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The Cerro Caquilluco–Cerrillos Negros Giant Rock Avalanches (Tacna, Peru)

  • January 2015
DOI:10.1007/978-3-319-09057-3_159
Authors:
Giovanni Crosta at Università degli Studi di Milano-Bicocca
Giovanni Crosta
P. Frattini at Università degli Studi di Milano-Bicocca
P. Frattini
Valbuzzi Elena
Valbuzzi Elena
Valbuzzi Elena
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Reginald L. Hermanns at Norwegian University of Science and Technology
Reginald L. Hermanns
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Abstract and Figures

Giant rock avalanches have been recognized and mapped in southern Peru, to the N of Tacna. The Cerro Caquilluco rock avalanche complex has a total volume of about 15 km3 and a length of 43 km, extending from 3,900 to 530 m a.s.l. Mapping the internal structures, the scar features and the depositional lobes allowed to suggest that the rock avalanche complex developed as a sequence of successive failures affecting tuffaceous and conglomeratic formations forming a gently dipping monoclinalic slope. Assessment of lobes volume constrained the reconstruction of the source areas for the multiple failures and the successive rock avalanche simulations. Seismic triggering is suggested, whereas H/L vs volume relationships suggest a high mobility comparable to that of extremely mobile volcanic rock avalanches. .
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159
The Cerro Caquilluco–Cerrillos Negros Giant
Rock Avalanches (Tacna, Peru)
Giovanni B. Crosta, Frattini Paolo, Valbuzzi Elena, and Reginald L. Hermanns
Abstract
Giant rock avalanches have been recognized and mapped in southern Peru, to the N of Tacna.
The Cerro Caquilluco rock avalanche complex has a total volume of about 15 km
3
and a
length of 43 km, extending from 3,900 to 530 m a.s.l.. Mapping the internal structures, the scar
features and the depositional lobes allowed to suggest that the rock avalanche complex
developed as a sequence of successive failures affecting tuffaceous and conglomeratic
formations forming a gently dipping monoclinalic slope. Assessment of lobes volume
constrained the reconstruction of the source areas for the multiple failures and the successive
rock avalanche simulations. Seismic triggering is suggested, whereas H/L vs volume
relationships suggest a high mobility comparable to that of extremely mobile volcanic rock
avalanches.
Keywords
Rock avalanche Failure sequence 3D runout simulation Peru
159.1 Introduction
The Arica Bend which coincides with the Peruvian-Chilean
border region is characterized by one of the largest relief
contrasts on Earth with depth of the subduction trench
ranging from 5,000 to 6,000 m b.s.l. and mountain tops
ranging from 5,500 to 6,300 m a.s.l.. Local relief contrasts
are pronounced due to the incision of deep canyons into
several million old uplifted surfaces, preserved because of
the extremely dry climate (precipitation averaging a few mm
and less per year). Systematic mapping in northern Chile and
Southern Peru (Crosta et al. 2012a,b;2014) revealed that
various gigantic landslides are present in the area suggesting
that landsliding has been a major factor in controlling
erosion.
In this paper, a rock avalanche complex located in
Southern Peru (Tacna) is presented. The aim of the paper is
to show the results of detailed photo-interpretation analysis,
to reconstruct the volume and sequence of rock-avalanche
events that occurred at the site.
