Along-strike variations of morphotectonic features in the Western Foothills of Taiwan: tectonic implications based on stream-gradient and hypsometric analysis

Abstract

The Taiwan mountain belt provides a key example for analyzing the relation between morphology and tectonics in an active compressional environment. There is a striking geometrical relationship between the location of salients and reentrants of the mountain belt and the presence of major crustal inhomogeneities such as structural highs in the foreland basin. Major along-strike variations of structural styles occur in range-front rock units of the fold-thrust belt. Stream-gradient and hypsometric analysis has been carried out to highlight the along-strike variations of morphotectonic features and to illustrate the relative activities in different tectonic regimes of the Western Foothills. The normalized stream length-gradient index (SL index) and the Hack profile are two of the stream-gradient indices that effectively reflect the tectonic uplift in a region. De-trending and residual analysis of the hypsometric integral (HI) eliminates the spatial dependency of small drainage basins, and the residual hypsometric integral corresponds to differential uplift activity. Integration of stream-gradient indices and hypsometry provides more robust tectonic interpretation. Five morphotectonic provinces have been identified in the Western Foothills based on stream-gradient indices and hypsometry. Each province is bounded by a series of N140°E-trending transfer fault zones, either inherited from the Eurasian passive margin and/or newly formed in the sedimentary cover in response to the presence of basement highs within the foreland basin (the Peikang and Kuanyin highs). The morphotectonic features as well as deformation styles vary within each province. A high uplift but low shortening rate typifies the crustal deformation style in the transpressional regime north of the Peikang Basement High. On the contrary, a low uplift but high shortening rate characterizes the crustal deformation style in the transtensional regime south of the Peikang Basement High. The higher geomorphic indices suggest that greater tectonic activity occurs in central than in southwestern Taiwan. This inference is also supported by the coseismic deformation of the 1999 Chi-Chi earthquake.