Beschreibung
Investigating fault rocks that are deformed by the Mw 5.5 Orkney earthquake in South Africa, is one scientific target in the ICDP (International Continental Drilling Program) supported “DSeis” project (Drilling into Seismogenic zones in deep South African Gold Mines). The overall goal of this project is to study the cause and the behavior of the earthquake rupture zone, its stress at great depth and its deep microbiological life (Ogasawara et al., 2016).
Until recently (June 2018), drilling took place in the Moab Khotsong mine in order to intersect an area that ruptured as a result of the Orkney earthquake in August 2014. In June 2017, the 817 m deep “Hole A” was drilled, intersecting sedimentary and volcanic rocks of the Jeppestown Subgroup, which is a subgroup from the larger West Rand Group [Antrobus, 1986]. At the end, an intersection of the target rupture zone did not occur. The 700 m deep “Hole B” (mother hole) was then drilled in November 2017. A major rupture zone was intersected, but with a significant core-loss within the center of the rupture zone. In April 2018, a deviated “Hole B” (branch hole) was successfully cored, starting at ca. 544 m from the mother “Hole B”.
We will perform a detailed microstructural, mineralogical and geochemical study of the drill core samples from the rupture zone as a function of distance to the active fault trace. We are planning to describe the characteristic damage features, deformation mechanisms and fluid-rock interaction processes that occurred during the earthquake rupture. The tools we will use are optical microscopes (including cathodoluminescence microscopes), scanning and high-resolution transmission electron microscopes (SEM/TEM), microprobe (EPMA), x-ray diffraction (XRD) -and x-ray fluorescence (XRF) analytics. Advanced diffraction techniques, particularly hard X-ray synchrotron and neutron diffraction will be applied for the determination of preferred orientation patterns in quartz within the fault rock. The study of minerals and element alteration, and (if possible) stable isotopes and clay minerals in fault rocks will be used to elucidate the origin, timing and mechanism of fluid infiltration, and the extent of fluid-rock interaction in the fault zone.
Our project, which includes the investigation of the drill cores taken in the deep gold mines, provides a great opportunity to contribute to the DSeis goals, as we will examine very recent deformation features and fluid-rock interaction processes in the young rupture zone that was probably affected the first time in a major dyke hidden at depth. It is an excellent contrast to previous scientific drilling projects that were also structured around a series of questions addressing basic issues in fault dynamics and weakening mechanisms.
