the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

crustal structure of the labrador sea conjugate margin and implications for the formation of nonvolcanic continental margins wide angle seismic studies have determined the detailed velocity structure along a 350 km long profile across the labrador margin combination of this model with a previously published cross section for the southwestern greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure the results indicate three distinct zones across the labrador margin similar to the structure of the conjugate greenland margin zone 1 represents 27 to 30 km thick continental crust thinning gradually seaward over 100 km distance farther seaward zone 2 is 70 80 km wide characterized by a distinct lower crust 4 5 km thick in which velocity increases with depth from 6 4 to 7 7 km s interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic under plating zone 3 represents two layered normal oceanic crust the cross sections from both margins are reconstructed to an early drift stage at chron 27 this demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of chrons 31 and 33 on both margins zone 1 shows a marked asymmetry with a gradual thinning of continental crust off labrador contrasted with a rapid thinning off greenland the abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt

tomographic images of the iberian subcrustal lithosphere and asthenosphere the s wave velocity structure of the lithopheric mantle and asthenosphere beneath iberia is displayed by means of tomographic images obtained from dispersion data of rayleigh waves propagating across the iberian region we have used long period data recorded at the broadband stations of the network of autonomously recording seismographs nars installed in the iberian peninsula on the occasion of the iberian lithosphere heterogeneity and anisotropy iliha project a total of 64 teleseismic events provided by the iliha array and 143 seismic paths have been studied surface wave dispersion analysis is carried out using various methods from methods for a correct acquisition of data and subsequent two station surface wave velocity measurements to inversion methods for velocity structure and methods for verifying the reliability of the inversion results the phase and group velocity dispersion curves of the fundamental mode rayleigh waves are the basic information from which we have obtained several earth models represented by shear wave velocity distributions with depth using these inversion results we display the most conspicuous features of the velocity stnicture of the iberian lithosphere asthenosphere system and propose a new regional model the iberian lithosphere asthenosphere ila model for the deep structure of the iberian peninsula down to a depth of 200 km we use a representation technique based on an iterative laplacian interpolation method for obtaining tomographic images of the subcrustal lithosphere and asthenosphere of iberia we find significant lateral velocity variations in the peninsula though these differences vary with depth interval a low velocity channel in the lithosphere 41 51 km appears with nonuniform velocity structure in contrast at the greatest lithospheric depths 51 81 km almost the entire peninsula shows a rather uniform velocity structure the asthenosphere 81 181 km is clearly a nonhomogeneous gross layer both laterally and with depth the relatively higher velocities span the shallower depths within the asthenosphere whereas the lower ones span the deeper part

groundwater flow conditions in a low angle fracture zone at finnsjon sweden abstract the performance and results of a series of hydraulic head measurements and differently designed tracer tests are described a combined interpretation of the results is utilized to estimate the natural groundwater flow conditions in a low angle major fracture zone zone 2 investigated at the finnsjon study site central eastern sweden the transport of tracer labelled flushing water used during drilling of a cored borehole was utilized to determine hydraulic connections and estimations of hydraulic parameters the breakthrough of tracer was monitored at a distance of 440 m from the injection point with a mean residence time of 37 days in order to determine transport parameters of the fracture zone a tracer test was performed in conjunction with an interference test the hydraulic fracture conductivity flow porosity equivalent fracture width and dispersivity were calculated for three different tracer routes tracer was also used to detect any bypass around the packers during the interference test a series of groundwater flow determinations are also described which used the point dilution technique in packed off borehole sections carried out under natural gradient conditions the groundwater head measurements indicated that a pressure gradient from high to low exists between the adjacent rocks and zone 2 where zone 2 is located closer to the ground surface the opposite situation prevails the upper highly conductive part of the zone was found to have high natural flow rates whereas in the lower highly conductive part the groundwater flow rates were very low close to the rate of molecular diffusion

hydrochemical investigations at finnsjon sweden abstract ground water collected from within and around a highly conductive low angle fracture zone in crystaline rock has been studied in relation to drilling and sampling methods water quality and hydrogeochemical interpretation air flush percussion drilling was demonstrated to be efficient down to 200 250 m at greater depths large amounts of drilling debris and increasing groundwater contamination from air dissolution become apparent a stepwise drilling sampling procedure using water flush rotary drilling in combination with water analysis carried out on site using a mobile laboratory is recommended for future studies the chemistry of the finnsjon ground waters shows a sharp contact between saline and non saline compositions this contact corresponding to a highly conductive low angle fracture zone zone 2 this zone is therefore a horizon along which ground waters of contrasting age and chemistry come into contact and partially mix this mixing has produced a highly supersaturated water resulting in the precipitation of calcite this is most evident in fractures within the more conductive upper part of zone 2 thus partly forming a seal between the two groundwater environments comparison of the finnsjon saline waters with other saline environments particularly in the fennoscandian shield has shown that the ground waters are dominantly marine in origin but with clear modifications resulting from water rock interaction apparent radiocarbon ages range from 9000 to 15000 years in those waters containing tritum levels below detection limits these ages are considered too young and are thought to have resulted from mainly carbon 14 dilution caused by groundwater mixing during the holocene when large quantities of glacial melt water were available this mixing process is also supported by the oxygen 18 data

