Uplift rate of the Mejillones Peninsula (Northern Chile) constrained by luminescence dating of coastal and alluvial deposits

The Mejillones Peninsula, in Northern Chile, shows a strong tectonic control on the topography. The geomorphology shows a well-preserved evidence of tectonic uplift and normal faulting. The uplift record is comprised by more of 300 beach ridges spanning over 30 km, limited by an 8 m high coastal cliff. These deposits are partially covered by a 30 m thick alluvial deposit derived from an active mountain front formed by the Mejillones normal fault. Cumulative vertical offset in the fault has built a 300 m fault scarp during the last 3.5 Ma. Uplift rate of the Mejillones Peninsula has been determined by cosmogenic nuclide dating of marine abrasion platform preserved in the uplifted block of the Mejillones Fault (Binnie et al. 2016). However, numerical dating of the beach ridges has not been performed; therefore, uplift rate estimation of the hanging wall is still missing. The absence of this uplift rate constrain put a strong limitation to understand the interplay role of fault slip and coastal uplift/subsidence in the landscape configuration of this peninsula. In this study we present the first systematic attempt of dating the beach ridge deposits. Optically Stimulated Luminescence (OSL) was selected as dating methodology. Ten samples from the coastal deposits were analyzed: six from beach ridge deposits and four from the current coastal cliff. The post-IR IRSL290 protocol on K-feldspar (Buylaert et al. 2012) was applied due to the low sensibility of the quartz available. Additionally, six samples extracted from the alluvial deposits were dated using OSL. First results from the post-IR IRSL signal have yielded equivalent doses spanning from 200 to 500 Gy, meaning ages from 71 ka to 406 ka. The obtained ages are in agreement with the position of the Marine Isotope Stages 3, 5 and 7. The samples from the beach ridge deposits yielded an average uplift rate of 0.769 ± 0.227 m/ka while the samples from the coastal cliff deposits yielded an average uplift rate of 0.146 ± 0.099 m/ka. These results suggest an increase in the uplift rate of the Mejillones Pampa during the last 250 ka.