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

the integration of seismic interpretations with gravity and magnetic 2 d modeling an example from the offshore faroes area the faroe shetland basin on the northwest european atlantic margin is one of the last frontier basins in the area figure 1 in recent years the basin s southeastern flank which lies in u k waters has attracted much attention primarily because of the foinaven and schiehallion paleocene oil discoveries that have an estimated 500 and 350 million bls of reserves respectively the northwestern flank of the basin which lies in faroese waters has remained a totally unexplored area this is partially due to the presence of an unknown thickness of basalt that covers the islands and the offshore area

frictional resistance of faults during accelerating and decelerating earthquake slip the dynamic friction along faults controls earthquake ruptures in the crust but many previous studies have quantified this value only for constant slip rates experiments accounting for the more realistic condition of changing slip rates suggest that faults undergo a sequence of strengthening weakening and healing during acceleration and deceleration of slip the dynamic friction of faults during earthquake slip is a critical control on earthquake ruptures in the crust extrapolation of brittle crack theories to natural earthquakes1 2 3 has led to the commonly held view that fault friction reduces during rapid earthquake slip a process known as slip weakening1 high velocity gouge experiments4 5 and recent analyses of thermal pressurization6 7 and frictional melting8 support such a notion however these studies dealt with constant rates of slip along faults here we present the results of experiments aimed at understanding the frictional behaviour of fault zone materials under variable slip rates conditions that are more representative of natural earthquakes our results show that faults undergo a sequence of strengthening weakening and healing during acceleration and deceleration of slip such a sequence may be explained by the extrapolation of rate and state frictional behaviour9 10 at low slip velocities to more realistic slip rates but involving different physical mechanisms and a different scale the initial strengthening should impose a barrier for rupture growth into large earthquakes the healing on decelerating fault motion may lead to pulse like earthquake ruptures11 12 13 14 and static stress drops that are low in comparison with the dynamic stress changes15

seismic stratigraphic analysis of the cenozoic sediments in the nw faroe shetland basin implications for inherited structural control of sediment distribution abstract the post basalt strata in the faroese area have been investigated based on interpretation of 2d and 3d reflection seismic data the post basalt package is divided into 5 units which have led to the constructions of 6 structural maps and 5 thickness maps within the 5 units 12 prograding sediment bodies have been identified based on the interpretation it is possible to obtain an overview during time of the location of depocentres and direction of prograding units within eocene time the depocentre was placed in the central part of the basin and the sediment influx was mostly from south and southwest during oligocene pliocene time the sediment influx was from north and northwest and the depocentre had moved in a westward direction closer to the faroe platform area emplacement of the cenozoic sediments in the faroese sector of the faroe shetland basin is controlled by thermal subsidence of the basin and local uplift of sediment source areas reactivation of older paleozoic and mesozoic structural elements seem to control the sediment path way and restrict the depositional areas various structural elements being re activated at different times caused considerable structural complexity understanding the older structural elements and their control on sedimentation is a potential tool for understanding deviations from normal thermal subsidence and for predicting the prospectivity in the post basalt succession in the faroe shetland basin

petrogenesis of the late cretaceous northern alberta kimberlite province abstract at present 48 late cretaceous ca 70 88 ma kimberlitic pipes have been discovered in three separate areas of the northern alberta the mountain lake cluster the buffalo head hills field and the birch mountains field the regions can be distinguished from one another by their non archetypal kimberlite signature mountain lake or in the case of kimberlite fields primitive buffalo head hills to evolved birch mountains magmatic signatures the dominant process of magmatic differentiation is crystal fractionation and accumulation of olivine which acts as the main criteria to distinguish between primitive and evolved group i type kimberlite fields in the northern alberta this is important from the viewpoint of diamond exploration because the majority about 80 of the more primitive buffalo head hills kimberlites are diamondiferous whereas the more evolved birch mountains pipes are barren of diamonds for the most part petrographically the buffalo head hills samples are distinct from the birch mountains samples in that they contain less carbonate have a smaller modal abundance of late stage minerals such as phlogopite and ilmenite and have a higher amount of fresh coarse macrocrystal 0 5 mm olivine consequently samples from the buffalo head hills have the highest values of mgo cr and ni and have chemistries similar to those of primitive hypabyssal kimberlite in the northwest territories based on whole rock isotopic data the buffalo head hills k6 kimberlite has 87sr 86sr and end values similar to those of south african group i kimberlites whereas the birch mountains legend and phoenix kimberlites have similar end values between 0 and 1 9 but distinctly higher 87sr 86sr values 0 7051 0 7063 the lack of whole rock geochemical overlap between kimberlite and the freshest least contaminated mountain lake south pipe rocks reflects significant mineralogical differences and mountain lake is similar geochemically to olivine alkali basalt and or basanite intra field geochemical variations are also evident the k4 pipe buffalo head hills and xena and kendu pipes birch mountains are characterized by anomalous concentrations of incompatible elements relative to other northern alberta kimberlite pipes including chondrite normalized rare earth element distribution patterns that are less fractionated than the other kimberlite samples from the buffalo head hills and birch mountains the xena pipe has similar major element chemical signatures and high al clinopyroxene similar to or trending towards the mountain lake pipes in addition k4 and kendu have higher 87sr 86sr and lower end than bulk earth and plot in the bottom right quadrant of the nd sr diagram we suggest therefore that the k4 and kendu pipes contain a contribution from old lree enriched low sm nd lithosphere that is absent from the other kimberlites are affected by crustal contamination or both based on xenocryst populations the northern alberta kimberlite province mantle is dominated by carbonate saturated lherzolitic mantle higher levels of melt depletion characterize the buffalo head hills mantle sample despite high diamondiferous to barren pipe ratios in the buffalo head hills pipes mineral indicators of high diamond potential such as g10 garnet diamond inclusion composition chrome spinels and high sodium eclogitic garnet are rare

k ar and 39ar 40ar whole rock dating of zeolite facies metamorphosed flood basalts the upper paleocene basalts of the faroe islands ne atlantic

trans atlantic correlation of the palaeogene volcanic successions in the faeroe islands and east greenland before continental break up in the ne atlantic the faeroe islands and central east greenland were within a distance of 100 120 km chemical and lithological data for complete sections through the 5 km thick piles of contemporaneous palaeogene flood basalts in the faeroe islands and in the nansen fjord area in east greenland show very similar basalt compositions and evolution patterns with time the faeroes lower basalt formation and the equivalent nansen fjord formation in east greenland form a pre break up succession overlain by a sediment horizon a syn break up succession consists of the faeroes middle and upper basalt formations and the equivalent milne land formation in east greenland in which five intervals can be correlated with a compositional evolution from ti rich magnesian basalts and picrites at the base to a dominance of morb like low ti basalts at the top the successions were generated in the same mantle melting column beneath a thinning continent with a rift zone that eventually ruptured the continent the evolution pattern is very similar to that seen on the se greenland margin but spreading according to the palmason model of 1973 was not yet established the pre and syn break up successions formed volcanic megasystems stretching across the rift zone with areal extents of 70 000 and 220 000 km 2 and volumes of 120 000 and 250 000 km 3 rocks from the pre and syn break up successions can be discriminated based on a simple major element plot the overlying succession was 3 3 5 km thick in e greenland but was thin or absent in the faeroes the energy source for the melting appears to have been concentrated on the greenland side

sub volcanic development of kimberlite pipes evidence from the lace and voorspoed group ii kimberlites south africa abstract the lace and voorspoed kimberlites occur on the kaapvaal craton south africa and form part of the kroonstad group ii kimberlite orangeite cluster the lace kimberlite is composed of a main pipe and a satellite blind pipe the latter of which does not reach the current land surface 30 m below the current land surface and is not observed connecting with the main pipe at depth the main pipe increases in size from 100 m to 250 m in diameter at depth the voorspoed kimberlite pipe is the largest of the cluster and is dominantly infilled with massive layers up to 200 m thick of resedimented volcaniclastic kimberlite rvk coherent kimberlite ck identified at all three pipes is described here in order to constrain their formation distinct volcaniclastic kimberlite vk at the lace main pipe is also described here and is characterised by a highly clastic fragmental quartz rich and mud rich interclast material the vk is interpreted to form by resedimentation of extra crater deposits into a deep 1600 m open vent the ck from both lace occurrences is characterised by coherent to globular segregationary textures generally high proportions of crustal xenoliths and the complete absence of karoo basalt xenoliths which are ubiquitous in vk infilling south african cretaceous kimberlites the increase in pipe size with depth associated with the occurrence of ck at the main pipe and the occurrence of ck within the satellite blind pipe strongly favours an interpretation of formation in a hypabyssal setting rather than by pyroclastic processes ck is also described from the voorspoed kimberlite where it occurs at depth along the south western margin of the pipe the ck is xenolith rich the xenolith population is entirely from the immediate sidewall lithologies and has an incipient globular segregationary texture the contact with the rvk in the centre of the pipe is sharp indicating that the ck was cross cut by a period of eruption prior to the formation of spatially associated rvk the pipe margins at similar depths along the northern and eastern sections of the pipe are characterised by the occurrence of contact breccias which have a gradational change to lower degrees of brecciation away from the pipe the ck at voorspoed is interpreted to form by the intrusion of magmatic kimberlite at depth which invades a previously formed contact breccia and incorporates large proportions of crustal xenoliths from the immediate sidewall the ck occurring at the lace and voorspoed kimberlite are interpreted to form in a sub volcanic hypabyssal setting during a stage of pre conditioning of the country rock prior to eruptive emplacement of the pipe a model for the development of a sub volcanic embryonic pipe is presented for the formation of ck at the kroonstad kimberlite cluster

