Midcontinent Rift Research Articles

Clay minerals and sedimentary basin history

Clay minerals in the mud and soil that coat the Earth's surface are part of a clay cycle that breaks down and creates rock in the crust. Clays generated by surface weathering and shallow diagenetic processes are transformed into mature clay mineral assemblages in the mudrocks found in sedimentary basins. During metamorphism, the release of alkali elements and boron from clay minerals generates magmas that are subsequently weathered and recycled, representing the magma-to-mud pathway of the clay cycle. Volcanogenic clay represents an important but hitherto underestimated proportion of recycled clay. Within sedimentary basins, immature clays are transformed to mature and supermature clay assemblages by a series of reactions that generally obey the Ostwald Step Rule. Bedding-parallel microfabric generated by these reactions produce significant changes in the physical properties of deeply buried mudrocks. Clay minerals react to form equilibrium assemblages in 1 × 104 years in some hydrothermal systems, but immature clays may survive for up to 2 × 109 years in mid-continental rift basins. Clay mineral assemblages and the b cell dimension of K-white mica can be used to infer the geotectonic settings of sedimentary basins.

The Elk Creek Carbonatite, Southeast Nebraska–An Overview

A framework geophysical program in southeastern Nebraska during 1970 identified a near-circular feature having gravity relief of about 8 mgal and a magnetic anomaly of about 800 gammas. Analysis of the geophysical data provided a model of a cylindrical mass of indefinite length with a radius of 5500 ft (1676 m) and beveled at the basement surface at about 600 ft (183 m). At the approximate depth at which Precambrian rocks were expected, the initial test hole (2-B-71) encountered an iron-rich weathered zone overlying carbonate-rich rock. The carbonate rocks consist essentially of dolomite, calcite, and ankerite and lesser amounts of hematite, chlorite, phlogopite, barite, serpentine, pyrochlore, and quartz and contain barium, strontium, and rare earths. Total REE, P2O5, and 87Sr/86Sr ratios confirm the carbonatite identification. Texturally, the rocks range from fragmental to contorted to massive. Associated with the carbonatite are lesser amounts of basalt, lamprophyre, and syenite. Additional exploratory drilling has provided about 80,000 ft (24,384 m) of rock record and has penetrated about 3400 ft (1038 m) of carbonatite. The carbonatite is overlain by marine sediments of Pennsylvanian (Missourian) age. The surrounding Precambrian basement rocks are low-to medium-grade metamorphic gneiss and schist of island arc origin and granitic plutons. The Elk Creek carbonatite is located near the boundary between the Penokean orogen created at about 1.84 Ga (billion years) and the Dawes terrane (1.78 Ga) of the Central Plains orogen. This boundary strongly influenced the geometry of both the Midcontinent Rift System (1.1 Ga) and the Nemaha uplift (0.3 Ga). It is assumed that the emplacement of the Elk Creek carbonatite (0.5 Ga) was influenced similarly by the pre-existing tectonic sutures.

Kinematics, mechanics, and potential earthquake hazards for faults in Pottawatomie County, Kansas, USA

Many stable continental regions have subregions with poorly defined earthquake hazards. Analysis of minor structures (folds and faults) in these subregions can improve our understanding of the tectonics and earthquake hazards. Detailed structural mapping in Pottawatomie County has revealed a suite consisting of two uplifted blocks aligned along a northeast trend and surrounded by faults. The first uplift is located southwest of the second. The northwest and southeast sides of these uplifts are bounded by northeast-trending right-lateral faults. To the east, both uplifts are bounded by north-trending reverse faults, and the first uplift is bounded by a north-trending high-angle fault to the west. The structural suite occurs above a basement fault that is part of a series of north–northeast-trending faults that delineate the Humboldt Fault Zone of eastern Kansas, an integral part of the Midcontinent Rift System. The favored kinematic model is a contractional stepover (push-up) between echelon strike-slip faults. Mechanical modeling using the boundary element method supports the interpretation of the uplifts as contractional stepovers and indicates that an approximately east–northeast maximum compressive stress trajectory is responsible for the formation of the structural suite. This stress trajectory suggests potential activity during the Laramide Orogeny, which agrees with the age of kimberlite emplacement in adjacent Riley County. The current stress field in Kansas has a N85°W maximum compressive stress trajectory that could potentially produce earthquakes along the basement faults. Several epicenters of seismic events (<M2.0) are located within 10 km of the structural suite. One epicenter is coincident with the northwest boundary of the uplift. This structural suite, a contractional stepover between echelon northeast-trending right-lateral faults, is similar to that mapped in the New Madrid Seismic Zone, and both areas currently feature roughly east–west maximum compressive stress trajectory. Based on these similarities, the faults in Pottawatomie County have the potential for seismicity. The results demonstrate that mechanical analysis of minor structural features can improve our knowledge of local earthquake hazards.

