evolution of oceanic lithosphere a driving force study of the nazca plate the role of the evolution of oceanic lithosphere in driving oceanic plates is tested with elastic and viscous finite element modeling of the nazca plate results indicate that ridge push forces when distributed over that portion of the oceanic lithosphere giving rise to the force predict a stress state that varies more spatially than does previous modeling with ridge forces concentrated at the ridge crest in particular the component of stress normal to the ridge vanishes near the ridge for the new modeling predicted stresses for distributed ridge force models are still in qualitative agreement with the limited data on nazca intraplate stress stress states have also been calculated for a variety of slab pull models which include possible dependence on age rate and angle of subduction the predicted stresses vary substantially near the trench among these models with less variation farther from the trench combinations of ridge push and slab pull models are consistent with the earlier result of ah upper limit of about a factor of 2 for net slab pull over ridge push simple viscous modeling with only distributed ridge forces predicts relative motion between the nazca and south american plates that is in fairly poor agreement with the observed direction the fit to the observed direction could probably be improved by either including slab forces or the effect of the largely transform boundary with the antarctic plate

correction to an extensive region of off ridge normal faulting earthquakes in the southern indian ocean by eric a bergman john l nabelek and sean c solomon

the north wales earthquake of 19 july 1984 the intra plate earthquake on the lleyn peninsula north wales afforded a rare opportunity to obtain accurate focal parameters for the main shock and the aftershock sequence the focal mechanism does not correlate with movement on any of the large faults showing surface expressions in the area but does indicate a nw se oriented maximum compressional stress direction in agreement with measurements made in other parts of britain aftershock activity was concentrated around the focus of the main shock at depths of between 21 and 24 km the depth indicates an unusually deep transition between brittle and ductile zones that is possibly a consequence of low heat flow in the region

electromagnetic induction fields in the deep ocean north east of hawaii implications for mantle conductivity and source fields summary we have analysed a thirty six day recording of the natural electric and magnetic field variations obtained on the deep ocean floor north east of hawaii the electromagnetic fields are dominated by tides which have an appreciable oceanic component especially in the east electric and north magnetic components the techniques of data analysis included singular value decomposition svd to remove uncorrelated noise there are three degrees of freedom in the data set for periods longer than five hours indicating a correlation of the vertical magnetic field and the horizontal components suggesting source field inhomogeneity tensor response functions were calculated using spectral band averaging with both svd and least squares techniques and rotated to the principal direction one diagonal component determined mainly by the north electric and east magnetic fields is not interpretable as a one dimensional induction phenomenon the other diagonal term of the response function indicates a rapid rise in conductivity to 0 05 mho m 1 near 160 km no decrease in conductivity below this depth is resolvable polarization analysis of the magnetic field indicates moving source fields with a wavelength near 5000 km model studies suggest that the two dimensionality in the response function may be caused by motion in the ionospheric current system

fault plane solutions of intraplate earthquakes in brazil some constraints on the regional stress field abstract determination of intraplate stress fields is fundamental for a better understanding of the forces driving the plates in the south american plate few earthquakes away from the plate boundaries have been recorded by enough stations to have the nodal planes well determined the fault plane solutions of four recent events in central and northern brazil were determined these solutions were compared with previously determined solutions of other events in central and southeast brazil a statistically reliable estimation of the stress field for all of brazil is not yet possible on the basis of these data near the atlantic coast however the data are compatible with a regional stress field with the maximum compressive stress oriented parallel to the expected direction of ridge push or of south america s absolute plate motion mendiguren and richter 1978 farther to the west the stress field changes the direction of the maximum compressive stress seems to be aligned n s rather than e w almost all earthquakes analysed show reverse or strike slip focal mechanisms one event in central brazil however shows normal faulting thus the stress field in brazil is certainly not uniform and in some areas local sources of stress may predominate over regional fields

an extensive region of off ridge normal faulting earthquakes in the southern indian ocean an unusually large number of off ridge earthquakes have occurred within a broad region astride the southeast indian ridge the source mechanisms of nine of these earthquakes in lithosphere from 4 to 27 m y old have been determined with a formal inversion technique based on matching observed and synthetic p and sh waves all events occur in the oceanic mantle from 8 to 22 km below the seafloor the seismic moment and duration of faulting obtained from the inversion constrain the stress drop to lie between a few tens of bars and a few hundred bars for most of the events all of the off ridge earthquakes are characterized by normal faulting mechanisms with the t axes nearly parallel to the nearby ridge axis in sharp contrast to typical mid ocean ridge or intraplate earthquakes a variety of possible tectonic processes which might produce normal faulting in young oceanic lithosphere are considered as possible causes including bending stresses thermal stresses of various origins differential subsidence at a fracture zone off ridge volcanism latitudinal plate motions on a nonspherical earth basal shear stress from small scale convection and plate wide perturbations of the stress field due to the continental collision between india and asia no single mechanism is adequate to explain all the observed off ridge normal faulting earthquakes an apparent asymmetry in the observed seismicity with respect to the ridge suggests that the earthquakes on the antarctic and indian plates are caused by different processes the antarctic plate events can be satisfactorily attributed to the release of thermal and bending stresses associated with a residual depth anomaly between the kerguelen plateau and the southeast indian ridge a region which has been suggested as the site of a horizontal pipe for flow in the asthenosphere from the kerguelen hot spot to the vicinity of amsterdam and st paul islands on the southeast indian ridge the off ridge earthquakes in the indian plate are speculatively attributed to part of the plate wide response of the lithosphere to the ongoing continental collision

in situ stress natural fracture distribution and borehole elongation in the auburn geothermal well auburn new york hydraulic fracturing stress measurements and a borehole televiewer survey were conducted in a 1 6 km deep well at auburn new york this well which was drilled at the outer margin of the appalachian fold and thrust belt in the appalachian plateau penetrates approximately 1540 m of lower paleozoic sedimentary rocks and terminates 60 m into the precambrian marble basement analysis of the hydraulic fracturing tests indicates that the minimum horizontal principal stress increases in a nearly linear fashion from 9 9 0 2 mpa at 593 m to 30 6 0 4 mpa at 1482 m the magnitude of the maximum horizontal principal stress increases in a less regular fashion from 13 8 1 2 mpa to 49 0 2 0 mpa over the same depth range the magnitudes of the horizontal principal stresses relative to the calculated overburden stress are somewhat lower than is the norm for this region and are indicative of a strike slip faulting regime that at some depths is transitional to normal faulting as expected from the relative aseismicity of central new york state however analysis of the magnitudes of the horizontal principal stresses indicates at least to a depth of 1 5 km that frictional failure on favorably oriented preexisting fault planes is unlikely orientations of the hydraulic fractures at 593 and 919 m indicate that the azimuth of the maximum horizontal principal stress at auburn is n83 e 15 in agreement with other stress field indicators for this region the borehole televiewer log revealed a considerable number of planar features in the auburn well the great majority of which are subhorizontal dips 5 and are thought to be bedding plane washouts or drill bit scour marks in addition a smaller number of distinct natural fractures were observed on the borehole televiewer log of these the distinct steeply dipping natural fractures in the lower half of the sedimentary section at auburn tend to strike approximately east west while those in the upper part of the well and in the precambrian basement exhibit no strong preferred orientation the origin of this east west striking fracture set is uncertain as it is parallel both to the contemporary direction of maximum horizontal compression and to a late paleozoic fracture set that has been mapped to the south of auburn in addition to these planar features the borehole televiewer log indicates paired dark bands on diametrically opposite sides of the borehole throughout the auburn well processing of the borehole televiewer data in the time domain revealed these features to be irregular depressions in the borehole wall as these depressions were consistently oriented in a direction at right angles to the direction of maximum horizontal compression we interpret them to be the result of stress induced spalling of the borehole wall breakouts

orientation of in situ stresses in the oceanic crust two in situ measurements of principal stress directions have been made in dsdp holes 504b south of the costa rica rift on the nazca plate and 597c west of the east pacific rise on the pacific plate in both cases the orientations of in situ principal stresses determined from borehole breakouts are consistent with the stress directions inferred from intraplate earthquakes located near the sites

