Implications from geophysical data for the structural and tectonic evolution of the Superior and Yilgarn cratons

English Abstract:

Gravity, aeromagnetic and seismic tomographic data for the Superior and Yilgarn cratons suggest that: (i) Some terrane boundaries defined from geological, geochemical and geochronological data and aeromagnetic interpretation of upper crustal features of the Superior craton do not correspond to margins of disparate, lithospheric-scale blocks. Early terrane boundaries preserved in the deep crust and upper mantle at high angles to mapped terrane boundaries and to greenstone belts in the upper crust are identified. Many “terranes” represent reassembled fragments previously derived from disaggregation of older (proto)cratons. (ii) Long wavelength aeromagnetic, bouguer gravity and pseudogravity data portray regional domes at deep crustal levels in both cratons, even in areas dominated by linear greenstone belts and transcurrent shear zones in the upper crust. Differences in shear zone geometries in the Superior craton indicate decoupling between the upper and mid- to lower crust, although shear zones at all crustal levels are compatible with the same inferred bulk shortening. (iii) Most large mineral deposits occur on margins of paleo-rifts preserved in the upper mantle and above deep crustal structures and/or on rims of deep domes at a high angle to greenstone belts for which there is little upper crustal expression. (iv) Broad regional ductile shear zones develop prior to formation of the mapped discrete ductile to brittle-ductile shear and faults. On deep reflection seismic profiles (i) dipping reflectors cutting the upper mantle and lower crust are interpreted to represent mantle and lower crustal imbricates formed during bulk shortening and not fossil subduction zones, and (ii) both cratons preserve an early extensional and not a thrust/accretionary architecture. Some adjacent blocks differing in age and metamorphism in both cratons (eg. Abitibi-Opatica, Narryer-Youanmi) display basement-cover relationships and are not tectonically juxtaposed exotic terranes. Many observations are incompatible with previous, actualistic, subduction/arc accretion models, and shortening associated with regional folding and shearing in both the Superior and Yilgarn cratons is interpreted as the result of cratonic mobilism. Outcomes provide a new framework and approach for mineral exploration.