UNIVER SIDAD DE CONCEPCIÓN
DEPARTAMENTO DE CIENCIAS DE LA TIERRA
10° CONGRESO GEOLÓGICO CHILENO 2003
KNICKPOINT MIGRATION IN MAIN RIVERS AND ITS TRIBUTARIES
OF ATACAMA DESERT (NORTH CHILE): TECTONIC AND
CLIMATIC SIGNATURES.
RIQUELME R1, 2, 3, DARROZES J1, HERAIL G2, CHARRIER R3.
1, 2, 3
, Convenio Universidad de Chile-IRD, Chile ([email protected]).
LMTG, UMR 5563, Univ. Paul Sabatier, 38 rue de 36 Ponts, 31400 Toulouse France ([email protected]).
2
IRD, UR 104, LMTG, 38 rue de 36 Ponts, 31400 Toulouse France ([email protected]).
3
Depto. Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile ([email protected]).
1
INTRODUCTION
Rivers network answers to abrupt base-level fall through an upstream migration (e.g. Whipple
and Tucker, 1999). This migration is marked by a stair known as knickpoint. The created
knickpoint transmit the base-level fall to all the point along the river profile and generally to all
the tributaries. This migration is registered in the topography by a moving knickpoint frontier,
know as knickzone, during time which separate areas adjusted to the new base level and areas
that not have. The abrupt base-level fall is generally connected to four principal phenomena:
tectonic, climate, stream capture and eustatism. The behaviour of knickpoint migration has
implication for the understanding of the landscape evolution and the dynamic between
tectonics, climate, and erosion/sedimentation.
Figure 1. The fore-arc and the
Altiplano-Puna zones at the
Atacama Desert level showing the
main physiographic units
STUDIED AREA
This study point out the influence, in the Atacama Desert (North Chile, 26°-28° SL), of these
phenomena through morphometric and field/imagery analyses of drainage network. The chosen
area is located in northern Chile and it reflects the gradual transition between a steeply deeping
subduction slab in north and a flat-slab subduction in south. We can divide the region
Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad.
in five North-South oriented physiographic units from West to East (Fig.1): the Coastal
Cordillera; the Central Depression; Precordillera; the Preandean Depression and the
Altiplano-Puna. In the northern part the Central Depression and the Precordillera occupy a
forearc position. In the southern part, coincident with the subduction changes, the AltiplanoPuna, the Central Depression and Preandean Depression disappear southward and leave place
in the central cordillera.
GEOMORPHOLOGIC ANALYSIS
To identify the main phenomena which control the geomorphologic evolution of our zone we
have used a combination of morphometric analysis (thalweg profile, slope versus downstream
relationship, log-bin averaged drainage area versus slope relationship) and ASTER/Landsat
imagery analysis.
Figure 2. Some longitudinal thalweg profiles of the Precordillera region in the Atacama
Desert. A. Streams with heads at altitudes smaller than 4000 m, showing a nickpoint for a 16
km2 drainage area. b. Streams with heads at altitudes greater than 4000 m, which do not show any
nickpoint.
The simplest manner to analyze a drainage network is to look at the river profiles of our zone
(Fig.2). We identified zone close to the steady state i.e. the main rivers (Fig.2b) and zone very
far away from steady state and we point out a knickpoint in all the tributaries (Fig.2a). The
analyse of slope downstream of different drainage basin for various scale indicates the
independence of the slope in function of the downstream distance (Fig.3) which can be
interpreted by drainage network in steady state (Lague, 2001) but the profile give an opposite
information due to the presence of knickpoint in tributaries.
Figure 3. The local slope doesn't depend
of the downstream distance. Example
taken from the El Salado River drainage
system in the Precordillera of the El
Salvador area.
We can explain this ambiguity by a system in detachment limited were the main rivers
respond to the global outlet while the tributaries respond to the local outlet i.e. the junction
between the tributary and the main river. To explain this phenomenon we must involve
different vertical incision rate between the main rivers and their tributaries. If we analysed
now the satellite imageries we observe that all the main river have their origins on the
volcanoes which limit the precordillera (e.g. Fig.4). These volcanos have more than 4000 m
high and at this altitude Aceituno (1993) demonstrate strong climatic variations. It shows in
particular that the Amazonian airstreams are blocked by the reliefs. These reliefs thus
concentrate moisture and the rainfall on these mountains; during this time for peak of lower
altitude (< 4000m) the rainfall fall to those of the Atacama desert i.e. around 4mm/y. All of
these information lead to a great flow variation between the main river with a high flow and a
high vertical incision rate and the tributaries with lower flow and incision rate. In this case we
can explain why we have a system in detachment limited with knickpoint in the tributaries.
