styles of failure in late holocene highstand prodelta wedges on the adriatic shelf on the continental shelves of the northern mediterranean basin the late holocene highstand systems tract hst prograded un der the influence of major rivers after the attainment of the present sea level highstand about 5 5 cal kyr bp on the adriatic shelf the thickness distribution of the late holocene hst reflects the location of major deltas on the western side of the basin and the geostrophic cir culation which prevents a more uniform sediment dispersal toward the center of the basin very high sediment accumulation rates 1 to 10 cm year resulted in the construction of a hst depocenter up to 35 m thick this shore parallel depocenter is affected by failure of limited displacement over as much as 40 of its extent gas impregnation is common in the topset region and occurs at very shallow levels a few meters below the sea floor five areas are characterized by a variety of sea floor and subsurface crenulations although locally some of these crenulations have an intriguingly regular geometry sediment failure is the most plausible mechanism for their formation sediment failure better explains the large variety of geometries that characterizes the coastal mud prism of the late holocene hst furthermore we observe that these crenulations occur only where the downlap surface at the base of the hst is disrupted and affected by geometries that are con sistent with fluid escape processes this relationship suggests that the basal surface acts as a weak layer for sediment failure failure occurred in variable water depths from the northern slope of the modern po prodelta 10 20 m water depth to the narrow shelf offshore bari 40 110 m water depth in all these areas the proximal part of the hst prodelta wedge is intensely gas charged the thickness and age of the sediment sections affected by failure are slightly differ ent from place to place but appear everywhere younger than 5 5 cal kyr bp where failure affects the entire hst the detachment occurs on the downlap surface at its base failure geometries characterize the head region whereas compressional features such as pressure ridges and mud diapirs dominate in the toe region ranging in depth between 70 and 110 m where failure is limited to the upper few meters of the hst there is a clear lithologic change decrease in carbonate fraction and grain size across the basal surface this lithological change reflects a switch in sediment supply from local apennine rivers below to po derived mud this change occurred at the onset of the little ice age documenting the indirect control of short term climate change and hu man impact on sediment architecture the deformations affecting the late holocene hst in the various areas show differences in internal geometry but appear everywhere to be characterized by limited downward displacement and can be attri buted to shear dominated retrogressive failure it is suggested that some degree of consolidation occurred immediately after mobilization possibly induced by the escape of fluids nowhere has failure evolved into disintegration and flow likely because the type of cyclic loading that triggered it was not prolonged over a long enough interval short lived radionuclides in the uppermost stratigraphic layers which postdate the failure in the area offshore ortona allowed us to quantify systematic changes in sediment accumulation rates as a func tion of the underlying deformed sea floor in areas of wavy sea floor troughs show sediment accumulation rates of greater than 16 mm yr a figure that is fourfold the rate measured on the flanks of the troughs

a physical basis for the frictional melting of some rock forming minerals abstract under conditions of coseismic slip 0 1 2 0 m s 1 within the upper crust differences in the shear yield strengths fracture toughnesses melting points and to a lesser extent thermal conductivities of rock forming minerals result in a hierarchy of friction melting susceptibilities in many so called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase whilst quartz and feldspar tend to survive as clasts this can be explained in terms of the disequilibrium flash melting of those mineral phases possessing the lower shear yield strengths fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes at high strain rates typical of coseismic faulting the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses with progressive comminution during slip leading to fusion the friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall rock lithology and earthquake magnitude if frictional melting results in fault lubrication

seafloor undulation pattern on the adriatic shelf and comparison to deep water sediment waves abstract on the adriatic shelf a large field of seafloor and subsurface undulations typically 300 m wide 2 5 m high and several km long parallel to the bathymetric contour affects the progradational clinoforms of the late holocene prodelta wedge above a regional downlap surface that shows evidence of deformation and fluid escape the undulations are associated with mud reliefs that occur farther seaward in elongated swarms perpendicular to the regional slope and to the crests of the undulations a quantitative measurement of key morphobathymetric parameters such as wavelength height asymmetry and wave form indexes was performed on 13 selected seismic reflection profiles perpendicular to the undulations these measurements are determined with respect to water depth or downslope distance from the shallowest undulation the results highlight substantial differences from deep water sediment waves these differences include 1 most undulations show a staircase morphology with sub horizontal upslope limbs of 100 300 m delimited seaward by sharp edges and steep downslope limbs 2 landward or upslope dipping limbs do not develop 3 there is no consistent downslope decrease in wavelength or amplitude as opposed to most cases of sediment waves related to overflowing turbidity currents 4 there is a consistent downslope loss in crest continuity approaching areas where mud reliefs and sediment deformation are evident at the seafloor 5 many of the undulations show differences in angle of climb and in some cases a single undulation at the seafloor results from the burial and merger of more undulations in the underlying section these differences compared to bottom current or turbidity current sediment waves are augmented by the observation that the adriatic shallow water undulations are also typically associated with gas charged sediment fluid escape sediment deformation and localised failure planes it is therefore suggested that seafloor undulations evolved in response to sediment deformation and were successively amplified by differential deposition from bottom currents crossing an irregular seafloor in general we feel that case by case solutions should be found to explain the origin and evolution of seafloor wavy features based on the geologic stratigraphic setting the recognition of possible weak layers and the co existence of a variety of other deformation geometries that may occur within or adjacent to the field of wavy features the adriatic example can reinforce recent suggestions that besides purely depositional bedforms there exist also wave like deformation structures and multi process sediment waves faugeres et al 2002 multi process generated sediment waves on the landes plateau bay of biscay north atlantic marine geology 182 279 302

modern state and dynamics of the sea of azov coasts abstract results of the modern state researches of the sea of azov coasts and coastal processes influencing their evolution are presented a multidisciplinary study of coastal processes allowed us to identify main types of coast evolution abrasion accumulation and stability predominance of abrasion is the main feature of modern dynamics of the azov shores not only the parent rock shores are subjected to erosion but also accumulative forms are eroded the eastern coast of the sea retreats most rapidly the intensity of the abrasion and landslide processes is due to the weak resistance of the rocks composing the coast to the wave forcing in particular intense abrasion occurs in those parts of the coast where wave forcing is strong together with the surges caused by onshore winds while the beaches are narrow or completely absent at the same time the percentage of beach forming fractions in eroded soils is low and the major part of the abrasion material in the form of suspended matter is transported to the deep parts of the sea in general the state and dynamics of the azov coasts are influenced by abrasion accumulative processes sea level fluctuations changes in the wind wave regime parameters and fluctuations in the biogenic beach forming material volumes the biogenic factor is the most important in the balance of deposits of accumulation forms spits and bay bars anthropogenic activity also affects the current state of the shores the transport of beach forming alluvial sediments into the sea is reduced because of the regulated river discharge the natural evolution of onshore processes is disrupted during the development of the coast agricultural activities industrial and resort construction removal of sand and shell material construction of shore protection facilities

