isotopic and ree studies of lunar basalt 12038 implications for petrogenesis of aluminous mare basalts we report sr nd and sm isotopic studies of lunar basalt 12038 one of the so called aluminous mare basalts a precise internal rb sr isochron yields a crystallization age of 3 35 0 09 ae and initial87sr 86sr 0 69922 2 2 error limits 1ae 109 years 87rb 0 0139ae 1 an internal sm nd isochron yields an age of 3 28 0 23ae and initial143nd 144nd 0 50764 28 present day143nd 144nd is less than the chondritic value i e nd 0 2 3 0 4 where nd is the deviation of143nd 144nd from chondritic evolution expressed as parts in 104 at the time of crystallization nd 3 2ae 1 5 0 6 we have successfully modeled the evolution of the sr and nd isotopic compositions and the ree abundances within the framework of our earlier model for apollo 12 olivine pigeonite and ilmenite basalts the isotopic and trace element features of 12038 can be modeled as produced by partial melting of a cumulate mantle source which crystallized from a lunar magma ocean with a chondrite normalized ree pattern of constant negative slope chondrite normalized la yb 2 2 for this hypothetical magma ocean pattern a plot of i sr versus nd for the apollo 12 basalts clearly shows the influence of varying proportions of olivine clinopyroxene orthopyroxene and plagioclase in the basalt source regions a small percentage of plagioclase 5 in the 12038 source apparently is responsible for low i sr and nd in this basalt aluminous mare basalts from mare crisium luna 24 and by inference mare fecunditatis luna 16 occupy locations on the i sr nd plot similar to that of 12038 implying that some basalts from three widely separated lunar regions came from plagioclase bearing source regions a summary of model calculations for mare basalts shows a record of lunar mantle solidification during the period when ree abundances in the lunar magma ocean increased from 20 chondritic to 100 chondritic although there is a general trend from olivine to clinopyroxene dominated source regions with progressive magma ocean evolution significant mineralogical heterogeneities in mantle composition apparently formed at any given stage of evolution as evidenced in particular by the three apollo 12 magma types

the tectonic development of the western margin of the gulf of elat aqaba rift abstract along the western coast of the gulf of elat a 30 km wide shear belt of subparallel faults trending n s to ne sw is developed this shear belt is observed mainly within the precambrian basement terrain sinistral movements on these faults have been recognized based on offsets of magmatic bodies and lithological contacts in rocks of precambrian age the cumulative displacement measured independently at several localities across the belt attains a total of 24 km early neogene volcanism in the form of long dikes trending nw se is known from the eastern desert of egypt of the gulf of suez and along the red sea coast of saudi arabia volcanic bodies of a similar pattern have been identified in eastern sinai these bodies mainly dolerites and basalts intruding mesozoic sediments are parallel to each other and to the gulf of suez they are tens to hundreds of kilometers long and spaced several tens of kilometers apart dating k ar of bodies in sinai saudi arabia and in the eastern desert of egypt suggest an age of 20 22 m y for this extensive volcanism all the dikes are sinistrally displaced by the individual faults of the eastern sinai shear belt the amount of movement recorded using the precambrian markers is identical to movement recorded by offsets of the early neogene dikes this proves that the total 24 km shear of this fault system postdates the 20 22 m y volcanic phase it is assumed that the recorded movements are part of the 105 110 km shear persumed to exist along the gulf of elat which is also younger than 20 22 m y preliminary results suggest that a similar amount of movement is observable along a shear zone developed on the eastern coast of the gulf of elat thus only some 60 km movement are taken up by faults in the gulf itself the evolving model suggests that 1 a the intrusion of very long basaltic dikes in a nw se direction on both sides of the red sea accompanied the initial stage of development of the red sea gulf of suez rift valley 2 b a basic change in the geotectonic regime occurs after the intrusion of this volcanic suite large scale horizontal movements are initiated being recorded as a 105 110 km sinistral movement along the arava jordan rift this movement implies a synchronous opening of the red sea