post seismic and interseismic fault creep i model description we present a model of localized aseismic fault creep during the full interseismic period including both transient and steady fault creep in response to a sequence of imposed coseismic slip events and tectonic loading we consider the behaviour of models with linear viscous non linear viscous rate dependent friction and rate and state dependent friction fault rheologies both the transient post seismic creep and the pattern of steady interseismic creep rates surrounding asperities depend on recent coseismic slip and fault rheologies in these models post seismic fault creep is manifest as pulses of elevated creep rates that propagate from the coseismic slip these pulses feature sharper fronts and are longer lived in models with rate state friction compared to other models with small characteristic slip distances in rate state friction models interseismic creep is similar to that in models with rate dependent friction faults except for the earliest periods of post seismic creep our model can be used to constrain fault rheologies from geodetic observations in cases where the coseismic slip history is relatively well known when only considering surface deformation over a short period of time there are strong trade offs between fault rheology and the details of the imposed coseismic slip geodetic observations over longer times following an earthquake will reduce these trade offs while simultaneous modelling of interseismic and post seismic observations provide the strongest constraints on fault rheologies

the structure of the faeroe shetland trough from integrated deep seismic and potential field modelling in the faeroe shetland trough thick basaltic lava units make it difficult to image underlying structure with conventional seismic profiles we have therefore used potential field modelling to estimate crustal structure along the fast deep seismic line and other regional transects we introduce the additional constraints of crustal isostasy to gravity models and the observations of the polarity of magnetic remanence within the lava pile to magnetic models the crustal stretching factor ranges up to 3 along the fast traverse and our model also suggests that a sedimentary section with a thickness of up to 3 4 km is present beneath a c 2 km basalt layer on the faeroese shelf

surface interpolation within a continental flood basalt province an example from the palaeogene faroe islands basalt group abstract the faroe islands basalt group fibg is a continental flood basalt province with four proven lithohorizons that have abundant spot height data the spline interpolation method was used to construct spatial surfaces for the lithohorizons the resultant surfaces conform to dip and strike estimates that were previously modelled by hand and confirm the methods reliability the surfaces should be used as guides only but can be quickly and easily updated as new information becomes available the surfaces have been used for a variety of different tasks for example obtaining gross thickness estimates producing isopach maps and constructing geological cross sections an additional benefit of the spatial surfaces is the ability to constrain disparate observations in a stratigraphic framework which will be particularly important for future studies attempting to understand for example the temporal and spatial development of continental flood basalt volcanism the spline interpolation method applied in this study can be equally used for other stratigraphic horizons e g chemohorizon chronohorizon biohorizon and in other continental flood basalt provinces e g deccan traps columbia river basalt group

fractal dimension of molten surfaces as a possible parameter to infer the slip weakening distance of faults from natural pseudotachylytes high velocity friction experiments on gabbro and monzodiorite using a rotary shear high velocity friction apparatus have revealed that frictional melting and progressive growth of a molten layer along a fault cause slip weakening eventually reaching a nearly steady state the melting surface at the host rock molten layer interface is initially very flat but it becomes more complex and rounded in shape towards the steady state owing to the selective melting of minerals with lower melting points and the gibbs thomson effect this change in the melting surface topography can be quantitatively expressed by the fractal dimension d as determined by the divider method from about 1 0 near the peak friction to around 1 1 near the steady state friction the ultimate fractal dimension at steady state friction tends to decrease with increasing heat production rate presumably due to more rapid and uniform melting a systematic correlation of d with mechanical behavior of the fault during frictional melting may provide a way of estimating slip weakening distance and heat production rate at steady state friction by measuring d for natural pseudotachylytes on slip surfaces with different displacements the weakening distance is of vital significance in relation to fault instability and the heat production rate is related to the fault strength the experimental studies point to ways to estimate these difficult quantities for natural faults

the mechanics of lubricated faults insights from 3 d numerical models 1 the weakening mechanisms occurring during an earthquake failure are of prominent importance in determining the resulting energy release and the seismic waves excitation in this paper we consider the fully dynamic response of a seismogenic structure where lubrication processes take place in particular we numerically model the spontaneous propagation of a 3 d rupture in a fault zone where the frictional resistance is controlled by the properties of a low viscosity slurry formed by gouge particles and fluids this model allows for the description of the fault motion in the extreme case of vanishing effective normal stress by considering a viscous fault response and therefore without the need to invoke in the framework of coulomb friction the generation of the tensile mode of fracture we explore the effects of the parameters controlling the resulting governing law for such a lubricated fault the viscosity of the slurry the roughness of the fault surfaces and the thickness of the slurry film our results indicate that lubricated faults produce a nearly complete stress drop i e a very low residual friction coefficient 0 01 a high fracture energy density eg few 10s of mj m2 and significant slip velocities vpeak few 10s of m s the resulting values of the equivalent characteristic slip weakening distance d0eq 0 1 0 8 m depending on the adopted parameters are compatible with the seismological inferences moreover in the framework of our model we found that supershear ruptures are highly favored in the case of enlarging gap height we can have the healing of slip or even the inhibition of the rupture quantitative comparisons with different weakening mechanisms previously proposed in the literature such as the exponential weakening and the frictional melting are also discussed

temperature fields generated by the elastodynamic propagation of shear cracks in the earth w 1 the temperature maximum shifts toward the crack tip for faults having slip zone thickness of the order of centimeters or less the onset of thermally induced phenomena e g frictional melting thermal pressurization etc may occur at any point along the rupture depending on the degree of slip localization and rupture duration in the absence of significant increases in the pore fluid pressure localized fault slip may raise temperature by several hundred degrees sufficient to cause melting the onset of frictional melting may give rise to substantial increases in the effective fault strength due to an increase in the effective fault contact area and high viscosity of silicate melts near solidus the inferred transient increases in the dynamic friction viscous braking are consistent with results of high speed rock sliding experiments and might explain field observations of the fault wall rip out structures associated with pseudotachylites possible effects of viscous braking on the earthquake rupture dynamics include 1 delocalization of slip and increases in the effective fracture energy 2 transition from a crack like to a pulse like rupture propagation or 3 ultimate rupture arrest assuming that the pulse like ruptures heal by incipient fusion the seismologic observations can be used to place a lower bound on the dynamic fault friction this bound is found to be of the order of several megapascals essentially independent of the earthquake size further experimental and theoretical studies of melt rheology at high strain rates are needed to quantify the effects of melting on the dynamic fault strength index terms 3210 mathematical geophysics modeling 5104 physical properties of rocks fracture and flow 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling 8010 structural geology fractures and faults

attenuation of radiated ground motion and stresses from three dimensional supershear ruptures 1 radiating shear and rayleigh waves from supershear ruptures form mach waves that transmit large amplitude ground motion and stresses to locations far from the fault we simulate bilateral ruptures on a finite width vertical strike slip fault of width w and half length l with l w breaking the surface of an elastic half space and focus on the wavefield out to distances comparable to l at distances much smaller than w two dimensional plane strain slip pulse models i e models in which the lateral extent of the slip zone is unbounded accurately predict the subsurface wavefield amplitudes in the shear mach wedge of those models are undiminished with distance from the fault when viewed from distances far greater than w rupture is accurately modeled as a moving point source that produces a shear mach cone and on the free surface rayleigh wave mach fronts geometrical spreading of the shear mach cone occurs radially and amplitudes there decrease with the inverse square root of distance the transition between these two asymptotic limits occurs at distances comparable to w similar considerations suggest that rayleigh mach waves suffer no attenuation in the ideally elastic medium studied here the rate at which fault strength weakens at the rupture front exerts a strong influence on the off fault fields only in the immediate vicinity of the fault for both sub rayleigh and supershear ruptures and at the mach fronts of supershear ruptures more rapid weakening generates larger amplitudes at the mach fronts

slip dependent friction law and nucleation processes in earthquake rupture abstract matsu ura m kataoka h and shibazaki b 1992 slip dependent friction law and nucleation processes in earthquake rupture in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 135 148 we developed a physical model that interprets slip strengthening and slip weakening processes in brittle shear fracture by considering the microscopic interaction between statistically self similar fault surfaces our model successfully describes general features of the constitutive behaviour observed in laboratory experiments of stick slip with the progress of fault slip frictional stress first increases rapidly up to a peak value and then decreases gradually to a constant level the first slip strengthening process can be explained by the increase of frictional resistance in transition from the in phase contact to the random phase contact of fault surfaces the latter slip weakening process can be explained by the decrease of frictional resistance due to the abrasion of surface asperities the trade off between these two processes determines the peak stress and the critical displacement over which the slip weakening proceeds we examined the nucleation process proceeding in a seismogenic zone with the increase of external shear stress through numerical simulations in our model the seismogenic zone is represented by a broad weak zone with a locally strong part asperity on a fault plane the basic equations governing the nucleation process are the equation of equilibrium and the constitutive relation between frictional stress and fault slip we obtained the following results the nucleation process first proceeds quasi statically at the weak portions with the increase of external shear stress then if the constitutive relation at the asperity has a high and narrow peak of stress brittle dynamic rupture occurs in a limited area around the asperity when the constitutive relation has a low and broad peak the dynamic rupture of the asperity does not occur the process is completely aseismic when the peak of stress is high and broad the dynamic rupture of the asperity leads to the catastrophic rupture which extends over the whole region from these results it is concluded that diversity in the nucleation process can be ascribed to difference in the variation of constitutive behaviour along the fault

