the effects of 10 to 160 gpa shock on the magnetic properties of basalt and diabase hypervelocity impacts within the solar system affect both the magnetic remanence and bulk magnetic properties of planetary materials spherical shock experiments are a novel way to simulate shock events that enable materials to reach high shock pressures with a variable pressure profile across a single sample ranging between 10 and 160 gpa here we present spherical shock experiments on basaltic lava flow and diabase dike samples from the osler volcanic group whose ferromagnetic mineralogy is dominated by pseudo single domain titano magnetite our experiments reveal shock induced changes in rock magnetic properties including a significant increase in remanent coercivity electron and magnetic force microscopy support the interpretation that this coercivity increase is the result of grain fracturing and associated domain wall pinning in multidomain grains we introduce a method to discriminate between mechanical and thermal effects of shock on magnetic properties our approach involves conducting vacuum heating experiments on untreated specimens and comparing the hysteresis properties of heated and shocked specimens first order reversal curve forc experiments on untreated heated and shocked specimens demonstrate that shock and heating effects are fundamentally different for these samples shock has a magnetic hardening effect that does not alter the intrinsic shape of forc distributions while heating alters the magnetic mineralogy as evident from significant changes in the shape of forc contours these experiments contextualize paleomagnetic and rock magnetic data of naturally shocked materials from terrestrial and extraterrestrial impact craters this article is protected by copyright all rights reserved

bayesian time lapse inversion a new fast inversion approach for time lapse seismic data is developed where the uncertainty of the inversion results is an integral part of the solution the inversion method estimates changes in the elastic material properties of a reservoir because of production of hydrocarbons including uncertainty bounds on these estimates the changes in elastic properties then can be related to changes in hydrocarbon saturation and reservoir pressure by using rock physics relations the inversion operates directly on the difference between a repeat survey and a baseline survey this is advantageous with respect to the uncertainty calculation because an estimate of the seismic uncertainty can be obtained directly from the difference data in zones not affected by production the method is formulated in a bayesian setting and the solution is represented by explicit expressions for the posterior expectation and the covariance of the elastic parameter changes the explicit analytical form of the posterior distributi

the mesoproterozoic copperwood sedimentary rock hosted stratiform copper deposit upper peninsula michigan copperwood is located in gogebic county upper peninsula michigan within the porcupine mountains copper district it was discovered in 1956 and in 2008 orvana minerals corp began exploration and development work with production projected in the near future stratiform copper mineralization is hosted by gray to black shales and siltstones of the basal nonesuch formation copper mineralization is overlain by laminated gray siltstones of the nonesuch formation and is underlain by red bed sandstones conglomerates and minor siltstones of the copper harbor formation these rocks are part of a thick section of clastic sedimentary rocks that comprise the upper fill of the mesoproterozoic 1 1 1 0 ga midcontinent rift the copper deposit is a tabular body that averages 2 5 m thick lacks significant basal undulations and dips 8 to 12 northward on the southwest limb of the western syncline an open fold with a shallow plunge to the northwest chalcocite is the only cu bearing mineral in the orebody and occurs as concentrations along stratigraphic laminations and as disseminated grains within the orebody there is no mineral or metal zoning cu ag and s are strongly enriched zn is very slightly enriched and pb and co lack enrichment gangue minerals in the orebody are quartz clinochore muscovite illite plagioclase k feldspar calcite and hematite which are consistent with temperatures of less than 130 c only one significant fault a shallow nw dipping reverse fault with displacement of up to 7 m has disturbed the section the mineralized zone at copperwood is stratigraphically equivalent to that at the white pine mine but copperwood lacks the structural complexity and the hydrothermal overprint manifested by the native cu mineralization at white pine reported canadian national instrument 43 101 compliant measured indicated and inferred resources are 33 million metric tonnes mt of 1 60 cu and 4 1 ppm ag for total contained cu of about 1 165 billion lb and 4 32 moz of ag the characteristics of copperwood are consistent with those of reduced facies or kupferschiefer type sedimentary rock hosted stratiform copper deposits the proposed genetic model is chalcocite replacement of pyrite in unlithified sediments during diagenesis as a result of emanating upward focused compaction driven cu bearing saline basinal waters with cu leached from the underlying red bed paleoquifer the simplicity of the mineralizing event and the undeformed nature of the deposit distinguish copperwood from most other examples of reduced facies or kupferschiefer type sedimentary rock hosted stratiform copper deposits throughout the world

