INITIATION OF FOLDING AND BOUDINAGE IN WRENCH SHEAR AND TRANSPRESSION

A linear stability analysis is performed on a deforming layered three- dimensional linear viscous system. The system consists of a single lay er of viscosity mu embedded in a medium of viscosity mu'. The layer is oriented normal to the z-axis and both the layer and medium are subje cted to uniform simple shearing parallel to the y-axis (wrench shear) with superposed uniform shortening parallel to x and extension paralle l to z (transpression). The stability of cylindrical perturbations of the form zeta = A(t) cos (alphax - betay) is examined. It is found tha t fold-type perturbations are unstable and pinch-and-swell disturbance s are unstable in some cases. For the case of wrench shear alone the f astest growing buckling disturbances are oriented at 45-degrees to the positive y-axis, while the fastest growing pinch-and-swell disturbanc es have positive growth rates and are oriented 90-degrees to the fold axes. Additional shortening parallel to the x-axis (transpression) cau ses the fold axes to initiate at lower angles to the y-axis. Pinch-and -swell disturbances may or may not be unstable in transpression depend ing on the magnitude of stretching parallel to the z-axis.