Heat flow in the Nipigon arm of the Keweenawan rift, northwestern Ontario, Canada

In the Archean Superior Province, the Nipigon Embayment, in the area of Lake Nipigon north of Lake Superior, is covered by MidProterozoic sediments intruded by Keweenawan diabase sills. It has been interpreted as a failed arm of the ca. 1100 Ma Keweenawan rift. Six new heat flow values in this area show that the region of low heat flow associated with the Keweenawan rift in Lake Superior extends northwards along the western margin of the Nipigon Embayment. The average heat flow in the Nipigon area (39 ± 5 mWm−2) is only slightly lower than that of the adjacent western Superior Province (42 ± 8 mWm−2), indicating that the volume of Keweenawan mafic intrusives in the Nipigon crust is small.

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 εNd values, and are oriented approximately north–south with steep westerly or near vertical dips. The older sills are more evolved, usually have εNd 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 εNd values, whereas sill compositions are similar to widespread basalt composition type 4 typical of most North Shore Volcanic Group flows. The 47th Avenue sill in Duluth is an evolved single intrusion North Shore Hypabyssal Group diabase sill with trough banding, sharp lower and upper contacts, and a spectacular fractured and undulating roof zone containing blocks of the overlying ferroandesite flow.

Lithosphere mapping beneath the North American plate

Major- and trace-element analyses of garnets from heavy-mineral concentrates have been used to derive the compositional and thermal structure of the subcontinental lithospheric mantle (SCLM) beneath 16 areas within the core of the ancient Laurentian continent and 11 areas in the craton margin and fringing mobile belts. Results are presented as stratigraphic sections showing variations in the relative proportions of different rock types and metasomatic styles, and the mean Fo content of olivine, with depth. Detailed comparisons with data from mantle xenoliths demonstrate the reliability of the sections. In the Slave Province, the SCLM in most areas shows a two-layer structure with a boundary at 140–160 km depth. The upper layer shows pronounced lateral variations, whereas the lower layer, after accounting for different degrees of melt-related metasomatism, shows marked uniformity. The lower layer is interpreted as a subcreted plume head, added at ca. 3.2 Ga; this boundary between the layers rises to <100 km depth toward the northern and southern edges of the craton. Strongly layered SCLM suggests that plume subcretion may also have played a role in the construction of the lithosphere beneath Michigan and Saskatchewan. Outside the Slave Province, most North American Archon SCLM sections are less depleted than similar sections in southern Africa and Siberia; this may reflect extensive metasomatic modification. In E. Canada, the degree of modification increases toward the craton margin, and the SCLM beneath the Kapuskasing Structural Zone is typical of that beneath Proterozoic to Phanerozoic mobile belts. SCLM sections from several Proterozoic areas around the margin of the Laurentian continental core (W. Greenland, Colorado–Wyoming district, Arkansas) show discontinuities and gaps that are interpreted as the effects of lithosphere stacking during collisional orogeny. Some areas affected by Proterozoic orogenesis (Wyoming Craton, Alberta, W. Greenland) appear to retain buoyant, modified Archean SCLM. Possible juvenile Proterozoic SCLM beneath the Colorado Plateau is significantly less refractory. The SCLM beneath the Kansas kimberlite field is highly melt-metasomatised, reflecting its proximity to the Mid-Continent Rift System. A traverse across the continent shows that the upper part of the cratonic SCLM is highly magnesian; the decrease in mg# with depth is interpreted as the cumulative effect of metasomatic modification through time. The relatively small variations in seismic velocity within the continental core largely reflect the thickness of this depleted layer. The larger drop in seismic velocity in the surrounding Proton and Tecton belts reflects the closely coupled changes in SCLM composition and geotherm.