sedimentological and palyno environmental appraisal of the late quaternary sediments north eastern bornu basin abstract an integrative study has been carried out on some quaternary sediments of the chad formation in tuma 1 and sa 1 wells they are located between 11 n and 13 43 38 n 8 21 49 and 14 40 22 e sedimentological study entailed textural analysis and thin section preparation on the disaggregated samples while the palynological study involves the recovering of the age diagnostic species which gave clues to the paleoenvironmental settings for these sediments the sediments consist predominantly of terrigenous and argillaceous mudstone with thin sandstone interbeds of point bar deposits of laterally migrating moderate to high sinuosity river channels the dominant sandstones are angular to subrounded moderately poor sorted fine modal mineralogically and texturally immature the presence of botryococcus braunii freshwater algae suggests a freshwater lacustrine environment

factorial correspondence analysis a useful tool in palaeogeographical reconstructions example from late cretaceous calciturbidites of the northwestern external rif morocco

the midcontinent rift in the lake superior region with emphasis on its geodynamic evolution abstract the midcontinent rift is a middle proterozoic continental rift which records about 15 m y of extension subsidence and voluminous volcanism in the period 1109 1094 ma in the central part of north america during that time the crust was nearly totally separated and as much as 25 km of subaerial basalts accumulated in a deep central depression following extension and volcanism a longer period of subsidence resulted in development of a post rift sedimentary basin in which as much a 8 km of fluvial and lacustrine clastic rocks were deposited partial inversion of the central depression occurred about 30 50 m y after extension to produce the current configuration of a central horst composed mostly of thick volcanic accumulations between shallower flanking basins

the sonju lake layered intrusion northeast minnesota internal structure and emplacement history inferred from magnetic fabrics abstract the sonju lake intrusion sli in northeastern minnesota is a layered mafic complex of keweenawan age 1096 1 0 8 ma related to the midcontinent rift the cumulate paragenesis of the intrusion is recognized as broadly similar to the skaergaard intrusion a classic example of closed system differentiation of a tholeiitic mafic magma the sli represents nearly closed system differentiation through bottom up fractional crystallization geochemical studies have identified the presence of a stratabound 50 100 m thick zone anomalously enriched in au pge similar to the pge reefs of the skaergaard intrusion this pge enriched zone is hosted within oxide gabbro cumulates about two third of the way up from the base of the intrusion we present a petrofabric study using the anisotropy of magnetic susceptibility ams to investigate the emplacement and flow patterns within the sonju lake intrusion petrographic and electron microprobe studies combined with ams and hysteresis measurements indicate the primary source of the magnetic signal is pseudo single domain psd magnetite or titanomagnetite low field ams was measured at 32 sites within the sonju lake intrusion which provided information about primary igneous fabrics the magnetic fabrics in the layered series of the sonju lake intrusion are consistent with sub horizontal to inclined emplacement of the intrusion and show evidence that the cumulate layers were deposited in a dynamic environment well aligned magnetic lineations consistently plunging shallowly toward the southwest indicate the source of the magma is a vertical sill like feeder presumably located beneath the finland granite the finland granite acted as a density trap for the sonju lake magmas forcing lateral flow of magma to the northeast the strongly oblate magnetic shape fabrics indicate the shallowly dipping planar fabrics were enhanced by compaction of the crystal mush