deep crustal structure and flexure of the arabian plate beneath the zagros collisional mountain belt as inferred from gravity observations the zagros mountain belt region a neogene continental collisional zone between arabia and iran that is characterized by a suture a folded belt and a foreland basin is unusual among active mountain belts in that its 6 12 km thick sedimentary cover is shortening predominantly by folding while the basement is apparently thickening along numerous high angle reverse faults we have used over 9000 gravity measurements in the region in conjunction with limited seismological observations and geological cross sections of the uppermost 10 km of crust to infer the deeper crustal structure beneath the region crustal models constrained by the bouguer anomalies and other seismological and geological data are characterized by a moho that dips about 1 to the northeast beneath the folded belt and increases in dip to about 5 near the main zagros thrust mzt the moho depth increases from 40 km beneath the leading edge of the foreland basin the mesopotamian foredeep and persian gulf to as much as 65 km beneath the mzt alternative models that incorporate a subducted oceanic crust that is attached to the underthrusting arabian crust deviate little in their moho configurations from the above simpler models negative isostatic residual anomalies are interpreted to indicate local overcompensation beneath the foreland basin and near the folded belt and beneath the mzt we show that underthrust sedimentary rocks of the converging margins along the suture zone may account for the negative isostatic residual anomalies near the mzt however to explain the rest of the isostatic anomalies requires other forces to be acting on this collisional plate boundary for example elastic flexure models required a combination of topography and subsurface loads or downward force the origin of which is not clear to approximate the bouguer or isostatic residual anomalies associated with the foreland basin and the folded belt the topography load of the zagros mountain belt is insufficient to cause the required deflection of the underthrusting arabian plate a combination of isostatic elastic flexure and horizontal compression forces acting on the edge of the arabian craton and the transitional lithosphere of central iran appears to best model the crust of the zagros region we propose a model in which the lower crust of the converging arabian plate located beneath the zagros is being shortened and thickened plastically between the seismogenic rigid upper crust and a rigid uppermost mantle lid the mantle lid probably decoupled from the lowermost crust un der thrusts the iranian crustal blocks thus confined from below the horizontally compressed plastic lower crust hydraulically depresses the moho and raises the faulted and folded brittle upper crust in isostatic equilibrium

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

magnetotelluric studies in and adjacent to the northumberland basin northern england abstract during the past decade broadband magnetotelluric mt soundings with d c resistivity soundings at some sites have been undertaken in three separate field studies in and around the northumberland basin a region of great interest to earth scientists on account of the proposed location there of the iapetus suture as a result of an increase in cultural noise during this period the data from the last two studies have been processed using a new robust constrained impedance tensor estimation program the resulting apparent resistivity and phase data from these studies together with those from the first broadband study and some earlier mt responses from the region have now all been modelled using an interpretative modelling procedure new information has been provided by the mt models on basement depths and by integrating these new estimates with those from gravity modelling and seismic studies both on land and offshore a detailed basement topography map has been compiled for the region the deep eletrical resistivity structure has been modelled along a nw se traverse from the weardale granite of the alston block across the northumberland basin to the southern uplands of scotland underlying the more conductive sedimentary rocks the basement rock is found to have resistivities which range from about 100 m in the northumberland basin to more than 1000 m in the alston block and probably of the same order in the southern uplands a mid crustal conductor exists along the whole traverse which is well resolved and has a southward dip beneath the weardale granite under the northumberland basin the conductor is less well resolved and thus an apparent northward dip can only be regarded as tentative comparison of the pseudo 2d and full 2d models resulting from this study and from earlier mt and magnetovariational mv studies in southern scotland with new mt and joint mt and mv inversions of livelybrooks et al phys earth planet inter 81 67 84 1993 for a subset of the same response estimates reveals both comparable and disparate features the features common to the models presented in this paper and at least one of the inversions of livelybrooks et al include differences in the resistivity and thickness of the upper crust in each of the alston block the northumberland basin and the southern uplands and a conducting zone at mid crustal depths which vary along the traverse from about 10 km near the southern edge of the northumberland basin to more than 30 km under the alston block and about 30 km in the southern part of the southern uplands the main difference between the pseudo 2d and the 2d forward models and the 2d inversions of livelybrooks et al is in the resistivity of the alston block the former being more than 1000 m whereas the latter is about 400 500 m a value which is unexpectedly low for a crust of known granitic composition

