Study of Magnetic Fabrics and Paleomagnetism Across Northern Transect of Taiwan Mountain Belt and Thier Implications
Abstract:Anisotropy of magnetic susceptibility (AMS) can be regarded as a useful tool for understanding the variation of finite strain pattern for regional deformation. In order to evaluate the interrelationship between maximum metamorphic temperature and deformation during mountain building, oriented samples of low-grade metamorphic rocks across the northern Taiwan were collected. In addition to the study of magnetic susceptibility anisotropy, experiments of natural remanent magnetization, hysteresis loop, and temperature-function magnetic susceptibility were also conducted.
Results show that K1 orientation of magnetic ellipsoids suggested NW-SE compression, which is consistent with plate convergence direction. Both deformation intensity and anisotropy increase from the west to east with abnormally strong intensity and oblate strain near major faults. Magnetic fabrics have grouped six-stage from Type I to VI upon increasing strain. Study area can be divided into four domain A to D by geological characteristics and distribution of magnetic fabric. Rocks in Domain A begin to be influenced by horizontal tectonic strain. Ellipsoid is oblate and K1 is in northeast-southwest orientation, indicating NW-SW compression. Magnetic fabric belongs to Type II. K3 orientation in Domain B started to be affected by cleavage. The shape of ellipsoid is mainly oblate. Magnetic fabric is classified as Type II-III. The shape of ellipsoid in Domain C gradually converts to prolate. Distribution of K3, influenced by cleavage development, becomes a girdle in NW-SE orientation. It is treated as Type III-IV. In Domain D, though both anisotropy and deformation intensity are increased, the direction of K3 is still concentrated in vertical, not in horizontal. Results might be the reflective of discontinuous strain response to different kinematic mechanisms between the Backbone Range and the Hsueshan Range.
Thermopaleogeomagnetic records of pyrrhotite remanence on both limbs of the Chungling Anticline failed the fold test, indicating the post-fold exhumation. It means while the metamorphic temperature was cooled down below the Curie temperature 320℃ of pyrrhotite, there was no remarkable folding deformation anymore. Later the Hsueshan Range was uplifted passively and eroded.