The temporal evolution of North American kimberlites

North American kimberlite magmatism spans a period of time in excess of 1 billion years from Mesoproterozoic kimberlites in the Lake Superior and James Bay Lowlands region of Ontario to Eocene kimberlites in the Lac de Gras field, N.W.T. Based on a compilation of more than 150 robust radiometric age determinations, several distinct kimberlite emplacement patterns are recognized. In general, the temporal pattern of kimberlite emplacement in North America can be broadly subdivided into five domains: (1) a Mesoproterozoic kimberlite province in central Ontario, (2) an Eocambrian/Cambrian Labrador Sea Province in northern Québec and Labrador, (3) an eastern Jurassic Province, (4) a central Cretaceous corridor and (5) a western mixed domain that includes two Type-3 kimberlite provinces (i.e. multiple periods of kimberlite emplacement preserved in the Slave and Wyoming cratons). For some provinces the origin of kimberlite magmatism can be linked to known mantle heat sources such as mantle plume hotspots and upwelling asthenosphere attendant with continental rifting. For example, the timing and location of Mesoproterozoic kimberlites in North America coincides with and slightly precedes the timing of 1.1 Ga intracontinental rifting that culminated in the Midcontinent Rift centered in the Lake Superior region. Many of the kimberlites in the Eocambrian/Cambrian Labrador Sea province were emplaced soon after the opening of the Iapetus Ocean at about 615 Ma and may also be linked to mantle upwelling associated with continental rifting. The eastern Jurassic kimberlites record an age progression where magmatism youngs in a southeast direction from the ∼200 Ma Rankin Inlet kimberlites to the 155–126 Ma Timiskaming kimberlites. The location of several kimberlite fields and clusters in Ontario and Québec lie along a continental extension of the Great Meteor hotspot track and represents one of the best examples in the world of kimberlite magmatism triggered by mantle plumes. The central Cretaceous (103–94 Ma) corridor extends for more than 4000 km from Somerset Island in northern Canada through the Fort à la Corne field in Saskatchewan to the kimberlites in central USA. This is the first recognized corridor of kimberlite magmatism of this magnitude. The possible westward younging of Cretaceous to Eocene corridors of kimberlite magmatism could reflect major changes in plate geometry during subduction of the Kula–Farallon plate.

A hydrogeologic model of stratiform copper mineralization in the Midcontinent Rift System, Northern Michigan, USA

AbstractThis paper presents a suite of two‐dimensional mathematical models of basin‐scale groundwater flow and heat transfer for the middle Proterozoic Midcontinent Rift System. The models were used to assess the hydrodynamic driving mechanisms responsible for main‐stage stratiform copper mineralization of the basal Nonesuch Formation during the post‐volcanic/pre‐compressional phase of basin evolution. Results suggest that compaction of the basal aquifer (Copper Harbor Formation), in response to mechanical loading during deposition of the overlying Freda Sandstone, generated a pulse of marginward‐directed, compaction‐driven discharge of cupriferous brines from within the basal aquifer. The timing of this pulse is consistent with the radiometric dates for the timing of mineralization. Thinning of the basal aquifer near White Pine, Michigan, enhanced stratiform copper mineralization. Focused upward leakage of copper‐laden brines into the lowermost facies of the pyrite‐rich Nonesuch Formation resulted in copper sulfide mineralization in response to a change in oxidation state. Economic‐grade mineralization within the White Pine ore district is a consequence of intense focusing of compaction‐driven discharge, and corresponding amplification of leakage into the basal Nonesuch Formation, where the basal aquifer thins dramatically atop the Porcupine Mountains volcanic structure. Equilibrium geochemical modeling and mass‐balance calculations support this conclusion. We also assessed whether topography and density‐driven flow systems could have caused ore genesis at White Pine. Topography‐driven flow associated with the Ottawan orogeny was discounted because it post‐dates main‐stage ore genesis and because recent seismic interpretations of basin inversion indicates that basin geometry would not be conductive to ore genesis. Density‐driven flow systems did not produce focused discharge in the vicinity of the White Pine ore district.

