Tephrochronologic Constraints on Temporal Distribution of Large
Landslides in Northwest Argentina
Reginald L. Hermanns, Martin H. Trauth, Samuel Niedermann,1
Michael McWilliams,2 and Manfred R. Strecker
Institut für Geowissenschaften, Universität Potsdam, Postfach 601553, D-14415 Potsdam, Germany
(e-mail: [email protected])
ABSTRACT
Two morphologic settings in the northwestern Argentine prone to giant mountain-front collapse—deeply incised
narrow valleys and steep range fronts bordering broad piedmonts—were analyzed through detailed investigations of
fossil landslides and related fluvio-lacustrine sediments. Nine different rhyodactic tephra layers were defined by
geochemical fingerprinting of glass, morphology of pumice, stratigraphic relationships, and mineralogy. The age of
three tephra could be determined either directly by 40Ar/39Ar dating or relatively by 14C dating of associated sediments:
Paranilla Ash (723 5 89 ka), Quebrada del Tonco Ash (∼30 ka), and Alemanı́a Ash (∼3.7 ka). These units permit
correlation of several spatially separate landslide deposits. Landslide deposits in narrow valleys were generated in the
late Pleistocene between 40 and 25 ka and in the Holocene since ca. 5 ka and correspond to periods characterized
by increased humidity in subtropical South America. In contrast, the age of large landslides in piedmont regions is
significantly greater but more difficult to define by tephrochronology. However, selected deposits from this second
environment have cosmogenic nuclide exposure ages of 140–400 ka. Because of the large distance of the collapsed
mountain fronts from eroding streams and because of important Quaternary displacement along the mountainbounding faults, we suggest that strong, low-frequency seismic activity is the most likely trigger mechanism for most
of the landslides in this environment.
Introduction
1979; Adams 1981; Schuster et al. 1992; Crozier et
al. 1995). Establishing a temporal framework for
large landslide events is often hampered by a lack
of datable material, especially in arid and semiarid
regions. This dilemma, however, is eased when
landslide deposits are associated with volcanic tephra layers and lacustrine sediments that may serve
as marker horizons.
Situated east of the Central Andean volcanic
zone (CVZ), the Argentinian Andes and the adjacent foreland between 247309and 277309S represent
an ideal region to develop a tephra-based chronostratigraphy of catastrophic mass movements in an
active mountain belt (fig. 1). This region is tectonically active and contains more than 50 voluminous landslide deposits. In addition, this part of the
Andes is characterized by an arid climate, which
guarantees a high preservation potential for landslide deposits. In the CVZ, more than 60 volcanoes
have been repeatedly active throughout the Quaternary (Francis and de Silva 1989). Historic tephra
Large mountain-front collapse resulting in deposits
of several million cubic meters is a common phenomenon in tectonically active orogens (e.g.,
Shreve 1966; Plafker and Ericksen 1978; Keefer
1984). Dating such catastrophic mass movements
is essential for the evaluation of mechanisms conditioning and triggering landslides, the assessment
of their recurrence, and finally, for the appraisal of
future landslides as potential natural hazards. The
need to understand landslide mechanisms as well
as spatial and temporal landslide distribution is particularly important in regions frequently affected
by earthquakes, which may ultimately trigger such
large mass movements in rocks already preconditioned for failure (e.g., Nikonov and Shebalina
Manuscript received November 30, 1998; accepted August
31, 1999.
1
GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473
Potsdam, Germany.
2
Geophysics Department, Stanford University, Stanford,
California 94305-2215, U.S.A.
[The Journal of Geology, 2000, volume 108, p. 35–52] q 2000 by The University of Chicago. All rights reserved. 0022-1376/2000/10801-0003$01.00
35
36
R. L. HERMANNS ET AL.
nary activity, thus potentially representing the result of seismic triggering (Fauque and Strecker
1988; González Dı́az and Mon 1996; Hermanns and
Strecker 1999). Although the timing of these avalanches is still poorly understood, there is another
influence on controlling landsliding. Radiocarbondated deposits of landslide-dammed lakes indicate
that some avalanches were generated during a more
humid period between 40 and 25 ka (Trauth and
Strecker 1999). However, in order to judge the relationship between the geomorphic setting of
mountain-front collapse and landslide frequency,
climate, and paleoseismic activity in the northwest
Argentinian Andes, a more detailed chronology of
mountain-front failures is needed. A large data set
of the landslides with volumes between 0.03 and
0.375 km3 (Hermanns and Strecker 1999), based on
detailed satellite image analysis, air photo interpretation, and fieldwork, permits a statistically representative analysis of the temporal distribution of
these voluminous mountain-front failures. In this
article, we present new chronostratigraphic data,
based on tephra that will be grouped on the basis
of geochemical studies of glass shards, pumice morphology, stratigraphic relations, and mineralogy.
These tephras will allow us to date several landslide
events and describe their temporal clusters and various morphologic settings.
Figure 1. Tectonic provinces (different shades of gray)
of the central Andes in Argentina, Bolivia, and Chile and
location of volcanoes with Quaternary activity (filled circles), modified after Jordan et al. (1983) and de Silva and
Francis (1991).
in the eastern Andean ranges and the foreland from
vulcanian- and subplinian-type eruptions indicate
ash transport toward the east-southeast, with an
areal coverage 1112,000 km2 (Glaze et al. 1989; Felpeto and Ortiz 1997). Although recent eruptions
produced only thin ash layers that were subsequently eroded (Felpeto and Ortiz 1997), older Quaternary eruptions produced deposits thick enough
to be preserved and have stratigraphic relations to
large landslide deposits.
Most of the landslide deposits with volumes 1106
3
m in the Argentinian Andes result from rock avalanches (sturzstroms; Hermanns and Strecker
1999). Previous studies show that such deposits lie
in clusters (fig. 2), always along lithologically,
structurally, and topographically preconditioned
mountain fronts (Wayne 1994; Hermanns and
Strecker 1999; Strecker and Marrett 1999). In addition, the avalanche deposits often exist in the
vicinity of faults having signs of multiple Quater-
Geologic Setting
The CVZ of the central Andes (fig. 1) is one of three
important Andean provinces to experience Cenozoic volcanism (e.g., Thorpe et al. 1982), whereas
the Andean sector between 287 and 337S is amagmatic (Jordan et al. 1983). In its southern part, the
CVZ has a transverse zonation with a calc-alkaline
association in the west and shoshonitic volcanics
in the east (Déruelle 1978). In both segments,
rhyodacitic to rhyolitic volcanics also exist (e.g.,
Francis et al. 1989; de Silva 1991; Coira and Kay
1993). Silicic eruptions were dominated by explosive ignimbrite volcanism, resulting in several major resurgent caldera complexes (de Silva 1989).