the state of stress on some faults of the san andreas system as inferred from near field strong motion data we present a simple method to calculate the stress produced on an earthquake fault during rupture this method allows the complete evaluation of the stress spatio temporal history over the fault we apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the san andreas fault system over the last 20 years the imperial valley earthquake of 1979 the morgan hill earthquake of 1984 the loma prieta earthquake of 1989 and the northridge earthquake of 1994 we use for this study the tomographic models of the fault rupture obtained from the inversion of the near field seismic data recorded during these earthquakes the results obtained show that the static and the dynamic stress drops vary greatly over the fault the peak values obtained for the four earthquakes studied range from about 20 to 100 mpa these high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault the regions of the fault which break with a high stress drop are also the regions where slip is large this suggests that most of the slip produced in a large earthquake takes place over the strong areas of the fault low slip regions tend to break with low stress drops after the earthquake the shear stress is increased over a significant portion of the fault which corresponds to low slip regions aftershock activity tends to be concentrated in these areas of stress increase the apparent strength of the fault before the earthquake that is the local shear stress increase which is required for rupture is also extremely heterogeneous the rupture velocity seems to be inversely correlated with this apparent fault strength the rupture accelerating over the weak areas of the fault and slowing down over the high strength areas

formation of coastline features by large scale instabilities induced by high angle waves along shore sediment transport that is driven by waves is generally assumed to smooth a coastline this assumption is valid for small angles between the wave crest lines and the shore as has been demonstrated in shoreline models but when the angle between the waves and the shoreline is sufficiently large small perturbations to a straight shoreline will grow here we use a numerical model to investigate the implications of this instability mechanism for large scale morphology over long timescales our simulations show growth of coastline perturbations that interact with each other to produce large scale features that resemble various kinds of natural landforms including the capes and cuspate forelands observed along the carolina coast of southeastern north america wind and wave data from this area support our hypothesis that such an instability mechanism could be responsible for the formation of shoreline features at spatial scales up to hundreds of kilometres and temporal scales up to millennia

eustatic and tectonic control on deposition and lateral variability of quaternary regressive sequences in the adriatic basin italy abstract during the late pleistocene sea level variations related to glacio eustatic cycles induced the deposition of four regressive sequences on the western continental shelf of the adriatic basin the four sequences are bounded by erosion surfaces that formed during sea level fall sub aerial exposure of the shelf and subsequent sea level rise and erosional reworking the sequences and the erosion surfaces at their top numbered 1 4 top down are correlated on distances in the order of 300 km in the central and south adriatic core control on deposits of the most recent depositional sequence sequence 1 suggests that each sequence records a fourth order ca 100 120 kyr cyclicity of sea level change within each depositional sequence progradational units representing highstand to forced regression are dominant whereas transgressive deposits are absent or reduced in both thickness and lateral extent sequences are thus referred to as regressive sequences deposition during sea level fall occurred on an overall subsiding shelf through the dispersal of fine grained sediment supplied from the adjacent alpine and apennine chains this resulted in stacked forced regression deposits thick enough to survive complete removal during subsequent phases of sub aerial and marine erosion erosion and or non deposition were enhanced only where tectonic uplift and morphological highs further reduced the remaining accommodation space resulting in local thickness reduction or in the non preservation of the regressive sequences at larger scale thickness variability and depocenter distribution north and south of the gargano promontory an uplifted block bisecting the adriatic foreland reflect differential deposition and preservation of regressive deposits as a function of contrasting tectonic setting and resulting physiography of the shelf and upper slope

the gargano promontory an important italian seismogenic tsunamigenic area abstract gargano is a small mountainous promontory protruding into the southern adriatic and from a structural point of view it is part of the apulia foreland deformative structures both inland and offshore the promontory are quite common and can partially be associated with a substantial quaternary uplifting seismogenically this area is well known because several historical strong earthquakes have been reported to occur here and recent important seismic sequences have been recorded this paper is a contribution to evaluate the seismogenic as well as the tsunamigenic potential of this region particular attention has been given to study the major events some of which were able to produce tsunamis the epicenter location of some of these earthquakes computed solely on the basis of macroseismic data would point to focal regions on land but this is questionable if tsunami generation is taken into account as regards the tsunamigenic potential statistical analysis is used to evaluate the local geographical distribution of expected tsunamis and the mean return periods it is found that large events are expected on average every 228 years while a tsunami of any size is generated every about 84 years which is in good agreement with the experimental observations

a diode laser system for heating minerals for u th he chronometry we have developed a diode laser 25 w 808 nm system for he extraction from minerals for u th he chronometry the laser beam is delivered via a 600 m fiber cable and focused using a binocular microscope temperatures necessary for he release from apatite 500 600 c and zircon 1100 1300 c encapsulated in pt foil tubes are attained by heating to 0 5 w for 30 s and 1 25 2 5 w for 20 min respectively using a defocused beam heating at these powers does not result in measurable u and or th loss from apatite as noted by the preservation of the distinct th u in multiple splits of two different durango apatite crystals analyses of durango and the california institute of technology internal standard apatite 97mr22 yield u th he ages of 32 8 1 8 ma 1 n 11 and 4 6 0 5 1 n 5 respectively well within accepted ages the u th he age and th u of five fish canyon tuff zircon aliquots yield 29 3 2 2 ma 1 and 0 6 0 03 respectively and are indistinguishable from ages produced by resistance furnace he extraction heating of unencapsulated minerals shows that the diode laser couples well with optically opaque minerals e g hornblende biotite muscovite garnet and basalt groundmass suggesting that diode lasers offer a cheap small low maintenance alternative to nd yag and ar ion lasers for 40ar 39ar cosmogenic noble gas and stable isotope studies

u and th zonation in fish canyon tuff zircons implications for a zircon u th he standard the inability to accurately determine the distribution of u and th in zircon crystals analysed using u th he is a major source of error and limits the confidence in subsequent data interpretation the fish canyon tuff fct zircon standard shows u th he age reproducibility in excess of 10 we have tested the extent to which this is due to u and th zonation using a combination of cathodoluminescence and ion probe analysis of a population of fct zircons primarily we find that fct zircons exhibit extreme u and th zonation and the population has large inter crystalline variability furthermore the net cathodoluminescence intensity from the fct zircons is negatively correlated with u and th concentration allowing cathodoluminescence emission to be used as a proxy for u and th zonation this correlation was exploited to constrain the u and th zonation within the crystal population and to show that the poor age reproducibility of the fct zircons is consistent with the observed zonation

