is the proterozoic ladoga rift se baltic shield a rift the southern part of the baltic shield hosts a series of mafic dykes and sills of mesoproterozoic ages including a ca 1 53 1 46 ga sheet like gabbro dolerite sills and the salmi plateau basalts from the lake ladoga region based on chiefly geochemical data the region is conventionally interpreted as an intracratonic ladoga rift graben we question the validity of this geodynamic interpretation by analyzing regional geophysical data crustal structure heat flow bouguer gravity anomalies magnetic anomalies and mantle vs velocities we provide a complete list of tectonic magmatic and geophysical characteristics typical of continental rifts in general and demonstrate that except for magmatic and perhaps some gravity signature the lake ladoga region lacks any other rift features we also compare the geophysical data from the lake ladoga region with similar in age midcontinent and valday rifts and provide alternative explanations for mesoproterozoic geodynamic evolution of the southern baltic shield we propose that mesoproterozoic mafic intrusions in southern fennoscandia may be associated with a complex deformation pattern during reconfiguration of a part of nuna columbia supercontinent which led to magma intrusions as a series of mafic dykes along lithosphere weakness zones and ponding of small magma pockets within the cratonic lithosphere consequent magma cooling and its partial transition to eclogite facies could have led to the formation of a series of basement depressions similar to intracratonic basins of north america while spatially heterogeneous thermo chemical subsidence with phase transitions locally speeded by the presence of subduction related fluids could have produced a series of faults bounding graben like structures

seismic images of the megathrust rupture during the 25th october 2010 pagai earthquake sw sumatra frontal rupture and large tsunami 1 the mentawai segment of the sumatra subduction zone is locked and likely to produce a large earthquake in the near future a part of this locked zone ruptured on 12 september 2007 producing twin earthquakes of mw 8 5 and 7 9 recently a third earthquake of mw 7 8 occurred on the 25th october 2010 sw of pagai island sumatra the earthquake generated an unexpected very large tsunami on pagai islands with run up height of up to 8 m here we present seismic reflection and bathymetry images from the 2010 epicentral region acquired before the earthquake we find that the frontal thrust is the main active fault in this region and might have ruptured up to the seafloor at 6 km water depth uplifting the water column and producing a large tsunami furthermore finite fault models indicate that this earthquake ruptured the frontal section of the subduction zone which is generally believed to slip aseismically and be incapable of producing large earthquakes the presence of aftershocks near the subduction front further confirms that the frontal section of the subduction zone is not aseismic if the rest of the mentawai locked zone ruptures the frontal section of the subduction zone during a megathrust then the resulting tsunami in the indian ocean could be devastating

possible resonance effect in the distribution of earthquake damage in mexico city the destruction caused by the earthquake of 19 september 1985 in mexico city shows three remarkable features1 first a concentration of damage in the former lake bed second a peculiar distribution of high and low damage areas alternating within a few city blocks and third a selectivity for buildings between five and fifteen storeys high while the last point is understood by civil engineers2 and the first confirms the relevance of the soft ground the second feature is the most puzzling to a physicist such a pattern evokes the idea of a standing wave with low and high damage areas corresponding to nodes and antinodes this suggests a resonance phenomenon of the waterlogged ground moving collectively from the navier equations of classical elasticity it is known3 that a transfer of the incoming seismic wave energy to longitudinal waves takes place if a sharp change from hard to soft ground exists solving then the two dimensional poisson equation for the longitudinal waves within the ancient lake boundaries we show that for the observed seismic wave period of 2 s a standing wave solution exists which explains qualitatively the damage distribution pattern

