Syn-eruptive normal faults in tephra rings and formation mechanisms

Abstract

This study systematically characterizes normal faults in tephra rings at Weizhou and Xieyang volcanic islands in southern China. Detailed field observations, measurements and analyses indicate that these faults were likely formed during phreatomagmatic eruptions based on the following evidence: i) faults in tephra rings are closely spaced and fault cores commonly have no fault gouge, debris and secondary infill, ruling out long-term faulting mechanisms in extensional settings; ii) faults are usually accompanied by soft-sediment deformation structures, indicating most volcanic layers were in soft during faulting; iii) faulted tephra deposits are consistently thicker on the downdropped hanging-wall than on the uplifted footwall, indicating that the faults formed during the deposition of the tephra; iv) unlike late-stage strike-slip faults, which crop out in many places on the two islands, the normal faults generally occur adjacent to the volcanic craters, implying local control. Repeated phreatomagatic eruptions produced alternating and unstable ash and lapilli deposits on the tephra rings. These layers failed during the eruptions, resulting in the formation of syn-eruptive normal faults in the tephra rings.