The North Pole Dome: a non-diapiric dome in the Archaean Pilbara Craton, Western Australia

Abstract

The northeastern part of the Archaean Pilbara Craton is characterised by an ovoid dome and basin pattern of domal granites separated by synformal greenstones (supracrustal rocks). This ovoid pattern is variably preserved at various structural and stratigraphic levels, with the most deeply eroded domes being granite dominated, and the least eroded domes being greenstone dominated. The North Pole Dome (NPD) is an example of a relatively high-level dome that has a flanking syncline preserving some of the youngest rocks of the craton (Fortescue Group) to be involved in the doming process. At the apex of the NPD is a small intrusive granite that was considered by some workers to be the top of a large underlying domal granite batholith. The NPD comprises mostly dense basaltic greenstones that range in age from approximately 3500 to 2700 Ma. The approximately 3460 Ma North Pole Monzogranite, a volumetrically insignificant intrusive granite body, intrudes the greenstones in the apex of the dome. No marginal shear zones occur around the intrusion. A new model, based on a detailed gravity traverse across the monzogranite, suggests that the granite intrusion is plug-like, up to 1.5 km thick and does not represent the exposed top of a larger underlying domal batholith. Results from potential field modelling show that the dome is relatively flat bottomed, with a base around 5.5–6.5 km deep. The NPD has no significant granitic material within the dome, but like all greenstones, is underlain by felsic crust (granite) below its base. The development of the NPD (and flanking syncline) was a multistage process. The first stage of doming involved relatively minor doming/tilting, possibly associated with the emplacement of the monzogranite, because palaeocurrents of synchronous volcanic rocks flowed radially outward from the dome. It is likely that this doming was minor as there are no recorded unconformities in the Warrawoona Group (in the NPD) above these volcanic rocks. A major dome-forming event (tilting >20°) occurred in the period between 3240 and 2772 Ma, and was unrelated to the emplacement of the small granite plug (diapirism). Regional folding and refolding from horizontal compression deformed the area into a domal shape (Type I fold interference pattern). Uplift and erosion of the dome was superseded by extension and deposition of flood basalts (Fortescue Group) that flowed towards the dome. Three further stages of shortening folded the regional unconformity and the underlying and overlying units, further amplified the underlying dome, developed the flanking Marble Bar Syncline, as well as fold interference patterns in the Fortescue Group. The NPD was developed over a 800 Ma time frame, ostensibly by a process of fold interference due to multiple stages of horizontal compression. This work shows that diapirism was not the cause of the development of the domal geometry of the NPD, and its flanking syncline, rather folding and refolding due to horizontal compression was the principal controlling factor.