To the east of the volcanic region is the intraAndean Puna plateau and the fault-controlled valleys of the Cordillera Oriental and the northern
Sierras Pampeanas (fig. 1). The ca. 4000-m-high
Puna plateau is characterized by intervening ranges
of reverse-fault-bounded basement blocks that
reach elevations of about 6000 m (fig. 2a, 2b). The
Cordillera Oriental is a fold and thrust belt (Mon
1976; Grier et al. 1991) of Precambrian basement
and overlying unmetamorphosed Cambrian to Tertiary sediments (Reyes and Salfity 1973; Omarini
Journal of Geology
TEMPORAL DISTRIBUTION OF LARGE LANDSLIDES
1983; Salfity and Marquillas 1994); it is cut by
deeply incised valleys (fig. 2a, 2b). In contrast, the
Sierras Pampeanas are late Cenozoic Laramide-type
uplifts composed of late Proterozoic metamorphic
basement rocks that contrast with highly erodible
Tertiary clastic sediments in the adjacent intramontane basins (Caminos 1979; Mon 1979; Jordan
et al. 1983). The basins are further characterized by
alluvial-fan deposits and coarse gravel associated
with multiple, gently inclined pediments that abut
the steep mountain fronts (Strecker et al. 1989).
Along tectonically active mountain fronts and in
areas where antecedent rivers cross the uplifting
ranges of the Cordillera Oriental, voluminous landslide deposits often associated with lacustrine and
terrace deposits exist (fig. 2a, 2b) (Hermanns and
Strecker 1999; Strecker and Marrett 1999). Common to all Cenozoic sedimentary deposits in the
intramontane basins is their association with intercalated volcanic ash layers related to multiple
eruptions in the CVZ (Marshall and Patterson 1981;
Strecker et al. 1989; Grier and Dallmeyer 1990;
Malamud et al. 1996).
Methods
In the study area, 26 tephra samples were taken
from nine different Quaternary sedimentary sequences associated with landslide deposits. At the
outset, we did not know the stratigraphic relations
among these tephra. Our task was therefore to characterize the samples geochemically and use statistical analyses of these data supplemented by scanning electron microscope (SEM) observations of the
glass morphology, stratigraphic relations, and mineralogy of the tephra (table 1) to identify groups of
samples that were deposits of single eruptions.
Once their identity as time-synchronous marker
horizons was known, these tephras were named after type localities. Finally, we could apply limited
chronological data from 40Ar/39Ar and 14C dating in
a more efficient manner.
The chemical composition of volcanic glass contained in such pyroclastics is one of the most distinctive and consistent characteristics by which tephra from individual eruptions can be identified
(e.g., Sarna-Wojcicki 1976; Westgate and Gorton
1981; Bogaard and Schmincke 1985). Our tephrochronological study is therefore based mainly on
electron-microprobe analysis (EMA) of volcanic
glass shards (table 2).
Tephra samples were disaggregated in water and
wet sieved at intervals of 63, 200, and 500 mm. The
200–500-mm-size fraction was placed in an ultrasonic bath to remove adhering material and sieved
37
again with water and acetone. Mafic minerals were
removed using a magnetic separator. Glass shards
were picked from the felsic concentrate, and biotite
from the mafic concentrate, avoiding samples with
vesicles, crystalline intergrowth, or alteration.
EMA was carried out according to the principles
outlined in Frogatt (1992), using a 15-kV excitation
potential, a 20-nA beam current, and a 10-mm beam
diameter. Up to 22 glass shards from each of the
26 tephra beds were analyzed for Na, Si, Mg, Al, K,
Ca, Ti, Mn, and Fe. The beam was defocused to
minimize volatilization of water or dispersion of
sodium. Up to nine single measurements per glass
shard were performed, using commercially available silicate and oxide minerals as standards. Oxide
concentrations for each sample generally ranged between 90% and 99%. In order to eliminate measurements of disproportionately strongly altered
glass shards, CO2 and H2O contents were measured
by infrared absorption. The resulting percentages
of volatiles were subtracted from the ideal volatile
free value of 199% of the EMA measurements. All
totals below this value were not included in further
analysis. All EMA data were normalized to 100%
to facilitate proper comparison (table 2).
The large number of analyses per sample allows
a statistical analysis of the data, involving an examination for outliers, alteration trends, and multiple populations—a standard procedure in tephrochronologic investigations with enormous data sets
(e.g., Smith and Nash 1976; Sarna-Wojcicki et al.
1987). The distribution of sodium in a few samples
tended to have a weak negative skew, suggesting a
loss during glass hydration (fig. 3; Cerling et al.
1985). After eliminating outliers, the median of the
measurements of all elements was used as a measure of central tendency, which is less sensitive to
outliers and skewness than arithmetic means
(Swan and Sandilands 1995). Before statistical analysis, the data were pretreated by log-ratio transformation (e.g., Aitchison 1984, 1986). This ensures
data independence (on an oxide-to-oxide basis) and
avoids the constant sum normalization constraints
imposed by EMA of volcanic glass with variable
composition:
yi = log (x i /xd),
where yi denotes the transformed score (i =
1, 2, ) , d 2 1) of some raw oxide score xi. The
procedure is invariant under the group of permutations of the components, and any component (oxide) can be used as divisor xd. In this study, we
adopted the log-transformation approach and selected Al2O3 as the divisor. The oxide Al2O3 has
Journal of Geology
TEMPORAL DISTRIBUTION OF LARGE LANDSLIDES
only a small variance within the data set and is
generally little affected by alteration processes;
hence, it is the most appropriate divisor. Using the
mobile K2O as divisor (e.g., Stokes et al. 1992) did
not lead to significantly different results.
