Incremental strain history of the Taiwan orogenic belt, measured from syntectonic pressure shadows

Incremental strain histories from syntectonic fibers in pyrite pressure shadows in the eastern Central Range of Taiwan indicate a progressive change from down dip to along strike extension during deformation. One accepted model of Taiwan explains the orogen as an arc-continent collision where continental margin material enters a doubly-vergent wedge on the west side, or prowedge, and advects from west to east through the wedge along hyperbolic paths, so the longest residence time and deepest burial is recorded by rocks east of the divide. Previously observed changes in cleavage plane orientation centered on the divide of the Central Range support the hypothesis that the orogen is doubly-vergent, with opposing stress regimes on either side of the wedge. Samples were collected from the slates of the Eastern Central Range to evaluate the variation in stretching direction through time in samples that record the deepest advection paths through the wedge. In general, incremental strain histories quantified from curved pressure shadows in cleavage-parallel samples within the Eocene slates show a change in incremental strain orientation as strain accumulates. Passively deformed, antitaxial fibers around pyrite grains were subdivided into short linear segments, where the relative orientation of each records the orientation of the stretching at the time of formation and the combined record of all stretch increments within a pressure shadow yields a chronological history of stretch variations. The eastern Taiwan samples record a close to 90 degree shift in stretching direction during deformation. The cumulative incremental strain history starts with early elongation oriented at a high angle to horizontal then gradually rotates (counter-clockwise when looking west) to late elongation oriented near-parallel to horizontal. This trend from down-dip to along-strike extension is seen regionally along the eastern edge of the Central Range and is consistent with the variation in the stretching lineation from downdip to along-strike from west to east across Taiwan. The changing extension direction seen in the strain history shows that the rocks were advected laterally through a displacement field with systematic variations in kinematics relative to the largely fixed geometry of the orogen. A change from downdip to along-strike extension could indicate strain partitioning within a critical wedge model related to the modest (~10 degree) amount of oblique convergence.