A SEQUENCE OF FOLDING AND DEXTRAL TRANSPRESSION IN THE SEINE RIVER CONGLOMERATES.

CZECK, Dyanna M. and HUDLESTON, Peter J., Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis MN, 55455, czec0019@tc.umn.edu

GEOLOGICAL INTRODUCTION

The Superior Province is comprised of approximately east - west trending subprovinces that are defined by lithological contrasts, metamorphic grade, and structural boundaries (Card and Ciesielski, 1986). A popular tectonic model for this province is the successive accretion of island arcs and their associated sediments (Hoffman, 1989). This study focuses on the boundary between the metavolcanic Wabigoon and the metasedimentary Quetico subprovinces near Mine Centre, Ontario. The Quetico contains mostly amphibolite facies metasediments, and the Wabigoon contains greenschist facies metavolcanics. Distinctive rocks found along this boundary are the Seine River Metasedimentary Group including the polymictic Seine River conglomerates. From stratigraphic evidence, the Seine River conglomerates formed late following the deposition of the volcanic sequences (Poulsen et al., 1980). Preliminary attempts at absolute dating have confirmed this (Davis et al., 1989). Previous structural work along this boundary described a dominant subvertical foliation transecting large folds (Poulsen, 1986; Tabor and Hudleston, 1991). These features are interpreted to have formed during dextral transpression.

STRUCTURAL INTRODUCTION

The term "transpression" was first introduced by Harland (1971) to describe motion that is obliquely convergent, or motion partitioned into both convergent and strike-slip. In Harlandıs original paper, he described the development of folds and strike-slip structures due to oblique convergence. A specialized case of transpression was introduced by Sanderson and Marchini (1984). This idealized definition of transpression has been used to describe homogeneous deformation consisting of orthogonal simple shear and pure shear components with constant volume and confined deformation boundaries, possibly corresponding to the strain in deep ductile shear zones that accommodate oblique convergence. The dominant foliation along the Wabigoon - Quetico boundary seems to be related to this more specific type of transpression based on the flattening fabrics and asymmetric features indicating simple shear in the subhorizontal plane. One of the questions addressed by this study is the relationship between the folds and the dominant foliation. Clearly, it has been demonstrated that folds can form in transpression Harland (1971). Are the folds at this boundary consistent with the rest of the transpression fabric, or are they an earlier structural feature?

FOLIATION/ BEDDING INTERSECTIONS

The Seine Group has well defined bedding indicated by fine grained layers interbedded with the pebble conglomerates. In this analysis, the major fold in the Seine River conglomerates was recognized on the basis of structural facing between bedding and the dominant foliation (Shackleton's rule ­ see Borradaile (1976)). This fold is consistent with the observed "way up" indicators in the conglomerates based on graded bedding. If this fold were to have formed as a result of the transpression, the foliation/ bedding intersections should be consistent throughout the field area. In fact, the plunge of the intersection varies throughout the field area. The intersection angle is generally small because both foliation and bedding are steep; however, locally the angle is quite large nearing 75° at the hinge. This suggests a more complex history, and that the folding occurred prior to the major transpressive event.

TECTONIC IMPLICATIONS

As described elsewhere in the Superior province, the sequence of initial folding followed by homogeneous transpression (and possibly later stage amplification of the strike- slip shear component along discrete shear zones) seems reasonable for the Wabigoon- Quetico boundary. It remains an interesting question whether the folds and the homogeneous transpression were two entirely abrupt, separate events or rather an evolution of structural styles during different stages of the same oblique collisional event. The fact that the initial folding is seen in the Seine Group has important implications for the relative timing between this change in structural style and Seine deposition. Some researchers have suggested that the Seine Group could have been syntectonically deposited in pull-apart basins formed as a result of wrenching (Poulsen, 1986). The early folding of the Seine seems to indicate that it was deposited earlier in the deformation sequence, prior to at least some of the folding. Additionally, the overall geometry of the interconnected faults seems to indicate that during dextral wrenching, the Seine Group's "basin" would have been in a compressive rather than an extensional regime required of a pull-apart basin. This suggests that a different sedimentation/ tectonic model for the Seine Group is needed.


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