Paleomagnetism of Middle Proterozoic mafic intrusions and Upper Proterozoic (Nankoweap) red beds from the Lower Grand Canyon Supergroup, Arizona

Paleomagnetic data from lavas and dikes of the Unkar igneous suite (16 sites) and sedimentary rocks of the Nankoweap Formation (7 sites), Grand Canyon Supergroup (GCSG), Arizona, provide two primary paleomagnetic poles for Laurentia for the latest Middle Proterozoic (ca. 1090 Ma) at 32°N, 185°E (dp=6.8°, dm=9.3°) and early Late Proterozoic (ca. 850–900 Ma) at 10°S, 163°E (dp=3.5°, dm=7.0°). A new 40Ar/ 39Ar age determination from an Unkar dike gives an interpreted intrusion age of about 1090 Ma, similar to previously reported geochronologic data for the Cardenas Basalts and associated intrusions. The paleomagnetic data show no evidence of any younger, middle Late Proterozoic tectonothermal event such as has been revealed in previous geochronologic studies of the Unkar igneous suite. The pole position for the Unkar Group Cardenas Basalts and related intrusions is in good agreement with other ca. 1100 Ma paleomagnetic poles from the Keweenawan midcontinent rift deposits and other SW Laurentia diabase intrusions. The close agreement in age and position of the Unkar intrusion (UI) pole with poles derived from rift related rocks from elsewhere in Laurentia indicates that mafic magmatism was essentially synchronous and widespread throughout Laurentia at ca. 1100 Ma, suggesting a large-scale continental magmatic event. The pole position for the Nankoweap Formation, which plots south of the Unkar mafic rocks, is consistent with a younger age of deposition, at about 900 to 850 Ma, than had previously been proposed. Consequently, the inferred ∼200 Ma difference in age between the Cardenas Basalts and overlying Nankoweap Formation provides evidence for a third major unconformity within the Grand Canyon sequence.

Evaluation of Duluth Complex anorthositic series (AS3) zircon as a U-Pb geochronological standard: new high-precision isotope dilution thermal ionization mass spectrometry results

U-Pb zircon geochronology is increasingly called upon to achieve the resolution of absolute time at the 0.1% to 1% level for rocks of Phanerozoic to Hadean age. Doing so requires accurate calibration of the several methods (conventional isotope dilution thermal ionization mass spectrometry [ID-TIMS], Pb evaporation, high-resolution ion microprobe [e.g. SHRIMP], and laser ablation inductively coupled plasma mass spectrometry [LA-ICPMS]) currently in use, in numerous laboratories, for the analysis of U and Pb isotopes in accessory minerals. Toward this end, the geochronological community would benefit from the establishment, distribution and widespread analysis of one or more standard reference materials. Among the candidates is natural zircon from the Duluth Complex anorthositic series of the Midcontinent Rift system of North America. These zircons, first dated by conventional ID-TIMS at 1099.1 ± 0.5 Ma, have been subsequently adopted as a geochronological standard by a number of high resolution ion microprobe facilities. A new and independent analysis of the systematics of a large set of single zircons (n = 27) from the same mineral separate yields indistinguishable 207Pb/ 206Pb, upper intercept, and U-Pb concordia dates for the AS3 zircons. The concordia date, based on a subset of 12 concordant and equivalent zircons, of 1099.1 ± 0.2 Ma (±1.2 Ma considering systematic uncertainties in Pb/U tracer calibration and U decay constants) is indistinguishable from previously published results. We further document the absence of inherited Pb in the AS3 zircons, and discuss strategies for avoiding certain domains within the AS3 zircons exhibiting small amounts of radiation-induced, surface and fracture-correlated, recent Pb loss. Although the AS3 zircons do not represent the ideal (and elusive) homogeneous closed U-Pb system, we conclude that these and similar zircons from the Duluth Complex anorthositic series can provide a suitable geochronological reference standard for numerous U-Pb zircon analytical methods, given appropriate preparation guided by the results of this study. Our high-precision data set also serves as a useful confirmatory test of the currently accepted U decay constants.