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

the igneous petrology and magmatic evolution of the midcontinent rift system klewin k w and shirey s b 1992 the igneous petrology and magmatic evolution of the midcontinent rift system in p a ziegler editor geodynamics of rifting volume ii case history studies on rifts north and south america and africa tectonophysics 213 33 40 the 1100 ma midcontinent rift system of north america is one of the world s great igneous provinces containing over one million km3 of mafic volcanic and plutonic rocks volcanic rocks range from picrite to rhyolite although tholeiitic basalt predominates plutonic rocks range from peridotite to granite with plagioclase rich gabbros and troctolites being most abundant the igneous history of the rift can be divided into early and late stages flows of the early stage form a trend having low alumina high magnesia picrite as the primitive endmember whereas flows from the late stage form a trend having high alumina olivine tholeiite as the primitive endmember the early basalts have enriched nd isotopic compositions and were derived largely from old heterogeneous subcontinental lithosphere the late basalts have chondritic to depleted nd isotopic compositions and were derived from large degree partial melts of a mantle plume that had mixed with isotopically depleted asthenospheric mantle apparently melting began initially in the subcontinental lithosphere as it was stretched subsequently a lower mantle plume which had entrained asthenospheric mantle during its ascent rose beneath the rift and melted at large degrees to yield the voluminous later basalts

petrology of the keweenawan mamainse point lavas ontario petrogenesis and continental rift evolution the mamainse point formation forms a 5250 m thick sequence of flood basalts ranging from high mgo picrites and basalts to basaltic andesites that formed in the keweenawan 1100 ma midcontinent rift of north america this section is the most continuous and complete sequence of keweenawan lavas and records almost the entire igneous history of the rift the section consists of numerous stratigraphic groups of lavas that have distinct chemical characteristics a variety of petrogenetic processes including partial melting of different sources at decreasing depth periodically replenished tapped and fractionated magma chambers crustal contamination and polybaric fractionation were responsible for the evolution of the magmas the dominant petrogenetic process changes with stratigraphy which suggests that the physical evolution of the rift controlled which process operated at any time the initial fracturing of the crust presumably allowed high mgo liquids generated at depth to be erupted the early stage of rifting was apparently slow and allowed the formation of long lived magma chambers in which complex processes such replenishment mixing and assimilation contamination could occur the late stage of rifting however was apparently vigorous and allowed voluminous magmas from throughout the crust to be erupted these conclusions from the chemostratigraphy agree with the physical stratigraphy which indicates a change in tectonic style from an early stage of broad subsidence to a late stage of active graben formation

the chengwatana volcanics wisconsin and minnesota petrogenesis of the southernmost volcanic rocks exposed in the midcontinent rift the southernmost exposed rocks of the north american midcontinent rift system 1100 ma consist of 3000 m of mafic volcanic flows and minor interflow sediment exposed along the st croix river in minnesota and wisconsin the flows are mostly high fe tholeiitic basalt with plagioclase phenocrysts and ophitic to subophitic clinopyroxene abundant secondary chlorite epidote and actinolite indicate the group was metamorphosed to greenschist facies 350 c low sodium m4 site and tetrahedral aluminum aliv contents of actinolite indicate low pressure metamorphism 0 25 gpa and imply a geothermal gradient of 45 50 c km low magnesium mg 0 37 0 58 and ni contents 36 185 ppm indicate the basalts have undergone significant fractionation and are not primary mantle melts incompatible element abundances are inversely correlated with mg and most samples plot within either high or low trace element groups e g ti p zr the basalts are enriched in the light rare earth elements and th and are v

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

chemostratigraphy of the neoproterozoic alona bay lavas ontario a basal sequence of flood basalt lavas associated with the neoproterozoic midcontinent rift system crops out in alona bay along the southeastern shore of lake superior in ontario the alona bay lava succession is about 1200 m thick and lies just north of the well studied contemporaneous mamainse point formation detailed chemostratigraphy of the alona bay lavas suggests they are grossly correlative with the basal portion of the mamainse point formation for instance like the basal part of mamainse point formation the alona bay section contains numerous high mgo lavas and can be subdivided into 4 5 groups with distinct chemical characteristics chemical variations within the alona bay groups are largely the result of fractional crystallization likely at moderate pressures one small group of alona bay lavas also carries the compositional imprint of crustal contamination the remaining inter group chemical distinctions at alona bay are the consequence of temporal changes in partial melting and source ch

