William F. Cannon

Crustal structure of the Grenville front and adjacent terranes

Under the auspices of the Great Lakes International Multidisciplinary Program on Crustal Evolution, approximately 320 km of deep seismic reflection data were collected in Lake Huron along a profile that extends east from the Manitoulin terrane across the Grenville front to the interior of the Grenville orogen. The Manitoulin terrane is characterized by a series of gently east-dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer. Imaged by strong reflections at the western end of a spectacular band of moderately east-dipping reflections, the Grenville front clearly truncates Manitoulin terrane structures to the west. These data are interpreted in terms of a speculative two-stage model involving (1) creation of a major decollement during northward collision of an allochthonous terrane with the southern Superior cratonic margin (1.83-1.89 Ga; Penokean orogeny) and (2) northwest-directed stacking of microterranes at the southeastern margin of the craton, followed by crust-penetrating ductile imbrication under high-pressure-high-temperature conditions leading to the ramping of deeply buried rocks to the near surface (1.0-1.3 Ga; Grenvillian orogeny).

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.

Closing of the Midcontinent rift-A far—field effect of Grenvillian compression

The Midcontinent rift formed in the Laurentian supercontinent between 1109 and 1094 Ma. Soon after rifting, stresses changed from extensional to compressional, and the central graben of the rift was partly inverted by thrusting on original extensional faults. Thrusting culminated at about 1060 Ma but may have begun as early as 1080 Ma. On the southwest-trending arm of the rift, the crust was shortened about 30 km; on the southeast-trending arm, strike-slip motion was dominant. The rift developed adjacent to the tectonically active Grenville province, and its rapid evolution from an extensional to a compressional feature at ca. 1080 Ma was coincident with renewal of northwest-directed thrusting in the Grenvllle, probably caused by continent-continent collision. A zone of weak lithosphere created by rifting became the locus for deformation within the otherwise strong continental lithosphere. Stresses transmitted from the Grenville province utilized this weak zone to close and invert 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.

GLIMPCE Seismic reflection evidence of deep-crustal and upper-mantle intrusions and magmatic underplating associated with the Midcontinent Rift system of North America

Abstract Deep-crustal and Moho reflections, recorded on vertical incidence and wide angle ocean bottom Seismometer (OBS) data in the 1986 GLIMPCE (Great Lakes International Multidisciplinary Program on Crustal Evolution) experiment, provide evidence for magmatic underplating and intrusions within the lower crust and upper mantle contemporaneous with crustal extension in the Midcontinent Rift system at 1100 Ma. The rift fill consists of 20–30 km (7–10 s) of basalt flows, secondary syn-rift volcaniclastic and post-basalt sedimentary rock. Moho reflections recorded in Lake Superior over the Midcontinent Rift system have times from 14–18 s (about 46 km to as great as 58 km) in contrast to times of about 11–13 s (about 36–42 km crustal thickness) beneath the surrounding Great Lakes. The Seismically complex deep-crust to mantle transition zone (30–60 km) in north-central Lake Superior, which is 100 km wider than the rift half-graben, reflects the complicated products of tectonic and magmatic interaction of lower-crustal and mantle components during evolution or shutdown of the aborted Midcontinent Rift. In effect, mantle was changed into crust by lowering Seismic velocity (through intrusion of lower density magmatic rocks) and increasing Moho (about 8.1 km s−1 depth.

Speculations on the origin of the North American Midcontinent rift

Cannon, W.F. and Hinze, W.J., 1992. Speculations on the origin of the North American Midcontinent rift. In: P.A. Ziegler (Editor), Geodynamics of Rifting, Volume II. Case History Studies on Rifts: North and South America and Africa. Tectonophysics, 213: 49–55. 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.

Origin of deep crystal reflections: seismic profiling across high-grade metamorphic terranes in Canada☆

Abstract In an attempt to better understand the origin of deep crustal reflections LITHOPROBE has sponsored or co-sponsored Seismic reflection surveys across tracts of high-grade metamorphic rock in the Archean Superior craton, the Proterozoic Grenville orogen and the Phanerozoic Cordilleran orogen. Common to these three diverse terranes are near-surface zones of prominent Seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology. At some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks (stratified assemblages, sills, etc.), whereas in other regions it was generated by extreme attenuation, stretching and ductile flow of weakly layered or irregularly organized rocks. It seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust, with reflections originating at lithological boundaries and zones of mylonite.

Extraterrestrial demise of banded iron formations 1.85 billion years ago

In the Lake Superior region of North America, deposition of most banded iron formations (BIFs) ended abruptly 1.85 Ga ago, coincident with the oceanic impact of the giant Sudbury extraterrestrial bolide. We propose a new model in which this impact produced global mixing of shallow oxic and deep anoxic waters of the Paleoproterozoic ocean, creating a suboxic redox state for deep seawater. This suboxic state, characterized by only small concentrations of dissolved O 2 (~1 μ M ), prevented transport of hydrothermally derived Fe(II) from the deep ocean to continental-margin settings, ending an ~1.1 billion-year-long period of episodic BIF mineralization. The model is supported by the nature of Precambrian deep-water exhalative chemical sediments, which changed from predominantly sulfide facies prior to ca. 1.85 Ga to mainly oxide facies thereafter.

Immediate and long-term fire effects on total mercury in forests soils of northeastern Minnesota.

Within the Boundary Waters Canoe Area Wilderness in northeastern Minnesota, soils were collected from 116 sites in areas of primarily virgin forest with fire-origin stand years (year of last recognizable stand-killing wildfire) that range from the 1759 to 1976. Median concentrations for total mercury in soils for this span of 217 years range from 0.28 +/- 0.088 ppm (1759) to 0.09 +/- 0.047 ppm (1976) for A-horizon soils and from 0.23 +/- 0.062 ppm (1759) to 0.09 +/- 0.018 ppm (1976) for O-horizon soils. A separate study of soils collected from 30 sites within an area that burned in a 2004 wildfire at Voyageurs National Park, northern Minnesota, suggested that high soil burn severity resulted in significant mercury loss from both organic and mineral soils. Integrated data from these two studies and additional regional soil data demonstrate that older forests have progressively higher mercury concentrations in O-horizon soils (r(2) = 0.423) and A-horizon soils (r(2) = 0.456). These results support the hypotheses that an important factor for mercury concentrations in forest soils is time since stand-replacing fire and that high soil burn severity has the potential to reduce the concentration of mercury in burned soils for tens to hundreds of years.

Metallogeny of the midcontinent rift system of North America

Abstract The 1.1 Ga Midcontinent rift system of North America is one of the worlds 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.