Architecture of interacting deformation band fault zones in siliciclastic rocks and its influence on geomechanical property distribution

Abstract

Spatial organization of strain-hardening zones and the adjacent damage zones associated with faulting of porous siliciclastic rocks influence the across-strike variation of geomechanical properties. The across-fault geomechanical property distribution is expected to change when fault zones of distinct orientations interact. We performed outcrop-scale structural analysis along scanlines trending perpendicular to fault strike to constrain the extent of fault zones oriented dominantly along NE-SW and NW-SE. Young's Modulus (E) and P-wave velocity (Vp) measurements on deformation bands (DBs) and host rocks along such scanlines are further utilized to better characterize the fault zones. The selected outcrop exposes Cretaceous siliciclastic rocks in Rio do Peixe Basin, NE Brazil. In this study, Vp measurements coupled with detailed structural investigation have been integrated with grain size analysis for the first time. We observe that DBs oriented parallel to the major fault sets show grain size reduction along with higher E and Vp relative to the adjacent host rocks, indicating cataclasis and strain-hardening. The expected variation of geomechanical properties across the relatively late-stage NE-SW-trending fault zones gets obscured at sites of intense interaction with the early formed NW-SE-striking fault zones. In such areas, E of DBs consistently increases with proximity to the NW-SE-oriented fault zones revealing that the structural position of pre-existing fault zones mostly controls the geomechanical property distribution at sites of intense fault interaction. Similarly, we observe a direct relationship between Vp and grain size of the host rocks across the late-stage faults where there is relatively less fault interaction. However, this relationship does not exist within zones of intense fault interaction. As DBs are prevalent mostly in coarse-grained rocks, Vp in coarse-grained host rocks increases with proximity to the constrained fault zones. An integrated approach adopted in our study is crucial in delineating the effects of fault interaction on the distribution of geomechanical properties.