migration of the seismic aseismic transition during uniform and nonuniform extension of the continental lithosphere we investigate the thermal and seismic implications of two extreme configurations of lithospheric extension one extreme treats the extension as being approximately uniform with depth the upper crust extending by the rotation of fault bounded blocks above a diffusely deforming lower crust in the other extreme crustal extension occurs only above a low angle fault dip less than about 10 very low angle shear zones that initially move aseismically within a uniformly extending lithosphere may be raised through the depth range that is normally seismically active without cooling appreciably the condition for this to occur is that extension should take place faster than about 10 14 s in contrast extension accommodated entirely above a very low angle shear zone rapidly brings that zone to temperatures at which it should be seismically active even if initial movement on it were aseismic the different thermal histories implied by the two configurations of extension offer the possibility of distinguishing between modes of extension by observations of temperature time and pressure temperature paths on rocks close to low angle faults

a geoelectric section across ireland from magnetotelluric soundings abstract a magnetotelluric survey across ireland has been completed the 40 sites were positioned on a line normal to the predominant caledonian geological trend i e ne sw of the country the average site separation was 5 km and frequencies from 640 to 0 01 hz were sampled bostick niblett and occam inversions of the data suggest that the carboniferous limestone of the main part of the profile is underlain by ordovician volcanic rocks in a block and trough structure with a wavelength of 15 km this supports previous gravity and magnetic interpretations at greater depths which vary significantly along the traverse an anomalous mid lower crustal conductor is apparent this conductor is probably a complex manifestation of the caledonian tectonic zone known as the iapetus suture in this respect it is significant that comparable conducting zones of similar geometry have previously been found in the suture region in southern scotland and northern england

conrad deep a new northern red sea deep origin and implications for continental rifting a previously unknown deep here called conrad deep was discovered during an extensive geophysical survey of the northern red sea in june 1984 conrad deep is located at 27 03 n 34 43 e only 90 km south of the gulf of suez and is the most northern deep yet discovered in the red sea it is located within a well developed axial depression which also contains charcot deep 100 km to the south the axial depression is associated with abundant recent deformation and is situated at the peak of a regional heat flow high extending across the rift conrad deep is typical of the small northern type red sea deep it is 10 km long 2 km wide and has a maximum depth of 1460 m it is associated with high and variable heat flow values and large magnetic anomalies there is no evidence of a dense brine layer detailed analysis of the geophysical data implies that the deep probably results from a very recent

recent studies of time variations of natural electromagnetic fields in scotland abstract a series of geomagnetic induction studies has been undertaken in scotland since 1973 it includes the operation of two geomagnetic arrays one over northern scotland and the other over southern scotland subsequent individual station and small scale geomagnetic array studies and three sets of magnetotelluric soundings which traverse scotland from the lewisian foreland to the english border the problems associated with the interpretation of induction data from an island located in the subauroral region are discussed qualitatively and the manner in which both coast and source field effects can be accounted for is described the geomagnetic deep sounding data gds from all the observation sites have been collated and examples of hypothetical event contour maps and traverses across the great glen and of individual events from the northern array are presented they indicate that significant lateral variations in electrical conductivity structure within the crust and upper mantle are associated with the major geological faults in the region examples of the results of the magnetotelluric soundings are also presented together with an outline of the procedure used for one and two dimensional modelling models of the geo electric structure in both northern and southern scotland have been obtained these show distinctive features which are compatible with the qualitative interpretation of the magnetovariational data for example the major granitic blocks are highly resistive while regions of relatively low resistivity exist at upper crustal depths near the great glen highland boundary and southern uplands faults a zone of low resistivity exists at lower crust uppermost mantle depths throughout much of the region the lowest value occurring under the southern uplands

separation of local and regional information in distorted gds response functions by hypothetical event analysis summary electromagnetic investigations are usually intended to examine regional structures where induction takes place at a given period range however the regional information is often distorted by galvanic eiects at local conductivity boundaries bahr 1985 and groom bailey 1989 developed a physical distortion model for decomposing the mt impedance tensor based upon local galvanic distortion of a regional 2 d electromagnetic eld we have extended their method to predict the magnetic variation elds created at an array of sites the magnetic response functions at periods around 1000 s may be distorted by large scale inhomogeneities in the upper or middle crust in this period range the data measured by a magnetometer array contain common information that can be extracted if the data set is treated as a unit for example by using hypothetical event analysis with this technique it is always possible to recover the regional strike direction from distorted data even if a strong spatially varying regional vertical eld component is present in the data set the determination of the regional impedance phases on the other hand is far more sensitive to deviations from the physical distortion model the approach has been used to investigate the iapetus data set for the array which covers an area of 200 km 300 km in northern england southern scotland the technique revealed a common regional strike azimuth of ca n125 0 e in the period range 500 2000 s this direction diiers from the strike indicated by the induction arrows which seem in uenced mainly by local current concentrations along the east weststriking northumberland trough and a ne sw striking mid crustal conductor both impedance phases are positive and diier by ca 1 0 0 which supports the assumptions of distortion elds in the data set and that the regional structure is 2 d