Principal component analysis (PCA) and hierarchical cluster analysis (CA) have been used to identify groups of ash layers based on the chemical composition of glass shards (e.g., Sarna-Wojcicki 1976;
Smith and Nash 1976; Sarna-Wojcicki et al. 1987;
for a review of the principles of PCA and CA see
Swan and Sandilands 1995). The advantage of CA
over other methods such as discriminant function
analysis is that an a priori set of known samples is
not required. Each of the groups identified using
CA is considered to represent a single volcanic
event and can therefore be used as a stratigraphic
marker horizon. The CA and PCA were carried out
using Matlab routines contained in the PLS Toolbox provided by Eigenvector Research, Manson,
Washington. The routine pca was used to perform
PCA on autoscaled data. Autoscaling (or variance
scaling) essentially puts all variables on an equal
basis in the analysis. This is important for variables
showing large differences in the absolute values of
mean and variance. The PCA redefines the nineelement coordinate system by ranking each new
perpendicular axis based on the maximum separation of the samples. It is generally found that the
data can be adequately described using far fewer
factors than the original variables. The variance
that each principal component (PC) captures can
determine the number of PCs to remain in the
model; the leftover variation is considered noise
(table 3). Unfortunately, there is no simple method
for automating the determination of the number of
PCs to retain in the model. However, a rule of
thumb is to start from the smallest eigenvalue and
to go back to the larger ones, looking for a sudden
jump in the values. It is then appropriate to choose
those PCs for the model that include this jump.
The routine cluster performs k-means clustering on
autoscaled data using Mahalanobis distances based
on raw principal component scores. The Mahalanobis distance is a measure of the differences between the means of k multivariate groups.
39
Applying the PCA on the log-ratio-transformed
and autoscaled data listed in table 1, it is appropriate to choose the first two PCs for the model
(fig. 4). The scores for PC 1 and PC 2 provide a firstorder relation between the different ash layers (fig.
5), defining distinct and well-separated tephra clusters. However, there are a number of randomly distributed data points without any relationship to a
single tephra group. Next, we employed cluster
analysis on autoscaled data to define the groups of
ash layers. The graphical output of cluster analyses,
the dendrogram choosing the first two PCs, displays information regarding the Mahalanobis distance between samples and groups of samples (fig.
6). For example, g63 is the sample with the greatest
distance from the other samples in the dendrogram.
Other samples, such as g43, g37, g47, g45, g35, g65,
and g21, are in close proximity to each other and
are very similar, suggesting that they represent a
single volcanic event.
Finally, in order to develop a chronostratigraphy
for the tephra layers and avalanche deposits, selected biotites from two ash layers were dated by
the incremental-heating 40Ar/39Ar method. Calculated ages are based on multiple crystal analyses.
In addition, AMS-14C dates from peat, mollusks,
and charcoal sampled from fluvio-lacustrine sediments associated with landslide deposits were used
for further age constraints. All 14C ages cited here
were converted to calendar years using the principles outlined in Stuiver and Reimer (1993) to
compare ages obtained by different dating methods.
Tephra Classification
Tephra in northwestern Argentina is made of pumiceous, fine- to coarse-grained glass particles with
an admixture of lapilli and lithic fragments. The
thickness of tephra layers varies between a few centimeters and 2 m; generally, tephra layers in the
study area thicken from north to south. All tephras
are dominantly white and rhyodacitic in composition, with SiO2 contents between 66.8% and
72.8% and total alkali contents ranging from 8.4%
to 8.9%. The tephras contain significant amounts
of quartz, potassium feldspar, and plagioclase; bi-
Figure 2. a, Topographic map showing distribution of mountain ranges, intramontane basins, and narrow valleys
and indicating type localities of the defined tephra layers (table 1). b, Generalized geologic map of the study area and
distribution of large landslide deposits (after unpublished map by J. Sosa Gomez, pers. comm.; Allmendinger et al.
1983; Strecker et al. 1989; Marrett et al. 1994; Hermanns and Strecker 1999). S.L.B. = Sierra Laguna Blanca, Q =
Quebrada. Open circles denote rock-avalanche deposits in narrow valleys; filled circles correspond to rock-avalanche
deposits in piedmont environments. Selected rock-fall deposits are marked by a circle with a cross. Letters indicate
positions of profiles illustrated in figure 8.
Table 1.
Compilation of Location, Glass Morphology, Biotite Contamination, and Age of Sampled Tephra
Sample
Location
(latitude/longitude)
Quebrada La Yesera Tuff:
g09
g67
267009S/657459W
257579S/657449W
Glass
morphology
Y–bubble wall junction,
double concave plates
Y–bubble wall junction,
double concave plates
Biotite
contaminationa
Age (ka)
um
)
um
)
El Peñon Tuff:
g71
g61
267299S/677499W
277109S/667099W
Lapilli tuff
Elongated, tubular vesicles,
thick vesicles walls
um
pm
1200
Cerro Paranilla Ash:
g05
267059S/657449W
Elongated, tubular vesicles,
thick vesicles walls
Elongated, tubular vesicles,
thick vesicles walls
Elongated, tubular vesicles,
thick vesicles walls
Elongated, tubular vesicles,
thick vesicles walls
um
)
um
723 5 89
um
)
um
)
)
g07
257349S/657589W
g25
267059S/657449W
g31
257349S/657589W
)
Ruinas del Rincón Ash:
g63
267389S/667259W
Irregularly shaped, oval vesicles, thin walls
pm
)
El Paso Ash:
g04
257599S/657459W
Elongated, tubular vesicles,
thick vesicles walls
Elongated, tubular vesicles,
thick vesicles walls
um
54 5 221
pm
)
277049S/667489W
257599S/657459W
257349S/657589W
Elongated, tubular vesicles
Elongated, tubular vesicles
Elongated, tubular vesicles
pm
pm
pm
133
277049S/667499W
Irregularly shaped, oval vesicles, thin walls, strong
intergrowth with biotite
pm
11.31 5 .13
277109S/667089W
Elongated, tubular vesicles,
thin vesicles walls
Elongated, tubular vesicles,
thin vesicles walls
Elongated, tubular vesicles,
thin vesicles walls
Elongated, tubular vesicles,
thin vesicles walls
pm
)
pm
)
pm
)
pm
13.63 5 .07
pm
)
pm
)
um
!3.63 5 .07
um
)
um
)
pm
)
um
!5.93 5 .05
g17
Quebrada del Tonco Ash:
g15
g29
g39
Villa Vil Ash:
g33
Buey Muerto Ash:
g27
g49
277049S/667489W
g69
257599S/657469W
g73
267319S/657449W
Alemanı́a Ash:
g21
a
277109S/667079W
267059S/657449W
g35
277049S/667499W
g37
267319S/657449W
g43
267059S/657449W
g45
267389S/667249W
g47
277109S/667089W
g65
257429S/657429W
um = unimodal; pm = polymodal.
