ESurfEarth Surface DynamicsESurfEarth Surf. Dynam.2196-632XCopernicus PublicationsGöttingen, Germany10.5194/esurf-5-125-2017Autogenic versus allogenic controls on the evolution of a coupled fluvial
megafan–mountainous catchment system: numerical modelling and comparison
with the Lannemezan megafan system (northern Pyrenees, France)MouchenéMargauxmmouchene@tulane.eduvan der BeekPeterhttps://orcid.org/0000-0001-9581-3159CarretierSébastienMouthereauFrédéricUniversité Grenoble Alpes, CNRS, ISTerre, CS-40700, 38058
Grenoble, FranceGET, Observatoire Midi Pyrénées, Université de Toulouse,
CNRS, IRD, 14 avenue E. Belin, 31400 Toulouse, FranceDepartment of Geology, FCFM, Universidad de Chile, Santiago, Chilenow at: Department of Earth and Environmental Sciences, Tulane University,
New Orleans, LA 70118, USAMargaux Mouchené (mmouchene@tulane.edu)21February2017511251438August201617August20169January201730January2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://esurf.copernicus.org/articles/5/125/2017/esurf-5-125-2017.htmlThe full text article is available as a PDF file from https://esurf.copernicus.org/articles/5/125/2017/esurf-5-125-2017.pdf
Alluvial megafans are sensitive recorders of landscape
evolution, controlled by both autogenic processes and allogenic forcing, and
they are influenced by the coupled dynamics of the fan with its mountainous
catchment. The Lannemezan megafan in the northern Pyrenean foreland was
abandoned by its mountainous feeder stream during the Quaternary and
subsequently incised, leaving a flight of alluvial terraces along the stream
network. We use numerical models to explore the relative roles of autogenic
processes and external forcing in the building, abandonment and incision of a
foreland megafan, and we compare the results with the inferred evolution of
the Lannemezan megafan. Autogenic processes are sufficient to explain the
building of a megafan and the long-term entrenchment of its feeding river on
time and space scales that match the Lannemezan setting. Climate, through
temporal variations in precipitation rate, may have played a role in the
episodic pattern of incision on a shorter timescale. In contrast, base-level
changes, tectonic activity in the mountain range or tilting of the foreland
through flexural isostatic rebound do not appear to have played a role in the
abandonment of the megafan.
Introduction
Alluvial fans and megafans are prominent geomorphic objects of remarkably
conical shape, constructed by the accumulation of sediments at the outlet of
mountain valleys. They occupy a key position in the sediment routing system
and, as such, have been widely used as recorders of external forcing on
landscape evolution in a variety of settings. Controls on the building and
incision of these deposits, through alternating phases of aggradation and
erosion, have been shown to be related to climatic changes (Barnard et al.,
2006; Arboleya et al., 2008; Assine et al., 2014), tectonic activity
(DeCelles and Cavazza, 1999), base-level oscillations (Harvey, 2002) or a
combination of those factors (Abrams and Chadwick, 1994; Dade and Verdeyen,
2007; Schlunegger and Norton, 2014). Laboratory experiments reproducing
alluvial fan dynamics have helped understanding of the respective roles of
these controls on fan morphology, facies changes and cyclic
erosion–deposition processes (Kim and Muto, 2007; Nicholas et al., 2009;
Rohais et al., 2011; Guerit et al., 2014). Both analog and numerical
modelling studies have shown evidence for autogenic processes that could be
of critical importance in fan evolution (Humphrey and Heller, 1995;
Coulthard et al., 2002; Nicholas and Quine, 2007). Temporary sediment
storage on the fan results in cyclic behaviour, with alternating phases of
deposition and incision in the absence of external forcing (e.g. Coulthard et
al., 2002). This behaviour is expressed in the thresholds (in run-off, slope
or shear stress) and defined and implemented in the models. A critical value
must be reached and exceeded for transport to be effective; after some
further time steps, this parameter value decreases below the threshold and
deposition occurs again (Schumm, 1979; Roering et al., 1999; Whipple and
Tucker, 1999; DiBiase and Whipple, 2011).
Another level of complexity, often overlooked in previous experiments with
alluvial systems, comes from the strong coupling and feedbacks between the
source catchment and the basin. The specific response time and amplitude of
each part of the system to a given forcing may differ and this results in a
complex, oscillating erosion signal (Densmore et al., 2007; Humphrey and
Heller, 1995; Babault et al., 2005; Carretier and Lucazeau, 2005). Numerical
modelling by Pepin et al. (2010) suggested that autogenic processes play a
key role in the evolution of such a coupled system subjected to constant
external forcing. For these authors, permanent autogenic entrenchment can
occur in a coupled catchment–fan system without changes in boundary
conditions and external forcing when (i) the transport threshold (critical
shear stress) is significant and (ii) progradation is limited by an open
boundary with fixed elevation (e.g. a large river system at the foot of the
fan).
