Pre-Timiskaming Folding in the Archean Southern Abitibi Greenstone Belt, Ontario and Quebec, Canada: Structural Constraints and Conundrums

Ben M. Frieman, Colorado School of Mines
Yvette D. Kuiper, Colorado School of Mines
Thomas Monecke, Colorado School of Mines
Nigel M. Kelly, Colorado School of Mines

Preliminary structural observations from the southern Abitibi subprovince (SAS) of Ontario and Quebec, Canada, suggest that tight to isoclinal folding occurred before deposition of the ~2680-2670 Timiskaming episode sedimentary rocks. Rocks affected by pre-Timiskaming structure display little to no internal deformation and the mechanism of folding is therefore unclear. The SAS is one of the largest continuous, best exposed, and well-studied Archean greenstone belts in the world. The geological history of the SAS is characterized by a relatively long phase (>2750-2695 Ma) of seafloor volcanism followed by uplift, sedimentary basin formation, and deformation. In the SAS, two major episodes of sedimentary basin formation are recognized based on temporal and stratigraphic constraints: the ~2695-2685 Ma Porcupine episode and the ~2680-2670 Ma Timiskaming episode. Porcupine episode sedimentary rocks are dominated by turbiditic facies rocks deposited in a submarine setting. Timiskaming episode sedimentary rocks contain fluvial to alluvial conglomerate and sandstone units. Throughout the study area, Timiskaming episode rocks unconformably overlie older volcanic greenstone and Porcupine episode rocks. Here we document a number of localities that expose this unconformity and define pre-Timiskaming structure throughout the SAS, including from west to east: the Kenogami, Perron, Doig, Grace, and Rouyn unconformities. At these localities, layers with opposite facing directions in the older volcanic greenstone are truncated by the Timiskaming unconformity, suggesting the existence of isoclinal folds prior to sedimentation. Some localities display back-to-back facing relationships between Timiskaming and older stratigraphic units while others display relatively high angle angular unconformities. Fold hinges are rarely observed in the field and are commonly inferred from map patterns. Based on these observations, pre-Timiskaming folds are characterized by 1-10 kilometer wavelengths and shallow plunges. Post-Timiskaming deformation resulted in the development of penetrative fabrics in all rocks of the SAS and is characterized by flattening fabrics and either a steeply plunging or subhorizontal lineation, especially along long-lived, crustal deformation zones that crosscut the SAS. Typically, no penetrative fabric is associated with the pre-Timiskaming folds away from these deformation zones. In general, formation of isoclinal folds requires substantial pure shear strain, or, more commonly, simple or general shear strain, all of which typically result in associated penetrative fabrics such as foliations, lineations, and/or shear fabrics. The absence of such fabrics in association with pre-Timiskaming isoclinal folds and the preservation of pristine primary structures such as volcanic pillows in these folded rocks remain problematic. The potential mechanisms for formation of these folds will be discussed.