Earthquake Rupture at Focal Depth,Part I: Structure and Rupture of the Pretorius Fault,TauTona Mine,South Africa |
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| Authors: | V Heesakkers S Murphy Z Reches |
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| Institution: | (1) School of Geology and Geophysics, University of Oklahoma, Norman, OK, USA;(2) Present address: Chevron ETC, 1500 Louisiana St, Houston, TX 77002, USA;(3) Present address: AngloGold Ashanti, Carletonville, Gauteng, South Africa; |
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| Abstract: | 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-strike-slip 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., Earthquake Rupture at Focal Depth, Part II: Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault, TauTona mine,
South Africa 2011, this volume). |
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