Structural evolution of slate belts: examples from Tawain and eastern Pennsylvania

Open Access
Author:
YEH, EN-CHAO
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 30, 2003
Committee Members:
  • Donald Myron Fisher, Committee Chair
  • Derek Elsworth, Committee Member
  • Terry Engelder, Committee Member
  • Chris Marone, Committee Member
  • Derrill Maylon Kerrick, Committee Member
Keywords:
  • Taiwan
  • tectonics
  • thermochronology
  • inclusion
  • kinematics
  • fold
Abstract:
The Taiwan orogen is a consequence of oblique collision between the Luzon island arc and the Asian margin. To characterize the kinematic evolution within the Taiwan orogen, detailed structural mapping, strain analyses, and 40Ar-39Ar dating were conducted along the eastern Central Range, Taiwan. The earliest deformation within the eastern Central Range involves west-vergent folding and development of a slaty cleavage/schistosity with fiber overgrowths and progressive deformation of chlorite-mica aggregates. This deformation was followed by left-lateral shearing and east-vergent backfolding that deformed the earlier fabrics. A regional west-to-east variation exists in extension direction from down-dip to along-strike across Taiwan. The regional extension pattern reflects a triclinic displacement field fixed to the geometric framework across Taiwan. The sinistral shear is indicative of strain partitioning associated with the margin-parallel shearing. Observations of the structure history within the Taiwan orogen are generally consistent with the predictions of doubly-vergent wedge model. By using 40Ar-39Ar dating, the matrix and whole-rock mixing ages of a pre-Tertiary basement sample containing syntectonic biotite fibers within the eastern Central Range is determined as 9.8±3.8Ma and 4.1~3.3Ma, respectively. The whole-rock mixing age of a slate sample from the eastern slate belt is determined as 28~33Ma. Our dating results establish that the development of current fabrics within Taiwan is due to the ongoing collision. The deformation instability due to the mechanical response of rock anisotropy developed in the foreland is the reason for multiple cleavages in the hinterland during ongoing collision. A new method based on mechanical models of inclusion deformation during folding is presented. Given inclusion-shape distributions on the limbs and the fold geometry, this method can differentiate between folding mechanisms and provide an estimate of the strain in folds. The method can also help to constrain the deformation behavior of inclusions and to investigate the rheologic properties of inclusions and surrounding matrix. Numerical simulation of porphyroclast deformation by forward and inverse modeling was completed on an anticline from Slatedale, Pennsylvania. Results show that chlorite-mica aggregates deformed by a single-slip system during flexural-slip folding reproduce better observed data than other models. The shortening of this anticline is estimated about 60%.