Late Pleistocene thrust tectonics in the north Peruvian forearc revealed by Terrestrial Cosmogenic Nuclides surface exposure dating, field structural data and seismic profiles

Abstract

Precise dating of geomorphic markers that are subject to thrust-related deformation, such as alluvial fan surface are crucial for quantifying forearc deformation, especially in a context of weak interseismic coupling on the subduction interface as in northern Peru. In this study, we document the late Pleistocene tectonic history of the Amotape massif, which is a prominent basement relief in the northern Peruvian forearc. To achieve this, we combine a morphometric analysis and Terrestrial Cosmogenic Nuclides (TCNs) surface exposure dating of alluvial fan surface with subsurface structural data. TCNs (¹⁰Be and ²⁶Al) surface exposure ages of alluvial fan surfaces on the northwestern flank of the massif range from 35.1 ± 1.1 ka to 59.3 ± 2.1 ka for ¹⁰Be and from 27.3 ± 2.0 ka to 48.9 ± 2.5 ka for ²⁶Al. Both cosmogenic nuclides provide consistent results. We calculated a mean weighted age of 39.0 ± 0.3 ka for an alluvial fan that is offset by a major NW-verging thrust. Our results provide an estimated slip rate of 1.15 ± 0.46 m/ka along the Amotape thrust front since the late Pleistocene. The surface deformation along the Amotape thrust front correlates with the top portion of a deep-seated contractional wedge involving the crustal basement, according to subsurface structural data and regional seismicity. This integrated analysis demonstrates active shortening in the onshore crustal basement of the northern Peruvian forearc where no active thrust reaching the surface has been described before.