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Plate-tectonic processes at ca. 2.0 Ga: Evidence from >600 km of plate convergence
Geology ( IF 4.8 ) Pub Date : 2019-11-19 , DOI: 10.1130/g47070.1 An Yin 1 , Günther Brandl 2 , Alfred Kröner 3
Geology ( IF 4.8 ) Pub Date : 2019-11-19 , DOI: 10.1130/g47070.1 An Yin 1 , Günther Brandl 2 , Alfred Kröner 3
Affiliation
We addressed when plate-tectonic processes first started on Earth by examining the ca. 2.0 Ga Limpopo orogenic belt in southern Africa. We show through palinspastic reconstruction that the Limpopo orogen originated from >600 km of west-directed thrusting, and the thrust sheet was subsequently folded by north-south compression. The common 2.7–2.6 Ga felsic plutons in the Limpopo thrust sheet and the absence of an arc immediately predating the 2.0 Ga Limpopo thrusting require the Limpopo belt to be an intracontinental structure. The similar duration (∼40 m.y.), slip magnitude (>600 km), slip rate (>15 mm/yr), tectonic setting (intracontinental), and widespread anatexis to those of the Himalayan orogen lead us to propose the Limpopo belt to have developed by continent-continent collision. Specifically, the combined Zimbabwe-Kaapvaal craton (ZKC, named in this study) in the west (present coordinates) was subducting eastward below an outboard craton (OC), which carried an arc equivalent to the Gangdese batholith in southern Tibet prior to the India-Asia collision. The ZKC-OC collision at ca. 2.0 Ga triggered a westward jump in the plate convergence boundary, from the initial suture zone to the Limpopo thrust within the ZKC. Subsequent thrusting accommodated >600 km of plate convergence, possibly driven by ridge push from the west side of the ZKC. As intracontinental plate convergence is a key modern plate-tectonic process, the development of the Limpopo belt implies that the operation of plate tectonics, at least at a local scale, was ongoing by ca. 2.0 Ga on Earth. INTRODUCTION The time of onset for plate tectonics on Earth has been hotly debated, with estimates ranging from >4.0 Ga to ca. 0.85 Ga (e.g., Korenaga, 2013; Cawood et al., 2018). The most commonly used proxies of plate-tectonic processes are rock records of subduction-zone–like low geothermal gradients (e.g., Hopkins et al., 2008) and modern-arc geochemical signatures (e.g., Martin et al., 2014). Since non-plate-tectonic models (e.g., Bédard, 2006; Moore and Webb, 2013) could also explain these proxies, determining the onset of plate tectonics on Earth requires direct field-based geological evidence of largescale (more than hundreds of kilometers) horizontal crustal motion. In order to address this issue, we reexamined the tectonic development of the >600-km-long Limpopo orogenic belt in southern Africa (Fig. 1). Our result shows that modern plate tectonics, expressed as intracontinental plate convergence, occurred on Earth by ca. 2.0 Ga. GEOLOGICAL SETTING The Limpopo belt consists mostly of supracrustal assemblages intruded by ca. 3.2 Ga and 2.7–2.6 Ga felsic plutons (e.g., Kröner et al., 2018). These protoliths were penetratively modified by orogen-scale ductile folding, granulitefacies metamorphism, and anatexis first at 2.7– 2.6 Ga and later at 2.02 ± 0.02 Ga (Kramers and Mouri, 2011; Brandt et al., 2018). The Limpopo belt of Mason (1973) (also known as the central zone of the Limpopo metamorphic complex) is bounded by the 5–10-km-wide, right-slip TuliSabi and left-slip Palala mylonitic shear zones in the north and south against the northern and southern marginal zones, respectively (Fig. 2). The marginal zones are composed of granuliteto amphibolite-facies granitic-greenstone associations with the metamorphic grades decreasing away from the Limpopo belt (Mason, 1973; van Reenen et al., 2019). Granuliteamphibolite–facies metamorphism occurred at 2.7–2.6 Ga and 2.0 Ga, respectively, in the northern marginal zone, whereas similar highgrade metamorphism appears to have occurred only at 2.7–2.6 Ga in the southern marginal zone (Kramers and Mouri, 2011; van Reenen et al., 2019). Both zones have yielded ca. 2.0 Ga Rb-Sr biotite and feldspar cooling ages, which are in contrast to the 2.7–2.6 Ga Rb-Sr biotite ages obtained from Zimbabwe-Kaapvaal craton (ZKC) rocks outside the marginal zones farther away from the Limpopo belt (van Breemen and Dodson, 1972). The origin of the Limpopo belt has been attributed to 2.7–2.6 Ga north-south ZimbabweKaapvaal collision (Light, 1982), protracted north-south terrane accretion from 2.7 Ga to 2.0 Ga (Barton et al., 2006), two-stage, glacierlike, gravity-driven ductile flow from east to west at 2.7–2.6 Ga and 2.0–1.8 Ga (McCourt and Vearncombe, 1987; van Reenen et al., 2019), and ca. 2.0 Ga intracontinental transpression (Schaller et al., 1999). A key piece of evidence for the 2.7–2.6 Ga Limpopo orogenic event is the presence of seemingly undeformed 2.7–2.6 Ga plutons (van Reenen et al., 2019), but such an interpretation does not consider the possible role of rheological contrasts among different lithologies in the Limpopo belt in controlling the style and distribution of deformation. A major issue with the existing models is that they all assume the orogen-bounding