In the northern foreland of the Pyrenees (France), the Lannemezan megafan
was built during the Miocene by the erosional products of the mountainous
Neste river catchment, and was abandoned during the Quaternary (Mouchené
et al., 2017). The respective roles of climate and tectonics in this
evolution remain unresolved. In this study, we seek to test hypotheses on
the mechanisms at play in the abandonment and incision of the Lannemezan
megafan through numerical modelling of alluvial megafan construction and
abandonment. Although the complexity of this natural case might not be fully
reproduced by the numerical model, we run a series of model scenarios to
explore the respective effects of potential forcing factors, including
autogenic dynamics, climate change, tectonic tilting, and base level change,
on trends and patterns of incision (time and space scales, amplitudes) of
the megafan. Disentangling the respective signals of autogenic processes and
allogenic forcing requires understanding of (i) the wavelength and amplitude
of each signal, (ii) the possible buffering effects of the response times of
the fan and of the mountainous catchment, and (iii) the
amplification–reduction factors introduced by the coupling of the system.
The Lannemezan megafan
Whereas the drainage network in the Pyrenean range is regularly spaced and
mostly transverse to the structural trend, rivers of the northwestern
foreland spread in a radial pattern over the convex topography of large
Miocene alluvial fans (Fig. 1). The Lannemezan megafan is the most prominent
geomorphic feature of the northern Pyrenean foreland, with a surface of
13 000 km2 and a mean slope of 0.3∘. Its characteristic
semi-conical shape is outlined by the Garonne River (to the south, east and
north) and by the radial river network on its surface. It was built during
the middle Miocene (Biteau et al., 2006), while orogenic activity within the
Pyrenean range had already waned (Vergès et al., 2002; Sinclair et al.,
2005). Molasse-type deposits of middle- to late-Miocene age, with rounded
pebbles and boulders in an abundant clayey and sandy matrix, make up most
of the megafan volume (Paris, 1975; Azambre et al., 1989). These are capped
by the Lannemezan Formation, consisting of (i) an upward-fining,
stratified clay and sand sequence that contains strongly weathered gravel and
pebbles in a very fine matrix, dated by a hipparion-bearing fauna at its
base as late Miocene to Pliocene (“Pontico-Pliocene”; Paris, 1975;
Azambre et al., 1989) and (ii) a Quaternary sheet of very similar
composition.
The Lannemezan megafan and Neste catchment in the central northern
Pyrenees (inset map shows location in southern France).
The Neste River exits the mountain range at the apex of the megafan and thus
most probably provided the material for building the megafan. Comparing sediment
volumes and thermochronology-derived exhumation rates, Mouchené (2016)
showed that the relatively small Neste catchment could have exported a
sufficiently large sediment flux to produce all of the megafan deposits.
Contrary to the neighbouring Garonne valley, the Neste valley is believed to
have mostly been exempt of glaciers during the Quaternary, and no glacial
features are known on the megafan itself (e.g. Delmas, 2015).
The Lannemezan megafan is currently disconnected from its source catchment
and is being incised. The Neste turns sharply to the east at the megafan
apex and incises the fan head ∼ 100 m vertically before
merging with the larger Garonne River at its mountainous outlet (Fig. 1);
this drainage pattern suggests the capture of the Neste by the Garonne.
During incision, the rivers of the northern foreland (including the Neste,
Garonne and fan rivers) left a series of alluvial terraces. Mouchené
et al. (2017) dated the abandonment of the fan and the onset of incision at
≥ 300 ka from 10Be and 26Al cosmogenic nuclide dating of the
fan surface and terraces along the Neste valley. The episodic abandonment of
these river terraces during incision may be related to changing fluvial
dynamics during shifts between Quaternary cold and warm phases (Mouchené
et al., 2017).
Model description
We use a recent version of the CIDRE code, which models landscape evolution
in a continental setting (Carretier et al., 2015). We recall here the main
characteristics of the code and refer the reader to Carretier et al. (2015)
and references therein for further details.
At the beginning of each time step, a specified volume of water is
distributed homogeneously over the cells making up the model surface. The
propagation of water and sediment is performed in cascade, from the highest
to the lowest cell and following decreasing elevation, to ensure mass
conservation. A multiple-flow algorithm is used to propagate the water flux
to downstream cells proportionally to the slope in each direction (Murray
and Paola, 1997; Coulthard et al., 2002; Carretier et al., 2009), allowing a
distributary drainage pattern to develop.
Mass balance
During a time step ∂t, the elevation z of a grid cell
changes as follows:
∂z∂t=-ϵ+D+U,
where ϵ is a local erosion (detachment or entrainment) rate,
D is a local deposition rate and U is an uplift or subsidence
rate.
The local deposition rate D is defined as
D=qsL,
with qs the incoming sediment flux per unit width and
L the transport length. The transport length L determines
the proportion of incoming sediment flux that is deposited in the cell: a
large L results in little deposition, such as a steep slope or high
water discharge would favour in natural settings. The cell outflux per unit
width qs is the sum of the sediment detached from a given
cell plus the sediment eroded upstream that crossed this cell without
being deposited; it is thus non-local (e.g. Tucker and Bradley, 2010). This
approach is generalized for both hillslope and fluvial processes by
specifying ϵ and L in both cases.
