Determining the maximum overburden along thrust faults using a porosity versus effective confining pressure curve

Abstract

This study aims to quantify the tectonically induced thrusting and erosion. Here, we propose and evaluate a laboratory-based method for determining the maximum effective stress ever sustained by a sedimentary-rock formation. The porosities of nineteen samples collected from two deep boreholes of the Taiwan Chelungpu-Fault Drilling Project were measured under different confining stresses. The relation between measured porosity and confining pressure (compaction curve) is bilinear in log–log scale. The maximum effective stress of the tested rocks was determined from the intersection point of the two straight lines of the compaction curve. The maximum effective stresses of the rocks below the Sanyi Fault (reverse fault) are close to the in-situ vertical effective stress. Above the Sanyi Fault (hanging wall), where the rocks have been thrust, the maximum effective stresses of the rocks are significantly greater than the in-situ stress with an average value of 29.48MPa. That is, the eroded thickness of the formation on the hanging wall of the Sanyi Fault is 2482m, a value close to that determined from the geological profile (2773m). It is inferred that the total displacements along the thrust Sanyi Fault is 11.5km, a value close to that determined from the geological profile (14–16km). These results indicate that the proposed laboratory-based method is potentially useful for determining the thrusting and erosion in a fold–thrust belt and the total displacements along a thrust fault.