159.2 Cerro Caquilluco–Cerrillos Negros
Rock Avalanche
The Cerro Caquilluco–Cerrillos Negros rock avalanche
complex affected the upper part of a SW dipping paleosur-
face (6–9°) cut by a disconnected and regular primitive
drainage network organized in a series of SW trending
parallel valleys. This network developed within the lower
Miocene pinkish tuffaceous deposits of the Huaylillas for-
mation, whereas the main landslide scarp lies within the
underlying conglomerates of the Upper Moquegua formation
(lower Oligocene; Acosta et al. 2010). The same type of
G.B. Crosta (&)F. Paolo V. Elena
Department of Earth and Environmental Sciences, University of
Milano-Bicocca, P.zza Della Scienza 4, 20126 Milan, Italy
e-mail: giovannibattista.crosta@unimib.it
F. Paolo
e-mail: paolo.frattini@unimib.it
V. Elena
e-mail: elena.valbuzzi@unimib.it
R.L. Hermanns
Norwegian Geological Survey (NGU), Trondheim, Norway
G. Lollino et al. (eds.), Engineering Geology for Society and Territory –Volume 2,
DOI: 10.1007/978-3-319-09057-3_159, ©Springer International Publishing Switzerland 2015
921
landscape is found to the southeast of Tacna and Arica
where a series of structures (Huaylillas anticline, Oxaya
anticline and Sucuna homocline) and formations (Oxaya
ignimbrite, Azapa conglomerates and sandstones) charac-
terize the area (Farias et al. 2005).
The major rock avalanche in the recognized sequence has
a total length of about 43 km, a source area width and length
of about 4 and 5.1 km, respectively. The computed
fahrböschung is equal to 4.6°with an H/L ratio of about
0.08, close to the extreme right hand extreme of the values
computed from the literature (Sosio et al. 2012) according to
the classical (H/L) versus volume empirical relationships
(Fig. 159.2). Deposition occurred along most of the trans-
portation area and is evidenced by a series of wide lobes
(10–65 m high) and levees (from a few meters to some tens
of meters high) with an average extent of 8 km transversally
to the flow direction. Flow structures are visible all over the
transportation area and several lobes can be mapped out.
The longest lobe has only pristine morphology in the
most distal part of the accumulation area (Cerrillos Negros,
Fig. 159.1). Here the deposit is represented by a unique
tongue shaped deposit, 11 km long, 3 km wide and 25–60 m
thick (rough volume estimate 1.15 km
3
), deposited along the
piedmont surface (average slope: 2°). Features are typical of
rock avalanches deposited on regular smooth surfaces: lat-
eral levees, longitudinal and transversal ridges and furrows.
The morphology of this lower lobe is contrasting to the
upper lobes, which smooth out the landscape. The minimum
volume involved in the giant rockslide-avalanche complex
amounts to about 10 km
3
. The extreme runout could be
explained assuming a large single failure event with a
detachment from a part of the slope located below the
present day upper scarp. In the middle part of the deposition/
transportation area few lateral levees are preserved and deep
valleys have been eroded into the deposit. Considering the
relationship with the piedmont deposits (Audin and Bechir
2006; Hall et al. 2012) and the faults cutting through the area
the deposit could be up to 2–2.3 Ma old (Fig. 159.2).
The head scarp area is affected by smaller and likely
younger rock avalanche lobes overlying the more massive
rock-avalanche lobe complex (Fig. 159.1). These features
are well preserved also to the west of the main slide com-
plex, along the E–W trending high scarp cutting the old
paleosurface, where more pristine rock avalanche lobes with
more blocky surfaces overlie older lobes characterized by a
smooth topography cut by high scarps. These deposits
locally present a matrix reach in silt and clay fraction.
Several conditioning factors of the slope instabilities have
been recognized: the SW dipping of weak formations (tuffs and
conglomerates), the presence of ENE trending sinistral faults
offsetting the primitive drainage network close to the headscarp
(see Fig. 159.1 and David et al. 2005), the intense seismicity, a
wetter climate, the continuous uplift (min. 0.04–0.3 mm a
−1
).
Fig. 159.1 Large landslides
inventory of Southern Peru.
