Midcontinent Rift Research Articles

A possible southeastern extension of the Midcontinent Rift System located in Ohio

In 1988 a stratigraphic test core drilled in southwestern Ohio penetrated a previously unknown structure and sedimentologic unit. Core analyses disclosed a sequence of lithic arenite and siltstone of probable pre‐Phanerozoic age, deposited within an alluvial‐fluvial environment. A seismic reflection profile across the core site shows a sequence of strong, horizontal Paleozoic reflectors unconformably overlying eastward dipping, layered units of poor reflectivity. Beyond the core hole total depth, a regime of excellent reflectors is seen. The pre‐Mount Simon (Upper Cambrian) sedimentary sequence encountered has been defined as the type section of the Middle Run Formation. Point counter analyses of typical Middle Run Formation samples indicate that this sequence is similar in composition to middle Proterozoic units associated with the Midcontinent Rift System in the Lake Superior region. The newly discovered structure in Ohio is also similar to half‐graben structures composing the Lake Superior Basin portion of the Midcontinent rift. On the basis of similarities in lithology, stratigraphy, structure, and proximity to regional gravity anomalies, the Middle Run Formation and its apparent half‐graben basin is proposed as evidence for the probable extension of the Midcontinent Rift System southward, from its normally accepted termination in southeastern Michigan, into southwestern Ohio.

Drainage, topographic, and gravity anomalies in the Lake Superior Region: Evidence for a 1100 Ma mantle plume

A topographic dome, radial drainage pattern, and regional negative gravity anomaly, all centered on Lake Superior, are vestiges of a 1100 Ma mantle plume that formed the Midcontinent Rift System of central North America. The topographic and gravimetric relations suggest that the dome is maintained isostatically by fundamental changes imprinted on the lithosphere by the plume, including magmatic underplating of the crust and depletion of the upper mantle. These changes occurred at distances of up to 500 km or more from the center of the plume, well beyond the margins of the rift.

Tectonic setting of the Midcontinent Rift system

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.

The Midcontinent rift in the Lake Superior region with emphasis on its geodynamic evolution

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 igneous petrology and magmatic evolution of the Midcontinent rift system

The 1100 Ma Midcontinent rift system of North America is one of the world's great igneous provinces, containing over one million km 3 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 lithosphère. 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.

Geophysical investigations and crustal structure of the North American Midcontinent Rift system

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.

Relations between sill intrusions and bedding-parallel extensional shear zones in the Mid-continent Rift System of the Lake Superior region

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.

Speculations on the origin of the North American Midcontinent rift

The Midcontinent rift is an example of lithospheric extension and flood basalt volcanism induced when a new mantle plume arrived near the base of the lithosphere. Very large volumes of basaltic magma were generated and partly erupted before substantial lithospheric extension began. Volcanism continued, along with extension and deep rift subsidence, for the ensuing 15 m.y. Much of the basaltic magma, including some of the earliest flows, was formed by partial melting of isotopically primitive asthenosphere contained in the plume head. The intense but relatively short duration of rifting and magmatism is a result of the dissipation of thermal and mechanical energy in the plume head. As the plume head spread beneath the lithosphere, it stretched the overlying lithosphere radially away from the Lake Superior region, the triple junction of the rift system, and partially melted to form the great volume of basalt and related intrusive rocks of the region. The plume arrived beneath a continent that was under compression as a result of the ongoing Grenville orogeny that affected a large region east of the rift. That compression prevented full continental separation and eventually returned the region to compressional tectonics as the energy of the plume head waned.

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.

