Fluid–rock interactions in seismic faults: Implications from the structures and mineralogical and geochemical compositions of drilling cores from the rupture of the 2008 Wenchuan earthquake, China

Abstract

We describe the structural features and mineralogical and geochemical compositions of the fault rocks recovered from boreholes at the Golden River site on the Yingxiu–Beichuan fault, which activated and slipped along a 240km-long main surface rupture zone during the 2008 Wenchuan earthquake. The fault, which accommodated co-seismic slip, cuts granitic rocks from the Pengguan complex, in which this earthquake most likely nucleated. Fault rocks, including cohesive cataclasite, unconsolidated breccia and three fault gouges with distinct colors, were identified from the drilling cores. On-going uplift and erosion in the area means that the fault rocks, formed at different depth, were exhumed to the shallow surface during the uplift history of the Longmenshan fault zone. A clear change from fracturing and comminution in the cataclasites and breccia to more pervasive shear/formation of fine grained materials in the gouges has been observed. The gouges are distinct and have accommodated significant displacement in multiple increments of shear. Furthermore, fault rocks recovered from the boreholes display numerous features indicative of fluid infiltration and fluid–rock interaction. Toward the fault core, clay minerals have replaced feldspars. The element enrichment/depletion patterns of the fault rocks show general fluid infiltration trends, such as 1) mobile elements are generally depleted in the fault rocks, 2) the microstructural, mineralogical and geochemical results of the fault rocks consistently indicate that pervasive fluid infiltration and fluid–rock interactions altered feldspars and mafic minerals to clay minerals. The fluid was Mg2+- and Fe2+-rich, facilitating formation of chlorite. Isocon analyses further reveal that a large rock volume has been lost, which is attributed to the removal of mobile elements associated with fluid infiltration and perhaps enhanced by pressure solution. These results reflect the accumulated effects of cataclasis and fluid infiltration/fluid–rock interaction during the long evolution history of the Longmenshan fault zone.