application of satellite magnetic anomaly data to curie isotherm mapping a boundary condition on temperature at depth in the continental crust can in principle be obtained by mapping of the curie isotherm where it forms the base of the magnetic crust an approach to mapping magnetic crustal thickness using satellite magnetic anomaly data is described because magnetic crustal thickness is small with respect to satellite altitude inversion of the anomaly data yields magnetization models expressing regional variations in the vertical integral of magnetization this parameter is inversely correlated with regional heat flow in the western united states suggesting that areas of low apparent magnetization are areas of thin magnetic crust resulting from a shallow curie isotherm for a simple model of a uniformly magnetized magnetic crust or intra crustal layer curie depth is linearly related to apparent magnetization if the relation can be calibrated by two or more curie depth estimates regional apparent curie isotherm configuration can be mapped in an alternative approach a family of curie depth models can be computed using assumed magnetization values in each case the curie depth configuration is used as a boundary condition in a thermal model that model is selected which gives a heat flow distribution best fitting observations conductive heat flow models for a traverse across the rio grande rift using a finite element technique and for a map grid using a one dimensional thermal model approximation are given as examples

contributions of cretaceous quiet zone natural remanent magnetization to magsat anomalies in the southwest indian ocean the magsat magnetic anomalies over the southwest indian ocean are modeled using a combination of induced plus viscous remanent magnetization im vrm and natural remanent magnetization nrm two broad roughly parallel sw to ne trending triple peaked positive anomalies dominate the region one lying south of africa and the other north of antarctica although these anomaly peaks generally correspond with the agulhas plateau maud rise mozambique plateau astrid ridge and madagascar ridge conrad rise conjugate pairs the im vrm contribution from structural characteristics i e crustal thickness accounts for only about 20 of the anomaly amplitudes a spatially variable but observationally constrained nrm contribution in cretaceous quiet zone kqz crust is required to account for the location shape and amplitude contrast of these anomalies many crustal features in the southwest indian ocean near antarctica have little geophysical data to constrain their structure but do have tectonic conjugates near africa for which much more geophysical data are generally available using geophysical and geological constraints from one member to model the magnetization structure of its conjugate reproduces the observed magsat reduced to pole anomalies over both structures very well this suggests that no significant alteration in their magnetization structure has occurred since the features split models of these conjugate structures show that im vrm reproduces the magsat anomalies associated with non kqz crust but that both im vrm and a dominant nrm component are required to explain the anomalies associated with kqz crust

decomposition decom approach applied to wave field analysis with seismic reflection records seismic common depth point cdp gathered data contain waves of various reflection phases other than the primary events of interest such waves often form distinctive separate peaks in the velocity spectrum derived from the cdp gather and their propagation modes can often be identified to exploit such separation and identification a method is developed to decompose the cdp gathered data into several phases the method combines normal moveout nmo removal space time filtering and nmo restoration the space time operator central to this method is shown to have several useful properties its frequency response passes one quadrant pair in the frequency wavenumber domain and rejects the other a set of marine reflection records is used to demonstrate the usefulness of the decomposition approach particularly for separating primary from nonprimary waves and waves of possible marine pssp mode from all others both examples may bear some exploration significance

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

coherent noise in marine seismic data three mechanisms that might produce noise patterns on stacked sections are examined direct and trapped waves that propagate outward from the seismic source cable motion caused by the tugging action of the boat and tail buoy and scattered energy from irregularities in the water bottom and sub bottom different noise patterns can be observed on shot profiles and common midpoint cmp gathers these patterns can be diagnostic of the dominant mechanism in a set of data field data suggest that the dominant noise is from waves scattered within the shallow sub bottom this type of noise is enhanced by cmp stacking moreover it can be as strong as surface wave noise on stacked land seismic data as well moveout filtering is best for suppressing the noise while preserving signal since the scattered noise does not exhibit a linear moveout pattern on cmp sorted gathers moveout filtering must be applied either to traces within shot records and common receiver gathers or to stacked traces our data example demonstrates that although it is more costly moveout filtering of the unstacked data is particularly effective because it conditions the data for the critical data dependent processing steps of predictive deconvolution and velocity analysis modified journal abstract

