Long-term reactivation and morphotectonic history of the Zambezi Belt, northern Zimbabwe, revealed by multi-method thermochronometry

Highlights

• First apatite and zircon (U-Th)/He data for the Zambezi Belt

• Multi-thermochronometer data constrain morphotectonic history from mid-Paleozoic.

• Reactivation of Zambezi Escarpment Fault in Carboniferous, Jurassic and Paleogene

• Reactivation caused denudation of large Zambezi Belt-Zimbabwe Craton crustal block.

• Karoo Cabora Bassa Basin experienced Triassic-Cretaceous breakup-related unroofing.

Abstract

Neoproterozoic-early Paleozoic Pan-African mobile belts that formed during the amalgamation of Gondwana, such as the Zambezi Belt, are inherently weak zones that are susceptible to reactivation by later tectonism. With the exception of Karoo rifting, however, the post-Pan-African morphotectonic history of the Zambezi Belt is poorly constrained. Here, we use multiple low-temperature thermochronometers on samples collected across major structures in northern Zimbabwe to reveal the temporal and spatial pattern of tectonism and denudation in this portion of the Zambezi Belt. Thermal history modelling suggests that a large crustal block encompassing part of the Zambezi Belt and northern margin of the Zimbabwe Craton experienced differential denudation during three main Phanerozoic episodes. This denudation of the Archean-Proterozoic basement was associated with reactivation of the Zambezi Escarpment Fault, which demarcates the southern margin of the Karoo Cabora Bassa Basin. In contrast, other major structures within the region remained relatively stable. The results highlight the value of using a multi-thermochronometer approach, where essentially zircon (U-Th)/He data preserve evidence of late Carboniferous Karoo rifting, apatite fission track data record the thermal effects of Jurassic tectonism associated with Gondwana breakup, and the apatite (U-Th-Sm)/He data reveal Paleogene reactivation of the basin-bounding fault. Thermal history modelling also suggests that the Cabora Bassa Basin experienced Gondwana breakup-related denudation, potentially associated with a proposed concurrent major regional drainage reversal. Since the Cretaceous, the basin has experienced limited sedimentation and erosion.

Introduction

Neoproterozoic-early Paleozoic Pan-African mobile belts developed between older cratons during the amalgamation of Gondwana (e.g. Kröner and Stern, 2004; Stern, 1994). The Zambezi Belt is one such orogenic belt that formed together with its western continuations, the Lufilian Arc and Damara Belt, during the collision of the Kalahari (Zimbabwe and Kaapvaal cratons sutured by the Limpopo Belt; Fig. 1) and Congo cratons (Coward and Daly, 1984; John et al., 2004). Beginning in the late Carboniferous, with the development of the Karoo rift basin system during the progressive fragmentation of Gondwana (Daly et al., 1989), these zones of Pan-African crustal weakness were reactivated. In the study region within the Zambezi Belt of northern Zimbabwe, the Zambezi Escarpment Fault was exploited during this period of extension to form the Cabora Bassa Basin to the north (Johnson et al., 1996; Shoko and Gwavava, 1999; Fig. 1).

Later reactivation of the inherently weak and thus susceptible Zambezi Belt (Lenardic et al., 2000), however, is poorly constrained (see Section 2.1 for more details). Low-temperature thermochronology (LTT) provides an effective means of revealing periods of normal fault movement due to the different amounts of denudation on either side of the reactivated structure (e.g. Ehlers and Farley, 2003; Emmel et al., 2012; Foster and Gleadow, 1992; Raab et al., 2002). Denudation of the uplifted footwall will result in crustal cooling that can cause rejuvenation of a LTT system (Ehlers and Farley, 2003). As this denudation and crustal cooling will not occur in the hanging wall, reactivation can lead to contrasting LTT ages either side of the structure (Foster and Gleadow, 1992). In particular, inverse thermal history modelling of footwall samples can be used to identify the onset of cooling, related tectonic denudation and in turn, the timing of reactivation (e.g. Gillespie et al., 2017; Raab et al., 2002). One other LTT study has been carried out within the Zambezi Belt of northern Zimbabwe where the forward modelled apatite fission track (AFT) data suggest that the Zambezi Escarpment Fault was reactivated in the Jurassic (Noble, 1997).

Here, by sampling across major structures—the Zambezi Escarpment Fault, the Red Fault, the Great Dyke fracture system and the thrust fault that marks the Zimbabwe Craton-Zambezi Belt boundary—in other regions of northern Zimbabwe, we demonstrate that multi-thermochronometer data can provide a more comprehensive morphotectonic evolution for the Zambezi Belt. By employing AFT, apatite (U-Th-Sm)/He (AHe) and zircon (U-Th)/He (ZHe) thermochronometry, we reconstruct the thermal history of a sample through a range of potentially ~30–220 °C (equivalent to <1–8 km assuming a typical mobile belt geothermal gradient of 25 °C/km (Nyblade et al., 1990) and surface temperature of 20 °C). Indeed, the different temperature sensitivities of the AHe, AFT and ZHe systems—typically ranging from ~30–90 °C (Flowers et al., 2009), ~60–110 °C (Gleadow et al., 2002) and <50–220 °C (Guenthner et al., 2013; Johnson et al., 2017; Mackintosh et al., 2017), respectively—provide insight into different portions of the morphotectonic history.

We present evidence suggesting that the Zambezi Belt of northern Zimbabwe experienced three main Phanerozoic cooling episodes, which we interpret as the morphotectonic response to different phases of reactivation of the Zambezi Escarpment Fault.