aspo se sweden a natural groundwater flow model derived from hydrogeochemical observations abstract hydrogeochemical studies form an essential part of current geoscientific investigations carried out in conjunction with excavations of an underground experimental facility as part of the swedish programme to dispose of high level spent nuclear fuel in crystalline bedrock a large part of the hydrogeochemistry has been devoted to the detailed investigation of each individual zone hydraulically selected tested and sampled for hydrogeochemical characterisation the data have been used to describe the chemistry and origin of the aspo groundwaters models have been developed to illustrate groundwater mixing and equilibrium modelling approaches have been employed to understand water rock interaction processes hydrogeochemical observations have been integrated with known geological and hydrogeological parameters to construct a groundwater flow model for the island

late cretaceous extensional unroofing in the funeral mountains metamorphic core complex california new field and geochronologic data document the existence of late cretaceous extensional structures in the death valley region california nevada we have mapped two major low angle ductile shear zones that omit stratigraphy in the footwall of the funeral mountains metamorphic core complex intervening strata have been strongly attenuated although stratigraphic offset across the shear zones is only 1 5 km the presence of a large metamorphic discontinuity suggests that the amount of unroofing must be much greater the timing of shear zone formation attenuation and subsequent northwest vergent folding is constrained by u pb geochronology on 1 prekinematic or synkinematic and 2 postkinematic pegmatites deformation was taking place by 72 ma and had ended by 70 ma these results support earlier petrologic and geochronologic data that suggested substantial unroofing of the funeral mountains in late cretaceous time and add to a growing body of evidence for widespread mesozoic extension in the hinterland of the sevier thrust belt

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

ocean continent boundary in the iberia abyssal plain from multichannel seismic data abstract the ocean continent boundary of the west iberia margin is marked by a basement ridge trending n s four segments of this ridge are recognized each of them being progressively offset westward from 40 n to 43 n because the setting and seismic character of the ridge in the iberia abyssal plain are similar to those of the galicia margin ridge which is made of serpentinized peridotite we think that the southern segments of the ridge are also made of the same mantle material the segmentation of the ridge suggests that the northward propagation of the continental break up during the north atlantic opening in early cretaceous times was discontinuous each segment possibly corresponding to a propagation step east of the ridge the basement of the whole iberia abyssal plain consists of highly thinned continental crust locally resting on a seismic reflector that resembles the s reflector previously recognized off galicia by analogy with the galicia margin we propose that the tilted crustal blocks lay on serpentinized peridotite derived from the upper mantle the s reflector corresponding to the contact between crustal rocks and serpentinite

the ocean continent boundary off the western continental margin of iberia crustal structure west of galicia bank a seismic refraction transect across the galicia bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin parallel peridotite ridge pr immediately west of the pr oceanic crust is only 2 5 3 5 km thick but farther west oceanward it thickens to 7 km the pr caps a similar to 60 km wide lens shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust when superimposed on a reflection time version of the velocity model the s reflector is clearly intracrustal at its east end westward s cuts down to lower crustal levels eventually coinciding with the top of the serpentinized peridotite lens original crust mantle boundary these observations render almost impossible the seafloor exposure of the pr by s acting as a top to the west detachment fault numerical models of melting and borehole subsidence information constrain our rifting model the easternmost continental crust experienced a total stretching factor of 4 3 most likely in two stages it probably occurred over similar to 25 m y with the highest fate of stretching at the beginning of the main earlier rift phase valanginian 141 135 ma the 3 4 7 km thick continental crust depending on whether serpentinized peridotite is assigned to crust or mantle which may include melt products requires stretching factors 6 more than 11 7 and a rift duration of more than 25 13 m y the thin oceanic crust immediately west of the pr is explained by conductive cooling of the mantle during the long prebreakup stretching phase which temporarily caused reduced melting immediately after breakup

saline minewaters of the keweenaw peninsula northern michigan their nature origin and relation to similar deep waters in precambrian crystalline rocks of the canadian shield

cooling history of granulite samples from the ocean continent transition of the galicia margin implications for rifting abstract zircon and apatite fission track ages were obtained on twogranulite samples that were recovered from the sea floor in theocean continent transition area of the galicia margin northatlantic using the french submersible nautile zircon agesindicate that the rocks cooled through about 2508cincarboniferous to early permian time 307 42 ma and287 35 ma hence the granulites do not represent the preriftlower crust but were in an upper crustal position long beforeriftingstarted apatitesyieldedearlycretaceousages 126 6 7myr and 129 13 4 myr indicating cooling through 90 308ccoeval with the main rifting phase that preceded continentalbreakup we assume that the granulite samples originate from atectonic breccia cropping out near one of the sample locations this breccia formed along a synrift detachment accommodatingcontinentalbreakupandfinalexhumationofthegaliciamargin speridotite ridge terra nova 10 96 100 1998 ahedbhedcheddhedref markerfig markertable markerref end ref start correspondence tel fax 41 61 2673610 3613 e mail fuegenschuh ubaclu unibas ch