comparative precambrian stratigraphy and structure along the mid continent rift the mid continent rift is a geophysically identified tectonic structure that has been traced from kansas northeastward into the lake superior district a related arm has been identified by gravity as extending from eastern lake superior southeastward into the lower peninsula of michigan this rift is of precambrian keweenawan age and began developing approximately 1 1 b y b p for most of its extent this continental class rift system is buried below phanerozoic strata outcrops are found only in east central minnesota northwestern wisconsin and the upper peninsula of michigan the most accepted tectonic model of the mid continent rift is that of a central horst partly covered by clastic rocks bounded by high angle faulting and flanked by basins filled with clastic rocks this model was developed in conjunction with field studies in the southwestern lake superior area and generally has been adopted for minnesota iowa nebraska and kansas a comparable model has yet to be presented for the related arm extending into southern michigan in the lake superior outcrop belt the oldest recognized rifted rocks are layered basalts of middle keweenawan age overlying these basalts is an upper keweenawan thick sedimentary sequence composed of the oronto older and bayfield younger groups these sedimentary rocks are almost entirely clastic with texture ranging from conglomerate to shale mudstone rock colors are principally reds and browns with secondary shades of green and white the oronto group is more immature mineralogically and texturally and more complex structurally than the bayfield group the oronto group also is differentiated by one of its units the nonesuch formation which contains organic shales that bleed liquid hydrocarbons from subsurface fractures and vugs in a michigan copper mine conditions of eposition for both groups are closely allied with an aquatic continental environment specific paleogeography ranged from alluvial fan and plain to deltaic flood plain fluvial and lacustrine southwest and southeast of the lake superior outcrop belt stratigraphic relationships are known principally from fewer than a dozen significant wells drilled into the precambrian within the mid continent rift trend analysis of the proposed upper keweenawan red clastic sedimentary rock column within these wells suggests that the lake superior stratigraphy can be provisionally correlated throughout the mid continent rift trend

geophysical investigations and crustal structure of the north american midcontinent rift system abstract geophysical investigations have had a prominent role in studying the 1100 ma midcontinent rift system because of limited surface exposures and deep drilling of the rift rocks gravity and magnetic anomaly data delineate the rift system its central volcanic filled graben and overlying and adjacent post rift sedimentary basins and the major faults of the system over its 2000 km length in central north america crustal seismic studies indicate a high velocity crust that is thickened to as much as 55 km along the rift deeply penetrating seismic reflection data obtained across the rift show that the crust was thinned to less than one half of its original thickness during rifting and the present crust has been thickened by addition of up to a total of 30 km of mantle derived volcanic rocks within grabens and clastic sediments deposited in overlying sag basins as well as by intrusions into the crust and crustal underplating these data also confirm the occurrence of a post rift regional compressional event that locally caused broad folding of the rift rocks and reactivation of the graben normal faults as high angle reverse faults with vertical throws of several kilometers geophysical data indicate that the rifting event had a profound effect on the entire vertical range of the crust and that structures and thus the tectonic evolution are similar along the entire length of the rift

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

correlation of magnetic anomalies in east central minnesota and northwestern wisconsin constraints on magnitude and direction of keweenawan rifting similarity of east striking magnetic anomalies in east central minnesota and northwestern wisconsin on opposite sides of the midcontinent rift system implies that archean and lower proterozoic rocks in these areas are correlative and have been offset by middle proterozoic rifting the anomalies are displaced about 60 km along a direction perpendicular to the rift axis this distance is regarded as a minimum estimate of crustal spreading associated with rifting this model provides improved constraints on the magnitude and direction of crustal separation associated with the midcontinent rift system

reply to comment by j l mateos o novaro t h seligman and j flores on can horizontal p waves be trapped and resonate in a shallow sedimentary basin summary using a complete mathematical formulation we show that the trapping of horizontal p waves in a very soft shallow alluvial layer is a minor effect these waves do not have a stable way of propagation since in order to exist they require an incident wave and are therefore incapable of resonating in the lateral direction when confined in a basin of limited extent

pg shingles preliminary results from the onshore glimpce refraction experiment abstract in 1986 a series of near vertical seismic reflection lines and coincident seismic refraction lines were recorded across the great lakes in the glimpce experiment from a refraction point of view this experiment was unique as this was the first time in north america that long range refraction lines up to 350 km were recorded with a trace spacing of less than 100 metres the source of energy was a large air gun some preliminary results of the nature of the onshore recordings made by the university of western ontario are presented in this paper observations of many of the in line profiles show that the character of the pg crustal phase is not simple but has a shingle like pattern which may be caused by wide angle reflection effects from numerous long and short reflectors within the crust theoretical studies indicate that the amplitudes of the wide angle reflected signals from thin high velocity layers are in general much larger than those from thin low velocity layers supercritical reflected waves are not possible from the top of a low velocity layer the closely spaced traces show in minute detail the manner in which one travel time branch fades and the next one arrives the results seem to indicate that the pg phase in our data set has little direct wave component but instead is made up of a whole series of shingled or fish scaled wide angle reflection components each with successively larger apparent velocities these results are in agreement with the findings of the near vertical reflection component of the experiment