the late holocene gargano subaqueous delta adriatic shelf sediment pathways and supply fluctuations the gargano subaqueous delta formed on the eastern and southeastern sides of the gargano promontory in the western adriatic this subaqueous deposit represents the southernmost portion of the late holocene highstand systems tract hst growing along the western side of the adriatic as an extensive wedge of deltaic and shallow marine mud the late holocene hst rests above a regional downlap surface that marks the time of maximum landward shift of the shoreline attained around 5 5 cal kyr bp at the end of the late pleistocene holocene sea level rise high resolution seismic stratigraphic and tephra correlation indicate the presence of a thin basal unit recording condensed deposition between 5 5 and 3 7 cal kyr bp over much of the basin above this unit sediment accumulation rates increased to high values as much as 1 5 cm yr 1 reflecting the stabilisation of relative sea level and the forcing from high frequency climatic or anthropogenic changes affecting river dynamics the late holocene mud wedge of which the gargano subaqueous delta is a significant component reaches up to 35 m in thickness and has a volume of ca 180 km3 the shore parallel thickness distribution of the mud wedge reflects the dominant oceanographic regime of the basin and the asymmetric location of the mostly western sediment sources with a combined modern delivery of 51 7 106 t yr 1 of mean suspended load in sections perpendicular to the coast the late holocene mud wedge appears composed of forestepping clinoforms with gently dipping foresets typically 0 5 the gargano subaqueous delta is characterised by a submarine topset in water depths shallower than 25 28 m and accounts for about 1 7th of the total volume of the late holocene mud wedge despite the absence of direct river supply to the gargano area in the area of maximum interaction between shore parallel currents and basin morphology progradation occurs onto a flat and barren bedrock outcrop in about 50 80 m water depth the rapid transition from a thickness of 30 m of late holocene mud to nil is a good indication of the role of southward flowing bottom hugging shelf currents in causing the redistribution of sediment along the adriatic inner shelf additional evidence of this regime comes from 1 the most recent sigmoid defined at seismic stratigraphic scale deposited since the onset of the little ice age showing a shore parallel thickness distribution and a main depocentre to the southeast of the gargano promontory 2 the maximum values of sediment accumulation rates over the last century documented by 210pb measurements defining a narrow shore parallel belt immediately seaward of the depocentre of the most recent sigmoid the gargano subaqueous delta grows from the outbuilding of progressively younger progradational sigmoids that tend to parallel the previous ones the gargano subaqueous delta differs from other documented late holocene subaqueous deltas because its growth reflects 1 sediment transport dominated by bottom currents sub parallel to the strike of the composing clinoforms 2 a complex supply regime including the po delta 350 km to the north and several coalescing apennine rivers acting as line source 3 several alternating intervals of enhanced outbuilding and condensed deposition and 4 an in phase growth of the most recent sigmoid with the major progradation of the po delta during the little ice age

morphometric analysis and genetic implications of pro deltaic sea floor undulations in the northern alboran sea margin western mediterranean basin abstract several fields of seafloor undulations from pro deltaic wedges in the northern margin of alboran sea western mediterranean basin are described in this paper a series of elongated subparallel bathymetric undulations are distinguished in the foreset to bottomsets regions of the holocene pro deltaic wedges related with the guadalfeo and verde seco rivers multibeam data surficial sediment samples and high resolution seismic profiles have been used in this study and integrated in a geographical information system quantitative measurement of key morphometric parameters wave length height asymmetry length and wave form index was performed on multibeam data and seismic profiles using the arcgis software wave form index and depth data were analysed statistically and a cluster analysis was carried out in each area according to these results we infer that sea floor undulations in the study areas are best explained by the influence of strong sediment flows normal to bathymetric contours probably coupled with the effect of subsequent slow sediment deformation processes such as sediment creeps the influence of bottom current activity is considered less likely as the position of river mouths and the very high slope gradients seems to be the key factors which control the development of these sea floor undulations the alboran sea sea floor undulations are compared with those of other fields in shallow water environments and the different processes of formation evaluated

frictional melting of gabbro under extreme experimental conditions of normal stress acceleration and sliding velocity 1 with the advent of high velocity shear apparatus several experimental studies have been performed in recent years improving our understanding of the evolution of fault strength during seismic slip however these experiments were conducted under relatively low normal stress 20 mpa and using small cylindrical samples where a large gradient in slip velocity exists across the sliding surface given the above limitations the extrapolation of these experimental results to natural conditions is not trivial here we present results from an experimental study on gabbroic rocks using a newly developed rotary shear apparatus capable of reaching higher normal stress up to 50 mpa on ring shaped samples 30 50 mm internal external diameter and allowing precise control of the imposed slip velocity function the results confirm that steady state shear stress during the melt lubricated phase of the experiment depends on normal stress in the form of a power law equation as predicted by theoretical models however the exponent appears closer to 0 5 contrary to the theoretical prediction of 0 25 we observe no systematic dependence of shear stress on acceleration but increasing deceleration drastically decreases the recovery of friction during final slip we find that the slip weakening distance decreases inversely with increasing normal stress in agreement with theoretical considerations and decreases with increasing slip rate extrapolation of the slip weakening distance to natural conditions predicts a slip velocity for ancient seismic events of 0 3 1 m s when compared with field estimates these values compare well with seismological estimates

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

gas seepage and assumed mud diapirism in the italian central adriatic sea abstract an assemblage of acoustic and topographical features recorded on high resolution 3 5 khz sub bottom profiler records are discussed a close relationship between locally elevated sea bed mounds or ridges and the several acoustic features is found in the adriatic sea off ortona italy these features include acoustically turbid or opaque zones assumed to represent gas charged sediments acoustically transparent sediments assumed to represent sediments with relatively high pressured pore water free of large gas bubbles acoustic fountains assumed to represent 1 2 m wide sediment columns of relatively high pressured pore water where the gas charged pore water may migrate vertically through the sediment acoustic voids assumed to be up to 50 m wide sediment zones near the sea bed containing high pressured gas charged pore water finely layered sediments assumed to represent sediments with sharp density contrasts caused by varying gas concentrations a function of grain size and stagnant pore water gas seepage to the water column originating at the acoustic voids the positive topographical sea bed features are assumed to represent sediment clay diapiric structures the mechanism of diapir formation and its association with the gas charged sediments and gas seepages is discussed diapirism is assumed to result when plastic clay near the sea bed is charged with gas migrating from below bubbles forming and concentrating in the plastic clay causes local density reversals whereby the clay flows upwards due to buoyancy thus forming the elevated topographical features and the associated gas seepages