seismotectonics of northeastern united states and adjacent canada data for local earthquakes recorded by a network of stations in northeastern united states and adjacent canada were analyzed to study the seismicity the relationship between earthquakes and known faults the state of stress and crustal and upper mantle velocity structure in addition portable seismographs were deployed in the field to study aftershocks as a result accurate locations for about 364 local earthquakes 2 mb 5 and 22 focal mechanism solutions were determined a comparison of the spatial distribution of these events 1970 1979 with historical earthquakes 1534 1959 reveals that seismic activity in the northeast is relatively stationary in space those areas that have had little or no seismicity historically are relatively aseismic today whereas the historically active areas are also active today the instrumental locations historical seismicity and focal mechanism solutions show an internal consistency that help us distinguish two distinct seismogenic provinces 1 the adirondack western quebec province is a northwesterly trending zone of seismic activity about 200 km wide and at least 500 km long extending from the se adirondacks into western quebec canada thrust faulting on planes striking nnw to nw appears to predominate and the inferred axis of maximum horizontal compression is largely uniform and trends wsw nearly parallel to the calculated absolute plate motion of north america little or no seismicity is found where anorthosite outcrops at the surface correlations between gravity anomalies and earthquake locations suggest that seismic activity in this zone is localized to regions of steep ne or sw gradient in bouguer anomalies this zone does not appear to extend southeastward to boston as proposed by some workers 2 the appalachian province is a northeasterly trending zone of seismic activity extending from northern virginia to new brunswick canada highangle reverse or thrust faulting on n to ne trending planes appears to predominate the western margin of this province however appears to be relatively aseismic we attribute this relative lack of activity to one or more of the following the presence of igneous activity postdating rifting of africa from north america the occurrence of intense metamorphism during the acadian orogeny which may have annealed preexisting faults and the predominance of ductile as opposed to brittle deformation in the geologic past the inferred axis of maximum horizontal compression along the eastern margin of the appalachians is rather uniform and trends w wnw almost perpendicular to the magnetic lineations offshore we suggest that this w wnw compression reflects the gravitational force arising from horizontal density variations in the oceanic lithosphere as it cools and moves away from spreading centers

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

near surface stress and displacement in a layered elastic crust in situ stress measurements on the san andreas fault are modeled in terms of a solution to the equations of three dimensional elastic equilibrium in a layered medium crustal stresses are expressed in terms of surface deformations in particular vertical stress gradients are shown to be proportional to horizontal gradients of vertical surface deformation in light of constraints derived by this relation it is proposed to reinterpret the observed near surface in situ vertical stress gradients on the san andreas fault as partly a manifestation of a layered crust with vertically increasing rigidity extrapolation to depth of the revised stress gradients suggests that the san andreas fault can be regarded as a low stress phenomenon compatible with estimates of shear stress obtained from heat flow data

visco elastic modelling of lithospheric stresses arising from density contrasts summary a method has been developed for application in conjunction with finite element analysis which can be used to model the stress distributions arising from lateral changes in thickness and density of lithospheric layers it is especially applicable to those situations where there are large lateral variations in density and rock materials are comparatively weak such as the upper crust of active and passive continental margins the full effect of gravitational body forces is incorporated in a visco elastic finite element model which is solved within an iterative scheme that preserves the observed or designed shape of the model if so desired further deformations can be studied in the pre stressed model to compare the method with others used previously it was applied to a passive continental margin a model in which the lower crust had a viscosity a factor of 10 smaller than that of the upper crust provided a stress distribution consistent with observational evidence of structure and tectonics the average shear stress of 60 mpa predicted in the continental crust was close to that derived by artyushkov using an analytical method

evolution of lithospheric strength after thrusting modeling of an eroding thrust belt indicates that immediately after thrusting the lithosphere is about twice as strong as unthickened cratonic lithosphere but after 10 to 20 m y it becomes weaker than cratonic lithosphere reaching a minimum of 40 to 70 of cratonic strength 30 to 100 m y after thrusting this may explain why rifting of thrust belts commonly occurs several tens of million years after thrusting ceases relative weakening results from 1 elevation of the geotherm caused by burial of radioisotope enriched surface rocks and 2 replacement of strong mantle rocks by weak crustal rocks in the lithospheric column low denudation rates enhance weakening by allowing radioisotope enriched rocks to remain buried longer our modeling indicates that 1 magmatism after thrusting is favored by high concentrations of heat producing elements in the lower plate and by slow denudation rates and 2 factors that affect denudation rate such as climate and the development of external drainage can significantly influence evolution of a thrust belt

the mechanism of continental splitting abstract continental splitting is the response of the continental lithosphere under favourable conditions to deviatoric tension and a hot partially fused upper mantle beneath it has commonly been suggested that rift systems such as that of east africa may possibly represent splitting in the process of occurring here the hot upper mantle has probably formed by convective upwelling from deep in the mantle allowing the escape of heat from the earth s interior by hotspot activity this causes volcanism and isostatic uplift of the heated and thinned lithosphere and the crustal tension subsequently develops in response to the surface load caused by the uplift and the upthrust of the isostatic compensation at depth it is suggested here that the crustal tension causing continental splitting does not normally originate as in east africa but rather by the suction force acting on the overriding plates at trenches at periods in earth history when subduction occurs on the opposite sides of a large continental mass such as pangaea the critical evidence on the splitting mechanism is mostly deeply buried at passive margins and inaccessible to study some margins such as the north biscay one were apparently formed under much quieter volcanic and tectonic conditions than continental rift zones whereas others such as those of the icelandic region underwent extensive continental volcanism indicating a hot upper mantle beneath the icelandic types of margin probably acted as foci for continental splitting by dyke emplacement with dykes propagating laterally from them through the more quietly developed parts of the margin to link with existing plate boundaries normal plate motions occur in response to the steady escape of heat from the deep interior through formation and cooling of the oceanic lithosphere which can be recycled whereas continental splitting is seen as the result of the spasmodic escape of heat from the deep interior through the continental lithosphere

intraplate lithosphere strength and heat flow the magnitude levels of intraplate stresses and the strength of intraplate lithosphere are critical for understanding the mechanisms of intraplate deformation the decay of stress in the ductile lower lithosphere and its resulting amplification in the brittle upper lithosphere constitute an important mechanism for providing stress levels in the upper brittle lithosphere sufficient for fracture such stress transfer and the subsequent lithosphere deformation are controlled by lithosphere rheology a model is examined here for the deformation of viscoelastic lithosphere with temperature and stress dependent rheology subjected to laterally applied stress for different lithosphere thermal structures the geotherm is found to be of critical importance in determining the strength of the lithosphere and the extent of the resulting lithosphere deformation for lithosphere with heat flow 75 mw m 2 0 25 kbar tensile stress is sufficient to cause geologically significant deformation while for compressional deformation either greater stress levels or a hotter lithosphere are necessary

growth and differentiation of the continental crust declining radiogenic heat production since the archaean has resulted in a secular evolution from a regime of numerous fast moving small thin torsionally weak plates to the present regime of larger thicker torsionally stronger plates moving at an average rate of less than one sixth of the archaean rate this has been accompanied by episodic changes in geological effects by 2500 ma b p about 85 of the present crustal mass had grown by the addition and amalgamation of mafic and calc alkaline rocks in oceanic arcs at an average rate of 11 17 pg a during the early proterozoic the first large cratons were stabilized and locally thickened and differentiated the proterozoic was an era of little continental growth falling average sea level and intracontinental deformation by 700 ma b p cratons had become much more stable marginal accretionary terrians had begun to develop with an average phanerozoic growth rate of 1 64 pg a and blueschists and ophiolites sensu stricto witness the advent of the extant plate tectonic regime

early cretaceous volcanism on the northeastern american margin and implications for plate tectonics volcanic and igneous rocks interbedded with lower cretaceous sedimentary rocks were penetrated in four exploratory oil wells in the orpheus graben scotian shelf offshore eastern canada the lower unit of the volcanogenic sequence is composed of basalt as sills or feeder dikes or flows a few metres to a few tens of metres thick the upper unit consists of tens of metres of pyroclastic and volcaniclastic rocks in which volcanic tephra including glass is mixed with quartz rich sediment interbedded with thin lava flows these rocks show dips of as much as 36 suggesting that they occur as volcanic cones the basalts are geochemically similar to within plate oceanic alkali basalts particularly in their rare earth element ree patterns and trace element composition radiometric ages of 102 to 125 5 ma for the basalts are in good agreement with ages of interbedded sediments dated by palynomorphs as aptian the volcanic rocks are located on continental crust along the possible extension of two oceanic crustal features the newfoundland azores fracture zone and the fogo seamount chain the orpheus graben alkaline volcanism may be an expression of a transform fault propagating into the continent as a result of rejuvenation during plate motion changes in the late lower cretaceous when continental spreading was initiated in the southern atlantic and labrador greenland regions mesozoic volcanism on the atlantic continental margin was episodic with a periodicity of 50 70 ma it is broadly time correlative with major episodes of intraplate oceanic volcanism in both the atlantic and pacific oceans indicating that the volcanism is mantle driven