two dimensional model for site effect studies of microtremors in the san fernando valley a new model is developed for the resonant response of ground motion in enclosed sedimentary basins in their study of deeply embanked sediment filled valleys bard and bouchon 1980 a b 1985 show that reflections and refractions at the valley s edges generate important conversion of incident sv waves to horizontally propagating p waves which establish lateral resonance patterns our model is related to these results through the use of two approximations reflecting edges and horizontal p waves this model allows us to calculate lateral resonances in a given sedimentary basin for any shape of its borders we apply this model to the actual shape of san fernando valley and account for the large differential amplifications reported by kagami et al 1986 in their two dimensional study of microtremors in this basin we obtain a map of amplifications in the surface of the san fernando valley that shows a remarkable coincidence with the microtremor experimental data

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

resonant response models for the valley of mexico i the elastic inclusion approach summary the model developed by bard and bouchon to study the earthquake response of open ended 2 d sediment filled valleys with simple geometries is generalized to describe completely enclosed basins with arbitrary geometries by solving the basin form that corresponds to the valley of mexico it is possible to understand via a two step process model several of the observed features of the 1985 september 19 earthquake that devastated downtown mexico city here a general approach that allows the treatment of the particular seismic behaviour of the valley of mexico which is a large sediment filled valley containing an extremely soft ground urban area with a very high water content using collective resonance methods that vary from elastic to acoustic models is established this will lead through a series of papers to a theory that takes into account the role played by any valley s collective modes for microzonation and seismic damage predictions

resonant response models for the valley of mexico ii the trapping of horizontal p waves summary the strong ground motion recorded during the 1989 april 25 earthquake ms 6 9 shows many of the previously observed features in the soft lacustrine clays over which downtown mexico city is built a dominant response frequency around 0 4 hz enormous time prolongation of the seismic signal with characteristic beating phenomena and conspicuous differential amplification following the two step process suggested in part i seligman et al 1989 we add two new theoretical aspects the generation of horizontally propagating sp waves in the layered structure beneath mexico city for incoming sv waves beyond critical incidence and the subsequent lateral trapping of these sp waves in the lacustrine clays the time evolution of the lateral resonant pattern produced by these waves is calculated for the realistic geometry of the lake beds synthetic accelerograms and fourier spectra at different sites within the clay region are thus obtained our results can explain the monochromaticity and the long duration of the strong motion observed systematically in the soft clays of the valley of mexico

seismic history of minnesota and its geological significance abstract twelve earthquakes have been documented in minnesota in the last 120 yr the first nine were felt whereas the last three all in 1979 were detected instrumentally by a six element seismic array which has recently been put into operation estimated magnitudes range from 0 1 instrumental only to 4 8 with four of magnitude 4 3 or greater the highest intensity values were vi to vii depths where obtainable are estimated at 5 to 20 km the best documented event occurred on 9 july 1975 near morris minnesota with a magnitude of 4 6 a maximum intensity of vi and a felt area of 82 000 km 2 covering parts of four states the event was recorded to epicentral distances of at least 38 the epicenters show a clear relationship to tectonic features of the state four epicenters lie along the newly defined great lakes tectonic zone an east northeast trending belt extending across several states and into canada the zone separates 3 000 to 3 600 m y rocks of a gneissic terrane to the south from 2 700 m y rocks of a greenstone granite terrane to the north four other events lie on known major northwest trending faults in the greenstone granite terrane two and possibly three events are associated with the western margin of the midcontinent rift system

comment on can horizontal p waves be trapped and resonate in a shallow sedimentary basin by f j sanchez sesma and f luzon summary we show that most of the arguments in the above paper are either incorrect or irrelevant to the point the authors are trying to make we show that their results have no bearing on the model proposed by our group as they claim they discuss the seismic response of a valley with a 2 d trapezoidal cross section in a vertical plane whereas we dealt with a closed basin with a 2 d cross section but of arbitrary geometry and in the horizontal plane even more significantly the width of the valley they use is much smaller than the wavelength of the horizontal p waves that can resonate thereby precluding any possibility of them being trapped therefore their arguments do not clarify the issue posed in the title of their article