the bangui magnetic anomaly its geological origin satellite magnetometer data have revealed a long wave length magnetic anomaly in the vicinity of bangui central african republic substantiation of the anomaly has been made through analysis of air and ground magnetic surveys a correlative gravity anomaly has also been observed in ground gravity data the anomaly in an area of highly metamorphosed precambrian rocks does not correlate with any surficial features but is definitely crustal in origin an interpretation consistent with all available geophysical and geological data is that the anomaly could be the result of an early intrusion into the crustal rocks of this region followed by subsidence and deformation

automated editing and true amplitude stacking of seismic data lateral changes in recording conditions often require that trace amplitudes be balanced with consequent loss of information on the lateral amplitude variation of reflected energy we present a quality control and automated editing algorithm that recognizes source and geophone coupling problems and different noise levels along the survey line problem traces are discarded and true amplitude stacking of the remaining ones is possible with constant scaling factors application of the algorithm to one of the vancouver island lithoprobe profiles gives a nominal signal to noise improvement of 15 db and a better understanding of recording problems in the field our analysis shows that the varying strength of a reflection from near the top of the subducting juan de fuca plate cannot be explained by changing recording conditions alone results suggest that the extra effort involved in the automated optimization of common midpoint stacks of low signal to noise deep seismic data is warranted only if lateral amplit

magsat and pogo magnetic anomalies over the lord howe rise evidence against a simple continental crustal structure magsat average scalar and pogo reduced to pole magnetic anomaly data both show prominent positive signatures over the lord howe rise although generally assumed to be continental in nature the lord howe rise cannot simply be submerged ordinary continental crust which would produce a negative anomaly contrast with respect to the surrounding higher susceptibility oceanic crust three dimensional modeling of the plateau using the known crustal structure and assuming an induced origin for the satellite elevation anomaly leads to a model in which the lowest crustal layer in an otherwise continental crustal structure has an unusually high susceptibility in the range cgs 0 008 0 010 si 0 10 0 13 replacement or alteration of the lowest layer by a high susceptibility rock type may be related to the subsidence of the plateau

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting

satellite magnetic anomalies and the middle america trench abstract quantitative modeling of the middle america trench satellite elevation crustal magnetic anomaly using seismic epicenter data to constrain the slab geometry and a magnetization representing induced and viscous contributions similar to that found for the aleutian arc slab agrees well with reduced to pole magsat data incorporating minor contributions from crustal sources in mexico and central america produces only minor improvement in the agreement between the modeled and observed anomaly supporting the idea that it is the slab which is the major source body for the middle america trench anomaly

satellite and surface geophysical expression of anomalous crustal structure in kentucky and tennessee abstract an equivalent layer magnetization model obtained from inversion of long wavelength satellite magnetic anomaly data indicates a very magnetic source region centered in south central kentucky the magnetization maximum nearly coincides with a gravity high elongated north south and extending into tennessee previous refraction profiles suggest that the source of the gravity anomaly is a large mass of rock occupying much of the crustal thickness the outline of the source delineated by gravity contours is also discernible in aeromagnetic anomaly patterns taken together the geophysical data suggest a large localized mass of intracrustal rock which is both dense and very magnetic a simple magnetization density model is given which accounts for the gravity and long wavelength aeromagnetic anomalies due to the body we interpret it as a mafic plutonic complex and several lines of evidence are consistent with a rift association the body is however clearly related to the inferred position of the grenville front it is bounded on the north by the fault zones of the 38th parallel lineament the inferred mean magnetization 4 a m of the body is large but not inconsistent with values reported by others for deep crustal bodies associated with long wavelength magnetic anomalies such magnetization levels can be achieved with magnetic mineralogies produced by normal oxidation and metamorphic processes and enhanced by viscous build up especially in mafic rocks of alkaline character