Hillslope processes
The approach used by Carretier et al. (2015) is different from the
non-linear diffusion model proposed by previous authors (Roering et al.,
1999; Carretier et al., 2009, 2014). Instead, in this model the elevation
variation results from the difference between a local detachment rate and a
deposition rate using Eqs. (1) and (2), where ϵ erosion rate and transport
length L are defined as
ϵ=κSL=dx1-(S/Sc)2,
where κ is an erodibility coefficient, S is the steepest slope and
Sc is a critical slope. If the slope is steeper than
Sc, ϵ is set such that S=Sc. The
detachment rate is proportional to the local gradient, but the deposition
rate (qs/L in Eq. 2) depends on the slope and critical slope:
when S≪Sc, most of the sediment entering a cell is
deposited there and when S∼Sc, L becomes infinity and
there is no deposition on the cell.
Fluvial processes
For fluvial processes, a detachment algorithm including a threshold is used
for sediment and bedrock:
ϵ=K(ktqmSn-τc)pL=ξq,
where K is an erodibility coefficient, q is water
discharge per unit flow width on the cell, S is slope, and the
exponents m, n, and p are positive. kt is
the shear stress parameter so that ktqmSn=τ (shear
stress) and Eq. (5) takes the classic form of the excess shear-stress
formula (Tucker, 2004). τc is the critical shear stress
required for clast detachment. p is set to 1 in our experiments
(following Lavé and Avouac, 2001). The transport length L
depends on particle size and density (included in the coefficient ξ).
This law implies that the deposition rate decreases when the water discharge
per unit width q increases.
For fluvial processes, the flow width w can be set to the cell width
dx or to a river width such as
w=kwQ0.5,
where kw is a coefficient depending on the lithology and Q is the
total water discharge at a river section. Flow-width variation is critical
in the modelling of alluvial-fan evolution because it plays a role in
avulsion processes, in the changing flow dynamics (a change in flux geometry
may lead to overflowing and a shift to distributive flow) and in incision
patterns (leaving alluvial terraces in some cases).
Cover effect
Erosion of sediment is different from that of bedrock (Eqs. 3 and 5),
and within the bedrock different layers can be defined by their respective
erodibility and detachment or slope thresholds (κ and Sc
for hillslope processes and K for fluvial processes). During a
time step dt, different layers can be eroded on a given cell: the
erosion of each layer consumes part of dt so that less time remains to erode
the underlying layer. This time reduction is taken into account by
multiplying dt by (1-volume layerwdxϵdt)
between layers. In this way, the “cover effect” of a sediment layer
covering the bedrock (e.g. Whipple and Tucker, 2002; Lague, 2010) can be
taken into account.
Lateral erosion
Flowing water can erode lateral cells, which are topographically above them
and placed in a lateral direction perpendicular to each downstream
direction. The lateral sediment flux Qsl is defined as a
fraction of the flux in the considered direction (e.g. Murray and Paola,
1997; Nicholas and Quine, 2007):
Qsl=αQs,
where α is a bank erodibility coefficient; it is specified for sediment and
implicitly determined for bedrock layers, proportional to their
erodibility (i.e. αsediment/αbedrock=Ksediment/Kbedrock with K from Eqs. 5 and 7).
Model set-up
The model simulates the evolution of a 100 × 150 km region split into a
foreland zone (100 × 100 km) and an uplifting mountain zone
(100 × 50 km; Fig. 2). The grid cell size is 500 × 500 m. The dimensions are
chosen to allow megafan building on an area matching that of the Lannemezan
megafan and to permit competing catchments to develop during the drainage
network growth phase; they correspond to a compromise between computing time
and spatial resolution. Our model has much larger dimensions than previous
experiments on coupled catchment–foreland systems (Tucker, 2004; Nicholas et
al., 2009; Pepin et al., 2010; Langston et al., 2015) and the
foreland / mountain width ratio is much higher than in previous work (Pepin et
al., 2010). The initial surface is a horizontal grid with a Gaussian
elevation noise (σ= 0.5 m) so we can study the system dynamics from
the start of drainage network growth.
The model grid consists of a mountain section, subjected to constant
uplift U, and of a flat foreland section; both sections are subjected to
precipitation. B.C. is boundary conditions; neither water nor sediment can
cross a closed B.C., while an open B.C. corresponds to transverse rivers of
fixed elevation capable of transporting both sediment and water fluxes out of
the grid.
Although the convergence stopped in the early Miocene, the Pyrenees are
still a high-relief mountain range, with long-term exhumation rates and
present-day uplift rates both on the order of 0.1–0.3 mm yr-1 (e.g.
