Basin and Paleoclimate Evolution of the Pampa del Tamarugal Forearc Valley, Atacama Desert, Northern Chile
This work details the Neogene to Recent evolution of the Pampa del Tamarugal (20?30? - 21?45?S), a northward-trending forearc basin located between the Coastal Cordillera to the west and the Precordillera to the east in the hyper-arid core of the Atacama Desert of northern Chile. Tectonic, sedimentary, and paleoclimatic implications, and their relationships to one another, are considered. Deformation structures within the Upper Oligocene and Miocene lithologies reveal a complex and long lived tectonic history. Fault mapping reveals north-, and northwest-trending reverse faults, east-trending reverse faults, as well as north-trending normal faults ubiquitous within the basin during the Lower and Middle Miocene, and concentrated during the Upper Miocene in the southwestern sector of the study area. Surface stratigraphy and seismic facies analysis indicate a short Middle Miocene humid interval, resulting in lacustrine and playa deposits over extensive regions. Increased aridity after ~15 Ma is here related to a global desiccation and after ~11 Ma is tied to orographic blocking of precipitation. Long-wavelength rotation of surfaces at the eastern limit of the basin accompanied uplift of the Altiplano plateau to its east. Conservative estimates place the magnitude of uplift at 1155 ? 475 m between 11 and the present, with highest rates between 11-5.3 Ma. This rotation is not accompanied by any significant surface-breaking faults over this same distance. Our preferred mechanism of uplift is lower or middle crustal ductile flow related to the underthrusting of the Brazilian Shield beneath the Altiplano from the east, although delamination of dense lower crust is also considered. Despite long-lasting hyperaridity during Pleistocene, there was perennial stream flow within the hyperarid core of the desert, which does not exist under modern climate conditions. Nineteen radiocarbon dates from deposits on terraced fluvial deposits indicate ages from 16,380 to 13,740 cal yr BP, synchronous with other regional evidence for wetter conditions. The stream flow responsible for these deposits likely represents the most important groundwater recharge event of the last 18,000 years. Sea surface temperature gradient changes in the equatorial Pacific was perhaps the major driver of hydrologic change in the Atacama on both centennial and millennial timescales.
paleoclimate; hyper-aridity; seismic stratigraphy; Atacama Desert; tectonics; Andes mountains
dissertation or thesis