The Arltunga Nappe Complex, macdonnell ranges, Northern Territory, Australia∗

Abstract

The Arltunga Nappe Complex trends east‐west along the northeastern margin of the Amadeus Basin, approximately in the centre of the Australian continent. The nappes formed in a stratigraphic sequence consisting of the crystalline Precambrian Arunta Complex overlain unconformably by Upper Proterozoic Heavitree Quartzite and carbonate rocks of the Bitter Springs Formation. The uppermost and largest nappe was transported at least 24 km across strike and developed by a combination of recumbent folding and overthrusting. The lower nappes formed by overthrusting alone. The nappes root to the north in a belt of crystalline rocks of the Arunta Complex, originally in the amphibolite facies, that are retrograded to the greenschist facies. North of the retrograded zone a belt of rocks belonging to the granulite facies and amphibolite facies is exposed. There is a sharp discontinuity in the style and pattern of deformation along the thrust fault surface that defines one of the lower thrust nappes. The autochthonous rocks beneath the thrust are folded about east‐west axes and the southern limb of each fold is overturned; the allochthonous rocks above the thrust are isoclinally folded about north‐south axes and display a prominent northerly lineation due to the elongation of quartz grains. Structurally higher in the inverted Heavitree Quartzite at the base of the uppermost nappe, elongated conglomerate pebbles also trend north‐south. The overall structural pattern indicates movement from north to south. The main folding of Upper Proterozoic and Palaeozoic sedimentary rocks in the northeastern Amadeus Basin occurred in the Carboniferous and this was probably also the age of nappe formation. Strong supporting evidence is given by Carboniferous K/Ar dates on muscovites from Heavitree Quartzite in the nappe complex. Bouguer gravity anomalies increase strongly in the 10 km northward from the root zone. The nappes apparently developed near the surface of the crust in a strongly deformed zone that dips northward through the crust into the mantle. At the same time a large part of the Upper Proterozoic and Palaeozoic sedimentary rocks was detached from the nappes and travelled southward into the Amadeus Basin on a major thrust surface.