displacement transfer from borders to interior of a plate a crustal transect of iberia abstract a n s crustal transect of the whole iberian plate is presented the displacement transfer during the alpine deformation from the border ranges cantabrian and betic chains toward the interior ones was analyzed by means of balanced crustal sections inconsistencies in shortening values between upper and middle lower crust in the cantabrian mountains basque cantabrian zone imply a mid crustal detachment transferring orogenic shortening southward toward the spanish central system depending on the shortening value attributed to the central system this mid crustal detachment could involve the entire iberian plate connecting the cantabrian mountains and the betic chain the very small crustal root in the central system can be explained as a result of the tectonic load caused by the upper crustal shortening that was transferred from the border ranges we conclude that a simple shear model rather than a pure shear with lithospheric folding better explains the whole iberia deformation finally the total alpine shortening of the iberian plate is estimated between 267 and 292 km

reviewing the configuration and extent of the midcontinent rift system abstract uncertainty exists in the configuration and extent of the midcontinent rift system mrs because of deficiencies in geophysical data and limited information from outcrops and basement drill holes additional ambiguity is caused by misunderstanding the definition of continental rifts six principal problematic regions in mapping the mrs are described gravity and magnetic data supported by drill hole and seismic reflection data show that the eastern lake superior rift segment of the mrs continues south from lake superior and connects to a much narrower rift in northern lake michigan the eastern margin of this transition is ill defined because of the lack of definitive anomalies and supporting seismic and drill hole data but is interpreted to occur near the u s canada border the rift segment in southeastern michigan intersects the grenville front and likely continues eastwards in modified form to near the boundary with canada cross cutting gravity and magnetic signatures may reflect grenvillian overthrusts near the terminus of the mrs in michigan the proposed southerly extensions of both branches of the rift system into oklahoma and ohio are based primarily on positive gravity anomalies but neither postulated extension appears to be associated with rifted troughs rather the gravity anomalies of the western branch are related to intrusive mafic rocks and those of the eastern branch are most likely related to deep crustal metamorphic rocks thrust into juxtaposition with less dense crust by grenville orogenesis recent paleomagnetic investigations in conjunction with high resolution radiometric dating imply that the mrs developed during the rapid southward movement of laurentia during a quiescent period along its eastern continental margin massive magmatic activity accompanying the rifting was likely due to rising mantle material that was displaced by subducted lithosphere along the southern margin the heated crust was made more ductile fostering rifting due to extensional stresses the nipigon embayment remains as a possible candidate for an early third branch of the mrs but current evidence is insufficient to include the fort wayne rift as part of the mrs future studies of the mrs would be well served by new age dating and high resolution seismic studies of the lithosphere

carbonate submarine fan facies along a paleozoic prograding continental margin western united states abstract mass transport deposits though common in carbonate basins normally occur as widespread sheets or debris wedges as in the devonian of canada and elsewhere or in modern interplatform troughs of the bahamas reexamination of a seaward prograding cambrian and ordovician continental margin section in central nevada reveals a 150 m thick interval whose facies resemble current models of submarine fan deposition the upper 100 m of sediment is assigned to an inner fan setting and is characterized by submarine slides and several entrenched channels 10 m deep and 500 m wide the channels are filled with disorganized boulder bearing conglomerates but are not arranged in any well defined thinning and fining upward sequences stratigraphically below this interval are thinning and fining upward organized pebble to cobble bearing channelized conglomerates 30 to 50 m thick these channels are 1 to 5 m deep 20 to 100 m wide and rapidly coalesce laterally and vertically the conglomerates grade la erally into and are interbedded with thin and discontinuously bedded ripple laminated and graded calcarenites similar to detrital overbank levee and interchannel deposits these sedimentary units probably represent a system of braided channels in a mid fan position below the braided channels are 10 to 20 m of thickening and coarsening upward cycles of virtually nonchanneled beds beds in the cycles are composed of calcarenites exhibiting bouma divisions these carbonate sand sequences are interpreted to represent prograding outer fan lobes the recognition of carbonate submarine fan sequences raises several questions 1 what sedimentologic and tectonic conditions are conducive to fan development in carbonate provinces 2 do these conditions resemble those for clastic fan development or do carbonate provinces have unique requirements by recognizing carbonate submarine fans and the geologic conditions that control their sediment dispersal patterns areas of maximum sediment accumulation may be predicted as an aid in exploring for petroleum reservoirs of deeper water carbonate environments end of article last page 913

