Elsevier

Geomorphology

Volume 104, Issues 3–4, 15 March 2009, Pages 117-133
Geomorphology

Large rockslides in the Southern Central Andes of Chile (32–34.5°S): Tectonic control and significance for Quaternary landscape evolution

https://doi.org/10.1016/j.geomorph.2008.08.008Get rights and content

Abstract

The distribution and age of large (> 0.1 km2) Pliocene to recent rockslides in the Chilean Cordillera Principal (32–34.5 S), the Southern Central Andes, has been analyzed to determine the rockslide triggering mechanisms and impact on regional landscape evolution. Most of the rockslides appear in the western Cordillera Principal and cluster along major geological structures. Variographic analyses show spatial correlation between rockslides, geological structures and shallow seismicity. A relative chronosequence was calibrated with existing 14C and 40Ar/39Ar dates and new cosmogenic nuclide exposure ages for selected rockslides. Rockslide-induced sediment yield was estimated with empirical relations for rockslide area distributions. Throughout the Quaternary, rockslides have delivered sediment to streams at rates equivalent to denudation rates of 0.10 ±0.06 mm a 1, while estimates using short term (20 a) seismicity records are 0.3 0.2+ 0.6 mm a 1. The estimates of sediment transfer and the spatial distribution of rockslides reflect a landscape in which tectonic and geological controls on denudation are more significant than climate.

Introduction

The Southern Central Andes (SCA) of Chile host some of the highest peaks (> 6000 m) and largest relief (> 2000 m) of the Andes, and have experienced combinations of compressional, extensional, and shear tectonics throughout the Late Cenozoic (Giambiagi et al., 2003, Charrier et al., 2007). Large rockslides (catastrophic bedrock involved landslides > 0.1 km2 in area) appear as prominent morphological features in the region (e.g. Abele, 1984), and exceed 1 km3 of mobilized rock. They influence catchment hydrology and in part control the genesis, transport and delivery of sediment to the lowlands. A thorough analysis of Andean rockslide distribution and triggers has not been conducted despite the prominent role they may play in landscape evolution, sediment transfer, and hazards in the area.

We hypothesize that Quaternary rockslides in the SCA have been triggered by shallow seismicity. To test the hypothesis, we produced a new, field-based inventory of large rockslides for the Chilean SCA high-relief zone between latitudes 32 and 34.5 S, and compared rockslide occurrence patterns to lithology, geological structures, and seismicity records. If the link between rockslide distribution and shallow seismicity can be demonstrated, it will be possible to use patterns of recent rockslides to predict the location of unrecognized (blind or unmapped) active faults. In addition, an inventory of large rockslides compiled over a broad area and long time can provide estimates of sediment generation at geodynamical space and time scales (Malamud et al., 2004a), overcoming the short-term bias present in most landslide inventories used in landscape evolution analyses (e.g. Hovius et al., 1997, Lavé and Burbank, 2004).

A multi-criteria relative chronosequence is developed for the rockslides and calibrated with previous and new geochronological data. This temporal framework, coupled with statistical analysis of rockslide areas and volumes, permits the estimation of long-term (105–106 a) and short-term (decadal) rockslide derived sediment fluxes. The estimated sediment yields help to quantify the role of rockslides in this region relative to other active orogens (e.g., Burbank et al., 1996, Burbank et al., 2003).

Section snippets

Late Cenozoic geologic setting

The SCA have formed above a transition between flat slab and steep subduction angle of the Nazca Plate (Fig. 1). Between 27 S and 33°S subduction is sub-horizontal and modern volcanism is absent, while between 33°S and 46°S the subduction angle reaches 30° (Cahill and Isacks, 1992). Orogen-scale morphostructural units in the Chilean SCA are, west to east: Coastal Cordillera, Central Depression, and Cordillera Principal (CP, Fig. 1). The Western CP (WCP, Fig. 2) comprises Oligocene–Miocene

Mass wasting in the SCA

Mass wasting is ubiquitous throughout the Andes (Blodgett et al., 1998, Schuster et al., 2002) but only a few areas have been studied systematically in terms of slide age, triggering mechanism, and link to tectonics or climate (e.g. Puna, NW Argentina, Hermanns et al., 2000, Hermanns et al., 2001, Hermanns et al., 2006). In Chile, attention has focused on high frequency small-volume historical landsliding close to transportation and energy corridors and urban areas (Hauser, 2000, Naranjo and

Methodology

Topographic maps at scales of 1:25,000 and 1:50,000 and DEMs at 30 and 90 m resolution provided the base for the inventory. Remote mapping following methodologies of Hovius et al. (1997), Korup et al. (2004) and Korup, 2005, Korup, 2006 was performed using aerial photographs at scales 1:30,000, 1:60,000, and 1:70,000, and ASTER and Landsat images. For inventory purposes, a 0.1 km2 area covered by the rockslide deposits was our minimum cut-off (Abele, 1974), which approximately corresponds to

Results

Summarized and complete versions of the rockslide inventory are provided in Table 2 and the Appendix, respectively. Within the ∼ 15,000 km2 area studied, 378 rockslides were identified (Fig. 3). Rockslides that are relevant in terms of size, age or other features are presented in Table 3. The SDF contains detailed descriptions of areas where specific spatial relationships between rockslide location and local geological and topographical features are present.

Most rockslides are located in the CP

Seismic versus climatic factors as trigger for large rockslides in the SCA

While it is likely that some large rockslides and the majority of debris flows correspond to weather-induced failures, the distribution of the majority of rockslides do not follow any known climate patterns, past or present. The lack of an increasing north–south trend in rockslide density does not support a link with the strong southward increase in precipitation in the WCP throughout the late Cenozoic (Lamy et al., 1999, Zech et al., 2006; Fig. 6c). Nor is any west–east trend evident in the

Conclusions

We propose and provide the first test of a hypothesis that links rockslides with Late Cenozoic structures and present-day shallow seismicity (< 20 km deep), suggesting a causative relationship between active tectonics in the western Cordillera Principal and large rockslides. Based on an inventory of large Late-Pliocene to Holocene rockslides, we propose that shallow seismicity is the primary trigger for large rockslides in the southern Central Andes of Chile. Using two methods to estimate

Acknowledgments

Thanks are due to C. Beaumont, R. Charrier, A. Fock, E. McDonald, R. Rauld, R. Farías, and M. Zentilli for stimulating discussions. JLA was funded by a Dalhousie Graduate Scholarship and a Geological Society of America Student Grant (#7864-05). ACOA-AIF Award 1005052 and NSERC-Discovery Grant awarded to JCG supported cosmogenic measurements, computational analysis and fieldwork. G. Yang and A. Reuther assisted in 10Be and 36Cl sample processing at Dalhousie University. R. Finkel supervised 10Be

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