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect we present a wide angle seismic refraction study of an 80 40 km region of the southern iberia abyssal plain south of galicia bank an intersecting grid of two e w and four n s wide angle reflection refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean continent transition oct these structures can be systematically linked to variations in acoustic basement morphology and to results from ocean drilling program odp boreholes lateral changes in the velocity structure of the basement occur abruptly over distances of 20 km where complex variations may be found thinned upper continental crust 2 5 km thick with velocities of 5 0 6 6 km s is limited to a series of n s fault blocks immediately south of galicia bank this crust is underlain by a high velocity layer 7 3 7 9 km s of weakly serpentinized i e 0 25 peridotite which exists throughout the eastern part of the survey area basement within the oct appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally in the southeast sector of our survey where basement topography deepens and becomes subdued continental fault blocks are absent instead basement contains an upper layer of more pervasively serpentinized i e 25 45 peridotite that is 2 km thick this layer is characterized by low velocity at the top of basement 4 2 km s that increases rapidly with depth and it probably corresponds to a seismically unreflective layer previously identified in reflection profiles to the south of our survey in the western section of our survey beneath a series of elevated basement ridges velocities are reduced within both the upper basement layer 3 5 6 0 km s and lower layer 6 4 7 5 km s these changes suggest that both upper and lower layers have become more highly serpentinized with values of 60 100 in the upper layer and 25 45 in the lower layer probably during the last stages of rifting and immediately before formation of oceanic crust a normal or slow spreading oceanic crustal structure is not found within the survey region thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at odp site 897 and east of the j magnetic anomaly

geochemistry and origin of saline groundwaters in the fennoscandian shield chemical and isotopic analyses of water from drill holes and mines throughout the fennoscandian shield show that distinct layers of groundwater are present an upper layer of fresh groundwater is underlain by several sharply differentiated saline layers which may differ in salinity relative abundance of solutes and o h sr and s isotope signature saline groundwater can be classified into four major groups based on geochemistry and presumed origin brackish and saline waters from 50 200 m depth in coastal areas around the baltic sea exhibit distinct marine chemical and isotopic fingerprints modified by reactions with host rocks these waters represent relict holocene seawater inland three types of saline groundwater are observed an uppermost layer of brackish and saline water from 300 900 m depth saline water and brines from 1000 2000 m depth and superdeep brines which have been observed to a depth of at least 11 km in the drill hole on the kola peninsula u s s r electrical and seismic studies in shield areas suggest that such brines are commonly present at even greater depths the salinity of all inland groundwaters is attributed predominantly to water rock interaction the main solutes are cl ca na and mg in varying proportions depending on the host rock lithology the abundance of dissolved gases increases with depth but varies from site to site the main gas components are n2 ch4 up to 87 vol and locally h2 the 13c value for methane is highly variable 25 to 46 and it is suggested that hydrothermal or metamorphic gases trapped within the surrounding rocks are the most obvious source of ch4 the uppermost saline water has meteoric oxygen hydrogen isotopic compositions whereas values from deeper water plot above the meteoric water line indicating considerably longer mean residence time and effective low temperature equilibration with host rocks geochemical and isotopic results from some localities demonstrate that the upper saline water cannot have been formed through simple mixing between fresh water and deep brines but rather is of independent origin the source of water itself has not been satisfactorily verified although superdeep brines at least may contain a significant proportion of relict precambrian hydrothermal or metamorphic fluids

deep structure in the vicinity of the ocean continent transition zone under the southern iberia abyssal plain we present results from a seismic and magnetic study of acoustic basement beneath the southern iberia abyssal plain an ocean continent transition zone oct characterized by subdued basement relief and weak magnetization lies seaward of thinned continental crust its western edge is a region with isochron parallel ridges and generally higher basement magnetizations the width of the oct decreases to the north the seaward change in basement morphology coincides with changes in seismic velocity structure but in both regions anomalously high velocities at shallow depths within acoustic basement suggest that the oct consists largely of serpentinized peridotite our magnetic and seismic data support the hypothesis of exhumed upper mantle more than that of ultraslow sea floor spreading for the origin of the oct

geological setting and deformation history of a low angle fracture zone at finnsjon sweden abstract in 1984 the swedish nuclear fuel and waste management company skb initiated detailed investigations involving the geological geophysical hydrological and geochemical character of a fracture zone for this purpose a low angle ssw dipping fracture zone in a foliated granodiorite of svecokarelian age approximately 1 8 ga within the sub cambrian peneplain of northeastern uppland central eastern sweden was identified and investigated by extensive drilling the initial formation of this zone which is estimated to have taken place approximately 1 7 ga ago as a result of ductile deformation was followed by shearing during ductile brittle transitional conditions and subsequently by brittle deformation it formed as a thrust in a 20 30 km wnw ese trending strike slip fault zone the fracture zone is approximately 100m wide with an anastomosing shear pattern and is displaced by subvertical faults late reactivation of the zone has occurred preferentially in the upper part which is now open permitting water transport the palaeo flow paths indicated by bedrock alteration and fracture infillings show that the hydraulic flow system in the rock mass has become increasingly restricted during geologic time flow now occurs predominantly along discrete zones which were initiated more than 1 7 ga ago and since then have been repeatedly reactivated the most intense penetrative fracturing associated with hydrothermal activity occurred more than 1 0 ga ago fractures and fracture zones most likely to conduct water i e forming transport pathways between a repository and the biosphere tend to be those with a long and complex tectonic history

