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 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 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

water washing of proterozoic oil in the midcontinent rift system whole oil gas chromatography shows that petroleum inclusions trapped in 1 05 ga veins in the midcontinent rift experienced variable water washing but no significant biodegradation or phase separation by comparing our results with published experimental data we estimate that water petroleum ratios varied from 200 1 to much greater than 9000 1 extreme water washing with water petroleum ratios very much greater than 9000 1 likely produced pyrobitumen solid hydrocarbon that was locally trapped as primary inclusions in vein calcite hydrothermal ore deposits with their extremely high water rock ratios are one of the best places to search for end member water washed petroleum sedimentary basins however with their variable water rock ratios also have a wide range of water petroleum ratios which significantly affect the composition of crude oils along the flow path between source rock and trap water petroleum ratios may be a useful concept to help evaluate crude oil compositions in a variety of settings

relations between sill intrusions and bedding parallel extensional shear zones in the mid continent rift system of the lake superior region abstract the mid continent rift system is a continuous u shaped feature extending from kansas north through the lake superior region and then south through michigan the logan sills on the northwest side of the lake were intruded during the rifting stage 1109 ma deformation features in the country rock adjacent to the logan sills indicate two episodes of shear strain the first consists of minor thrusts and mesoscopic shear bands both suggesting top to the south southwest movement parallel to the bedding the second rotates the previously formed shear bands in the opposite sense top to the north northeast and is localized at the basal contact between the sills and the country rock we attempt to explain the relations observed with a tectonic model in which sill intrusions take advantage of bedding parallel shear zones that are the surface expression of a south dipping detachment plane the detachment plane steps up in the layered proterozoic sedimentary sequence and allows extension during rifting in this scenario the first episode of shear strain might be related to bedding parallel extension and the second to shear stress induced by the intrusion of magma along the detachment from the south the polarity of both events is supported by small scale kinematic indicators such as veins fiber growths and foliation

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

organic matter and copper mineralization at white pine michigan u s a mauk j l and hieshima g b 1992 organic matter and copper mineralization at white pine michigan u s a in p a meyers l m pratt and b nagy guest editors geochemistry of metalliferous black shales chem geol 99 189211 indigenous and exogenous organic matter in sediments of the north american midcontinent rift were involved in precipitation of cu bearing minerals during main and second stage mineralization events at white pine michigan samples of the proterozoic nonesuch formation from the white pine mine and adjacent areas were investigated petrographically and geochemically in order to determine the influence of organic matter on the precipitation of cu and to determine whether mineralization influenced thermal maturity of organic matter during main stage mineralization introduction of cupriferous fluids into the lower nonesuch fro formed cu bearing sulfides by replacement of pyrite sulfur may have been added to the system via abiogenic sulfate reduction with organic matter providing a direct source of reducing potential in basal ore horizons where all sulfide had been incorporated into chalcocite during main stage mineralization native cu also precipitated locally with organic matter presumably acting as a direct reductant later compressional faulting at white pine provided conduits for additional cupriferous fluids these fluids deposited second stage cu mineralization inclusions of liquid petroleum and solid pyrobitumen in veins filling fractures associated with compressional faults record migration of petroleum into the white pine district from the deeper hotter axis of the rift penecontemporaneous with second stage fluids some of this petroleum may have acted as a reductant during precipitation of native cu in basal ore horizons biomarker compositions of bitumens and isotopic compositions of individual n alkanes from petroleums and bitumens suggest that these materials were all derived from organic matter deposited during sedimentation of the nonesuch fro biomarker data demonstrate that bitumens from the white pine area have significantly higher levels of thermal maturity than bitumens from unmineralized strata outside the mine area therefore cu mineralization at white pine coincides with a thermal anomaly seep petroleums from the northeast and southwest domains of the mine contain different n alkane distributions we propose that this difference reflects elevated thermal maturity of seep petroleums spatially associated with second stage mineralization in the southwestern domain of the mine