Elongated, tubular vesicles,
thick vesicles walls
Elongated, tubular vesicles,
thick vesicles walls
Elongated, tubular vesicles,
thick vesicles walls
Elongated, tubular vesicles,
thick vesicles walls
Elongated, tubular vesicles,
thick vesicles walls
Elongated, tubular vesicles,
thick vesicles walls
Elongated, tubular vesicles,
thick vesicles walls
)
)
Journal of Geology
TEMPORAL DISTRIBUTION OF LARGE LANDSLIDES
otite is a minor or accessory constituent. In some
cases, accessory minerals such as zircon and apatite
were identified as intergrowths in biotite.
Tephra classification is based on chemical glassshard fingerprinting because of the unambiguity of
homogenous glass composition within a tephra
layer (e.g., Sarna-Wojcicki 1976; Westgate and Gorton 1981). Although these glass shards generally
have rhyolitic compositions, precise EMA revealed
significant differences, used to group several tephra
layers (table 1). Further discrimination is based on
EMA compositions of biotites; unimodal biotite
compositions were found for nearly half of the samples and are taken as evidence for primary air-fall
deposition. Conversely, redeposition of tephra layers is indicated by biotites with polymodal compositions (table 1).
Of the 26 tephra samples, seven related tephra
groups and two individual tephra could be identified (tables 1, 4). Among the different groups, one
tephra unit occurs only locally, whereas the others
are widely distributed throughout the entire area.
Since their exact eruption source is unknown, the
tephra are named after type localities. Terminology
is used after Fisher and Schmincke (1984). The term
“ash” is used for unconsolidated tephra with a grain
size smaller than 2 mm; tuff is the consolidated
equivalent of ash.
Table 2.
41
Figure 3. Histogram of Na2O concentration of EMA of
glass in ash layer g67 (60 samples).
Quebrada La Yesera Tuff. This tuff (g09, g67) is
a fine-grained and slightly lithified tephra deposit;
mafic minerals only occur as accessories. The glass
shards typically show Y-shaped bubble wall junctions or double concave plates (fig. 7a). This tuff
occurs as a continuous layer around 20 m below
the top of a terrace at the confluence of the Calchaquı́es and Santa Marı́a rivers in the southern
Median Glass Shard Major Element Composition of Tephra Layers in Northwest Argentina
Sample
n
Na2O
SiO2
MgO
Al2O3
K2O
CaO
TiO2
MnO
FeO
g09
g67
g71
g05
g07
g25
g31
g63
g61
g04
g17
g15
g29
g39
g33
g27
g49
g69
g73
g21
g35
g37
g43
g45
g47
g65
14
49
3
17
25
14
21
31
7
6
30
11
8
14
4
8
20
7
13
26
45
24
58
38
23
12
3.58
3.61
2.41
2.37
2.30
2.51
3.12
3.29
3.02
2.39
2.26
3.83
3.61
3.63
2.36
3.28
2.84
1.87
2.08
3.57
2.59
3.54
2.81
2.96
2.99
3.42
77.93
77.61
77.48
78.09
77.96
77.66
77.12
75.55
77.82
79.01
78.99
77.74
77.94
77.80
77.43
78.26
78.63
79.67
79.54
78.07
78.74
77.90
78.59
78.49
78.46
77.82
.10
.11
.06
.10
.09
.10
.09
.13
.05
.05
.04
.04
.04
.05
.14
.04
.05
.05
.05
.05
.04
.04
.05
.04
.04
.04
13.22
13.34
13.41
13.04
13.06
13.04
12.97
14.17
12.70
12.90
13.08
12.84
13.06
13.12
13.18
12.81
13.01
13.23
13.21
12.91
13.28
13.02
13.22
13.15
13.21
13.02
3.89
4.06
4.96
4.84
4.94
4.90
4.95
4.62
4.91
4.16
4.35
4.39
4.36
4.31
4.74
4.45
4.25
4.01
4.06
4.30
4.26
4.36
4.21
4.24
4.17
4.39
.51
.50
.87
.73
.73
.73
.73
1.08
.81
.83
.69
.48
.50
.50
1.03
.52
.52
.51
.51
.51
.52
.49
.50
.50
.50
.51
.10
.11
.07
.14
.13
.13
.14
.12
.08
.08
.07
.09
.08
.08
.17
.06
.09
.07
.07
.07
.07
.07
.07
.07
.08
.07
.06
.08
.04
.05
.06
.05
.06
.09
.04
.05
.05
.12
.10
.09
.04
.08
.05
.07
.09
.09
.09
.08
.09
.10
.08
.10
.62
.61
.60
.76
.76
.80
.78
.89
.55
.54
.47
.44
.42
.49
.90
.47
.49
.49
.44
.47
.47
.47
.46
.45
.46
.50
Note.
n = number of analyses. Values are in weight-percent oxide, recalculated to 100% on a fluid-free basis.
42
R. L. HERMANNS ET AL.
Cordillera Oriental (fig. 2a). In the Quebrada La Yesera (Cafayate section) and Casa de Los Loros sections (fig. 8g, 8h), the terrace is overlain by two
generations of rock-avalanche deposits. In the Quebrada La Yesera, these rock avalanches are in turn
overlain by the deposits of a landslide-dammed
lake; stratigraphic relations show that this lake was
most likely caused by the Casa de Los Loros rock
avalanche farther downstream. The age of the Quebrada La Yesera Tuff is not well constrained; the
AMS-14C age of mollusk shells from lake deposits
define a minimum age of 32,480 5 150 yr (Trauth
and Strecker 1999). The high degree of consolidation and deformation features suggests, however,
that the terrace deposits and the intercalated Quebrada La Yesera Tuff may be older than the unconsolidated Cerro Paranilla Ash (see next section)
sampled in a similar setting.
