On the effect of crustal layering on ring-fault initiation and the formation of collapse calderas

摘要

Collapse calderas can be attributed to subsidence of the magma chamber roof along bounding sub-vertical normal faults (ring-faults) after a decompression of the magma chamber. It has previously been shown that for ring-faults to initiate, and thus facilitate collapse, the stress field both at the surface and around the magma chamber must satisfy specific critical conditions. Here, we present new numerical models that use a Finite Element Method to investigate the effects of crustal layering on local stress field distribution. Results are compared with existing criteria for ring-fault initiation. Different subsurface scenarios were simulated by varying the stiffness (Young's modulus) of layers placed above the magma chamber, and the host rock in which the chamber is seated. We consider depressurisation of a magma chamber, so as to simulate magma withdrawal. Results indicate that mechanical layering is a further first-order variable in the rare achievement of stress conditions required for ring-fault formation, and may be influential in facilitating or inhibiting caldera collapse. We show that for a given geometrical set-up, the magnitude and position of maximum tensional stress at the Earth's surface are influenced by the occurrence and relative distribution of mechanically stiff or soft lithologies above the magma chamber. For example, tensional stress at surface may be reduced by the presence of stiff layers (e.g. lavas), or increased by soft layers (e.g. pyroclastic units) compared to generic simulations using a homogeneous background medium. Overall we find that the presence of mechanically soft material promotes surface fracture initiation. In addition we suggest that the position of peak tensional stress at surface derived by the numerical models does not represent that related to the position of the bounding ring-fault, but is instead related to the position of initial tensional fractures appearing prior to the collapse faults.