Expanding the Late Quaternary morphochronology of Atacama's coastal alluvial fans by Schmidt hammer exposure dating reveals spatially distinct genesis

Late Quaternary alluvial fans along the coast of the hyperarid Atacama Desert have been exploited as palaeoenvironmental archives by several studies. However, a regional palaeoenvironmental interpretation remains challenging as established geochronological frameworks are based on a limited number of sites where fan aggradation was constrained by numerical dating. Consequently, it is unclear to what extent fan aggradation and progradation is controlled by large-scale allogenic versus individual autogenic forcing. To therefore expand the regional alluvial fan morphochronology along the south-central coast, the applicability of cost-effective Schmidt hammer exposure dating (SHD) was explored on boulders of plutonic lithology. The independently exposure-dated coastal alluvial fan (CAF) Paposo served as regional age-calibration site for SHD of 11 telescopic CAFs between 24.25°S and 25.25°S. SHD was complemented by the assessment of the CAFs’ terrestrial and shelf morphometry and the hydromorphometric and climatic catchment properties. The results of the successfully calibrated SHD suggest that weathering in the coastal desert allows for age ranges extending over the Late Quaternary. The expanded morphochronology reveals a distinct fan evolution segmenting a synchronous morphogenesis south of 24.85°S from a diachronous one in the north. Spatial segmentation is reflected in specific catchment hydromorphometric characteristics and differences in the relationships between terrestrial fan versus shelf gradients – the latter indicating a dominant allogenic control of eustatic sea-level changes at least in the south. While spatiotemporal variability in Late Quaternary precipitation and tectonic activity may constitute additional relevant factors, diachronism of northern CAF evolution could also be the response to individual autogenic fan dynamics. • Successful age-calibration of Schmidt hammer rebound on multi-stage coastal alluvial fan • Age range of Schmidt hammer exposure dating extending in costal Atacama over Late Quaternary • Expanded morphochronology reveals synchronous (south) versus diachronous (north) fan evolution • South: fan versus shelf gradients indicate dominant allogenic control of eustatic sea-level changes • North: inconclusive palaeoenvironmental interpretation – allogenic and/or autogenic forcing