Morphologic evolution of the Central Andes of Peru

Abstract

In this paper, we analyze the morphology of the Andes of Peru and its evolution based on the geometry of river channels, their bedrock profiles, stream gradient indices and the relation between thrust faults and morphology. The rivers of the Pacific Basin incised Mesozoic sediments of the Marañon thrust belt, Cenozoic volcanics and the granitic rocks of the Coastal Batholith. They are mainly bedrock channels with convex upward shapes and show signs of active ongoing incision. The changes in lithology do not correlate with breaks in slope of the channels (or knick points) such that the high gradient indices (K) with values between 2,000-3,000 and higher than 3,000 suggest that incision is controlled by tectonic activity. Our analysis reveals that many of the ranges of the Western Cordillera were uplifted to the actual elevations where peaks reach to 6,000 m above sea level by thrusting along steeply dipping faults. We correlate this uplift with the Quechua Phase of Neogene age documented for the Subandean thrust belt. The rivers of the Amazonas Basin have steep slopes and high gradient indices of 2,000-3,000 and locally more than 3,000 in those segments where the rivers flow over the crystalline basement of the Eastern Cordillera affected by vertical faulting. Gradient indices decrease to 1,000-2,000 within the east-vergent thrust belt of the Subandean Zone. Here a correlation between breaks in river channel slopes and location of thrust faults can be established, suggesting that the young, Quechua Phase thrust faults of the Subandean thrust belt, which involve Neogene sediments, influenced the channel geometry. In the eastern lowlands, these rivers become meandering and flow parallel to anticlines that formed in the hanging wall of Quechua Phase thrust faults, suggesting that the river courses were actively displaced outward into the foreland.

Mesozoic–Cenozoic Evolution of the Western Margin of South America: Case Study of the Peruvian Andes

“…But later, the Cordillera Blanca Batholith was uplifted more or less along the same fault relative to the highly deformed Mesozoic strata to the west. The fault and evidence for its motion can be observed at outcrop (see Figure 13 in [7]); displaced Quaternary gravel terraces testify to a very young activity along this fault. The net displacement along the fault as shown in Figure 15 amounts to 3 km.…”

“…Cooling ages of this batholith range between 16 and 9 Ma in the tonalites and diorites, and between 6 and 2.7 Ma in the leucogranodiorite [70,71]. This intrusion now forms peaks reaching more than 6000 m. Steep faults at the western margin of the batholith (see [7], their Figure 13) raised the intrusive rocks next to the Cenozoic volcanics of the Cordillera Negra and are, combined with the low erodibility of the intrusive rocks responsible for the impressive local relief in the area.…”

“…Part of this uplift occurred in the context of the intrusion of the Cordillera Blanca Batholith. As discussed in [7], the Cordillera Blanca detachment fault (CBDF in Figure 19B), which manifests itself by a WSW dipping 1 km thick mylonite belt [94], is suspected to be a syn-intrusion normal fault that accommodated the rise of the central part of the batholith, comparable to structures such as known e.g. in the Alps ("Pedra Rossa shear zone" of [95] in the roof of the Bergell pluton).…”

“…The area of central Peru between the Western and Eastern Cordillera is characterized by relatively low local relief and elevations not exceeding 4000 m [7]. This high plateau, the Central Highlands, links to the Altiplano of Bolivia in the SE.…”

“…Active incision of these rivers explains they flow on bare rock [7]. In the core of the Western Cordillera, Jurassic and Cretaceous strata [80] are folded and faulted [17,18,26] and form a narrow thrust belt, the so-called Marañon fold-and-thrust belt.…”