A detachment fold model for fault zones in the Late Archean Abitibi greenstone belt

The Abitibi belt is one of the largest and most extensively studied Late Archean greenstone belts. The structural geology of the Abitibi belt consists of one generation of upright to slightly overturned, doubly plunging first-order folds with half-wavelengths of 20-60 km, and E-W-striking, steeply dipping fault zones that are parallel to the fold limbs. Two of the main fault zones are continuous for hundreds of kilometers. Previous tectonic models for the Abitibi belt interpret the fault zones to have formed as extensional growth faults bounding a volcanic-sedimentary basin, which were reactivated as thrusts during subsequent crustal shortening. Other models propose that the fault zones represent tectonic sutures, implying that the Abitibi belt is a collage of exotic terranes. However, distinct geological terranes have not been geologically demonstrated. We propose a new detachment fold model for the deformational history of the southern Abitibi belt, in Ontario, that explains the formation of the fault zones during the single, well-documented folding event that deformed the entire region. The internal structure of the fault zones, documented here with emphasis on the Porcupine-Destor fault zone, consists of isoclinally folded, strongly schistose, highly metamorphosed rock, cross-cut by numerous fault segments. We interpret that the upper crust (greenstones) was folded above a proposed detachment in the lower part of the volcanic stratigraphy. The fault zones would be, in essence, highly evolved detachment anticlines. Ultramafic metavolcanic rock that crops out within the fault zones would represent material from the detachment horizon that was emplaced in the cores of the detachment anticlines. The numerous segments that make up the mapped fault zones would be linked faults that formed within the isoclinal detachment anticlines to accommodate folding of the rheologically complex greenstones. The detachment fold model is compared to the results of analogue experiments designed to investigate crustal-scale folding, using viscous and frictional materials. Detachment folds are produced in the brittle upper crustal analogue on the limbs of folds formed in the ductile middle and lower crust analogues. The experimentally produced structures scale to the structures in the study area and indicate the detachment fold model for the southern Abitibi is mechanically viable.