geologic and geochemical evidence for the nature and development of the middle proterozoic keweenawan midcontinent rift of north america abstract general models of active continental rifting must take into account the large plateau basalt provinces many of which have been precursors of or associated with continental breakup the keweenawan middle proterozoic midcontinent rift mcr of north america 2300 km long is one such major continental rift but it aborted before significant crustal separation was achieved these plateau basalt rifts including the mcr differ from the east african type in being dominated by broad subsidence rather than rift valleys tholeiitic flood basalts rather than alkalic central volcanoes and large positive gravity anomalies geologic and geophysical evidence is reviewed which contradicts some recent models which have been proposed for the development of the mcr the basic causes of the pre rift subsidence the u shaped nature of the structure and the abrupt termination of its activity are obscure a triple junction in the usual sense does not appear to have been formed

initial scalar magnetic anomaly map from magsat a scalar magnetic anomaly map between 50 geographic latitude was derived from magsat data acquired during the november 1979 to june 1980 mission lifetime separation of anomaly fields from core and external fields was accomplished utilizing techniques developed for pogo data mayhew 1979 langel et al 1980 comparison with a similar map from pogo data shows that the basic anomaly patterns are essentially reproducible except in a few regions such as the atlantic and pacific at latitudes south of 15 increased resolution due to a lower measurement altitude is evident in the magsat map

the bulk magnetization contrast across the ocean continent boundary in the east coast of north america summary the intermediate wavelength 600 3000 km magnetic anomalies of earth derived from satellite data indicate that both continental crust and oceanic lithosphere are magnetic however no well defined and consistent magnetic anomaly is associated with the ocean continent boundary emphasizing that the bulk magnetization the vertically integrated magnetization contrast of oceanic lithosphere and continental crust is not appreciable an upper limit of 37000 a is estimated for the bulk magnetization contrast using the low altitude aeromagnetic and marine magnetic data from across the east coast of north america

european tectonic features observed by magsat abstract regional three dimensional magnetic models have been developed to characterize the principal european long wavelength magnetic anomalies represented on the improved magsat magnetic anomaly map of europe the magnetic models were constrained by regional variations in geology and geophysical parameters e g geologic boundaries crustal thickness heat flow because only limited measurements of magnetization are available on lower crustal and uppermost mantle rock samples our results are useful in constraining and understanding the overall magnetization of these regions illustrations of these include 1 geologic provinces across the tornquist teisseyre tectonic zone 2 regions of thin crust and high mantle heat flow in south central europe 3 the kursk voronezh magnetic anomaly and 4 the ladoga gulf of bothnia zone the region of the tornquist teisseyre tectonic zone that marks the boundary between the fennoscandian baltic shield and metastable europe is a major magnetic discontinuity in south central europe the regional magnetic variations appear to be directly related to variations in the lower crustal thickness and possibly also to heat flow in addition the famous kursk ukraine iron ore deposit produces a prominent bullseye anomaly at satellite altitude the kiruna magsat anomaly is modelled as having a large deep body as its source the high p wave velocity basal crustal layers encountered in rift e g the tornquist teisseyre tectonic zone itself and continental arc e g the ladoga gulf of bothnia zone settings of europe appear to be nearly non magnetic

crustal structure of the midcontinent rift system results from glimpce deep seismic reflection profiles interpretation of great lakes international multidisciplinary program on crustal evolution glimpce seismic reflection profiles indicates that the midcontinent keweenawan 1100 ma rift system of volcanic rocks and postvolcanic and interbedded sedimentary rocks extends to depths as great as 32 km about 10 5 s reflection time along profiles crossing western central and eastern lake superior and the northern end of lake michigan this area may overlie the greatest thickness of intracratonic rift deposits on earth times to moho reflections vary along strike from 11 5 to 14 s about 37 46 km depth in the west to 17 s about 55 km in the center and 13 to 15 s about 42 49 km in the eastern end of lake superior the prerift crust however was thinned 25 30 km beneath the central rift compared with its flanks providing evidence for crustal extension by factors of about 3 4 the midcontinent rift system differs from phanerozoic rifts in having total crustal thicknesses equal to or greater than the surrounding presumably unextended regions