The Cerro Caquilluco–Cerrillos
Negros rock avalanches are
highlighted with blue border
Fig. 159.2 Relative runout (H/L) versus volume for rock avalanches
in the study area (large red dots) compared with rock avalanches in
different settings (literature data from Sosio et al. 2012 and references
therein). See Fig. 159.4 for symbols
922 G.B. Crosta et al.
This area is about 230 km away from the subduction trench;
M7 to 9 earthquakes occur on average every 100 years on the
subduction segment. Megathrust earthquakes are common in
the area on geological time scales, nevertheless, no large
historical landslides have been reported during multiple his-
toric subduction earthquakes in S-Peru and N-Chile
Based on geomorphological interpretation, we recon-
structed a possible rock avalanches sequence. We suggest
that the first rock avalanche event corresponds to the
Cerrillos Negros rock avalanche. Reconstruction of pre-
failure morphology was accomplished by mimicking the
preserved morphology close to the source area, and by
removing the deposited volumes from the rock avalanche
path. For this, we made the hypothesis that the old paleo-
surface was already eroded by valleys progressively moving
upstream during a wetter climate, as suggested by Hoke
et al. (2007) for similar conditions in northern Chile. The
reconstruction of the pre-event morphology required several
attempts to fit the eroded and the deposited volumes. Finally,
a mobilized volume of about 10.2 km
3
was obtained.
Fig. 159.3 Proposed rock
avalanche sequence in the head
scarp zone. Retrogressive events
are progressively smaller,
following a power-law decay. For
eastern sequence (red), the power
law exponent is −1.21. For
western sequence (blue), the
exponent is −1.73
Fig. 159.4 Numerical model of
the Cerrillos Negros rock
avalanche; areconstructed pre-
event; bdetachment scar; crock
avalanche depth from DEM
comparison; drock avalanche
depth from numerical model
159 The Cerro Caquilluco–Cerrillos Negros 923
For the successive scenarios of slide retrogression, we used
the morphologies obtained by previous scenarios as pre-
failure morphologies, and we calculated, by difference with
current topography, the lobe volumes. The volumes of single
rock avalanche episodes decrease from the first to the last
event, roughly following a power-law decay (Fig. 159.3). This
behavior is comparable to that described by Utili and Crosta
(2011) for retrogressive instabilities in rocky cliffs.
159.3 Numerical Simulation
Quasi-3D numerical simulations have been carried out to
verify the different scenarios in terms of spreading area and
maximum runout by using the SPH (Smooth Particle
Hydrodynamics) code DAN 3D (McDougall and Hungr
2004). Different equivalent fluid models were tested and a
frictional rheology was selected.
In order to reach the Cerrillos Negros tongue, a value of
5°was used for the basal friction angle, with a pore pressure
ratio of 0.1 (i.e., the water pore pressure equals 10 % of the
total pressure). The results of the simulation show a larger
extent of the depositional areas with respect to actual
deposition (Fig. 159.4). In particular, the simulated runout
extends to the neighboring valleys.
Although a deposition in these neighboring valleys was
not recognized, it may be possible that the deposits have
been successively eroded. On the other hand, it is also
possible that the reconstructed pre-failure morphology is not
exact, leading to lateral over-runout of the model. The basal
friction angle is comparable to the measured H/L value, thus
confirming the extreme runout of the rock avalanche.
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... The instability has developed along the western flank of the Peruvian Andes, an area characterized by steep slopes and extremely dry climate for the last million years (Placzek et al., 2010). A preliminary geomorphological description of this exceptional site can be found in Audin and Bechir (2006) and Crosta et al. (2015). The Caquilluco landslide is a large complex of imbricated paleo deep-seated landslides and paleo rock-avalanches. ...
... It affects the western flank of a large anticline trending NW-SE, which developed during compressive deformation propagation along west-verging blind reverse faults (e.g. the Calientes fault, Hall et al., 2012). The total volume of rock material that has been mobilized is about 15 km 3 (Crosta et al., 2015). From the head scarp (~3800 m a.s.l.) to the most distal rock-avalanche lobe (600 m a.s.l.), avalanche deposits are covering more than 40 km in distance along-strike and about 15 km in width (Fig. 1B). ...
View
... In this context, a seismo-tectonic forcing has been often a preferred interpretation to explain the triggering of paleolandslides Table 1 (e.g. Crosta et al., , 2015Mather et al., 2014;McPhillips et al., 2014;Pinto et al., 2008;Strasser & Schlunegger, 2005 However, the paleo-seismicity of this fault system is largely unknown and such link would remain thus speculative. Along the coastal cordillera of northern Chile, Crosta et al. (2017) proposed a seismic origin, linked to the mega-earthquakes of subduction, for the triggering of the El Magnifico rock-avalanche paleo cluster. ...