a new method of surface modeling and its application to dem construction a new method of surface modelling based on the fundamental theorem of surfaces smts is presented eight different test surfaces are employed to comparatively analyze the simulation errors of smts and the classical methods of surface modeling in gis including tli triangulated irregular network with linear interpolation spline idw inverse distance weighted and kriging numerical tests show that smts is much more accurate than the classical methods smts theoretically gives a solution to the error problem that has long troubled dem construction as a real world example smts is used to construct a dem of the da fo si coal mine in shaan xi province china its root mean square error rmse is compared with those of dems constructed by the four classical methods the results show that although smts also has a higher accuracy in the real world example the improvement of accuracy is less distinct than that expected from the numerical tests the accuracy loss seems to be caused by location differences between sampling points and the central points of lattices of the simulated surfaces two alternative ways are proposed to solve this problem

fusion by earthquake fault friction stick or slip field observations of pseudotachylites and experimental studies of high speed friction indicate that melting on a slipping interface may sig nificantly affect the magnitude of shear stresses resisting slip we investigate the effects of rock melting on the dynamic friction using theoretical models of shear heating that couple heat transfer thermodynamics of phase transitions and fluid mechanics results of laboratory experiments conducted at high order of m s slip velocities but low order of mpa normal stresses suggest that the onset of frictional melting may give rise to substantial increases in the effective fault strength presumably due to viscous effects however extrapolation of the modeling results to in situ conditions suggests that the efficiency of viscous braking is significantly reduced under high normal and shear stresses when transient increases in the dynamic fault strength due to fusion are not sufficient to inhibit slip decreases in the effective melt viscosity due to shear heating and melting of clasts drastically decrease the dynamic friction resulting in a nearly complete stress drop thermal runaway the amount of energy dissipation associated with the formation of pseudo tachylites is governed by the temperature dependence of melt viscosity and the average clast size in the fault gouge prior to melting clasts from a coarse grained gouge have lower chances of survival in a pseudotachylite due to a higher likelihood of non equilibrium overheating the maximum temperature and energy dissipation attainable on the fault surface are ultimately limited by either the rock solidus via viscous braking and slip arrest or liquidus via thermal runaway and vanishing resistance to sliding our modeling results indicate that the thermally activated fault strengthening and rupture arrest are unlikely to occur in most mafic protoliths but might be relevant for quartz rich rocks especially at shallow 5 7 km depths where the driving shear stress is relatively low

coherence of mach fronts during heterogeneous supershear earthquake rupture propagation simulations and comparison with observations 1 we study how heterogeneous rupture propagation affects the coherence of shear and rayleigh mach wavefronts radiated by supershear earthquakes we address this question using numerical simulations of ruptures on a planar vertical strike slip fault embedded in a three dimensional homogeneous linear elastic half space ruptures propagate spontaneously in accordance with a linear slip weakening friction law through both homogeneous and heterogeneous initial shear stress fields in the 3 d homogeneous case rupture fronts are curved owing to interactions with the free surface and the finite fault width however this curvature does not greatly diminish the coherence of mach fronts relative to cases in which the rupture front is constrained to be straight as studied by dunham and bhat 2008a introducing heterogeneity in the initial shear stress distribution causes ruptures to propagate at speeds that locally fluctuate above and below the shear wave speed calculations of the fourier amplitude spectra fas of ground velocity time histories corroborate the kinematic results of bizzarri and spudich 2008a 1 the ground motion of a supershear rupture is richer in high frequency with respect to a subshear one 2 when a mach pulse is present its high frequency content overwhelms that arising from stress heterogeneity present numerical experiments indicate that a mach pulse causes approximately an 1 7 high frequency falloff in the fas of ground displacement moreover within the context of the employed representation of heterogeneities and over the range of parameter space that is accessible with current computational resources our simulations suggest that while heterogeneities reduce peak ground velocity and diminish the coherence of the mach fronts ground motion at stations experiencing mach pulses should be richer in high frequencies compared to stations without mach pulses in contrast to the foregoing theoretical results we find no average elevation of 5 damped absolute response spectral accelerations sa in the period band 0 05 0 4 s observed at stations that presumably experienced mach pulses during the 1979 imperial valley 1999 kocaeli and 2002 denali fault earthquakes compared to sa observed at non mach pulse stations in the same earthquakes a 20 amplification of short period sa is seen only at a few of the imperial valley stations closest to the fault this lack of elevated sa suggests that either mach pulses in real earthquakes are even more incoherent that in our simulations or that mach pulses are vulnerable to attenuation through nonlinear soil response in any case this result might imply that current engineering models of high frequency earthquake ground motions do not need to be modified by more than 20 close to the fault to account for mach pulses provided that the existing data are adequately representative of ground motions from supershear earthquakes

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

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

a critical analysis of the structure and tectonic significance of rift oblique lineaments transfer zones in the mesozoic cenozoic succession of the faroe shetland basin ne atlantic margin abstract nw se trending rift oblique lineaments transfer zones9 occur along the length of the ne atlantic margin previous workers have suggested that these lineaments played an important role in providing conduits and or barriers to sedimentation during the cretaceous and palaeocene it has also been suggested that they were active as discrete basin wide strike slip faults this study uses a well calibrated 3d seismic survey of regional extent to critically assess the structural and stratigraphic evidence for three rift oblique lineaments in the uk sector of the faroe shetland basin victory clair and judd lineaments structures previously attributed to basin wide strike slip deformation can be more simply explained as igneous intrusions hydrothermal vent complexes gas chimneys and or faults that transfer extensional strain between en echelon rift segments there is little evidence to suggest that activity along discrete basin wide lineaments controlled palaeocene sedimentation within the faroe shetland basin rather sediment transport and deposition at this time are likely to have been controlled by along and across strike variations in the magnitude of thermal subsidence which in turn reflect the 3d nature of the underlying mesozoic rift architecture

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

energy partitioning during seismic slip in pseudotachylyte bearing faults gole larghe fault adamello italy thedeterminationoftheearthquakeenergybudgetremainsachallengingissueforearthscientists asunderstandingthepartitioningofenergyisa keytowardsthe understandingthe physicsofearthquakes here weestimatethe partitionofthe mechanical workdensity intoheatandsurfaceenergy energyrequiredtocreatenewfracturesurface duringseismicsliponalocationalongafault earthquakeenergypartitioningisdeterminedfromfield and microstructural analyses of a fault segment decorated by pseudotachylyte solidified friction induced melt produced during seismic slip exhumed from a depth of 10 km typical for earthquake hypocenters in the continental crust frictional heat per unit fault area estimated from the thickness of pseudotachylytes is 27 mj m 2 surface energy estimated from microcrack density inside clast i e cracked grains entrapped in the pseudotachylyte and in the fault wall rock ranges between 0 10 and 0 85 mj m 2 our estimates for the studied fault segment suggest that 97 99 of the energy was dissipated as heat during seismic slip we conclude that at 10 km depth less than 3 of the total mechanical work density is adsorbed as surface energy on the fault plane during earthquake rupture 2008 elsevier b v all rights reserved

on the deterministic description of earthquakes 1 the quantitative estimate of earthquake damage due to ground shaking is of pivotal importance in geosciences and its knowledge should hopefully lead to the formulation of improved strategies for seismic hazard assessment numerical models of the processes occurring during seismogenic faulting represent a powerful tool to explore realistic scenarios that are often far from being fully reproduced in laboratory experiments because of intrinsic technical limitations in this paper we discuss the prominent role of the fault governing model which describes the behavior of the fault traction during a dynamic slip failure and accounts for the different and potentially competing chemical and physical dissipative mechanisms we show in a comprehensive sketch the large number of constitutive models adopted in dynamic modeling of seismic events and we emphasize their prominent features limitations and specific advantages in a quantitative comparison we show through numerical simulations that spontaneous dynamic ruptures obeying the idealized linear slip weakening sw equation and a more elaborated rate and state dependent friction law produce very similar results in terms of rupture times peaks slip velocity developed slip and stress drops provided that the frictional parameters are adequately comparable and more importantly that the fracture energy density is the same our numerical experiments also illustrate that the different models predict fault slip velocity time histories characterized by a similar frequency content a feeble predominance of high frequencies in the sw case emerges in the frequency ranges 0 3 1 and 11 50 hz these simulations clearly indicate that even forgiving the frequency band limitation it would be very difficult virtually impossible to discriminate between two different but energetically identical constitutive models on the basis of the seismograms recorded after a natural earthquake

the destiny of a clast within a molten pseudotachylyte vein pseudotachylytes are important markers that can indicate the thermal state during a coseismic slip failure and provide indirect information about the level of stress and sliding velocities attained during that time window on the other hand survivor fragmented clasts embedded in the quenched material are also very impor tant in that they provide information about the energy spent to create new fracture surfaces in this paper i study the temperature evolution of clasts subjected to the heat dissipated by a just formed molten pseudotachylyte pt vein in particular i find the analytical solutions for the temperature evolution within the pt vein the surrounding undamaged host rock and inside a clast according to the proposed model the numerical results show that the clasts tend to preferentially melt in the inner part of the pt vein i e they are completely assimilated by the pt in contrast some clasts can survive far from the pt vein center my solutions although based upon a simplified model can provide a theoretical framework to predict the maximum size of the survived clasts at a given distance from the pt center the distribution of these survivor clasts follows a power law relation in terms of their radius and its features generally agree with field and laboratory observations

