Mineralogy and geochemistry of suspected mud volcano fluid migration pathways within the footwall of the active Chishan Fault, southern Taiwan

During a 2008 investigation in the footwall of the active Chishan thrust fault in southern Taiwan, foliated, dark, planar bands were mapped as cross-cutting Plio-Pleistocene mudstones of the Gutingkeng Formation. It was originally hypothesized that these bands were localized shear zones associated with the Chishan fault, but normal sense-of-shear indicators observed in this thrust fault zone proved problematic. We developed a new hypothesis that the dark bands are inactive fluid migration pathways from now-eroded mud volcanoes in the over-pressurized footwall of the Chishan fault. Dark band and wall rock mudstone samples, along with sediment from several currently active mud volcanoes along the Chishan fault, were gathered in March 2009 and tested for mineralogical and chemical variations. X-Ray diffraction revealed no difference in mineralogy between the dark bands of mudstone and the wall rock around them. You et al. (2004) recorded a high concentration of boron in Chishan fault mud volcano fluids, and preliminary results of our rock and sediment samples using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) indicate elevated concentrations of boron in the active mud volcano sediments and dark band mudstones versus the unaltered mudstones. Since fluid pathways will often coincide with permeable shear zones, the overprinting of sheared rock fabrics and fluid/mud transport is likely in these localized structures.