The crustal scale architecture of the Eastern Succession, Mount Isa: The influence of inversion

Abstract

The three-dimensional crustal architecture of the eastern part of the Mount Isa Inlier is investigated from serial cross-sections constructed using geological map data, revised chronostratigraphy, gravity, magnetics, worms (multiscale wavelet edges of potential field data) and seismic data. The top part of the crust consists of rift and platform type metasediments that were deposited in three cover sequences from 1850 to 1610 Ma. These rocks constitute the Mount Isa Eastern Succession, and they were intruded by mafic–felsic plutons, dykes and sills of various ages before and during the Isan Orogeny ( ca. 1.6–1.5 Ga). The Eastern Succession overlies a felsic metamorphic basement, which in turn sits on a tonalitic–gabbroic lower crust. The depositional basin architecture for the Eastern Succession was controlled by major N–S trending structures that penetrated the lower crust, and accommodated E–W extension. These structures also underlie major upper crustal structures such as the Mitakoodi Culmination and Snake Creek Anticline that were formed by contraction in the Isan Orogeny. Positive inversion may therefore have been a key process in the evolution of the eastern part of the inlier, and governs its architecture at the crustal scale. Inversion involved reactivation of basement-penetrating structures, which localised contractional structures in the cover sequences above, as well as influencing pluton emplacement. The felsic metamorphic basement may have been penetratively deformed during inversion. The spatial association between the basin-controlling and contractional structures suggests that either early extensional displacements were completely reversed by later contraction, or that much of the Eastern Succession has remained essentially parauthochthonous relative to the basement.