contribution of deep seismic profiling to the knowledge of the lower crust in france and neighbouring areas abstract in france and neighbouring areas the crust is essentially formed of segments of the variscan orogen 380 300 ma and older formations cratonic basins began to form in the permian but extensive subsidence only occurred from the lias in basins such as the paris and aquitaine basins and also along the margin of the alpine tethys ocean the pyrenees and alps range eventually formed during the tertiary as a result of the collision between the european and african continental plates large grabens were also rifted in the rhine and rhone valleys deep seismic profiles show remarkable features common to the crust of the whole area although it underwent such a complicated geological history the most striking feature is the occurrence of conspicuous flat reflections densely distributed in the lower part of the crust below a comparatively transparent upper crust these reflections hereafter called laminations have been found to occur on most profiles within a large area covering western europe although they vary in depth thickness and energy dipping reflections have frequently been observed in the upper crust some of them extend across the lower crust and even penetrate into the upper mantle they may be reasonably interpreted as variscan compressional features by correlation with surface geology and comparison with similar features found in deep seismic profiles from other regions the origin of the flat laminations which generally lie in the lower part of the crust is more controversial this paper attempts to use the well known geological history of the region to place some constraints on the hypotheses which may be advanced to explain the formation of these laminations these features may be explained by the following tectonothermal events 1 1 compression which prevailed during the variscan orogeny may have formed the laminations as a result of delamination and shearing near the bottom of the crust associated with melting intrusion and metamorphism during the different stages of the crustal evolution 2 2 the thermal equilibration of the upper mantle which occurred in the permian at the end of the variscan orogeny may have caused metamorphism and magmatism in the overlying crust 3 3 a regional thermal event which might be associated with subordinate extension occurred during the rifting of the alpine tethys and the initiation of mesozoic cratonic basins in late triassic middle jurassic times it may also have caused metamorphism and magmatism in the lower crust while the first model implies conservation of the crust material the other two suggest that the crust mantle transition has been somewhat uplifted since the end of the variscan orogeny the latter hypothesis is one way to explain the undeniable occurrence of seismic events below the present moho reflection the above three models are not mutually exclusive and all these phenomena may well have affected the crust to some extent however the data at hand are still insufficient to determine what parts tectonic and nontectonic processes have played in the creation of the laminations in the lower crust and the moho further experiments are needed to check these hypotheses

delamination of eclogitized lower crust control on the crust mantle boundary in the central fennoscandian shield abstract the crustal structure of the central part of the fennoscandian shield is dominated by a high velocity lower crust hvlc vp 7 3 7 5 km s at the depth of 40 63 km in an area of ca 400 km 600 km formed in the palaeoproterozoic svecofennian orogen at 1 91 1 87 ga in the area of the hvlc the moho is deeper than 50 km and attains depths of up to 63 km in eastern finland at the archaean proterozoic boundary zone we suggest that the present moho dates back to late svecofennian time and was determined by delamination of high density eclogitized lower crust after tectonic collisional thickening of crust densities deduced from seismic and gravity data suggest a very small or completely lacking density contrast between lower crust and uppermost mantle new geological evidence from kimberlite hosted garnet xenocrysts supports the presence of a garnet population with compositions in agreement with crustal eclogitic garnets in addition small clinopyroxene garnet mineral aggregates microxenoliths show omphacitic clinopyroxene compositions therefore the presence of eclogites in the high velocity lower crust is implied although the eclogite proportion as estimated from seismic velocity data is only a few percent in the present hvlc with results on near incidence and wide angle seismic studies and geological and geochemical data on heat producing elements we modeled the svecofennian collision and the thermal evolution of the crust to provide temperature constraints for the delamination model the constructed thermal model simulates the genesis of svecofennian synkinematic at 1 89 1 88 ga and postkinematic 1 88 1 87 ga granitoids as well as the general metamorphic grade of the surface rocks and lower crustal xenoliths moreover the narrow distribution of the ages of the svecofennian mafic ultramafic magmatism ca 1 89 1 87 ga dispersed around the hvlc area is explained by the homogeneous warming of the lithosphere the conditions of lower crustal delamination were estimated with a rayleigh taylor convective instability model constrained with lower crustal and upper mantle rheological data delamination of a lower crustal eclogite layer 5 30 km thick takes place in about 7 50 ma if the rock dynamic viscosity is 1 1021 pa s it requires temperatures of 1000 c in the lower crust and upper mantle in the svecofennian crust such temperatures were attained at about 1860 1850 ma i e after the orogenic peak we conclude that the moho of the hvlc area was controlled by delamination of a ca 5 10 km thick layer of high density lowermost crust into the mantle such a density controlled process may have controlled the moho depth also in other proterozoic shield areas