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Full-text available
  • Oct 2011
The quadrangles of Pachia (36-v) and Palca (36-x) are located in the Western edge of Western Cordillera in the central Andes, between the parallels 69º 30’ to 70 º 30’ west longitude and 17º 30’ to 18º 00’ south latitude, showing an approximate area about 5000 km2. Politically, this area corresponds to the northern province of Tacna and also includes the districts of Inclan, Alto de la Alianza, Calana, Pachia and Palca, and the southern province of Tarata. The studied area presents a wide climatic variety, from arid, tropical and cold weather, and depends of the geomorphology of the region. Thus, we can observe three main geomorphologic units: Coastal plains, Western edge of Western Cordillera and Western Cordillera. These units are controlled by the Incapuquio System Fault, placed in NW-SE of regional alignment. The Coastal plains are characterized by having a low relief that change dramatically to abrupt, until the boundary with the beginning of the Western edge of the estern Cordillera, where topography begins to includes highs mountain peaks and strong deepening, typical features from the Western Cordillera morphology. This geomorphologic unit includes the Barroso mountains chain, with the highest peak in about 5900 masl, (Tacora volcan). The features and kind of relief present in these geomorphologic units depends of the presence of the Incapuquio System Fault, kind of rock and its mechanical properties and the climatic conditions which they are exposed. The area, which the Arequipa basin sediments have deposited, have experimented a complex geodynamic evolution until nowadays. The Arequipa basin has as settlement the Mal Paso metamorphic Basement rocks and that belongs to the Arequipa Massif, and the older ages corresponds to lower Paleozoic. These metamorphic rocks are ortogneisses banded and only outcrops around Huacano Chico. Above and erosional discordance, conglomerates, shales, limestones and sandstone from the mississipian Ambo Group are overlying. From late Triassic to upper Pliensbachian, the first volcanic event was created in this region, represented by the Chocolate volcanic arc growing, which are the main rock materials of the Chocolate Formation. These rocks suggest us the beginning of the development of the Arequipa Mesozoic basin by a back-arc extension regime (Martinez et al., 2005; Carlotto et al., 2009; Mamani et al., 2010). During Toarcian to Bathonian, under several tectonically distensive regimes, intercalated limestones, sandstones and conglomerates from Socosani Formation are deposited, and paleoenvironmentally correspond to huge shallow carbonated platforms. Since Callovian to early lower Cretaceous, the Arequipa basin accommodation space receive very thick layers of continental and deltaic sediments in a more than 1700 m stacking, called Yura Group.
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Geología de los cuadrángulos de Pachía (36-v) y Palca (36-x), escala 1:50 000.
Technical Report
Full-text available
  • Jan 2011
Los cuadrángulos de Pachía (36-v) y Palca (36-x) se hallan en la Ladera oeste de la Cordillera Occidental de los Andes centrales, entre los paralelos 69º 30’ a 70º 30’ de longitud oeste y 17º 30’ a 18º 00’ de latitud sur, presentando un área aproximada de 5000 km 2 . Políticamente, el área de estudio está ubicada al norte y noreste de la ciudad de Tacna. Territorialmente, corresponde la parte noreste de la provincia de Tacna y abarca parcialmente los distritos de Inclán, Alto de la Alianza, Calana, Pachía y Palca, y el sur de la provincia de Tarata.