Stable isotope (C, S, N) and molecular studies on the Precambrian nonesuch Shale (Wisconsin-Michigan, U.S.A.): Evidence for differential preservation rates, depositional environment and hydrothermal influence

The organic-rich, Middle Proterozoic Nonesuch Formation is part of a thick volcaniclastic rift-fill sequence (Mid-Continent Rift System, northern Wisconsin and Upper Peninsula Michigan) with a mild thermal history. Despite stratigraphic/sedimentologic similarities between the east (Michigan) and west (Wisconsin) study areas, distinctions in organic petrologic, elemental and stable isotopic parameters are noted. Geologic arguments and predictable relationships among organic carbon content, bulk and molecular pyrolysate composition and petrologic parameters are sufficient to invoke differential preservation of the same (or similar) primary producers as the principal cause of variability. Isotopic signals indicating precipitation of calcite via intense organic production (planktonic blooms) is evident among carbonate laminites. Incorporation or concentration of organic degradation by-products is evident, particularly in the west, from carbon and nitrogen stable isotope data. It is proposed that whereas preservation of organic productivity of a particular organism may be nearly complete in one organic facies in the east and west, organic remains in other intervals in the west were subject to extensive degradation and reconstitution to form protokerogens. Implications of the study include establishing parameters for recognizing pre-Devonian lacustrine vs. marine euxinic systems, clues to the complexity of Middle Proterozoic aquatic systems and in recognizing the input of hydrothermal fluids to water bodies and early sediments.

Organic matter and copper mineralization at White Pine, Michigan, U.S.A.

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 Fm. 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 Fm.. 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.

Lithoprobe: new perspectives on crustal evolution

Lithoprobe is Canada's national, collaborative, multidisciplinary earth science research program directed toward an enhanced understanding of how the North American continent evolved. Research in its eight transects or study areas, which span the country from Vancouver Island to Newfoundland and geological time from 4 Ga to the present, is spearheaded by seismic reflection surveys. These, combined with many other studies, are providing new insight into the varied tectonic processes that have been active in forming the continent. Results from the Southern Cordillera transect show that Mesozoic crustal growth occurred in the central and eastern Cordillera by the accretion and amalgamation of exotic terranes, the collision of which resulted in the generation of crustal-scale antiforms and duplexes. After the principal periods of compression, this area was affected by a major episode of extension that led to the unroofing of the metamorphic core complexes. Farther to the west, past and present subduction processes have eroded the lower lithosphere of accreted terranes and left underplated sediments and oceanic lithosphere. The Lithoprobe East transect, covering the Paleozoic Newfoundland Appalachians and Mesozoic rifted Atlantic margin, reveals three lower crustal blocks, each with distinctive reflection signatures on marine seismic data. Structures of the geologically established tectono-stratigraphic domains, imaged clearly by new onshore reflection data, sole at upper crustal to mid-crustal levels, suggesting that much of the surface stratigraphy is allochthonous to the lower crustal blocks. At the ocean–continent transition, interpretations suggest underplating of thinned continental crust by basaltic melt during the rifting process.In Lake Superior, data from the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE) transect reveal the complex structures of the late Middle Proterozoic Keweenawan rift, which is up to 35 km deep, that almost split North America. The GLIMPCE data in Lake Huron show a spectacular series of east-dipping crustal-scale reflections that coincide with the Grenville front tectonic zone. These and other data have led to a two-stage model involving collision of an exotic terrane with the southern Superior cratonic margin in the late Early Proterozoic followed by stacking–crustal penetrating imbrication and ramping associated with the Middle Proterozoic Grenvillian orogeny. The Archean Kapuskasing structural zone, a prominent northeast-trending feature that cuts obliquely across the dominant east-west structures of the Superior Province, is interpreted as a thin thrust sheet, soled by a variably reflective décollement, above which about 70 km of crustal shortening has occurred to bring mid-crustal to lower crustal rocks to the surface, and below which the Moho deepens. The shortening may have been accomplished by brittle faulting and erosion at levels above 20 km and ductile folding or faulting in the lower crust. Preliminary studies in the Archean Abitibi greenstone belt indicate that two major fault zones, the Larder Lake–Cadillac and Porcupine–Destor, which host significant mineralization, were generated by crustal-scale thrust and (or) strike-slip tectonics. Archean crustal sections are as structurally diverse and complex as their Proterozoic and Phanerozoic counterparts. The reflection Moho has highly variable characteristics as imaged within transects and among different transects. Crustal and Moho reflectivity observed in the various transects is caused by a wide range of features, including fault–shear zones, lithologic contacts, compositional layering, fluids in zones of high porosity, and anisotropy.