deep crustal reflections in australia 1957 1973 ii crustal models summary in a companion paper we described experiments conducted by the australian bureau of mineral resources bmr between 1957 and 1973 to record deep crustal seismic reflections at 24 of the 46 sites investigated events were recorded at times up to 10 13 s at some sites only one or two shots were recorded where more comprehensive investigations were made the events were consistent with deep reflections though they lacked continuity in many cases the results were sufficiently encouraging that we have now analysed the events on the hypothesis that they represented deep crustal reflectors including the moho in 1976 bmr introduced digital recording and successfully carried out several long deep crustal profiles since then this paper analyses only the recordings made with analogue equipment prior to 1973 in a few places the reflection structure could be compared with nearby refraction traverse results some events correlated well with first or second order discontinuities in the refraction velocity depth curves while for others there was no correspondence it is difficult to generalize as to which types of refractors give rise to reflections the refraction surveys show that the moho is not always a boundary with a sharp change in velocity across it in some areas in queensland the deepest reflector correlates well with the refracting moho for reflection shots not near refraction surveys we converted travel times to depths using simplified velocity models based on refraction surveys in similar geological environments and on isostatic balance considerations using gravity and elevation data in central australia if the deepest reflector is the moho a relatively thin and light crust is implied however comparison with refraction models makes it seem probable that a heavier thicker crust could exist with the moho being deeper than the deepest reflector also in the tasman geosyncline it is probable that the moho is deeper than the deepest reflector

crustal structure of central saudi arabia determined from seismic refraction profiling abstract based on a two dimensional ray path interpretation of travel time and wave amplitude ratios a detailed crustal velocity structure of central saudi arabia has been derived using the deep seismic refraction profile of saudi arabia the crustal model derived significantly differs from previous interpretations carried out on the same data set since more energetic crustal phases were considered in this analysis the two dimensional crustal model derived shows that the crust consists of four distinct layers it is approximately 42 km thick under the arabian platform and thins gradually in a southwest direction to about 38 km under the shield the upper crust consists of two layers the first one has a p wave velocity of about 6 1 km s and is about 3 km thick thinning to about 1 km under the platform the second layer has a p wave velocity of about 6 2 km s and is about 14 km thick but thins to about 7 km under the platform the intermediate crustal layer has a p wave velocity of about 6 4 km s is about 7 5 km thick under the shield under the platform it is about 16 km thick with a velocity of about 6 3 km s the lower crust has a p wave velocity of about 6 9 km s and is about 15 km thick a decrease in the upper mantle pn velocity from 8 2 to 8 1 km s seems to accompany the crustal thinning the moho discontinuity is sharply defined beneath the shield and gently dips towards the northeast a significant crustal inhomogeneity in the region separating the afif terrane from the ar rayn terrane is interpreted as a thrusting system rather than a distinct vertical discontinuity in the crustal structure as previously interpreted this region between the two terranes which is of different crustal composition is probably the result of thrusting and emplacement of nappes of upper crustal materials of the afif terrane onto the ar rayn foreland the nature and location of this zone were constrained by travel time and wave amplitude ratio measurements the detailed velocity crustal model derived in this study is of significant importance in earthquake studies tectonics and determining the evolution of the arabian plate

rupture process of a multiple main shock sequence analysis of teleseismic local and field observations of the tennant creek australia earthquakes of january 22 1988 on january 22 1988 three large intraplate earthquakes with ms 6 3 6 4 and 6 7 occurred within a 12 hour period near tennant creek australia these earthquakes which occurred over a small interval of time and within a small volume of space present a unique opportunity to study the rupture process of the class of intraplate earthquakes that occur as multiple main shocks broadband displacement and velocity records of body waves from teleseismically recorded data are analyzed to determine source mechanisms depths and complexity of rupture of each of the three main shocks hypocenters of an additional 150 foreshocks and aftershocks constrained by local arrival time data and field observations of surface rupture are used to complement the source characteristics of the main shocks in order to derive as complete a description of the rupture process as possible the interpretation of the combined data sets suggests that the overall rupture process involved unusually complicated stress release as locations of the main shock hypocenters progressively moved from west to east we infer that the first and third main shocks denoted as ms1 and ms3 produced the southeast northwest trending scarps observed at the western end the kunayungku fault and at the eastern end the east end of the lake surprise fault respectively of the rupture zone the epicenter of the only immediate foreshock was located in the gap between these two fault scarps ms1 nucleated near this epicenter and ruptured upward and to the northwest from a depth of 6 5 km ms3 ruptured predominantly to the se at a depth of 4 5 km the second main shock ms2 is inferred to have produced the deformation of the southwest trending central scarp segment the western end of the lake surprise fault from the sense of thrusting seen at the surface and from the distribution of aftershock hypocenters the south dipping nodal planes derived from waveform modeling are identified as the fault planes for earthquakes ms1 and ms3 in contrast the dip of the central fault scarp is reversed relative to the dips of the western and eastern fault scarps the rupture process of ms2 turns out to be commensurately complex and sufficiently explains the geological complexity ms2 consisted of three subevents the southeast dipping nodal plane of the first two subevents is coplanar with a southeast dipping plane implied by locations of aftershocks which did not break the surface choice of the north dipping plane as the rupture plane of the third subevent consistent with the surface deformation and coplanar with a second plane delineated by aftershocks would imply conjugate faulting the majority of the aftershocks are concentrated near the edges of the fault planes and there is an absence of activity in the center of the planes the areas of absent activity may represent the failed asperities of the main shocks in which substantial stress relief occurred the rupture process of each main shock is characterized by the rapid release of energy followed by a much slower release of moment and by aftershock zones whose dimensions exceed the inferred dimensions of the rupture these characteristics suggest that substantial slow slip occurred on each of the three fault interfaces that was not accompanied by major energy release the first main shock nucleated at the deepest part of its aftershock zone and ruptured upward in contrast ms2 and ms3 nucleated at depths that were one third to one half of the maximum depth of their aftershock zones this variation of focal depth and the strong increase of moment and radiated energy with each main shock imply that lateral variations of strength were more important than vertical gradients of shear stress in controlling the progression of rupture

