The Pilbara (NW Australia) and Kaapvaal (Africa) cratons exhibit pristine 3.6-2.7 Ga crust; as such, they provide a critical window in the mechanical behavior of the early Earth’s lithosphere. The fundamental characteristics of these terranes are deformed, steeply dipping greenstone synclines (i.e., keels) folded between massive Tonalite–trondhjemite–granodiorite (TTG) domes. An ongoing debate is whether the terranes’ current dome-and-keel geometries and structural fabrics represent a different crustal deformation mechanism in the early Earth, or typical plate tectonic related structures heavily modified by subsequent deformation. We focus on two observations of current map patterns within the greenstone belts along the margins of the Shaw dome that we suggest may be the result of modern thrust sheet-style structures that were subsequently folded into their current geometry. 1) Map patterns of complexly faulted and folded strata in the Lallah Rookh-Western Shaw (LRWS) corridor are like cross-section patterns of antiformal stacks in thrust sheet hanging walls. We suggest the complexly folded and imbricated strata in the greenstone keels may represent tilted antiformal duplexes in the hanging wall of a regionally extensive thrust sheet. 2) The elongate Shaw dome in the south-center EPGGT is bound by the Mulgandinnah shear zone (MSZ) to the west and the Split-Rock shear zone (SRSZ) to the east. Our initial explorations back-rotating the mylonitic foliation and lineation data of these shear zones from previously published data (Zegers et al., 1998) onto a continuous planar structure displays a primarily NE-SW trending, top-to-the SW linear fabric from both the SRSZ and MSZ, with a possible NW-SE overprinting fabric in the MSZ. While more detailed structural investigations of the shear zones bounding the Shaw dome are necessary to assess the viability of this reconstruction, we suggest the shear zones may represent an initial sub-planar shear zone along the base of a regional thrust that was folded into its current geometry by subsequent deformation. Our model for the tectonic history of the EPGGT involves a modern-style structural evolution, where the current structural geometries are formed by: 1) The development of a greenstone basin; 2) SW-directed thin-skinned emplacement and thrust stacking over a granitic terrane; and 3) cross-folding of the ductile crust via a younger tectonic event(s) causing the older shear zones to appear parabolic in map view. This hypothesis predicts that the shear zones, which form contacts between the granites and greenstones, are a continuous folded detachment fault that should show similar trends in kinematic data (lineation direction, shear sense, etc.) when unfolded, and that complexly folded and faulted geometries in greenstone belts with near vertical bedding may be rotated thrust belt structures. 

Reference(s)

Zegers, TE, de Keijzer, M, Passchier, CW and White, SH 1998), The Mulgandinnah Shear Zone; an Archean crustal scale strike-slip zone, eastern Pilbara, Western Australia: Precambrian Research, v. 88, p. 233–247.