analysis of holocene sedimentary features on the adriatic shelf from 3d very high resolution seismic data triad survey very high resolution vhr 3d seismic data were collected on the adriatic shelf offshore the city of ortona in order to define the small scale geometric complexity of late holocene deposits three groups of units are distinguished from the bottom up 1 units u1 to u3 forming the transgressive system tract tst 2 units u4 and u5 representing the condensed interval deposited on the maximum flooding surface mfs and the basal unit of the high stand system tract hst and 3 u6 to u10 forming the progradational units of the hst the muddy hst deposits show seafloor and subsurface sedimentary features over up to 40 of its extent on the adriatic shelf in the area offshore ortona the hst shows shore parallel undulations in water depth of ca 30 70 m and mud reliefs in water depth of ca 70 110 m the mud reliefs are subcircular in plan view and occur in swarms perpendicular to the regional contour that are connected to each other by shore parallel junctions the genesis of these features is interpreted as a combination of two different mechanisms 1 sediment deformation in relation with the condensed section and 2 depositional process related to shore parallel currents interacting with topographic irregularities the evolution of the hst sedimentary pattern is closely related to a key unit u5 that influences both the seaward transition from linear to complex features and the stacking pattern

frictional melt and seismic slip 1 frictional melt is implied in a variety of processes such as seismic slip ice skating and meteorite combustion a steady state can be reached when melt is continuously produced and extruded from the sliding interface as shown recently in a number of laboratory rock friction experiments a thin low viscosity high temperature melt layer is formed resulting in low shear resistance a theoretical solution describing the coupling of shear heating thermal diffusion and extrusion is obtained without imposing a priori the melt thickness the steady state shear traction can be approximated at high slip rates by the theoretical form ss n1 4 a under a normal stress n slip rate v radius of contact area r a is a dimensional normalizing factor and w is a characteristic rate although the model offers a rather simplified view of a complex process the predictions are compatible with experimental observations in particular we consider laboratory simulations of seismic slip on earthquake faults a series of high velocity rotary shear experiments on rocks performed for n in the range 1 20 mpa and slip rates in the range 0 5 2 m s 1 is confronted to the theoretical model the behavior is reasonably well reproduced though the effect of radiation loss taking place in the experiment somewhat alters the data the scaling of friction with n r and v in the presence of melt suggests that extrapolation of laboratory measures to real earth is a highly nonlinear nontrivial exercise

tephra layers in late quaternary sediments of the central adriatic sea abstract magnetic susceptibility curves of ten cores collected in the western adriatic shelf and in the mid adriatic deep allowed us to recognize thirty tephra layers whose age based on physical and biostratigraphic data encompass a time interval ranging from less than 70 ka to the present chemical features of the tephra defined by sem eds analyses performed on glass shards indicate alkaline affinity mainly with k na 1 and trachytic compositions with two minor benmoreitic and phonolitic groups these characteristics are consistent with campanian and subordinate etnean provenances because of chemical and age constraints the glass shards can be referred to six tephra layers already described in the literature such as c20 c14 citara ischia c10 campanian ignimbrite c2 nyt neapolitan yellow tuff ams pf agnano monte spina phlegrean fields from campanian area and y1 from etna volcano indicating an age ranging from about 70 to 4 ka ams pf and c2 nyt dated 4 4 and 12 3 ka respectively are the most widespread tephra beds in the study cores the occurrence of ams pf never recorded in marine sequences up to now represents a very good marker for holocene adriatic sediments c2 nyt already recorded in the central tyrrhenian sea in lacustrine sediments from southern italy and in the campanian area is a good stratigraphic marker for inter sea and sea land correlations y1 tephra dated 14 2 ka also occurs in on land sediments of central italy and in marine sediments from the tyrrhenian and ionian seas it now represents one of the most widespread stratigraphic markers in the central mediterranean region

on the transient behavior of frictional melt during seismic slip s n and g d t were supported by a european research council starting grant project acronym usems and by a progetti di eccellenza fondazione cassa di risparmio di padova e rovigo we are grateful to nick beeler and to an anonymous referee for their constructive reviews and their help to improve the clarity of the manuscript

fracturing and hydrothermal alteration in normal fault zones large normal fault zones are characterized by intense fracturing and hydrothermal alteration displacement is localized in a slip zone of cataclasite breccia and phyllonite surrounding corrugated and striated fault surfaces slip zone rock grades into fractured but less comminuted and hydrothermally altered rock in the transition zone which in turn grades abruptly into the wall rock fracturing and fluid flow is episodic because permeability generated during earthquakes is destroyed by hydrothermal processes during the time between earthquakes

the argus multicollector noble gas mass spectrometer performance for 40ar 39ar geochronology we describe a new high sensitivity multicollector noble gas mass spectrometer argus that has been specifically designed for simultaneous collection of all ar isotopes and hence is ideally suited for 40ar 39ar geochronology the instrument uses a detector housing that holds five faraday collectors one collector is equipped with a 1011 ohm resistor for 40ar and the remaining four are fitted with 1012 ohm resistors for 36ar to 39ar argus has a mass resolution of 225 250 and a measured sensitivity of 7 10 14 mol v at 200 a trap current during the course of a 1 month run cycle 145 air calibrations yielded a weighted average 40ar 36ar of 300 67 0 07 2 eight data points rejected and mean square weighted deviation 1 18 the ages of three mineral standards taylor creek rhyolite sanidine heidelberg biotite and alder creek sanidine are within error of the accepted ages and uncertainties for two are improvements on previously published data argus decreases analytical time while allowing more measurements to be made at higher precision compared to standard peak jumping single collector mass spectrometers

frictional melting processes and products in geological materials introduction and discussion this special issue is the outcome of a theme session convened by the above authors entitled frictional melting processes and products in geologic materials held at the 1990 geological society of america annual meeting in dallas texas u s a the goals of the session were threefold to bring together researchers working on various aspects of frictional melting in rock to provide a forum for the exchange of new ideas and to encourage the production of a thematic set of papers dedicated to the topic of the eight papers presented here the first two spray swanson are primarily processand mechanism oriented the next magloughlin maddock o hara deal primarily with chemical aspects of host rock pseudotachylyte relations the last three mainly provide descriptions of naturally occurring koch and masch techmer et al and artificially produced kennedy and spray friction melts natural pseudotachylytes are described from strike slip zones magloughlin swanson thrusts and their listric counterparts koch and masch techmer et al and connecting lateral ramps o hara thus it appears that there exists an association between pseudotachylyte generation and relatively long lived largedisplacement faulting and shearing the prime objective of the meeting was to promote the integration of different approaches

thin pseudotachylytes in faults of the mt abbot quadrangle sierra nevada physical constraints for small seismic slip events abstract we document the occurrence of pseudotachylyte solidified melt produced during seismic slip along strike slip faults in the lake edison granodiorite of the mt abbot quadrangle sierra nevada california and provide constraints on ambient conditions during seismic faulting the pseudotachylytes are less than 0 3 mm thick and are found in faults typically up to 1 cm in thickness total measured left lateral offset along sampled faults is approximately 20 cm field and microstructural evidence indicate that the faults exploited pre existing mineralized joints and show the following overprinting structures with inferred ambient temperatures mylonites are more or less coeval with quartz veins 400 c cataclasites and pseudotachylytes 250 c more or less coeval with epidote veins and zeolite veins