propagation of lg waves in the north australian craton influence of crustal velocity gradients abstract aftershocks of three large earthquakes near tennant creek in the northern territory of australia provide a unique opportunity to study regional seismic phases in the north australian craton three portable digital seismographs were operated near the source zone to provide control on earthquake locations and seven recorders formed a line between tennant creek and the seismic array at alice springs which is 430 km to the south eleven earthquakes of sufficient size for analysis were recorded during a one week deployment two months following the main shocks the group velocity observed for lg waves along this profile of 3 7 km sec is at the high end of the normal range and appropriate for a stable continental interior we estimate attenuation in discrete frequency bands by measuring the decay of amplitude as a function of distance from the source and assuming the common r 5 6 relationship for geometrical spreading however this assumption leads to frequency dependent q values q 230 36 f 0 66 0 12 that are abnormally low for a stable continental region the apparent q is also not consistent with other observations such as the high lg group velocity the positive bias in m l estimates obtained for other parts of australia using standard attenuation relations the size of felt areas for large central australian earthquakes nor with the high frequency content of seismic reflection data collected near our profile some crustal models for northern australia derived from seismic refraction work show a gradient zone between 30 and 55 km rather than a sharp moho discontinuity we therefore investigate the effect of gradient zones at the crust mantle transition on the decay of lg amplitudes using synthetic seismograms calculated with the wavenumber integral method the modeling suggests that some of the s wave energy that would be trapped in a crustal wave guide with a sharp lower boundary can leak out in the presence of a gradient and so enhance the attenuation in addition there are focusing effects due to the formation of caustics from the gradient zones which cannot be represented by a single analytic amplitude correction

p wave velocity features of the lithosphere under the eromanga basin eastern australia including a prominent mid crustal conrad discontinuity abstract the crustal structure of the central eromanga basin in the northern part of the australian tasman geosyncline revealed by coincident seismic reflection and refraction shooting contrasts with some neighbouring regions of the continent the depth to the crust mantle boundary moho of 36 41 km is much less than that under the north australian craton to the northwest 50 55 km and the lachlan fold belt to the southeast 43 51 km but is similar to that under the drummond and bowen basins to the east the seismic velocity boundaries within the crust are sharp compared with the transitional nature of the boundaries under the north australian and lachlan provinces in particular there is a sharp velocity increase at mid crustal depths 21 24 km which has not been observed with such clarity elsewhere in australia the conrad discontinuity in the lower crust the many discontinuous sub horizontal reflections are in marked contrast to lack of reflecting horizons in the upper crust further emphasising the differences between the upper and lower crust the crust mantle boundary moho is characterised by an increase in velocity from 7 1 7 7 km s to a value of 8 15 0 04 km s the depth to the moho under the canaway ridge a prominent basement high is shallower by about 5 km than the regional moho depth there is also no mid crustal horizon under the canaway ridge but there is a very sharp velocity increase at the moho depth of 34 km the ridge could be interpreted as a horst structure extending to at least moho depths but it could also have a different intra crustal structure from the surrounding area the sub crustal lithosphere has features which have been interpreted from limited data as being caused by a velocity gradient at 56 57 km depth with a low velocity zone above it because of the contrasting crustal thicknesses and velocity gradients the lithosphere of the central eromanga basin cannot be considered as an extension of the exposed lachlan fold belt or the north australian craton the lack of seismic reflections from the upper crust indicates no coherent accoustic impedance pattern at wavelengths greater than 100 m consistent with an upper crustal basement of tightly folded meta sedimentary and meta volcanic rocks the crustal structure is consistent with a pericratonic or arc back arc basin being cratonised in an episode of convergent tectonics in the early palaeozoic the seismic reflections from the lower crust indicate that it could have developed in a different tectonic environment

