Luminescence dating in northern Chile: Implications for the Quaternary tectonic and geomorphologic evolution of the Mejillones Peninsula

Abstract

The geomorphology of the Mejillones Peninsula, northern Chile, evidences a Quaternary tectonic uplifting process. Upper plate faults located close to the peninsula show recent activity. However, there is no consensus about the uplift rate of the Mejillones Peninsula in the millennial timescale and numeric ages of uplifted sediments or sediments related to fault activity are scarce. These geochronological data are crucial to establish the geologic history of the upper plate faults and improve the understanding on plate tectonics processes in Chile. The main aim of this Ph.D. thesis is to contribute to a better knowledge of the subduction process and upper plate deformation relationship through the quantification of the coastal uplift and the upper plate fault activity in the Mejillones Peninsula for the Late Quaternary applying luminescence dating techniques to quartz and potassium feldspar. To achieve this, 31 sediment samples were collected from four localities: marine- coastal sediments from the Pampa Mejillones, an alluvial deposit associated to the Mejillones Fault, alluvial and eolian deposits associated to the Naguayán Fault and a colluvial deposit associated to the Salar del Carmen Fault. Topographic profiles from the Mejillones Peninsula and the studied faults were obtained by means of a differential GPS. Trenches were excavated in the fault traces for paleoseismic analysis and their fault scarps were modelled with high-resolution 3D techniques. Optically stimulated luminescence (OSL) signals from quartz and post-infrared infrared stimulated luminescence (pIRIR) signals from K-feldspar were analyzed to determine sediment burial ages. From the comparison between OSL and pIRIR ages, it was concluded that the OSL ages from quartz are largely underestimated. This is attributed to the fact that the quartz OSL signals lack a strong fast OSL component and that predominant medium and slow components show thermal instability. On the other hand, pIRIR signals are close to stability, showing low, though variable, fading rates (0.7-6.77 %/decade), and bleachable, presenting low residual doses in modern analogues (between 2 and 6 Gy). The alluvial sediments spatially associated with the Mejillones Fault yielded fading-corrected pIRIR ages between 87.4 ± 6.6 and 163.4 ± 18.4 ka. For the sediments associated with the Naguayán fault, the fading-corrected pIRIR ages were between 10.4 ± 1.3 and 44.1 ± 4.7 ka. The fading-corrected pIRIR ages of the colluvial sediments associated with the Salar del Carmen Fault resulted between 14.7 ± 1.0 and 131.6 ± 74.2 ka. For the coastal sediments from the Mejillones Pampa, fading-corrected pIRIR ages were from 70.6 ± 5.1 to > 330 ka. From the paleosismological study of the Naguayán and Salar del Carmen Faults, it is concluded that the activity of these faults has been continued during the Late Pleistocene and Holocene. The activity rates of both faults classify them as slow faults, with slip rates of ~0.06 m/ka and earthquakes recurrences of 10-20 ka. Based on stratigraphic references such as colluvial wedges, it was possible to estimate coseismic displacements of up to 2 m, translating into magnitudes of paleoearthquakes of up to Mw7.2. In the specific case of the Naguayán Fault, at least four events could be identified, two of which occurred less than 40 ka ago. According to the pIRIR ages obtained for the studied sediments, Pampa Mejillones has uplifted at rates ranging from 0.25 to 0.5 m/ka between 400 and 100 ka. For the last 70 ka, the pIRIR age obtained in a coastal marine deposit suggests an uplift acceleration to 1.01 m/ka. The activity of the Mejillones Fault, estimated in 0.26 m/ka, partially controls the disposition, formation and preservation of the paleocoastlines, in combination with the variation of relative sea level. Based on these data, an evolution model of the Mejillones Peninsula during the Late Pleistocene is proposed according to which the coastal deposits found in the Pampa Mejillones record coastline progradation and retrogradation as a consequence of the combination of tectonic uplift, eustatic sea level variation and upper plate fault activity. An excess in the uplift rate over the subsidence produced by the activity of the Mejillones Fault would elevate the Pampa Mejillones surface, generating the current coastal cliff. Therefore, coastal uplifting process and the upper plate fault activity, especially the potential reactivation of Mejillones and Naguayán faults, must be considered as a potential seismic hazard to the urban and productive infrastructure of Mejillones and industrial complexes located in the Coastal Cordillera.