Factors Affecting Transpression Folding as Inferred From Numerical Analysis


Three-dimensional folding behavior of an elasto-viscous layer induced by transpression, and biaxial compression on the boundary is studied. The factors studied include the strain rate, elastic competency contrast, and viscous competency contrast. It was found that the influence of elastic competency contrast fades as the strain rate slows until finally folding behavior is completely controlled by viscous behavior. If biaxial compression is applied, induced folding along each principal stress direction is dependent. The resulting wavelength in any particular direction will not be affected by initial shortening applied in another direction. The final fold form is the supposition of fold forms in two individual directions.

For transpression, the resulting fold axis may deviate from being perpendicular to the direction of boundary displacement. As the strain rate decreases, wavelength increases. The fold axis can be curved at slow strain rates. The directions of principal stress and the fold axis are nearly perpendicular to one another in most cases. However, for medium strain rates ( ε = 10-13 ~ 10-14 sec-1), the angle of deviation from the mutual perpendicular direction may reach 20 degrees. This deviation possibly accounts for the orientation of slaty cleavage not being perpendicular to the fold axis. Accordingly, interpretations or assertions are made based on the results of numerical analyses for the folding structure of southeastern Taiwan.

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