Unraveling the key factors controlling active faulting in Tertiary and Quaternary sequences

Abstract

The effects of across-fault variation of mechanical properties and along-fault lithological heterogeneity on the seismicity of active faults is poorly understood. Moreover, the role of chemical processes on the mechanical behavior of active faults has not yet received sufficient attention. Such phenomena have more relevance, especially when faults juxtapose chemically reactive carbonate-dominated host rocks. In this study, we examine the key controls on active faulting by utilizing excellent outcrops of an intrabasinal strike-slip fault cutting across Tertiary and Quaternary sequences. We integrate structural, geomechanical and mineralogical investigations of faults and adjacent host rocks geographically exposed at different structural positions along the length of a 30 km-long, NE-SW-striking Galera Fault (GF) in Guadix-Baza basin, SE Spain. Our results show that fault and host rocks along the GF become carbonate-rich predominantly consisting of calcite towards NE. The thickness of sedimentary cover increases towards SW, where gypsum and phyllosilicate minerals are dominant. Calcite precipitation in fault rocks leads to reduction of porosity and significant enhancement of P-wave velocity (Vp), relative to the adjacent host rocks, and makes the fault zone more prone to seismic slip. On the contrary, phyllosilicates including clay mineral assemblages reduce the strength of the fault rocks and promote aseismic creep. The relationship between Vp and modified estimate of Brittleness Index used in this study indicate that the mineral assemblage of the fault as well as the adjacent host rocks influence seismicity. Our novel approach predicts more seismic activity towards the NE-part of GF due to dominance of calcite-rich rocks and proximity to the basement. This is validated by earthquake records in the studied area, which document shallow earthquakes concentrated towards the NE-part of GF. Thus, our study highlights the importance of adopting interdisciplinary approaches and understanding of the basin architecture controlling the distribution of host rocks to comprehend complex processes associated with active faulting.