Pleistocene alluvial deposits dating along frontal thrust of Changhua Fault in western Taiwan: The cosmic ray exposure point of view

To tackle the history of active thrusts, it is necessary to open the observation window on time scales on the order of 10 ⁴-10 ⁵ years by studying the surface morphologies resulting from their activities. Because fluvial systems are particularly sensitive to recent environmental changes, geomorphic features such as alluvial terraces are frequently used as markers to gauge tectonic deformation. Together with the measurement of cumulative displacements, the chronological framework of emplacement and abandonment of these geomorphic markers is thus fundamental to determine long-term fault slip-rates. In Taiwan, the geomorphic features associated with fault activity have been studied in detail with a high level of resolution; however, the use of deformed and partially preserved alluvial terraces is often hampered by the absence of well-documented ages. The purpose of this paper is two-fold. First, we take the opportunity to review the chronological constraints that have been published in Taiwan so far. Second, we present how the cosmogenic dating method (in situ-produced ¹⁰Be) can be used to constraint the chronological framework of alluvial deposits over a Pleistocene time scale. Thanks to a comparison of our cosmogenic-derived ages with existing data, we present a consistent regional chronological framework for the Pakua-Tadu area along the Changhua Fault, one of the most active frontal thrusts in the Western Foothills of the Taiwan mountain belt. We also discuss its relationships with global eustatism and its tectonic implications for the timing of propagation of the deformation front during the last 450kyr.