electromagnetic induction fields in the deep ocean off california oceanic and ionospheric sources summary a 56 day long measurement of seafloor electromagnetic field fluctuations is separated into parts coherent and incoherent with the horizontal magnetic field at a terrestrial observatory 1700 km distant but at the same geomagnetic latitude least squares methods show that the m2 and n2 ocean tides are significant in the seafloor data but not on land spectra of the remaining data show high seafloor electric to seafloor magnetic field coherences but markedly lower seafloor to land values especially for the horizontal electric field model studies show that the correlated parts of the seafloor spectra are consistent with induction by broad scale ionospheric sources while the uncorrelated parts are explained by a combination of a barotropic long wave continuum and an internal wave component the oceanic electromagnetic field is discussed in terms of its modal structure and wavelength dependence and implications for seafloor magnetotellurics may be profound

mechanisms of crustal deformation thirty sixthwilliam smith lecture the energy source for crustal deformation is isotopic heating and secular cooling of the mantle in a true solid heat would be lost to the surface by conduction however solid state creep processes allow the earth s solid mantle to exhibit a fluid behaviour thus thermal convection can convert heating into directed motion variations in temperature lead to variations in densitythrough thermal expansion and contraction although the resultant body forces are vertical horizontal variations in temperature lead to horizontal body forces at an ocean ridge the variations in the thickness of the lithosphere lead to the elevation of the ridge the result is a substantial horizontal ridge push force this force along with the trench pull force on the descending lithosphere is the primary force driving plate tectonics lithospheric thinning must occur beneath continental rifts and plateau uplifts i suggest that lithospheric thinning is caused by the diapiric penetration of hot asthenospheric rock to the crust mantle boundary this penetration may also cause lithospheric stoping and plateau uplifts lithospheric thinning leads to large tensional stresses above the zone of thinning these forces are generally much larger than the compressional forces generated by crustal thinning the forces generated by lithospheric thinning are quite large and may be responsible for the propagation of continental rifts in island arcs and andean type orogenic belts both lithospheric thinning and crustal thickening result in large tensional forces probably the most complex zone of crustal deformation is the continental collision variations in types of continental collisions are discussed

offset boreholes in the rocky mountains of alberta canada recent northeast directed updip bedding plane displacements of several centimetres are indicated by borehole offsets in road cuts at eight localities in the southern canadian rocky mountains these displacements are thought to record release of tectonic stresses because the displacements are congruent with the present day regional stress field

mantle upflow under north america and plate dynamics in the development of plate dynamics there are two fundamental questions first do the plates slide over a passive asthenosphere or are they driven by a convecting upper mantle second are all ocean ridges passive pull apart plate margins or do some lie above upcurrents in the mantle and in that sense become active margins the combination of stress orientations in the crust of north america with other geophysical and geological information favours an active mantle interpretation both for the north american plate and for the mantle upflow which lay beneath the east pacific rise in tertiary times and which now lies beneath the western united states

petrology of the grenville swarm of gabbro dikes canadian precambrian shield the grenville tholeiitic dikes of late proterozoic or early paleozoic age cut marbles and gneisses of the grenville province within and adjacent to the ottawa rift structure where traversed the swarm consists of about 40 large dikes up to 100 m wide representing a crustal extension of 1 km the principal minerals are olivine augite pigeonite plagioclase magnetite ilmenite k feldspar and quartz crystals of pigeonite around olivine and complex augite pigeonite composite grains suggest that the reactions olivine melt pigeonite and pigeonite melt augite have taken place conspicuous zoning and grain to grain variation in the composition of augite are consistent with fractional crystallization but crystal melt equilibrium during crystal growth is indicated by a restricted range in pyroxene paleotemperatures of 1180 1060 c rock textures subophitic combined ophitic subophitic and equigranular in dike centres are related to dike width and are determined principally by the influence of

why large earthquakes do not nucleate at shallow depths although by definition tectonic earthquakes may nucleate anywhere within the seismogenic layer in almost all cases those earthquakes that become large nucleate near the base of the seismogenic layer because frictional strength and stress drop should increase with depth and in situ stress measurements suggest that the ambient stress increases with depth we have examined spontaneous rupture models with gradients of both average stress drop and average strength relative to ambient stress to determine whether this explains the observed phenomena we report here that ruptures that are nucleated with in the low stress drop shallow regions of the model are inhibited from propagating but those that are nucleated within the high stress drop regions can propagate over the entire fault plane

analysis of states of stress between provinces of constant stress recent studies have indicated the existence of crustal provinces or domains within which the state of stress shows little apparent horizontal variation between provinces the state of stress may undergo substantial changes often over remarkably short distances as analyzed here the transition in stress state between adjacent domains is caused by shear tractions acting on the base of the elastic brittle layer and is mediated by a stress field within a zone between the provinces that can be determined by solving the equations of force equilibrium and stress compatibility the component of horizontal stress oriented perpendicular to the provincial boundary changes smoothly across the transition zone from one province to the other whereas the horizontal stress oriented parallel to the boundary shows smaller but discontinuous changes the horizontal component of shear stress is unlikely to change from one province to another because of the implausible nature of the required basal traction thus in this regard states of stress in adjacent domains appear to be coupled the coincidence of a number of stress transition zones with boundaries defined by heat flow suggests that in many cases the basal tractions are the result of thermally induced mass transport below the elastic brittle layer in the direction of increasing horizontal compressive stress this analysis provides a quantitative relationship between changes in the stress field measured in the upper portion of the crust and shear stresses acting on the base of the elastic brittle layer

origin of the lithospheric tension causing basin formation most fault controlled basin formation within plate interiors occurs by normal faulting in response to horizontal deviatoric tension in the continental crust it is suggested that the tension originates either from the plate boundary forces acting at trenches or as a result of isostatically compensated uplifted regions such as east africa the tension produced by both mechanisms is greatest in high heat flow regions where the upper elastic part of the lithosphere is thinned and weakened particularly widespread tension in the continental lithosphere occurs when subduction takes place on opposite sides of a large continental mass such as pangaea in the early mesozoic where it led to widespread graben formation and in association with hot spot activity to continental splitting

moho beneath the north sea compared on normal incidence and wide angle seismic records the mohorovicic discontinuity the moho is properly defined as the step in the velocity of horizontally travelling p waves from about 7 km s 1 to about 8 1 km s 1 its detailed structure varies from place to place but it is universally interpreted as the transition from crust to mantle normal incidence reflections originating at about moho depth have been reported sporadi cally during the past 20 yr during the past 10 yr continuous seismic reflection profiles have been obtained to this depth both on the continents and in the deep sea and at the same time striking improvements have been made in the methods available for interpretation of refraction experiments here we report a comparison between the moho determined by a long range refraction experiment across the north sea and the results of a short normal incidence reflection profile shot along the same line fig 1 we believe this to be the first time that such a direct comparison has been made

structure of the new madrid seismic source zone in southeastern missouri and northeastern arkansas new seismic reflection data in the southwestern part of the new madrid seismic zone nmsz show that a major zone of disrupted reflectors and a large antiform coincide with a prominent trend of earthquake epicenters between caruthersville missouri and marked tree arkansas we speculate that the antiform may be caused by igneous intrusions the strong correlation between the seismicity in this trend and the extent of the disrupted zone provides a geologic basis for defining the source zone for large earthquakes in this part of the nmsz

brittle failure in the upper mantle during extension of continental lithosphere i present a model for the stress required to extend or stretch continental lithosphere the lithosphere is considered to have four rheological fields quartz in the brittle field quartz in the ductile field olivine in the brittle field and olivine in the ductile field as continental lithosphere extends the required stress initially decreases due to nearly isothermal thinning of the strong upper mantle olivine near the brittle ductile transition the stress required to continue extending the crust may then increase due to cooling of the upper mantle however the required stress can increase to no more than about 1 3 times the stress required to start extension this contrasts with an earlier study that required exponentially increasing stress to continue extension my model improves on this earlier study by considering the effects of brittle failure in the upper mantle the rheology of the continental lithosphere should be no barrier to its continued extension regardless of the rate of extension