saline groundwaters in the canadian shield a first overview abstract the chemical and isotopic compositions of saline groundwaters and brines in the canadian shield are described the occurrence of such waters is very widespread not related to any specific rock type or mineralization and generally can be expected at depths below 1 km the most saline brines have salinities exceeding 200 g l 1 dissolved solids and their chemistries tend to suggest that in the past saline waters penetrated the canadian shield the original chemistry of these solutions was subsequently modified through strong rock water interactions and it appears that their final chemical compositions are independent of the original chemistries stable isotope analyses indicate that these waters are not simple evaporated seawater sedimentary basin brines hydrothermal solutions or residual ore forming fluids although any of these could be a precursor of the brines secondary isotope exchange reactions and the hydration of primary silicates accompanied by the formation of clay minerals could have modified not only the primary isotopic composition but may have also profoundly influenced the chemistries of these brines to be effective these processes would have had to occur in closed systems

crustal structure of the goban spur rifted continental margin ne atlantic summary we present results that constrain the crustal structure across a typical non volcanic rifted continental margin the goban spur in the ne atlantic traveltime and amplitude modelling of wide angle ocean bottom seismograph profiles shows that the continental crust thins in a seaward direction to about 7 km before it breaks to allow a new oceanic spereading centre to develop the crustal velocity structure the stretching factors and the density distribution inferred from gravity anomalies are consistent with bulk pure shear stretching for the development of the rifted continental margin the seismic velocity structure of the thinned continental crust suggests that thinning was accompanied by only limited igneous intrusion and extrusion in the upper crust the adjacent oceanic crust is abnormally thin 5 4km this is explained most readily by accretion at a very slow spreading centre approx 11 mm yr full rate although additional cooling by conductive heat loss from the parent mantle as it rose slowly beneath the stretched and thinned continental lithosphere together with minor igneous intrusion into and surface flows onto the adjacent continental crust may also have reduced the oceanic igneous thickness immediately adjacent to the continent ocean transition

extension of the exmouth plateau offshore northwestern australia deep seismic reflection refraction evidence for simple and pure shear mechanisms the response to extensional stresses that deformed and finally ruptured the northwest australian passive margin was achieved by an interplay of simple and pure shear mechanisms the results of a two ship seismic reflection and refraction experiment show that the outer exmouth plateau is dominated by large rotated blocks bounded by deeply penetrating normal faults some of which are heavily intruded magmatic underplating attended this extension but is absent in the central region where a set of prominent subhorizontal mid crustal arrivals are interpreted to be detachment surfaces extensive exploration seismic coverage drilling and sampling suggest that deformation on the outer plateau postdates that in the central plateau we propose that initial deformation of the region was dominated by a simple shear detachment system along low angle fault planes this passed laterally and temporally into a region of pure shear deformation characterized by high angle normal faults and magmatism and this became the locus of final rupture

seismic reflection evidence for seismogenic low angle faulting in southeastern arizona focal mechanisms for large m 6 earthquakes in extensional terranes suggest that seismogenic normal faults have dips that range from approximately 30 degree to approximately 70 degree geologic relations suggest that low angle faults have accommodated large scale upper crustal extension these disparate observations are often reconciled by arguments that low angle faults move aseismically or rotate to low angles from initially high angles seismic reflection data from the tucson basin in southeast arizona image a low angle normal fault the santa rita fault that crops out along the trend of late quaternary fault scarps caused by large magnitude m approximately 6 7 7 6 earthquakes velocity independent dip analysis from shot records of the santa rita fault indicates that it has a true dip of approximately 20 degree to a depth of at least 6km this observation suggests that low angle extensional faults indeed may be seismogenic and that actual mechanisms for accommodation of upper crustal extension depend on local conditions of stress and preexisting geologic structure

plate reconstruction of the bay of biscay a plate reconstruction of the bay of biscay to a predrift stage is proposed on the basis of 1 an aptian albian tectonic pattern of the basque cantabrian region in northern spain which suggests major left lateral strike slip movements and 2 a continental margin that matches predrift stages plate reconstruction results from the juxtaposition of the two continental margins of the bay of biscay so that the eastern north spanish margin lies laterally adjacent to the southwest french margin landes plateau and the western north spanish margin borders the eastern part of the meriadzek terrace in the armorican margin the restoration obtained is supported by both the implied amount of clockwise rotation of iberia 37 very close to the 35 suggested by paleomagnetism and the positive matching of north south oriented transform structures from the south biscay margin with northeast southwest oriented transform structures from the north biscay margin

refining the laurentide ice sheet at marine isotope stage 3 a data based approach combining glacial isostatic simulations with a dynamic ice model abstract while glacial geologic records track the retreat history of the laurentide ice sheet constraints on the ice sheet build up history are more elusive as the laurentide ice sheet expanded to its maximum extent at the last glacial maximum 26 ka it erased most evidence of previous glaciations however an increasing number of dated non glacial deposits point to a relatively confined laurentide ice sheet during marine isotope stage 3 mis 3 60 26 ka suggesting rapid ice growth rates leading to the last glacial maximum in this study we demonstrate that predictions of relative sea level change due to glacial isostatic adjustment involving a late and rapid growth of the laurentide ice sheet are consistent with sea level bounds associated non glacial deposits of mis 3 age in canada we also run a simple dynamic ice model that adopts the paleotopography generated from the glacial isostatic adjustment simulation and show that using accumulation rates similar to that of the present day arctic rapid glaciation rates starting at mid mis 3 44 ka are predicted