origin of olivine in kimberlite phenocryst or impostor abstract kimberlite hosts two populations of olivine that are distinguished on the basis of grain size and morphology the populations are commonly described genetically as xenocrysts and phenocrysts olivine xenocrysts or macrocrysts are thought to derive from disaggregation of mantle xenoliths whereas the smaller euhedral olivine crystals are presumed entirely cognate to the kimberlite melt recent studies of zoning patterns of euhedral olivine in kimberlite have however cast doubt on the actual origins of the smaller olivine crystals here we elucidate the nature and origins of the textural and chemical zonation that characterize both populations of olivine macrocrysts olivine i and euhedral crystals olivine ii specifically we show that both olivine i and olivine ii feature chemically distinct overgrowths resulting from heterogeneous crystallization onto pre existing olivine xenocrysts our analysis limits the total volume of olivine crystallized during transport to 5 in contrast to previous estimates of 25 the reduced extent of olivine crystallization allows for closer reconciliation of crystallized olivine compositions and estimates of mg s for primitive kimberlite melts it also places constraints on processes involving orthopyroxene assimilation by kimberlite melt if olivine crystallization and orthopyroxene assimilation are coupled then orthopyroxene assimilation is limited to 7 larger masses of orthopyroxene assimilation i e 25 are possible only if kimberlite magmas originate at super liquidus 100 oc conditions and sub equal amounts of olivine crystallization occurs

crustal structure of the ne rockall trough from wide angle seismic data modeling two wide angle seismic lines located in the northern rockall trough were acquired in may 2000 one line line e crosses the trough from the continental shelf off lewis to normal oceanic crust west of lousy bank in nw se direction the other line line d intersects with line e crosses the wyville thomson ridge in a sw ne direction and ends in the faeroe shetland basin sonobuoy data and expanding spread profiles acquired in the same area have been remodeled analysis of the seismic data using travel times and amplitudes reveals an up to 5 km thick sedimentary basin including an up to 1 5 km thick basaltic layer which is present in most of the trough further conclusions of this study are that the rockall trough is underlain by highly stretched continental crust of 13 km thickness the crust thickens to 24 km beneath lousy bank which is interpreted to be of continental nature beneath the hebrides continental shelf a three layer continental crust of 26 km is modeled an up to 12 km thick high velocity layer is observed underneath the ocean continent boundary and is interpreted as magmatic underplating resulting from excess volcanism during rifting no evidence for an underplate layer could be distinguished beneath the trough area modeling of the structure of the wyville thomson ridge revealed no existing igneous core of the ridge confirming existing theories that it is a compressional structure

fault geometry rupture dynamics and ground motion from potential earthquakes on the north anatolian fault under the sea of marmara summary using the 3 d finite element method we develop dynamic spontaneous rupture models of earthquakes on the north anatolian fault system in the sea of marmara turkey considering the geometrical complexity of the fault system in this region we find that the earthquake size rupture propagation pattern and ground motion all strongly depend on the interplay between the initial static regional pre stress field and the dynamic stress field radiated by the propagating rupture by testing several nucleation locations we observe that those far from an oblique normal fault stepover segment near istanbul lead to large through going rupture on the entire fault system whereas nucleation locations closer to the stepover segment tend to produce ruptures that die out in the stepover however this pattern can change drastically with only a 10 rotation of the regional stress field our simulations also reveal that while dynamic unclamping near fault bends can produce a new mode of supershear rupture propagation this unclamping has a much smaller effect on the speed of the peak in slip velocity along the fault finally we find that the complex fault geometry leads to a very complex and asymmetric pattern of near fault ground motion including greatly amplified ground motion on the insides of fault bends the ground motion pattern can change significantly with different hypocentres even beyond the typical effects of directivity the results of this study may have implications for seismic hazard in this region for the dynamics and ground motion of geometrically complex faults and for the interpretation of kinematic inverse rupture models

fluidization and melting of fault gouge during seismic slip identification in the nojima fault zone and implications for focal earthquake mechanisms 1 the record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active nojima fault in japan the fault rocks formed at about 3 km depth and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite each layer is thinner than a few millimeters and corresponds to one seismic slip event the very thin slip zone width suggests that some mechanisms of slip weakening operated and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method it is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains once fluidization occurs the frictional resistance decreases abruptly to nearly zero even before thermal pressurization reaches its extreme state when fault gouge is melted frictional resistance is governed by the viscosity of melt which depends mainly on temperature chemical composition h2o concentration and the volume fraction of unmelted grains for each pseudotachylyte layer we estimated the temperature of melt using various temperature indices and measured volume fraction of unmelted grains we synthesized these data to reconstruct the change in viscosity during seismic slip events the melt viscosity is very high 107 9 pas during initial melting due to the combined effect of low temperature 750 800 c and large volume fraction of solid grains thus seismic slip is inevitably restrained instantaneously once this mechanical barrier is overcome the viscosity reduces continuously and dramatically as slip increases at 1000 c the viscosity reduces to 104pas and eventually to 10pas at 1280 c thus stress drops almost completely and rupturing tends to run away

the 3d facies architecture of flood basalt provinces and their internal heterogeneity examples from the palaeogene skye lava field quantifying the facies architecture of flood basalt provinces is important as it can be used to understand the physical volcanology and rock property variations throughout the lava sequence the 3d facies architecture and internal heterogeneity of the skye lava field for example provides important insights into the evolution of the british tertiary igneous province and valuable information to aid in the exploration of potential offshore reservoirs underlying significant flood lavas along the north atlantic margin the volcanic stratigraphy of the talisker bay area of skye comprises 1 lower compound braided lavas flow lobes 3 the transitional sequence 7 4 km 3 and the upper sequence 17 0 km 3 the lower sequence lavas formed on the flanks of a shield volcano and were sourced from the ne volcanological features such as lava feeder tubes pahoehoe textures and lobes indicate a scale of volcanism similar to that of present day hawaii the within flow heterogeneity of the basalts is characterized using an intrafacies scheme allowing comparison of variations in lithofacies with characteristic geophysical rock properties of compressional wave velocity and density

petrology of hypabyssal kimberlites relevance to primary magma compositions abstract hypabyssal kimberlites are similar in their mineralogy and geochemistry across the world and thus form a distinct magma type that is produced repeatedly in time and space this small volume magma is derived from the asthenosphere 200 650 km and as it nears the surface consists of olivine macrocrysts i e xenocrysts 25 vol olivine phenocrysts 25 vol entrained in a ba ti mg rich al poor carbonate silicate liquid containing high but unknown amounts of dissolved co2 ch4 and h2o hypabyssal kimberlites are shown to be hybrid rocks formed by mixing with and assimilation of mantle derived lherzolite and harzburgite orthopyroxene is not stable in these magmas and its resorption leads the precipitation of primary liquidus olivine the magma crystallizes rapidly at crustal levels to uniform holocrystalline rocks that do not contain glass in addition to spinel perovskite monticellite apatite and phlogopite kinoshitalite primary carbonates are typically present as low pressure liquidus phases serpentines primary polygonal serpentine and chrysotile and carbonate commonly crystallize together from volatile rich co2 h2o deuteric residua as a polycrystalline mesostasis or discrete segregations the latter are not in filled vesicles kimberlite magmas have a very long crystallization interval ranging from 1200 to 300 c and there is a continuum from the magmatic crystallization of silicate and oxide minerals to carbo hydrothermal residua forming the calcite serpentine mesostasis it is concluded that there are no hypabyssal kimberlites which are representatives of the initial primary magma all kimberlites are hybrid and contaminated magmas which have undergone crystallization in the mantle prior to crystallization of the groundmass mesostasis assemblages

the petrogenesis of group 2 ultrapotassic kimberlites from finsch mine south africa abstract major trace element and radiogenic isotope results are presented for a suite of hypabyssal kimberlites from a single pipe at the finsch mine south africa these are group 2 kimberlites characterised by abundant phlogopite serpentine diopside they are ultrabasic sio2 6 9 igneous rocks they exhibit a wide range in major element chemistry with sio2 27 6 41 9 wt and mgo 10 4 33 4 wt 87 sr 86 sr i 0 7089 to 0 7106 nd is 6 2 to 9 7 and they have unradiogenic 207pb 204pb i contents which ensure that they plot below the pb ore growth curve they have high incompatible and compatible element contents a striking positive array between y and nb which indicates that garnet was not involved in the within suite differentiation processes and a negative trend between k nb and nb contents which suggests that phlogopite was involved in addition some elements exhibit an unexpected order of relative incompatibility for different trace elements which suggests that the intra kimberlite variations are not primarily due to variations in the degree of partial melting the effects of fractional crystallization are difficult to establish because for the most part they have been masked by the entrainment of 50 60 mantle peridotite thus the finsch kimberlites are interpreted as mixtures of a melt component and entrained garnet peridotite with no evidence for significant contamination with crustal material the melt component was characterised by high incompatible element contents which require both very small degrees of partial melting and source regions with higher incompatible element contents than depleted or primitive mantle since the melt component was the principal source of incompatible elements in the kimberlite magma the enriched nd sr and pb isotope ratios of the kimberlite are characteristic of the melt source region the melt fractions were therefore derived from ancient trace elements enriched portions of the upper mantle most probably situated within the sub continental mantle lithosphere and different from the low 87sr 86sr garnet peridotite xenoliths found at finsch within the sub continental mantle lithosphere old incompatible element enriched source regions for the kimberlite melt fraction are inferred to have been overlain by depleted mantle material which became entrained in the kimberlite magma