age of native copper mineralization keweenaw peninsula michigan amygdaloidal flood basalts and conglomerates are the host for substantial deposits of native copper within the portage lake volcanics in the keweenaw peninsula of michigan a wide variety of gangue minerals are associated with the regional hydrothermal alteration mineralization event application of the rb sr method to amygdule filling microcline calcite epidote and chlorite suggests an age of mineralization between 1 060 and 1 047 m y or asymp 20 m y these results are supported by a fission track age on epidote of 1 044 or 169 m y the age of native copper mineralization determined in this study is consistent with geologic evidence which suggests that mineralization postdated the deposition of most or all of the overlying freda sandstone variable initial 87 sr 86 sr ratios between the low rb sr phases suggest mixing of isotopically distinct sources of sr during the generation and equilibration of the hydrothermal solutions

tectono stratigraphic evolution of the early proterozoic wisconsin magmatic terranes of the penokean orogen the early proterozoic penokean orogen developed along the southern margin of the archean superior craton the orogen consists of a northern deformed continental margin prism overlying an archean basement and a southern assemblage of oceanic arcs the wisconsin magmatic terranes the south dipping niagara fault suture zone separates the south facing continental margin from the accreted arc terranes the suture zone contains a dismembered ophiolite the wisconsin magmatic terranes consist of two terranes that are distinguished on the basis of lithology and structure the northern pembine wausau terrane contains a major succession of tholeiitic and calc alkaline volcanic rocks deposited in the interval 1860 1889 ma and a more restricted succession of calc alkaline volcanic rocks deposited about 1835 1845 ma granitoid rocks ranging in age from about 1870 to 1760 ma intrude the volcanic rocks the older succession was generated as island arcs and or closed back arc basins above the south dipping subducti

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

estimation of lava extrusion and magma production rates for two flood basalt provinces utilizing the statistical approach of cox and dairymple 1967 it is shown that the geomagnetic polarity time scale of harland et al 1982 may contain errors of 0 85 1 0 m y for middle late miocene times a revised time scale based on the work of mcdougall et al 1984 is correlated with the magnetostratigarphic units of the columbia river basalts united states lava extrusion rates reached a peak of 275 km3 kyr for a period of 0 5 m y around 15 8 ma during the columbia volcanic episode it is suggested that even higher rates of lava extrusion may have occurred in the deccan province central india 65 ma a simple energy budget calculation indicates that the reunion hotspot could provide enough heat to melt the total volume of rock needed to produce the deccan traps during the 5 m y it would have taken the subcontinent to traverse the hotspot

crustal scale thrusting and origin of the montreal river monocline a 35 km thick cross section of the midcontinent rift in northern michigan and wisconsin a structurally simple 35 km thick north facing stratigraphic succession of late archean to middle proterozoic rocks is exposed near the montreal river which forms the border between northern wisconsin and michigan this structure the montreal river monocline is composed of steeply dipping to vertical sedimentary rocks and flood basalts of the keweenawan supergroup middle proterozoic along the south limb of the midcontinent rift and disconformably underlying sedimentary rocks of the marquette range supergroup early proterozoic these rocks lie on an archean granite greenstone complex about 10 km of which is included in the monocline this remarkable thickness of rocks appears to be essentially structurally intact and lacks evidence of tectonic thickening or repetition tilting to form the monocline resulted from southward thrusting on listric faults of crustal dimension the faults responsible for the monocline are newly recognized components of a well known regional fault system that partly closed and inverted the midcontinent rift system resetting of biotite ages on the upper plate of the faults indicates that faulting and uplift occurred at about 1060 20 ma and followed very shortly after extension that formed the midcontinent rift system