seismic crustal structure of the proterozoic north australian craton between tennant creek and mount isa seismic recording along a 600 km traverse across the southern part of the north australian craton indicates that lateral velocity variations exist in the middle lower crust taken together with other data from the craton there is evidence for midcrustal velocities increasing from east to west and from south to north these trends could result from a greater proportion of high velocity granulites being emplaced at higher levels in the crust between tennant creek and mount isa the velocity structures near the surface are compatible with an assemblage of younger and older metamorphic rocks intruded by granites near surface 0 5 km p wave velocities on the tennant creek inlier increase from 5 47 km s at the surface to about 6 2 km s at 6 km depth and correspond with low grade metamorphics of the warramunga group interspersed with weathered granites overlying amphibolite facies rocks of older metamorphic domains in the mount isa geosyncline velocities of 6 03 6 15 km s correspond to the leichhart metamorphics interspersed with granites in the middle crust p wave velocities of 6 85 km s occur at a depth of 26 km near tennant creek whereas such velocities are not evident until depths of about 37 km near mount isa there is therefore a trend for midcrust velocities to increase from east to west the lower crust is characterized by velocities of 7 3 7 5 km s and the upper mantle velocities of 8 16 8 20 km s are reached at depths of 51 54 km exposed granulite facies rocks in central australia are low in heat producing elements and are compatible with the low surface heat flow generally measured in precambrian australia these data taken together with laboratory velocity measurements on worldwide samples reported elsewhere indicate that middle to lower crustal rocks could be an assemblage of granulite facies rocks varying from pyroxene granulite in the middle crust through hornblende granulite to garnet granulite in the lower crust

geophysical differences in the lithosphere between phanerozoic and precambrian australia abstract cannikin atomic bomb recordings indicate that there are differences in travel times from the aleutian islands test site to phanerozoic and precambrian provinces in australia of up to 1 1 s explosion seismic studies in central and southeastern australia enable travel time corrections for crustal and upper mantle structure to be made to recordings of such teleseismic events structure in the upper 60 km can account for at most about 0 2 s of the residual difference but attempts to constrain the remaining residual time to the region above the lehmann discontinuity at about 200 km depth are difficult to reconcile with explosion seismic models regional differences in seismic velocity structure between phanerozoic and precambrian australia therefore appear to exist at depths greater than 200 km electrical conductivities within the mantle have been investigated using two methods long period electromagnetic depth sounding using magnetometer arrays demonstrates that conductivities increase at about 200 km under phanerozoic australia but not until about 500 km depth under precambrian australia shorter period magnetotelluric measurements can only resolve shallower structures these too indicate a similar trend but with sub crustal conductivities increasing at less than 100 km under phanerozoic australia magma at these depths and shallower may be the source for cainozoic volcanism in eastern australia under precambrian central and northern australia magnetotelluric investigations indicate that pronounced conductivity increases do not occur until depths of 150 200 km are reached oceanic magnetic observations indicate that the australian lithospheric plate as a whole is separating from antarctica at a rate of about 7 cm yr the seismic and conductivity structures under the continental region of this plate indicate that lateral inhomogeneities possibly extend to depths as great as 500 km and are probably caused by the passage of eastern australia over a hot spot hawaiian studies indicate that hot spots are not local features but result from large scale disturbances in the mantle conductivity increases commencing in the depth range 100 250 km may give an indication of uppermost zones within which the palaeozoic lithospherc has been substantially modified resulting in elevated surface heat flow volcanism and seismic travel time anomalies

the conrad discontinuity and the top of the reflective lower crust do they coincide abstract in order to investigate an often quoted coincidence of an intracrustal velocity jump the conrad discontinuity and lower crustal reflections found in tectonothermally young areas 54 reflection seismic profiles have been analysed and combined with 50 refraction seismic profiles adjacent to the reflection lines such a relationship is however contradicted by the observation that in some places a reflective lower crust is found while a velocity discontinuity is missing most of the compared data sets clearly demonstrate that the velocity jump if present is found more than 2 km below the uppermost reflections from the lower crust it is thus concluded that the two phenomena are not related occasionally both a reflective lower crust rlc and a conrad discontinuity can be observed in this case the differences in depth between the two can be explained by the structure of the velocity increase as in the example of the kapuskasing uplift in canada due to the overall depth dependent increase of seismic velocity a genetic correlation of high velocity and reflectivity is negligible this conclusion is supported by the observed highly reflective lower crust of the urach low velocity body by explaining the rlc with thermorheological arguments the upper limit of reflections simply appears as a thermal boundary depending only on temperature material and fluid content the theological models presented are explaining a rlc with and without the presence of a velocity discontinuity

origin of major dissolved ions in groundwater within the lower pra basin using groundwater geochemistry source rock deduction and stable isotopes of 2h and 18o hydrochemical and stable isotopes 18o and 2h analyses of groundwater samples were employed to establish the origin of major dissolved ions in groundwater within the lower pra basin results showed that the major processes responsible for chemical evolution of groundwater include silicate sio4 4 dissolutions ion exchange reactions sea aerosol spray and pyrite fes2 and arsenopyrite feass oxidations the groundwater is strongly acidic to neutral with ph generally range from 3 5 to 7 0 ph units and mean 5 9 0 5 approximately 89 of boreholes had ph values outside the world heath organization who guidelines for drinking water quality 2004 guideline value for drinking water due principally to natural biogeochemical processes and therefore not suitable for potable purposes electrical conductivity ec range from 57 6 to 1 201 s cm with mean 279 3 198 8 s cm total dissolved solids tds range from 32 to 661 mg l with mean 151 7 106 8 mg l with 98 6 of groundwater as fresh tds ca2 mg2 k and hco3 cl so4 2 respectively a plot of 18o against 2h showed that ground and surface waters clustered on or closely along the global meteoric water line suggesting that the waters emanated principally from meteoric source with evaporation playing an insignificant role on the infiltrating water

