STRAIN FABRICS ALONG AN ARCHEAN SUBPROVINCE BOUNDARY, ONTARIO, CANADA: EVIDENCE FOR A MODIFIED TRANSPRESSION MODEL.
Czeck, D. M. and Hudleston, P. J., Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455
The goal of this work has been to study the processes that led to accretion of island arcs in the Archean Superior Province especially along a boundary between two subprovinces, the metavolcanic Wabigoon and metasedimentary Quetico. We have documented that the current kinematic model of transpression needs to be modified and have proposed a revised model to be further tested.
The most widely accepted kinematic model based on field observations is transpression (Hudleston et al. 1988). Transpression involves pure shear in the vertical plane and simple shear, perpendicular to pure shear, in the horizontal plane. This model requires that the stretching lineation be either vertical or horizontal, depending on the angle of collision and the amount of deformation (Fossen & Tikoff 1993). In the field, most rock fabrics that one would expect to see with transpression are found. However, the mineral lineations are not vertical or horizontal, they plunge between 0-90°. This requires that a more complex kinematic model be proposed. While there is no unique kinematic solution to describe oblique lineations within a transpression zone, some models can be refuted by additional field evidence. Two likely models have been addressed.
The first model to create oblique lineations involves transpression with an additional thrust component (Merle 1986). Thrusting has been thought to be a dominant part of the tectonic history due to evidence of early recumbent nappe structures (Poulsen et al. 1980) which suggest similar strain partitioning to that described by Merle (1986). Although this model explains oblique lineations, it does not create a range of oblique lineations. Also, there is no evidence for asymmetrical features on the vertical plane perpendicular to the foliation plane. Therefore, this model seems unlikely.
The second model to create oblique lineations is derived from a new physical laboratory experiment. If small amounts of material are heterogeneously extruded along with transpression, a range of lineations form. This is consistent with a model of anastomosing shear zones (Poulsen 1986) which create lozenge shapes. These shear zones surround less intensely strained blocks which rotate due to the noncoaxial strain component; these rotations create similar extrusion outlets based on lozenge geometry. This model is consistent with the observation that the lineations have a wide range of plunges. In order to test this transpression/ heterogeneous extrusion model, the intensity of strain and the lineations need to be correlated in the field. To be consistent with the model, the most oblique lineations should be found near the intensely strained areas whereas vertical lineations should be found in the centers of the blocks.
Fossen, H. & Tikoff, B. 1993. The deformation matrix for simultaneous simple shearing, pure shearing and volume change, and its application to transpression- transtension tectonics. Journal of Structural Geology 15, 413-422.
Hudleston, P. J., Schultz-Ela, D. D. & Southwick, D. L. 1988. Transpression in an Archean greenstone belt, northern Minnesota. Canadian Journal of Earth Sciences 25, 1060-1068.
Merle, O. 1986. Patterns of stretch trajectories and strain rates within spreading-gliding nappes. Tectonophysics 124, 211-222.
Poulsen, K. H. 1986. Rainy Lake Wrench Zone: An example of an Archean Subprovince boundary in Northwestern Ontario. In: Tectonic evolution of greenstone belts Technical Report (edited by de Wit, M. J. & Ashwal, L. D.) 86-10. Houston TX, Lunar and planetary Inst., 177-179.
Poulsen, K. H., Borradaile, G. J. & Kehlenbeck, M. M. 1980. An inverted Archean succession at Rainy Lake, Ontario. Canadian Journal of Earth Sciences 17, 1358-1369.
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