hydrothermal flow systems in the midcontinent rift oxygen and hydrogen isotopic studies of the north shore volcanic group and related hypabyssal sills minnesota abstract rift related lavas of the north shore volcanic group nsvg are intruded by plutonic rocks of the duluth complex along the unconformity between the nsvg and the underlying proterozoic metasedimentary rocks animikie group and archean volcano sedimentary and plutonic rocks heat associated with the emplacement of the mafic intrusions generated fluid flow in the overlying plateau lavas 18o values for whole rocks from the nsvg and hypabyssal sills range from 5 5 to 17 7 and 5 3 to 11 5 respectively and most values are higher than those considered normal for basaltic rocks 5 4 to 6 0 in general there is a positive correlation between whole rock 18o and water content which suggests that elevated 18o values are related primarily to secondary mineral growth and isotopic exchange during hydrothermal alteration and metamorphism 18oh2o values computed from amygdule filling minerals such as smectite chlorite and epidote found in low to high temperature metamorphic zones range from 1 to 6 with an average value of 3 smectite in the lower grade zones gives computed dh2o values between 26 and 83 whereas epidote in the higher grade zones gives dh2o values of 15 to 6 fluid isotopic compositions computed from epidote and smectite values are suggestive of the involvement of at least two fluids during the early stages of amygdule filling fluid d and 18o values determined from epidote at the higher metamorphic grades indicate that seawater dominated the deeper portions of the system where greenschist facies assemblages and elevated 18o values were produced in flow interiors as well as margins smectite isotopic compositions suggest that meteoric water was predominant in the shallower portions of the system the increase in 18o values of massive flow interiors with depth is interpreted as a result of rock interaction with a fluid of constant oxygen isotopic composition with increasing temperature the stable isotopic data are supportive of previous suggestions that seawater was involved in the hydrothermal system associated with the midcontinent rift although the origin of the seawater remains problematic it appears that marine incursions may have occurred during the late stages of portage lake volcanism and periodically thereafter

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

the structure and subsidence of rockall trough from two ship seismic experiments coincident multichannel seismic reflection and refraction profiles acquired northwest of britain during a two ship seismic experiment in the southern rockall trough provide evidence for a thin 6 km syn rift crust below rockall trough the variation of seismic velocity with depth is consistent with either oceanic crust similar to that found in other parts of the north atlantic or with thinned continental crust heavily intruded by syn rift igneous rocks the syn rift crust beneath rockall trough is now buried by nearly 5 km of sediments that are intruded by tertiary volcanics varying in extent from isolated sills to a large sill complex extending 100 km laterally the basement imaged on seismic profiles exhibits a band of subhorizontal reflectors that extend laterally over distances of up to 40 km which they interpret as sediments intercalated with submarine lava flow generated during the opening of the trough estimates of stretching from crustal thinning and from subsidence indicate that rockall trough has undergone extensive rifting beta 6 upwelling asthenosphere beneath the thinned lithosphere generated at least 1 3 km thickness of melt as it decompressed the molten rock rose upward from the mantle until it was in part extruded as lava flows and in partmore intruded into the crust less

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

mantle heterogeneities and their significance results from lithoprobe seismic reflection and refraction wide angle reflection studiesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent lithoprobe contribution 1486 within lithoprobe s 10 transects data from more than 20 000 km of multichannel seismic mcs reflection profiling and 12 refraction wide angle reflection r war surveys were acquired while the

ultra shallow seismic imaging of the top of the saturated zone this is the published version copyright 2010 by the american geophysical union all rights reserved

oceanography chemical twins separated how the chemical composition of sea water changes on its journey through the world s oceans is poorly understood systematic measurements of dissolved trace metals across the pacific ocean suggest that these metals may help track sources and mixing of water masses

seismic attribute inversion for velocity and attenuation structure using data from the glimpce lake superior experiment a simultaneous inversion for velocity and attenuation structure using multiple seismic attributes has been applied to refraction data from the 1986 glimpce lake superior experiment the seismic attributes considered include envelope amplitude instantaneous frequency and travel time of first arrival data instantaneous frequency is converted to t using a matching procedure which approximately removes the effects of the source spectra the derived seismic attributes are then used in an iterative inversion procedure referred to as aft inversion for amplitude instantaneous frequency and time uncertainties and resolution of the velocity and attenuation models are estimated using covariance calculations and checkerboard resolution maps a simultaneous inversion of seismic attributes from the glimpce data results in a velocity model similar to that of previous studies across lake superior a central rift basin and a northern basin are the most prominent features with an increase in velocity near the isle royale fault although there is an indication of the central and northern basins in the attenuation model for depths greater than 4 km the separation is not evident for shallower depths this may result from microfractures masking compositional variations in the attenuation model for shallower depths attenuation q values range from approximately 60 near the surface to near 500 at 10 km depth a relationship between inverse q and velocity of q 0 0210 0 0028 v was found with a correlation coefficient of 0 96 this suggests a nearly linear inverse relationship between q l and velocity beneath lake superior which supports previous laboratory results the inverted velocity and attenuation models provide important constraints on the lithology and physical properties of the midcontinent rift beneath lake superior