stationary gravity wave structure in flows with directional wind shear most theoretical studies of flow over mountains assume that the wind direction is constant for two dimensional orographic ridges this is not an issue since only the component of the wind across the ridge forces internal gravity waves a special case of rectilinear flow is when the wind component changes sign i e the wind direction changes discontinuously through 180 degrees such a point is a critical level for all stationary gravity waves for isolated three dimensional mountains the gravity wave response comprises waves of all azimuthal orientations and each of these may have a different critical level height depending on whether the wind backs or veers with height using a combination of ray tracing methods and stationary phase solutions the structure of the stationary hydrostatic gravity wave field generated by an isolated mountain is studied for a simple wind field which backs with height in contrast to the behaviour for unidirectional flow wave action does not accumulate indefinitely beneath the critical level but is continuously advected downwind by the component of the wind parallel to the phase lines instead of the wave energy becoming unbounded following a ray as in the 2d case the wave energy decays to zero along a trailing wake zone downwind of the mountain along phase lines this is just an extension of the asymptotic wake found in the linear solutions of smith for constant flow over an isolated mountain the work described here underlines the desirability of including the effects of selective critical level absorption in the gravity wave drag parametrization schemes of numerical weather prediction models

relocation of teleseismically recorded earthquakes near tennant creek australia implications for midplate seismogenesis the three ms 6 tennant creek australia earthquakes of january 22 1988 were preceded by small and moderate earthquakes in 1986 and 1987 we have used the method of joint hypocenter determination to estimate the positions of teleseismically recorded 1986 1987 earthquakes the 1988 main shocks and aftershocks with respect to a system of surface fault scarps the 1986 1987 shocks and the 1988 main shocks nucleated near the center of the zone of surface scarps where fault segment boundaries at depth are implied by complexities in the distribution of scarps at the surface this suggests that the fault segments that ruptured in 1988 were already in existence in 1986 1987 which is consistent with the hypothesis that strong midplate earthquakes occur on preexisting faults the redetermined 1986 1987 hypocenters are however also consistent with the hypothesis that midplate seismicity is localized by stress concentration due to bulk rheological heterogeneity of the crust because they are situated on a regionally prominent gravity anomaly the teleseismically recorded seismicity of the tennant creek region prior to the 1988 main shocks has the temporal pattern of a swarm followed by a lull the concentration of swarm earthquakes between two scarps is consistent with models in which precursory swarm earthquakes correspond to faulting that is spatially distinct from the site of primary main shock faulting the time interval between swarm and main shocks is similar to intervals between intermediate term precursory swarms and main shocks in regions that have much higher rates of tectonic loading this similarity suggests that the time intervals between precursory swarms and subsequent main shocks are not strongly influenced by the rate of tectonic loading but are determined primarily by time dependence of the failure process the spatial distribution of teleseismically recorded aftershocks is in most respects like that of aftershocks recorded by a local network of portable stations in the half year following the main shocks the set of teleseismically recorded aftershocks like the set of locally recorded aftershocks includes some events that occurred well away from the causative faults of the main shocks at the length scale of the 1988 main shock rupture the distribution of aftershocks occurring more than one year after the main shocks is not representative of the distribution of earlier aftershocks

constraints on locations of large intraplate earthquakes in the northern territory australia from observations at the warramunga seismic array three earthquakes of ms 6 3 6 4 and 6 7 near tennant creek northern territory australia on january 22 1988 produced 35 km of surface rupture in the precambrian shield the surface deformation consists of low angle 20 30 thrust faulting on 2 main scarps separated by a 7 km gap the kunayungku western fault and the eastern end of the lake surprise eastern fault dip to the south while the western end of the lake surprise fault dips to the north although these earthquakes are among the largest ever recorded on the australian continent there is no record of earthquakes larger than magnitude 5 within 500 km prior to 1987 data from the warramunga wra seismic array located 30 km east of the rupture show no local activity between 1965 and 1986 two small events in the fault area were observed in february 1986 and were followed by a series of 6 mb 4 to 5 2 earthquakes in january 1987 and their aftershocks the 1986 1987 and january 22 1988 mainshocks all nucleated within a zone about 5 km across from north to south but elongated radially away from wra by up to 30 km although the distribution of these events is in the direction of poorest hypocentral resolution we can place meaningful constraints on the epicenters by examining the range of solutions that fit the p arrival time data the 1986 and largest 1987 events were restricted to the western part of the fault zone the three mainshocks progressed from west to east suggesting that the kunayungku scarp was produced by the first mainshock and the west and east ends of the lake surprise scarp by the second and third respectively