diamond s depth distribution systematics abstract the thermobarometric analysis of inclusions in lithospheric diamonds has shown that these diamonds may originate from a wide range of depths with a global mode at 175 15 km studies based on diamond depth distribution at global scale however cannot clarify if this mode reflects a real concentration of diamonds preferential sampling of materials from this level by ascending kimberlites or simply a statistical distribution within the hard limits imposed by diamond stability lithosphere thickness and mantle adiabat under typical cratonic thermal regimes we addressed this problem by comparing depth distributions for peridotitic diamonds from the three localities that have been the most prolific for diamond geobarometry cullinan kimberley and voorspoed south africa with those of mantle xenocrysts from the same kimberlite sources the revised p t estimates indicate that the diamonds were formed at t higher equal or lower than the ambient geotherm recorded by the xenocrysts these conditions may represent old mantle thermal regimes or local thermal perturbations related to infiltration of parental fluids or melts nonetheless the studied diamonds show similar depth distributions for the different localities with a distinct mode at 180 10 km the similarity of these distributions with that calculated for peridotitic diamonds worldwide as well as the lack of systematic correlation with kimberlite sampling efficiency as recorded by mantle xenocrysts suggests that this mode has genetic significance based on observed depth distributions and thermodynamic modeling of coh fluids diamond forming processes are predicted to become less efficient with decreasing depth from at least 165 km in addition diamond endowment near the base of the lithosphere may be negatively affected by infiltration of carbon undersaturated melts or fluids after diamond formation considering the poor correlation between diamond and xenocryst depth distributions in single kimberlites or kimberlite clusters even limited xenocryst records from diamond favorable depths especially from the 160 190 km interval may correspond to significant diamond potential

how natural remanent magnetization of basaltic units can dominate the reduced to pole magnetic value a case study from the faroe islands abstract early attempts to utilize magnetic data to understand the volcanic and subvolcanic succession on the faroese continental shelf have shown that conventional interpretation and modelling of magnetic data from this area leads to ambiguous results interpretation of the aeromagnetic data on the faroese continental shelf shows that some previously identified basement highs coincide with reduced to pole magnetic highs whereas others coincide with negative or mixed magnetic features similarly igneous centres are characterized by different polarity magnetic anomalies palaeomagnetic analysis of the onshore volcanic succession has demonstrated that the thermoremanent magnetization of the basaltic lavas is stronger than the induced magnetism and both reversely and normally magnetized units are present we have tested this with 2 d profile modelling using the palaeomagnetic information to correlate high amplitude magnetic anomalies with basalt successions containing changes in magnetic polarity this approach has enabled us to map the termination of the differently magnetized units offshore and thereby extend the mapping of the faroe island basalt group on the faroese platform and into adjacent areas

extreme frictional processes in the volcanic conduit of mount st helens usa during the 2004 2008 eruption abstract the 2004 2008 eruption of mount st helens saw the extrusion of seven high viscosity spines and formation of discrete shear zones along the conduit margin at spine 7 this shear zone consists of four structurally distinct layers the outer surface gouge l1 crosscuts a dark banded layer l2 which grades into a moderately sheared layer l3 and undeformed rock l4 inside the spine field observations porosity measurements geochemistry mineralogy microstructure crystal size and shape distribution kinetic properties and magnetic analyses chart the evolution of deformation processes and products throughout the eruption gouge formation was concomitant with characteristic microseismic drumbeats at depths 0 5 1 km in addition the seismic record shows two larger earthquakes with similar seismic signatures in august 2006 which we conclude represent larger slip amounts along the conduit margin of spine 7 extensive slip resulted in frictional heating on the order of several hundreds of degrees melting the highly viscous crystalline ascending magma plug and forming a pseudotachylyte high ambient temperatures in the conduit resulted in near equilibrium melting and slow recrystallisation thus impeding the development of signature pseudotachylyte characteristics and hindering identification thus frictional melting and recrystallisation in ascending magma plugs may be a common but unidentified phenomena at composite volcanoes worldwide

effects of permeability and porosity evolution on simulated earthquakes numerical simulations are a fundamental tool to access the typical conditions attained during earthquake instabilities and to simulate the large number of dissipative processes taking places during faulting in this study we consider a single degree of freedom spring slider system a simplified fault model which can describe the whole seismic cycle and the dynamics of a fault with spatially homogeneous properties we assume a rate and state dependent friction in which we incorporate the effects of pore fluid pressure thermally pressurized as a consequence of the frictional heat produced during sliding we explore in a single framework the role of the time variations of the porosity permeability or both ultimately leading to changes in hydraulic diffusivity which has been recognized as one of the key parameters in thermally pressurized faults our synthetic ruptures show that the changes in the hydraulic diffusivity only due to porosity variations do not markedly affect the earthquake recurrence cycle time the traction evolution and the thermal history of the fault on the contrary when the evolutions of both the porosity and the permeability are accounted for the cycle time is significantly reduced this result has a clear implication in the context of the hazard assessment

new paleomagnetic pole and magnetostratigraphy of faroe islands flood volcanics north atlantic igneous province abstract a paleomagnetic sampling was carried out along four sections altogether 86 lava flows 548 samples in the north atlantic igneous province outcropping in faroe islands denmark the four polarity zones in the 700 m thick exposed part of the faroes lower formation can be correlated with the geomagnetic polarity time scale as c26n c25r c25n c24r the seven lava flows erupted during c25n indicate a very low eruption rate in the upper part of the faroes lower formation of 1 70 kyr the faroes middle and upper formations composite thickness 2300 m are all reversely magnetized corresponding to c24r the eruption rate at the onset of middle formation volcanism was very high as evidenced by several thick lava sequences recording essentially spot readings of the paleomagnetic field the shift in eruption rate between the faroes lower and middle formations and evidence that onset of the faroes middle formation volcanism took place in c24r are of particular importance placing onset of middle formation volcanism in close temporal relation to north atlantic continental break up and the late paleocene thermal maximum after grouping flows recording the same field directions we obtained 43 independent readings of the paleomagnetic field yielding a paleomagnetic pole with coordinates 71 4 n 154 7 e a95 6 0 k 14 n 43 age 55 58 ma the pole is supported by a positive reversal test paleosecular variation estimated as the angular standard deviation of the virtual geomagnetic pole distribution 21 7 3 9 2 8 is close to expected for the given age and paleolatitude our new faroes paleomagnetic pole is statistically different from the majority of previously published poles from the british and faroes igneous provinces and we suggest that these older data should be used with care

magnetic remanence constraints on magnetic inversion models a magnetic anomaly is produced when a rock unit has a magnetic contrast with a laterally adjacent rock unit the cause of this magnetic contrast might be produced by a change in the magnetic susceptibility and or magnetic remanence of the source bodies after vine and matthews 1963 demonstrated that magnetic anomalies observed over ocean basins record a chronology of ocean floor generation it was readily apparent that in some instances the genesis of a magnetic anomaly can be dominated by the remanence component surprisingly few investigators have acknowledged that magnetic remanence can have a similar influence on continental magnetic anomalies the incorporation of magnetic remanence data into a magnetic inversion scheme continues to present a major problem in this note we outline the problem and present some approaches that might be used to derive relevant remanence information

ambient noise tomography reveals basalt and sub basalt velocity structure beneath the faroe islands north atlantic abstract ambient noise tomography is applied to seismic data recorded by a portable array of seismographs deployed throughout the faroe islands in an effort to illuminate basalt sequences of the north atlantic igneous province as well as underlying sedimentary layers and precambrian basement rayleigh wave empirical green s functions between all station pairs are extracted from the data via cross correlation of long term recordings with phase weighted stacking implemented to boost signal to noise ratio dispersion analysis is applied to extract inter station group travel times in the period range 0 5 15 s followed by inversion for period dependent group velocity maps subsequent inversion for 3 d shear wave velocity reveals the presence of significant lateral heterogeneity up to 25 in the crust main features of the final model include i a near surface low velocity layer interpreted to be the malinstindur formation which comprises subaerial compound lava flows with a weathered upper surface ii a sharp velocity increase at the base of the malinstindur formation which may mark a transition to the underlying beinisvor formation a thick laterally extensive layer of subaerial basalt sheet lobes iii a low velocity layer at 2 5 7 0 km depth beneath the beinisvor formation which is consistent with hyaloclastites of the lopra formation iv an upper basement layer between depths of 5 9 km and characterized by s wave velocities of approximately 3 2 km s consistent with low grade metamorphosed sedimentary rocks v a high velocity basement with s wave velocities in excess of 3 6 km s this likely reflects the presence of a crystalline mid lower crust of archaean continental origin compared to previous interpretations of the geological structure beneath the faroe islands our new results point to a more structurally complex and laterally heterogeneous crust and provide constraints which may help to understand how continental fragments are rifted from the margins of newly forming ocean basins