Possible crustal contamination of Midcontinent Rift igneous rocks: examples from the Mineral Lake intrusions, Wisconsin

Interlayered mafic–telsic intrusions from the Mineral Lake intrusive complex in northwest Wisconsin reflect the typical bimodal basalt–rhyolite compositional pattern of the Midcontinent Rift flood basalt province in the Lake Superior region. The later felsic intrusions were emplaced between the mafic intrusions and overlying basalt flows, and postemplacement fractional crystallization produced gradational mineralogical and geochemical variations. Isotopic and trace-element data for the Mineral Lake intrusions are consistent with mantle sources for both mafic and felsic intrusions, with compositional differences explained by the extent of fractional crystallization and crustal contamination or mantle source characteristics.εNd–εSr plots of analyzed Midcontinent Rift igneous rocks define three largely separate isotopic fields that suggest separate sources. However, the spread in isotopic data and a spider diagram plot of mafic samples from the εNd = εSr = 0 field suggest a crustal component and derivation from depleted rather than chondritic mantle. Evolved felsic rocks plotting in two negative εNd – positive εSr fields can be explained by derivation from separate enriched mantle sources or crustal contamination or both.

Variations in the reflectivity of the moho transition zone beneath the Midcontinent Rift System of North America: Results from true amplitude analysis of GLIMPCE data

True amplitude processing of The Great Lakes International Multidisciplinary Program on Crustal Evolution seismic reflection data from the Midcontinent Rift System of North America shows large differences in the reflectivity of the Moho transition zone beneath the axial rift, beneath the rift flanks, and outside of the rift. The Moho reflection from the axial rift has a discontinuous, diffractive character marginally stronger (several decibels) than an otherwise transparent lower crust and upper mantle. Beneath the axial rift, Moho is interpreted to be a synrift igneous feature. Beneath the rift flanks, the reflectivity of the Moho transition is generally well developed with two identifiable boundaries, although in places it is weakly reflective to nonreflective, similar to Moho outside the rift. The two boundaries are interpreted as the base of essentially intact, although stretched, prerift Archean crust (upper boundary) and new synrift Moho 1–2 s (6–7 km) deeper (lower boundary). Beneath the rift flanks, the layered reflection Moho transition results from the preexisting crustal composition and fabric modified by synrift igneous processes and extensional tectonic/metamorphic processes. The geologic evidence for extensive basaltic magmatism in the rift is the basis for interpreting the Moho signature as a Keweenawan structure that has been preserved for 1.1 b.y. Extension and magmatism appear to enhance reflectivity in the lower crust and Moho transition zone only where stretching factors are moderate (rift flanks) and not where they are extreme (axial rift). This leads to the prediction that the reflectivity across analogous volcanic passive continental margins should be greatest beneath the moderately stretched continental shelves and should decrease towards the ocean‐continent boundary.

Timing and origin of midcontinent rift alkaline magmatism, North America: evidence from the Coldwell Complex