lg coda q and its relation to the structure and evolution of continents a global perspective tomographic maps of lg coda q q lg c variation are now available for nearly the entire african eurasian south american and australian continents as well as for the united states q lg c at 1 hz q 0 varies from less than 200 to more than 1000 and q lg c frequency dependence varies between 0 0 and nearly 1 0 q 0 appears to increase in proportion to the length of time that has elapsed since the most recent major episode of tectonic or orogenic activity in any region a plot of q 0 versus time since that activity indicates that a single q 0 time relation approximates most mean q 0 values those that deviate most from the trend lay in australia the arabian peninsula and the east african rift the increase in q 0 with time may be due to a continual increase in crustal shear wave q q caused by the loss of crustal fluids and reduction of crustal permeability following tectonic or orogenic activity extrapolated values of q lg c at 5 hz using q 0 and values measured at 1 hz and assuming that is constant in all regions between 1 and 5 hz show a similar percentage wise increase with times that has elapsed since the most recent activity other factors that can reduce q 0 in continental regions include thick accumulations of sediment especially sandstone and shale of mesozoic age and younger severe velocity gradients at the crust mantle transition and possibly lateral variations in the depth thickness and severity of those gradients severe and large increases of q in the mid crust of some regions can cause relatively large values of even if the frequency dependence of q is small

a review of crust upper mantle structure in the precambrian areas of australia and implications for precambrian crustal evolution the western two thirds of the australian continent are underlain by precambrian rocks seismically this region is characterised by negative teleseismic travel time residuals indicating a thick cold tectosphere despite extensive platform cover and phanerozoic crustal reworking in a few intracratonic sedimentary basins the precambrian crust can be classified broadly into archaean and proterozoic cratons and proterozoic mobile belts the archaean cratons have a two layered crust 25 35 km thick with observed velocities in the lowermost crust in the range 6 8 7 2 km s 1 the moho is a distinct feature with a velocity contrast of 1 0 km s 1 vertical reflection probes in the southern yilgarn block yielded record sections that have numerous laterally discontinuous but distinct reflections especially in the lower crustal layer that are especially concentrated around the boundary between the two crustal layers and the moho the seismic reflections and velocities are consistent with a predominantly felsic crust with mafic lenses constituting 20 35 of the lowermost crust and with the metamorphic grade ranging from granulite in the upper to middle crust to garnet granulite and even eclogite at the base of the crust the proterozoic mobile belts and cratons a have multi layered crust 45 50 km thick velocities below 25 km depth are almost always 7 0 km s 1 and reach 7 4 7 6 km s 1 in the lowermost crust velocities such as these are characteristic of gabbroic basaltic underplate the velocity contrast is 1 0 km s 1 at the moho which is less distinct than in archaean terranes record sections from vertical reflection probes show a highly reflective crust with no correlation between seismic layers in the refraction models and the distribution of vertical reflections the contrasting character of the crust in archaean and proterozoic provinces favours a tectonic model for the evolution of precambrian crust in which predominantly felsic archaean protocrust was tectonically reworked and underplated to produce proterozoic crust the simple two layered protocrust became more complicated in the process the introduction of vast amounts of underplate especially near the crust mantle boundary thickened the crust and destroyed the character of the moho it overprinted the reflection character of the crust so that the bands of reflections now do not correlate with the boundaries between the crustal layers the velocities in the lower crust are high because of the underplate

deep crustal reflection results from the central eromanga basin australia abstract from 1980 to 1982 deep seismic reflection profiles were recorded across the central eromanga basin in eastern australia to study the regional structure stratigraphy and geological history of the eromanga basin and infra basins the reflection data were recorded to 20 s to obtain additional information on the nature and structure of the crust below the sediments and their relationship to the development of the basins the seismic sections show good quality reflections from the deep crust as well as from the sedimentary layers based on the character strength coherence continuity and spatial distribution of the reflections the sections can be divided into four zones the tope zone between 0 and 2 5 s shows fairly uniform coherent and continuous events which correlate with the mesozoic and late palaeozoic sediments the zone from 2 5 to 8 s 4 to 22 km does not show any primary reflections and is interpreted as the highly deformed metasedimentary and metavolcanic rocks of the early palaeozoic thomson orogen underlying the sediments without any recognisable reflection or diffraction patterns in this zone it is difficult to say whether the faulting and folding observed in the sediments extend into the upper crustal basement the deeper zone of numerous reflection segments between 8 and 12 5 s 22 to 36 km is interpreted as thin laminae of alternating low and high velocity intermediate and basic rocks and correlates with the lower crust bounded by refraction velocity discontinuities the lowest zone of no reflections below 12 5 s corresponds with the upper mantle the reflection character and thickness as well as the refraction velocity structure of the crust under the central eromanga basin area are significantly different from those of the precambrian crust under the georgina basin to the northwest it is proposed that the crust under the eromanga basin is extensionally attenuated crust which had been intruded by sills of basaltic melt from the underlying asthenosphere

the mid crustal horizon under the eromanga basin eastern australia abstract long line explosion seismic recordings indicate that a major mid crustal p wave velocity increase exists under the eromanga basin which contrasts the crust of this region from its neighbouring tectonic provinces the velocity increase is shown to be a regional feature and to correspond to the upper boundary of a zone of discontinuous seismic reflections characteristic of the lower crust the velocity increase of 0 35 0 6 km s occurs over a small depth range at an average depth of 24 km this velocity gradient produces conspicuous wide angle reflections at distances greater than 60 km from the shot point the seismic data present the clearest evidence to date of the existence of a major mid crustal horizon in continental australia a feature not prominent in other cratonic provinces which have been investigated in detail the velocity in the lower crust below the horizon is 6 7 7 0 km s the horizon represents a boundary between the upper and lower crust which could imply a different tectonic history for these two zones

