Using ignimbrites to quantify structural relief growth and understand deformation processes: implications for the development of the Western Andean Slope, northernmost Chile

摘要

Large-volume ignimbrites are excellent spatial and temporal markers for local deformation and structural relief growth because they completely inundate and bury the underlying paleotopography and leave planar surfaces with relatively uniform, low-gradient slopes dipping less than 2°. Using one of these planar surfaces as a reference frame, we employed a line-balanced technique to reconstruct the original morphology of an ignimbrite that has undergone postemplacement deformation. This method allowed us to constrain both the amount of posteruptive deformation and the topography of the pre-eruptive paleolandscape. Our test case was the unwelded surface of the 21.9 Ma Cardones ignimbrite, located on the western slope of the Central Andes in northernmost Chile (18°20′S). By reconstructing the original surface slope of this ignimbrite, we demonstrate that the pre–21.9 Ma topography of the Western Andean Slope was characterized by structural relief growth and erosion in the east, and the creation of accommodation space and sedimentation in the west. The paleoslope at that time was dissected by river valleys of up to 450 ± 150 m deep that accumulated great thicknesses (>1000 m) of the Cardones ignimbrite, and likely controlled the location of the present-day Lluta Quebrada as a result of differential welding compaction of the ignimbrite. Our reconstruction suggests that growth of the Western Andean Slope had already started by ca. 23 Ma, consistent with slow and steady models for uplift of the Central Andes. Subsequent deformation in the Miocene generated up to 1725 ± 165 m of structural relief, of which more than 90% can be attributed to fault-related folding of the ∼40-km-wide Huaylillas anticline. Uplift related to regional forearc tilting is less than 10% and could have been zero. The main phase of folding likely occurred in the mid- to late Miocene and had ceased by ca. 6 Ma.