basaltic volcanism mantle plumes and the mechanics of rifting the parana flood basalt province of south america dynamic modeling of continental extension between south america and africa shows that the mechanics of rifting played an important role in determining the pattern of volcanism within the parana and etendeka flood basalt provinces on the brazilian and namibian margins the key feature of the model is the development of a horizontal pressure gradient in the lower crust during the early stages of extension which provided a mechanism for transporting magma generated beneath the incipient sea floor spreading axis into the parana province 100 200 km distant the horizontal pressure gradient developed as a consequence of the dynamic interaction of preexisting weaknesses in the middle crust and upper mantle during rifting the model accounts for the large quantity of basalt the asymmetric distribution of basalt on the conjugate margins and the northward migration of the eruptive center with time the rapidity of magma genesis is in agreement with models of decompression melting during rifting the model indicates that although elevated asthenosphere temperatures associated with the tristan plume account for the volume of melt generated the mechanics of rifting control the location and style of emplacement the model suggests that extension in the region began ca 150 155 ma and lasted about 25 m y

overview of the fracture zone project at finnsjon sweden abstract at the finnsjon site central sweden a 100 m wide low angle fracture zone in granodiorite zone 2 has been studied by means of borehole investigations this has included core and geophysical logging as well as hydraulic tests and groundwater sampling zone 2 was developed at 1 7 1 6 ga as a ductile shear zone at a depth of approximately 10 15 km and repeated reactivation has occurred during precambrian time and later today the upper boundary of the zone is defined in nine boreholes at depths ranging from 100 to 295 m pumping tests have demonstrated that the upper part of the zone is highly water conductive with a transmissivity in excess of 10 3 m 2 s 1 from tracer tests the equivalent single fracture hydraulic conductivity was calculated to be 0 03 0 2 ms 1 this extremely high transmissivity and conductivity over such a large areal extent can be speculated to be the result of recent opening of zone 2 during the last inland ice period in situ measurements of the natural groundwater flow show that the flow is high 67 90 m 3 m 2 year 1 at the upper boundary of zone 2 whereas near stagnant conditions occur at the lower boundary and in the relict saline water beneath the zone isotope data suggest a complex origin for the saline water the hydraulic head measurements as well as the groundwater chemistry strongly suggest that the low angle zone 2 acts as a hydraulic barrier preventing ground water from percolating downward through the zone these studies have helped to focus on repository safety assessment problems which hitherto had been oversimplified for the benefit of performance models furthermore the results have shown that these low angle large scale fracture zones are likely to be even more important controls than steep zones when considering bedrock stability rock stress regimes and lateral groundwater flow patterns quantification of such phenomena should provide an invaluable input into the various safety assessment performance codes for model validation and siting criteria for a repository

palynological evidence for recycling of upper devonian into lower cretaceous of the moose river basin james bay lowland ontario cored drill hole on a high electromagnetic conductivity anomaly near the onakawana lignite field penetrated 50 3 m of pleistocene cretaceous and devonian strata spore pollen floras from the cretaceous interval indicate an early middle albian age and a correlation with the lower part of the mattagami formation spores acritarchs and tasmanitids from the devonian interval indicate that the upper part of the long rapids formation is marine and is frasnian famennian in age multiple recycling events are documented between the devonian and cretaceous and between the cretaceous and pleistocene provenances being local near the margins of the basin

a transect across australia s southern margin in the otway basin region crustal architecture and the nature of rifting from wide angle seismic profiling abstract the otway basin in southeastern australia formed on a triangular shaped area of extended continental lithosphere during two extensional episodes in cretaceous to miocene times which ultimately led to the separation of australia and antarctica the velocity structure and crustal architecture of the otway continental margin has been interpreted from offshore onshore wide angle seismic profiling data along a transect extending from near the northern otway basin margin with palaeozoic outcrop to the deep ocean basin under the southern ocean along this transect the otway continental margin ocm transect the onshore half graben geometry of early cretaceous deposition gives way to a 5 km thick basin sequence p wave velocity 2 2 4 6 km s extending down the continental slope offshore to at least 60 km from the shoreline at 120 km from the nearest shore sonobuoy data indicate a 4 5 km sedimentary sequence overlying 7 km of crustal basement rocks above the moho at 15 km depth water depth 4220 m conspicuous strong moho reflections are evident under the continental slope at about 10 2 s twt basement is interpreted to be attenuated faulted palaeozoic rocks of the delamerian and lachlan orogens intruded with jurassic volcanics that thin from 16 km onshore to about 3 5 km at 120 km from the nearest shore these rocks comprise a 3 km section that has a velocity of 5 5 5 7 km s overlying deeper basement with a velocity of 6 15 6 35 km s over the same distance the moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore and then to about 12 km in the deep ocean at the limits of the profile water depth 5200 m the continent ocean boundary cob is interpreted to be at a prominent topographic inflection point at the bottom of the continental slope in 4800 m of water p wave velocities in the lower crust are 6 4 6 8 km s above a transition to the moho with an upper mantle velocity of 8 05 km s there is no evidence of massive high velocity 7 km s intrusives underplate material in the lower crust nor any syn rift or early post rift subaerial volcanics indicating that the otway continental margin can be considered a non volcanic margin similar in many respects to some parts of the atlantic ocean margins e g the nova scotia newfoundland margin off canada and the galicia bank off the iberian peninsula

