Abstract

Landforms on the Pacific slope of the Cordillera Occidental (Western Cordillera) in southernmost Peru reflect the episodic epeirogenic uplift of the Central Andes since the earliest Miocene. In the geomorphic history, three major stages are distinguished. Initial uplift followed a mid-Tertiary period of tectonic and magmatic quiescence in which a low-lying, subdued landscape formed through dissection of an Eocene subaerial volcano-plutonic terrain. Uplift and resulting erosional beveling of this landscape, coinciding in their later stages with the explosive reactivation of the magmatic arc, gave rise to the regionally extensive, lower Miocene Altos de Camilaca surface (I), which was mantled soon after its formation by ignimbrites of the Huaylillas Formation. Major relief was achieved in the middle and early-Late Miocene, when a succession of uplift episodes generated a sequence of four discrete pediments, constituting the Multiple Pediment Stage (II). Uplift was accompanied by widespread eruption of ignimbrites of the Chuntacala Formation. Subsequently, in the Pliocene, pedimentation was succeeded as the major erosional process by canyon incision across the lower slopes of the cordillera. During this Valley and Terrace Stage (III), the immediate littoral margin experienced the development of two major terraces, contemporaneous with a transition from ignimbrite eruption to predominantly andesitic volcanism in the High Cordillera in the early Pleistocene. During this time, it is probable that epeirogenic uplift, rather than glacio-eustatic sea-level changes, exerted the dominant control on landform development.

In the absence of evidence for large-scale crustal compression in this immediate region since the Cretaceous, it is inferred that Neogene uplift resulted from episodic thickening of the continental crust through “underplating” by basic and/or intermediate magmas. The Neogene chronology of landform development herein defined is remarkably similar to that outlined in other transects of the Central Andes, implying that the continental margin has, on a broad scale, responded uniformly to plate subduction since the early Miocene. However, the low relief evident in the mid-Tertiary in the study area, and not, for instance, in northern Chile (27°S), suggests that crustal thickening through the Neogene was more rapid, and more extensive, in the vicinity of the Arica deflection than in transects to the north and south. The estimated Neogene uplift rate for the transect was in the range 0.06 to 0.19 mm/yr.

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