regional north american gravity and magnetic anomaly correlations summary correlation of gravity and magnetic anomalies combined with other geological and geophysical data is useful for enhancing the quality of geological interpretation of potential anomaly fields maps produced by equivalent point source inversion are used to investigate visual spatial correlations of surface free air gravity and pogo satellite magnetic anomalies and regional heat flow and tectonic data for north america and adjacent marine areas a quantitative analysis of regional potential field anomaly correlations at satellite elevations is also considered utilizing poisson s theorem in a moving window linear regression between derivatives of the anomalous gravity and magnetic fields an inverse relationship is observed between long wavelength gravity and magnetic anomalies over continental terrain negative gravity and positive magnetic anomalies are areas characterized by relatively thick crust and high magnetization an example is a prominent magnetic high which corresponds to a trend of gravity minima extending from the anadarko basin to the cincinnati arch negative magnetic and positive gravity anomalies characterize thinner crust and regions of higher heat flow such as the cordillera of north and central america and specifically the yellowstone geothermal region although gravity and magnetic anomalies over oceanic areas show poor correlation the sign of the statistical correlation generally is positive 1 crustal correlation considerations the utility of long wavelength gravity anomalies covering several degrees of surface area for geological analysis is well recognized magnetic anomalies which have significant energy in wavelengths of several hundred kilometres also have been identified as originating from within the earth s crust and perhaps the uppermost mantle the geological interpretation of these regional anomalies is hindered by the effect of anomaly superposition and source ambiguity which is inherent to the analysis of potential fields an approach to minimizing these limitations especially in continental terrains is to evaluate the correlation between anomalous gravity and magnetic fields the basis of correlation analysis is the hypothesis commonly validated in continental areas that variations in lithology and physical properties of the crystalline crust are reflected

comment on paleozoic reactivation structures in the appalachian ouachita marathon foreland far field deformation across pangea by j craddock d malone r porter j compton j luczaj a konstantinou j day and s johnston earth science reviews 169 2017 1 34

deep crustal structure of the precambrian basement beneath northern lake michigan midcontinent north america a deep seismic reflection profile in northern lake michigan midcontinent north america provides a cross section of the crust across the 1850 ma penokean orogen in which an early proterozoic island arc complex was deformed between two converging archean continental masses the island arc crust is about 40 km thick and has a few kilometres of intensely reflective rocks near its base above which it is variably reflective to transparent the archean terranes have thicker crust as much as 50 km the lower 20 25 km of which is strongly reflective abrupt offsets of moho near terrane boundaries may have been preserved since accretion during the penokean orogeny

tectonic setting of the midcontinent rift system abstract the continental lithosphere which hosts the midcontinent rift mcr system comprises a crustal assemblage ranging in age from 3 6 to 1 5 ga the most voluminous igneous activity along the mcr occurred within late archean 2 7 ga crust presumably due to magmatism associated with a rising mantle plume two major arms of the rift extend to the southeast and the southwest where rifting magmatism and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3 6 to 1 5 ga tensional reactivation of pre existing crustal discontinuities may have influenced the location of parts of the mcr however direct cause and effect are difficult to distinguish for want of sufficient exposures the most active phases of magmatism and rifting occurred between 1109 and 1087 ma a u pb age of 1097 ma for the southern end of the mcr shows that magmatic activity was synchronous throughout its length there is evidence of rifting and magmatism prior to 1109 ma but there is little evidence of magmatism after 1087 ma rifting may have effectively ceased by that time rifting may have ceased as a result of compressional forces being exerted on the foreland during the grenville collisional event spanning 1030 970 ma and compressional reactivation of normal faults may have resulted in partial inversion of the mcr chemical data and u pb and sm nd isotopic data for igneous rocks along the rift are similar to those for mcr rocks in the lake superior area which are best explained in terms of a mantle plume model however since the distal arms of the mcr were developed in proterozoic lithosphere and did not involve nearly as much magmatism as in the lake superior area mantle plume sources may not have been as important in formation of the distal magmas data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of proterozoic lithosphere during rifting

generation of inhomogeneous p waves in a layered medium abstract the generation of inhomogeneous horizontally propagating p waves in a stratified elastic medium were analyzed for incident sv waves beyond the critical angle using realistic values of s and p wave velocities and considering both clay and alluvial sedimentary layers overlying a bedrock half space we obtained enormous amplifications for the horizontal displacements at the free surface these amplifications are of the order of several thousands for the fundamental mode of the system which systematically dominates all other frequencies

