Effects of changing stress states on the development of caldera-bounding faults: Geological evidence from Kumano caldera, Japan

Collapse of a large caldera can cause spatial and temporal perturbations of stress, and formation of "caldera faults." The stress variations influence the direction of slip vectors on the fault planes; hence, stress estimation is important for the study of caldera-forming processes. In our paleostress estimation, the stress variations in the collapse of the ca. 14 Ma Kumano caldera in Japan have been revealed. A stress inversion method based on the Wallace-Bott hypothesis was used to compute the orientation of the principal stress axes (sigma(1)>=sigma(2)>=sigma(3)) and the stress ratio phi=(sigma(2)-sigma(3))/(sigma(1)-sigma(3)), where 0 <=theta <= 1. The caldera faults formed simultaneously with the caldera-forming ash-flow tuff eruption. Therefore, paleostress solutions obtained from slip data measured on such faults show the spatial and temporal changes of the stress at the time of the caldera collapse. The computed stress ratio phi characterizes a pair of stress fields. In the early stage, the stress field with phi similar to 1.0 shows a semi-radial trajectory of stress sigma(2) and an eastern concentric trajectory of stress sigma(3). This stress regime, resulting from pre-collapse tumescence, counteracts the gravitational force and thus produces smaller net vertical stress. The regional tumescence above an inflated magma chamber is the most plausible source of the stress field, and it is consistent with the timing of the caldera formation. In the late stage, the stress field with phi similar to 0.5 shows the semi-radial trajectory of stress sigma(2) and the west-convex and concentric trajectory of stress sigma(3). Change of the stress ratio phi from 1.0 to 0.5 implies that increase in the relative magnitude of the stress sigma(1) caused the deeper subsidence of the caldera floor. Stress variations may be of significant value for reconstructing the structural history of the caldera. (c) 2004 Elsevier B.V. All rights reserved.