40Ar/39Ar constraints on the age of metamorphism in the Witwatersrand Supergroup, Vredefort dome (South Africa)

Gibson, Roger L.; Reimold, W. U.; Phillips, David; Layer, Paul W.

doi:10.2113/1030175

Cited by 19 publications

(12 citation statements)

References 29 publications

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“…The sedimentation was followed by up to 3 km thick tholeiitic flood basalts from the Ventersdorp Supergroup during the Platberg rifting. This was followed by up to 2 km of local rift sediments (GIBSON et al, 2000b). Most of the Kaapvaal Craton was covered by a shallow sea 2.6 Ga ago, after which a period of slow subsidence was followed by deposition of the 2-3 km thick Pretoria Group.…”

Section: The Witwatersrand Basinmentioning

confidence: 99%

“…The basin is a 350 9 200 km NE trending structure that developed in a foreland thrust setting during the convergence of the Zimbabwe plate and the Kaapvaal Craton, during the later stage of the West Rand, about 2.9 Ga ago (GIBSON et al, 2000b;ROBB et al, 1997). The basin accumulated the Witwatersrand Supergroup sediments of 2.71-2.97 Ga ago, a 7-10 km thick sequence of terrigenous sedimentary rocks, comprising mainly of sandstones and mudrock, together with minor conglomerate horizons (ARMSTRONG et al, 1991;GIBSON et al, 2000b;ROBB et al, 1997). The region experienced continuous thrusting into the Central Rand stage until 2.7 Ga BP (Gibson et al, 2000b).…”

Section: The Witwatersrand Basinmentioning

confidence: 99%

“…The basin accumulated the Witwatersrand Supergroup sediments of 2.71-2.97 Ga ago, a 7-10 km thick sequence of terrigenous sedimentary rocks, comprising mainly of sandstones and mudrock, together with minor conglomerate horizons (ARMSTRONG et al, 1991;GIBSON et al, 2000b;ROBB et al, 1997). The region experienced continuous thrusting into the Central Rand stage until 2.7 Ga BP (Gibson et al, 2000b). The sedimentation was followed by up to 3 km thick tholeiitic flood basalts from the Ventersdorp Supergroup during the Platberg rifting.…”

Section: The Witwatersrand Basinmentioning

confidence: 99%

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Earthquake Rupture at Focal Depth, Part I: Structure and Rupture of the Pretorius Fault, TauTona Mine, South Africa

Heesakkers

Murphy

Reches

2011

Pure Appl. Geophys.

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We analyze the structure of the Archaean Pretorius fault in TauTona mine, South Africa, as well as the rupture-zone that recently reactivated it. The analysis is part of the Natural Earthquake Laboratory in South African Mines (NELSAM) project that utilizes the access to 3.6 km depth provided by the mining operations. The Pretorius fault is a *10 km long, oblique-strikeslip fault with displacement of up to 200 m that crosscuts fine to very coarse grain quartzitic rocks in TauTona mine. We identify here three structural zones within the fault-zone: (1) an outer damage zone, *100 m wide, of brittle deformation manifested by multiple, widely spaced fractures and faults with slip up to 3 m; (2) an inner damage zone, 25-30 m wide, with high density of anastomosing conjugate sets of fault segments and fractures, many of which carry cataclasite zones; and (3) a dominant segment, with a cataclasite zone up to 50 cm thick that accommodated most of the Archaean slip of the Pretorius fault, and is regarded as the 'principal slip zone' (PSZ). This fault-zone structure indicates that during its Archaean activity, the Pretorius fault entered the mature fault stage in which many slip events were localized along a single, PSZ. The mining operations continuously induce earthquakes, including the 2004, M2.2 event that rejuvenated the Pretorius fault in the NELSAM project area. Our analysis of the M2.2 rupture-zone shows that (1) slip occurred exclusively along four, pre-existing large, quasi-planer segments of the ancient fault-zone; (2) the slipping segments contain brittle cataclasite zones up to 0.5 m thick; (3) these segments are not parallel to each other; (4) gouge zones, 1-5 mm thick, composed of white 'rock-flour' formed almost exclusively along the cataclasite-host rock contacts of the slipping segments; (5) locally, new, fresh fractures branched from the slipping segments and propagated in mixed shear-tensile mode; (6) the maximum observed shear displacement is 25 mm in oblique-normal slip. The mechanical analysis of this rupture-zone is presented in Part II (HEESAKKERS et al.,

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Section: The Witwatersrand Basinmentioning

confidence: 99%

Section: The Witwatersrand Basinmentioning

confidence: 99%

Section: The Witwatersrand Basinmentioning

confidence: 99%

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Earthquake Rupture at Focal Depth, Part I: Structure and Rupture of the Pretorius Fault, TauTona Mine, South Africa

Heesakkers

Murphy

Reches

2011

Pure Appl. Geophys.