morphologies and emplacement mechanisms of the lava flows of the faroe islands basalt group faroe islands ne atlantic ocean the palaeogene faroe islands basalt group fibg comprises three eruptive sequences or formations all emplaced into a subaerial environment during the development of the extensive continental flood basalt province that stretches from east greenland through the faroe islands and into the faroe shetland basin the beinisvor formation having a tabular classic facies architecture is composed of a sequence of simple flows each comprising a single sheet lobe the beinisvor formation is overlain by the distinctly contrasting malinstindur formation that has a compound braided facies architecture the enni formation occurs at the top of the sequence and consists of a mixture of simple and compound flows with tabular classic and compound braided facies architectures respectively surface and internal characteristics of the sheet lobes of the beinisvor and enni formations indicate emplacement through inflation which is more obvious for the tube fed compound flows of the malinstindur and enni formations the difference between the simple and compound flow sequences of the fibg is most likely linked to the manner in which the lava was supplied during the eruption and the eruptive style of the volcanic system the sheet lobes were erupted over laterally extensive areas from fissure systems which had a continuous supply of lava which contrasts with the tube fed compound flows which were erupted in a gradual piecemeal manner from point sourced low shield volcanoes with limited areal extents

accuracy of interpolation techniques for the derivation of digital elevation models in relation to landform types and data density one of the most important scientific challenges of digital elevation modeling is the development of numerical representations of large areas with a high resolution although there have been many studies on the accuracy of interpolation techniques for the generation of digital elevation models dems in relation to landform types and data quantity or density there is still a need to evaluate the performance of these techniques on natural landscapes of differing morphologies and over a large range of scales to perform such an evaluation we investigated a total of six sites three in the mountainous region of northern laos and three in the more gentle landscape of western france with various surface areas from micro plots hillslopes and catchments the techniques used for the interpolation of point height data with density values from 4 to 109 points km2 include inverse distance weighting idw ordinary kriging ok universal kriging uk multiquadratic radial basis function mrbf and regularized spline with tension rst the study sites exhibited coefficients of variation cv of altitude between 12 and 78 and isotropic to anisotropic spatial structures with strengths from weak with a nugget sill ratio of 0 8 to strong 0 01 irrespective of the spatial scales or the variability and spatial structure of altitude few differences existed between the interpolation methods if the sampling density was high although mrbf performed slightly better however at lower sampling densities kriging yielded the best estimations for landscapes with strong spatial structure low cv and low anisotropy while rst yielded the best estimations for landscapes with low cv and weak spatial structure under conditions of high cv strong spatial structure and strong anisotropy idw performed slightly better than the other method the prediction errors in height estimation are discussed in relation to the possible interactions with spatial scale landform types and data density these results indicate that the accuracy of interpolation techniques for dem generation should be tested not only in relation to landform types and data density but also to their applicability to multi scales

experimental study on diamond dissolution in kimberlitic and lamproitic melts at 1300 1420 c and 1 gpa with controlled oxygen partial pressure to evaluate the dissolution processes of diamond crystals in kimberlitic and lamproitic magmas a series of diamond dissolution experiments were carried out in the graphite stability field at 1300 1420 c and 1 gpa under the wi mw and hm buffers dissolution agents used include an aphanitic kimberlite from wesselton mine south africa and a lamproite from mount north west kimberley australia with increasing run duration diamond morphology changed from a sharp octahedron through combinations of octahedron and tetrahexahedroidal forms to spherical tetrahexahedroid having rounded faces negatively oriented trigons were formed on the octahedral 111 face as the degree of diamond dissolution increased the trigons changed from smaller shallow types to larger deep types the dissolution rate in the kimberlitic solvent at 1300 c was 0 12 mm h under the hm buffer 0 0034 mm h under the mw buffer and 0 0017 mm h under the wi buffer whereas that at 1420 c was 0 014 mm h under the wi buffer in the lamproitic solvent the dissolution rate was 0 0024 mm h at 1420 c under the wi buffer the data indicate that diamond dissolves in silicate melts as carbonate ions through an oxidizing reaction the degree of dissolution strongly depends on temperature oxidation state and the compositional dependence of co 2 solubility in the melts

effects of supershear rupture speed on the high frequency content of s waves investigated using spontaneous dynamic rupture models and isochrone theory 1 in this paper we achieve three goals 1 we demonstrate that crack tips governed by friction laws including slip weakening rate and state dependent laws and thermal pressurization of pore fluids propagating at supershear speed have slip velocity functions with reduced high frequency content compared to crack tips traveling at subshear speeds this is demonstrated using a fully dynamic spontaneous three dimensional earthquake model in which we calculate fault slip velocity at nine points locations distributed along a quarter circle on the fault where the rupture is traveling at supershear speed in the in plane direction and subshear speed in the antiplane direction this holds for a fault governed by the linear slip weakening constitutive equation by slip weakening with thermal pressurization of pore fluid and by rate and state dependent laws with thermal pressurization the same is also true even assuming a highly heterogeneous initial shear stress field on the fault 2 using isochrone theory we derive a general expression for the spectral characteristics and geometric spreading of two pulses arising from supershear rupture the well known mach wave and a second lesser known pulse caused by rupture acceleration 3 we demonstrate that the mach cone amplification of high frequencies overwhelms the de amplification of high frequency content in the slip velocity functions in supershear ruptures consequently when earthquake ruptures travel at supershear speed a net enhancement of high frequency radiation is expected and the alleged low peak accelerations observed for the 2002 denali and other large earthquakes are probably not caused by diminished high frequency content in the slip velocity function as has been speculated

is the initial thermal state of a fault relevant to its dynamic behavior the prediction ofimpending earthquakes undoubtedly remains one of the most pursued goals of modern seismology within the framework of a deterministic description of earthquake faulting the initial state of the fault system and the choice of the governing model describing its rheological behavior play a fundamental role in the description of the earthquake recurrence in classical models of faulting this initial stateisbasicallydescribedbytheinitialshear stressdistribution priortothenextearth quakeevent andbytheinitialslidingvelocity inthispaper byassumingarate state andtemperature dependent rheology weinvestigatewhether theinitialthermalstate of the fault can also have a significant role in earthquake dynamics our numerical results clearly demonstrate that the initial temperature greatly influences the cosesimic slip and thus the earthquake magnitude the released stress and thus the radiated energy andtheintereventtime i e theearthquakerecurrence despitetheremainingissueson theconceptofearthquakecyclicity ourresultscancontributetothelivelydebateonthe deterministic hazard assessment illuminating that the temperature field also plays a fundamental role in earthquake dynamics not only because it controls possible phase changes and the chemical environment of the fault zone but also because it affects the response of a brittle fault and earthquake cycles

volatile fluxes during flood basalt eruptions and potential effects on the global environment a deccan perspective abstract we examine the role that flood basalt eruptions may have played during times of mass extinction through the release of volcanic gases continental flood basalt provinces have formed by numerous eruptions over a short period of geologic time characteristically a few million years within this period a short lived climactic phase that lasts about 1 ma typically emplaces a large proportion of the lava volume this phase consists of a series of huge eruptions each yielding 10 3 10 4 km 3 of magma each eruption lasted on the order of a decade or more and built an immense p hoehoe dominated lava flow field by eruptive activity along fissures tens to hundreds of km long high fire fountains emanating from vents along the fissures at times sustained eruption columns that lofted gas and ash into the upper troposphere and lower stratosphere while the lava flows covered huge areas the combination of large eruption magnitudes maintained high effusion rates during eruptions and the repeated nature of the characteristic large scale eruptive activity occurs in earth history only during periods of flood basalt volcanism based on recent analogs and determination of volatile contents of ancient flood basalt lavas we estimate that individual eruptions were capable of releasing 10 000 tg of so 2 resulting in atmospheric loadings of 1000 tg a 1 during a sustained decade long eruptive event we apply this model of flood basalt volcanism to estimate the potential mass of co 2 and so 2 released during formation of the 65 ma deccan province the deccan lava pile contains the record of hundreds of enormous p hoehoe flow fields erupted within a period of about 1 ma consequently atmospheric perturbations associated with so 2 emissions from just one of these long lasting eruptions were likely to have been severe and constantly augmented over a decade or longer by contrast the amounts of co 2 released would have been small compared with the mass already present in the atmosphere and thus much more limited in effect individual eruptions were followed by hiatuses of hundreds to thousands of years during which the gas contributions to the atmosphere would be recycled it is clear that the nature and potential atmospheric impact of a series of huge volume repeated long term degassing events requires further investigation in conjunction with appropriate climate models

a geophysical study of the hatton bank volcanic margin a summary of the results from a combined seismic gravity and magnetic experiment abstract this paper summarizes the results of a geophysical study of the hatton bank volcanic margin in the north atlantic normal incidence and synthetic aperture profiles saps show seaward dipping reflectors interpreted as subaerial basalt flows on the continental slope and in the adjacent ocean basin velocity depth profiles derived from expanding spread profiles esps shot along strike show elevated velocities in the lower crust on the continental slope the high velocities are interpreted as evidence for igneous underplating or intrusion gravity modelling shows high densities in the lower crust supporting a model of igneous accretion an ocean bottom seismometer obs and multichannel refraction line shot perpendicularly to strike through the midpoints of the esps further constrained the velocity structure across the margin the velocity model was refined as each dataset was processed clearly showing that multiple datasets were required in order to constrain the velocity structure tightly in particular esps were found to produce misleading results in places where the assumption that the sub surface structure is one dimensional inherent in the esp technique was not valid the final velocity model is used to infer the thickness of igneous material accreted across the hatton margin which is estimated to increase westwards from zero at the top of the bank to 18 km at the bottom of the continental slope and then thin rapidly to 11 km in the adjacent iceland basin as the oceanic crust thins its seismic velocity decreases suggesting a decrease in percentage partial melting these observations strongly support a model in which volcanic margins are produced by elevated asthenospheric temperatures