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Geomorphic evidence for post-10 Ma uplift of the western flank of the Central Andes 18°30'-22°S
Article
Full-text available
  • Oct 2007
The western Andean mountain front forms the western edge of the central Andean Plateau. Between 18.5° and 22°S latitude, the mountain front has ~3000 m of relief over ~50 km horizontal distance that has developed in the absence of major local Neogene deformation. Models of the evolution of the plateau, as well as paleoaltimetry estimates, all call for continued large-magnitude uplift of the plateau surface into the late Miocene (i.e., younger than 10 Ma). Longitudinal river profiles from 20 catchments that drain the western Andean mountain front contain several streams with knickpoint-bounded segments that we use to reconstruct the history of post-10 Ma surface uplift of the western flank of the central Andean Plateau. The generation of knickpoints is attributed to tectonic processes and is not a consequence of base level change related to Pacific Ocean capture, eustatic change, or climate change as causes for creating the knickpoint-bounded stream segments observed. Minor valley-filling alluvial gravels intercalated with the 5.4 Ma Carcote ignimbrite suggest uplift related river incision was well under way by 5.4 Ma. The maximum age of river incision is provided by the regionally extensive, approximately 10 Ma El Diablo-Altos de Pica paleosurface. The river profiles reveal that relative surface uplift of at least1 km occurred after 10 Ma.
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Late Cenozoic deformation and uplift of the western flank of the Altiplano: Evidence from the depositional, tectonic, and geomorphologic evolution and shallow seismic activity (northern Chile at 19°30′S)
Article
Full-text available
  • Aug 2005
We analyze the west vergent thrust system (WTS) along the western flank of the Altiplano in northern Chile (18°S–21°S). In our study area (19°20′S–19°50′S), the WTS consists of three thrust propagation monocline folds (flexures) developing growth strata. The relative uplift accommodated by the flexures is rapid between 26 and 8 Ma (0.1 mm/yr), diminishing to 0.02 mm/yr after 8 Ma. Approximately 2000 m of relative surface uplift was accommodated by the flexures since the late Oligocene. Sedimentological and geomorphological analysis shows that westward tilting of the forearc occurred after 10 Ma, coeval with the shifting of deformation from the Altiplano to the sub-Andean zone, where the underthrusting of the Brazilian Craton would have resulted in crustal thickening, surface uplift in the orogen, and westward ductile subcrustal flow. Forearc tilting is accommodated by east vergent thrusts (ETS) issued from the Benioff zone beneath the Central Depression emerging into the Western Cordillera, contributing 500–1400 m of surface uplift. The WTS connects the ETS in the brittle-ductile crustal transition (∼25 km depth), continuing farther east as the Altiplano low-velocity zone, configuring the western Altiplano as a crustal-scale fault bend fold. Forearc tilting would be caused by westward ductile flow in the lower crust pushing the rigid forearc in the ETS. Meanwhile, between 19°S and 21°S, the WTS accommodates dextral strike slip, and ∼3 km of N-S shortening occurred in the Coastal Cordillera. Transcurrence and strain partitioning are probably the result of slight plate convergence obliquity, strong coupling within the interplate zone, westward continental concavity, and high elevation opposing horizontal contraction.
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Large rock avalanches in southern Perù: the Cerro Caquilluco – Cerrillos Negros rock slide - avalanche (Tacna, Tomasiri, Perù)
Conference Paper
Full-text available
  • Apr 2012
The Andean bent which coincides with the Peruvian-Chilean border region is characterised by one of the largest relief contrasts on earth with depth of the subduction trench ranging from 5000 to 6000 m below sea level and mountain tops ranging from 5500 to 6300 m a.s.l.. The western flank of the Andes is subdivided in 4 major geologic zones (i.e. Coastal Cordillera, longitudinal Basin or depression, the Precordillera or western escarpment and western Cordillera). Local relief contrasts are also pronounced due to the incision of deep canyons into several million old uplifted surfaces, preserved because of the extremely dry climate with precipitation averaging a few mm and less per year. The Lluta collapse (minimum age of 2.5 Ma; volume 26 km3) is one of the largest non-volcanic non-marine landslides on Earth and has been mapped in that area (Wörner et al., 2002). Systematic mapping in northern Chile and Southern Peru has revealed that this is not the only gigantic landslide in the area but that further landslides of similar size occurred in the area, located both along the canyon slopes and along the western escarpment of the Cordillera. This suggests that landsliding has been a major factor in controlling erosion. This contribution describes first results on mapping a giant landslide complex in southern Perù called the Cerro Caquilluco – Cerrillos Negros Tomasiri rock slide – avalanche complex. The systematic mapping we have carried out in the area is presented in a further contribution to this conference. The Cerro Caquilluco – Cerrillos Negros Tomasiri rock slide – avalanche complex affected the upper part of a SW dipping paleosurface (8� to 9� ) cut by a disconnected and regular primitive drainage network organized in a series of SW trending parallel valleys. This network developed within the lower Miocene pinkish tuffaceous deposits of the Huaylillas formation, whereas the main landslide scarp lies within the conglomerates of the Upper