Paleostress magnitudes associated with development of mountain belts: Insights from tectonic analyses of calcite twins in the Taiwan Foothills

Determinations of stress magnitudes based on inversion of calcite twin data from fault-related folds in the Taiwan Foothills show a decrease of differential stresses during synfolding erosion and exhumation, suggesting that paleodepth of burial largely controlled the levels of differential stresses sustained by rocks. Regional frictional conditions linked to shallow and/or deep decollement tectonics probably also influenced stress magnitudes. Combination of the twin inversion technique with fracture analysis and rock mechanics data provides first-order estimates of principal stress values related to the major Pleistocene shortening event in Taiwan. These stress estimates are compared to previous stress estimates in fold-thrust belts and to available data on differential stress magnitudes in foreland and hinterland domains of orogens. At the scale of an entire orogenic system the most striking point is a decrease of differential stresses from the hinterland toward the foreland. This decrease not only reflects tectonic stress attenuation away from the plate boundary but also suggests a major control by the depth of deformation. Differential stresses estimated from natural deformation consequently provide an independent support to the depth-dependent strength and the frictional behavior of the upper continental crust deduced from laboratory experiments and stress measurements in deep boreholes.