origin and implications of two verwey transitions in the basement rocks of the vredefort meteorite crater south africa abstract two populations of magnetite exist in the shocked basement rocks of the vredefort meteorite impact crater one associated with original crustal genesis and metamorphism around 3 0 ga and the other related to the impact itself at 2 02 ga pre impact magnetite is mostly micron to millimeter in size lying within the multidomain to pseudo single domain range the second population of magnetite is less than 10 m in size and formed within the interstices of planar deformation features or within the reaction rims of biotite both of which were created during impact our study shows that each of these populations possesses specific verwey transition temperatures one around 124 k associated with pre impact magnetite and the other around 102 k associated with impact related magnetite the high temperature verwey transition is attributed to stoichiometric magnetite while the low temperature verwey transition to non stoichiometric magnetite pre impact rocks containing both verwey transitions are ubiquitous throughout the crater pseudotachylites formed during impact have a single verwey transition spanning temperatures from 94 to 111 k heating the basement rocks above 550 600 c for 3 min or above 500 c for 1 h irreversibly modifies the 124 k verwey transition by shifting it to lower temperatures based on these findings it is possible that no wholesale heating of the crater occurred above 550 600 c for 3 min or above 500 c for 1 h during or since the time of impact although some places of more localized heating are identified an unresolved problem remains to reconcile these data with temperatures thought to exist in the crust during and after impact

pre and poststack migration of glimpce reflection data abstract glimpce deep seismic reflection profiles across the midcontinent rift system beneath lake superior reveal a central asymmetric rift with an enormous thickness of volcanic and sedimentary rocks true amplitude cmp processing poststack and prestack migration and forward modelling are used to improve images of steeply dipping faults unconformities and other discontinuities in the deep seismic data with prestack migration important steeply dipping structural features of the lake superior data set are revealed for the first time improved images of high angle normal faults later reactivated as reverse faults provide key structural information for understanding the evolution of the rift system results illustrate that structural interpretations of complex deep reflection records such as those recorded by glimpce should always be based on migrated data furthermore depth migrated sections provide useful starting models for forward velocity modelling of seismic data

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

magsat equivalent source anomalies over the southeastern united states implications for crustal magnetization the magsat crustal anomaly field depicts a previously unidentified long wavelength negative anomaly centered over southeastern georgia examination of magsat ascending and descending passes clearly identifies the anomalous region despite the high frequency noise present in the data using ancillary seismic electrical conductivity bouguer gravity and aeromagnetic data a preliminary model of crustal magnetization for the southern appalachian region is presented a lower crust characterized by a pervasive negative magnetization contrast extends from the new york alabama lineament southeast to the fall line in southern georgia and eastern alabama coincident with the brunswick terrane the model calls for lower crustal magnetization contrast of 2 4 a m northern georgia and the carolinas are modeled with contrasts of 1 5 a m large scale blocks in the upper crust which correspond to the blue ridge charlotte belt and carolina slate belt are modeled with magnetization contrasts of 1 2 a m 1 2 a m and 1 2 a m respectively the model accurately reproduces the amplitude of the observed low in the equivalent source magsat anomaly field calculated at 325 km altitude and is spatially consistent with the 400 km lowpass filtered aeromagnetic map of the region

keweenaw hot spot geophysical evidence for a 1 1 ga mantle plume beneath the midcontinent rift system the proterozoic midcontinent rift system of north america is remarkably similar to phanerozoic rifted continental margins and flood basalt provinces like the younger analogues the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad asthenospheric thermal anomaly which the authors call the keweenaw hot spot great lakes international multidisciplinary program on crustal evolution seismic reflection profiles constrain end member models of melt thickness and stretching factors which yield an inferred mantle potential temperature of 1 500 1 570c during rifting combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material the isotopic signature of keweenawan volcanic rocks presented in a companion paper by nicholson and shirey this issue is consistent with the model of passive rifting above an asthenospheric mantle plume