causes of high crustal conductivity beneath the iapetus suture zone in great britain summary two magnetic variation anomalies in southern scotland and northern england have been linked to the position of the iapetus suture previous magnetotelluric measurements along a 140 km profile that crosses both anomalies have been supplemented by new high quality broad band observations groom bailey decomposition of the impedance and hypothetical event analysis of the magnetic variation data agree in defining different strike directions for the two halves of the mt profile n50 e for the northern part over the southern uplands and n90 e for the southern part over the northumberland trough te and tm mode apparent resistivity and phase data at representative frequencies have been inverted for the two sub profiles separately using the smooth inverse of smith booker 1990 the uppermost layer has a low resistivity is variable in thickness and correlates well with the known position and thickness of carboniferous sedimentary rocks the second layer has a very high resistivity thousands of m and reaches the surface where the lower palaeozoic metamorphic rocks of the southern uplands crop out a relatively rapid transition to low resistivities occurs at depths of between 8 and 16 km the conducting layer appears to be quasi continuous but where the profile crosses the anomalies identified by magnetic variation measurements the conductance increases and the upper surface is shallower the spatial coverage of the magnetic variation data has enabled us to extrapolate conductive features away from the line of section and project the electrical image onto the nec vertical incidence seismic reflection profile in the north sea there is excellent agreement between a number of features in the acoustical and electrical images the shallowing of the low resistivity layer to form a narrow wedge like feature corresponds to the offshore position of the stublick fault while to the north of the fault the top of the layer coincides with a south dipping reflector thought to be a thrust however the zones of high conductance and high lower crustal reflectivity do not in general correlate the good conductor beneath he northumberland trough spans two zones which were differentiated on ihe nec profile in terms of their reflectivity the shape of the conductor s upper surface in the vicinity of the stublick fault agrees well with the model of chadwick holliday 1991 who proposed that the iapetus suture was a whole crustal shear with a gently dipping central ramp the coincidence between the present day low resistivity layer and a surface of weakness that was active 300 400 myr ago is much more readily explained in terms of mineralogy the presence of graphite than the presence of fluids there is also strong support for interpreting the northern conductor in the same way its upper surface is relatively flat and occupies the position predicted as the horizontal detachment surface over which wedges of the southern uplands rocks were thrust its northeastward extension is sampled by a group of carboniferous vents from which xenolith suites of crust and upper mantle origin have been obtained the middle to upper crust is believed to be represented by quartzo feldspathic gneisses that contain abundant graphite

basement control on the location of strike slip shear in the southern uplands of scotland in the sw part of the southern uplands terrane the moniaive shear zone has been superimposed above a geophysically defined basement discontinuity deformation was a protracted event commencing in the early wenlock and ending with granite intrusion at about 392 ma at surface the shear zone separates contrasting suites of granitic plutons these have age diverences and isotopic characteristics indicative of a lower crustal and possibly mantle break coincident with the crustal discontinuity deduced from geophysical modelling it is proposed that sinistral reactivation of the basement structure caused the development of the shear zone within the overlying allochthonous thrust belt this would require the basement structure to have been established beneath the developing southern uplands terrane by the wenlock

effects of finite rifting times on the development of sedimentary basins abstract most thermo mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post rift development of the basin this assumption greatly simplifies the mathematical treatment but is not in accord with what is found in nature where 10 to 50 m y rifting events commonly accompany the formation of sedimentary basins and continental margins the effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer this technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced the effect of a finite duration extension event is that heat is lost during rifting increasing the syn rift subsidence at the expense of the post rift lateral heat flow which was not included in previous studies of the effect of finite rifting times has a significant effect on the subsidence history distribution of sediments and thermal history in particular the post rift subsidence is decreased by more than 25 for a 20 m y rifting event and by more than 10 15 for a rifting event as short as 10 m y this will significantly decrease the subsidence rates in the post rift stage and implies that inferences concerning the structure development and thermal history of the basin derived from using curves to interpret backstripped subsidence can be greatly in error variations in syn rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors such as the flexural response of the lithosphere to sediment loading to affect the final width of the basin the total amount of sediments that accumulate and the basin stratigraphy