Jolivet et al., 2007; Nguyen et al., 2016), possibly controlled by the
ongoing isostatic response to post-orogenic erosion. Because isostasy is
currently not included in the model, we impose block uplift of the mountain
part of the model at a constant rate of 0.3 mm yr-1 (except in
experiments where this is explicitly modified, see below). Homogeneous
precipitation is applied at a constant rate over the entire model (P= 1 m yr-1);
this parameter is modified in some experiments (Experiment 2a, b, c).
The sides of the mountain block (southern border and southern third of the
eastern and western borders) are closed; neither water nor sediment can exit
the grid through these. The other boundaries are open, corresponding to
transverse rivers of fixed elevation (0 m), and are able to transport both
sediment and water fluxes out of the grid. This is an approximation, as
these transverse rivers will obviously have a downstream slope and therefore
not be at a fixed constant elevation. However, the major rivers of the
Pyrenean foreland have very low gradient (< 0.5 %) and lie
significantly lower than the streams incising the megafan.
Building the megafan. Temporal evolution of the mean elevation
change in the foreland and mountain and map views of the mountain and
foreland landscapes (black lines are 100 m contour lines, water flux in blue
shades) at the time steps marked with arrows, through the megafan building
phase. (A) Drainage network initiates and propagates in the mountain block
through headward incision, while sediments are deposited along the front by
regularly spaced streams. (B) Deposits merge in the foreland to form a
bajada fed by a decreasing number of rivers as the mountain streams
enlarge their basins. (C) Mean elevation in the range stabilizes and
aggradation continues in the foreland, dominated by outflux of a central,
main channel as the more lateral streams are drained directly toward the
borders. (D) As aggradation continues, limited incision can occur along the
borders of the fan (here on the western border) but (D') those streams are
quickly refilled. (E) Similarly, temporary incision can happen near the
apex. (F) After 15.3 Myr the megafan is built.
We conducted a series of trial runs to adjust the relevant parameters in
order to reproduce the first-order morphological traits of the northern
Pyrenean foreland. In particular, the values for transport length
(L) for the erodibility of bedrock and sediments (respectively
kbr and kall) and the critical shear
stress (τc) need to be established. These parameters are critical
in the relief evolution but are generally poorly constrained.
Giachetta et al. (2015) provided a compilation of values for erodibility of a set of
lithologies in the context of the Iberian peninsula. However, these were
used for models where the critical shear stress is zero and should be
significantly different (approximately 1 to 2 orders of magnitude
larger) when τc > 0. The erodibility coefficient
also depends on the value of m (e.g. Carretier et al., 2009); we
therefore used a different value than that of Giachetta et al. (2015). In
any case, the erodibility of sediment should be larger than that of the
bedrock, the ratio between the two critically influencing the landscape
morphology. We thus tested this ratio and the transport length L in
order to reproduce the first-order characteristics of the northern Pyrenean
landscape. Resemblance between the model and the ASTER DEM of the Lannemezan
area was evaluated based on a number of morphological parameters (which need
to agree within 30 % to be accepted): in the range and foreland we
assessed maximum, minimum, and mean elevations, river spacing, and relief
(i.e. valley-to-ridge elevation difference); in the foreland we also
assessed the length, width and northward slope of the megafan. The duration
of the megafan-building and -incision phases is also compared to the
evolution of the Pyrenean foreland as described by previous authors (Crouzel
1957; Azambre et al., 1989; Mouchené, et al., 2017). The best-fit
parameters used for the model runs are presented in Table 1.
Pepin et al. (2010) suggested that the critical shear stress should be significant for
permanent incision to occur. We thus fixed a positive value for
τc (15 Pa) following Lavé and Avouac (2001) and
Pepin et al. (2010).
Parameters used in the experiments. (a) shows the fixed parameters
for all model runs. τc is the critical shear stress;
Kbr and Kall are the bedrock and sediment
erodibility respectively; m, n, and p are coefficients for the
fluvial erosion law; L is the transport length; and α is the
lateral erosion coefficient. (b) shows the model settings for the
experimental runs and results.