The Coldwell Complex represents the largest alkaline intrusion associated with the Midcontinent Rift System in North America. This complex contains a plethora of rock types that have previously been subdivided into three intrusive centers. A detailed U-Pb zircon/baddeleyite age study of five samples indicates that the majority of the complex was emplaced into “cold” Archean crust at 1108±1 Ma and likely experienced a rapid cooling history. These data, combined with published U-Pb zircon/baddeleyite results for other rift related igneous activity, document the contemporaneous production and emplacement of tholeiitic and alkaline magmas at the onset of rifting. The Sr-Nd-Pb isotopic compositions of selected minerals from different phases of the complex display considerable scatter that is best explained by the presence of magmas with different initial isotopic compositions. The initial Sr and Nd isotopic compositions for clinopyroxene and plagioclase from one of the earliest gabbro phases (eNd=+0.5 to +1.6; eSr=+2.4 to +3.1) are identical to published data for primitive olivine tholeiites from the rift and indicate that the majority of magmas, both tholeiitic and alkaline, have a uniform, nearly chondritic isotopic composition. This very reproducible isotopic composition for rift magmatism can be explained by the dominance of a well-mixed mantle plume signature in magma genesis. The shift in isotopic compositions observed for the more evolved granite and syenite samples (eNd=−4.6 to −6.4; eSr=+10.2 to +13.8) combined with a less radiogenic Pb isotopic signature is consistent with derivation of these magmas from or interaction with an older granulite facies lower crust. The chondritic isotopic signature typical of most MRS volcanic and plutonic rocks is quite distinct from published results on associated carbonatites (eNd=+2.1 to +4.5; eSr=−8.0 to 2212;11.5) indicating the presence of at least two distinct subcontinental mantle isotopic reservoirs in this region.

Crustal structure of the Midcontinent rift system in eastern Lake Superior from controlled-amplitude analysis of GLIMPCE deep reflection seismic data

The reprocessing of line F of the GLIMPCE deep marine reflection seismic survey according to controlled-amplitude principles provides new insights on the crustal structure of the Midcontinent rift system in eastern Lake Superior. The insertion of refraction static corrections in the processing sequence was crucial to recover the lateral continuity and amplitude strength of reflectors under rough lake-bottom topography. Coherent noise consisting mainly of waves scattered by irregularities on die lake bottom and first-order water reverberations was best attenuated by use of multiple passes of velocity filtering in different seismic domains rather than by trace editing. Overall, the variations in reflection style and strength on our final stacked and trace envelope sections allow for the identification of broad geological domains. Lava flows and postrift sediments can be traced uninterrupted across most of the rift basin, which is bounded to the north by a complex region of secondary sagging, felsic volcanism, and (or) intrusives, and to the south by the Keweenaw fault. The crust–mantle transition zone is characterized by both a smooth amplitude anomaly and by discontinuous packages of diffractions and reflections. It is overlain by an anomalous transparent lower crustal domain beneath the central basin. These observations suggest that, during rifting, magma rose from a layer of underplated material located at the base of the crust to the surface through numerous small feeders. They do not, however, fully exclude die possibility of complete crustal separation.

Petrology of the Great Abitibi Dyke, Superior Province, Canada

The Great Abitibi Dyke (GAD) which can be traced northeast, for > 700 km, across the Abitibi Belt in the southeastern Superior Province of the Canadian Shield, is composed of olivine gabbro to monzodiorite, weakly saturated to undersaturate d in silica. All rocks of the GAD can be derived by mainly plagioclase and olivine fractionation from a parental magma corresponding in composition to chilled margin samples. Two units can be distinguished , a marginal unit (Unit 1) representing 0-50% crystallization and a central unit (Unit found over about half of the dyke length, representing 50-70% crystallization. Modelling, using Pearce elemental ratio analysis, quantifies the fractionation history and allows mass-balance calculations over the present exposure level of the dyke. The approximate balance between the amounts of cumulate and fractionated rocks suggests that Unit 1 differentiated essentially in situ as a closed system. In contrast, Unit 2 rocks were formed by loss of substantial plagioclase and olivine from the parental magma. This fractionate must have either been lost to depth or left behind in an external chamber. Unit 2 rocks show depletion in plagioclase and enrichment in mafic minerals along strike towards the southwest (deeper exposure level?), a trend explained by density stratification. Regional variation in Unit 1 chemistry is interpreted in terms of lateral magma injection towards the northeast from the locus of Keweenawan rift volcanism. Feldspar, olivine, and augite compositions are linearly correlated with equilibrium temperature and extent of magma evolution. The parent magma had a trace element chemistry corresponding to a 'within-plate' setting and was probably derived from an incompatible-element enriched mantle similar to the source for ocean island basalts (OIBs).

