Role of erosion and isostasy in the Cordillera Blanca uplift: Insights from landscape evolution modeling (northern Peru, Andes)
Introduction
In mountain ranges, surface uplift is usually assumed to be the result of shortening and crustal thickening. Surprisingly, in northern Peru, uplift of the footwall of an active normal fault is responsible for the formation of the highest Peruvian summits in the Cordillera Blanca (Fig. 1). Several models have been proposed (Dalmayrac and Molnar, 1981; McNulty and Farber, 2002) to explain this unusual situation, but the processes driving both the Cordillera Blanca uplift and extensional deformation along the Cordillera Blanca normal fault (CBNF) remain poorly constrained. The CBNF trends parallel to the Andean range and is the most spectacular normal fault in the Andes (Fig. 1; Margirier et al., 2017): the CBNF is ~200 km long and shows ~7 km of vertical offset in total (Margirier et al., 2016), it has been active since ~5.4 Ma (Bonnot, 1984; Giovanni, 2007). The CBNF is located above the Peruvian flat-slab (Barazangi and Isacks, 1976), a section of the convergent plate boundary between the Nazca Plate and the South American Plate characterized today by near-horizontal subduction geometry. The Cordillera Blanca and the Cordillera Negra form, respectively, the hanging wall and the footwall of the CBNF (Fig. 1).
The Cordillera Blanca fast exhumation rate (~1 mm/yr) has been previously linked to motion on the CBNF (e.g., Bonnot, 1984; McNulty and Farber, 2002; Giovanni, 2007; Margirier et al., 2015). New thermobarometry data and erosion rates reconstruction based on thermochronological data indicate a recent, i.e. Early Pleistocene, increase in erosion rate in the Cordillera Blanca (~2–0 Ma; Margirier et al., 2016). Margirier et al. (2016) suggested that an important isostatic contribution from glacial erosion may explain the recent exhumation of the Cordillera Blanca batholith. Indeed, the removal of such a mass of material represents a significant upward unloading on the lithosphere, which should drive substantial flexural uplift. This unloading and flexural uplift would have also generated large differential stresses in the lithosphere, which could have caused the reactivation of pre-existing structures such as the CBNF. Previous studies demonstrated that the flexural uplift driven by alpine-type valley incision could reach rates similar to those caused by tectonic processes (Montgomery, 1994; Small and Anderson, 1995; Cederbom et al., 2004; Stern et al., 2005). Recently Braun et al. (2014) proposed that erosion-driven isostatic rebound should scale with the density of surface rocks: denser rocks, such as a granitic body intruded in sedimentary rocks, rebound and therefore are exhumed faster than the surrounding less dense rocks.
The rapid uplift of the Cordillera Blanca, the large volume of eroded rocks since the emplacement of the Cordillera Blanca batholith and its location in the footwall of an active normal fault in a compressive plate boundary, make the Cordillera Blanca the perfect place to question the nature and efficiency of potential feedbacks between erosion and uplift along the CBNF. The aim of this paper is thus (i) to test whether the increase of erosion rate suggested for the last 2 Ma in the Cordillera Blanca (Margirier et al., 2016) could be due to an increase of rock uplift rates since 2 Ma rather than a change of climate and/or erosion process and (ii) to quantify the importance of isostatic rebound associated with valley incision and erosion of denser rocks to explain the uplift of the Cordillera Blanca and to test the adequacy of a flexure-driven model in such a setting. To address this issue, we have attempted to model landscape evolution in the Cordillera Blanca using a landscape evolution model, in this case based on the FastScape algorithm (Braun and Willett, 2013).
Section snippets
Geologic and climatic context
The Cordillera Blanca hosts the highest Peruvian summits with a cluster of 6000 m peaks (Fig. 1). It hosts a large 14–5 Ma granitic pluton (zircon U-Pb; Mukasa, 1984; Giovanni, 2007) emplaced at ~3 km depth into deformed Jurassic sediments (Margirier et al., 2016). The Cordillera batholith is elongated (150 × 15 km) and trends parallel to the Andean range (Fig. 1A). Based on apatite fission-tracks and (U-Th/He) dating several studies gave estimations of exhumation rates ranging between 1 and
Model
We used the FastScape algorithm (Braun and Willett, 2013) to solve the stream power law to predict landscape evolution following a set of tectonic forcing (uplift) and initial topography (geomorphic setting). Because of the optimum ordering of the nodes, the algorithm is implicit in time and computationally very efficient, requiring only O(n) operations where n is the number of points used to discretize the topography. Consequently, FastScape can be used repetitively, even if using a very high
Role of tectonics, erosion and initial topography
Here we aim to test (i) if the increase of erosion rate suggested for the last 2 Ma in the Cordillera Blanca (Margirier et al., 2016) could be due to an increase in uplift rates since 2 Ma rather than a change of climate and/or erosion process (glacial erosion vs fluvial erosion) and (ii) the role of initial topography in the present day Cordillera Blanca drainage divide location. In these inversions, the four parameters that we wanted to constrain by inversion of the thermochronological ages,
Paleogeography
The substantial surface uplift and resulting erosion in the Cordillera Blanca make it difficult to study the paleogeography in this area based on remnants geomorphological features. However, the drainage network geometry and the drainage divide location provide information on the topography of the Cordillera Blanca in the past and its evolution. Notably, the Cordillera Blanca drainage divide is located in the eastern part of the range whereas both the higher peaks and higher uplift rates are
Conclusions
Our study provides new constraints on the erosion efficiency, elastic thickness of the lithosphere, temperature gradient in the crust and uplift rates in the Andes of northern Peru. The absolute rock uplift rates obtained at the end of the models for the Cordillera Blanca (ranging from 1.5 to 2.5 mm/yr) are coherent with Quaternary slip rates documented on the CBNF (5.1 ± 0.8 mm/yr to 0.6 ± 0.2 mm/yr, Schwartz, 1988; Siame et al., 2006; Margirier et al., 2017; Gérard et al., n.d.). Our results
Acknowledgements
We thank Emily Richards for proofreading the manuscript and English improvements, Jessica Stanley and Benoît Bovy for their help with the inversions on the cluster. We acknowledge the work of the editor and of our two anonymous reviewers for their critical and helpful reviews. Last but not least, Audrey Margirier acknowledges Blockzone team for the nice boulders in Potsdam.
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