(a)τcKbrKallmnpLα15 Pa0.5 10-34 10-30.60.710.30.01(b)Settings Results ExperimentPrecipitationPrecipitationBase levelUplift ratePermanentTime of entrenchment afternumberrateoccurrenceentrenchmentend of building at 15.3 Myr(m yr-1)(fraction of time step)(m a.s.l.)(mm yr-1)(kyr)1-default1100.3Yes270Climate2a10.5100.3No–2a22100.3Yes1802b1sinusoidal00.3No–2c10.500.3Yes150Base level3a11-500.3Yes250Uplift4a1100Yes5004b1100.1Yes3104c1101Yes500Tilting5a1100 to 0.68No–5b1100 to 2No–ResultsMegafan building
We successfully reproduced the first-order morphology of a fluvial megafan
constructed on a low-elevation, stable foreland from the erosional products
of a slowly uplifting mountain-range-like block (Fig. 3). The drainage
network initiates from the area of transition between the mountain and
foreland blocks (Fig. 3, A). In the foreland, it propagates outward and fans
aggrade. Evenly spaced rivers (every ∼ 10 km) build small fans
and progressively lengthen their watershed towards the hinterland through
headward incision. The fans quickly merge into a bajada, on top of
which the flow is distributive (Fig. 3, B). At around 7.65 Myr, the mountain
range becomes fully connected (i.e. all cells of the mountain block are
connected to the base level through the river network) and the mountain
outflux is dominated by a few large rivers (∼ five). In the
meantime, aggradation continues in the foreland with a markedly conical
pattern. The rivers situated at the easternmost and westernmost ends of the
mountain range bend sharply to follow an along-strike course and quickly
reach the open model boundaries, constrained by their short distance to a
base-level outlet. In the following time steps, their watershed will
increase in size by retreat of the drainage divide towards the middle of the
range and the mountainous outlets of these streams will migrate towards the
nearest border (Fig. 3, C). At this stage, the foreland deposits are mainly
provided by a single central channel, the flow of which distributes
sediments largely over the whole foreland, now clearly defining a megafan
(the flow spanning 180∘ over the foreland).
Several episodes of temporary entrenchment (< 50 m) occur during the
building phase. They either concern the lower parts of the fan being incised
by headward incision (Fig. 3, D) or the apex being incised by the main stream
(Fig. 3, E). In both cases, within a few hundred thousand years the main
stream has brought sufficient material to the entrenched zone to refill it
and to overflow and become distributive again (Fig. 3, D and D'). This cyclic
pattern is expected on megafans (Leier et al., 2005) and shows that the code
mimics the natural fluvial dynamics of these settings.
In the long term, the mean elevation stabilizes in both the foreland and the
mountain. From about 9 Myr, mean elevation stabilizes in the range (Fig. 3, B)
but the elevation change remains slightly positive, which means that the
relief is eroded at a slower rate than the applied uplift rate (i.e. true
topographic steady state is not reached). Aggradation continues in the
foreland, although at a slow pace (0.015 to 0.02 mm yr-1 of mean
elevation change); the timescale of aggradation is thus larger than that of
the relief development. This is consistent with the observations of Babault
et al. (2005) from an analog model. For them, aggradation in the foreland
influences erosion of the mountain range by modifying the relative uplift
rate (i.e. the difference between the uplift applied to the mountain block
and the aggradation rate). Erosion of the range balances the continuously
varying relative uplift rate, creating a “dynamic equilibrium” (Babault et
al., 2005). A steady-state equilibrium (in which erosion rate equals uplift
rate in the mountain) cannot be reached in such a landscape as long as
aggradation occurs in the foreland.
Autogenic entrenchment
If the same conditions are maintained, natural entrenchment of the main
stream occurs rapidly over a timescale that is 2 orders of magnitude
smaller than the fan-building timescale (Fig. 4). Contrary to the building
phase, during which episodes of temporary entrenchment occurred but were
followed by refilling and overflow, the incision starting at 15.3 Myr near
the apex is sufficient to constrain the main stream avulsions to the eastern
half of the fan for the subsequent time steps (Fig. 4, G, H, I). A small
stream that developed on the eastern foot of the fan episodically captures
the main flow and is thus progressively deepened and incised through
headward incision.
Autogenic entrenchment. Temporal evolution of the mean elevation
change in the foreland and mountain and map views of the mountain and
foreland landscapes (black lines are 100 m contour lines, water flux in blue
shades) at the time steps marked with arrows, through the autogenic incision
phase (note change in timescale between this figure and Fig. 3). Starting
from the landscape obtained at 15.3 Myr (see F in Fig. 3), the fan is
incised when the main flow reaches the position of a small stream (e.g. G,
I) but continues to grow (H) when the main flux overflows and migrates again
on the fan. (J) After several of these cycles, the main flow is finally
captured permanently in the stream. (K) As the main river now incises
laterally towards the mountain front, a secondary stream is captured. (L) At
the end of the experiment a large valley is incised along the front of the
range. Topographic profiles across this valley (right panel) show that about
120 m of incision occurs in the foreland at the time of capture (black
profiles, inset J); subsequently the valley is mostly enlarged by lateral
erosion in the foreland, deepened and enlarged near the apex, and markedly
deepened in the mountain (red profiles, inset L; note that horizontal scale
is different for each profile).
Still, the main flow remains highly distributive and overflows
this path several times before finally being permanently captured at around
15.57 Myr
(Fig. 4, J). This event triggers a rapid incision phase, reaching nearly 150 m
of incision close to the apex and larger amounts further downstream. The
mean elevation change in the foreland drops dramatically upon entrenchment
and the fan is subsequently eroded (mean elevation change remains
negative for the rest of the experiment; Fig. 4). Erosion sharply increases
in the mountain, especially in the watershed of the now connected main
stream (Fig. 4). Figure 5 suggests that this incision leads to an increase
in the relief due to rapid incision in the riverbed and little to no
increased erosion on the hillslopes and ridges.