subsurface structure of se scotland from broadband magnetotelluric measurements abstract broadband 1000 0 01 hz magnetotelluric mt soundings have been undertaken at 22 sites in se scotland twelve of these were located in a 12 km 6 km array near duns where previously a deep resistivity sounding had been undertaken using the habberjam square array technique these sites lie to the se of an mt traverse which crossed both the southern upland fault and the previously determined southern uplands magnetovariation anomaly the results from the duns array have been used for a comparison of the resistivity and mt sounding techniques and for extension southward of the 2d modelling of the traverse data in addition several 1d mt inversion procedures have been implemented including one recently developed from the zohdy resistivity inversion method no significant differences were found between the models resulting from application of these different algorithms the advantages of close station spacing and broadband soundings are clearly demonstrated by the duns study in which static shift at one site and downward biasing at another have been easily detected comparison of the electrical sounding methods demonstrates that apart from the usefulness of tensorial resistivity soundings for studies of the surficial layers electrical resistivity structure can be obtained more easily and with much greater depth and lateral resolution by means of broadband mt soundings an interesting feature in the electrical resistivity structure in the duns area is the presence of a good conductor at about 1 km depth at the most easterly sites this requires more extensive study further examination of the resolution of the published 2d model for the mt traverse across the southern uplands fault confirms the presence of a conducting zone at shallow depth 3 5 km in the neighbourhood of the fault the similarity in electrical resistivity structure along this traverse and along subsequent parallel traverses in central scotland and in ireland is noted

submarine rifting at mid ocean ridges an analytical model for the formation of mid ocean ridges is presented in which the central rift zone is interpreted as a belt of localized extension of width 2w centred on the ridge axis upwelling of the asthenosphere occurs passively below the zone of extension at the rate required to conserve mass at distances greater than w from the ridge axis the lithospheric plates move at a uniform horizontal velocity u0 the model is thermally and mechanically consistent and predicts the surface heat flow and topography across the rift zone plus the vertical thermal structure and the thickness of the lithosphere below the zone of extension measurements of ridge heat flow and topography can be used to constrain estimates of the width of the zone of intrusion this width may be as large as 100 km comparable to the thickness of the lithosphere and that of mantle plumes as proposed in some models of mantle convection a comparison of the thermal structure at a ridge crest as produced passively by horizontal extension and actively by a mantle convection cell shows a qualitative similarity this similarity illustrates the difficulty in distinguishing cause and effect with regard to plate formation and motion

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

a geophysical approach to the granite batholith under the eastern southern uplands scotland from our interpretation of the bouguer gravity and aeromagnetic anomalies in south east scotland we conclude that a massive granite batholith underlies the greater part of the eastern southern uplands the granite model which we computed earlier from gravity anomalies in the tweeddale area fits the observed magnetic anomalies closely if a normal magnetization of 0 095 a m 1 is assigned similar to values found for exposed local granites further gravity modelling shows that apart from the tweeddale boss the granite shallows to less than 1 km near lammer law in east lothian and extends north of the lammermuir fault a model for the east lothian volcanics was computed from their aeromagnetic anomalies then their gravitational effect was combined with that estimated for the devonian and carboniferous sediments and the result stripped off the observed gravity field the residual gravity anomalies were used to generate a two dimensional model for the granite north of the lammermuir fault the expected tectonic consequences of a massive granite batholith in the eastern southern uplands are compared with the known development of faults and sedimentary basins around its margins