Magmatism of the mid-Proterozoic Gardar Province, South Greenland: chronology, petrogenesis and geological setting

A lithospherically weak zone embracing the southern margin of the Greenland Archaean craton and the Ketilidian Julianehåb batholith underwent repeated rifting during the interval 1350–1140 Ma, accompanying breakup of Palaeopangaea. The Gardar Province comprises the area affected by the faulting and asssociated alkaline magmatism. While an estimated 2–5 km of Proterozoic cover has been eroded, rift-fill successions have been preserved in early fault-bounded basins. The orientation of dyke swarms changed from WNW–ESE to nearly NE–SW during Gardar times. The principal swarms are inferred to occupy zones of lithospheric thinning and graben development. Central-type intrusive complexes, largely of syenites and nepheline syenites, reached shallow levels. The principal magmatic evolution was from transitional olivine basalt through to phonolites. Accompanying silica-oversaturated magma generation involved greater degrees of crustal assimilation. Anorthositic xenoliths in the Gardar intrusions imply the presence of an extensive anorthositic complex at depth, regarded as an integral part of the North American Proterozoic massif anorthosite association. The most primitive Gardar basalts are themselves relatively evolved, probably as a result of olivine±pyroxene fractionation during crustal underplating. The Gardar basic rocks are troctolitic with elevated Al 2O 3/CaO ratios: their incompatible element patterns suggest a significant input from lithospheric sources. The Ca-deficient nature of the Gardar basalts is attributed to an origin involving lithospheric mantle depleted by previous melting events. Trace element and isotopic signatures suggest considerable heterogeneity in the mantle sources which are ascribed to differential metasomatism of clinopyroxene-poor peridotite sources by small-fraction asthenospheric melts. The ultramafic lamprophyre/carbonatite association that recurred throughout the period is inferred to have originated from melting of metasomites deep within the lithospheric mantle. Affinities between the alkaline intrusions over an interval of >100 Ma characterize the Gardar as a coherent magmatic province and support the contention that the magmas are largely of lithospheric origin. The energy required to generate the very large requisite volumes of primitive magmas may have been supplied by successive mantle plumes. The Gardar magmatism pre- and post-dates the ∼1.27 Ga Mackenzie Igneous Events of North America but wholly pre-dates the ∼1.1 Ga Keweenawan magmatism associated with the Midcontinent Rift.

Post-rift deformation of the Midcontinent rift under Grenville tectonism

Using a thin-viscous-sheet model, we investigate the hypothesis that compressive stresses resulting from the Grenville orogeny caused uplift along the Midcontinent rift. The objective of this study is to investigate factors involved in shaping the post-rift structure of the Midcontinent rift including: the size and location of the colliding terranes and the convergence angle between the colliding terranes and the North American craton. We find a linear relationship between the size of the terrane and the deformation within the Midcontinent rift. The calculations predict comparable deformation along both the eastern and western arms of the Midcontinent rift. Our results demonstrate that the geologically indicated NE–SW deformation can be produced without a multistage compression event because of the nonlinear interaction of far-field compressive stress and the mechanically weak rift. Therefore, it is not possible to determine which compression direction is representative of the compression associated with the Grenvillian orogenic event based on the post-rift pattern of deformation.

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 character. With time and development of the Midcontinent rift, degrees of melting increased; mean pressures of melting decreased, reducing garnet control; and lithospheric source contributions waned. Similar temporal variations during flood basalt evolution have been documented elsewhere.

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.

Seismic heterogeneity and anisotropy in the Western Superior Province, Canada: insights into the evolution of an Archaean craton

Abstract The Superior Province, which forms the nucleus of North America, is the largest preserved Archaean crustal block in the world and may have originated as the result of a widespread crustal accretion event ( c. 2.7 Ga) manifested in Archaean cratons worldwide. An understanding of the accretionary evolution of this craton is the objective of the continuing Lithoprobe transect in the Western Superior Province, Canada. The geophysical components of the transect include seismic reflection and refraction, magnetotellurics and teleseismic experiments. The Teleseismic Western Superior Transect (TWiST) was designed to explore the structural and physical properties of the subcrustal lithosphere and their implications for proposed accretionary models. A north-south-trending array of 17 broadband three-component seismometers was deployed between May and November 1997. Surface-wave analyses, SKS-splitting studies and travel-time tomography show variations in the velocity structure and anisotropy between the southern end of the transect, a region affected by Keweenawan rifting, and the northern part, which lies in the Proterozoic Trans-Hudson shear zone. Surface waves reveal evidence for a thin high-velocity layer, 5–20 km thick, beneath a 37–43 km thick crust and above c. 250 km of high-velocity continental root. This thin layer is also visible in wide-angle refraction data from the southern end of the line and may be evidence of underplating during terrane accretion. Discrepancies in the Love and Rayleigh waves and surface-wave particle motions show evidence for an anisotropic mantle. SKS analysis shows large amounts (up to 2 s) of shear-wave splitting with a roughly eastwest trend in the fast-shear-wave polarization direction for most stations. This conforms with crustal deformation trends. Stations in the younger Trans-Hudson orogen show much less splitting. Detailed analysis at a permanent station in the Western Superior shows evidence for two layers of anisotropy. A thinner upper layer is aligned with the surface geology, indicating crust-mantle coupling during craton formation, whereas a thicker lower layer is aligned with the direction of absolute plate motion. Tomographic results show a featureless mantle beneath the Sachigo proto-craton and more heterogeneity towards the south end of the line. A steeply dipping slab-like feature in the lithosphere correlates with wide-angle refraction and deep-reflection seismic profiles. A similar high-velocity feature continues well into the transition zone, but its origin remains to be understood. Towards the southern end of the line there is a deep-seated low-velocity anomaly, which may be associated with Keweenawan plume activity. As a whole, the seismic results show many features that support ideas of subduction-related accretion of a thick stable Archaean tectosphere. There are, however, interesting details that are to date unique to the Western Superior Province. These include thicker than normal Archaean crust, a slab-like velocity anomaly in the mantle transition zone, and large SKS splitting in the Archaean Superior Province but little splitting in the surrounding Trans-Hudson Proterozoic shear zone.

