A tale of two synclines: Rifting,inversion and transpressional popouts at Lake Julius,northwestern Mt Isa terrane,Queensland

This study reviews the origin of two approximately east‐west‐trending synclines in the Lake Julius area at the eastern edge of the Leichhardt Rift. The genesis of one of these structures can be found in a north‐south shortening event (D₁) that occurred at the beginning of the compressional Isan Orogeny (at ca 1600 Ma). Metasediments in a cross‐rift were rammed against a competent buttress defined by the pre‐existing rift architecture, producing the approximately east‐west‐trending Somaia Syncline and its associated axial‐plane slaty cleavage. In contrast, the Lake Julius Syncline was produced by reorientation of an originally approximately north‐south‐trending (D₂) fold, in a transpressional zone adjacent to a strike‐slip fault, at the end of the Isan orogeny. The effects of late fault movement can be partially reconstructed, based on correlations assuming that regionally developed trains of upright folds formed during the peak of the Isan Orogeny (D₂). These folds have been offset, as well as having been tightened and disrupted at the same time as fault movements took place. The overall pattern of movement in the Lake Julius region can be explained as the result of an ‘indentor’ ramming into the ancient edge of the Leichhardt Rift, which acted as a buttress.