Cerro Paranilla Ash. The ash (g05, g07, g25, g31)
is relatively coarse grained and contains abundant
biotite crystals up to 1 mm in diameter. The average TiO2 content of the biotites typically ranges
from 4.8% to 4.9%, compared with values between
2.8% and 4.2% in biotites from other tephras. Glass
shards are pumiceous with elongated, tubular vesicles and thick vesicle walls (fig. 7b). The ash also
occurs in the Quebrada del Tonco and 90 km to the
south (figs. 8f, 8j, 9a). In the Quebrada del Tonco
(fig. 2a), it covers eroded Cretaceous strata and is
in turn overlain in direct contact by rock-avalanche
debris. Stratigraphically, this tephra layer is below
the Tonco Ash. Immediately west of Cerro Paranilla (fig. 8f), the Cerro Paranilla Ash occurs within
terrace gravel overlain by landslide debris. It is also
found as a lens in reworked landslide deposits,
which in turn are overlain by two younger landslide
deposits. Biotites from the Cerro Paranilla Ash bed
were dated with the 40Ar/39Ar method, resulting in
a plateau age of 723 5 89 ka and a concordant isochron age of 763 5 136 ka.
El Peñon Tuff. This tuff (g71) typically contains
Table 3.
Figure 4. Eigenvalues and cumulative proportion of total variance explained by the first eight principal components (cf. table 3).
biotite-bearing pumice lapilli up to 2–3 cm in diameter (fig. 7c). It is poorly lithified and appears to
be reworked as indicated by the large number of
lithic fragments. The tuff forms a continuous layer,
2–3 cm thick, about 80 m below the top of an uplifted pediment remnant on the western piedmont
of Sierra Laguna Blanca (fig. 8d). The 247 eastdipping pediment remnant contains a sequence of
at least three rock-avalanche deposits. A 21Ne-exposure dating of the avalanche deposits results in
minimum ages of ca. 140–200 ka for these deposits
(Hermanns 1999; Niedermann and Hermanns
1999). However, since the rock-avalanche deposits
do not show any evidence of deformation, the tilted
pediment and intercalated El Peñon Lapilli Tuff are
probably significantly older.
Of similar geochemical composition is an unlithified ash lens (g61) with tubular pumiceous
shards and elongated, tubular vesicles and thick
vesicle walls. The ash occurs in a basin on top of
the youngest of five landslide deposits (figs. 8a, 9b)
Tabular Output from the Principal Component Analysis
Principal
component
number
Eigenvalue
of Cov (X)
1
2
3
4
5
6
7
8
4.36e
1.50e
9.97e
6.05e
3.41e
1.17e
6.57e
1.88e
1
1
2
2
2
2
2
2
00
00
01
01
01
01
02
02
Percentage variance
captured this PC
Percentage variance
captured total
54.45
18.73
12.46
7.57
4.26
1.47
.82
.24
54.45
73.19
85.65
93.22
97.48
98.94
99.76
100.00
Note. For each principal component, the table gives the value of the associated eigenvalue of the covariance matrix Cov (X) (i.e.,
correlation for autoscaled data), the percent variance captured by the PC, and the cumulative variance captured.
Journal of Geology
TEMPORAL DISTRIBUTION OF LARGE LANDSLIDES
Figure 5. Data points for the first versus second principal component (PC) calculated for individual ash layers
from table 2.
in the western Aconquija piedmont (Hermanns and
Strecker 1999). It lies 2 m below the top of the basin
fill. In this area, the age is relatively defined by the
few-decimeters-thick carbonate cementation. Pediment surfaces several tens of kilometers away
with similarly advanced carbonatization have 40Ar/
39
Ar and fission-track ages of 0.6–1.2 Ma (Strecker
1987). This ash is significantly older than the Buey
Muerto and the Alemanı́a ashes because of its stratigraphic position. In contrast to the tephra sample
from the Sierra Laguna Blanca pediment, this tephra is unlithified and does not contain lapilli fragments; it is believed to belong to the same eruptive
event because of its geochemical composition and
similar age at both localities. The state of lithification may be because of different depositional histories, and the difference in grain size because of
the distance to eruptive centers. While Sierra Laguna Blanca lies close to active Quaternary volcanoes (some tens of kilometers), the distance from
these centers to the Sierra Aconquija is 1100 km.
Ruinas del Rincón Ash. The ash (g63) is exposed
in an erosional cut at the base of a landslidedammed basin in the piedmont of Sierra Chango
Real at the southeastern Puna border (fig. 2). The
ash is therefore older than the Villa Vil Ash sampled
from a surficial layer on the landslide debris (fig.
8c). It is unlithified and contains irregularly shaped
pumiceous glass characterized by spherical to oval
vesicles with thin walls (fig. 7d). The ash is 5 cm
thick and has been reworked.
El Paso Ash. The ash (g04, g17) occurs at two
43
localities (fig. 9c). It is unlithified and contains tubular pumiceous shards with elongated vesicles and
thick vesicle walls. It overlies the older of two landslide deposits (fig. 8g). Biotites provide a 40Ar/39Ar
plateau age of 54 5 221 ka, which is not significantly different from a zero age. The young age is
corroborated by the age of mollusks from lake sediments, 32,480 5 150 yr, that also overlie the El
Paso landslide. Because of the duration of this lake
(roughly 3000 yr, estimated by varve counts; Kleinert et al. 1997), this ash has a maximum age of
about 36 ka. The El Paso ash correlates geochemically with a tephra layer sampled 135 km to the
south in the footwall area of the Sierra Aconquija
reverse fault. At this locality, it documents a minimum offset of this fault of 10 m since deposition.
Quebrada del Tonco Ash. The ash (g15, g29, g39)
occurs as a 15-cm-thick continuous layer in a small
basin on the landslide deposits in the Quebrada del
Tonco (fig. 8j). It belongs to a widespread tephra
layer of polymodal biotite composition, indicating
retransportation (fig. 9d). The glass is pumiceous,
tubular shaped, and contains elongated vesicles.
About 200 km south at Villa Vil, the same unit
exists as a lens in reworked landslide debris (fig.
8b). The chronostratigraphic position of the Que-
Figure 6. Dendrogram showing relationships between
individual samples and sample groups, using Mahalanobis distances based on raw principal component scores
of autoscaled EMA data. Data were log-ratio transformed
using Al2O3 as divisor, after removal of outliers, in order
to ensure independence (on an oxide-to-oxide basis).
Lower values indicate more similar geochemical composition; larger values indicate more different composition.
44
Table 4.