how the crust meets the mantle lithoprobe perspectives on the mohorovicic discontinuity and crust mantle transition application of regional geophysical and geological methods throughout two decades of canada s lithoprobe project provides new opportunities to analyze the mohorovicic discontinuity moho and crust mantle transition the transect format employed during lithoprobe in which 10 specified regions of canada were targeted for approximately a decade each between 1984 and 2003 permitted teams of scientists to focus on geological geophysical and tectonic issues for each transect as a primary objective was to enhance knowledge of the structure of the crust and lithosphere an obvious target in each transect was the nature and origin of the moho and crust mantle transition accordingly the combined results provide new perspectives on the moho and the relationship of the moho to the crust mantle transition perhaps the most important result is that the continental geophysical moho is a deceptively simple feature it has a variety of signatures at different scales that preclude a single universally applicable interpretation in methods that provide large scale information such as regional seismic studies it is a relatively abrupt refraction velocity contrast that often displays a dramatic downward decrease in seismic reflectivity however its origin in a geological or tectonic sense is perhaps best determined by careful analyses of structural details near the geophysical moho which are complex and varied in some areas within canada it appears that the geophysical moho may be old and perhaps remains from the time the crust formed in other areas it appears to be a relatively young feature that was superimposed onto older crustal fabrics

the grenville front tectonic zone results from the 1986 great lakes onshore seismic wide angle reflection and refraction experiment the grenville front which marks the orogenic boundary between the archean superior structural province and the much younger grenville province to the southeast is one of the major tectonic features of the canadian shield within canada it is approximately 1900 km in length extending from the north shore of lake huron across ontario and quebec to labrador in 1986 a major coincident onship near vertical reflection and onshore wide angle reflection refraction experiment glimpce great lakes international multidisciplinary program on crustal evolution was conducted along a series of lines across the great lakes one of the lines line j ran across georgian bay and lake huron for a distance of 350 km and crossed the grenville front tectonic zone gftz the seismic signals from the air gun array source were well recorded by the onshore stations up to distances of 250 km with a seismic trace spacing of 50 62 5 m the gftz had a profound effect on the nature of the reflector patterns observed on the onshore seismic sections data recorded by the stations on the east end of the line indicate that the crustal p phases are very complex and form a shinglelike pattern of reflected waves data recorded by stations at the center and at the western end of the line show that the pg phases are normal and lack the shinglelike appearance this character of arrivals was also observed on the corresponding s wave sections a combined p and s wave forward modeling analysis shows that the gftz is composed of bands of reflectors dipping at angles of 20 35 extending to the lower crust these reflectors were also well imaged on the coincident near vertical reflection data reflectors under the britt domain to the east of the gftz have a shallower dip than those along the zone the structure of the crust under the manitoulin terrane to the west of the gftz is laterally homogeneous with a major intracrustal reflector at a depth of 17 20 km below the surface poisson s ratio is slightly higher to the east of the grenville front compared to the region to the west the travel times of the pmp signals indicate that the moho may be deeper under the gftz than under the surrounding regions our results give added support to tectonic theories that the grenville front owes its origin to a continental collision process

crustal architecture of the inverted central lapland rift along the hukka 2007 profile we have studied the lateral velocity variations along a partly buried inverted paleo rift in central lapland northern europe with a 2d wide angle reflection and refraction experiment hukka 2007 the experiment was designed to use seven chemical explosions from commercial and military sites as sources of seismic energy the shots were recorded by 102 stations with an average spacing of 3 45 km two dimensional crustal models of variations in p wave velocity and vp vs ratio were calculated using the ray tracing forward modeling technique the hukka 2007 experiment comprises a 455 km long profile that runs nnw sse parallel to the kittila shear zone a major deformation zone hosting gold deposits in the area the profile crosses paleoproterozoic and reactivated archean terranes of central lapland the velocity model shows a significant difference in crustal velocity structure between the northern distances 0 120 km and southern parts of the profile the difference in p wave velocities and vp vs ratio can be followed through the whole crust down to the moho boundary indicating major tectonic boundaries upper crustal velocities seem to vary with the terranes compositional differences mapped at the surface the lower layer of the upper crust displays velocities of 6 0 6 1 km s both paleoproterozoic and archean terranes are associated with high velocity bodies 6 30 6 35 km s at 100 and 200 350 km distances the central lapland greenstone belt and central lapland granitoid complex are associated with a 4 km thick zone of unusually low velocities 6 0 km s at distances between 120 and 220 km we interpret the hukka 2007 profile to image an old partly buried inverted continental rift zone that has been closed and modified by younger tectonic events it has structural features typical of rifts inward dipping rift shoulders undulating thickness of the middle crust high velocity lower crust and a rather uniform crustal thickness of 48 km