We have here an orographic climatic control of the river incision. But this phenomenon
doesn’t explain the existence of the knickpoints in the whole region, it just explain the
evolution of the knickpoint.
Figure 4. Orographic control of the difference of the incision rate for the drainage system development. A. The
main stream of the El Salado Rivers (El Salado canyon) passes only some meters to the west of the Pedernales
Salar without capturing it. B. However, this canyon is born to the north in the hillslope of a
high volcano (the Doña Inés volcano, >5000 m). This indicates that most of the water supply
comes from the high altitude. Consequently the greater incision capacity is recorded in the
streams with heads at high altitudes as showing in the figure 2.
An other phenomenon must explain the knickpoint creation, one possibility is a base level
lowering but in our zone it never exceeds 150m and we observe knickpoint at 200 km of the
ocean which have an altitude of more than 250m. So the base level lowering not explains the
knickpoint formation. An over possibility is a local tectonic control due to a fault. On the field
two evidence of fault activity have been describe:
1.- One in the Atacama Fault System (Riquelme et al., 2002) in the Coastal Cordillera where
the displacement is evaluated near 300m. But the activity of the fault began near 18 My and
stop at 10 My. The fault activity termination is characterize by the installation of
pedimentation surface witch reflect an ancient base level. This base level can be observed
with a slope upper than those of the river profiles who try to the new base level. This indicate
an uplift post 10 My.
2.-One in the Precodillera but with displacement lower than 50 m (Audin et al, accepted)
which not explain the knickpoint observed here, and knickpoint are located in front of the
faults, so these two examples of tectonic activity have not generated the knickpoints.
A manner of identifying the origin of these knickpoints is to analyse the area-slope
relationship of the Salado river combine to the associated river profile (Fig.5). In this case, I
observe evidence of uplift rate change, weak at the beginning (upper Miocene) to high rate at
the present day time. The natural example of the Salado River is a good example of
theoretical modelling of Snyder et al. (2000). This uplift corresponds in fact to a tilting toward
the west estimated at least at 1° by various field arguments : The tilting of the pedimentation
surface (in the precordillera) and the tilting of playa deposits (in the central depression and
coastal cordillera). This tilting explains the increase of topography, more than 500m, during
the upper Miocene to the present day period and it is consistent with local climatic change
induce by the uplift; the volcanoes cross the orographic limit define by Aceituno (1993).
Figure 5. (left) For the El Salado canyon the local slope-drainage area relationships defines a straight line
displaced at 300 km2. This type of drainage area-local slope relationships has been obtained in numerical models
for a knickpoint migrating upstream as the channel responds to the uplift rate changes. (rigth) In the El Salado
canyon the 300 km2 drainage area corresponds to a knickpoint in the thalweg profile.
CONCLUSION
The use of the morphometric measuring instruments associated field analysis made possible
to show the strong interaction between climate and tectonics in the Atacama Desert. It permits
to point out the general tilting of the fore arc region due to the Andean belt uplift. These uplift
generate climatic change witch induce a local acceleration of the vertical incision (only for the
main river), this feedback effect induce the fast readjustment of the main river to the new base
level while the tributaries preserve the knickpoints which are the witnesses of the tilting.
REFERENCIAS.
Aceituno, P., 1993. Aspectos generales del clima en el Altiplano sudamericano. El Altiplano. Ciencia y
Conciencia en los Andes. Actas del II Simposio Internacional de Estudios Altiplanicos, 19 al 21 de Octubre de
1993, Arica, Chile (Charrier R., Aceituno P., Castro M., Llanos A., Raggi L. Eds.), 390 p.
Audin L., Hérail G., Riquelme R., Darrozes J., Martinod J., Font E., 2003, Geomorphic markers of faulting and
neatectonic activity along the Western Andean margin, Northern Chile., Quaternary Science Revue, accepted.
Lague, D., 2001. Dynamique de l’érosion continentale aux grandes échelles de temps et d’espace: modélisation
expérimentale, numérique et théorique. PhD, Université de Rennes I, France, 143 pp.
Riquelme R., Martinod J., Hérail G., Darrozes J., Charrier R. (2003) A geomorphological approach to
determining the Neogene to Recent tectonic deformation in the Coastal Cordillera of northern Chile (Atacama).,
Tectono., 361, 255-275.
Snyder, N.P., Whipple, K.X., Tucker, G.E., Merritts, D.J., 2000. Landscape response to tectonic forcing: DEM
analysis of stream profiles in the Mendocino triple junction region, northern California, GSA Bulletin, 112 (8),
1250-1263.
Whipple, K.X., Tucker, G.E., 1999. Dynamics of the stream-power river incision model : Implications for heigth
limits of mountain ranges, landscape response timescales, and research needs, Journal of Geophysical Research,
104 (B8), 17, 661-17,674.