comments about the age of the canada basin abstract the canada basin appears to have formed over a 60 ma interval during the cretaceous period the opening process can be divided into two parts continental breakup widespread in hauterivian through aptian time about 113 131 ma ago along what was about to become the polar margin of north america and seafloor formation active during the extended cretaceous interval of normal geomagnetic polarity to marine magnetic anomaly 33 time about 118 79 ma ago presently available age controls indicate that all parts of the north american polar margin could have formed at about the same time two styles of margin formation are evident from brock to axel heiberg island there appears to be an extended 70 ma stratigraphic hiatus and northeastward from ellef ringnes island extensive tholeiitic igneous activity associated with continental breakup from banks island to northeastern alaska the breakup interval is abbreviated 20 30 ma and few igneous rocks are observed a progressive difference may be indicated in the amount and or rate of crustal stretching during margin formation with relatively less slower stretching having occurred along the segment southwest of brock island and relatively more faster stretching having taken place along the segment to the northeast

regional jointing in the northern michigan basin the orientations of joints were measured at 142 locations across the northern michigan basin in rocks of cambrian to devonian age four major vertical joint sets were found throughout the area the average trends of the four sets are 052 134 091 and 001 there are small random fluctuations in mean orientation of all the sets but they remain remarkably consistent in orientation throughout the area across lithologic and stratigraphic boundaries and across a regional strike change around the michigan basin of about 115 the joint pattern does not seem to be related to structural trends in the precambrian basement inferred from geophysical data the joint set at 134 has the same trend as a series of major folds in the paleozoic rocks of the michigan basin suggesting that this joint set may be related to the folding the joint set at 052 is parallel with the maximum horizontal compressive stress that exists today throughout the mid continent region this joint set may be extension joints formed in response to the present stress regime

upper crustal stresses and vertical stress migration in eastern canada diversified phenomena are contributing to our knowledge of various aspects of the stress field and the origin of the crustal stresses in eastern canada both in situ stress measurements to 2100 m and earthquake fault plane solutions in the upper crust 5 20 km indicate that the maximum horizontal stress is greater than the vertical absence of earthquakes in the lower crust 20 km implies a comparatively lower deviatoric horizontal stress in this layer stress directions as determined from both in situ measurements and earthquake fault plane solutions indicate an overprint confined to the ene octant which correlates with the direction predicted by modeling of plate tectonic stresses thus spreading mid atlantic ridge stress is considered to be one of the important contributors to the stress field in eastern canada viscoelastic relaxation of this stress is considered to enhance the deviatoric stress level in the upper crust while simultaneously reducing that in the lower crust the crust acts as a stress guide within which stresses in the upper crust that are lost through brittle failure i e earthquake stress drop can be replenished from the lower crust moreover the directionality predicted by modeling of spreading mid atlantic ridge stress is maintained incomplete postglacial rebound also contributes to deviatoric compression other possible contributors to the upper crustal stress field are basal drag and membrane stress but the relative contributions from these sources are not properly understood

numerical modeling of intraplate deformation simple mechanical models of continental collision we study a model of continental collision in which one of the continents acts as a rigid die indenting the other plate which flows as an incompressible viscoplastic medium we consider two extreme cases of plane deformation 1 plane strain which corresponds to an infinitely thick lithospheric plate and 2 plane stress corresponding to a very thin plate deformation of the lithosphere a thick plate should be intermediate between those extremes we found that the flow in the plane strain case is quite similar to that obtained by slip line theory the plane stress results are quite different since in this case most of the plate shortening is taken up by the thickening of the lithosphere we also explored the role of boundary conditions on the flow in particular the role of the side walls containing the flow of the lithosphere in the case of a free lateral boundary the main feature is a flow of matter toward this free wall and a s like pattern for the horizontal stress field for a rigid wall on the other hand the plane strain and the plane stress results are quite different in the first case there is a large return flow on the sides of the punch the material being extruded along the only free surface available in the plane stress case the return flow disappears and the material displaced by the penetration of the die tends to thicken the plate the role of a nonlinear constitutive relation is studied for power law creep as the power of the flow limit increases the flow retains its general features but the deformation localizes creating sharper contrasts between high and low strain rate areas in plane stress the effect of nonlinearity is to increase the contrasts in vertical motion available data for asia are discussed in the light of the new results

intraplate seismicity of the southern part of the pacific plate we present a compilation of all the known seismicity of the part of the pacific plate located south of the equator this includes historical earthquakes dating back to 1937 in addition to the gilbert islands swarm of 1981 1983 we identify 60 events at 32 epicenters in the magnitude range 4 3 6 9 our results indicate that the level of seismicity of the southern pacific plate is comparable to that of other plates including continent bearing ones most of the seismic activity is concentrated in the youngest parts of the plate but a few large events are present in cretaceous lithosphere with the exception of events from the gilbert islands swarm earthquakes located in lithosphere 35 ma and older are usually compatible with the release of horizontal compressional stress due to the gravitational process known as ridge push in younger areas the situation is much less clear cut the final geometry of the focal mechanisms is controlled by local tectonics in particular due to the miocene reorientation of the spreading at the east pacific rise the bulk of the pacific plate is particularly vulnerable to ridge push and this explains the numerous strike slip events and the absence of magnitude 6 events in most of the plate correlations of epicenters with zones of weakness or bathymetric features are frequent but not universal finally a number of areas with high seismic release are identified as potential targets for future exploration

observations of the electrical asthenosphere beneath scandinavia the recent recognition that long period i e of the order of hours electromagnetic induction studies could play a major role in the detection of the asthenosphere has led to much interest amongst the geophysical and geological communities of the geomagnetic response functions derived for differing tectonic environments experiments carried out on the ocean bottom have met with considerable success in delineating the electrical asthenosphere i e a local maximum in electrical conductivity minimum in electrical resistivity in the upper mantle in this paper observations of the time varying magnetic field recorded in three regions of scandinavia northern sweden kiruna kir northern finland northeastern norway kevo kev and southern finland sauvamaki sau are analysed in order to obtain estimates of the inductive response function c for each region the estimated response functions are compared with one from the centre of the east european platform eep and it is shown that the induced eddy currents at periods of the order of 103 104 s in the three regions flow much closer to the surface than under the platform centre specifically at a period of 3000 s these currents are flowing at depths of the order of kev 120 km kir 180 km sau 210 km eep 280 km implying that the transition to a conducting zone of 0 2 s m occurs at around these depths one dimensional inversion of ckev and ckir shows that there must exist a good conducting zone of 0 1 1 0 s m under each of the two regions of 40 km minimum thickness at depths of kev 105 115 km kir 160 185 km this is to be contrasted with eep where the d profile displays a monotonically decreasing resistivity with depth reaching 0 1 s m at 300 km finally a possible temperature range for the asthenosphere consistent with the deduced conducvitity is discussed it is shown that at present there is insufficient knowledge of the conditions water content melt fraction etc likely to prevail in the asthenosphere to narrow down the probable range of 900 1500 c