The 1.1-Ga Midcontinent Rift System, central North America: sedimentology of two deep boreholes, Lake Superior region

Abstract The Midcontinent Rift System (MRS) of central North America is a 1.1-Ga, 2500-km long structural feature that has been interpreted as a triple-junction rift developed over a mantle plume. As much as 20 km of subaerial lava flows, mainly flood basalts, are overlain by as much as 10 km of sedimentary rocks that are mostly continental fluvial red beds. This rock sequence, known as the Keweenawan Supergroup, has been penetrated by a few deep boreholes in the search for petroleum. In this paper, two deep boreholes in the Upper Peninsula of Michigan are described in detail for the first time. Both the Amoco Production #1-29R test, herein referred to as the St. Amour well, and the nearby Hickey Creek well drilled by Cleveland Cliffs Mining Services, were 100% cored. The former is 7238 ft (2410 m) deep and the latter is 5345 ft (1780 m) deep. The entirety of the stratigraphic succession of the Hickey Creek core correlates very well with the upper portion of the St. Amour core, as determined by core description and point-counting of 43 thin sections selected out of 100 studied thin sections. Two Lower Paleozoic units and two Keweenawan red bed units—the Jacobsville Sandstone and the underlying Freda Sandstone—are described. The Jacobsville is largely a feldspatholithic sandstone and the Freda is largely a lithofeldspathic sandstone. Below the Freda, the remaining footage of the St. Amour core consists of a thick quartzose sandstone unit that overlies a heterogenous unit of intercalated red bed units of conglomerate, sandstone, siltstone, and shale; black shale; individual basalt flows; and a basal ignimbritic rhyolite. This lower portion of the St. Amour core presents an enigma, as it correlates very poorly with other key boreholes located to the west and southwest. While a black shale sequence is similar to the petroleum-bearing Nonesuch Formation farther west, there is no conglomerate unit to correlate with the Copper Harbor Conglomerate. Other key boreholes are distributed over a 1300-km distance along the better known southwest arm of the triple-junction MRS, and can be correlated rather well with the units that are exposed in the Lake Superior region. However, a definitive explanation of the anomalous, deeper St. Amour stratigraphy is elusive and any explanation is tenuous. A possible explanation for this anomalous stratigraphy may be the geographic proximity of the St. Amour borehole to the Keweenawan Hot Spot (mantle plume), the suggested thermal force behind the development of the MRS. Similarly, a drastic change in structural architecture may be explained by this geographic relationship. Thus, within the locale of this rifting center, complexities of expansion tectonics may well be responsible for igneous and sedimentary sequences that differ considerably from those found farther west along the rift arm.