R. L. HERMANNS ET AL.
Averaged Chemical Composition (and 1j Errors) of Tephra Groups
Na2O
Quebrada La Yesera Tuff
Cerro Paranilla Ash
El Peñon Tuff
El Paso Ash
Quebrada del Tonco
Ash
Buey Muerto Ash
Alemanı́a Ash
3.60
2.64
2.71
2.33
(.02)
(.36)
(.43)
(.10)
SiO2
77.77
77.75
77.65
79.00
(.23)
(.39)
(.24)
(.02)
MgO
.11
.10
.05
.05
(.00)
(.01)
(.01)
(.01)
Al2O3
13.28
12.95
13.06
12.99
(.08)
(.18)
(.50)
(.12)
K2O
3.98
4.89
4.93
4.26
(.12)
(.06)
(.04)
(.13)
CaO
.50
.67
.84
.76
(.01)
(.13)
(.04)
(.10)
TiO2
.11
.15
.08
.08
(.00)
(.04)
(.01)
.01)
MnO
.07
.05
.04
.05
(.01)
(.01)
(.00)
(.00)
FeO
.61
.80
.57
.50
(.00)
(.07)
(.04)
(.05)
3.69 (.12) 77.83 (.10) .04 (.01) 13.01 (.15) 4.35 (.04) .49 (.01) .08 (.01) .10 (.01) .45 (.03)
2.52 (.66) 79.03 (.69) .05 (.00) 13.06 (.20) 4.19 (.20) .52 (.01) .07 (.01) .07 (.01) .47 (.02)
3.15 (.41) 78.26 (.38) .04 (.00) 13.11 (.15) 4.28 (.08) .51 (.01) .07 (.00) .09 (.01) .47 (.02)
brada del Tonco Ash is best defined in the Cafayate
section (fig. 8g), where it is overlain by lake deposits
containing mollusk shells dated at 32,480 5 150 yr.
Villa Vil Ash. This redeposited ash (g33) occurs
within sediments of a landslide-dammed lake at the
southeastern Puna border (figs. 2a, 8b). The ash typically shows strong intergrowths of tiny biotites
with irregularly shaped pumice. The pumiceous
glass has spherical to oval vesicles and thin vesicle
walls. AMS-14C-dated organic material from lake
deposits 5 m above the Villa Vil Ash yields a minimum age of 1311 5 132 yr for both the tephra and
the landslide (Fauque 2000).
Buey Muerto Ash. This ash (g27, g49, g69, g73)
belongs to an extensive deposit (fig. 9e) characterized by tubular pumiceous glass with elongated,
tubular vesicles with thin walls. It has polymodal
biotite compositions at all sites and occurs in the
western piedmont of Sierra Aconquija (fig. 8a)
within a terrace deposit that covers a rock-avalanche deposit. In the vicinity of Villa Vil (fig. 8b),
an equivalent ash occurs within reworked landslide
debris, whereas at the Cafayate section, it was
found within reworked lake deposits (fig. 8g). The
ash is constrained by peat deposits with an age of
3632 5 70 yr (Beta-122114) overlying it in direct
contact, which are in turn overlain by the Alemanı́a
Ash (fig. 8e). This age correlates with an ash sampled in the Lerma basin south of Salta (fig. 2a), dated
at 4400 5 190 to 6300 5 250 yr by over- and underlying organic horizons, respectively (Malamud
et al. 1996).
Alemanı́a Ash. This ash (g21, g35, g37, g43, g45,
g47, g65) is widespread in Holocene deposits (fig.
9f). It contains small amounts of tiny and thin biotites, which are difficult to analyze by EMA. Similar to other tephras, the pumiceous glass typically
has elongated, tubular vesicles and thick vesicle
walls. East of Cerro Paranilla (fig. 8f), it occurs
about 1 m below the top of an alluvial-fan deposit.
At Alemanı́a (fig. 2), it is 1 m below the top of a
terrace gravel overlying lacustrine sediments related to a rockfall (fig. 8i). The maximum age of
3632 5 70 yr (confined by the dated underlying
peat deposit in the Cumbres Calchaquı́ basin; fig.
8e) is consistent with the age of charcoal of
4540 5 40 yr (Beta-129760) sampled about 1.20 m
below the ash-bearing terrace gravel at Alemanı́a
and with mollusk shells sampled from an unknown
position of underlying lake sediments having an age
of 5926 5 50 yr (fig. 8i; Wayne 1999).
Discussion
In most cases, the occurrence of large landslides in
the study area is restricted to two different environments: (1) narrow valleys with steep walls characterized by rock anisotropies oriented toward the
valley and/or young thrust faults and (2) broad piedmont regions with alluvial fan–covered pediments
in front of steep mountain fronts characterized by
a variety of rock-strength anisotropies and active
range-bounding faults (Hermanns and Strecker
1999; Strecker and Marrett 1999). The oldest landslides documented exist in unrestricted piedmont
regions, whereas late Pleistocene and Holocene
landslide clusters prevail in narrow and deeply incised valleys.
Ages of Landslides. All landslide deposits in piedmont regions rest on top of alluvial fans or fancovered pediments. Possible tephra layers predating
these landslides are most likely covered or eroded
and were not found in shallow depth exposures.
Tephra postdating the landslide deposits lie on the
deposit surfaces or are, rarely, preserved together
with other detritic and air-transported sediments
in small basins behind the frontal rims of lobate
deposits. The morphology of landslide source areas
indicates that the piedmont landslide deposits are
significantly older than the landslides clustered in
narrow valleys (fig. 10). Typically, breakaway scarps
and sliding surfaces are eroded or poorly preserved.
The inference of an old age for these deposits is also
supported by the degree of soil development on the
avalanche deposits. For example, the frontal rims
and lateral levees of the Aconquija slides are cemented by several decimeters of carbonate hori-
Journal of Geology
TEMPORAL DISTRIBUTION OF LARGE LANDSLIDES
45
Figure 7. Scanning electron micrographs of glass shards of ash units g67, g07, and g63, and field photograph of the
lapilli tuff g71. a, Mostly bubble wall Y-junction shards and a few bubble wall shards with large radius or curvature.
b, Tubular pumiceous shard with elongated, tubular vesicles or “capillaries” and thick vesicle walls. c, Lapilli tuff
with 2–3-cm large lapilli (arrows indicate thickness of tephra layer). d, Irregularly shaped pumiceous shard with
spherical to oval vesicles and thin vesicle walls.
zons, related to eolian carbonate input and indicating an age of several hundred thousand years
(Fauque and Strecker 1988). Similar horizons associated with tephra incorporated in old pediment
surfaces in the neighboring Calchaquı́ Valley were
dated by the 40Ar/39Ar and fission-track methods
and are on the order of 0.6–1.2 Ma (Strecker 1987).