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

stress amplification in the upper crust and the development of normal faulting abstract finite element analysis of lithosphere deformation incorporating failure criteria based on the modified griffith theory for an elastic upper crust and a visco elastic rheology for the lower crust and lithospheric mantle demonstrates that normal faulting as a result of comparatively small horizontal tensile stresses acting throughout the lithosphere can occur by the phenomenon of stress amplification the use of a simple newtonian viscosity or of power law creep for the visco elastic material does not significantly change the results the time to failure is dependent upon the magnitude of the applied stress and the effective viscosity of the underlying material for an applied stress of 20 mpa and a constant viscosity of 1023 pa s failure is predicted after 1 19 ma with a fault plane hade of about 30 a decrease of one order of magnitude in the viscosity of the lower crust results in a slightly shorter time to failure whereas an increase of one order of magnitude results in a very much greater failure time

the origin of tectonic stress in the lithosphere abstract the sources of lithospheric stress and their distinctive features are briefly reviewed it is suggested that there are two main categories of lithospheric stress renewable stress which persists despite continuing stress relaxation and non renewable stress which can be dissipated by relief of the initial strain the two most important types of renewable stress arise from plate boundary forces and from isostatically compensated loads non renewable stress systems include bending stresses membrane stresses and thermal stresses an important phenomenon generating large stresses at shallow depth is stress amplification caused by lower lithospheric creep this applies to renewable stresses but not to the non renewable type it is suggested that only renewable stresses contribute significantly to tectonic activity however bending and thermal stresses are locally important in subducting lithosphere

geomagnetic variation sounding of the asthenosphere beneath the juan de fuca ridge two ocean bottom magnetometers have been deployed at sites on either side of the juan de fuca ridge 20 and 40 km from the crest near latitude 47 n twelve days of data were recorded simultaneously at the two sites and these data have been used to determine the electrical conductivity structure beneath the region the vertical field variations at both stations are of very low amplitude in the period range 0 25 to 8 hours with no detectable phase reversal across the ridge electric fields in the ocean layer have been estimated from the attenuation of sea floor horizontal fields with respect to victoria and the resulting impedance spectra inverted to find resistivity as a function of depth using the technique of oldenburg 1979 resistivity models which fit the data within the specified bounds are characterized by a comparatively resistive crust and upper lithosphere overlying a low resistivity zone between depths of 55 and 75 km which in turn appears to be underlain by a higher resistivity section the average resistivities are used to estimate temperature and melt fraction on the basis of the effective medium theory of shankland and waff 1976 for a basalt melt fraction within an olivine matrix for the low resistivity zone and depending on the water content of the melt 0 to 10 the estimated temperature range is 1400 c to 1250 c and the corresponding melt fraction 0 05 to 0 09 the data suggest that the melt fraction decreases below 80 km the conductivity profile from magnetotelluric data on 72 ma ocean floor in the north central pacific filloux 1977 has similar characteristics to the 1 ma old conductivity structure near the juan de fuca ridge but with the low resistivity layer at depths of 140 to 220 km in both cases the resistivity minimum should be equated with the region of maximum partial melt within the asthenosphere rather than with the base of the lithosphere the increase in depth to this zone with increasing lithosphere age is in general agreement with plate tectonic concepts

thermoelastic stress how important as a cause of earthquakes in young oceanic lithosphere we present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere less than 35 m y in age in support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m y where cooling rates are relatively high most near ridge earthquakes have focal depths between the moho and the depth of the 800 c isotherm thrust and strike slip events dominate the shallowest seismicity while the focal mechanisms of the deeper events are generally characterized by normal faulting to assess the importance of thermal stress in the generation of near ridge earthquakes we model the lithosphere as an elastic half space in which the response to cooling is computed using the method of thermoelastic displacement potentials a key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere a further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half space cooling models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust and normal faulting earthquakes near mid ocean ridges the addition of a ridge push stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m y but probably contributes little to the stress field at younger ages though these simple models are limited by several simplifying assumptions particularly in the immediate vicinity of the ridge axis they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere

geologic evidence for recurrent moderate to large earthquakes near charleston south carolina multiple generations of earthquake induced sand blows in quaternary sediments and soils near charleston south carolina are evidence of recurrent moderate to large earthquakes in that area the large 1886 earthquake the only historic earthquake known to have produced sand blows at charleston probably caused the youngest observed blows older late quaternary sand blows in the charleston area indicate at least two prehistoric earthquakes with shaking severities comparable to the 1886 event

effects of three dimensional structure on magnetotelluric sounding curves significant errors may result when applying one dimensional 1 d or two dimensional 2 d interpretation methods to magnetotelluric mt data collected in three dimensional 3 d environments both depths and resistivities can be grossly incorrect if 1 d or 2 d methods are applied in a 3 d setting we present examples of mt sounding curves generated using a 3 d modeling program madden and park 1982 and illustrate some interpretation pitfalls if 3 d effects are not considered the 3 d effects discussed herein are attributed to a surface layer heterogeneity and can be readily identified in mt data from a well designed mt survey the mt survey must include stations chosen to yield information about regional structure alternatively carefull examination of a geologic map will help the intepreter estimate the regional effects present in the data

on the state of stress in new england as determined from earthquake focal mechanisms assuming only that slip occurs in the direction of resolved shear stress on fault planes we have found a wide range of uniform stress tensors each of which is consistent with ten diverse focal mechanisms for recent moderate size earthquakes in new england and vicinity thus it is possible that the variations in focal mechanisms are due to the occurrence of specific pre existing zones of weakness rather than to inhomogeneous stresses acceptable stresses are broadly consistent with those established for other parts of the eastern united states namely northeast or east northeast directed maximum compression and nearly vertical minimum compression

magnetotelluric measurements over the alpha ridge abstract magnetic and telluric field variations were recorded on the arctic ocean sea ice over the alpha ridge in april and may 1983 the magnetotelluric measurements were made near the base camp of operation cesar canadian expedition to study the alpha ridge to investigate the deep structure and electrical properties of the lithosphere and upper mantle beneath the ridge thirty days of usable data were obtained within the period range 120 20 000 s during the experiment the base station drifted across a deep 2000 m median valley or graben lying parallel to the ridge axis and then over a steep rise on its northern flank one data set was obtained over the graben and the remainder of the measurements were made as the station travelled back and forth over the rise the directional properties of both the geomagnetic induction arrows and the magnetotelluric impedance tensors were consistent with changes in water depth along the drift path one dimensional inversions of e polarization mt data revealed a resistive lithosphere about 70 85 km thick with a conductive 10 m upper mantle asthenosphere below sea water depths estimated in the inversion process agreed remarkably well with the known bathymetry indicating that the ocean floor contained less than 100 m of highly conducting sediments along most of the drift path two dimensional models required simulation of the topography of the entire section of the alpha ridge between the canada and makarov basins in order that a satisfactory fit be obtained to the h polarization data two dimensional forward modelling indicates that the anisotropic character of the impedance measurements can entirely be attributed to the bathymetry of the ridge the data provided no indication of structural inhomogeneity within the oceanic crust and lithosphere consistent with results from companion seismic refraction and gravity surveys the thickness of the lower conducting region was not resolved because reliable long period data t 20 000 s were not available for analysis impedance estimates at these periods were poorly determined and subject to rapidly rising tensor skew they were also subject to bias errors caused by water movements in particular the m2 tide

seismicity seismic reflection studies gravity and geology of the central virginia seismic zone part i seismicity the central virginia seismic zone is an area of persistent low level seismicity in the piedmont province of that state its north south dimension is 120 km and it extends some 150 km in an east west direction from richmond to lynchburg the results of instrumental monitoring there by a seismic network during the past 5 yr are reported herein to provide a useful constraint on geologic structural models considerable care must be exercised in the location of earthquakes and in the specification of the accuracy of those locations our procedures for accomplishing those tasks were as follows first a velocity model specific to the network locale was developed from seismic refraction and reflection data second evaluation of the detection capability of the network indicated that the detection of seismic events should be virtually complete down to a magnitude of 1 5 third the locational capability of the network was assessed by locating blasts originating from three different quarries operating within the network area as if they were earthquakes of unknown location the average calculated locations when compared to the known blast sites indicated average differences of 2 km horizontally and 4 km vertically except for the northeastern portion there the location capability degrades to fair for epicenters 4 km and to poor for estimation of focal depths 8 km the resultant decrease in locational capability is probably due to the presence of low velocity coastal plain sediments fourth the location of all of the earthquakes relative to each other was tested by use of special joint hypocenter calculations the recalculated locations differed from the original locations in a random manner on the average that difference was 3 km and it indicates the absence of serious bias introduced by combining earthquake locations derived from different subnetworks of seismographs ninety six pre network pre 1978 earthquakes for the most part felt events with noninstrumental locations and 34 network determined hypocenters 1978 1982 for mostly smaller not felt shocks form the basic data set of this study the spatial distribution exhibited by the 130 hypocenters is diffuse both vertically and horizontally such a pattern favors multiple rather than singular seismogenic structures seventy five percent of the focal depths are in the upper one third of the crust at depths of 11 km or less these multiple shallow sources will be shown in part 4 of this sequence of papers to be along and above a master detachment fault