a high resolution magnetotelluric survey of the iapetus suture zone in southwest scotland summary magnetic variation mv surveys show a 40 km wide zone of high electrical conductivity in the middle crust extending across the southern uplands of scotland from the north sea to galloway a new magnetotelluric mt survey across the conductor in galloway was designed to improve the resolution of structure in the depth range 1 15 km and at the same time assess the improvement in resolution that could be achieved in a practical situation by optimizing data acquisition processing analysis and modelling a 40 km long profile was measured doubled for part of its length at a separation of 1 km with a typical site spacing of 2 km or less it was supplemented by shorter parallel and orthogonal lines giving a total of 40 sites galvanic distortion models involving regional 2 d structures gave poor fits to the impedance tensors both for the array as a whole and for individual sites confirming the picture from induction arrow maps that many sites were influenced by 3 d structures on scales of a few kilometres and upwards other indicators of the electrical strike regional and local mv measurements the spatial structure of the groom bailey regional impedance phase and of the rotated off diagonal phase gave values more in agreement with the geological strike of n52 e which was adopted as the most appropriate coordinate system into which to resolve the data the mt impedances were accordingly rotated into directions n38 w tm and n52 e te and inverted using the 2 d code of rodi mackie the investigation explored the sensitivity of the outcome to a wide range of starting model resistivities and roughness parameters and to different data subsets the te mode data were insensitive to structural detail and the best fit achieved was unacceptable suggesting the influence of structure outside the plane of the section the tm mode data could be fitted satisfactorily by varying the model roughness and gave the best control on the geometry of the conductive bodies forward modelling showed that the model generated from the tm mode also fitted the mv data adequately the inversion resolved the conductive zone into distinct blocks with edges matching faults mapped in the surface geology the leadhills fardingmullach and orlock bridge faults and the moffat valley lineament in the resistivity image the faults are vertical structures disrupting other features to a depth of 15 km which agrees with the view that although they originated as thrusts they were reactivated by strike slip motion during oblique closure of the iapetus ocean within some of the blocks are highly conductive regions no more than 4 km wide the most likely explanation is graphite or other metallic mineralization localized in shear zones

surficial structures of the northern red sea axial valley from 23 n to 28 n time and space evolution of neo oceanic structures abstract during recent french cruises in the northern red sea the surficial structures of the axial valley were studied by means of bathymetry magnetics and single channel reflection seismics new compilation maps have been established and comparison of the various data allow different segments of the northern red sea axial valley to the distinguished where only intra evaporitic deformations 24 25 n large volcanic outcrops mabahiss deep 25 30 26 n or several recent isolated volcanic intrusions 26 27 n characterize different stages of evolution this new information allows reconstruction of the evolution of oceanization cells from a simple intra evaporitic deep to a limited oceanic rift zone such as those observed in the central red sea a major characteristic of this evolution also depicted in other young oceanic rifts is a succession of volcanic and tectonic phases the major event in the northern red sea axial valley is a recent episode of voluminous but well localized magmatism it would represent a new stage in the red sea evolution and thus may have some implications on earlier phases

evidence from the northern red sea on the transition from continental to oceanic rifting abstract the northern red sea is an active rift in the last stages of continental rifting which is beginning to undergo the transition to oceanic seafloor spreading the rift can be divided into two distinct regions the marginal areas consist of a series of bathymetric terraces and as indicated by geophysical data are underlain by a series of rotated fault blocks the axial depression which forms the axis of deep water is a continuous region 10 to 25 km wide near the center of the sea which is characterized by high heat flow a large gravity minimum and very deformed sediments both regions are divided along strike into segments roughly 60 km long by cross trending transfer zones the presently active tectonics are characterized by the concentration of extension and deformation in the axial depression and by the emplacement of a number of large intrusions the intrusions occur at distinct sites located at the intersection of transfer zones with the faults bounding the rotated blocks in the marginal areas and at deeps located in the axial depression halfway between transfer zones although extension is becoming concentrated in the axial depression geophysical data imply that at least the upper few kilometers of the crust in that region consists of non magnetic continental type material and that this ca apace has been ruptured by large intrusions only at the deeps a wide range of crustal seismic velocities has been observed in the northern red sea including velocities normally associated with oceanic crust the velocity variations appear spatially related to corresponding large variations in basement rock type observed on shore in the eastern desert of egypt the northern red sea is in an intermediate stage between a continental rift valley and a young ocean basin and delineation of its structure and tectonics allows a model to be developed for the evolution of a rifted continental margin rifting starts in a series of linked half grabens 50 60 km long and formed over detachment surfaces which probably penetrate no deeper than the mid or lower crust this style of rifting can accommodate at least the amcunt of extension represented by the gulf of suez 35 km at some greater amount of extension the rift acquires a more symmetric shape probably through the development of antithetically faulted blocks in the hanging wall extension in this stage is still distributed throughout the rift and is accommodated in the upper crust by rotation of fault blocks at the stage represented by the northern red sea extension becomes concentrated in an axial depression and begins to be partially accommodated by large intrusions which form deeps the position of the deeps is controlled by the transfer zones established in the earliest stages of rifting with further extension the deeps serve as nucleation points that develop into small discrete cells of seafloor spreading these cells grow and coalesce to form a continuous mid ocean ridge spreading center such as is found in the southern red sea