New Kimberlite Discoveries in Kansas: Magnetic Expression and Structural Setting

A total of 13 kimberlites have been located in a two-county area in northeastern Kansas along the NNE-trending trace of the Proterozoic Midcontinent Rift System, an aborted rift extending from the Lake Superior region southwestward into Oklahoma.The Kansas Geological Survey drilled three new kimberlites in 1999. They are the Baldwin Creek and Tuttle kimberlites located in Riley County and the Antioch kimberlite in Marshall County. They were discovered as part of a systematic ground follow-up of private aeromagnetic data (200–400-m-line spacing) collected by Cominco American Inc. in the early 1980's and donated to the Kansas Geological Survey in 1999. Six exposed kimberlites were discovered prior to 1970 and another four in the early 1980's, one of which is exposed at the surface.Cross-cutting major tectonic elements in NE Kansas are ideal for accommodating the intrusion of kimberlites. The major structural elements are regional NNE-trending, high-angle normal and reverse faults associated with the 1.10 Ga Midcontinent Rift System, and regional, older, high-angle normal and reverse, NW-SE striking, cross-cutting faults which offset portions of the rift. The presence of the rift is identified in regional gravity and magnetic data sets and evidenced in core and cuttings recovered from drill holes that penetrate Precambrian basement. Sedimentary rocks up to about 800 m (3,040 ft) thick and ranging in age from Ordovician to Permian overlie the Proterozoic basement.Ground magnetic surveys were conducted to model the kimberlites and to define targets for drilling and core recovery. These detailed ground magnetic data show that final emplacement of the kimberlite bodies is controlled by N40°W-striking structures, rather than the NNE-trending structures. Modeling of the magnetic anomalies shows that the contacts between the kimberlite bodies and the country rock are steeply dipping.The kimberlites are believed to be Late Cretaceous in age (about 90 my) and range in character from crater and diatreme facies in Riley County to possible hypabyssal facies in Marshall County. The three new kimberlites are buried under a shallow soil cover ranging in thickness up to 8 m (26 ft).

Zircon U-Pb geochronology of the Ottawan Orogeny, Adirondack Highlands, New York: regional and tectonic implications

Both single and multigrain U-Pb zircon thermal ionization mass spectrometry (TIMS) as well as sensitive high resolution ion microprobe (SHRIMP ll) dating of two suites of Adirondack granites have yielded ages constraining the principal tectonomagmatic events of the Ottawan Orogeny to the interval ca 1090–1035 Ma. The earliest of these consists of mildly A-type hornblende granites of the Hawkeye granite suite, multigrain samples of which define a tight age cluster of ca 1103–1093 Ma. Assemblages and fabrics in this suite demonstrate that it experienced the high-grade effects of the Ottawan Orogeny, thereby fixing the maximum age of the latter at ca 1090 Ma. The second suite consists of Lyon Mt. Granitic Gneiss, six samples of which cluster tightly at ca 1060–1045 Ma. Fabrics associated with this suite indicate a late- to post-tectonic origin thus fixing the minimum age for the Ottawan Orogeny. Especially critical are two samples of ca 1047 Ma fayalite granite that are essentially undeformed and must post-date tectonism. In addition, an undeformed pegmatite dike yields an age of 1034±8 Ma confirming the termination of Ottawan orogenesis by that time. It is suggested that the genesis of the Hawkeye suite is related to athenospheric heating of the crust due to far-field effects of contemporaneous magmatic events at the Midcontinent rift. Lyon Mt. Granitic Gneiss is interpreted as the result of deep crustal melting following delamination of the overthickened Ottawan orogen. Together with the results of metamorphic investigations, these two suites define a counterclockwise P–T–t loop for the Adirondacks during the Ottawan Orogeny. Geochronological and tectonic investigations from the Grenville Province of Canada and the northern Blue Ridge Province of the Appalachians demonstrate the presence of strong Ottawan deformation, magmatism, and metamorphism in these areas and emphasize the large scale and marked intensity of this event. In Canada, the crust responded to the Ottawan collision principally by imbricating into large northwest-directed thrust slices rather than the fold nappes of the Adirondacks. This is consistent with the apparent absence in the Canadian foreland of Hawkeye age magmatism and resultant rheological weakening of the crust.