Temporal elevation change of three locations in the mountain: bed
of the main feeding river (red crosses), on nearby slope (black crosses) and
ridge (blue crosses, see map view in right panel for locations). Following
the entrenchment (marked with vertical grey line), the river rapidly
incises, increasing (temporarily) the relief, as ridge elevations are not
affected by the incision episode. The hillslope response is slow and lags
behind that of the riverbed.
The main stream then laterally erodes its right bank in the foreland,
tending towards an along-strike flow direction without further vertical
incision (Fig. 4, K, L). Incision occurs in this bank and oblique to it,
eventually capturing a secondary stream of the mountain range.
External forcing
Subsequent experiments start from the topography obtained at the end of the
“building phase” at 15.3 Myr and aim at evaluating the respective roles of
different external factors in the incision pattern of the megafan. We
consecutively explore the influence of changing parameters related to
climate (precipitation rate and frequency of precipitation events), base-level change and tectonics (uplift rate and style). These models are run for
500 kyr to evidence the effects of external factors on this specific
timescale, which corresponds to the abandonment and incision timescale of
both the model and the Lannemezan megafan. Parameters used for these
experiments are summarized in Table 1.
Precipitation rate and style
Decreasing (Experiment 2a1) or increasing (Experiment 2a2) the precipitation
rate only results in decelerating or accelerating the processes observed in
the original experiment. The same evolution is observed in experiment 2a1 as
using the default settings, but the evolution is slower and the model does
not reach the permanent entrenchment stage after the 500 kyr simulation. In
the experiment with increased precipitation rate (2a2), erosion is enhanced
and results in important widening of the valleys, but scattered deposition
in the lower valleys create instabilities that perturb the model results.
In experiment 2b, we set the precipitation rate to follow a sinusoidal
distribution with 100 kyr cycles to simulate the Quaternary climatic cycles.
In this case, the trends of mean elevation change in the mountain and in the
foreland are inversely correlated (Fig. 6). The mean elevation of the
foreland slightly increases through the experiment but remains stable in
periods of low run-off, as the sediment supply from the mountain is halted.
The mountain is eroded in periods of maximum run-off, whereas the elevation
increases (at the uplift rate) in periods of minimum run-off. There is a
slight delay in the mountain response to the variations in precipitation: as
the run-off starts to increase, the elevation in the mountain continues to
rise at the uplift rate for another time step (10 kyr) before it starts to
decrease (Fig. 6). Similarly, there is a small lag between maximum run-off and
minimum mean elevation change (Fig. 6). This delay corresponds to the
response time of the mountain to cyclic precipitation rate changes and is
consistent with works by Carretier and Lucazeau (2005) and Braun et al. (2015),
who suggested a 1 to 30 kyr offset between forcing and response to
rainfall variability at orbital (Milankovitch) timescales. In our
experiment, the same delay is observed in the foreland, although the signal
is less clear for periods of high run-off (Fig. 6).
Experiment 2b (sinusoidal precipitation). Temporal evolution
of (a) run-off and
(b) mean elevation change in the foreland and (c) in the
mountain. The trends of mean elevation change in the mountain and in the
foreland are inversely correlated, and they show a slight delay relative to the
change in run-off, corresponding to the response time. (d) Temporal elevation
change of the central valley floor in the mountain (location marked by red
cross on inset map) during Experiment 2b. The incision (blue arrows) related
to humid periods (blue vertical lines are maximum run-off) is (over-)
compensated for in drier periods (grey vertical lines correspond to no run-off)
by the uplift, so that the elevation generally increases through the
experiment. The maximum incision is delayed from the maximum run-off
(response time). Temporary deposits at the valley outlet around 230 kyr dam
the valley and trigger rapid backfilling of it, responsible for the high
elevation between 230 and 310 kyr; the following incision episode removes this
dam.
At the end of the 500 kyr simulation, no permanent entrenchment is observed
on the megafan. The small amount of incision that occurs on the fan when
precipitation decreases is more than compensated for by renewed sediment influx
from the mountain as precipitations start to increase again. The incision of
the riverbed in the mountain in periods of high run-off is more than
compensated for by the uplift in periods of low run-off (dominated by the
applied uplift; Fig. 6)
Base-level change
A 50 m drop in base level is applied at the beginning of Experiment 3a. This
leads to erosion in the foreland through headward incision of a number of
streams, developing mostly on the western and eastern borders and persisting
until the end of the experiment. Connection between the main feeder channel
and the largest incising stream on the eastern border happens earlier than
in the default model (at around 250 kyr) but the subsequent landscape
evolution is very similar in both cases, although more incised streams
remain at the end of this experiment (Fig. 7).
Model configuration at the end (t= 15.80 Myr) of Experiment 3a
(50 m drop in base level at the onset of the incision phase). The megafan is
incised by headward incision of a number of streams on its western and
eastern borders (and marginally on the northern border).