growth linkage and termination processes of a 10 km long strike slip fault in jointed granite the gemini fault zone sierra nevada california abstract field based structural analysis of an exhumed 10 km long strike slip fault zone elucidates processes of growth linkage and termination along moderately sized strike slip fault zones in granitic rocks the gemini fault zone is a 9 3 km long left lateral fault system that was active at depths of 8 11 km within the transpressive late cretaceous sierran magmatic arc the fault zone cuts four granitic plutons and is composed of three steeply dipping northeast and southwest striking noncoplanar segments that nucleated and grew along preexisting cooling joints the fault core is bounded by subparallel fault planes that separate highly fractured epidote chlorite and quartz breccias from undeformed protolith the slip profile along the gemini fault zone shows that the fault zone consists of three 2 3 km long segments separated by two zones of local slip minima slip is highest 131 m on the western third of the fault zone and tapers to zero at the eastern termination slip vectors plunge shallowly west southwest and show significant variability along strike and across segment boundaries four types of microstructures reflect compositional changes in protolith along strike and show that deformation was concentrated on narrow slip surfaces at or below greenschist facies conditions taken together we interpret the fault zone to be a segmented linked fault zone in which geometrical complexities of the faults and compositional variations of protolith and fault rock resulted in nonuniform slip orientations complex fault segment interactions and asymmetric slip distance profiles

a high resolution late holocene palaeo environmental record from the central adriatic sea a multi proxy study of a holocene sediment core rf 93 30 from the western flank of the central adriatic in 77 m of water reveals a sequence of changes in terrestrial vegetation terrigenous sediment input and benthic fauna as well as evidence for variations in sea surface temperature spanning most of the last 7000 yr the chronology of sedimentation is based on several lines of evidence including ams 14c dates of foraminifera extracted from the core palaeomagnetic secular variation pollen indicators and dated tephra the temporal resolution increases towards the surface and for some of the properties measured is sub decadal for the last few centuries the main changes recorded in vegetation sedimentation and benthic foraminiferal assemblages appear to be directly related to human activity in the sediment source area which includes the po valley and the eastern flanks of the central and northern appenines the most striking episodes of deforestation and expanding human impact begin around 3600 bp late bronze age and 700 bp medieval and each leads to an acceleration in mass sedimentation and an increase in the proportion of terrigenous material reflecting the response of surface processes to widespread forest clearance and cultivation although human impact appears to be the proximal cause of these changes climatic effects may also have been important during these periods signs of stress are detectable in the benthic foram morphotype assemblages between these two periods of increased terrigeneous sedimentation there is smaller peak in sedimentation rate around 2400bp which is not associated with evidence for deforestation shifts in the balance between terrigenous and authigenic sedimentation or changes in benthic foraminifera the mineral magnetic record provides a sensitive indicator of changing sediment sources during forested periods of reduced terrigenous input it is dominated by authigenic bacterial magnetite whereas during periods of increased erosion anti ferromagetic minerals haematite and or goethite become more important as well as both paramagnetic minerals and super paramagnetic magnetite analysis of the alkenone u37k record provides an indication of possible changes in sea surface temperature during the period but it is premature to place too much reliance on these inferred changes until the indirect effects of past changes in the depth of the halocline and in circulation have been more fully evaluated the combination of methods used and the results obtained illustrate the potential value of such high resolution near shore marine sedimentary sequences for recording wide scale human impact documenting the effects of this on marine sedimentation and fauna and potentially disentangling evidence for human activities from that for past changes in climate

depositional and erosional bedforms in late pleistocene holocene pro delta deposits of the gulf of patti southern tyrrhenian margin italy abstract multibeam bathymetry high resolution seismic profiles and seafloor samples have been analyzed to characterize depositional and erosional dynamics recorded pro deltaic deposits and outer shelf sediments along of a sector of the ne sicilian margin southern tyrrhenian sea the deltaic deposits cover an area of ca 15 km 2 in front of the mazzarra river and are morphologically characterized by waveforms trending overall along strike and incised cross strike gullies of variable length the gullies are shallow and characterized by small coaxial erosive scours in the inner middle shelf whereas they become larger and deeper in the outer shelf upper slope in relation to the marked increase of slope gradients at the shelf break here the wider gullies are characterized by a frame of crescent shaped bedforms interpreted as cyclic steps indicating the occurrence of sedimentary gravity flows in supercritical regime prodelta waveforms are widespread between 50 and 120 m with wave lengths of 34 110 m and wave heights of 0 5 3 m morphometric characterization and spatial distribution of the waveforms suggests a main role in their genesis played by hyperpycnal flows although we cannot discard the possible effect of internal waves or slow deformation processes i e creep depositional and erosional features similar to those observed on the seafloor are evident in the subsurface stratigraphy revealing the onset and growth of the mazzarra delta since the last glacial maximum the post glacial sea level rise caused lateral shifts of the mazzarra river mouth controlling migration of depositional lobes and intensity of seafloor incision and sediment reworking ultimately resulting in the observed wavy bedforms extending from the inner shelf to the upper slope

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

rough faults distributed weakening and off fault deformation 1 we report systematic spatial variations in fault rocks along nonplanar strike slip faults cross cutting the lake edison granodiorite sierra nevada california sierran wavy fault and lobbia outcrops of the adamello batholith in the italian alps lobbia wavy fault in the case of the sierran fault pseudotachylyte formed at contractional fault bends where it is found as thin 1 2 mm fault parallel veins epidote and chlorite developed in the same seismic context as the pseudotachylyte and are especially abundant in extensional fault bends we argue that the presence of fluids as illustrated by this example does not necessarily preclude the development of frictional melt in the case of the lobbia fault pseudotachylyte thickness varies along the length of the fault but the pseudotachylyte veins thicken and pool in extensional bends we conduct a quantitative analysis of fault roughness microcrack distribution stress and friction along the lobbia fault numerical modeling results show that opening in extensional bends and localized thermal weakening in contractional bends counteract resistance encountered by fault waviness resulting in an overall weaker fault than suggested by the corresponding static friction coefficient the models also predict static stress redistribution around bends in the faults which is consistent with distribution of microcracks indicating significant elastic and inelastic strain energy is dissipated into the wall rocks due to nonplanar fault geometry together these observations suggest that damage and energy dissipation occurs along the entire nonplanar fault during slip rather than being confined to the region close to the dynamically propagating crack tip