large scale magmatic pulses drive plant ecosystem dynamics the 6 6 km gross thickness of the palaeogene lava field of the faroe islands basalt group was erupted in the initial phases of north atlantic rifting thin interlava sedimentary rocks yield palynofloras that vary in composition and diversity with the duration of the interlava period long term trends in plant ecological succession occur within the record each reflecting initially rapid and subsequently slowing eruption tempo tio 2 and mgo plots derived from the basalt lava flows show corresponding fractionation trends these link melt column processes to vegetation ecosystem dynamics via controls on eruption tempo thermal support and substrate disturbance supplementary material further data are available at www geolsoc org uk sup18538

thellier palaeointensity experiments on faroes flood basalts technical aspects and geomagnetic implications based on palaeomagnetic and rock magnetic results 108 samples from 27 flows from palaeogene flood basalts of faroe islands were chosen for whole rock thellier palaeointensity experiments altogether 90 samples were rejected due to either chemical alterations or typical multidomain md behaviour evidenced by ptrm tails af pre treatment was used to reduce the effect of md grains on thellier experiments only five flows 18 samples yielded acceptable palaeointensity estimates with flow mean vdms ranging from 3 5 to 7 4 10 22 am 2 modest selection criteria imposed on all published 5 160 ma palaeointensity data left only 15 palaeomagnetic dipole moments eight from whole rock samples six from submarine basaltic glass and one from single plagioclase crystals more data are needed before the intriguing differences between results from different materials can be put into a geomagnetic context 2002 elsevier science b v all rights reserved

dynamic seismic ruptures on melting fault zones 1 we present a physical model that describes the behavior of spontaneous earthquake ruptures dynamically propagating on a fault zone and that accounts for the presence of frictional melt produced by the sliding surfaces first we analytically derive the solution for the temperature evolution inside the melt layer which generalizes previous approximations then we incorporate such a solution into a numerical code for the solution of the elastodynamic problem when a melt layer is formed the linear slip weakening law initially governing the fault and relying on the coulomb friction is no longer valid therefore we introduce on the fault a linearly viscous rheology with a temperature dependent dynamic viscosity we explore through numerical simulations the resulting behavior of the traction evolution in the cohesive zone before and after the transition from coulomb friction and viscous rheology the predictions of our model are in general agreement with the data from exhumed faults we also find that the fault after undergoing the breakdown stress drop controlled by the slip weakening constitutive equation experiences a second traction drop controlled by the exponential weakening of fault resistance due to the viscous rheology this further drop enhances the instability of the fault increasing the rupture speeds the peaks in fault slip velocity and the fracture energy density

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

use of low frequencies for sub basalt imaging many prospective passive ocean margins are covered by large areas of basalts these basalts are often extremely heterogeneous and scatter the seismic energy of the conventional seismic reflection system so that it becomes difficult to obtain information on deeper reflectors since high frequencies are scattered more than low frequencies we argue that the acquisition system for sub basalt targets should be modified to emphasize the low frequencies using much larger airguns and towing the source and receivers at about 20 m depth in the summer of 2001 we obtained seismic reflection data over basalt in the northeast atlantic using a system modified to enhance the low frequency energy these new data show deep reflections that are not visible on lines shot in the same places with a conventional system

the geology and emplacement of the volcaniclastic infill at the voorspoed group ii kimberlite orangeite pipe kroonstad cluster south africa abstract the voorspoed group ii kimberlite pipe is atypical in terms of south african kimberlite pipes located in the central region of the kaapvaal craton south africa the voorspoed pipe is one of six pipes comprising the kroonstad kimberlite cluster reconstruction of the palaeo stratigraphy at the time of emplacement of the kroonstad kimberlites indicates that significant post emplacement erosion has occurred and the pipes are currently exposed at 1600 m depth from the original land surface the volcaniclastic kimberlite vk infilling the pipe is distinctly different from typical tuffisitic kimberlite infilling other south african type kimberlite pipes three textural volcaniclastic kimberlite varieties are observed massive volcaniclastic kimberlite mvk bedded volcaniclastic kimberlite bvk and fine grained volcaniclastic kimberlite fgvk layers the bvk and fgvk are volumetrically insignificant and the bulk of the pipe is infilled with mvk the mvk can be further sub divided into seven varieties the two dominant varieties are described in detail which include main mvk mmvk and the juvenile rich mvk jrmvk the mvks are typically massive clastic poorly sorted and supported by an interclast material consisting of juvenile and xenocrystic crystals in a mud rich matrix magmaclasts within the mvks are spherical crystalline with a typical coherent hypabyssal texture and different mineralogical varieties are typically juxtaposed bvk units are composed of several normally graded beds of volcaniclastic material fgvk layers are well sorted relative to the mvks and all constituents are typically the evidence suggests that the volcaniclastic infill has formed through complex processes involving re working of extra crater tephra back into the vent and coeval pyroclastic deposition it is also likely that there is significant mixing of the resedimented and pyroclastic deposits distinction between deposits formed through re working of material through repeated explosive eruptions within the vent and that formed through varying degrees of re working extra crater tephra from the surface with possible coeval pyroclastic eruption is probably impossible for this reason we interpret the mmvk as being deposited by dominantly resedimentation of tephra whereas the jrmvk has been deposited dominantly by pyroclastic processes the presence of bvk and fgvk deposited deep within the pipe through slumping of the crater margins at the surface suggests that a crater 2000 m in depth was periodically open the identification of the complex layered variable internal structure of the volcaniclastic infill at the voorspoed has significant implications for diamond ore evaluation in south africa typical south africa kimberlite pipes are infilled with tuffisitic kimberlite which are characterised by their homogenous nature over large vertical intervals 1000 m this is not the case at the voorspoed pipe where mvk layers are variable in thickness

rupture speed and slip velocity what can we learn from simulated earthquakes abstract in this paper we consider a wide catalog of synthetic earthquakes numerically modeled as spontaneous fully dynamic 3 d ruptures on extended faults governed by different friction laws including slip dependent and rate and state dependent equations we analyze the spatial correlations between the peak of fault slip velocity vpeak and the rupture speed vr at which the earthquake spreads over the fault we found that vpeak positively correlates with vr and that the increase of vpeak is roughly quadratic we found that near the transition between sub and supershear regimes vpeak significantly diminishes and then starts to increase again with the square of vr this holds for all the governing models we consider and for both homogeneous and heterogeneous configurations moreover we found that on average vpeak increases with the magnitude of the event vpeak m00 18 our results can be incorporated as constraints in the inverse modeling of faults

an efficient mechanism to avert frictional melts during seismic ruptures abstract we consider the spatio temporal evolution of temperature due to frictional heating caused by the spontaneous propagation of 3 d dynamic seismic ruptures on planar faults in our numerical experiments which characterize typical crustal earthquakes we assume that fault friction is controlled by different linear and nonlinear slip dependent friction laws in this paper we confirm that a necessary condition to prevent melting is to have a nearly complete breakdown stress drop our simulations which employ a nonlinear slip dependent governing equation recently inferred from laboratory experiments by sone and shimamoto 2009 reproduce such a dramatic fault weakening and represent a plausible explanation for the prevention of melting during earthquake ruptures we also demonstrate that low friction alone although necessary is not a sufficient condition to avert melts the linear or classical slip weakening sw law would produce melting even assuming the same lengthscales and frictional levels to avoid melting with linear sw law we have to impose a specific value of the sw distance this reveals the prominent role of the time evolution of traction within the cohesive zone where the stress release is realized and of the value of the fracture energy density

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

fault structure wear mechanisms and rupture processes in pseudotachylyte generation abstract fault generated pseudotachylyte is found within both cataclastic and mylonitic host rocks suggesting that rapid catastrophic displacements have occurred at a variety of depths within paleoseismogenic zones pseudotachylyte bearing fault zones represent a composite of structural features associated with the process of earthquake rupture propagation and coseismic slip the development of multiple pseudotachylyte veins in fault linkages duplexes sidewall ripouts en echelon arrays and brittle zones suggests repeated rupturing during a series of characteristic earthquakes each earthquake as a coseismic slip event can be subdivided into initial rupture acceleration stable sliding and final deceleration stages these evolve through distinctive sequences of wear and deformation mechanisms that vary with sliding velocity duration of slip total displacement and the hydrodynamics of the developing fault zone slip is thought to proceed toward surface refinement and possible frictional melting following the propagation of leading shear fracture process zones the passage of the initial process zone of oblique fracturing would be followed by linkage to a throughgoing structure with asperity reduction through brecciation comminution and refined cataclastic flow for frictional melting in an abrasive wear dominant model at greater depths in the presence of mylonitic anisotropy slip would proceed through initial layer parallel surfaces and duplex linkages with rapid surface refinement through plastic smearing and laminar flow for frictional melting in an adhesive wear dominant model