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

magmatic activity and plate motion during the latent stage of midcontinent rift development the keweenawan midcontinent rift of north america records signifi cant continental rifting between ca 1110 and 1085 ma and preserves the most detailed paleomagnetic record of plate motion of any continent in precambrian time u pb dates from extrusive and intrusive rocks of the western lake superior basin suggest a latent stage of reduced magmatic activity from ca 1106 to 1100 ma that places constraints on the dynamics of rift development and the record of plate motion however it has remained unclear whether this stage is a feature of the entire 2500 km long rift the succession of picritic and basaltic lava fl ows at mamainse point in the eastern lake superior basin may be the most continuous and best exposed record of rift related volcanism and magnetic reversals but its age and duration relative to the latent stage has been uncertain due to a lack of radioisotopic dates we present a weighted mean 206 pb 238 u date of 1100 36 0 25 ma on zircon crystals isolated from a newly discovered tuff within the upper reversed polarity portion of the stratigraphy below the great conglomerate this date indicates that eruptive activity at mamainse point continued during the interval of diminished magmatic activity in the western lake superior basin this result strengthens the chronostratigraphic framework of rift development while explaining the preservation of additional geomagnetic reversals at mamainse point and the record of progressively decreasing paleomagnetic inclination that is indicative of rapid paleogeographic change

sea level control of mixed carbonate siliciclastic gravity flow deposition lower part of the keeler canyon formation pennsylvanian southeastern california depositional cycles present in a mixed carbonate and quartz gravity flow sequence exposed in the death valley owens valley of southeastern california may record the response of this system to third order 0 7 1 4 m y sea level changes during late pennsylvanian time time equivalent outer shelf and basin margin deposits elucidate the following key elements of this depositional system 1 a distally steepened ramp profile 2 an unchanneled basin margin sequence which accumulated as a line sourced apron system and 3 penecontemporaneous deposition of carbonate and fine grained quartz sediment in both the shallow and deep water regimes basin margin facies consist of mudstones turbidites debris flows and megabreccias organized into thickening and coarsening upward packages lime mudstone intervals 3 7 m thick alternate with mixed carbonate siliciclastic gravity flow intervals 10 30 m thick time equivalent outer ramp facies show a similar organization of alternating silty bioclastic wackestones grainstones 10 40 m thick containing a diverse open marine fauna and rhythmically bedded lime mudstone 15 60 m thick which is barren of fauna an upward increase in debris flows and megabreccias within basin margin cycles indicates the involvement of large scale ramp processes and cannot be explained by simple progradation and abandonment of depositional lobes the absence of megabreccia caps on many of the cycles argues against autocyclic progradation and collapse of the platform although larger scale coarsening and thickening upward trends within the basin margin succession are attributed to autocyclic processes instead we propose that cycles develop in both the shallow and deep water regimes by changes in relative sea level drowning of the outer shelf during relative highstands rhythmically bedded lime mud stone would yield starved basin deposits lime mudstone as sea level falls both carbonates and siliciclastics prograde closer to the shelf edge silty bioclastic wackestones grainstones thus activating the mass transport system mixed clastic gravity flow packages increased storm and wave influence on the outer ramp upper slope triggers debris flows and occasional large scale collapses producing megabreccias most models of turbidite deposition have stressed reciprocal sedimentation with sea level change bioclastic turbidites during highstands and silicilclastic turbidites during lowstands instead our model for mixed clastic deposition emphasizes the importance of the build up profile the position of sea level relative to the shelf margin and the rate of change of sea level

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

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

sequence stratigraphy and biological signals statistical studies of benthic foraminifera from liassic series abstract a study of benthic foraminifera from the toarcian in quercy aquitaine basin france shows that micropaleontological assemblages are influenced by sea level changes the percentages of coiled and straight nodosarridae lenticulina citharina differ during periods of low sea level to high sea level statistical analysis of all the taxa in the successive levels of a section multiple factorial analysis of correspondences methods of automatic clustering show that there is a perfect distribution of the samples into groups corresponding to the type of systems tract differences within the foraminiferal benthic biocoenosis allow the successive systems tracts to be separated the first axis in the multiple analysis of correspondences coincides with a gradient of biotic capacity and seems to be connected with the sea level changes the second axis could be linked with depth the renewal rate of taxa and the factorial diversity of the sampling levels show fluctuations associated with 4th 3rd and 2nd order variations of the sea level these connections are caused by intricate processes with ecological biogeographical and bioevolutionary influence however the micropaleontological assemblages can be good indicators of systems tracts and unconformities this is particularly important within deposits that have homogeneous lithology

