Doubly vergent orogenic wedge model providing mechanism for regional-scale cleavage formation
Speaker: Wang Szu-Hua
Abstract
A doubly vergent wedge model was proposed to interpret the nature of continental collision zones and active margins. The mechanical model was basically set to be a two-dimensional, plane-strain deformation of a laterally uniform, rigid-plastic layer attached to two converging, nearly rigid underlying plates. Continuous convergence of the two plates would eventually lead to a development of an asymmetric wedge of the overlying layer. Whereas the ¡§pro-wedge¡¨ facing the subducting plate has a minimum taper angle, the ¡§retro-wedge¡¨ on the opposing side, by contrast, is steeper and owns a maximum permissible taper angle. While applying this model to natural orogens, other processes should be incorporated, such as the effect of isostasy, thermally activated viscous lower crust, surface denudation, and sedimentation at active margins. Taiwan is a good example of doubly vergent orogenic wedge development. Due to abrupt stress orientation change beneath the topographic divide, cleavages show distinct patterns on opposite sides of the orogenic wedge. There exists only one cleavage in the pro-wedge in the western Taiwan, while a second crenulation cleavage is observed in the retro-wedge in the eastern Taiwan. Advection of a cleavage across the topographic divide in a doubly vergent wedge provides an explanation for the occurrence of orogen-scale cleavage fans in natural systems such as Taiwan.
Reference
Willett, S.D., Beaumont, C., and Fullsack, P., 1993. Mechanical model for the tectonics of doubly vergent compressional orogens. Geology, 21, p. 371¡V374.
Fisher, D. M., Willett, S.D., Yeh, E.-C., and Clark, M. B., 2007. Cleavage fronts and fans as reflections of orogen stress and kinematics in Taiwan. Geology, 35, p. 65¡V68.