the velocity structure of the australian shield from seismic travel times abstract travel times between sources and stations in the australian shield are used to develop p and s wave velocity models for the precambrian shield of western and central australia errors in travel times are minimized by using sources for which locations and origin times are constrained by local data or fault scarps apparent velocities between 1500 and 2200 km for p 8 8 km sec and s 4 8 km sec require a high velocity lid and the maximum range of observation of this branch 2200 km requires that the lid extend to a depth of 205 20 km because of the distribution of sources and receivers there are no observations from the shield at ranges greater than 2300 km so we use times from stations in phanerozoic eastern australia and published global summary times as a guide for interpretation at greater ranges allowing for known differences in teleseismic travel time residuals between the precambrian shield and eastern australian the p times are best fit by a model that has a small velocity inversion at the base of the lid and the s times are best fit by a model with a more substantial reduction in velocities however both p and s wave travel times can be satisfied with models that have velocities below 210 km that are concordant with global averages such as iasp91 thus the velocities inferred beneath the lid are low relative to those of the lid but not necessarily low relative to global averages

electrical conductivity profiles and implications for the absence or presence of partial melting beneath central and southeast australia abstract a previous paper presented measurements by australian magnetometer arrays of earth response to the magnetic daily variation source field and interpreted these data by simple representative models of one dimensional structure found by forward model fitting this paper now supplements the earlier interpretation by seeking to clarify the ranges of acceptable models which fit the data that is to demonstrate the extent of the non uniqueness of the interpretation searches for models which fit the data have been carried out on both a systematic and a random basis with similar results the major conclusion of the earlier paper is confirmed that there is a substantial difference in conductivity structure between central and southeast australia beneath central australia the structure is consistent with a traditional continental geotherm and published laboratory measurements on the temperature dependence of the electrical conductivity of recognized upper mantle crystalline olivine materials beneath southeast australia a higher conductivity by an order of magnitude in the depth range 200 300 km is most directly interpreted in terms of a small degree perhaps 5 of basalt melt such a partial melt zone under southeast australia is consistent with previous natural electromagnetic measurements for the area has earlier and independently been indicated by a variety of seismic studies and correlates with proposed thermal models which involve crustal intrusion from some sub lithospheric magma source

crustal structure of the fennoscandian shield a traveltime interpretation of the long range fennolora seismic refraction profile abstract the 1979 fennoscandian long range project fennolora was aimed at the determination of the detailed structure in the earth s mantle down to a depth of about 400 km observation distances reached almost 2000 km within scandinavia between shotpoints off the north cape and the southern coast of sweden to achieve an unbiased regarding the upper mantle structure a careful crustal survey was carried out along the entire profile at the same time beneath the fennoscandian shield i e the central section of the profile the crust is characterized by quite a smooth increase in p wave velocity down to the moho which lies at a depth of about 50 km in the southern half of the shield and at about 45 km further north the mean crustal velocity is 6 6 6 7 km s at the base of the crust the velocity increases gradually from about 7 km s to 8 0 8 4 km s in a 5 10 km thick crust mantle transition zone both in the south and in the north the relatively homogeneous crust of the baltic shield borders on areas with a more differentiated velocity structure first order discontinuities at shallower depth characterize the crust mantle boundary near the southeastern tip of sweden 38 km and under the caledonides in the north 43 km

petrology geochemistry and origin of a major australian 1880 1840 ma felsic volcano plutonic suite a model for intracontinental felsic magma generation in australia a major period of felsic volcanism and granite emplacement occurred between 1880 and 1840 ma towards the end of the early proterozoic these volcanics and granites covering at least 37 000 km2 are compositionally uniform over large areas the felsic magmas are predominantly i type being derived from infracrustal sources which have never been exposed to surficial weathering processes when compared with early archaean and phanerozoic i type granites the 1880 1840 ma suite has distinctively high levels of k2o rb zr la ce th u and low mgo and cao contents even at relatively low sio2 levels of 60 65 the 1880 1840 ma rocks are characterized by low initial 87sr 86sr ratios and negative nd values chemical and isotopic modelling on these volcanics and granites suggest that they formed from mantle derived sources that are around 2300 2000 ma old there is no evidence of a major archaean component in these granites the sources are interpreted to have formed as a result of polygonal small scale mantle convection which led to extensive underplating of archaean crust during this underplating event large volumes of incompatible element enriched mantle derived material accreted to the base of the lower crust and fractionated significantly the more fractionated part of this source was remelted between 1880 1840 ma to produce the voluminous felsic volcanics and comagmatic granites present in nearly every proterozoic domain of australia the actual underplating event between 2300 2000 ma is believed to have triggered the formation of early proterozoic sedimentary basins which occur in a polygonal pattern and appear to wrap around archaean nuclei the composition of these felsic melts and the tectonic setting operating during their emplacement precludes genesis by contemporaneous subduction of oceanic crust all are generated by processes of vertical accretion in an intracontinental environment