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

hydraulic testing and modelling of a low angle fracture zone at finnsjon sweden abstract the hydraulic characteristics of a gently dipping fracture zone in crystalline rock at the finnsjon test site in sweden are described the information is derived from single hole injection tests of different packer spacings including very detailed tests in 0 11 m sections in a few borehole segments in addition detailed hydraulic interference tests were performed by pumping in different parts of the zone and monitoring pressure changes in multiple section observation boreholes the results of the hydraulic testing show that the fracture zone is composed of two to five thin highly conductive subzones in the boreholes the uppermost subzone is consistently located close to the upper boundary of the major zone and can be correlated over several hundreds of metres there is evidence of both lateral and vertical hydraulic anisotropy along the zone the interference tests show that the fracture zone is delimited by semi permeable boundaries probably constituted by steeply dipping fracture zones the results of the hydraulic testing are used to simulate the pressure responses obtained in the observation boreholes by numerical modelling

dimensions of the late cretaceous paleocene northeast atlantic rift derived from cenozoic subsidence abstract the distribution of cenozoic subsidence across northeast atlantic volcanic margins have been evaluated to define the width of the rift zone and magnitude of extensional deformation the subsidence profiles are corrected for the effects of lower crustal magmatic bodies emplaced during continental break up the dimensions of the bodies have been derived from the crustal velocity structure the width of the late cretaceous paleocene northeast atlantic rift zone was more than 300 km and the lithospheric extension factor increases gradually towards the line of continental separation a large number of high quality seismic reflection data tied to scientific and commercial wells reveals that the initiation of extensional deformation preceded continental separation by 18 m y on the voring margin off norway these results show that the northeast atlantic volcanic margins commonly considered as typical volcanic margins indeed have similar dimensions as non volcanic margins and as continental rifts thus these margins contrast significantly with previously suggested evolutionary models based on narrow rift zones and formation during rapid lithospheric failure the wide rift is compatible with volume of igneous rocks observed along these margins and with a thermal anomaly similar to that associated with production of northeast atlantic oceanic lithosphere

large scale intrusion of shallow water into a vertical fracture zone in crystalline bedrock initial hydrochemical perturbation during tunnel construction at the aspo hard rock laboratory southeastern sweden on march 13 1991 construction of the entrance tunnel to the aspo hard rock laboratory opened a vertical fracture zone at a depth of 70 m this provides an opportunity to study geochemical changes resulting from shallow water inflow into a crystalline bedrock aquifer as anticipated during construction and operation of a deep repository for spent nuclear fuel chloride ion is a natural conservative tracer for mixing between the dilute cl 10 mg l 1 shallow groundwater and the saline cl 5000 mg l 1 native groundwater of the fracture zone a sharp dilution front corresponding to 80 dilution of the native groundwater indicated arrival of shallow groundwater in the entrance tunnel after 3 weeks in spite of this large input of shallow water the fracture zone remains predominantly anoxic major element hydrochemistry and carbon and oxygen stable isotope data indicate large inputs of alkalinity and biogenic co2 g input of organic carbon with shallow groundwater provides a possible energy and carbon source for anaerobic respiration there is no evidence for sulfate reduction and fe iii oxyhydroxide fracture minerals are the only other dominant electron acceptor observed

gabbro and related rock emplacement beneath rifting continental crust upb geochronological and geochemical constraints for the galicia passive margin spain the thinned continental crust of the west galicia margin is bound by a belt of serpentinized peridotites peridotite ridge lying about 300 km off the coast in the north atlantic ocean from this ridge a gabbro and a chlorite rock were studied in an attempt to substantiate rift related subcontinental magmatism occurring prior to sea floor spreading u pb dating of 13 different zircon fractions yields a precise age of 122 1 0 3 ma 2 for the emplacement of the chlorite rock protolith from which more than 50 of si and alkali calc alkali elements were lost during greenschist facies tectonometamorphism sr and nd isotope signatures suggest that the gabbro and chlorite rock protoliths were derived from mantle sources that were moderately depleted in lile relative to a chondritic reservoir no evidence for the presence of continental material in the magma source regions can be observed from the new zircon age of 122 1 0 3 ma and earlier determined39ar40ar age of 122 0 0 6 ma for amphibole from the same locality it can be documented that magma formation solidification and unroofing of the mantle rocks occurred during a short period of time of about 3 4 ma which means that the peridotite ridge detached from the continent and rose to the surface immediately after or even coevally with mantle melting

isotopic evidence for lithospheric thinning during extension southeastern great basin mafic rocks erupted during late cenozoic extension near las vegas nevada show temporal patterns of variation in nd and sr isotope ratios and in bulk chemistry the patterns indicate that depths of magma generation were varying through time and that lithospheric mantle was partially replaced with asthenospheric mantle as extension proceeded in alkalic rocks epsilon sub nd changed through time from minus 9 1 typical of lithospheric mantle in this area before the onset of major 16 ma extension to 6 4 typical of asthenospheric mantle after extension 4 6 ma near the end of the period of major extension 10 6 ma tholeiitic rocks erupted the epsilon sub nd of which ranged from minus 10 1 to minus 7 9 this indicates that the lithosphere had not thinned sufficiently by that time to bring asthenospheric mantle into the depth range of tholeiitic magma generation 33 50 km the lithosphere in the las vegas area appears to have thinned by about 50 less than would be predicted by the magnitude of upper crustal extension a factor of 3 or 4 and indicative of a nonuniform lithospheric response to extension

