Strike-slip faulting as a trigger mechanism for overpressure release through piercement structures. Implications for the Lusi mud volcano, Indonesia

Abstract

Piercement structures such as hydrothermal vent complexes, pockmarks, and mud volcanoes, are found in various geological settings but are often associated with faults or other fluid-focussing features. This article aims to investigate and understand the mechanisms responsible for the formation of piercement structures in sedimentary basins and the role of strike-slip faulting as a triggering mechanism for fluidization. For this purpose four different approaches were combined: fieldwork, analogue experiments, and mathematical modeling for brittle and ductile rheologies. The results of this study may be applied to several geological settings, including the newly formed Lusi mud volcano in Indonesia (Mazzini et al., 2007). Lusi became active the 29th of May 2006 on the Java Island. Debates on the trigger of the eruption rose immediately. Was Lusi triggered by the reactivation of a fault after a strong earthquake that occurred two days earlier? Or did a neighbouring exploration borehole induce a massive blow-out? Field observations reveal that the Watukosek fault crossing the Lusi mud volcano was reactivated after the 27th of May 2006 earthquake. Ongoing monitoring shows that the frequent seismicity periodically reactivates this fault with synchronous peaks of flow rates from the crater. Our integrated study demonstrates that the critical fluid pressure required to induce sediment deformation and fluidization is dramatically reduced when strike-slip faulting is active. The proposed shear-induced fluidization mechanism explains why piercement structures such as mud volcanoes are often located along fault zones. Our results support a scenario where the strike-slip movement of the Watukosek fault triggered the Lusi eruption and synchronous seep activity witnessed at other mud volcanoes along the same fault. The possibility that the drilling contributed to trigger the eruption cannot be excluded. However, so far, no univocal data support the drilling hypothesis, and a blow-out scenario can neither explain the dramatic changes that affected the plumbing system of numerous seep systems on Java after the 27-05-2006 earthquake. To date (i.e. April 2008) Lusi is still active.