Cosmogenic-nuclide dating of a series of landslides
on the western Sierra Laguna Blanca piedmont has
resulted in exposure ages of 389 1 17/ 2 24 to
139 1 16/ 2 22 ka, also documenting greater ages
for landslide deposits in piedmont regions (fig. 10;
Hermanns 1999; Niedermann and Hermanns
1999).
Multiple landslides in narrow valleys perpendicular or oblique to structural trends commonly occur together with terrace and lake deposits. Pre-
served erosional remnants of tephra-bearing
deposits are as old as 723 5 89 ka (Cerro Paranilla),
but such deposits are rare exceptions. Most avalanche deposits in these valleys cluster into two
distinct time periods (fig. 10). The earlier and quantitatively more important cluster occurs at about
33,000 yr. In the Cafayate section, the older of two
landslide deposits is overlain in up- and downstream directions by lake sediments. The lake sediments upstream have an age of 32,480 5 150 yr
(Trauth and Strecker 1999). These sediments are in
turn covered by a younger landslide mass that contains injection dikes of lake sediment, indicating
that the sediments at that time were still water
saturated (Hermanns and Strecker 1999). Varve
counts in an undeformed lacustrine section suggest
that the lake existed for about 3000 yr before the
Figure 8. Composite profiles of 10 localities (indicated in fig. 2), showing the stratigraphic relations of different deposits and
tephra layers. All sections are drawn in the same scale, except e. Note that tephra lenses sampled from redeposited landslide debris
(e.g., b) postdate stratigraphically lower landslides but do not necessarily predate stratigraphically higher landslides.
Journal of Geology
TEMPORAL DISTRIBUTION OF LARGE LANDSLIDES
47
Figure 9. Sample sites and minimum spatial distribution (hachured areas) of six tephra layers (a–f). Thickness of
tephra layer is given in brackets in all cases where the tephra exists as a continuous layer for several meters. Shaded
areas correspond to regions at 3000 m elevation. Crosses denote sampling localities.
second landslide covered the unconsolidated sediment (Kleinert et al. 1997). Landslide deposits from
the adjacent Casa de Los Loros section must be
relatively coeval because the lake sediments in the
Cafayate section from the downstream direction
resulted from a barrier located at the Casa de Los
Loros section (Hermanns and Strecker 1999). Landslides at Villa Vil and in the Quebrada del Tonco
also seem to correlate with this time of enhanced
landsliding activity. The deposits are overlain by
the Quebrada del Tonco Ash, which postdates the
older landslide deposits in the Cafayate section,
giving a minimum age for these deposits. The interpretation that these minimum ages are close to
the true age is supported by morphologic observations of the breakaway zones of the landslides.
All of these landslides and two landslides from the
Cerro Paranilla area were generated in coarse clastic sedimentary rocks, which dip toward the valley.
Breakaway scarps are well defined, and sliding sur-
48
R. L. HERMANNS ET AL.
Figure 10. Volcanic ash, radiocarbon, and cosmogenic-nuclide stratigraphy for the studied landslides. Sequences are
based on the chemical similarities of individual ashes, which displayed high degrees of correlation throughout all
the statistical analyses. Ca. = Casa, Q. = Quebrada, S. = Sierra, C. = Cumbres.
faces are only weakly eroded, attesting to a relatively youthful age. In addition, multiple breakaway surfaces and two separate lacustrine units
caused by landslides in the Quebrada del Toro dated
at 33,550 5 190 and 29,580 5 130 yr also suggest
pronounced landsliding activity in northwestern
Argentina during this period (Trauth and Strecker
1999). Ages of these landslide dams are probably
within an error range of several hundred to a few
thousand years, similar to the age of the related
lacustrine deposits. This interpretation is based on
observations from the Cafayate sections and empirical data of landslide-dammed lakes showing
that 85% of landslide-dammed lakes last !1 yr
(Costa and Schuster 1988).
The younger, less pronounced cluster of landslides is Holocene in age and is made up of the
youngest rock avalanche near Villa Vil, the large
rock fall at Alemanı́a, and possibly a rock avalanche
deposit at Brealito (fig. 10). Near Villa Vil, the landslide was dated with a minimum age of 1311 5
132 yr on organic material from a small landslidedammed lake (Fauque 2000). The lake deposit and
the Alemanı́a rockfall are overlain by the ∼3700-yr
Alemanı́a Ash. The landslide at Brealito is also inferred to be of Holocene age because of the pristine
morphology of the breakaway scarps, intact sliding
surfaces, and depositional morphology; 1-cm-deep
and sharp-edged grooves, produced by the movement of the rock mass on the smooth sliding surface, are perfectly preserved. This surface provides
a 21Ne-exposure age of !24,000 yr. However, this
age is considered to be too old, as no cosmogenic
Ne was detected unequivocally. All excess 21Ne
could be characterized as radiogenic Ne; hence, this
age represents the maximum analytical error (2j).
Coeval with this age group may also be a giant
granitic rockfall deposit with pristine surface mor-
Journal of Geology
TEMPORAL DISTRIBUTION OF LARGE LANDSLIDES
phology which was generated 14 km northwest of
Brealito (fig. 2a).
Significance of Temporal Landslide Distribution.
The pronounced differences in the temporal distribution of landslides in narrow valleys versus broad
piedmont regions result from distinct conditioning
parameters in both environments. Tectonically and
lithologically, both settings are similar and prone
to generating large landslides. Evidence for repeated
Quaternary fault movements along steep mountain
fronts in combination with a variety of rockstrength anisotropies suggests that paleoseismicity
played an important role in generating large rock
avalanches in these environments (Hermanns and
Strecker 1999). However, the late Pleistocene and
Holocene clusters of landslides in northwestern Argentina are coeval with increased humidity and
runoff in this part of the Andes (e.g., van der Hammen and Absy 1994; Grosjean et al. 1997; Turcq et
al. 1997; Trauth and Strecker 1999). With respect
to the total number, the most important landslide
clusters in valleys lie in the Quebrada de Cafayate
and in the Quebrada del Toro. Both valleys are characterized by large catchment areas of up to 19,800
km2 and important runoff (EVARSA 1994; Malamud et al. 1996) originating in regions with elevations 14000 m that were also affected by multiple
late Pleistocene glaciations (Fox and Strecker 1991).
