Extensional faulting as the explanation for the Deighton ‘Klippe’ and other Mount Albert Group outliers, Mount Isa Inlier, northwestern Queensland

Abstract

Recent work in the Mount Isa Inlier has suggested that the Mount Albert Group forming the Deighton ‘Klippe’ (or outlier) is the stratigraphic equivalent of, but now thrust over, the Mary Kathleen Group. New mapping indicates that three other outliers of the Mount Albert Group are underlain conformably and gradationally by the Mary Kathleen Group on their western sides, but are vertically or normally faulted against older rocks on their eastern or northeastern sides. One of the normal faults is clearly pre‐ or early D2, but post‐D2 movement on the other two has obliterated any evidence of an earlier history. The Deighton Outlier is in largely bedding‐parallel fault contact with the underlying Mary Kathleen Group, but the fault is folded by and therefore pre‐dates D1. Excision of stratigraphic units along the eastern limb of the outlier indicates the extensional nature of the fault contact. The evidence indicates southwesterly directed extension along pre‐ or early D2 (and possibly pre‐D1) southwest‐dipping low‐angle normal faults. The largely bedding‐parallel fault beneath the Deighton Outlier was originally a flat in this system. Extension was followed by north‐south D1 shortening, expressed as local D1 folds, and then by east‐west D2 shortening which produced regional northerly trending upright folds with a steep axial‐plane foliation (S2). The D2 shortening also steepened the gently dipping pre‐ or early D2 normal faults, so that they now appear as steep normal or vertical faults. Pre‐D2 extensional faulting is now known over 5000 km2 of the Mount Isa Inlier. The faulting may have initiated the basins in which the Mount Isa Group was deposited, and may also have channelled metal‐bearing brine into the basins to form the Mount Isa‐type Zn‐Pb‐Ag ore bodies.