Moquegua formation (lower Oligocene). The same type of landscape is found to the southeast of Tacna and Arica (Huaylillas anticline, Oxaya anticline and Sucuna homocline) The Cerro Caquilluco – Cerrillos Negros Tomasiri rock slide – avalanche complex has a total length of about 43 km, a source area width and length of about 4 km and 5.1 km, respectively. The computed fahrböschung is equal to 4.6� with an H/L ratio of about 0.08 and resulting in an extremely large excessive travel distance. The H/L value is well below the expected value computed according to the classical (H/L) vs volume empirical relationships presented in the literature. Deposition occurred along most of the transportation area and is evidenced by a series of wide lobes (10 to 65 m high) and levees (from a few meters to some tens of meters high) with an average extent of 8 km transversally to the flow direction. Flow structures are visible all over the transportation area and several lobes can be mapped out. Presently, we cannot determine if most of these failures belong to a consequent large retrogressive event or to different events separated in time. The longest lobe has only pristine morphology in the most distal part of the accumulation area. Here the deposit is represented by a unique tongue shaped deposit, 11 km long, 3 km wide and 25 to 60 m thick (rough volume estimate 1.15 km3) ), deposited along the piedmont surface (ave. slope: 2� ). This deposit presents a series of features typical of rock avalanches deposited on regular smooth surfaces, like: lateral levees, longitudinal and transversal ridges and furrows. The extreme runout of this failure could be explained assuming a single failure event or the detachment from a part of the slope located well below the present day upper scarp. In the middle part of the deposition/transportation area few lateral levees are preserved and deep valleys have been eroded into the deposit. Considering the relationship with the piedmont deposits and the faults cutting through the area the deposit could be up to 2-2.3 Ma old. The morphology of this lower lobe is contrasting to the upper lobes which smooth out the landscape. The minimum volume involved in the giant rockslide-avalanche complex amounts to about 9 km3. The headscarp area is affected by smaller and likely younger rock avalanche lobes overlying the more massive rock-avalanche lobe complex. These features are well preserved also to the west of the main slide complex, along the E-W trending high scarp cutting the old paleosurface, where more pristine rock avalanche lobes with more blocky surfaces overlie older lobes characterized by a smooth topography cut by high scarps. Conditioning factors of the slope instabilities could have been: the SW dipping of weak formations (tuffs and conglomerates), the presence of ENE trending sinistral faults offsetting the primitive drainage network close to the headscarp, the intense seismicity and/or a wetter climate, the continuous uplift (min. 0.04-0.3 mm/yr). This area is less than 230 km away from the subduction trench and magnitude 7 to 9 earthquakes occur on average every 100 years on the subduction segment. Megathrust earthquakes are quite common in the area on geological time scales and no big landslides of that site has been reported during multiple historic subduction earthquakes in southern Peru and northern Chile. Some results concerning slope stability analyses and runout modelling are presented to support possible failure mechanisms and to understand the exceptional avalanche mobility
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Recently active contractile deformation in the forearc of southern Peru
Article
In the Precordillera and Western Cordillera of southern Peru (14°-18°S), vast pediment surfaces have been abandoned through drainage diversion and river incision, with the major drainages carving deep canyons. Within this region, we have identified range-sub-parallel contractile structures that accommodate significant distributed crustal deformation. Young geomorphic features document both the presence and youthfulness of these contractile structures. Here, we determine exposure ages on geomorphic features such as pediment surfaces and fluvial terraces using in situ produced cosmogenic radionuclides, in conjunction with field and remote mapping. This chronologic data reveals that ancient surfaces have been preserved as a result of very low erosion rates. We measure this rate to be
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A model for the analysis of rapid landslide motion across three-dimensional terrain
Article
  • Dec 2004
A new numerical model for the dynamic analysis of rapid flow slides, debris flows, and avalanches has been developed. The model is an extension of an earlier algorithm and is implemented using a numerical method adapted from smoothed particle hydrodynamics. Its features include (i) the ability to simulate flow across complex three-dimensional terrain; (ii) the ability to allow nonhydrostatic and anisotropic internal stress distributions, coupled with strain changes through frictional relationships; (iii) the ability to simulate material entrainment; (iv) a choice of different rheological kernels, including frictional, plastic, viscous, Bingham, and Voellmy; (v) a meshless solution, which eliminates problems with mesh distortion during long displacements; and (vi) highly efficient and simple operation. The model has been tested by analysing a series of laboratory flume experiments with granular materials, both on straight and curved paths. The model is capable of accurately predicting the margins of various curving flows using a single set of input parameters. A preliminary analysis of a real rock avalanche case history is also included.