Uplift rate
Experiment 4a tests a scenario where uplift stops after the megafan building
phase. In this experiment, the headward-incising stream connected to the
eastern border captures a secondary river (at 300 kyr) before connecting to
the main central channel at the end of the experiment (500 kyr, Fig. 8). The
mean erosion rate in the range decreases steadily down to 0.19 mm yr-1
(value for last 10 kyr of the experiment).
Experiment 4a, the incised stream first connects through headward
incision to the secondary river (left, t= 300 kyr) before being connected to
the outlet of the central river at the end of the experiment (right,
t= 500 kyr).
Increasing the uplift rate to 1 mm yr-1 (Experiment 4c) quickly and
permanently increases the elevation in the range without significantly increasing the
aggradation in the foreland. This may be due to erosion
(detachment and/or transport) not responding rapidly enough to catch up with
this increase. Permanent entrenchment occurs at the end of the experiment
(500 kyr) through the same process as in the default experiment.
Tilting experiment
In Experiment 5, we seek to reproduce the effect of isostatic rebound on the
erosional pattern of the range and its foreland. At the moment, the CIDRE
model does not include flexure. We thus chose to simulate the first-order
effect of the flexural response to erosional unloading of the range through
simple linear tilting of the model. This corresponds to an uplift pattern
that increases linearly from zero at the northern boundary to a maximum fixed
value at the southern boundary.
To scale the tilting to the observed geomorphic characteristics of the
northern Pyrenean foreland, we estimate the potential tilting of the
Lannemezan megafan since the onset of incision. We use a scaling law between
fan area and fan slope to estimate the initial depositional slope of the
Lannemezan Formation that caps the Miocene deposits. We use this formation
because its base is the only mapped surface effectively preserved from
erosion since deposition. We use a digitized geological map and an ASTER DEM
(70 m resolution) to extrapolate the basal surface of the Lannemezan
Formation using ArcGIS software and we estimate its current slope at
0.5∘. Figure 9a shows area and slope data for the Lannemezan
megafan compared to data compilations from active and inactive alluvial fans
and megafans from the Alps, the Andes and the Himalayas (Horton and DeCelles,
2002; Guzzetti et al., 1997). The discrepancy of the Lannemezan data with
the scaling law suggests an estimated ∼ 0.4∘ tilt,
which is simulated by uplift increasing linearly from 0 at the northern
boundary to 2 mm yr-1 at the southern boundary of the model.
It should be noted that this scaling relationship suggests a depositional
angle of ∼ 0.1∘, which is within the range of
values for megafans (e.g. DeCelles and Cavazza, 1999) but is not consistent
with the slope observed in the default experiment (∼ 0.4–0.5∘).
We compare this result with the tilt estimated using
another often-used scaling relationship, between the catchment area and the
fan slope (e.g. Champagnac et al., 2008). Figure 9b shows area and slope
data for the Lannemezan megafan compared to data compilations from active
and inactive alluvial fans from the Alps (Guzzetti et al., 1997; Crosta and
Frattini, 2004; Champagnac et al., 2008). The discrepancy of the Lannemezan
data with this scaling law only suggests an estimated 0.13∘ tilt,
which will be simulated by uplift increasing linearly from zero at the
northern
boundary to 0.68 mm yr-1 at the southern boundary. We test both these
minimum (0.13∘, Experiment 5a) and maximum (0.4∘,
Experiment 5b) tilt scenarios.
(a) Scaling relationship between fan area and fan slope for
alluvial systems of the Alps, the Andes and the Himalayas; data from Guzzetti
et al. (1997; crosses) and Horton and DeCelles (2002; open circles). Thick
line is the best power-law fit to the combined data:
Sf= 2.42 Af(-0.30).
Tilt (vertical arrow) for the Lannemezan megafan (red circle) is estimated
as the difference between present-day slope and predicted slope from the
power-law fit. (B) Scaling relationship between drainage area and fan slope
for alluvial systems of the Alps; data from Guzzetti et al. (1997; crosses),
Crosta and Frattini (2004; triangles) and Champagnac et al. (2008; black
circle = Valensole, grey square = Chambaran). The Lannemezan
megafan–Neste system (red circle) lies slightly out of the relation (fit:
Sf= 10.4 Ab(-0.51±0.05)). Quaternary tilt (vertical arrow) of the Lannemezan megafan
surface is estimated at 0.13∘, the difference between present-day
slope and slope predicted by the power-law fit. Modified after Champagnac et al. (2008).
With the linearly increasing uplift, the megafan continues to grow; the mean
elevation change in the foreland is steady, positive and higher than in the
default experiment (> 0.2 mm yr-1, including uplift). In both
experiments, connection with the headward-incising stream and entrenchment
occur (at ∼ 280 kyr in the lower tilt experiment, and at
∼ 240 kyr in the higher tilt experiment) but only temporarily
affects this trend because as tilting continues the river outflows from
this path (at ∼ 320 and ∼ 260 kyr
respectively; Fig. 10). Deposition in the main path causes the stream to
overflow from this channel and resume distributive flow over the megafan,
but instabilities in the models blur the results (Fig. 10). This suggests
that overall tilting of the model prevents or limits permanent
entrenchment. The mean elevation change in the mountain is steady and
positive (∼ 0.25 and 1.2–1.3 mm yr-1
respectively), with peaks following the transient capture.