crustal thinning and subsidence in the north sea we present here the results of a detailed study of subsidence and crystal thinning across the central graben of the north sea basin two independent lines of evidence have been used to test how well the uniform extensional model1 2 can account for the evolution of the north sea basin a long range seismic profile was shot perpendicular to the central graben from edinburgh to southern norway and data were analysed from 11 exploration wells situated close to the seismic line fig 1 the estimates of the increase in surface area due to extension from the crustal structure determined using the seismic data and from the well subsidence patterns are both compatible with a simple uniform extensional mechanism for the post palaeozoic evolution of the north sea

the tasman sea earthquake of 25 november 1983 and stress in the australian plate abstract the tasman sea earthquake of 25 november 1983 was large enough m 0 1 1 10 18 nm to be recorded world wide and provide information on the state of intraplate stress in the lithosphere beneath the tasman sea the earthquake occurred beneath the abyssal plain at a depth of about 25 km and was associated with almost pure dip slip faulting the direction of the pressure axis of the focal mechanism is similar 139 degrees e of n to those observed from the nearby australian mainland hence both the oceanic tasman sea and continental australia appear to be part of the same stress regime however the direction of stress in this part of the australian plate does not coincide with the north south direction of motion of the plate and therefore forces other than the ridge push must be invoked to generate the stresses observed

the picton and varty lake ultramafic dikes jurassic magmatism in the st lawrence platform near belleville ontario the picton and varty lake ultramafic dikes that intrude ordovician limestone near belleville ontario consist of olivine or serpentinized pseudomorphs and ti phlogopite phenocrysts set in a groundmass of spinel perovskite serpentine calcite apatite and chlorite phlogopite exhibits a complex chemical variation and late stage ba enrichment up to 6 8 wt bao spinels are chemically zoned and commonly exhibit an atoll texture in which a magnesian aluminous chromite core is separated by a silicate zone from late stage magnesian aluminous titanomagnetite chemically similar to kimberlitic spinel atoll spinels from the picton dike also have a zone of titanian pleonaste surrounding a central grain of magnesian aluminous chromite at the centre of the atoll structures both dikes are extremely silica undersaturated with high k na and mg mg fe2 ratios petrographic and chemical features of the picton and varty lake dikes preclude simple classification as both dikes have aspects indicative of kimberl

a study of joints and stress release buckles in palaeozoic rocks of the niagara peninsula southern ontario joint sets in palaeozoic bedrock of the niagara peninsula occur at 005 045 085 and 135 orientations there is no consistent change in joint orientation with lithology rock age or location within the area drainage pattern analysis using airphotos and topographic maps indicates that glacial drift effectively masks underlying joint systems rare calcite and gypsum mineralization occurs along joints of 085 and 135 trend stress relief buckles indicate a contemporary maximum principal stress along an 050 trend joint systems in the niagara peninsula are similar in trend to those on the appalachian plateau

magnetic and electric fields produced in the sea during geomagnetic disturbances to understand geomagnetic effects on systems with long conductors it is necessary to know the electric field those systems experience for surface conductors such as power systems and pipelines this can easily be calculated from the magnetic field variations at the surface using the surface impedance of the earth however for calculating the electric fields in pipelines and submarine cables at the seafloor it is necessary to take account of the attenuating effect of the conducting seawater assuming that the fields are vertically propagating plane waves we derive the transfer functions between the electric and magnetic fields at the seafloor and the magnetic field variations at the sea surface these transfer functions are then used with surface magnetic field data to determine the power spectra of the seafloor magnetic and electric fields in a shallow sea depth 100 m and in the deep ocean depth 5 km for different values of the kp magnetic activity index for the period range considered 2 min to 3 hrs the spectral characteristics of the seafloor magnetic and electric fields for a 100 m deep sea are very similar to those of the surface fields for the deep ocean the seafloor spectra show a faster decrease in spectral density with increasing frequency compared to the surface fields the results obtained are shown to be consistent with seafloor observations assessment of the seafloor electric fields produced by different levels of geomagnetic activity can be useful in the design of the power feed equipment for submarine cables and cathodic protection for undersea pipelines

magnitude of crustal extension in the southern great basin strike slip faults in the southern great basin separate areas of cenozoic upper crustal extension from relatively stable tectonic blocks linear geologic features offset along the garlock fault las vegas valley shear zone and lake mead fault system allow reconstruction of the southern great basin to a pre extension configuration the sierra nevada mojave desert spring mountains and colorado plateau are treated as stable unextended blocks that have moved relative to each other in response to crustal extension with the spring mountains held fixed to the mojave block our reconstruction indicates a minimum of 65 extension 140 km between the southern sierra nevada and colorado plateau

upwarp of anomalous asthenosphere beneath the rio grande rift continental rifts are possible analogues of mid ocean ridges although major plate tectonic features are less clearly observed1 current thermal models of mid ocean ridges2 4 consist of solid lithospheric plates overlying the hotter less viscous asthenosphere with plate thickness increasing away from the ridge axis the lithospheric lower boundary lies at or near the melting point isotherm so that at greater depths higher temperatures account for lower viscosity lower seismic velocities and possibly partial melting upwarp of this boundary at the ridge axis concentrates heat there thus lowering densities by expansion and raising the sea floor to the level of thermal isostatic equilibrium at slow spreading ridges a major central graben forms owing to the mechanics of magma injection into the crust5 topography heat flow gravity and seismic studies support these models on the continents a low velocity channel has been observed although it is poorly developed beneath ancient cratons6 9 plate tectonic models have been applied to continental basins and margins10 12 but further similarities to the oceanic models remain elusive topographic uplift is often ascribed to airy type isostatic compensation caused by crustal thickening rather than thermal compensation in the asthenosphere here we discuss the rio grande rift in southwestern united states teleseismic p wave residuals show that regional uplift is explained by asthenosphere uplift rather than crustal thickening

the tasman project of seafloor magnetotelluric exploration experiment and observations abstract a major experiment in natural geomagnetic induction has been carried out by making seafloor magnetotelluric observations on a line of sites between australia and new zealand simultaneous observations were made by land magnetometers on the australian continent the large volume of data recorded shows a great variety of phenomena of electromagnetic induction in the earth and sea parkinson arrows and magnetotelluric impedances have been computed for the seafloor sites the parkinson arrows show a strong coast effect near the australian continent and a weaker coast effect over the submerged lord howe rise the occurrence of smoothly varying arrows across the tasman sea suggests a simple large scale pattern for electromagnetic induction in the tasman sea the interpretation of parkinson arrows at seafloor sites is shown to require special care due to the attenuation and phase shift effects caused by the ocean on seafloor horizontal magnetic field fluctuations the magnetotelluric impedances exhibit a strong anisotropy at all sites in the tasman sea the orientation of the impedance axes is constant across the sea and the anisotropy may be attributed to the two dimensional shape of the tasman sea calculations based on the distance scale of the magnetotelluric anisotropy interpreted as an ocean continent boundary effect give a lower bound for the integrated resistivity of the tasman sea oceanic lithosphere of order 107 m2 corresponding to an estimate of 2 102 m for its average resistivity

seismic profiling the continental lithosphere deep seismic reflection profiling is successfully being used worldwide to study the structure of the continental crust1 7 the moist8 profile recorded by birps british institutions reflection profiling syndicate was the first to show that the structure of the upper mantle could also be imaged given a powerful source and long recording times birps has recently acquired an ultra deep seismic reflection line drum deep reflections from the upper mantle the drum line was designed specifically to investigate the reflectivity of the lower continental lithosphere and was therefore recorded to a two way time twt of 30 s giving a depth of penetration into the lithosphere of 110 km this profile has now revealed strong reflections which are unequivocally from the lower lithosphere well below the base of the crust and which are as far as we are aware the deepest and most continuous structures imaged in the upper mantle by multi channel seismic reflection profiling

conductivity structure of oceanic upper mantle beneath the pacific plate summary ocean bottom magnetotelluric responses from three ages on the pacific plate have been inverted to reveal the presence of a conductive zone whose depth is a function of lithospheric age the conductivity achieves its maximum value 0 1 sm 1 at depths near 70 120 and 180 km for lithospheric ages of 1 30 and 72 myr respectively the existence of this conductive structure has firmly been established by constructing a number of different acceptable models and also by obtaining unique averages over depth of the conductivity an explanation in terms of partial melting in the upper mantle has produced lower bounds for the volume fraction of melt which must exist in order to produce the high conductivity beneath jdf t 1 myr the lower bound lies in the range 0 04 0 12 beneath cal t 30 myr it lies in the range 0 02 0 10 and beneath ncp t 72 myr it lies in the range 0 008 0 02 the distributions of partial melt show that the major portion of melt beneath jdf lies between 40 and 90 km and that melting has ceased by depths of about 170 km at all three sites geotherms have been computed and they are qualitatively similar to those predicted from a convective system where the temperature increases rapidly through a boundary layer to achieve a maximum value and then possibly decreases with depth the depth at which the maximum temperature is reached is about 125 km for cal and about 175 km for ncp and the maximum temperatures are estimated to be 1400 1500 c lastly as an independent check upon the partial melt explanation the location and magnitude of the seismic low velocity zone has been correlated positively with the computed distribution of partial melt at all three ocean bottom sites