a heat production model of the crust and upper mantle abstract experimental evidence on the relationship between seismic velocity and radiogenic heat production has been used to estimate the vertical distribution of heat sources in the continental lithosphere the derived differentiation rate of the radioelements with depth has been investigated to depths 100 km in various world wide tectonic provinces the results show that more pronounced upward differentiation steeper heat production distribution corresponds to regions of higher heat flow and usually higher crustal temperatures thus crustal distributions of radiogenic heat may be related to tectonic age and crustal evolution the vertical distribution of heat production can be traced even in the subcrustal lithosphere and confirms the upward concentration of a substantial part of the radioelements into the crust at the expense of the upper mantle reflecting the early stages of crustal separation while in the uppermost crust the distribution of the heat sources seems to be controlled by uranium and thorium in the upper mantle it is potassium which controls it data obtained may be important in the geothermal modelling deep temperature calculation and in determinations of moho heat flow

deep reflection experiments in northeastern australia 1976 1978 between 1976 and 1978 the australian bureau of mineral resources bmr recorded deep crustal reflection data at seven sites in northeastern australia over continuous profiles up to 15 km long by simply extending the recording time to 16 sec during normal sedimentary basin surveys the record sections show many events with variable strength continuity dip and spatial distribution by comparing the sections from the longer and the shorter perpendicular traverses it is possible to discriminate between primary reflections and diffractions multiples and other noise events based on their character the reflections can be grouped into zones which are interpreted in terms of the nature and structure of the crust most of the reflection sections show below the sedimentary reflections a thin 2 3 sec reflection free zone underlain by a thick 9 sec or more zone of numerous reflection segments which varies in thickness and the distribution of reflection segments the data thus suggest that the upper crust

aftershocks of the 1988 january 22 tennant creek australia intraplate earthquakes evidence for a complex thrust fault geometry summary on 1988 january 22 three earthquakes of ms 6 3 6 7 occurred in a 12 hr period near tennant creek northern territory in the proterozoic north australian craton and produced 32 km of surface rupture on two main scarps the lake surprise scarp is shaped like a boomerang with the east arm trending ese and the west arm wsw the kunayungku scarp lies 7 km to the northwest and trends ese locations of 211 aftershocks determined using data from portable seismograph arrays define a 10 km by 40 km aftershock zone elongated parallel to the trend of the surface ruptures in the western and eastern part of the fault zone aftershocks occur only south of the kunayungku and eastern lake surprise scarps respectively in the central section in contrast aftershocks lie primarily to the north of the western arm of the lake surprise scarp focal depths range from the surface to 8km in all sections the shallowest earthquakes lie closest to the scarps whereas deeper events lie further away in cross section the aftershocks lie in three distinct inclined zones which delineate fault surfaces ruptured by the main shocks aftershocks are concentrated near the edges of the inferred fault surfaces suggesting that stress was relieved in the centre of the faults by rupture during the main shocks the inferred fault plane dips are 45 to the ssw for the kunayungku fault 55 to the nnw for the western lake surprise fault and 35 to the ssw for the eastern lake surprise fault these dip directions are consistent with the sense of thrust movement inferred from surface deformation the reversal of fault dip and sense of movement between the central and end segments requires a complicated fault geometry the aftershock data would allow a second south dipping plane in the central section that does not intersect the surface this blind fault may be responsible for much of the moment release interpreted from the modelling of broad band teleseismic waveforms choy bowman 1989 rupture of the north dipping plane may have been controlled by a pre existing fault marked at the surface by a quartz ridge 5 km long and may be of secondary importance although the quartz ridge suggests previous faulting the absence of significant topographic relief together with estimates of erosion rates suggest an average repeat time in excess of 104yr preliminary interpretation of water withdrawal histories and water level data from a borefield adjacent to the eastern lake surprise scarp does not support a causal relationship between the borefield and the occurrence of the earthquakes the nearby warramunga wra seismic array provides a record of local earthquake activity from 1965 to the present the fault area was essentially aseismic until two small earthquakes in 1986 february and a series of six magnitude 4 5 earthquakes in 1987 january which were followed by more than 1000 aftershocks the frequency of occurrence of aftershocks declined very slowly in 1987 with p 0 2 in the modified omori formula whereas for the 1988 aftershock sequence p 1 the low p during 1987 may have been diagnostic of high stress in the source and water authority provided partial financial support for j r b and g g

observations of the phase sp at distances less than 40 summary a detailed analysis of short period shear waves using an array of three component seismometers near tennant creek in the centre of northern australia has found only isolated samples of the phase sp which for a mode conversion at the crust mantle boundary precedes the s arrival by about 6 s two strong phases commonly observed in the s wavetrain in the distance range 30 40 with a time separation of between 6 and 8 s have been determined to be the phases s and sp respectively contrary to the jeffreys bullen seismological tables sp is first generated at a minimum distance not of 40 but rather in the vicinity of 20

lg coda q in australia and its relation to crustal structure and evolution we have determined lg coda q lg c from ground motion recorded at seven broadband stations in australia using a stacked spectral ratio method in spite of the relatively small number of events and less than optimum station coverage we were able to use those data to obtain a tomographic map q lg c and its frequency dependence at 1 hz for almost the entire island continent