sedimentary structures offshore ortona adriatic sea deformation or sediment waves the late holocene mud wedge on the adriatic shelf offshore ortona italy shows undulating sub parallel seismic reflector sequences which extend several kilometres along strike and 100 200 m down dip in water depth between 20 and 80 m the amplitude of such undulations is up to 5 m and the undulations continue as stacked sediment packages downwards throughout the 35 m thick mud wedge the undulations are separated by 4 to 5 dipping boundary zones and at first glance these sediment undulations resemble the seafloor sedimentary structures visible in the humboldt feature offshore california there is an ongoing debate whether seafloor undulations are the result of deformation processes or sediment deposition and or reworking due to submarine shelf currents a dense net of recently reprocessed and digitally interpreted high resolution chirp seismic data on the adriatic shelf favours an interpretation that these undulations developed in the upper part of the stratigraphic section through sediment reworking rather than through deformation there are three lines of evidence for this 1 the spatial extent of the undulations coincides with higher seabed reflector amplitudes than found both on the shelf and in the distal part of the shelf if the features were solely caused by slope failure there should be no change of amplitudes 2 the seabed reflector amplitude is generally higher on the gently dipping or flat upslope limbs than on the steeper downslope limbs supporting a current origin that causes preferred deposition of sediments with higher acoustic impedance on the top 3 the boundaries between the undulations are dipping at angles that are much lower than the angle of internal friction of these sediments excluding that a simple model like the mohr coulomb under gravitational loading could describe the undulations as sediment deformation and failures

multi process generated sediment waves on the landes plateau bay of biscay north atlantic detailed analyses of recently collected bathymetric and sparker seismic data support a new interpretation of the landes plateau field of sediment waves located on the aquitaine upper continental slope bay of biscay the wave geometry previously described as the result of a major sediment failure is interpreted as a structure with a complex origin including the interaction of depositional and gravity deformation processes depositional processes are mainly recorded by the upslope migrating pattern of the waves resulting from oblique or sigmoid downlap reflections on the upslope flank of the waves and by toplapping and truncated reflections on the downslope flank hemipelagic and turbiditic sedimentation may be involved in the wave building as well as contouritic processes that could be related to the existing northward polar current and internal waves gravity deformations are syndepositional discontinuous and of low amplitude affecting thick layers which alternate with undeformed layers they seem to correspond to gentle sediment creeping or stretching associated with minor listric or compaction like faults and possible limited back rotation of sediment blocks these multi process generated sediment waves could be rather common on the continental margins as they could have been mistaken with either depositional or deformational structures

sub millennial scale climatic oscillations in the central adriatic during the lateglacial palaeoceanographic implications abstract a multi proxy study of a sedimentary core obtained from the central adriatic basin provides evidence of short term events of palaeoceanographic variability during the last glacial lateglacial period grip event stages gi 1 and gs 1 comparison of the adriatic results with other mediterranean records reveals a common sequence of changes in palaeoceanographic conditions variations in foraminiferal assemblage data and in stable isotopic compositions indicate two short term cold oscillations which interrupted an overall cooling trend that commenced at the beginning of the greenland gi 1 episode bolling allerod episode these cold events considered to be contemporaneous with gi 1b and gi d of the grip ice core record are indicated by distinctive foraminiferal assemblages which reflect episodes of high productivity under relatively restricted water circulation with in particular reduced vertical mixing during winter differences in the foraminiferal assemblages indicate that the younger of the two cold spells is characterized by reduced ventilation of bottom waters our results combined with published data from neighbouring marine basins suggest a widespread reduction in deep water formation in the central mediterranean which commenced during the gi 1b episode and reached its maximum effect during the onset of the gs 1 younger dryas episode moreover three intervals with reduced vertical mixing correspond to the two cold spells during gi 1 and the lower phase of the gs 1 episode and show close similarities with the palaeoceanographic changes reported for the north atlantic this similarity indicates that the mediterranean and north atlantic may have been responding more or less synchronously to common forcing mechanisms

pseudotachylytes in an ancient accretionary complex and implications for melt lubrication during subduction zone earthquakes abstract pseudotachylyte bearing fault zones found in the shimanto accretionary complex southwest japan developed during subduction or underplating at seismogenic depths the pseudotachylytes occur in narrow dark veins less than a few millimeters thick that are sharply bounded by foliated cataclasites that originated from a melange the microstructures of pseudotachylytes are represented by a fragment laden glass supported texture resulting from the rapid cooling of the frictional melt transmission electron microscopy reveals the presence of glass in which euhedral microcrystals of mullite are locally developed the compositions of the pseudotachylyte matrix and the characteristics of the unmelted grains and microlites in the matrix suggest that frictional melting occurred in an illite rich slip zone with a minimum melting temperature of 1100 c the viscosities of the frictional melt were calculated from the pseudotachylyte matrix composition as well as from the volume fraction and aspect ratio of the unmelted grains the viscosities at 1100 c range from 85 to 290 pa s and the corresponding shear resistance along a 1 mm thick melt layer at a slip rate of 1 m s was 0 1 0 3 mpa the formation of a melt layer in an illite rich slip zone can possibly induce large stress drops increase the slip rate and enhance rupture propagation which together could affect the earthquake magnitude in a subduction zone

slope failures and stability analysis of shallow water prodeltas in the active margins of western greece northeastern mediterranean sea sediment instabilities are common on the prodeltas of the seismically active continental margins of western greece sediment failures on the low angle 0 5 2 prodelta slopes manifest themselves as successions of peripheral rotational block slumps restricted to the foresets of the late highstand systems tract hst the individual slump blocks are about 80 150 m long and are bounded by growth faults acting as curved slip planes that extend to a mean depth of 10 15 m below seafloor shear planes develop in the lower part of muddy and or gas charged hst foresets deeper basal transparent muddy layers of the early hst bottomset together with the late pleistocene transgressive systems tract sequences tst are mostly unaffected on the steeper 2 6 fan delta slopes of the western gulf of corinth debris flows and avalanches with a significant retrogressive component dominate slope destabilisation sediment cores taken from landslide scarps and slide planes penetrated gas bubble releasing sediments thereby indicating that failure planes are in the late hst foresets upper part of the early hst bottomsets gas charged zone the foresets of the hst prodelta deposits display high water content 30 80 low bulk density 1 4 1 9 g cm 3 and relatively low values of undrained shear strength 3 20 kpa the water content of the hst distal muddy bottomsets is relatively higher 50 110 and bulk density relatively lower 1 3 1 7 g cm 3 with low values of shear strength 2 10 kpa the shear strength of the gas releasing sediment layer displays lower values 2 9 kpa relative to the overlying post failure muddy sediments of the late 100 300 years slope stability was calculated using the normalised soil parameter nsp method under undrained conditions for normally consolidated prodelta sediments this analysis indicates that instabilities could be induced by critical earthquake ground accelerations of 26 6 29 6 g for the hst foresets and 12 4 14 1 g for the basal transparent layer belonging to the early hst bottomsets consequently the early hst bottomsets has to be considered a potentially unstable layer since the regional peak ground accelerations pgas for the next 50 years are expected to range from 19 to 30 g moreover our results show that new glide planes in the prodeltaic sediment bodies of the seismically active continental margins of western greece will likely develop from the gas charged sediments of the lower part of the hst foresets to the upper part of early hst bottomsets