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Slope instabilities along the Western Andean Escarpment and the main canyons in Northern Chile
Conference Paper
  • Apr 2012
The western slope of the Andes of northern Chile – southern Perù is generally subdivided from the west to the east into the morphological units of: the Coastal Cordillera, Central Depression, the Western Escarpment-Precordillera and the Western Andean Cordillera. The western escarpment and Precordillera are formed by the Azapa coarsegrained clastic formation (sandstones, conglomerates, mudstones) and the Oxaya (rhyodacitic ignimbrites) and Diablo volcanoclastic formations (Oligocene and Miocene). Important uplift has been suggested between the deposition of the Oxaya and Diablo formations. The entire area has been characterized by a long-term hyperaridity (Atacama desert), initially established between 20 and 15 Ma, and this caused a strong difference between the long term continuous uplift and low denudation rates. This long sector of the central western escarpment and Precordillera is incised by deep canyons and subparallel drainage network in the upper part. The drainage network developed in two main phases: a lower–middle Miocene phase with formation of a parallel poorly structured drainage network cutting into the Oxaya formation, and presently well preserved; the canyons have been incised in the initial topography starting around 9 Ma and up to about 3.8 Ma with subsequent refilling episodes. Valley incision (ave. rate of 0.2 mm yr1 ) has been controlled by topographic uplift and less arid climate (after 7 Ma). As a consequence of these geologic and climatic settings the evolution of this area has been characterized by canyon incision and extremely large slope instabilities. These slope instabilities occur in the “interfluvial” sectors of the western escarpment and Precordillera and along the canyon flanks. Landslides affecting the preserved paleosurfaces, interested by the parallel drainage network in the Oxaya formation, involve volumes of various cubic kilometres (Lluta collapse, Latagualla Landslide) and can control the drainage network. These mega landslides can be classified as large block slides and can evolve in large rock avalanches. Their initiation seems to be strongly associated to the presence of secondary faults and large fractures transversal to the slope. Furthermore, most of these landslides show evidences suggesting a re-incision by the main canyon network. Landslides along the canyon flanks affect volumes lower than 1 km3 and can be mainly classified as large complex slumps. The deposits of these landslides often cross the valley and have been incised exposing undeformed bedrock material. At the same time large boulder fields and alluvial deposits infill the lower part of the canyons suggesting also a long history of dam breaching events. We present a landslide inventory in the area (about 220 km long and 80 km wide) between Pisagua (19.4� Chile) and Tacna (17.5� Perù) to the NE of the Arica bend. We mapped landslides, main tectonic structures and other morphological features. Mapping has been performed by use of satellite images, Google Earth® and field surveys performed in the last few years. We discuss two specific landslide sites, the Cerro Caquilluco–Cerrillos Negros rock slide–avalanche (Tacna, Tomasiri, Perù) and a small group of rock avalanches south of Iquique (Chile) in two other abstracts presented by the authors at this conference
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