The Central Gneiss Belt revisited: Forensic analysis of the metamorphic core complex hypothesis and an alternative explanation

The hypothesis that the Central Gneiss Belt (CGB) of the western Grenville orogen in Ontario is the footwall infrastructure of a metamorphic core complex (MCC) formed during late Ottawan orogenic extension was developed over the last 15 years by T. Rivers and co-authors. We challenge this interpretation based on an in-depth re-analysis that shows: 1) the CGB lacks essential characteristics of modern MCC’s; 2) the northern CGB was buried and heated after the supposed removal of the Ottawan orogenic lid (OOL); 3) rocks now at the surface were exhumed to 15-25 km before OOL removal with minimal exhumation for the next 100 My; 4) CGB rocks below the proposed detachment cooled slowly long after the suggested OOL removal time; 5) a proposed explanation for slow cooling involving delamination or rollback of lithospheric mantle accompanied by emplacement of voluminous mafic intrusions is not supported by the evidence. We offer an alternative explanation for CGB evolution that is compatible with the geological evidence and with predictions of numerical model GO-ST87, which considers gravitationally driven crustal spreading of a Grenville-style large hot orogen when convergence stops. Model crust is extruded to the northwest (i.e. toward foreland), with ductile infrastructure flowing faster than overlying superstructure, leading to top-to-southeast shear in the weak decoupling zone between them. This is opposite to the MCC hypothesis which proposes that the superstructure (OOL) was pulled off to the southeast. Model superstructure is then removed gradually by erosion leading to the observed slow exhumation and cooling of the CGB.