6SubsidenceAnalysisofthe__Wit.doc

6Subsidence Analysis of the Witwatersrand Basin, South AfricaThe worlds largest accumulation of gold is in the Witwatersrand basin of South Africa, so this area has attracted an enormous amount of geologic study. Debate still occurs, however, over basic aspects of basin history including the timing of introduction of the metals. For example, Phillips and co-workers (Phillips et al, 1989; Phillips and Myers, 1989) challenged the conventional view that the Au and U are detrital placer enrichments and argued that the Au was introduced during metamorphism. Sutton et al. (1990), Robb and Meyer (1991), and Minter et al. (1993), among others, have challenged this metamorphic model, supporting the syngenetic, placer model for introduction of the Au, followed by one or more episodes of local redistribution of the Au during metamorphism. The fill of the basin consists of three main units (Fig. 3), the largely volcanic Dominion Group, and the mostly clastic West Rand and Central Rand Groups. Conglomerate horizons from all three units have produced Au and U, but the Central Rand Group hosts by far the richest Au accumulations. In present extent, the Dominion strata cover about 15,000 km2, the West Rand about 42,000km2 and the Central Rand about 10,000 km2 (Tankard et al. 1982), thus their present basins are comparable in size. There is evidence, however, that the original extent of the West Rand depositional basin was much larger, particularly if its correlation to Pongola sediments as proposed by Beukes and Cairncross (1991) is confirmed.Several qualitative basin analyses of the Witwatersrand have appeared, based on lithologic and structural data. For example, Burke et al. (1986) concluded that the basin formed in a retroarc foreland setting resulting from subduction of oceanic crust beneath the Kaapvaal craton with the development of a continental volcanic arc along the craton margin in the hinterland of the Witwatersrand basin. They assigned the Dominion Volcanics and the West Rand Group to this phase of basin development. Subsequently, the Kaapvaal craton collided with the Zimbabwe craton, forming the source area for the gold-rich Central Rand Group. Winter (1987) similarly concluded the basin belonged to a retroarc foreland setting. Stanistreet and McCarthy (1991), Robb et al. (1991), and Jackson (1992) envisioned a more complex history, with the Dominion Group belonging to an early rift phase of basin development and the West Rand Group and its Pongola equivalents belonging to the thermal subsidence phase of a cratonic basin. In these models, the Central Rand Group marks a profound shift to foreland tectonics, which was followed in turn by extrusion of the flood basalts of the Klipriviersburg Group and impactogenal rifting that culminated in the Platberg Group. DeWit et al (1992) essentially followed this model in their review of Archean tectonics, but they pointed out that the presence of northward thrusting is inconsistent with a simple foreland model and suggested that collision occurred between a series of oceanic and continental fragments producing piggy-back basins.We can contrast these models that emphasize foreland tectonics with rifting-only models proposed by Bickle and Eriksson (1982) and by Clendenin et al. (1988). Bickle and Eriksson (1982) identified the Dominion Group with a phase of rapid mechanical subsidence and assigned the West Rand and Central Rand Groups to a later phase of slower thermal subsidence. Based on this assumption and the thicknesses of the groups, they calculated a total subsidence of 9 km, of which 7 km can be attributed to the thermal phase. They were able also to estimate a stretching factor() of 1.4 to 1.7. Apparently their calculations did not take into account corrections for compaction. Clendenin et al. (1988) assigned the entire sequence from the Dominion through the Central Rand Groups to what they called a pregraben protobasin, with the main graben development occurring in the overlying Ventersdorp Supergroup, but they made no estimates of relative amounts of subsidence or subsidence rates.Yet a third possible tectonic style is strike-slip-dominated transtensional and transcompressional basins. Stanistreet and McCarthy (1991) discussed the likelihood of the Central Rand Group representing a time of escape tectonics, in which the Witwatersrand block moved eastward along a set of major strike-slip faults. They called on the modern Maracalbo basin as an analogue. In a set of more detailed papers (McCarthy et al., 1990; Myers et al., 1999; Stanistreet and McCarthy, 1990), evidence was presented for synsedimentary fault activity in the Witwaterarand basin. This activity seems to have been dominated by steeply dipping faults that changed their senses of motion from normal to reverse and had a large lateral component of offset (see especially the block diagrams of Stanistreet and McCarthy, 1990). Such a structural style is characteristic of pull-apart basins, suggesting that Witwatersrand subsidence might be best modeled as occurring in a strike-slip-dominated terrane. Recent publication of high resolution U-Pb dates from single zircons (Barton et al., 1989; Robb et al., 1990; Armstrong et al., 1991) provides reasonably tight age constraints on events in the history of the Witwaterarand basin, and we have used these ages to attempt a more quantitative subsidence analysis. The ages for detrital zircons from each horizon show a younging-upward trend throughout the section, over a time span of 860 m. y. (Robb et al., 1991). Accordingly, we used the youngest zircon age for each conglomerate horizon to derive a maximum age curve. Three lava ages define a parallel minimum age curve (Fig. 4). Values interpolated from the minimum age curve were then used as the thickness versus age input for the subsidence program (Table 1). We realize that this curve has large error limits, but if the time lag between the youngest source rock and deposition is essentially constant, as shown in Figure 4, then our approach provides an accurate estimate of the relative timing of events. The poorest constraint is for the duration of the Central Rand Group. Our calculations give a maximum duration, and therefore a minimum subsidence rate. Subsequent data are likely to force an upward revision in the estimate of the subsidence rate, but the presence of multiple unconformities within the Central Rand Group indicates to us that it was deposited at a much slower rate than the West Rand group.New wordsinterpolate in5tE:pEuleit v. 插值，内插subsidence sEbsaidEns 沉降foreland fC:lEnd n. 前陆setting setiN n. 背景detrital di5traitEl a.碎屑的placer plAsE / 5pleisE n. 砂矿retroarc retrEua:k 弧后, 后弧craton kreitCn n. 克拉通arc a:k n. 弧hinterland 5hintElAnd n. 腹地collide kElaid v.