palynological fluorescence in hinterland reconstruction of a cyclic shallowing up sequence pliocene papua new guinea abstract variations in fluorescence characteristics of contemporaneous and reworked palynological particles are documented through a cyclic shallowing upward sequence of lower pliocene mudstones of the orubadi formation in the puri anticline of the papuan foreland basin papua new guinea fluorescence of palynological particles may be affected by age oxidation and thermal maturation and therefore provides information about tectonic and palaeogeographical events in the source area of the particles the new guinea fold and thrust belt palynological particles have been identified with different fluorescence properties and sources 1 rocks at the southern proximal edge of the fold and thrust belt not greatly affected by tectonic events 2 rocks at a more distant location within the fold and thrust belt affected by tectonic controlled changes in gradient orographic rainfall and fluvial transportation and 3 contemporaneous vegetation affected by tectonic controlled changes in altitude and fluvial transportation variation in the fluorescence of palynological particles from these sources allows interpretations to be made about timing of thrusting events changes in the weathering and transportation regime and the variations in accommodation space in the foreland basin

an axial view of a metamorphic core complex crustal structure of the whipple and chemehuevi mountains southeastern california a 135 km long nw se trending seismic refraction wide angle reflection profile provides a unique along strike view of the crustal structure of a belt of metamorphic core complexes in southeastern california the whipple chemehuevi and sacramento mountains metamorphic core complexes interpretation of the seismic data was done by two dimensional forward modeling of travel times and amplitudes the final model consists of 1 a thin 1 5 km veneer of upper plate and fractured lower plate rocks velocities of 1 5 5 3 km s 1 overlying a fairly homogeneous basement with velocities of 6 0 km s 1 2 a localized high velocity 6 4 km s 1 body situated directly beneath the whipple mountains 3 a 6 3 6 4 km s 1 middle crust that is thickest beneath the core complexes 4 a 6 65 0 15 km s 1 lower crust 5 crustal thickness of 27 km with a deeper crustal root 3 km beneath the whipple mountains metamorphic core complex and 6 a pn velocity of 8 0 0 10 km s 1 the crustal structure that underlies the belt of metamorphic core complexes provides new insights into the processes that control extension in the deep crust upper crustal velocities are higher beneath the whipple mountains where velocities increase to 6 4 km s 1 at 5 km depth than beneath the chemehuevi and sacramento mountains in addition midcrustal discontinuities rise 2 5 km beneath the whipple complex compared to the other complexes these observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the whipple mountains compared to rocks in the chemehuevi and sacramento mountains despite the enhanced uplift and extension in the whipple mountains the crust is thicker here 30 km than anywhere else along the colorado river extensional corridor this may be in part a relic of compressional and magmatic thickening during the mesozoic however we suggest that inflation of the crust during tertiary extension was the dominant mechanism both mantle derived magmatism and lateral ductile inflow in the crust are proposed

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

effects of gas lift pumping on borehole hydraulic conditions at finnsjon sweden abstract the recorded volumes of flushing water used during the drilling of cored boreholes and the amounts recovered at the surface have been used to estimate the potential quantities of flushing water and drilling debris injected into fractures the results show that significant amounts of flushing water and drilling debris may be injected into fractures during drilling for cored boreholes penetrating major fracture zones only about 7 8 of the total volume of flushing water used was recovered at the surface a total loss of flushing water occurred in all boreholes during penetration of these fracture zones the recovered amounts of drilling debris were also low higher percentages of drilling debris were recovered from the air flush percussion boreholes but on the other hand larger total amounts of debris were lost to the rock during gas lift pumping in borehole kf110 only very small estimated amounts of the flushing water 3 4 and drilling debris approximately 1 lost to the rock were recovered the gas lift pumping was also shown to have a minor influence on the borehole hydraulic parameters calculated from hydraulic tests before and after pumping

the s reflector west of galicia the seismic signature of a detachment fault the so called s reflector is a group of bright continuous reflections underlying the landward tilted fault blocks of the west galicia rifted margin s has been interpreted as the brittle ductile transition the lop of an intrusion a detachment fault and the crust mantle boundary to constrain the internal structure of the reflector we have carried out detailed analyses of these reflections we compare the waveforms of the seafloor reflection and its first multiple both to determine the amplitude of the seafloor reflection and to show hat the seafloor is in effect a spike in the reflectivity series so that the seafloor reflection can be used as the far field wavelet including both source and receiver ghosts we compare he waveform of the seafloor and 5 and show that within the resolution of our data s is a reflection from a step increase in acoustic impedance this result is confirmed through complex trace analysis and in particular the determination of the apparent polarity of s and the examination of the instantaneous frequency function s is consistently positive polarity and shows no significant frequency anomaly simple modelling shows that s is very unlikely to be a reflection from a thin layer we thus conclude that s is probably a single steplike interface from the varying frequency content of the data we determine a value for the effective q between s and the seafloor and use this to assess the amplitude loss due to attenuation and scattering we use a comparison between the seafloor and the s reflection to constrain the amplitude of s and estimate a reflection coefficient for s of at least 0 2 in places decreasing landwards by analogy with structures developed in the highly extended regions of the western united states we consider that the most likely interpretation of s is as a sharp west dipping detachment fault separating a granitic upper plate from a higher velocity lower plate locally probably serpentinized mantle