the effect of particle shape on suspension viscosity and implications for magmatic flows 1 the rheology of crystal bearing magma and lava depends on both the shape and volume fraction of the suspended crystals we present the results of analogue rheometric experiments on monodisperse suspensions of solid particles in a newtonian liquid in which particle volume fraction and aspect ratio rp are varied systematically we find that the effect of on viscosity is well captured by the maron pierce model and that this model is valid across the range of particle aspect ratios investigated 0 04 rp 22 i e strongly oblate to strongly prolate when the maximum packing fraction m is treated as a fitting parameter the value of m derived from fitting to our experimental data depends strongly and systematically on particle aspect ratio hence m represents an effective proxy for the influence of particle shape on suspension rheology we present a simple relationship for m rp which allows the viscosity of a suspension to be calculated as a function of and rp only we investigate the impact of accounting for crystal shape when modelling volcanic flows by simulating the eruption of magma carrying crystals of different aspect ratio and conclude that the effect of crystal shape should not be neglected

distinguishing sediment waves from slope failure deposits field examples including the humboldt slide and modelling results abstract migrating sediment waves have been reported in a variety of marine settings including submarine levee fan systems floors of fjords and other basin or continental slope environments examination of such wave fields reveals nine diagnostic characteristics when these characteristics are applied to several features previously attributed to submarine landslide deformation they suggest that the features should most likely be reinterpreted as migrating sediment wave fields sites that have been reinterpreted include the humboldt slide on the eel river margin in northern california the continental slope in the gulf of cadiz the continental shelf off the malaspina glacier in the gulf of alaska and the adriatic shelf a reassessment of all four features strongly suggests that numerous turbidity currents separated by intervals of ambient hemipelagic sedimentation deposited the wave fields over thousands of years a numerical model of hyperpycnal discharge from the eel river for example shows that under certain alongshore current conditions such events can produce turbidity currents that flow across the humboldt slide serving as the mechanism for the development of migrating sediment waves numerical experiments also demonstrate that where a series of turbidity currents flows across a rough seafloor i e numerical steps sediment waves can form and migrate upslope hemipelagic sedimentation between turbidity current events further facilitates the upslope migration of the sediment waves physical modelling of turbidity currents also confirms the formation and migration of seafloor bedforms the morphologies of sediment waves generated both numerically and physically in the laboratory bear a strong resemblance to those observed in the field including those that were previously described as submarine landslides

stress and deformation along wavy frictional faults locally inhomogeneous stress states are expected along faults owing to slip on geometrically irregular fault surfaces we use an analytical model of elastic deformation along a wavy frictional fault to evaluate the variation in local stress state as a function of surface roughness elastic modulus slip coefficient of friction and far field stress the total stress state along the fault may be described by the sum of a basic stress component resulting from frictional slip on a planar fault surface and a perturbed stress component resulting from the presence of roughness roughness produces a variation in normal stress across the fault surface and assuming roughness and modulus appropriate to crustal faults the normal stress should be reduced to a near zero magnitude locally such that separation of fault walls is likely the large variation in normal stress along the fault surface resulting from fault roughness may be responsible in part for complexity in moment release during large earthquakes and for lateral variation in seismic coupling along faults the variation in principal stress orientations and magnitudes along a fault increases with a decrease in the coefficient of friction of the fault the location and size of regions with a high likelihood for brittle failure depend on the orientation of the far field principal stress and fault friction the average orientation of the principal stresses in the region of likely failure is not the same as the far field principal stress orientation although inversion of earthquake and fabric data for stress orientation along a fault may be possible the model results suggest that inversion results are insufficient to determine far field stress states and fault friction without additional independent data

morphobathymetric analysis and evidence of submarine mass movements in the western gulf of taranto calabria margin ionian sea the western part of the gulf of taranto southern italy is a seismically active area with high sediment supply new swath bathymetry and sub bottom profiler chirp data were acquired on board r v ogs explora during the wgdt cruise the data were analyzed to describe the seafloor morphology and the acoustic facies of relevant morphologic features special attention was given to the features produced by mass wasting and soft sediment deformations the features identified include slides and slide scars debris flow deposits enigmatic dip slope trending sub parallel linear depressions and along slope undulations inferred to have been produced either by creeping or sediment deposition by hyperpycnal flows the description of these features contributes to the characterization of the calabria foreland basin system and provides an analogue for other basins similarly affected by a close relation between sediment supply and active tectonics

sand spit dynamics in a large tidal range environment insight from multiple lidar uav and hydrodynamic measurements on multiple spit hook development breaching reconstruction and shoreline changes

geologic evidence for multiple slip weakening mechanisms during seismic slip in crystalline rock we examine exhumed seismogenic faults to investigate the mechanisms that may have achieved dynamic fault weakening during ancient ruptures field and microscope observations imply more than one weakening mechanism must have been active during slip events on the faults pseudotachylytes that are continuous over the scale of field exposures are indicative of melt lubrication a fault breccia crosscutting earlier formed cataclasites was mobilized during faulting and possibly represents a pressurized fault rock that resulted from thermal pressurization or elastohydrodynamic lubrication in some faults pseudotachylytes are developed in patches several meters long cataclasites in which there is no evidence for melting are present immediately along strike from the pseudotachylyte patches by considering the energy required for melting we show that the pseudotachylytes must have formed during ruptures larger than the patches implying that the cataclasites also accommodated seismic slip the distribution of fault rock types shows that the frictional response to slip during a single event was spatially variable reworked pseudotachylytes also indicate that coseismic processes change over time at a point on a fault these observations emphasize that macroscopic dynamic fault weakening is a function of multiple coeval processes at microscales to mesoscales detailed observations of the discontinuous pseudotachylytes show that slip zone thickness is the critical parameter that controls active coseismic processes the frictional response of a fault to slip is therefore dependent on the internal structure of faults given the along strike heterogeneity of most mapped fault zones the coexistence of multiple slip weakening mechanisms in a single earthquake will be common

grenville age pseudotachylite in the lewisian laserprobe 40ar 39ar ages from the gairloch region of scotland uk infrared laserprobe 40 ar 39 ar dating has been used to date pseudotachylite and host rock minerals from a crush belt in the lewisian basement of scotland it has revealed complexity in the pseudotachylite data that is attributable to the presence of refractory host rock clasts and mineral fragments in the pseudotachylite in conjunction with the host rock mineral laserprobe 40 ar 39 ar data it has been possible to simplify the pseudotachylite data for the samples and the preferred ages for these are 980 39 ma 999 31 ma and 1024 30 ma 2 these ages are the first record of grenville aged brittle deformation in the lewisian further this study serves to illustrate the complexity of dating pseudotachylites and the advantages and limitations of the ir laserprobe applied to such materials