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

consortium for continental reflection profiling michigan surveys reprocessing and results reprocessing of consortium for continental reflection profiling data from central michigan with emphasis on deconvolution and velocity filtering before stack and more detailed velocity analysis has resulted in greatly improved imaging of a previously reported keweenawan rift buried beneath the paleozoic of the michigan basin the seismic stratigraphy of the sub paleozoic units can now be related to the basin filling middle keweenawan volcanic sequence and the upper keweenawan clastic assemblage in greater detail important structural and stratigraphic features can be seen for the first time for example reprocessing has imaged an unconformity within the upper keweenawan it is possible some of these newly imaged features are associated with a late keweenawan compressive episode furthermore reprocessing also reveals previously unrecognized sub keweenawan structures including a deeply penetrating reflection which may represent a major crustal fault and possible weak reflections from midcrustal 25 km and moho 43 km depths

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

source location and valley shape effects on the p sv near displacement field using a boundary integral equation discrete wavenumber representation method summary we use a numerical simulation to investigate the response of sediment filled valleys to p sv seismic excitation the calculation is carried out using the discrete wavenumber boundary integral equation method we study in particular the case of explosion sources located within a basin or in its vicinity we show that large amplification or defocusing effects are related to the source location and to the shape of the basin in the case of complex basins made up of several sedimentary layers waves propagating along the various interfaces play an important role in shaping the seismic response of the basin

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

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

major proterozoic basement features of the eastern midcontinent of north america revealed by recent cocorp profiling cocorp profiling in the eastern midcontinent of north america has 1 traced an extensive sequence of precambrian layered rocks beneath southern illinois indiana and western ohio 2 detected a broad zone of east dipping basement reflectors associated with the grenville front beneath western ohio and 3 discovered a wide region of west dipping reflectors penetrating most of the crust beneath eastern ohio the precambrian layered assemblage may be as much as 11 km thick beneath southern illinois extends at least 170 km in an east west direction and contains several strong reflectors that have a lateral continuity of 80 km or more industry seismic data indicate that the layering is extensive in a north south direction as well possible explanations for the layering include the silicic igneous rocks of the ca 1 48 ga eastern granite rhyolite province which are penetrated by basement drill holes throughout the region perhaps intermixed or underlain by mafic igneous or sedimentary rocks the 40 50 km wide zone of strong east dipping 25 30 reflectors beneath west central ohio corresponds to the position of the grenville front as determined from potential field and drill hole data these dipping reflectors in the upper and middle crust are interpreted to result from ductile deformation zones mylonites like those exposed at the grenville front in canada and imaged on the glimpce seismic reflection lines in lake huron both the cocorp and glimpce lines show a remarkably similar reflection geometry despite the more than 500 km separating the two profiles easternmost ohio appears to be underlain by pronounced west dipping 40 reflectors in the middle and lower crust which are also interpreted as marking a region of pervasive ductile deformation 80 km or more in width analogy with similar reflection packages elsewhere suggests that these reflections may mark a major collision zone the west dipping reflectors may be correlative with similar reflectors imaged on another cocorp survey in northern alabama the correlations suggested by these new results though tentative imply that the eastern midcontinent is composed of a relatively simple assemblage of crustal blocks

