Dome-and-keel provinces formed during Paleoproterozoic orogenic collapse—core complexes, diapirs, or neither? : Examples from the Quadrilátero Ferrífero and the Penokean orogen.

Abstract

Paleoproterozoic dome-and-keel provinces, in which troughs of deformed and metamorphosed Paleoproterozoic supracrustal rocks surround domes of Archean basement, continue to puzzle geologists. In current literature, some authors refer to the domes as diapirs (implying basement flowed vertically upward) and others consider them to be Cordilleran-type metamorphic core complexes (implying that the contact between basement and cover is an upwarped detachment). Geochronological studies suggest that dome emplacement occurred during extensional collapse of contractional orogens, seemingly supporting the core-complex analogy. However, structural analyses in the Quadrilátero Ferrífero (Brazil) and the Penokean orogen (Michigan) demonstrate that the domes resemble diapirs in shape and in terms of surface kinematics. The domes differ from diapirs, however, in that they did not flow penetratively within but were emplaced by movement along steeply dipping shear zones. We suggest that contrasts between domeand- keel and core-complex provinces reflect contrasts between Paleoproterozoic and Phanerozoic crustal structure. Specifically, during the Paleoproterozoic, when crust was warmer and supracrustal assemblages denser, core-complex–like detachment faulting and associated plutonism resulted in juxtaposition of hot basement beneath a denser, tectonically thickened supracrustal layer. The resulting viscosity contrast and density inversion triggered vertical rise of diapir-shaped basement domes by slip on steep shear zones.