deposits salento se italy nine discrete metre scale sequences of early pliocene to middle pleistocene age were deposited in the small novoli graben salento peninsula puglia s italy they consist of carbonate and mixed carbonate siliciclastic sediments deposited on a slowly subsiding foreland ramp skeletal concentrations and intervening less fossiliferous intervals have been examined to provide information on major environmental parameters and infer the dynamics of their changes taphonomic and palaeoecological analyses indicate that storm induced waves and currents reduced sediment input and settling behaviour of components were the main factors controlling the features of the various shellbed types the concentrations were formed below fair weather wave base in low stress inner to outer shelf environments and are often associated with surfaces or intervals that are characterized by sedimentary condensation vertical change in the fossil content within individual cycles indicates water depth changes that were parallel to climatic fluctuations hence may result from glacio eustatic sea level changes most sequences are bounded by subaerial karstic unconformities because of the regional setting of low subsidence rate the record of the relative sea level fluctuations is incomplete episodes of subaerial exposure and concomitant effects of vadose diagenesis are documented by i diagenetic changes leading to hardening of unconformity horizons ii local subvertical solution pits developed in the vadose zone below unconformity surfaces iii networks of polygonal cracks below unconformities and iv infilling of solution pits and polygonal cracks with vadose silt and marine sediment inwashed during transgressions following the subaerial stages lower sequences are characterized by tighter cementation and significant increase in moldic and vuggy porosity due to superimposition of the diagenetic effects of repeated high amplitude sea level fluctuations

source parameters of earthquakes and intraplate deformation beneath the shillong plateau and the northern indoburman ranges we determined the fault plane solutions and focal depths of 17 earthquakes beneath the shillong plateau and the northern indoburman ranges by combining results from the inversion of long period p and sh waveforms and amplitudes from polarities of first motions and from the identification of pp and sp phases on short period seismograms fault plane solutions of 15 earthquakes show mixtures of thrust and strike slip faulting but the p axes for these events are nearly horizontal and consistently oriented north northeast south southwest all of these earthquakes occurred at depths greater than 29 km beneath the shillong plateau one event occurred at a depth of 52 km the relatively large depths for earthquakes in an intraplate setting suggest that the indian lithosphere in this area is especially cold earthquakes beneath the northern indoburman ranges define a gently east southeast dipping zone from 30 to 45 km beneath the bengal basin to 40 to 90 km beneath the ranges this zone seems to steepen and connect with the zone of intermediate depth seismicity that dips eastward beneath burma these earthquakes cannot have occurred along the interface between a subducting indian plate and the overriding indoburman lithosphere because the p axes not the nodal planes are parallel to the north south trending seismic zone although a couple of the earthquakes might have occurred within the indoburman lithosphere most of this seismicity seems to have occurred within the indian plate recently and currently being subducted eastward beneath the indoburman ranges the consistent north northeast trend of the p axes implies that the orientation of maximum compressional strain in the indian plate throughout its northeastern pan is nearly perpendicular to that responsible for roughly north south trending folds of the indoburman ranges thus either recently in geologic time since 1 ma the orientation of maximum compression changed dramatically or more likely the deformation in the indoburman ranges is decoupled from that in the underlying indian plate meanwhile the seemingly identical northward displacement of india and the indoburman ranges with respect to south china must be accommodated farther east along the sagaing and other faults

sand waves echinocardium traces and their bathyal depositional setting monte torre palaeostrait plio pleistocene southern italy stacked cross sets up to 2 5 m thick produced by sand wave migration and meniscate trace fossils produced by echinocardium cordatum both considered in the literature as typical of shallow water marine depositional settings commonly occur in the bathyal plio pleistocene deposits of monte torre calabria southern italy the plio pleistocene sediments form two coarsening upward depositional sequences separated by an unconformity and by a palaeobathymetric gap of at least 300 m the lower sequence passes upwards from hemipelagic marls and thin bedded turbidites to thick bedded sandy turbidites then to sand wave deposits alternated with sandy turbidites and finally to base of slope megabreccias facies characteristics and relationships and the occurrence of deep sea faunal associations indicate deposition in the bathyal zone the facies of the upper sequence reflect a fan delta environment no deeper than a few tens of metres the depositional setting of the lower sequence where the sand wave deposits and meniscate trace fossils occur appears to have been a tectonically controlled seaway connecting the tyrrhenian and ionian seas this seaway became progressively narrower with time evolving into a strait the overall coarsening upward trend reflects the upward transition from a low to a high energy environment possibly caused by the tectonic narrowing of the seaway deposition and erosion from high concentration turbidity currents and from tidal bottom currents were important processes periods of tectonic activity producing first the uplift of the seaway margins and culminating with the uplift of the strait sequence itself are marked by scattered rockfall deposits the strait setting causing the development of powerful oxygenated bottom currents produced optimal conditions in the bathyal zone for the colonization of sandy bottoms by a single infaunal r selected species echinocardium sp

palaeoecology and taphonomy of spatangoid dominated echinoid assemblages a case study from the early middle miocene of sardinia italy abstract the early middle miocene marine sedimentary succession of the borutta formation cropping out at porto torres northern sardinia represents one of the most diverse and well preserved spatangoid dominated sedimentary successions in the miocene of the mediterranean area the echinoid bearing succession is studied with the twofold purpose of firstly reconstructing the palaeoenvironmental setting in which these echinoids occurred and secondly investigating factors influencing the preservation potential of spatangoid echinoids in the fossil record these goals are accomplished by 1 detailed logging of sedimentary facies 2 applying functional morphological interpretations of echinoid skeletons in order to reconstruct their behaviour and life habits 3 quantifying the relative abundance of taxa and taphonomic features of test preservation and 4 analyzing associated trace fossils within the beds the sedimentary succession is composed mainly of fine grained sandstones which are intensely bioturbated by thalassinoides like burrows spatangoid abundance is high throughout the profile declining only in intercalated dm thick rhodolith beds a 20 cm thick bed with a bedding surface of approximately 250 m2 was investigated in detail with respect to relative abundance of spatangoid taxa and taphonomic signatures the echinoids consist almost exclusively of deposit feeding irregular forms with a dominance of the spatangoids ova and brissopsis and the minute clypeasteroid echinocyamus the spatangoids opissaster holanthus metalia and hemipatagus as well as the clypeasteroid clypeaster marginatus are also present a low energy moderately deep outer sublittoral environment with high rates of bioturbation and episodes of sediment deposition by storms is inferred interspecific competition among deposit feeding echinoids appears to be limited by their different burrowing depths feeding strategies and resource partitioning the gregarious behaviour of spatangoid echinoids their habitat in quiet deeper environments and episodes of rapid deposition seem to represent the main factors leading to this spatangoid bearing deposit since both shallow and deep burrowing spatangoid echinoids are well represented as complete specimens mode of life does not appear to play a major role in their preservation potentials