The Muller anticline, Papua New Guinea; basement-cored, inverted extensional fault structures with opposite vergence

Abstract

Throughout the NW-SE trending Papuan Foldbelt, two dominant structural styles are present. In the northeast the 1 km thick, thrust-imbricate slices of Miocene limestone attest to the classical thin-skinned nature of the deformation. In the southwest, adjacent to the undeformed foreland, fold structures are 1 or 2 orders of magnitude greater in size, are strongly asymmetrical and commonly have relatively thicker stratigraphie sections on the crests. Such structures are the result of Mio-Pliocene compression that caused thrusting along faults that are hypothesized to have been extensional in the Mesozoic and Palaeogene. Geological traverses from the undeformed foreland heading northeast across the 150 km by 50 km Muller Range in western Papua New Guinea, provided detailed surface structural data. When combined with analysis of existing geological maps, exploration wells, regional gravity surveys and extensive seismic in the foreland, moderately constrained balanced and restorable cross-sections through the Muller anticline could be constructed. Interpretations of gravity data are consistent with the Muller anticline, being cored by basement and seismic data extrapolated beneath the mountains, show the outcrop of basement in the centre of the Muller Range to be elevated 8 km above regional. The eastern Muller anticline (EMA) has a shallow northeast limb and steeper southwest limb, suggesting a NE-dipping fault below, such that basement was thrust to the southwest. The Cecilia, Wai Asi, Juha and Lavani anticlines to the southwest of the EMA are interpreted to be formed by shortening of the whole sedimentary section to balance the shortening in basement. There are no such structures to the southwest of the western Muller anticline (WMA) where the southwest limb is gently dipping and the northeast limb is steep. Hence, the WMA is interpreted as having formed above a SW dipping fault along which basement was thrust to the northeast. The marked difference in shortening of the southwest limbs requires a tear fault between the EMA and WMA. The tear fault is here interpreted to coincide with the northwestern end of the Cecilia and Lavani anticlines and the marked change in strike of the Muller anticline. By analogy with other very large, asymmetrical, basement-involved structures in the Papuan Foldbelt it is suggested that the faults underlying the Muller anticline were active as extensional faults in the Mesozoic, soling at a mid-crustal detachment. The extensional faults beneath EMA and WMA had opposite vergence and were separated by a transfer zone. Subsequent compressional reactivation of the extensional faults in the Pliocene gave rise to the present Muller anticline.