The 14GA-CF1 seismic transect adds new detail to previous understandings of the subsurface linkages between the Mount Isa and Georgetown Provinces, including new detail of the longitudinal extent of the Mesoproterozoic? Millungera Basin. The transect also provides the first full crustal image of the continent-scale Diamantina Lineament which abruptly terminates the Mount Isa Province at its southern extremity. The latter is revealed as a crustal-scale south-dipping fault penetrating the MOHO at the crust/mantle boundary, forming the northern margin of a northeasttrending rift-like basin controlling deposition of the earliest Thomson Orogen successions.

The northern segment of the seismic profile images the eastern margin of the Mount Isa Inlier as a southeastdipping extensional fault system (displaying some later Isan Orogeny inversion) facilitating development of the (now subsurface) Julia Creek Basin during the latest Paleoproterozoic. The basin extends to a depth of ~6.5 s two-way travel time (TWT) (~20 km) and is inferred to comprise mainly moderately to poorly reflective meta-sediments, mafic lavas and sills of the Soldiers Cap Group, underlain by a basal relatively non-reflective sequence of uncertain affinity.

By comparison with the intersecting 07GA-IG1 seismic profile, the Julia Creek Basin section is here recognised as being contiguous with the Kowanyama Seismic Province of Korsch et al. (2012), while the underlying more reflective lower crustal section is recognised as a continuation of the Numil Seismic Province defined by Korsch et al. (2012) from the 07GA-IG1 profile. The now deeply-buried Numil crustal layer forming basement to the Julia Creek Basin displays a segmented, block-faulted character in the 14GA-CF1 profile, probably reflecting its fragmentation during basin development prior to its eventual foundering and burial by the Julia Creek Basin succession. An upper more reflective section of the Numil Province crust has also been delineated in the 14GA-CF1 profile, and has also been recognised in the orthogonal 07GA-IG1 deep seismic profile.

A zone running the length of the eastern margin of the Mount Isa Inlier has been identified where discrete Numil-Isa seismic and conductivity contrasts occur. However, no feature similar in character to the Gidyea Suture Zone defining the eastern margin of the Mount Isa Inlier on the 07GA-IG1 seismic profile has been identified from the 14GA-CF1 profile crossing the margin further to the northeast. Moreover, the Numil-Isa basement contrast is not evident at depth across this segment of the Mount Isa margin, where the Isa basement displays more Numil-style characteristics. These features point to a more complex history for the eastern Mount Isa margin than previously proposed, and highlight the need for further careful integration of existing and new geophysical, geological and isotopic datasets to resolve the origin, timing and significance of crustal-scale structures underpinning the region.

Bibliographic reference: Donchak, P.J.T. 2021. Notes on the geological interpretation of deep seismic transect 14GA-CF1, including inferences from other associated regional datasets. Queensland Geological Record 2021/04.