A gravity study of the northwestern boundary fault of the southern Kapuskasing Structural Zone

This study addresses the crustal structure and the tectonic evolution of the western part of the Archean Superior Province, based on the interpretation of its regional gravity field with constraints from Lithoprobe seismic data. In addition, by using techniques of enhancement of magnetic data, and integration of various geophysical datasets, the detailed structure of the metasedimentary English River belt is investigated in order to clarify the tectonic significance of this type of terrain, and to understand the processes accounting for its characteristics. The results indicate that post-accretionary processes had a dominant contribution in defining the current structure of the Western Superior Province (WS), especially at deeper levels of the crust. With the exception of the top 10 km, the crust of the WS is characterized by vertical density stratification and lateral mass homogeneity, reflecting post-accretionary reorganization of the crustal structure through processes of vertical mass transfer and ductile lateral flow, which are related to a major episode of mid- to lower-crustal softening that occurred between ca. 2.71 and 2.66 Ga. The topography of the crust-mantle interface was also significantly influenced by late-orogenic processes, such as crustal extension and vertical mass redistribution, although some of its features may have originated from the accretionary or collisional stages of the WS tectonic evolution. In addition, the WS preserves a record of post-orogenic modification of the lower crust through processes of magmatic intra- and underplating, possibly related to the development of the 1.1 Ga Midcontinent rift. Detailed investigation of the structure of the metasedimentary English River belt confirms that this terrain does not represent an ancient accretionary prism, as initially thought. Its structure is consistent with alternative interpretations of such terrains as remnants of syn-orogenic deposits accumulated in response to the deformation, uplift, and erosion of bounding volcanic arcs. The English River belt contains evidence of a major post-accretionary extensional episode at ca. 2.70 Ga that affected all levels of the crust, and which ultimately may explain the high-temperature, low-pressure metamorphism of the English River sediments., The northwestern boundary of the Kapuskasing Structural Zone is formed by the Saganash Lake Fault, which was previously interpreted as a northwest dipping normal fault. A gravity study was initiated in the Little Missinaibi Lake - Racine Lake area with the purpose of determining the nature of this fault at its southern end. Gravity observations were made with a Lacoste-Romberg Model G gravity meter along a 40 km profile that is normal to the trace of the fault. The Bouguer gravity values show an increase of over 35 mgal across the fault, reflecting the presence of uplifted deep crust southeast of the Saganash Lake Fault. 2.5D gravity modeling suggests that at its southern end the Saganash Lake Fault is a reverse fault with a dip of 60°-70° SE and a depth extent of 10-15 km. This result supports the model of a pop-up structure for the southern Kapuskasing Structural Zone.