Mel R. Stauffer

Manikewan: an early Proterozoic ocean in central Canada, its igneous history and orogenic closure

Abstract During Early Proterozoic time (Aphebian Period) much of the southwestern Churchill province of the Canadian shield was covered by a sea, called here the Manikewan Ocean. Although the original dimensions of this ocean are unknown, the highly deformed and metamorphosed rocks that represent its evolution and final closure as the Manikewan mobile belt now crop out over an area of at least 6 × 105 km2. Along strike (NE—SW) these rocks are buried at both ends by flat-lying Phanerozoic sediments, whereas, across strike, they are bounded by Archaean cratons, the Western craton to the northwest and Superior province to the southeast. Part of the Western craton and Manikewan mobile belt is overlain by the flat-lying, Middle Proterozoic, Athabasca sandstone, a remnant of a once wide-spread, predominantly fluvial group of rocks. Manikewan rocks can be divided into two major zones: 1. (1) The Cree Lake zone in the northwest consists of a predominantly meta-sedimentary succession which overlies unconformably the southeastern extension of the Western craton. This region appears to have been an ensialic stable platform to miogeosyncline during Aphebian time. 2. (2) The Reindeer Lake zone in the southeast consists of thick sequences of mainly mafic, island-arc type, meta-volcanic rocks (e.g., Amisk volcanics) and their meta-sedimentary equivalents. Locally these are overlain unconformably by thick molasse deposits (e.g., Missi Fm.). This region appears to have been an ensimatic eugeosyncline during at least part of Aphebian time. Numerous stocks and batholiths intrude the Manikewan mobile belt and form an integral part of it. They are mainly granitic and predominantly syn-tectonic. The largest, the Wathaman batholith is over 900 km long, making it the largest known Precambrian batholith. It lies along the junction of the Reindeer Lake and the Cree Lake zones. Radiometric dating is scanty, but what has been done indicates that closure of the Manikewan Ocean was well advanced by 1865 Ma and complete by 1800 Ma (Hudsonian orogeny).

Structure of a Paleoproterozoic continent-continent collision zone: a LITHOPROBE seismic reflection profile across the Trans-Hudson Orogen, Canada

Abstract An ~ 800 km reflection seismic profile across the Trans-Hudson Orogen, northern Saskatchewan and Manitoba, images crustal-scale tectonic imbrication in an unprecedented picture of Paleoproterozoic crustal accretion and continent-continent collisional tectonism. The profile is crudely symmetric about a crustal-scale culmination in the western part of an accreted juvenile collage (Reindeer Zone). Geologic and isotopic data suggest that this culmination is cored by microcontinental Archean basement. West of the culmination, highly reflective juvenile crustal elements dip westward into the lower crust, beneath the Wathaman Batholith and Archean continental crust of Hearne craton. To the east, strong reflections in the juvenile Reindeer Zone crust and reworked Archean foreland of the Thompson belt have eastward dips persisting into the middle crust and extending beneath the Superior craton. A continuous reflection Moho, well-defined for > 500 km in the western part of the profile, shows marked relief (12- > 15 s), including a prominent root below the crustal culmination. These imaged structures give evidence of substantial crustal shortening and thickening via large-scale imbrication consistent with collisional orogeny. W-dipping structures below the Wathaman Batholith and reworked Hearne craton may reflect subduction polarity in this part of the orogen. However, geological evidence suggests that E-dipping structures below Superior craton are largely related to late/post-collisional deformation, rather than to prior oceanic subduction polarity.

Seismic reflection images of high‐angle faults and linked detachments in the Trans‐Hudson Orogen

Postcollisional (1.8–1.7 Ga) intracontinental deformation in the Trans-Hudson Orogen (Canada) produced a series of orogen-parallel high-angle faults and folds. In seismic reflection profiles, the faults are imaged by subvertical zones of diffractions and truncated reflections that extend to 4–8 s (12–24 km). The folded and faulted upper part of the crust is underlain by laterally coherent shallow-dipping reflections that are locally bounded by discrete, highly reflective zones. These zones are interpreted as detachments (shear zones) and can be traced from the upper to lower crust, where some of them appear to pass into laterally continuous reflections that define the Moho. Two distinct regimes of postcollisional crustal deformation are inferred from the seismic images: high-angle faulting and lateral block extrusion in the upper crust and low-angle ductile shearing in the mid/lower crust. The surface geology indicates that the faults resulted in southwest (orogen-parallel) extrusion of the orogens internal zone relative to the bounding Archean Hearne and Superior cratons. Faulting was concurrent with the development of upright folds with trends that are subparallel to the extrusion direction. The seismic images suggest that the high-angle fold/fault structures are kinematically linked to low-angle detachments represented by laterally coherent, highly reflective zones. The detachment shear zones are inferred to have a top-to-the-southwest sense of shear associated with a subhorizontal, northeast-southwest extension direction, parallel to those observed for 1.83–1.80 Ga collisional shear zones exposed in major postcollisional fold culminations. Long-lived orogen-parallel extension is interpreted as a consequence of the boundary conditions imposed by the northeast trend of both the Superior and Hearne margins.

A cordilleran-type batholitic belt in the Churchill province in northern Saskatchewan

Abstract The Rottenstone and western La Ronge domains of the south—central Churchill province in Saskatchewan consist mostly of Hudsonian batholithic complexes spatially and structurally related to an Aphebian—Hudsonian volcanic arc. The batholithic belt is of comparable dimensions and compositional variation to Cordilleran-type Phanerozoic batholiths and appears to be emplaced in close proximity to a prior late Archean—early Aphebian continental crustal margin. A “quartz-diorite line” separating dominant quartz-diorite—tonalite—granodiorite plutonic rocks from dominant granodiorite—quartz-monzonite—granite intrusives can be defined. These general characteristics, as well as more detailed features of Aphebian deposition and Hudsonian thermotectonism within and on either side of the Rottenstone domain, suggest a continental margin, rather than an “intracontinental” location, for evolution of the Hudsonian orogenic belt of the south—central Churchill province and provide some insights into the possible development of this part of the Canadian shield as part of a Hudsonian Wilson Cycle.

