Elsevier

Tectonophysics

Volumes 728–729, 20 March 2018, Pages 119-129
Tectonophysics

Role of erosion and isostasy in the Cordillera Blanca uplift: Insights from landscape evolution modeling (northern Peru, Andes)

https://doi.org/10.1016/j.tecto.2018.02.009Get rights and content

Highlights

  • Inversion of the landscape evolution coupled with thermochronological data provides constraints on uplift rates in Peruvian Andes

  • Isostatic effect of eroding a denser rock mass represent a not negligible contribution to the Cordillera Blanca uplift

  • Cordillera Blanca drainage divide location controlled by paleogeography rather by uplift rates and precipitation distribution

Abstract

The processes driving uplift and exhumation of the highest Peruvian peaks (the Cordillera Blanca) are not well understood. Uplift and exhumation seem closely linked to the formation and movement on the Cordillera Blanca normal fault (CBNF) that delimits and shapes the western flank of the Cordillera Blanca. Several models have been proposed to explain the presence of this major normal fault in a compressional setting, but the CBNF and the Cordillera Blanca recent rapid uplift remain enigmatic. Whereas the Cordillera Blanca morphology demonstrates important erosion and thus a significant mass of rocks removal, the impact of erosion and isostasy on the evolution of the Cordillera Blanca uplift rates has never been explored. We address the role of erosion and associated flexural rebound in the uplift and exhumation of the Cordillera Blanca with numerical modeling of landscape evolution. We perform inversions of the broad features of the present-day topography, total exhumation and thermochronological data using a landscape evolution model (FastScape) to provide constraints on the erosion efficiency factor, the uplift rate and the temperature gradient. Our results evidence the not negligible contribution of erosion and associated flexural rebound to the uplift of the Cordillera Blanca and allow us to question the models previously proposed for the formation of the CBNF.

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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