Clay Formation and Fabric Development in the DFDP-1 Borehole, Central Alpine Fault, New Zealand

Abstract:

Samples retrieved by shallow drilling into two principal slip zones of the central Alpine Fault, New Zealand, offer an excellent opportunity to investigate clay formation, fabric development and fluid-rock interaction in an active fault zone. Here, we provide lithological and structural observations of five samples from borehole DFDP-1B, drilled during Phase 1 of the Deep Fault Drilling Project (DFDP-1) in 2011. Each sample’s mineralogical composition was determined by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). Furthermore, the preferred orientation of the clays was analyzed by x-ray texture goniometry (XTG).

The dominant clay mineral phases are illite and chlorite/kaolinite. Newly formed smectitic clays are observed solely in the cm-thick zones of fault gouge, indicating that these mineral reactions are restricted to the fault zone. We observe that smectite forms by transformation of very fine-grained material produced by cataclasis during slip. Clay fabric intensity of both illite and chlorite reveal that relatively strong fabrics are present in the cataclasites above the principal slip zone, but that the clay minerals in the gouge have a very weak preferred orientation. The weak fabric supports the notion that clay orientation is a result of authigenic mineral growth and not of strain-induced particle reorientation. It also indicates that fluids are able to pass through the gouge, presumably along variably spaced and interconnected fracture networks or between particle boundaries. Our analysis of samples retrieved by DFDP-1 drilling and sampling demonstrates intimate association of localized shear, comminution, and rapid fluid-rock interaction. It thus contributes to a growing body of evidence that alteration processes, particularly formation of frictionally weak smectitic clay minerals, may be a significant weakening mechanism within active shallow faults.