shear zones to have maintained their original geometry. As shown here, *E-mail: ayin54@gmail.com †Deceased on 20 May 2019 Published online 19 November 2019 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/2/103/4926764/103.pdf by UCLA Digital Collect Serv user on 12 January 2021 104 www.gsapubs.org | Volume 48 | Number 2 | GEOLOGY | Geological Society of America this assertion is inconsistent with the regional geologic relationships, which require the orogen to have been tightly folded by later compression. POSTOROGENIC FOLDING OF THE LIMPOPO OROGENIC BELT Four lines of evidence suggest that the Limpopo orogen was folded by north-south compression at ca. 1.97–1.88 Ga (Fig. 1). First, the 1.97–1.88 Ga Blouberg Formation, unconformably resting on top of the Limpopo belt, was folded and then overlain by undeformed ca. 1.88 Ga upper Waterberg Group strata (Figs. 3A and 3B). Rotation of the dominantly vertical Blouberg beds to their original horizontal positions requires the nearby vertical “left-slip” Palala shear zone to have a prefolding subhorizontal geometry with top-to-the-west thrust kinematics. The folding event postdates the youngest cooling age of the Palala shear zone at ca. 1.97 Ma (Schaller et al., 1999) and the youngest detrital zircon age of the Blouberg Formation at ca. 2.04 Ga (Corcoran et al., 2013) but predates a mafic dike intruding the undeformed overlying Waterberg Group strata at ca. 1.88 Ga (Fig. 3A; Hanson et al., 2004). Second, the lower Waterberg Group, located directly south of the Palala shear zone (unit Ptw in Fig. 2), was also folded by north-south compression between ca. 2.02 Ga and ca. 1.93 Ga (Dorland et al., 2006). This timing is similar to the aforementioned Blouberg folding event (Fig. 3A). Third, the Limpopo gneissic foliation defines an orogen-scale fold structure (e.g., McCourt and Vearncombe, 1987; see also Fig. 2), expressed by the gneissic foliation oriented at high angles to the orogen-bounding shear zones along the central section of the Limpopo belt, which becomes subparallel to the shear zones approaching the edges of the belt (Fig. 2). Rotation of the nearly vertical gneissic foliation near the orogen margins to a horizontal position would require the nearby bounding shear zones to have had a subhorizontal geometry before orogen-scale folding. This reconstruction is consistent with restoring the folded Blouberg and lower Waterberg strata mentioned above, which places the currently vertical Palala shear zone into a prefolding subhorizontal orientation. Fourth, gravity and seismic-reflection surveys show that the low-density Limpopo belt lies above the high-density ZKC craton, separated by a synformal thrust with the Tuli-Sabi and Palala shear zones as its surface traces (de Beer and Stettler, 1992). The inferred Limpopo belt thickness above the fault is <8 km (de Beer and Stettler, 1992). Although our own field observations corroborate the earlier work showing that the Tuli-Sabi (Fig. 3C) and Palala (Fig. 3D) shear zones are rightand left-slip features, respectively, in their current orientations (McCourt and Vearncombe, 1987), contradictory right-slip shear bands in the overall left-slip Palala shear zone were reported locally by Schaller et al. (1999). The minor right-slip indicators, used as the supporting evidence for a right-slip transpressional model of the Limpopo belt (Kramers et al., 2011), could have resulted from postshear folding or development of antithetic structures (Figs. 3E and 3F). TECTONIC MODEL Unfolding the currently oriented eastnortheast–trending Limpopo belt and the steep bounding Tuli-Sabi and Palala shear zones requires the Limpopo belt to have formed by west-southwest–directed thrusting, placing the Limpopo belt over the ZKC (Figs. 3E and 3F). Because the Limpopo rocks are dominated by supracrustal assemblages in the hanging wall that do not match the granitic-greenstone rocks of the ZKC and the marginal zones in the footwall (Kröner et al., 2018; van Reenen et al., 2019), the total motion on the Limpopo thrust must have exceeded 600 km (Fig. 1). This interpretation raises the question of whether the Limpopo belt was part of the ZKC or a separate craton. For the Limpopo belt to be a separate continent, its collision with the ZKC would require the presence of a magmatic arc immediately predating the ca. 2.0 Ga Limpopo thrust. The absence of such an arc in either the Limpopo belt or the ZKC and marginal zones rules out this possibility. In contrast, the presence of the diagnostic 2.7–2.6 Ga felsic plutons and the records of coeval granulite metamorphism in the hanging-wall Limpopo belt and footwall marginal zones (Kröner et al., 2018; van Reenen Figure 1. Tectonic map of southern Africa, modified from Hanson (2003) and McCourt et al. (2013). 2.25-2.0 Ga “regional granites” (McCourt et al., 2013) 0 400 800 km Congo Craton Tanzania Craton Kaapvaal Craton Roheboth Block Bangweulu Block 2.04-1.93 Ga Magondi belt (Hanson, 2003) Zimbabwe Craton 06o00’S 29o00’S Eastern edge of Kaapvaal before rifting (Hanson, 2003) 20o00’E 32o00’E 17o30’S Kalahari Desert 2.04-2.00 Ga Limpo
更新日期：2019-11-19
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