Mechanisms and patterns of O and H isotopic exchange during hydrothermal alteration of the North Shore Volcanic Group and related hypabyssal sills, Midcontinent Rift System, Minnesota

Hydrothermal flow systems developed in volcanic and interflow sedimentary rocks in response to the emplacement of ∼1.1Ga mafic plutonic rocks during intracontinental rifting in the midcontinent region. Oxygen and hydrogen isotopic studies of the North Shore Volcanic Group (NSVG) (northeastern Minnesota) and related hypabyssal sills demonstrate the importance of both permeability and temperature in controlling isotopic exchange in the rift-related hydrothermal system. Oxygen isotopic values of mineral separates and volcanic whole rocks range from 5.5‰ to 17.7‰, and show a strong correlation with whole rock water content and intensity of hydrothermal alteration. In the shallow portion of the stratigraphic sequence, elevated δ 18O values and mineralogical alteration are confined principally to highly permeable amygdaloidal flow tops and bottoms. Massive flow interiors show little or no perturbation from δ 18O values considered normal for fresh basaltic rocks. With increased depth and temperature, isotopic exchange becomes more pervasive, δ 18O values remain elevated, and oxygen isotopic profiles show no difference between flow margins and interiors. At depths where greenschist facies mineral assemblages are found, flow margins show a reversal to δ 18O values as low as 6.6‰ and δD values decrease to <−70‰. Both increased temperature and the involvement of a relatively D-depleted fluid at depth have played a role in lowering previously elevated δ 18O and δD values. Enhanced rates for hydrogen isotopic exchange relative to oxygen are suggested by whole rock δD values that tend to be uniform throughout flows that are less than ∼7 m thick. δD profiles that parallel those of δ 18O values are found in thicker flows. The correlation between degree of hydrothermal alteration and δ 18O values suggests that solution-recrystallization was the primary mechanism of isotopic exchange in the hydrothermal system. Individual feldspar phenocrysts from a single lava flow may show a range of δ 18O values from 10.5‰ to 17.7‰, and ranges in individual altered phenocrysts may be as large as 4.8‰. Feldspar isotopic heterogeneity suggests that permeability varied on a grain to grain scale, and that flow through microfractures was important in controlling isotopic variations. Coexisting plagioclase and pyroxene from hypabyssal sills show a very steep slope on a δ 18O plagioclase– δ 18O pyroxene plot, also suggestive of kinetically dominated exchange. Assuming a fluid-buffered system in the permeable flow margins, a duration of isotopic exchange on the order of 40,000 years is calculated. This time period is short compared to the lifetime of the intrusive activity in the rift, and suggests that fluid flow was either episodic in nature, or confined to the late stages of magmatic evolution.

00/02425 A process and apparatus for treating solid fuel materials

LITHOPROBE is Canada's national, collaborative, multidisciplinary geoscience project directed toward an enhanced understanding of how the North American continent has evolved. Research in its eight transects, which span the country and geological time, are spearheaded by seismic reflection studies. These, combined with other studies, provide new insight into the varied tectonic processes that have been active in forming the continent. The Archean Kapuskasing Structural Zone, a prominent feature which cuts obliquely across east-west Superior Province structures, has been imaged as a low-angle thrust sheet above a major décollement, along which 55–70 km of crustal shortening has occurred to bring mid-crustal rocks to the surface. The shortening was accomplished by brittle faulting and erosion at levels above 20 km and ductile shear and flow in the lower crust. The GLIMPCE transect in Lake Huron shows a spectacular series of east-dipping crustal-scale reflections that coincide with the Grenville Front Tectonic Zone. A two-stage model involves terrane collision with the cratonic margin in the Early Proterozoic followed by crustal-scale stacking and ramping associated with the Middle Proterozoic Grenvillian orogeny. In Lake Superior, GLIMPCE data reveal the structure of the Middle Proterozoic mid-continent rift that almost splits North America. The Lithoprobe East transect covering the Paleozoic Newfoundland Appalachians reveals three lower crustal. blocks, defined seismically on the basis of reflectivity characteristics imaged along the seismic profiles. Reflective upper crust can be related to geological structures at the surface; the thin-skinned tectonics implied have yet to be reconciled with the block tectonics of the lower crust. Results from the southern Canadian Cordillera show that the Mesozoic-Cenozoic east-west shortening in the eastern and central Cordillera occurred by compressional deformation forming crustal-scale antiforms that overrode a westward thinning cratonic North America; and in the western Cordillera by subduction processes that have tectonically eroded the lower lithosphere of accreted terranes and left underplated sediments and oceanic lithosphere. The reflection Moho has highly variable characteristics as imaged within transects and among different transects. Crustal and Moho reflectivity is caused by a wide range of features, including fault/shear zones, lithologie contacts, compositional layering, fluids in zones of high porosity, and anisotropy.