Uplift and Deformation of the Western Andean Slope, Northernmost Chile: Insights from Deformed Early Miocene Ignimbrites

Abstract:

In northern Chile and southern Peru, deformed early Miocene ignimbrites characterize the Western Andean Slope (WAS). In this study we use the emplacement and subsequent deformation of these large-volume ignimbrites to reconstruct the pre-eruptive paleotopography and constrain Miocene uplift rates across the WAS in northernmost Chile. Our results provide evidence that the uplift of the WAS began as early as Eocene-Oligocene time, with elevated rates of uplift during the middle-late Miocene.

In northernmost Chile, the WAS is covered by the early Miocene Oxaya Formation (22.7 Ma – 19.6 Ma), a series of ignimbrites and minor sediments that includes the large-volume (>3000 km³), 21.9 Ma Cardones ignimbrite. Following eruption, these ignimbrites were faulted, tilted, and folded into the long-wavelength Huaylillas anticline, which forms a steep slope rising ~2300 m over a distance of ~40 km.

Based on the assumption that large-volume ignimbrites like the Cardones infill existing topography to create a relatively uniform low-gradient surface slope of ~1°, we performed a line-balanced reconstruction of this surface. Results suggest the existence of a proto-WAS, with an elevation up to ~1800 m, prior to 21.9 Ma. This proto-WAS had local paleo-lows, in which the Cardones ignimbrite was able to reach thicknesses exceeding 1000 m. We interpret these lows as river valleys that transported sediment from the proto-WAS westwards towards the Central Basin, creating an Eocene-Oligocene (~50-22.7 Ma) clastic wedge, expressed today as the Azapa Formation.

After deposition of the Oxaya Formation, the WAS experienced <1° of westward tilting, faulting, and folding resulting in ~2000 m of uplift along the hinge of the Huaylillas anticline. U-Pb geochronology of zircons from intercalated volcanic units in the syn-tectonic Huaylas Basin on the eastern limb of the anticline suggest that the majority of this Miocene uplift occurred between 16 and 6 Ma.