[M-GI31] Drilling Earth Science

[MGI31-08] Introduction of a new drilling project “Probing the heart of
an earthquake and life in the deep subsurface (PROTEA)”

*Yasuo Yabe¹, Musa Manzi², Alba Gomez-Arias³, Devan Nisson⁴, Julio Castillo⁵, Raymond Durrheim², Hiroshi Ogasawara⁶, Thomas Kieft⁷, Scott Perl⁸ (1.Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, 2.School of Geosciences, University of the Witwatersrand, 3.Chemistry Department, University of the Free State, 4.Department of Geosciences, Princeton University, 5.Department of Microbiology and Biochemistry, University of Huelva, 6.College of Science and Engineering, Ritsumeikan University, 7.Biology Department, New Mexico Institute of Mining and Technology, 8.Origins and Habitability Laboratory, NASA Jet Propulsion Laboratory)

Keywords: New ICDP drilling project, Drilling to the strong motion source, Hypersaline brine ecosystem, High-resolution sill-dike structure, Talc formation, South Africa

ICDP-DSeis project is the first drilling project in the world that succeeded in recovering rock samples from an active, seismogenic fault on which high aftershock activity occurred viz. the 2014 Orkney earthquake (M5.5), South Africa. The aftershock zone was formed in an altered lamprophyre dike intruded into the 2.9 Ga altered andesitic basaltic lava of the Crown Formation. The aftershocks line up in streaks sub-parallel to the intersection lines of the lamprophyre dike and the seismic reflective planes. Synthetic seismic models from acoustic downhole and lithological information of Hole A drilled during the DSeis project suggest that the strong reflectors observed on 3D seismic data originate from the sills, which are associated with a dike-sill complex that intruded post deposition of the Witwatersrand Basin. Laboratory experiments using the recovered rocks demonstrated that the frictional properties of the fault were suitable for terminating the dynamic rupture propagation of the mainshock and for accommodating the high aftershock activity. However, they do not enable the nucleation and propagation of the dynamic rupture.
The DSeis holes intersected the fissure with hypersaline (24wt%) and moderate temperature (54℃) brine in the Onstott dike, which is different from the altered lamprophyre dike. Pore pressure in the fissure is only 10 MPa despite 3 km depth. δ¹⁸O and δ²H isotope values of the brine indicated that the brine has been isolated from the surface hydrological cycle. Alpha particle radiolysis of water along with anhydrous-silicate-to-clay alteration reactions over long residence times (1.2 Ga) estimated from ⁴⁰Ar excess caused the brine to be significantly more saline than any previously sampled Witwatersrand Basin fluid. The brine also contained a large dissolved organic carbon pool, primarily composed of Archean kerogen that has been radiolytically oxidized over this long subsurface residence. Other notable organics included volatile C₁-C₃ hydrocarbons bearing δ¹³C and δ²H signatures consistent with abiotic production. Cell concentration in the brine was ~100 cells/mL. Radiolytic reactions under the high U concentration within the Moab Khotsong rocks supply H₂ which has been identified globally as an energy currency (electron donor) for microbial populations in the deep subsurface. The occurrence of potential electron donor (H₂)/acceptor (e.g., NO₃) species over geologic time, alongside bioavailable sources of organic carbon, indicate support for diverse chemotrophic strategies.
To understand the above-mentioned findings by the DSeis further in-depth, we propose a new drilling project “Probing the heart of an earthquake and life in the deep subsurface (PROTEA)”. PROTEA will drill holes to explore the spatial extent of the hypersaline brine fissure to investigate the locality and universality of the ecosystem in the brine. In addition, a hole as long as 1800 m targeting the strong motion source of the mainshock will be drilled to recover rock samples from the heart of the earthquake and to measure spatial variations in stress state and pore pressure from the edges of the source fault to the heart of the earthquake. The borehole-tunnel fibre optic acoustic distributed sensing (DAS) surveys will be designed to better define the dike-sill complex and monitor seismicity.
The major open questions to be approached in PROTEA are: What is the structural architecture of the dike-sill complex? How was the dike-sill complex formed and altered? Why does the aftershock distribution correlate with the dike-sill complex? What frictional properties, stress state, and pore pressure enabled the nucleation and propagation of the dynamic rupture of the mainshock? Is there a relationship between seismicity and microbial activity? What metabolic pathways enable long-term microbial persistence in the hypersaline oligotrophic closed ecosystem and what is their role in the C, N, and S geochemical cycles?