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“…1). This basin consists of about 10 km thick sequence of sedimentary rocks, mainly sandstones, mudrocks, and gold-bearing conglomerates (ARMSTRONG et al, 1991;GIBSON et al, 2000;ROBB et al, 1997). The basin underwent low-grade metamorphism and was tectonically inactive for the last 2.5 Ga (ROBB et al, 1997).…”

Section: The Pretorius Fault-zonementioning

confidence: 99%

Earthquake Rupture at Focal Depth, Part II: Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault, TauTona Mine, South Africa

Heesakkers

Murphy

Lockner

et al. 2011

Pure Appl. Geophys.

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We analyze here the rupture mechanics of the 2004, M2.2 earthquake based on our observations and measurements at focal depth (Part I). This event ruptured the Archean Pretorius fault that has been inactive for at least 2 Ga, and was reactivated due to mining operations down to a depth of 3.6 km depth. Thus, it was expected that the Pretorius fault zone will fail similarly to an intact rock body independently of its ancient healed structure. Our analysis reveals a few puzzling features of the M2.2 rupture-zone:(1) the earthquake ruptured four, non-parallel, cataclasite bearing segments of the ancient Pretorius fault-zone; (2) slip occurred almost exclusively along the cataclasite-host rock contacts of the slipping segments; (3) the local in-situ stress field is not favorable to slip along any of these four segments; and (4) the Archean cataclasite is pervasively sintered and cemented to become brittle and strong. To resolve these observations, we conducted rock mechanics experiments on the fault-rocks and host-rocks and found a strong mechanical contrast between the quartzitic cataclasite zones, with elastic-brittle rheology, and the host quartzites, with damage, elastic-plastic rheology. The finite-element modeling of a heterogeneous fault-zone with the measured mechanical contrast indicates that the slip is likely to reactivate the ancient cataclasitebearing segments, as observed, due to the strong mechanical contrast between the cataclasite and the host quartzitic rock.

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“…2.06 Ga) and the Vredefort impact event (ca. 2.02 Ga) (Robb et al, 1997;Gibson et al, 2000;Friese et al, 2003). Studies of xeno-time from the Witwatersrand Supergroup suggest a potentially complex history of postdepositional fl uid fl ow (England et al, 2001;Kositcin et al, 2003); however, these studies were hampered by minute crystal sizes, the presence of radiogenic inclusions, and mixing of dates from multi-aged crystals.…”

Section: Introductionmentioning

confidence: 99%

Bushveld-aged fluid flow, peak metamorphism, and gold mobilization in the Witwatersrand basin, South Africa: Constraints from in situ SHRIMP U-Pb dating of monazite and xenotime

Rasmussen¹,

Fletcher²,

Muhling³

et al. 2007

Geol

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In situ U-Pb dating of monazite and xenotime in gold reefs and unmineralized greenschist facies sedimentary rocks from the Witwatersrand basin, South Africa, reveals two episodes of tectonothermal activity. A major event between 2.06 and 2.03 Ga is recorded in the Witwatersrand and Transvaal Supergroups in the northwestern and central basin, and broadly coincides with the ca. 2.06 Ga Bushveld event. In the central and southern basin, a previously unrecognized event has been dated between 2.14 and 2.12 Ga. The widespread geographic and stratigraphic occurrence of Bushveld-aged monazite and xenotime, including both auriferous reefs and unmineralized strata, indicates that metamorphism and fl uid fl ow associated with magmatism was pervasive, affecting most of the succession (>10 km thick) in the central and northern parts of the basin. The metamorphic phosphate dates, which are younger away from the complex, indicate a lag of 20-30 m.y. between emplacement and phosphate growth in the central basin (~100 km south), suggesting that heat related to magmatism was transferred southward at an average rate of 3-5 mm yr -1 . The absence of 2.06-2.03 Ga phosphates in the Welkom goldfi eld at the southern end of the basin implies that Bushveld-related heating and fl uid fl ow did not affect this part of the basin. The intergrowth of ca. 2.045 Ga monazite with gold in quartz-pebble conglomerate from the West Rand goldfi eld indicates that fl uid fl ow related to the Bushveld event caused mobilization of gold in the Witwatersrand basin.

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40Ar/39Ar constraints on the age of metamorphism in the Witwatersrand Supergroup, Vredefort dome (South Africa)

Cited by 19 publications

References 29 publications

Earthquake Rupture at Focal Depth, Part I: Structure and Rupture of the Pretorius Fault, TauTona Mine, South Africa

Earthquake Rupture at Focal Depth, Part I: Structure and Rupture of the Pretorius Fault, TauTona Mine, South Africa

Earthquake Rupture at Focal Depth, Part II: Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault, TauTona Mine, South Africa

Bushveld-aged fluid flow, peak metamorphism, and gold mobilization in the Witwatersrand basin, South Africa: Constraints from in situ SHRIMP U-Pb dating of monazite and xenotime

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