Post break-up tectonic inversion across the southwestern cape of South Africa: New insights from apatite and zircon fission track thermochronometry

Mark Wildman,Roderick Brown,Ron Watkins,Andrew Carter,Andrew Gleadow,Michael Summerfield

doi:10.1016/j.tecto.2015.04.012

Abstract

The south-west African margin is regarded as an example of a passive continental margin formed by continental rifting following a phase of lithospheric extension and thinning. Recent attention focused on this margin has included theoretical modelling studies of rift processes, plate kinematic studies of the opening geometry and timing, and empirical studies focused on documenting the crustal structure and offshore sedimentary record. Here, we examine the onshore geomorphic and tectonic response to rifting and breakup, with a specific focus on the SW Cape of South Africa. We present 75 new apatite and 8 new zircon fission track analyses from outcrop samples and onshore borehole profiles along the western margin of South Africa. The data are used to derive robust thermal histories that record two discrete phases of accelerated erosional cooling during the Early Cretaceous (150–130Ma) and Late Cretaceous (100–80Ma), respectively. Both periods of enhanced erosion are regional in extent, involved km-scale erosion, and extend well inland of the current escarpment zone, albeit with spatially variable intensity and style. The Late Cretaceous episode is also expressed more locally by tectonic reactivation and inversion of major faults causing km-scale differential displacement and erosion. The new AFT data do not exclude the possibility of modest surface uplift occurring during the Cenozoic, but they restrict the depth of regional Cenozoic erosion on the western margin to less than c. 1km. The inferred pattern and chronology of erosion onshore is consistent with the key features and sediment accumulation patterns within the offshore Orange and Bredasdorp basins. It is suggested that the Late Cretaceous event was triggered by a combination of regional dynamic uplift augmented along the western margin and in the SW Cape by local tectonic forces arising from dextral displacement of the Falkland Plateau along the Falkland–Agulhas Fracture Zone.

Highlights

The widespread occurrence of prominent coast-parallel escarpments along high-elevation passive margins is a strong indication that significant relief is generated along the edge of separating continental plates as a consequence of rifting with examples being southern Africa, western India and southeastern Australia (e.g., Matmon et al, 2002; Ollier, 1985; Summerfield, 2000)
The younger episode at c. 100–80 Mineral FT age (Ma), seen in previous Apatite fission track (AFT) studies across the southern (Tinker et al, 2008a), western (Kounov et al, 2009) and eastern margins (Brown et al, 2002), appears to be focused at localities within the SW Cape region that are controlled by differential displacement across significant normal faults. We suggest that this structural reactivation in places on normal faults along the western margin has caused the earlier thermal history record to be effectively removed by erosion
The data presented in this paper has significant implications for our understanding of the western continental margin of South Africa which has long been considered a ‘passive’ continental margin

Summary

Introduction

The widespread occurrence of prominent coast-parallel escarpments along high-elevation passive margins is a strong indication that significant relief is generated along the edge of separating continental plates as a consequence of rifting with examples being southern Africa, western India and southeastern Australia (e.g., Matmon et al, 2002; Ollier, 1985; Summerfield, 2000). Despite numerous studies focusing on the geodynamic and geomorphic processes involved during continental rifting (e.g., Braun and Beaumont, 1989; Brune et al, 2014; Gilchrist and Summerfield, 1991; Huismans and Beaumont, 2011; Japsen et al, 2006; Kooi and Beaumont, 1994; Ollier, 1985; van der Beek and Braun, 1998), the precise nature of the relationship between rifting processes and the mechanisms responsible for creating and maintaining topography is not fully understood and remains controversial (e.g., Blenkinsop and Moore, 2013; Green et al, 2013; Japsen et al, 2006; Rouby et al, 2013).

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