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

a magnetotelluric transect of the jacobsville sandstone in northern michigan eight magnetotelluric soundings were conducted across a basin of the jacobsville sandstone along a northwest southeast line beginning north of toivola michigan northwest of the keweenaw fault and ending near the village of keweenaw bay in michigan9s upper peninsula the soundings have provided an estimate of the thickness and resistivity of structures in the jacobsville sandstone as well as the resistivity stratigraphy and structure of the underlying strata one and two dimensional models indicate a northwestward thickening of the jacobsville sandstone a faulted basement uplift the keweenaw fault and a deep conductor at the southeast end of the profile

a new isostatic residual gravity map of the conterminous united states with a discussion on the significance of isostatic residual anomalies to display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust a new colored isostatic residual gravity map of the conterminous united states has been prepared using the gravity data set compiled for the gravity anomaly map of the united states society of exploration geophysicists 1982 the new isostatic residual gravity map is based on an airy heiskanen model of local compensation in which the surface load requiring compensation is defined by 5 min topographic and bathymetric data sets a colored first vertical derivative map of isostatic residual gravity further enhances the short wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field for the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features anomalies on isostatic residual gravity maps should not be casually interpreted in terms of undercompensation or overcompensation because large amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies we present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment in general highs correlate with intruded or accreted mafic material or with upthrusted crustal sections whereas lows occur over low density sedimentary or volcanic sections felsic intrusive bodies or down warped crustal sections although some longer wavelength anomalies such as the broad gravity high centered over montana could be manifestations of density contrasts deep in the mantle many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers so that their source bodies need not be deep the fact that certain of these broader anomalies have well defined boundaries which correlate with near surface geologic features increases the likelihood that their sources lie entirely within the lithosphere if so then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions we have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the united states these fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories

can horizontal p waves be trapped and resonate in a shallow sedimentary basin summary it has been proposed recently that horizontal p waves can be trapped and resonate laterally in a shallow sedimentary basin in this work it is shown both theoretically and numerically that in a sedimentary basin of finite lateral extent trapped horizontal p waves if they exist are not a stable way of propagation and therefore cannot resonate

structure of the southern keweenawan rift from cocorp surveys across the midcontinent geophysical anomaly in northeastern kansas this is the published version copyright 1984 american geophysical union all rights reserved

was the midcontinent rift part of a successful seafloor spreading episode the 1 1 ga midcontinent rift mcr the 3000 km long largely buried feature causing the largest gravity and magnetic anomaly within the north american craton is traditionally considered a failed rift formed by isolated midplate volcanism and extension we propose instead that the mcr formed as part of the rifting of amazonia precambrian northeast south america from laurentia precambrian north america and became inactive once seafloor spreading was established a cusp in laurentia s apparent polar wander path near the onset of mcr volcanism recorded by the mcr s volcanic rocks likely reflects the rifting this scenario is suggested by analogy with younger rifts elsewhere and consistent with the mcr s extension to northwest alabama along the east continent gravity high southern appalachian rocks having amazonian affinities and recent identification of contemporaneous large igneous provinces in amazonia

surface and near surface effects on seismic waves theory and borehole seismometer results a simple plane wave model is adequate to explain many surface versus borehole seismometer data sets using such a model we present a series of examples which demonstrate the effects of the free surface near surface velocity gradients and low impedance surface layers on the amplitudes of upcoming body waves in some cases these amplitudes are predictable from simple free surface and impedance contrast expressions however in other cases these expressions are an unreliable guide to the complete response and the full plane wave calculation must be performed large surface amplifications are possible even without focusing due to lateral heterogeneities or nonlinear effects both surface and borehole seismometer site responses are almost always frequency dependent ocean bottom versus borehole seismic data from the 1983 ngendei seismic experiment in the southwest pacific are consistent with both a simple plane wave model and a more complete synthetic seismogram calculation the borehole seismic response to upcoming p waves is reduced at high frequencies because of interference between the upgoing p wave and downgoing p and sv waves reflected from the sediment basement interface however because of much lower borehole noise levels the borehole seismometer enjoys a p wave signal to noise advantage of 3 to 7 db over nearby ocean bottom instruments