timing and duration of volcanism in the north atlantic igneous province implications for geodynamics and links to the iceland hotspot abstract we combine new and published 40 ar 39 ar age determinations from incremental heating experiments on whole rocks and mineral separates to assess the timing duration and distribution of volcanic activity during construction of the north atlantic igneous province we use these ages together with volume estimates of erupted magmas and their cumulates to calculate melt production rates for the early tertiary flood basalts of east greenland and the faeroes islands the lavas lie at opposite ends of the greenland iceland faeroes ridge the postulated iceland hotspot track and record volcanic activity leading up to during and following continental breakup between greenland and europe dominantly basaltic but also alkalic and picritic magmas were erupted from beneath thick continental lithosphere simultaneously at 61 ma from baffin island to the western and eastern margins of greenland to the faeroe islands and the western british isles a roughly circular area 2000 km in diameter volcanic activity was increasingly intermittent by 57 56 ma but at 56 1 0 5 ma the average melt production rate increased by more than an order of magnitude over previous levels from 3000 km 3 km of rift my coinciding with continental rifting and shallow decompressional melting of the mantle that fed subaerial volcanic activity along most of the eastern margin of greenland and its complement on the northwestern european margin offshore abnormally thick oceanic crust was produced along the line of plate separation the upper part of this crust comprises seismically imaged seaward dipping subaerially erupted lavas by 50 ma eruption rates had diminished drastically and volcanic activity had narrowed to a much restricted portion of the east greenland margin at the western end of the greenland iceland ridge that subsequently connected in an age progressive trend to present hotspot activity in southeastern iceland the startup mantle plume head and tail model with moderate excess temperature t 100 c and active upwelling best explains the time space and compositional aspects of volcanism in this province

a fault constitutive relation accounting for thermal pressurization of pore fluid 1 the heat generated in a slip zone during an earthquake can raise fluid pressure and thereby reduce frictional resistance to slip the amount of fluid pressure rise depends on the associated fluid flow the heat generated at a given time produces fluid pressure that decreases inversely with the square root of hydraulic diffusivity times the elapsed time if the slip velocity function is crack like there is a prompt fluid pressure rise at the onset of slip followed by a slower increase the stress drop associated with the prompt fluid pressure rise increases with rupture propagation distance the threshold propagation distance at which thermally induced stress drop starts to dominate over frictionally induced stress drop is proportional to hydraulic diffusivity if hydraulic diffusivity is 0 02 m2 s estimated from borehole samples of fault zone material the threshold propagation distance is 300 m the stress wave in an earthquake will induce an unknown amount of dilatancy and will increase hydraulic diffusivity both of which will lessen the fluid pressure effect nevertheless if hydraulic diffusivity is no more than two orders of magnitude larger than the laboratory value then stress drop is complete in large earthquakes

the nature and age of cenozoic contractional deformation within the ne faroe shetland basin abstract a number of large mainly ne trending and a few nne trending cenozoic to recent anticlines have been identified from commercial seismic reflection data within the ne faroe shetland basin regional seismic markers have enabled the timing of formation of these structures to be ascertained and they appear to have developed mainly during early to mid miocene times but there is also some evidence of activity through to early pliocene to recent and even of pre miocene activity the ages of the cenozoic folds within the ne faroe shetland basin partly overlap with those observed on the norwegian margin and around the faroe islands and the hatton rockall areas although in general these regions have an important older component of development mechanisms for the formation of the folds are controversial though results from this study indicate that the ne orientations of many of the folds in the ne faroe shetland basin are compatible with compression approximately orthogonal to the continental margin the change from ne to nne trending anticlines in the vicinity of the magnus and erlend lineaments transfer zones may suggest that the development of these folds was related to sinistral strike slip movement along these major structures speculatively quaternary strike slip movement along these zones may also have triggered significant slope failure and the formation of mud volcanoes and mud intrusions as evidenced by the close spatial association of the miller slide headwall and the pilot whale diapirs

exploration in the shetland faeroe basin using densely spaced arrays of ocean bottom seismometers recent exploration activity in the peripheral regions of the shetland faeroe basin offshore northwest scotland has led to the discovery of some of the largest oil reserves on the united kingdom uk continental shelf we present results from two ocean bottom seismometer profiles acquired by mobil north sea ltd across the center of the shetland faeroe basin these data provide a powerful tool for delineating long wavelength velocity variations and thus have potential for reducing the nonuniqueness associated with conventional seismic exploration methods analysis of the first arrival traveltime data using both forward and inverse ray based techniques produces a well constrained velocity depth model of the basin fill we estimate that the uncertainty in the velocity structure is 5 from a series of trial and error perturbations applied to the final models the velocity structure of the faeroe basin has three principal layers 1 a near surface layer with velocities in the range 1 6 to 2 2 km s 2 a 3

petrology of the hypabyssal kimberlite of the kroonstad group ii kimberlite orangeite cluster south africa evolution of the magma within the cluster abstract we report here a detailed petrographic study accompanied by the first whole rock major and trace element geochemical analyses as well as the first sr and nd isotope data for the kroonstad group ii kimberlite cluster hypabyssal kimberlite is described from the lace voorspoed and besterskraal north kimberlite pipes the lace kimberlite is a monticellite phlogopite kimberlite with mineralogy and geochemistry more similar to typical group i kimberlites the voorspoed kimberlite is an aegirine and sanidine bearing kimberlite and along with the geochemistry shows a more evolved composition relative to the lace kimberlite the besterskraal north kimberlite is a highly evolved type group ii kimberlite containing abundant k richterite sanidine aegirine and leucite the magma evolutionary trend is linked to a variation in olivine macrocryst abundance the lace unevolved magma contains abundant olivine macrocrysts and has not been affected by significant fractionation whereas the voorspoed and besterskraal north kimberlite contain far fewer olivine macrocrysts which indicates that significant fractionation has affected these magmas en route to the surface we evaluate the potential for fractionation to result in differentiation of a common parent magma or magma generated from a common source en route to the surface this process is complicated by the fact that olivine macrocrysts are essentially xenocrysts within the magma and the loss of these xenocrysts will not affect the primary magma composition furthermore we present two alternate hypotheses for the evolution of the magmas a contamination en route to the surface and b varying degrees of partial melting at the source the evolutionary trend is also linked with diamond characteristics in particular diamond parcels at the voorspoed pipe between 1906 and 1911 were deficient in large stones which indicate that fractionation has likely also removed the large stones from the voorspoed magma along with the olivine macrocrysts

the hatton basin and continental margin crustal structure from wide angle seismic and gravity data results from a wide angle seismic and gravity study between the rockall bank and the iceland basin in the north atlantic are presented crustal and sedimentary structures are resolved in the hatton basin and across the hatton continental margin hcm east of magnetic anomaly 24 the structure of the oceanic crust west of the anomaly is also determined gravity data support the seismic model in areas of good seismic coverage and are used to control the model where the wide angle seismic data are poor a two layer sedimentary sequence is present both in the hatton basin and across the continental margin the lower layer with p wave velocity of about 4 km s is interpreted as pre eocene synrift sediments and is up to 3 5 km thick a younger and thinner 1 2 5 km postrift sequence with a velocity of about 2 km s defines a strong velocity contrast which suggests an erosional unconformity surface the sedimentary structure is distinctly different from that in the rockall trough where a third intermediate layer vp 3 km s occurs the three layer crust characterized by two intracrustal reflections pip1 and pip2 varies from 30 km thick under the rockall bank to about 15 km below the hatton basin where it is stretched by a factor of 2 relative to onshore ireland the crust is thinnest below the hatton bank where the presence of a single intracrustal reflection indicates that the lower crustal layer thins to below the seismic resolution limit below the hcm a region of thick lower crust with anomalously high velocity vp 7 2 km s is resolved by the seismic and gravity data it is connected west of anomaly 24 to a region of oceanic crust which is thicker than in the iceland basin these relationships between the thick lower crust below the hcm and the oceanic crust in the iceland basin are interpreted as evidence for magmatic underplating consistent with previous models for the hcm the inferred unconformity surface between the synrift and postrift layers may be due to regional uplift driven by upwelling of hot asthenosphere before anomaly 24 early eocene time

the historical development of the magnetic method in exploration the magnetic method perhaps the oldest of geophysical exploration techniques blossomed after the advent of airborne surveys in world war ii with improvements in instrumentation navigation and platform compensation it is now possible to map the entire crustal section at a variety of scales from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale methods of data filtering display and interpretation have also advanced especially with the availability of low cost high performance personal computers and color raster graphics the magnetic method is the primary exploration tool in the search for minerals in other arenas the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications such as locating intrasedimentary faults defining subtle lithologic contacts mapping salt domes in weakly magnetic sediments and better defining targets through 3d inversion these new applications have increased the method s utility in all realms of exploration in the search for minerals oil and gas geothermal resources and groundwater and for a variety of other purposes such as natural hazards assessment mapping impact structures and engineering and environmental studies

temperature variations of constitutive parameters can significantly affect the fault dynamics abstract a rate and state dependent governing law with temperature dependent constitutive parameters is considered on the basis of laboratory inferences we model the whole seismic cycle of a homogeneous fault obeying to such a law by adopting a spring slider dashpot fault analog model we show that the variations of the parameter a accounting for the so called direct effect with the temperature cause the system to enter at high speeds in a conditionally stable regime and also in a velocity strengthening regime although we do not observe the complete cessation of slip we can see a severe reduction of the degree of the instability of the fault in particular the peaks of the sliding velocity are reduced as well as the developed temperature due to frictional sliding and the released stress during each instability event moreover the recurrence times are reduced of a factor of two with respect to a reference configuration where the canonical formulation of rate and state friction with temporally constant parameters is assumed the obtained results can help the interpretation of high velocities laboratory experiments and further illuminate the importance of the temperature in the context of seismic hazard assessment