insights into volcanic conduit flow from an open source numerical model 1 numerical models that calculate the fluid dynamics of explosive volcanic eruptions have been used with increasing frequency to understand volcanic processes and evaluate volcanic hazards yet those who develop such models rarely make them publicly available so that they can be verified used and possibly improved by other scientists in this paper i present a visual interactive open source numerical model that calculates steady state flow of magma and gas in vertical eruptive conduits and contains user friendly utilities for quickly determining physical thermodynamic and transport properties of silicate melts h2o gases and melt gas crystal mixtures the model represents an advance over previously published conduit models by incorporating a non arrhenian viscosity relation for hydrous silicate melts a relation between viscosity and volume fraction of gas that depends on capillary number and adiabatic temperature changes in the mixture using established thermodynamic relations for melts and h2o gas respectively volcanologists who have not had access to conduit models have frequently approximated conduit flow using an analytical equation for incompressible laminar newtonian pipe flow which predicts that the mass flux is proportional to the fourth power of conduit radius and inversely proportional to mixture viscosity the model presented here which is not much more difficult to use than a back of the envelope calculation shows that the pipe flow approximation significantly overestimates the sensitivity of mass flux to both conduit radius and mixture viscosity results from the model also show that viscous heating in the lower conduit which is not considered in most other models may increase the mass flux of large silicic eruptions by several percent and decrease the viscosity of the mixture at the fragmentation depth by a few tens of percent

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

faults smooth gradually as a function of slip abstract geometry and roughness of fault surfaces plays a central role in the dynamics and kinematics of faulting faults smooth with increasing slip but the degree of the smoothing has not previously been well constrained for natural faults we measure the roughness as a function of displacement for a suite of 16 faults with cumulative offsets ranging from 0 1 m to approximately 500 m we find that slip parallel roughness evolves gradually with slip for instance for segments of length 0 5 m h 2 10 3 d 0 1 where h is the rms roughness and d is the displacement on the fault strand with both quantities measured in meters the gradual nature of the smoothing is robust to varying lithology and erosion the weak function implies a decrease in the rate of gouge formation for a model with increasing slip for a model in which gouge is generated by abrading an asperity tip the relatively gradual evolution of roughness could be explained by lubrication by the accumulated gouge that mitigates the abrasional smoothing that occurs during slip and or re roughening processes

viscosity determinations of some frictionally generated silicate melts implications for fault zone rheology at high strain rates analytical scanning electron microscopy has been used to determine the major element compositions of some natural and artificial silicate glasses and their microcrystalline equivalents derived by the frictional melting of intermediate to acid protoliths the data show that the matrices of the friction melts which cool to form pseudotachylytes are relatively basic and hydrous even when their protoliths are intermediate to acid this is because frictional fusion involves the selective comminution and nonequilibrium melting of minerals based on their individual mechanical properties and melting points not the formation of minimum melts through equilibrium mineral interaction this means that hydrous ferromagnesian minerals e g micas and amphiboles melt preferentially to form the liquid matrix while feldspars and especially quartz more readily survive as clasts pseudotachylytes generated by frictional melting are therefore not bulk melts and as clast melt suspensions they cannot be considered as simple newtonian fluids the calculated viscosities of the friction melts are low for example at 1200 c most friction melts possess zero shear suspension viscosities of 102 104 dpa s 1 dpa s 1 p this is equivalent to the viscosities of tholeiitic and alkaline basaltic magmas at the same temperature these viscosities are maximum determinations because as clast melt suspensions friction melts may undergo shear thinning and exhibit pseudoplasticity at high shear rates i e during slip on a fault surface contrary to earlier suggestions where the bulk melting of intermediate to acid protoliths was believed to result in the generation of viscous friction melts that could act to inhibit continued sliding this work shows that most pseudotachylytes are partial melts possessing low viscosities the formation of highly fluid suspensions during slip may have profound effects on the dissipation of stored strain energy in the rocks surrounding a fault interface lubrication could facilitate an increase in the slip rate and the rate of energy dissipation this would be manifest as an increase in high frequency seismic wave radiation and vibrational

quantitative constraints on mid to shallow crustal processes using the zircon u th he thermochronometer despite the potential of zircon he thermochronometry for constraining rock thermal histories it remains less commonly exploited than the apatite he chronometer in part this is due to the more challenging analytical techniques required to extract he u and th here we present a new method for the routine determination of zircon u th he ages and demonstrate how it can be used to constrain thermal histories and to quantify cooling in different tectonic settings we present zircon u th he ages that place a firm upper limit on the extent of denudation induced cooling c 3 km on the se australian passive margin a region where synrift apatite fission track and apatite u th he ages have previously prevented quantitative constraint we have also used the zircon u th he thermochronometer to quantify the cooling of early tertiary mafic plutons from skye scotland where the rate and timing of cooling cannot be determined using other thermochronometers

transformation of cataclastically deformed rocks to pseudotachylyte by pervasion of frictional melt inferences from clast size analysis abstract optical contrast between clasts and surrounding dark coloured matrix in thin sections has been used in a computer aided method which has been applied for clast size analysis of pseudotachylyte exposed in a 25 km long brittle deformation zone in the precambrian terrain of western india while the number of clasts n2 equal to or coarser than size u shows a power law distribution n2 u d the area a1 occupied by the clasts of different size fractions shows a log normal distribution the clast size distribution pattern indicates transformation of cataclastically deformed rocks to pseudotachylytes by pervasion of a rock melt generated at the frictional slip planes it is inferred that the melt pervaded the permeable cataclastically damaged zones and so occupied the intergranular space this melt mixed with the comminuted material flowed further as a slurry to form the pseudotachylyte veins the clast laden final product retained the grain size distribution pattern of the parent comminuted material with minor modifications fusion of clast rinds due to heat transfer from the melt slightly modified the initial grain size distribution pattern because of the short duration of the heat transfer process combined with the low conductivity and high specific heat of rocks the temperature at the centre of the coarse grains remained at the initial temperature ti and reduction w of radius r by fusion was a negligible fraction of r as a consequence the population of coarse clasts retained the pre fusion size distribution pattern in contrast w r of the smaller grains was considerable the initial grain size distribution pattern did not survive in the finer fraction of the clasts this difference in thermal response of the smaller versus larger grains to the heat transfer process is revealed in clast size distribution graphs for the pseudotachylytes moreover the observed proportion of matrix can not be explained by cataclasis alone the extent of comminution in the parent material and size of the openings through which the clast laden melt flowed to form veins controlled the clast matrix ratio in the pseudotachylytes

pseudotachylytes generated during seismic faulting and eclogitization of the deep crust pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust pseudotachylytes in the bergen arcs of western norway contain microlites including omphacite garnet plagioclase and quartz this eclogite facies assemblage is stable at temperatures of about 800 c and pressures of 18 to 19 kilobars corresponding to depths of 60 kilometers or more the pseudotachylytes are exposed in grenvillian granulites that locally underwent fluid induced eclogitization and corresponding volume reduction of approximately 10 percent during the caledonian continental collision the pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process