a dynamic model of extension in the baltimore canyon trough region a dynamic model of extension in the baltimore canyon trough region of the central north atlantic indicates that rifting proceeded in two stages and was controlled by laterally offset preexisting weaknesses within the middle crust and upper mantle beneath the appalachian orogen the crustal weakness is hypothesized to result from thin skinned thrust faulting in the appalachian foreland and the upper mantle weakness results from crustal thickening beneath the appalachian hinterland during the early stages of rifting 225 185 ma the loci of extension in the crust and mantle lie within the respective preweakened regions and are connected by a shear zone in the lower crust in a manner similar to whole lithosphere simple shear extension and thinning of the upper crust at this time results in the formation of late triassic early jurassic rift basins in the appalachian piedmont and in the development of a horizontal pressure gradient which drives widespread ductile flow in the lower crust the pressure gradient may also have influenced magma transport moving melt generated beneath the eastern baltimore canyon trough into the more westerly triassic basin province during the late stages of rifting 185 175 ma extension in the crust shifts to a position overlying the preexisting upper mantle weakness as a result of thermal weakening by the upwelling asthenosphere in this region the loci of extension in the crust and mantle are then coincident and the style of rifting becomes similar to whole lithosphere pure shear the shift in the locus of crustal extension is accompanied by the cessation of subsidence and volcanism in the triassic basins the onset of rapid subsidence in the baltimore canyon trough and the formation of a basement hinge zone separating the highly thinned crust beneath the baltimore canyon trough from moderately thinned crust beneath the triassic basin province extension becomes increasingly focused beneath the eastern edge of the baltimore canyon trough until the onset of seafloor spreading approximately 50 m y after rifting began the resulting conjugate margins are asymmetric with most of the region encompassing the triassic basins stranded on the north american plate

the s reflector west of galicia spain evidence from prestack depth migration for detachment faulting during continental breakup the mechanism of continental extension and breakup and the role of detachment faults in these processes are currently the subject of intense debate one possible detachment fault is the s reflector imaged as an undulating in time locally discontinuous reflection on existing seismic profiles west of iberia here we present new images in depth of the four margin normal profiles across the west galicia rifted margin where the s reflector was originally defined and is best imaged it is shown that faults bounding wedge shaped units of late synrift sediments which hence were active during rifting immediately prior to breakup appear to detach at shallow levels onto the s reflector s itself appears as a continuous locally domal feature and does not generally appear offset the waveform of s is compatible with a reflection from a single interface such as a sharp tectonic boundary the depth sections show that s was active at 1 3 km below the seafloor during final rifting s is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of galicia furthermore the data provide constraints on the sense of shear of s analogy with detachment terranes the present synrift and structural dips of s and the identification of a breakaway to s imply that s accommodated top to the west shear toward the east of the profiles s becomes more complex possibly because of different phases of detachment faulting and the development of both incisement and excisement structures from the geometry of wedge shaped sedimentary units deposited during faulting above s it also appears that s was active at an angle of 20 or less and hence may be considered a genuine low angle normal fault

inhibiting magmatism on nonvolcanic rifted margins the conditions under which continental rifting may occur without an accompanying protracted period of synextensional magmatism are explored with a series of finite element models that dynamically simulate continental extension the models demonstrate that 5 7 km of new oceanic crust can be generated at the time of breakup while restricting synextensional magmatism to less than 2 m y prior to the end of the rifting episode if the extension rate exceeds 15 mm yr and the crust contains only minor lateral variations in strength under these conditions extensional strain in the mantle is localized during the early stages of rifting preventing diffuse lithospheric thinning melting does not begin until lithospheric necking is well established resulting in a narrow zone of melt production that limits magmatism to within 30 km of the ocean continent boundary once melting begins rifting progresses rapidly to breakup restricting the period of synextensional magmatism to as little as 1 6 m y prior to the onset of sea floor spreading because the models do not require either a cool mantle prior to rifting or pronounced conductive cooling of the mantle during rifting they are able to account for the generation of new oceanic crust at the time of breakup the model predictions are consistent with the structure and magmatic history of nonvolcanic rifted margins in the north atlantic ocean

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

patterns of continental extension along the conjugate margins of the central and north atlantic oceans and labrador sea the anomalously thin crust and subsidence history of passive margins suggest that significant extension occurs within continental lithosphere prior to the onset of seafloor spreading we estimate the amount of continental extension prior to seafloor spreading along the conjugate margins of the central and north atlantic oceans and labrador sea from the total tectonic subsidence of the margins in the central atlantic basin the integrated continental extension varies from 225 km for the break between the southern appalachian segment of the north american margin and africa to over 600 km for the break between the blake plateau region of north america and the cape verde terrace region of africa the original width of the extended zone varies from 240 to 770 km in the north atlantic and labrador basins the integrated extension varies from 60 km for the break between northern labrador and greenland to over 600 km for the break between newfoundland and europe the original width of the extended zones in these basins varies from 100 km to over 1460 km in general continental breakup involves two to three times less extension where it follows the preexisting structural grain than where it crosses the preexisting grain as a result seafloor spreading tends to begin first along segments which follow the structural grain extension tends to be symmetrically distributed between conjugate margins where the break follows the grain and asymmetrically distributed where the break crosses the grain failure parallel to the structural grain exhibits greater variability from margin to margin than failure across the grain this phenomenon appears to result from differences in the nature of preexisting weaknesses within the structural grain we conclude that the distribution of the continental extension along passive margins is to a large extent controlled by the orientation of the break relative to the structural grain and the nature of the preexisting weaknesses within that structural grain