Deciphering the building blocks of the eastern North Australia Craton

Interpretation of the nature and extent of basement rocks in many regions is hampered by lack of exposure and because potential field data tend to reflect the signatures of overlying upper crustal units, highlighting the need for tools that image deeper. In Northwest Queensland (NWQ) numerous crustal-scale seismic lines were collected from 1994 to 2019 within the eastern North Australia Craton. Although multiple interpretations have been published for some lines, definition of the underlying basement terranes has remained elusive. This project presents a new understanding of the basement terranes that form the fundamental building blocks of the Mount Isa region based on crustal-scale interpretation of all the major seismic lines, integrated with magnetotellurics and tomography, in the mid to lower crust. Surface geology and potential field data helped constrain the upper crustal interpretation. The major lower crustal blocks beneath the outcropping Mount Isa region include a broad, trapezoidal, N-S trending basement terrane (Pitta Pitta) that reaches its shallowest level beneath the Kalkadoon-Leichhardt Belt, and a 24-30 km thick terrane (Central Isa) that overlies Pitta Pitta and extends east beneath the Eastern Fold Belt. The wedge-shaped, eastern margin of these two terranes has been thrust over Numil crust along the Gidyea Structure. The large Pitta Pitta terrane has been overthrust from the west by the Altjawarra crustal block, and, the Central Isa terrane was later thrust west across the Pitta Pitta and Altjawarra basement terrane. The seismic data indicates that the Tennant basement terrane underlying the Tennant Creek region has limited depth extent as it forms an E-W trending, bivergent block overlying the Altjawarra to the south and Murphy basement terranes to the north.