Geodynamic evolution and thermal history of the central Flin Flon Domain, Trans‐Hudson Orogen: Constraints from structural development, 40Ar/39Ar, and stable isotope geothermometry

Shear zones in the Flin Flon area record five episodes of development. These have been studied using structural analysis and 40Ar/39Ar thermochronology. Deformation occurred as a result of collision between the Archean Rae-Hearne and Archean Superior plates during the Trans-Hudson Orogeny (1900 to 1700 Ma). The geodynamic evolution encompasses a transition from island arc supracrustal development (∼1900 Ma) to tectonic mountain building under brittleductile conditions (>1860 Ma), two episodes of ductile lineation-schistosity (L-S) shear zones on the prograde metamorphic path (1860–1810 Ma), and two episodes of retrograde displacement between 1790 and 1690 Ma, during final suturing of the Archean cratons with Paleoproterozoic assemblages. Metamorphism evolved in three stages. Contact aureoles to granitoid bodies formed at 1860–1840 Ma. Hornblende homfels facies 40Ar/39Ar ages indicate cooling through 500°C at ∼1840 Ma. This was followed by regional heating (burial), which produced regional peak thermal recrystallization at ∼1820–1790 Ma, under lower greenschist to lower amphibolite conditions. Lastly, retrograde metamorphism took place during unroofing between 1790 and 1690 Ma, with a regional cooling rate of 2 °C/m.y.

Penecontemporaneous Recumbent Folds in Trough Cross-Bedding of Pleistocene Sands in Saskatchewan, Canada

ABSTRACT Sets of trough cross-bedding in the glacio-fluvial sands and gravels of the Pleistocene Floral Formation near Saskatoon, Saskatchewan, are deformed into recumbent synclines by, the overturning of individual cross-strata. The direction of overturning of these folds is to the south and east, and is the same as the palaeocurrent direction indicated by the dips of cross-strata and the orientations of trough axes. These folds, which are exposed at several levels in the sequence have apical angles of from 30° to 60°, and approximately horizontal hinge lines. The trends of hinge lines vary greatly depending on position within an individual cross-bed set. Along the margins of troughs, hinge lines are at high angles to those near trough apices. All of the folds have upper contacts whic have been produced by erosion before deposition of the overlying layers. In general, the lower part of the folded bed is not deformed and the cross-stratification is tangential to the underlying layer. Each unit of deformed cross-bedding consists of one overturned fold, but in many places the hinge area and/or the upper limb of such a fold contains a series of smaller folds also with flat-lying axial planes. Considerations of kinematics of fold formation show that fold geometry is controlled largely by the shape of the shear profile, and by the initial inclination of the bedding. The distribution of the folds suggests that the process which caused the deformation occurred on several occasions during deposition of the Floral sands. The consistent pattern of the deformation and the lack of other types of deformed bedding suggest that liquefaction of the sediment was not common, that downslope slumping did not occur, and that ice-thrusting did not cause the deformation. It is believed that the folding was caused by frictional drag as a result of the passage of a mass of saturated sand over the surface of the cross-bedded sand. Such an interpretation is favoured also because this is the only mechanism which has produced overturning of cross-beds in laboratory experiments.

The shape distribution of splash-form tektites predicted by numerical simulations of rotating fluid drops

Splash-form tektites are glassy rocks ranging in size from roughly 1 to 100 mm that are believed to have formed from the splash of silicate liquid after a large terrestrial impact from which they are strewn over thousands of kilometres. They are found in an array of shapes including spheres, oblate ellipsoids, dumbbells, rods and possibly fragments of tori. It has recently become appreciated that surface tension and centrifugal forces associated with the rotation of fluid droplets are the main factors determining the shapes of these tektites. In this contribution, we compare the shape distribution of 1163 measured splash-form tektites with the results of the time evolution of a 3D numerical model of a rotating fluid drop with surface tension. We demonstrate that many aspects of the measured shape distribution can be explained by the results of the dynamical model.

Penecontemporaneous folds in cross-bedding: Inversion of facing criteria and mimicry of tectonic folds

Deformation of cross-bedding by downcurrent drag at or near the time of deposition of sediment can result in structures that resemble cross-bedding in the overturned limb of a tectonic fold. Penecontemporaneous deformation of individual cross-beds into recumbent folds also produces structures that may be easily mistaken for early phases of tectonic deformation in complexly folded terranes.

New Method for Mapping Fold Axial Surfaces

The common practice of mapping the axial trace of a fold by connecting points on a geological map where the strata within the fold undergo their most rapid rate of change of curvature is geometrically incorrect in most geological situations. A line drawn in this way will be oblique to or, in the case of an ideal similar-style fold, parallel, but not coincident, with the outcrop of the axial plane. A method for mapping the true axial trace of a cylindrical fold can be derived by considering the geometry of a cylinder (either circular or elliptical in right section) drawn tangent to a folded surface within the hinge zone of the fold. The axis of such a cylinder will lie along the axial plane. Furthermore, the cylinder will have an elliptical outcrop pattern (outcrop ellipse) on a plane topographic surface, the center of which will lie on the axial trace. The axial trace of a fold can be constructed by drawing the locus of the centers of outcrop ellipses for several stratigraphic contacts within the fold. This technique is nearly universal in its application and is largely independent of fold style.