paleomagnetism and upb geochronology of volcanic rocks from michipicoten island lake superior canada precise calibration of the keweenawan polar wander track abstract paleomagnetic poles have been determined for normally magnetized samples of late proterozoic volcanic rocks from 23 sites on michipicoten island lake superior canada the results are in general accord with an earlier less precise study by one of the authors and indicate a small polar shift between rocks of the mamainse point formation 186 e 38 n and the michipicoten island formation 175 e 26 n a quartz feldspar porphyry intruding the lower mamainse point formation but unconformably overlain by the michipicoten island formation was sampled for u pb zircon dating the zircons define an age of emplacement of 1086 5 1 3 3 0 ma this age and the pole positions are consistent with and supplement ages previously determined for the keweenawan paleomagnetic polar wander curve these data define an apparent polar wander rate of 14 cm a 1 this age combined with previously published u pb ages indicates a time span of 22 ma for most keweenawan igneous activity in addition the time of the keweenawan paleomagnetic reversal is constrained to be between 1086 5 1 3 3 0 and 1097 6 3 6 ma the age of the upper part of the reversely magnetized osler group the zircon population contained an inherited component from a source 1403 50 ma old this suggests that the porphyry magma was at least partly derived by melting of an anorogenic pluton emplaced during the mid proterozoic

paleomagnetism and orientation of precambrian dykes eastern lake superior region and their use in estimates of crustal tilting archean rocks form the eastern margin of the 1 1 ga old central north american rift along the eastern shore of lake superior and have been tilted westwards in response to rifting paleomagnetic and structural data from 2 6 ga old matachewan dykes suggest a westward crustal tilt of about 60 which agrees well with dips recorded in nearby keweenawan volcanics that rest directly on basement rocks the matachewan dyke swarm occurs throughout the east shore region of lake superior whereas keweenawan supracrustal sequences which give a more precise estimate of tilt are restricted to a few isolated shoreline patches estimates of crustal tilt can be obtained from the dykes on a regional basis thus generating a more complete picture of basement deformation adjacent to a major intracratonic rift

petrologic model of the northern mississippi embayment based on satellite magnetic and ground based geophysical data abstract a petrologic model of the northern mississippi embayment derived from gravity seismic and rift data is evaluated by converting the model to a magnetization model which is compared with satellite magnetic anomaly models a magnetization contrast of approximately 0 54 a m determined from the petrologic model of the embayment compares favorably to values of 0 62 a m and 0 45 a m from a magsat united states apparent magnetization contrast map and a published pogo magnetization contrast model respectively the petrologic model suggests that the magnetic anomaly low associated with the mississippi embayment may be largely due to the intrusion under non oxidizing conditions of low curie temperature gabbroic material at the base of the crust of the embayment near surface mafic plutons bordering the mississippi valley graben appear from aeromagnetic data to have higher magnetizations than the deeper gabbroic material however it is impossible to ascertain if this is due to compositional differences or similar material at shallower lower temperature depths these results indicate that variations in the curie temperatures of intrusions accompanying rifting may account for a large part of the wide range of magnetic anomalies associated with presently inactive rifts with normal heat flow

an equivalent layer magnetization model for the united states derived from magsat data long wavelength anomalies in the total magnetic field measured by magsat over the united states and adjacent areas are inverted to an equivalent layer crustal magnetization distribution the model is based on an equal area dipole grid at the earth s surface model resolution defined as the closest dipole spacing giving a solution having physical significance is about 220 km for magsat data in the elevation range 300 500 km the magnetization contours correlate well with large scale tectonic provinces