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

the opening of the atlantic the mesozoic new england igneous province and mechanisms of continental breakup abstract plume and tensionally induced rifting models have been proposed as causes of continental rifting and breakup mantle plumes should produce early lithospheric doming events associated with alkaline magmatism however the triassic alkaline rocks on the east north american e n a margin are structurally localized the uplifts are mostly jurassic in age and voluminous plateau magmatism post dates the initiation of taphrogenesis by 10 to 20 ma in addition there are no chains of anorogenic igneous rocks leading to any of the postulated triassic plumes these facts argue against the plume generated rifting model for the e n a margin but are consistent with the predicted effects of the tensionally induced rifting model the genesis of linear chains of alkaline rocks has been attributed to passage of a lithospheric plate over a mantle plume this explanation has been proposed for the cretaceous monteregian and white mountain intrusive series and the kelvin seamount chain however these rocks are distributed along structurally controlled e w trending lineaments the monteregians are localized by the presence of a paleorift ottawa bonnechere graben in the underlying grenvillian basement and the kelvin seamounts are emplaced along the early jurassic kelvin fracture zone the structures localizing the southern white mountain lineament are less well defined the spatial temporal and geochemical relationships along and between these lineaments are consistent with an origin as a southeastward propagating tensional event the intra plate stresses are probably the result of shifts in the pole of rotation of the north american plate due to irregular northward extensions of seafloor spreading during the cretaceous

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

gravity induced stresses near topography of small slope topographic modification of gravity induced near surface stresses results in significant departures from a lithostatic state a perturbation scheme provides approximate analytical solutions for plane strain of an elastic half space with an irregularly shaped free surface of small characteristic slope e the leading order effect of the topography is equivalent to that of a distributed normal load on a plane boundary and the correction at order e is due to a distributed shear traction on a plane boundary in the near surface region these two effects contribute at the same order to departures from the lithostatic stress field the contribution of the shearing has been neglected in most previous analyses we present explicit solutions for several particular geometries a symmetric ridge or valley an adjacent ridge and valley a shelf and a sharp crested ridge notable features include 1 for a sufficiently steep ridge the horizontal normal stress is compressive at the crest and decreases possibly becoming tensile with depth 2 large horizontal tension is induced in a valley bottom at greater depth these effects vanish and the stresses approach lithostatic similar analysis is applied to a half space under a far field tectonic compression or tension the result shows that the leading order effect of topography is equivalent to a distributed shear traction on a plane surface regional horizontal compression can be significantly reduced or even change to tension in the neighborhood of a topographic high

preferential rifting of continents a source of displaced terranes lithospheric rifting while prevalent in the continents rarely occurs in oceanic regions to explain this preferential rifting of continents we compare the total strength of different lithospheres by integrating the limits of lithospheric stress with depth comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of 3 also a thickened crust can halve the total strength of normal continental lithosphere because the weakest area acts as a stress guide any rifting close to an ocean continent boundary would prefer a continental pathway this results in the formation of small continental fragments or microplates that once accreted back to a continent during subduction are seen as displaced terranes in addition the large crustal thicknesses associated with suture zones would ironically make such areas likely locations for future rifting episodes this results in the tendency described as the wilson cycle of new oceans to open along the suture where a former ocean had closed

inversion of ocean bottom magnetotelluric data revisited three ocean bottom magnetotelluric data sets from sites on the pacific plate are reinterpreted the initial analysis found a correlation between the lithospheric age and the depth to a conductive zone beneath each site that work also suggested that the resistivity increased below the conductor this analysis which includes new methods for constructing one dimensional conductivity models shows that the postulated increase in resistivity is not demanded by the data it also reveals an unexpectedly large nonuniqueness inherent in the interpretation of these data the previously reported trends with lithospheric age still exist but they are not as strong as initially believed finally it is shown rigorously that the different age sites are distinct in that no one dimensional model can account for all three data sets

convective thinning of the lithosphere a mechanism for the initiation of continental rifting thinning of the lithosphere by heat carried convectively to its base is a possible mechanism for the initiation of continental rifting the lithosphere asthenosphere boundary marks the transition from a quasi rigid behavior of the mantle on geological time scales to a viscous one the boundary lies at or close to the solidus of the mantle material and its position is a function of temperature and pressure additional heat carried to the boundary will cause it to rise and the lithosphere to thin a one dimensional model is used to study the upward movement of the boundary from an initial equilibrium position as a consequence of an increase in the heat flux from the convecting mantle the ratio of the amount of thinning to the initial thickness is directly proportional to the increase in heat flux from the asthenosphere and inversely proportional to the difference between the final subcrustal geothermal gradient and the slope of the transition temperature profile the rate of thinning is determined by the initial and final equilibrium thicknesses the thermal diffusivity the transition temperature profile and the plume temperature profile it depends only slightly on any latent heat associated with the lithosphere asthenosphere transition during a large fraction of the time between the initial and final equilibrium states the lithosphere thins at a rate which is inversely proportional to the square root of time t even 300 km thick lithospheres may be thinned to 50 km or crustal thicknesses in tens of million years by the heat advected by mantle plumes this heat contributes to an enhanced surface heat flow because of the constant level of radioactive heat production in the crust the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to the initial heat flow from the asthenosphere for large increases in asthenospheric heat flow the lithosphere is almost thinned to the crust before any significant change of surface heat flow occurs uplift due to thermal expansion upon lithospheric thinning is of the order of 102 103 m a rayleigh taylor instability acting on the thinning lithosphere may further increase thinning rates and cause magmatic intrusions of the lithosphere and surface volcanism

hydraulic fracturing stress measurements at yucca mountain nevada and relationship to the regional stress field hydraulic fracturing stress measurements and acoustic borehole televiewer logs were run in holes usw g 1 and usw g 2 at yucca mountain as part of the nevada nuclear waste storage investigations for the u s department of energy eight tests in the saturated zone at depths from 646 to 1288 m yielded values of the least horizontal stress s h that are considerably lower than the vertical principal stress s v in tests for which the greatest horizontal principal stress s h could be determined it was found to be less than s v indicating a normal faulting stress regime the borehole televiewer logs showed the presence of long in excess of 10 m vertical drilling induced fractures in the first 300 m below the water table these are believed to form by the propagation of small preexisting cracks under the excess downhole fluid pressures up to 5 2 mpa applied during drilling the presence of these drilling induced hydrofractures provides further confirmation of the low value of the least horizontal stresses a least horizontal principal stress direction of n60 w n65 w is indicated by the orientation of the drilling induced hydrofractures n25 e n30 e and the orientation of stress induced well bore breakouts in the lower part of usw g 2 n65 w this direction is in good agreement with indicators of stress direction from elsewhere at the nevada test site the observed stress magnitudes and directions were examined for the possibility of slip on preexisting faults using these data the coulomb criterion for frictional sliding suggests that for coefficients of friction close to 0 6 movement on favorably oriented faults could be expected for coefficients of friction of 1 0 preexisting faults of all orientations should be stable laboratory studies on the yucca mountain tuffs reported elsewhere yield coefficients of friction ranging from 0 6 to 0 9