thermal and mechanical evolution of u s atlantic continental margin abstract the postrifting tectonic subsidence of the u s atlantic continental margin appears to be due to thermal contraction following continental breakup this subsidence is greatly increased by the effect of sediment loading models for the response of the lithosphere to loads show that the rigidity of the lithosphere is a strong function of temperature hence the flexural rigidity at the u s margin should vary in both space and time the thermal structure and subsidence of the margin has been modelled using a two dimensional extension model where the amount of extension has been allowed to vary across the margin and lateral conduction of heat has been included the equivalent elastic thickness of the lithosphere has been approximated as the depth to an isotherm thermoelastic effects of the cooling lithosphere as well as the flexural loading of the sediments have been included the lateral flow of heat and flexure have contrasting effects the lateral heat flow tends to cause uplift of the coastal plain region landward of the hinge zone and to increase the subsidence rate in the regions of the transitional crust in addition the effects of lateral heat flow modify the thermal history of the sediments and heat flow flexure on the other hand causes subsidence of the coastal plain and decreases the sediment accumulation beneath the transition zone the stratigraphy of the u s margin is seen to be the result of the complex interaction of these effects as their contributions vary through the history of the margin end of article last page 1671

lateral variations of conductivity structure across southern scotland and northern england abstract high frequency 100 0 01 hz magnetotelluric mt soundings have been carried out along a 140 km profile across southern scotland and northern england where previously longer period soundings had been undertaken examples of the results of the processing of data from 15 sites along the profile and of the application of a number of 1d inversion procedures are presented the latter include the joint inversion of d c resistivity and mt data at rookhope where borehole stratigraphy is also available although the data were severely contaminated by near field sources at some sites and the station spacing was too large in parts of the traverse for good resolution of model parameters by incorporation of results from the earlier studies it is possible to obtain a preliminary 2d model of the electrical resistivity structure of the upper crust in this region significant differences have been found in the three main geological zones crossed by the survey the alston block the northumberland basin and the southern uplands of scotland in the discussion special attention has been paid to four sites in the neighbourhood of the southern uplands fault for which the data have been subjected to tensor decomposition this has revealed two dominant regional azimuths one of which dominates at the shorter periods and corresponds to the strike of the southern uplands fault and the other at longer periods which corresponds to the strike of the well known southern uplands magnetovariational mv anomaly moreover in this region the electrical resistivity models indicate the presence of a highly conducting zone beginning at a depth of about 4 km at station cap which is located on the axis of the mv anomaly and extending to a depth of at least 25 km the depth to this conducting zone increases both to the north and the south of the mv anomaly axis resulting in a structure which has many features in common with those along parallel traverses in se scotland and in ireland in discussing the implications of the irish model another researcher proposed either metamorphosed sediments and or serpentinised island are crust as being possible sources of the high conductivity and other geophysical and geological data the probability that the arguments in that study are equally applicable to southern scotland is currently under consideration

inversion of electromagnetic induction data for the iapetus suture zone in the uk abstract the iapetus suture zone isz corresponding to the join between the european crust of northern england and the north american derived crust of scotland has been examined by numerous seismic and induction experiments over a 20 year period the university of edinburgh electromagnetic em induction group is in the process of building a database comprising magnetotelluric and magnetic variation stations from this region in this paper we present 2d inversions of selected induction sites along roughly north south traverses across the isz in northern england and southern scotland the first inversion involves both the transverse electric te and transverse magnetic tm modes for 10 audio magnetotelluric approximately 100 0 1 hz sites care has been taken to remove the effects of static shifts this inversion resolves a conductive feature beginning under the southern northumberland trough at depths of 10 km dipping to the south below the alston block in the second inversion 12 longer period magnetic variation sites were added to the original data set improving the resolution of this feature which correlates well with a dipping bright reflector detected by the north east coast nec seismic reflection experiment in addition the southern uplands anomaly is imaged no north dipping mid to deep crustal conductor is observed which would correlate to north dipping palaeozoic subduction under laurentia