Influence of Growth Strata on the Evolution of Faulted-related Folds by 2-D Distinct-element Models: Case Studies for Coseismic Deformation of the Chi-Chi Earthquake

The Fault-propagation folds are generally associated with blind faults that are of great interest from an academic point of view. They have recently been recognized as extremely important for their seismic hazard potential. The Chi-Chi earthquake produced many monoclinal scarps by fault-propagation folding which caused great damage. The trishear kinematic model of fault-propagation folding appears to approximately represent the geometric development of some structures like monoclines, comparatively little is known of the mechanical controls on their development. Thus we construct a series of distinct-element models that consist of bounded assemblies of elastic particles that simulate the brittle deformation associated with fault-related folding over a rigid footwall. Here we attempt to predict the broad-scale features and basic characteristics of distributed deformation developed above blind contractional faults at depth. The initial rock mass is modeled by a series of discrete, non-uniform-sized circular, elastic, frictional particles, bounded in shear and tension and capable of progressive fracturing during loading. The models reproduce the deformation patterns with an evolutionary slip of rigid the basement fault in different strength of parallel bond representing the cementation of granular particles. Two kinds of rock type, sandstone and mudstone, are tested numerically with the constraint of laboratory results. Then the mechanical strength of mudstone and sandstone is reduced to meet the weakened and unconsolidated sedimentary covers. We conclude that weak cover strength promotes cover flowage, wide zone of deformation and limited fault propagation while strong cover reproduce a narrow zone of deformation and faster fault propagation.