Array observations of PKP phases across the Kenya Rift: Implications for structure and tectonics

PKP travel-time residuals from teleseisms recorded by a 100 × 110 km array centered on the south-central portion of the Kenya Rift show that variations in residuals are as large parallel to the rift as perpendicular to it. Asymmetry in residuals observed across the rift is consistent with half-grabens facing in opposite directions separated by an accommodation zone. The largest time residuals are offset at least 40–50 km west of the rift axis, supporting a rifting model in which a dominant half-graben fault is listric, penetrates the lithosphere, and triggers intrusion from asthenospheric sources. Linear regression of residuals and Bouguer gravity anomalies shows a slope of −0.015 ± 0.002 s/ mgal; the slopes of similar regressions for the tectonically inactive Midcontinent Rift and Sierra Nevada are −0.0025 ± 0.001 s/ mgal. One explanation for the observed factor of 5 difference is the presence of partial melt under the Kenya Rift system.

Wisconsin gravity minimum: Solution of a geologic and geophysical puzzle and implications for cratonic evolution

An intense Bouguer gravity anomaly minimum extending across much of Wisconsin cannot be explained by the surface Phanerozoic sedimentary strata, the basement Precambrian geology, or the topography of the region. The most intense ({minus}100 mgal) part of the minimum coincides with the 1.47 Ga anorogenic granitic Wolf River batholith of northeastern Wisconsin. In southern Wisconsin, however, the densities of the Precambrian basement rocks, which are older than the batholith, provide no clue to the origin of the anomaly. The gradients of the minimum indicate that the source of the anomaly is in the upper crust. Furthermore, nearby deep seismic reflection data indicate that lower crustal structures do not significantly contribute to the gravity minimum. Thus, the minimum is appropriately interpreted as originating from the low-density Wolf River batholith that crops out only in northeastern Wisconsin but is buried beneath a veneer of older rocks in the southern and central parts of the state. Gravity modeling suggests that the batholith is at least 10 km thick and encompasses an area of {approximately}50,000 km{sup 2}. This interpretation provides an important clue to the origin of similar negative gravity anomalies of the Phanerozoic strata-covered craton. Also, the presence of this massive granitic bodymore » appears to have influenced the evolution of the craton - e.g., by controlling the location of the 1.1 Ga Midcontinent rift system and the Paleozoic Wisconsin arch. The fact that the Wolf River batholith is mostly buried suggests that central Wisconsin has been tectonically stable for the past 1.47 b.y. and that the Precambrian basement has been minimally eroded.« less

Relations between deformation and sediment-hosted copper mineralization: Evidence from the White Pine part of the Midcontinent rift system

Detailed studies over the past decade have significantly extended and revised our knowledge of the geologic history of the well-known White Pine mining district of northern Michigan, and indicate that the location of faults exerted a strong control on copper mineralization in this part of the Midcontinent rift system. At White Pine there is evidence for three episodes of faulting: (1) synsedimentary faulting, (2) subsequent high-angle, dominantly normal faulting, and (3) thrusting. Two stages of copper mineralization are present at White Pine and in the nearby Presque Isle syncline. The first, main-stage mineralization, formed a classic sediment-hosted stratiform copper deposit during early diagenesis. Synsedimentary faults may have provided important conduits for cupriferous brines flowing from underlying red beds of the Copper Harbor Conglomerate into the reduced silts and shales of the Nonesuch Formation, where main-stage copper sulfides and native copper were precipitated. The second stage of copper mineralization was synchronous with thrusting and introduced additional copper to the White Pine ore body and the Presque Isle syndine. Thrust faults and cogenetic tear faults provided conduits for second-stage mineralizing fluids. Collectively, these observations indicate strong control by regional deformation on fluid migration and mineralization in the rocks of the Midcontinent rift, similar to proposed relations between deformation and mineralization in other tectonic settings.