Cumulative deformation and original geometry of the Bushveld Complex

• Published in: Tectonophysics 2019-01, Vol.750, p.177-202
• Main Author: Basson, I.J.
• Format: Article
• Language: English
• Subjects: Bushveld Complex; Cooling; Cumulative deformation; Deformation; Displacement; Escape structures; Expulsion; Faults; Fluids; Geological faults; Geometry; Geophysical surveys; Geophysics; Gravity; Removal; Rock intrusions; Shearing; Structural history; Uplift
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2018.11.004

A structural review and re-interpretation of the Bushveld Complex, based on 254 georeferenced maps from research, technical reports and geophysical surveys, suggests that its current geometry differs markedly from its original geometry. Broadly N–S flattening, left-lateral transpressional shearing along an interconnected fault network, steepening of cumulate layering and 10 km of uplift, to the north of the northern margin of the complex's main chamber, were accompanied by an eastward-verging escape structure (Burgersfort Bulge) and syn-cooling gap areas on its Western Limb. Southward-verging deformation was transmitted from the Villa Nora Fragment, via thrusting in the Rooiberg area, generating 50–70 km of southward displacement, uplift and erosional loss of the Northern Limb's western portion. A string of anomalous occurrences of Bushveld Complex units and dense areas outlined by unconstrained gravity isoshells suggest a further 170 km southward displacement through the center of the main chamber. The Southern Limb underwent predominantly right-lateral, ductile offset from the Western Limb along the Crocodile River and Rustenburg Faults, expressed by an in-line series of NNW-trending magnetic highs to the south of their mapped terminations. Left-lateral shearing and progressive splaying of a NNE-trending dyke swarm, to the west of an inflection point, provides strain markers for this offset and 35° anticlockwise rotation of the Burgersfort Bulge and central parts of the main chamber. Although recent work suggests that the complex cooled within 1 Myr, late-intrusion/syn-cooling, SSE-wards movement of its deeper, ductile, central parts, within at least 5–10 Myr after its initial intrusion, the Transvaalide Fold-and Thrust event and the expulsion of fluids to form the high-Ti igneous suite (HITIS) and related bodies, appear to have closely post-dated the complex's intrusion. Removal of the effects of cumulative strain produces a more equant complex, shortly after its intrusion, occupying an orthogonal network of normal faults and perpendicular, strike-slip, transverse faults. [Display omitted] •Main chamber underwent N–S flattening, sinistral transpression, steepening of N margin•Burgersfort Bulge comprised early, eastward-verging escape structure.•At least 70 km S-verging displacement in Rooiberg area•Anomalous BC intersections, density isoshells imply 170 km S-ward displacement of centre.•Crocodile River Fault allowed 35° anticlockwise rotation of central