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The McEwan Lake fault: gravity evidence for a new structural element of the Kapuskasing zone
New constraints on the Slate Islands impact structure: Comments and Reply
Constraints on the nature of the Kapuskasing structural zone from the study of Proterozoic dyke swarms
Synopsis of paleomagnetic studies in the Kapuskasing structural zone
Broad-scale Proterozoic deformation of the central Superior Province revealed by paleomagnetism of the 2.45 Ga Matachewan dyke swarm
Paleomagnetism of dykes from the Groundhog River Block, northern Ontario: implications for the uplift history of the Kapuskasing Structural Zone
Regional variation in paleomagnetic polarity of the Matachewan dyke swarm related to the Kapuskasing Structural Zone, Ontario
The tectonic relationship of two Early Proterozoic dyke swarms to the Kapuskasing Structural Zone: a paleomagnetic and petrographic study
Paleomagnetism and orientation of Precambrian dykes, eastern Lake Superior region, and their use in estimates of crustal tilting
Compositional characteristics of the Kenora–Kabetogama dyke swarm (Early Proterozoic), Minnesota and Ontario
Paleomagnetism, structure, and longitudinal correlation of Middle Precambrian dykes from northwestern Ontario and Minnesota
THE AGE OF THE NORTH MOKKA ANTICLINE, AXEL HEIBERG ISLAND, CANADIAN ARCTIC ARCHIPELAGO: AN APPLICATION OF THE PALEOMAGNETIC FOLD-TEST
Structural analysis of shatter cones from the Slate Islands, northern Lake Superior
There have been many U-Pb, Rb-Sr., and K-Ar geochronologic studies of Keweenawan rocks. U-Pb results on zircons by Silver and Green show that most of the igneous activity (by volume) occurred 1,110 ± 10 m.y. ago in the Lake Superior region. This includes rocks of the upper normal magnetic polarity sequence as well as upper units of the underlying reversed magnetic polarity sequence, thus dating that reversed-to-normal change at 1,110 ± 10 m.y. ago. Many of the Rb-Sr and K-Ar results on the 1,110 m.y. old units are concordant although in many other instances there is clear discordance with the Rb-Sr and K-Ar ages being too young. Other units, not yet dated by U-Pb methods, also give young ages, potentially suggesting that upper Keweenawan igneous activity may have extended to as young as 900 m.y. ago. However, review of paleomagnetic pole positions for such units shows no evidence for such young crystallization ages; the paleomagnetic data are consistent with all younger units being about 1,110 m.y. old. K-Ar results suggest ages of 1,150-1,250 m.y. for stratigraphically older units (for example, Logan Sills) of the reversed sequence and, along with Rb-Sr results, for normal polarity dikes of the Sudbury dike swarm. Paleomagnetic pole positions are also consistent with early Keweenawan igneous activity occurring about 1,200 m.y. ago. Thus, we conclude that Keweenawan rifting and associated igneous activity began 1,200-1,225 m.y. ago, peaked at 1,110 m.y. ago, and ceased shortly thereafter.
More than 60 individual paleomagnetic poles have been obtained by various workers in the last 20 years from late Precambrian Keweenawan rocks of the Lake Superior region. Nearly all major formations and intrusive units have been subject to at least one paleomagnetic study. Keweenawan rocks thus represent paleomagnetically the world’s most intensely studied rock sequence, one that may span a time interval from about 1.2 to 1.0 b.y. ago. The large amount of paleomagnetic data coupled with locally excellent stratigraphic and structural control allows an examination of the extent to which factors other than continental displacement determine the distribution of Precambrian paleopoles. Keweenawan paleomagnetic poles of both normal and reversed polarity plot along a northeast-southwest trending band in the North Central pacific. Stratigraphic and radiometric evidence suggests that within this polar distribution there is a hairpin-shaped path open to the southwest (the so-called Logan Loop) along which there appears to be an anticlockwise polar movement with time. After filtering of the pole population using certain reliability criteria, the width of the better documented western arm of the loop decreases from 20 to 10 degrees of arc along an arc length of about 70 degrees. A smooth narrow polar path is thus produced by selecting those poles for which errors due to sampling density, structural correction, and unremoved secondary components are considered to be a minimum. Although much of the dispersion in pole position may be caused by uncertainties in the paleomagnetic data and associated geological constraints, the gross form of the loop appears to result from two superimposed effects: an apparent movement of the pole relative to the North American continent and a fictitious one arising from a violation in the assumption of a geocentric axial dipole to calculate pole positions. The latter effect is revealed by successive asymmetric reversals that can be explained neither by the presence of an unremoved secondary component nor by continental motion. The Keweenawan apparent polar wander path and that for a contemporaneous sequence from the Grand Canyon, Arizona, agree closely if only normal poles are used. In this case both paths have a similar form to the Logan Loop but are more subdued. While the Keweenawan reversed data also appear to follow an arcuate path, the arc is displaced to the northeast of the normal one as a possible consequence of non-geocentric dipole field behavior. However, both paleointensity and paleosecular variation results from Keweenawan igneous rocks are compatible with the usual assumption of a geocentric dipole and with a change to higher paleolatitudes during times of reversed polarity, but it is possible that some non-geocentric dipole model could also explain these data. Although a regional secondary component can be discounted as the cause of Keweenawan reversal asymmetry, other generally minor components are present with different directions and origins. They may be due to late Keweenawan igneous activity, burial of the Keweenawan sequence, Grenville tectonism, emplacement of copper-bearing ores, and in one instance, possible meteorite impact. Some of these magnetic overprints appear to have formed within a time period of about 1.0 to 0.8 b.y. ago and are thus important as they lie in an age interval poorly represented in North American paleomagnetic data.
Compilation maps of crustal time-terms and apparent thickness in the Lake Superior region reveal an area where the Moho is anomalously deep with crustal thicknesses ranging from about 45 to perhaps more than 50 km. This anomalous region is centered on the eastern and central parts of Lake Superior, on the axis of the Midcontinent Rift System. Another region of crustal thickening also occurs further west along the rift in Wisconsin, but the intervening region appears characterized by more normal crustal thicknesses of about 40 km. Seismic data from eastern Ontario indicate that a more concerted crustal seismic study of the Kapuskasing Structural Zone may be warranted.