flexural modeling of the midcontinent rift a basement profile obtained from seismic reflection data has been used to constrain a two dimensional flexural model of basin formation for the midcontinent rift at a latitude of 45 sup 0 25 n model parameters included the thickness of the elastic plate the basin width and the maximum basin thickness modeling suggests that flexure produced a deep narrow basin along the rift axis and that the crust was thinned at the time of basin formation to an elastic thickness of 9 6 km for a plate ruptured by rifting or 5 7 km for an unbroken plate with corresponding maximum basin thicknesses of 14 km and 16 km respectively the plate thickness depends most strongly on the basin width and is well constrained by the seismic data although erosion may have narrowed the basin the maximum basin thickness is poorly constrained because of the lack of seismic data for depths greater than about 10 km and because the strata at the center of the rift have been disturbed by a postrift compressional event which produced the st croix horst despite uncertainty about the basin thickness the load required to flex the crust to produce the midcontinent rift basin is too large to be attributed tomore the weight of the central flood basalts unless the basin subsided into a fluid less dense than the solidified basalts on the basis of seismic refraction data and by analogy with other rifts we hypothesize that a magnetic rift pillow intruded in the lower crust the basaltic pillow subsequently solidified to produce a large high velocity region in the lower crust centered under the rift axis as determined from deep seismic refraction this crystallization and cooling may be responsible for the sag phase of rift evolution as evidenced by laterally widespread occurrence of postvolcanic sediments copyright american geophysical union 1988 less

rift structure beneath the michigan basin from cocorp profiling cocorp consortium for continental reflection profiling surveys across the mid michigan gravity anomaly near the mcclure sparks no 1 deep well delineate a thick layered sequence of relatively strong reflectors that define a structural trough within the precambrian beneath the michigan basin these deep reflections which correlate spatially with the mid michigan gravity and magnetic high are interpreted to be a pile of basaltic extrusive rocks intercalated with clastic rocks and correlative with keweenawan volcanics exposed in the lake superior region to the northwest a reflection poor zone between the base of the paleozoic as found in the deep well and the top of the deep layered sequence probably corresponds to the upper keweenawan clastic assemblage direct evidence of faulting within the inferred volcanic sequence is lacking although sharp flexures structural benches and dips of as much as 10 suggest subsidence controlled by fault block motions reflections from beneath the inferred volcanic pile are rare the deepest clear event being at about 18 km there is no unequivocal evidence on these sections to indicate whether precambrian structures were reactivated to affect the overlying paleozoic strata of the michigan basin proper although data quality may be insufficient to detect such effects

a geophysical profile of the southern margin of the midcontinent rift system in western lake superior a 70 km long seismic reflection profile in western michigan provides new insight into the nature distribution and structure of the keweenawan supergroup volcanic and overlying sedimentary rocks and the controversial keweenaw fault along the southern boundary of the midcontinent rift system in the lake superior basin interpretation of the 5 s reflection data constrained by surface geology magnetics and gravity modeling shows that volcanic rocks which cropout north of the keweenaw fault dip northerly to depths of the order of 17 km south of the fault volcanic rocks overlain by 2 km of clastic sedimentary rocks thin gradually to the south as they dip upward at a shallow angle to the outcrop in the south range the volcanic pile within the basin thickens rapidly to the north of the keweenaw fault suggesting that the volcanics were deposted in an extensional fault bounded basin clear evidence of normal faulting is not present in the seismic reflection data because of a later compressional event the thickness of the volcanic filled basin implies that the upper crust was almost completely broken during the rifting event the available evidence is interpreted to show the keweenaw fault as a moderate to high angle reverse fault that occurs within the volcanic pile and breaks through to the surface along the abrupt change in thickness of the volcanic sequence there is no evidence from the seismic profiling for major faulting except for the keweenaw fault intrusions or folding of the keweenawan supergroup in this region

model of the midcontinent rift system in northeastern kansas recent drilling of the midcontinent rift system in northeastern kansas reveals a transposed keweenawan stratigraphy of mafic volcanic rocks overlying thick clastic sedimentary rocks a reprocessed version of cocorp kansas line 1 indicates that the drillhole penetrated a series of west dipping reflectors associated with a reverse fault bounding the east side of the rift basin reanalysis of the original cocorp line also reveals a west bounding reverse fault and evidence of crustal thickening this information is integrated with gravity and magnetic data to define a model of the rift in northeastern kansas the model consisting of an asymmetric basin bounded by reverse faults with thickened crust beaneath the rift is similar to models proposed for northern segments of the rift system and argues for homogeneity in the structural style and tectonic evolution along the length of the rift

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