a satellite magnetic model of northeastern south american aulacogens magnetic modeling of the amazon river and takatu aulacogens in northeastern south america illustrate the utility of satellite magnetic data in characterizing the properties and structure of the lithosphere specifically reduction of preliminary magsat scalar magnetic anomaly data to an equivalent condition of vertical polarization shows a general correlation between magnetic anomaly minima and the amazon river and takatu aulacogens surface gravity data demonstrate a correlative positive anomaly spherical earth modeling of the magnetic data indicates a less magnetic crust associated with the aulacogens which is compatible with previous studies over the mississippi river aulacogen and rio grande rift in north america

model of the southeastern margin of the mississippi valley graben near memphis tennessee from interpretation of truck magnetometer data the geometry and magnetic properties of the southeastern margin of the mississippi valley graben ate studied by analyzing detailed magnetic anomaly profiles that are based on measurements made along roads near memphis tennessee the closely spaced measurements were obtained with a truck mounted magnetometer system developed by the u s geological survey modeling results suggest that the graben margins represent both structural boundaries and conduits for ascending magma about 2 km of vertical offset associated with normal faulting occurs within an interpreted 5 5 km wide zone in which magnetic basement has an average dip of 20 nw into the graben this zone may contain several large normal faults formed during a stage of rifting in late precambrian or early paleozoic time subsequent erosion of the scarps probably aided in forming the gentle northwestward slope of the buried precambrian surface across the fault zone the high apparent susceptibility about 3 10 3 emu of magnetic basement associated with this fault zone and with the upblock suggests either that ascending magma intruded the upblock or that the two blocks differed lithologically prior to the formation of the graben

crustal structure of the midcontinent rift system in eastern lake superior from controlled amplitude analysis of glimpce deep reflection seismic data the reprocessing of line f of the glimpce deep marine reflection seismic survey according to controlled amplitude principles provides new insights on the crustal structure of the midcontinent rift

analysis and use of magsat satellite magnetic data for interpretation of crustal structure and character in the u s midcontinent abstract magnetic anomaly data acquired by nasa s magsat satellite have been processed analyzed and interpreted in terms of causative crustal sources major structures tectonic development geologic provinces crustal composition and magnetic properties in the central u s midcontinent an optimum data set was developed by cleaning correcting and editing and then reduced to a 1 1 latitude longitude grid at a common altitude of 400 km then wavelength filtered and reduced to the magnetic pole the resulting satellite anomalies have a magnitude range up to 15 nt in the region the relationship between these long wavelength anomalies and crustal sources is investigated a magnetic high over kentucky tennessee is associated with more mafic and thicker lower crust and a high over texas oklahoma and trending northeast through missouri may be due to a geologic province and anomalously magnetic lower crust a magnetic low over the mississippi embayment aulacogen can be explained by crustal thinning and less magnetic crust and one over georgia by less magnetic lower crust the magnetic anomaly gradient pattern generally follows and reflects the southern ancient rifted continental margin of the north american craton and there is a lack of apparent magnetic expression of the central north american rift system midcontinent geophysical anomaly

imaging the midcontinent rift beneath lake superior using large aperture seismic data the authors present a detailed velocity model across the 1 1 billion year old midcontinent rift system mrs in central lake superior the model was derived primarily from onshore offshore large aperture seismic and gravity data high velocities obtained within a highly reflective half graben that was imaged on coincident seismic reflection data demonstrate the dominantly magic composition of the graben fill and constrain its total thickness to be at least 30 km strong wide angle reflections are observed from the lower crust and moho indicating that the crust is thickest 55 60 km beneath the axis of the graben the total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north above the moho is a high velocity lower crust interpreted to result from syn rift basaltic intrusion into and or underplating beneath the archean lower crust the lower crust is thickest beneath the axis of the main rift half graben a second region of thick lower crust is found approximately 100 km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting the crustal model presented here resembles recent models of some passive continentalmore margins and is in marked contrast to many models of both active and extinct phanerozoic continental rift zones it demonstrates that the moho is a dynamic feature since the pre rift moho is probably within or above the high velocity lower crust whereas the post rift moho is defined as the base of this layer in the absence of major tectonic activity however the moho is very stable since the large abrupt variations in crustal thickness beneath the mrs have been preserved for at least a billion years less