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

state of stress in the northern gulf coast conspicuous structural features of the northern gulf coast are regional systems of listric normal faults formed contemporaneously with deposition periods of main fault movement range from eocene to miocene initial development of these contemporaneous fault systems is usually attributed to differential compaction and or gravity sliding within the sedimentary section during clastic sedimentation on a stable shelf evidence of recent reactivation of some fault systems indicates that sediments in the region are currently in a tensional state of stress infrequent low to moderate intensity earthquakes also are associated with the faulted region lithospheric flexure due to sediment loading has been suggested as a possible cause of this extensional regime for most passive continental margins sedimentation alone is not sufficient to cause flexurally induced faulting and earthquakes because the sediment load accumulates slowly enough for the stresses to relax however sedimentation rates in the northern gulf coast have been extremely high since the pleistocene 12 18 cm 100 yr a simple flexural calculation yields horizontal extension normal faulting in the lower gulf coast caused by the weight of pleistocene sediments although the relationship between basement upwarp and surface uplift may be complicated by subsidence due to compaction of sediment the location of the flexural bulge due to loading by pleistocene and younger sediments is generally consistent with areas of recent uplift determined from releveling data bending stresses caused by the total weight of mesozoic and cenozoic sediments predict horizontal compression for this region indicating that most of these stresses must have relaxed relaxation of stresses over time may also explain the absence of large earthquakes

on the electrical crust mantle structure in fennoscandia no moho and the asthenosphere revealed summary an interpretation of the geomagnetic inductive response function c 0 observed at kiruna in northern sweden is herein undertaken the bounds of acceptable solutions are initially discovered by a monte carlo random search procedure and the best fitting solutions are examined by the application of linear theory to the problem the data are shown to have a higher degree of internal consistency than that described by the estimated variances of each datum a further monte carlo inversion of the variance reduced data set gives solutions with well defined model parameters the two major features of the models are 1 a small or non existent electrical conductivity variation across the seismic moho boundary and 2 the unequivocal existence of an electrical asthenosphere under the fenno scandian shield beginning at a depth of between 155 185 km and of 60km minimum thickness both of these observations have seismic counterparts finally possible mantle temperature profiles are deduced which depend on the assumptions and laboratory data employed

stress relief buckles in the mcfarland quarry ottawa six quarry floor buckles in a west end ottawa bedrock quarry are described and a semi quantitative estimate of the stress field is inferred from them the maximum principal stress component is hori

paleomagnetism of the hearst dike swarm and implications for the tectonic history of the kapuskasing structural zone northern ontario prominent north to northwest trending dikes herein termed the hearst swarm west of the kapuskasing structural zone ksz in northern ontario yield a paleomagnetic pole position of 48 6 n 69 0 e dm 6 9 dp 3 6 60 samples from 12 dikes using paleomagnetic petrographic and structural criteria the hearst swarm is correlated with the 2 6 ga matachewan swarm east of the ksz a positive correlation is observed between the trend and paleomagnetic declination of hearst matachewan dikes and may result from an 15 post 2 6 ga rotation of terranes across the ksz paleomagnetic and structural data from the hearst dikes are consistent with the regional westward tilting of the crust near the southern ksz as first proposed in 1983 by percival and card

crustal doming and the mechanism of continental rifting abstract bott m h p 1980 crustal doming and the mechanism of continental rifting in j h illies editor mechanism of graben formation tectonophysics 73 1 8 a mechanical explanation of the relationship between crustal uparching and graben formation is suggested based on the geologically supported assumption that doming precedes rifting rifting and graben formation can occur under conditions of crustal tension by the vening meinesz wedge subsidence mechanism modified to apply to the uppermost 20 km or thereabouts of the continental crust if sediment filled troughs of around 5 km depth are to be formed by this mechanism then a persistent tension of about 200 mpa 2 kbar is required such a stress system may result from the combined effect of the topographic load of the uplifted region and the upthrust caused by the underlying low density upper mantle region below if the upper 10 to 20 km of the crust is elastic but the underlying region can deform slowly by visco elastic creep then stress differences of the order of 200 mpa can occur in the upper elastic crust by this process it is suggested that such a stress system rather than one arising from bending of the uparched crust or from plate boundary stresses may be the primary cause of the rifting

a stress province boundary and tractions on the north american plate stress provinces characterized by nearly constant directions and relative magnitudes of horizontal principal stresses are potentially informative of the forces acting on a lithosphere plate this applies most strongly to stress provinces which occupy a large part of a plate such as the mid continent stress province of north america identified by zoback and zoback1 we have used fractures known as breakouts in oil wells which are controlled by the in situ stress to show that this stress province extends through the western canadian sedimentary basin to the arctic coast there is a strong possibility that a uniform stress orientation with greatest compression ne sw is co extensive with the north american continent east of the rocky mountains excluding the appalachian and gulf of mexico stress provinces we further report new breakout orientations in the far northwest of canada which locate the boundary between the mid continent stress province and an alaskan stress province with largest compression nnw sse probably controlled by the subduction of the pacific plate under alaska

an early cambrian rift to post rift transition in the cordillera of western north america the upper proterozoic and lower palaeozoic wedge of miogeoclinal strata in the north american cordillera is widely regarded as evidence for a proto pacific passive margin1 3 the rifting history of this margin appears to have been protracted possibly spanning 200 myr in a tectonic setting that is not well understood3 quantitative subsidence analyses of lower palaeozoic strata between british columbia and utah however provide indirect evidence that the transition from rifting to post rift cooling occurred within a relatively short interval of time although probably not synchronously between 600 and 555 myr refs 4 6 this age is significantly younger than that implied in previous studies1 2 we describe here new field data which together with published geological data provide direct evidence of a latest proterozoic or early cambrian age for the rift to post rift transition the data support recent arguments for widespread initiation of passive margins around the edge of the north american craton close to the cambrian precambrian boundary7

statistical evaluation of mt and amt methods applied to a basalt covered area in southeastern anatolia turkey ilkisik o m and jones a g 1984 statistical evaluation of mt and amt methods applied to a basalt covered area in southeastern anatolia turkey geophysical prospecting 32 706 724 the efficacy of the magnetotelluric and audiomagnetotelluric mt amt methods for detailing the structure of a hypothetical geological section is investigated by using the singu lar value decomposition svd technique the section is representative of southeastern turkey which is mostly covered by basalt and is a prime area for oil exploration one of the geological units the germav shale at a depth of 600 m is a problem layer for electromagnetic surveys because of its very low resistivity on average 3 om and highly variable thickness across the area 200 900 m in the mt frequency range 0 0004 40 hz its total conductance or since its resistivity is known from resistivity log information its thickness is the best resolved model parameter the total depth to the germav shale and the resistivity of the cambrian precambrian basement are the marginally resolved parameters in the amt frequency range 4 10000 hz the resistivity of the surface basalt layer strongly affects the resolution of the other less important model parameters which are the total depth to the germav shale and the total conductance of the germav shale the errors in the measure ments determine the number of model parameters resolvable and are also important for interpretation of the geological model parameters to within a desired accuracy it is shown that statistical evaluation of the mt and or amt interpretations by using an svd factorization of the sensitivity matrix can be helpful to define the importance of some particular stage of the interpretation and also provides a priori knowledge to plan a pro posed survey arrangements of mt and amt observations together with some schlumber ger resistivity soundings on a large grid will certainly provide three dimensional detailed information of the deep geoelectric structure of the area

paleoseismic evidence for recurrence of earthquakes near charleston south carolina a destructive earthquake that occurred in 1886 near charleston south carolina was associated with widespread liquefaction of shallow sand structures and their extravasation to the surface several seismically induced paleoliquefaction structures preserved within the shallow sediments in the meizoseismal area of the 1886 event were identified field evidence and radiocarbon dates suggest that at least two earthquakes of magnitudes greater than 6 2 preceded the 1886 event in the past 3000 to 3700 years the evidence yielded an initial estimate of about 1500 to 1800 years for the maximum recurrence of destructive intraplate earthquakes in the charleston region

the west european stress field new data and interpretation abstract the mapped stress field of western europe reflects the tectonic process active there a traverse of stress measurements from the alps and through their northern foreland to the southern border of the lower rhine embayment identifies three distinct stress sub provinces the western alps the blocks on both flanks of the rhinegraben and the rhenish shield the alps have high magnitude stresses up to 35 mpa in the direction of maximum compression here called 1 h the general direction of 1 h is about 140 the foreland has the same directional trend of 1 h with a magnitude reduced to about 2 0 mpa local anomalies in magnitude and direction occur along the course of the rhinegraben which is a site of active sinistral shear the rhenish shield shows an internal zonation of the stress field the magnitudes of the stresses are low usually negative along the axis connecting the northern end of the rhinegraben with the rifting of the lower rhine embayment the direction there is about 150 on the eastern and western flanks of the shield the stress directions are essentially the same as in the southern blocks these zones are distant from the belt of active strain release consequently stresses of up to 4 0 mpa have accumulated