interpretation of seismic reflection gravity and magnetic data across middle proterozoic mid continent rift system northwestern wisconsin eastern minnesota and central iowa seismic reflection gravity and magnetic data were used along four profiles to study the mid continent rift system between northwestern wisconsin and central iowa mafic lavas in medial horsts are characterized by strong laterally continuous reflections that indicate a stratigraphic package as much as 10 km thick mafic roots extending beneath the reflective sequence to mid crustal depths were defined from gravity modeling in northwestern wisconsin southeastern minnesota and central iowa a strong normally directed remanent magnetization was modeled for most lavas although some lavas deep in the section of northwestern wisconsin and central iowa were modeled as magnetically reversed the horsts are apparently bounded by faults that are vertical or dip moderately inwar several of these faults originated as growth faults in fault bounded grabens but these faults were transposed into reverse faults by a late stage compressional event the medial horsts are flanked by several half graben basins that contain as much as 5 km of sedimentary material inferred to be lithologically akin to the oronto and bayfield groups of michigan and wisconsin several synclinal basins characterized by 2 4 km of sedimentary rocks are developed atop the medial horsts when viewed regionally the four interpretive profiles show a marked asymmetry with regard to the off axis placement of the deep mafic roots and the positions of postulated growth faults this asymmetry is consistent with the rift propagating as a series of northeast striking en echelon half grabens separated by northwest striking scissor faults or accommodation zones

rupture creep lamellae and crocodiles happenings in the continental crust from the observation of certain seismic reflectivity patterns in phanerozoic continental crusts a relationship is suggested between 1 rupture and seismicity generally restricted to the upper crust 2 crocodile tectonics a kind of vertical indentation observed by diverging plane reflectors preferably in the middle crust 3 creep and the formation of reflecting lamellae in the lower crust based on these observations and on viscosity depth estimates a conceptual model for the above mentioned structures is presented which is also compatible with observations in older crusts boundary conditions are given for rupture as well as for lamellae forming creep

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

possible extension of the midcontinent rift in west texas and eastern new mexico the midcontinent rift system forms one of the most prominent gravity features in north america the recognized geophysical anomaly extends in an arc from southern oklahoma to lake superior and then into southern michigan the midcontinent rift system was active between 1185 1010 ma as indicated in the lake superior region by age determinations on intrusive igneous rocks we suggest that the period of formation of the midcontinent rift was also a time of extensive igneous activity in texas and new mexico this activity is represented by intrusions beneath the central basin platform texas and new mexico intrusions which crop out at the pajarito mountain in the sacramento mountains new mexico a basaltic debris flow in the franklin mountains texas basalt flows at van horn texas and the crosbyton geophysical anomaly east of lubbock texas these bodies and other bodies located by geophysical anomalies and wells drilled into mafic precambrian rocks may be related to the midcontinent rift system

structural evolution of the hemlo greenstone belt in the vicinity of the world class hemlo gold deposit a complex history of volcano sedimentary deposition polyphase strain multiple intrusive events and various stages of porphyroblastesis is indicated for the hemlo gold deposit area within the hemlo greenstone belt structural elements can be assigned to at least six stages of development d1 d6 d1 generated small scale folds and low angle faults thrusts with no planar fabric except within strain aureoles around the earliest intrusions d2 was a progressive event resulting from northeast directed compression which generated regional predominantly s shaped folds early d2 penetrative planar and linear fabrics overturned stratigraphy and formation of an inflection in the strike of the greenstone belt mid d2 and development of high strain zones with dominant sinistral and local dextral shear sense late d2 d3 was a distinctly separate progressive event resulting from northwest directed transpression which generated variably penetrative east to northeast striking foliation s3 ductile de

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

metallogeny of the midcontinent rift system of north america abstract the 1 1 ga midcontinent rift system of north america is one of the world s major continental rifts and hosts a variety of mineral deposits the rocks and mineral deposits of this 2000 km long rift are exposed only in the lake superior region in the lake superior region the rift cuts across precambrian basement terranes ranging in age from 1850 ma to more than 3500 ma where exposed the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks beneath the center of lake superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection profiles this region hosts two major classes of mineral deposits magmatic and hydrothermal all important mineral production in this region has come from hydrothermal deposits rift related hydrothermal deposits include four main types 1 native copper deposits in basalts and interflow sediments 2 sediment hosted copper sulfide and native copper 3 copper sulfide veins and lodes hosted by rift related volcanic and sedimentary rocks and 4 polymetallic five element veins in the surrounding archean country rocks the scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals where hydrothermal sulfides occur i e shale hosted copper sulfides the source of sulfur was local sedimentary rocks magmatic deposits have locally supported exploration and minor production but most are subeconomic presently these deposits occur in intrusions exposed near the margins of the rift and include cu ni pge and ti fe v in the duluth complex u ree nb in small carbonatites and breccia pipes resulting from local hydrothermal activity around small felsic intrusions mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization most of the sulfide deposits in intrusions however contain sulfur derived from country rocks the interaction between magma and country rocks was important in generation of the magmatic cu ni sulfide deposits a mantle plume origin has been proposed for the formation of the midcontinent rift more than 1 million km3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines these brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the plume hydrothermal deposits were emplaced for at least 30 40 m y after rift magmatism and extension ceased this time lag may reflect either the time required to heat deeply buried rocks and fluids within the rift or may be due to the timing of post rift compression that may have provided the driving mechanism for expulsion of hydrothermal fluids from deep portions of the rift