Glacial advances could be dated in the Cordillera
Oriental at ca. 27,970 5 190 and after 5280 5 200
yr (Zipprich et al. 1998). Higher runoff in the course
of climate change would have resulted in enhanced
scouring, undercutting, and landsliding along the
structurally preconditioned mountain fronts and
valley walls. Similar relations have been demonstrated for enhanced landsliding activity in other
regions of the world (e.g., Reneau and Dethier
1996). Furthermore, greater humidity and enhanced seasonality may have increased groundwater flow, which increased seepage body forces
and lowered critical thresholds in rocks susceptible
to failure by changing the effective stress (Trauth
and Strecker 1999). Under such circumstances, in
addition to scouring, landsliding could have been
aseismic or triggered by even smaller earthquakes
with lower levels of ground motion.
The multiple occurrences of young landslide deposits in these valleys is remarkable, but the quantity of preserved landslide deposits in the stratigraphic record may not represent the total number
of Quaternary landslide deposits in these environments. Because of the highly dynamic environments in these narrow valleys, older deposits may
have been removed. For example, cut-and-fill terrace deposits upstream from the approximately
49
29,500-yr landslide deposits in the Quebrada del
Toro attest to repeated temporary base-level
changes in this area that may be caused by a repeated already-obliterated landslide barrier. Although there are no unambiguous relations between these deposits and landslide remnants, the
spatially limited occurrence of multiple-terraced
deposits immediately upstream from the landslide
area suggests a causative relationship. The dominant occurrence of late Pleistocene and Holocene
landslides is therefore a combined result of increased mass movements in the deeply incised valleys of the Cordillera Oriental at these times and
the low preservation potential of older deposits of
similar origin.
Although the ages of piedmont-region landslides
are more difficult to define, these landslide deposits
have a higher preservation potential. It is likely that
these environments are less sensitive to climatic
change, as no clear correlation of landslide occurrence with humid climates exists. The landslide
ages also do not correlate with any other climatedriven morphologic changes in the Argentinian Andes (Siame et al. 1997). Because of the large distance
between the avalanche sites and trunk streams, the
higher discharge of these rivers during wetter climatic conditions did not affect mountain fronts in
this setting. In addition, mountain-front failure in
this environment is not correlated with glacial processes, such as in the Alps, for example (e.g., Porter
and Orombelli 1981), because mountain-front failure occurred on slopes below the glaciated parts of
the ranges or because avalanche deposits are situated along mountain fronts without any sign of glacial overprint.
Instrumentally recorded large earthquakes are
rare in this region (Chinn and Isacks 1983). However, this region is characterized by the occurrence
of frequent low-magnitude events (Cahill et al.
1992; Assumpção and Araujo 1993). Nevertheless,
large prehistoric earthquakes must have occurred,
as documented by the formation of Quaternary
fault scarps along reverse-fault-bounded mountain
fronts and within late Pleistocene fan deposits
(Strecker et al. 1989; Cahill et al. 1992; Marrett et
al. 1994; Hermanns 1999). For example, the maximal 36,000-yr-old El Paso Ash, sampled from the
southern Sierra Aconquija reverse fault, indicates
a minimum displacement of 10 m, suggesting that
several strong, shallow-seated earthquakes have occurred along this mountain front. Because of the
low frequency of large landslides at mountain
fronts with broad piedmonts, it is plausible that
cliff collapses in this environment were most likely
triggered by large earthquakes with long recurrence
50
R. L. HERMANNS ET AL.
intervals. We cannot unambiguously prove the seismic origin of these landslides since no other independent dateable paleoseismic features are preserved. This is in line with observations by
Densmore and Hovius (1999), who suggested that
seismic events focus landsliding to ridge crests and
hilltops where effects of fluvial erosion are negligible, while climatic events trigger landslides near
hillslope toes.
Conclusions
Two settings with distinctive temporal distribution
for generating large landslides exist in northwestern Argentina: narrow valleys and broad piedmont
regions. While landslides in narrow valleys can often be accurately dated by tephrochronology, occur
in temporal clusters, and are relatively young, the
age of landslides in piedmont regions is more difficult to constrain because of the different depositional environment. In general, however, large
landslides in piedmont regions are significantly
older, and a tendency toward temporal clustering
in the late Pleistocene and Holocene is not observed. We suggest that the young landslide clusters and higher landsliding frequency in narrow valleys represent effects of climatic change affecting
the distant high-catchment areas. In contrast, large
landslides along tectonically active mountain
fronts paralleling broad piedmont regions cannot be
correlated with climate change; for these regions,
we suggest that the more likely trigger mechanism
was low-frequency/high-magnitude seismic events.
Landslides in these settings may thus represent effects of important paleoseismic events.
At a larger regional scale, the tephra associated
with landslide-related deposits in northwestern Argentina reveals an explosive Quaternary eruptive
history of the CVZ. Although the distal tephra record analyzed here is not complete and mainly involves late Pleistocene deposits, at least nine important rhyodacitic eruptive events are recorded.
The identification and correlation of tephra thus
provides a chronostratigraphic framework for Quaternary deposits in northwestern Argentina and defines three important temporal marker beds at
723 5 89, ∼30, and ∼3.7 ka.
ACKNOWLEDGMENTS
This work is part of the Collaborative Research
Center 276 “Deformation Processes in the Andes”
supported by the German Research Council in a
grant to M. Strecker. The success of this project
was made possible through the support of many
friends and colleagues, namely B. Aban, F. Aban, R.
Alonso, A. Bourdin, I. Capdevila, L. Fauque, R.
Gonzales, F. Hongn, J. J. Marcuzzi, J. A. Salfity, A.
Villanueva, and J. Viramonte. EMA and SEM were
carried out at the GFZ Potsdam and at the Naturhistorisches Museum Berlin. We appreciate help by
O. Appelt, P. Claeys, D. Rhede, and E. Wäsch. C.
Fischer prepared most samples for analysis. R. Naumann (GFZ Potsdam) performed XRF and infrared
absorption analyses. The AMS radiocarbon date
was performed by Beta Analytic, Miami, Florida.
We thank N. B. Gallagher and B. M. Wise from
Eigenvector Research, Manson, Wash., for support
while using the Matlab PLS Toolbox. We gratefully
acknowledge thorough reviews by T. Jordan and
three anonymous reviewers.
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