transgressive regressive facies cycles in late cretaceous calciturbidites from the mauretanian series beni ider thrust sheet northwestern external rif morocco application of the facies tract facies sequence concept

magnetization age from paleomagnetism of the copper harbor red beds northern michigan usa and its keweenawan geologic consequences the copper harbor formation on lake superior s keweenaw peninsula records the transition from volcanic to sedimentary infilling of north america s 1 1 ga keweenawan rift radiometric dating shows t

rift wide correlation of 1 1 ga midcontinent rift system basalts implications for multiple mantle sources during rift development magmatism that accompanied the 1 1 ga midcontinent rift system mrs is attributed to the upwelling and decompression melting of a mantle plume beneath north america five distinctive flood basalt compositions are recognized in the rift related basalt succession along the south shore of western lake superior based on stratigraphically correlated major element trace element and nd isotopic analyses these distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other mrs localities around western lake superior four of these compositions are also recognized at mamainse point more than 200 km away in eastern lake superior these regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood basalt magmatism during rift development extending a model originally developed for eastern lake superior in this refined model the earliest basalts were derived from small degrees of partial melting at great depth of an enriched ocean island type plume mantle source e value of about 0 followed by magmas representing melts from this plume source and interaction with another mantle source most likely continental lithospheric mantle e 0 the

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

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

further refinement to the timing of mesoproterozoic magmatism lake nipigon region ontario the midcontinent rift in the lake superior region of north america is one of the best preserved examples of an aborted precambrian intercontinental rift one that hosts a diverse suite of rock types in addition to the well studied and voluminous rift fill flood basalts although there is a growing database of high precision age information for the main volcanic packages and the largest mafic intrusions there is relatively little information available on the absolute timing of mafic ultramafic intrusions dyke swarms and alkaline complexes especially in the ontario portion of the rift we report new high precision u pb ages for 29 samples primarily collected in the lake nipigon area ontario from these new age results it is now possible to expand the known distribution of geon 15 magmatism in the region confirm an early stage of midcontinent rift mafic magmatism between 1150 and 1130 ma provide evidence that significant mafic ultramafic magmatism occurred in the lake nipigon region slightly earlier

polybaric fractionation in an evolving continental rift evidence from the keweenawan mid continent rift evaluation of magma compositions in the keweenawan mid continent rift shows that major element chemistry and variations in lava flow composition with stratigraphy were controlled by the physical evolution of the rift in particular evidence from the north shore volcanic group of minnesota reveals that primitive high al olivine tholeiite magmas saturated with olivine clinopyroxene and plagioclase at high pressure near the base of the crust were emplaced throughout the crust where they underwent lower pressure fractionation of olivine and plagioclase to again become triply saturated as the rift evolved tectonically magmas from successively deeper chambers were erupted until finally the deep seated voluminous primitive high al magmas were able to reach the surface thereby explaining the predominance of high al basalt in the upper volcanic sequences and plutonic rocks

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

geochemistry of north shore hypabyssal dikes and sills in the midcontinent rift of minnesota an example the 47th avenue sill this study presents geochemical data for several of the numerous small to large dikes and sills including the 47th avenue sill exposed along the shore of lake superior in and north of duluth minnesota these intrusions are late magmatic features of the proterozoic midcontinent rift system and together form the north shore hypabyssal group the dikes are geochemically distinct from the sills and when the two are exposed together the younger dike intrudes the older sill dikes are primitive with mg up to 68 have positive end values and are oriented approximately north south with steep westerly or near vertical dips the older sills are more evolved usually have end values near or below 0 and have the same gentle easterly dip as the thick sequence of north shore volcanic group flows they intrude dike compositions correlate best with a mixture of widespread basalt compositions types 4 and 5 with primitive geochemistry and positive end values whereas sill compositions are similar to widespread ba