Polyphase Archean to Paleoproterozoic deformation along the northwest margin of the Johannesburg Dome, Kaapvaal Craton

Abstract

Despite extensive gold exploration, the reasons for the dome shape of the Paleoproterozoic Johannesburg Dome (JD) remain elusive. We investigated the structural record of an outlier of Archean to Paleoproterozoic supracrustal rocks (the Zwartkops Hills Outlier, ZHO) overlying the JD to investigate its tectonic evolution. Steep gneissic fabrics in the basement pre-date the intrusion of ∼ 3225 Ma trondhjemite (new U–Pb zircon age). Two Archean deformation events affected West Rand Group rocks and pre-date Platberg Group deposition. To the south of the ZHO, a newly recognized, large-scale, recumbent fold formed during top-to-the-south shearing. In the central part of the ZHO, steeply W-dipping stratification results from E-W shortening. Two more deformation events are Paleoproterozoic in age and affected all units. A shallowly S-SW-dipping cleavage formed in the biotite zone of the greenschist facies is associated with N–S shortening and top-to-the-north shearing. The cleavage transects the large recumbent fold and is observed down to the upper 30 m of the basement. Finally, a weak N–S shortening event is inferred from kink folds and north-dipping axial planar cleavage. These data are consistent with the existing regional models of (i) south- and east-directed fold-and-thrust belts formed during sedimentation of the Archean Central Rand Group, (ii) north-verging schist belt formed during, or after the Transvaal Supergroup deposition, and (iii) impingement of the Bushveld Complex onto the JD during N–S shortening. These findings are integrated with previous regional sedimentological, seismic, and structural studies to suggest that the JD granitoid basement formed a paleo-high during the deposition of the Central Rand and Platberg groups. Once buried in the upper crust, the paleo-high acted as a rigid granitoid promontory that indented weak surrounding cover rocks during Paleoproterozoic N–S contraction events, creating the dome shape of the Johannesburg Dome.