Tilting experiments 5a (0.13∘ tilt) and 5b
(0.4∘ tilt). Connection with the headward incising stream occurs
(at 280 and 240 kyr respectively) but model instabilities in the channel,
interpreted as deposition, induce overflowing (at 320 and 260 kyr
respectively). Distributive flow over the megafan resumes and lasts until
the end of the experiment.
Figure 11 compares the evolution of a north–south topographic profile across
Experiment 5a (0.13∘ tilt) and the default model. The megafan
topography on this section is rather stable over the course of the default
experiment. However, the megafan slope increases significantly in experiment
5a, showing that the tilt affects the megafan slope without being fully
compensated for by erosion.
DiscussionMegafan building
In the model, the main steps of the megafan-building phases are (i) foreland
deposition starts with small fans that quickly merge into a
bajada; mountain watersheds merge so only a few streams are left;
(ii) the rivers situated near the boundary change their direction to reach
the shortest flow path to the border, and the central stream becomes the
dominant provider for foreland sedimentation; and (iii) the megafan grows in
response to cyclic flow dynamics (oscillating between channelized and
distributive flow) and reaches dynamic equilibrium.
The timescale of the building phase of the megafan is long (> 10 Myr)
when compared to active megafans of similar volumes deposited during
the Quaternary (e.g. in the Alps, Andes and Himalayas; Assine et al., 2014;
Fontana et al., 2014; Abrahami, 2015) but compares well to older systems
(e.g. Campanian–Maastrichtian Hams Fork formation in Utah; DeCelles and
Cavazza, 1999) and is consistent with the Lannemezan megafan building phase
encompassing the early middle Miocene to Pliocene (i.e. ∼ 15 Myr).
The long foreland (foreland length / mountain length = 2) allows for a large
fan to develop but requires the model parameters to be set in a way that
allows transportation over such great distance; in particular the parameter
L must be large enough. This required longer L, which may
be interpreted as a smaller settling rate (Davy and Lague, 2009), is
consistent with the downstream fining of sediment in the Lannemezan megafan
(Crouzel, 1957; Azambre et al., 1989), even though sediment fining is not
accounted for in CIDRE.
Evolution of a north–south topographic profile near the middle of
the model during Experiment 5 (region-wide tilting). Inset shows that with
default settings (Experiment 1), the slope of the megafan does not increase;
regional tilting (Experiment 5) is needed to create an increasing northward
slope through time.
Comparison between (a) DEM of the northern Pyrenees and foreland
at the longitude of the Lannemezan megafan and (b) the final model (contour
lines every 100 m) shown at the same scale. The proposed evolution for the
Lannemezan megafan is schematized below: (c) a preexisting river
Ariège–lower Garonne River flows through the foreland while the Neste
transports sediments deposited in the foreland through a distributive
pattern to build a megafan; (d) a tributary of the Ariège–lower Garonne
retreats headward and westward towards the apex of the Lannemezan megafan
while the megafan keeps growing; (e) the migration of the tributary leads to
sequential capture of (1) the upper Garonne and (2) the upper Neste and thus
the abandonment of the megafan; rivers incise together in the foreland
deposit leaving a series of alluvial terraces (preferably on the left bank
due to the direction of river migration indicated by white arrows).
The boundary conditions, open in the foreland and closed in the mountain,
play a key role in the development and evolution of the drainage network. In
particular, open boundary conditions on all three sides of the foreland
allow for (i) the central river to become dominant in the sediment flux
deposited in the foreland (thus creating a megafan) as the more lateral
rivers rapidly adapt their course to the shortest path reaching the
base level (along-strike to reach boundaries), and (ii) the conical shape to
develop (contrary to Pepin et al., 2010, where cyclic boundary condition on
lateral boundaries resulted in a more bajada-like landform). Open
boundary conditions in the mountain would result in strike-parallel
drainage, which shows that megafan building requires a relatively large
range. Thus, in natural settings transverse rivers with efficient fluvial
transport (to evacuate both water and sediments) appear necessary on all
sides for a river–fan system to be singled out and grow into a megafan
deposit. In the northern Pyrenean foreland, the Garonne–Ariège and Adour
rivers could have played this role, which suggests that they might have
existed prior to the Miocene onset of the megafan building.
In our model, the absence of subsidence in the foreland may have encouraged
the development of a fan covering a large area, which imposes overfilled
conditions in the foreland basin. High subsidence rate would have allowed
thick accumulation close to the range and thus limited its northward
extension (e.g. Allen et al., 2013). This hypothesis could be tested with
the addition of an algorithm for flexure (Simpson, 2006; Naylor and
Sinclair, 2008). Nevertheless, the overfill hypothesis may be justified by
the deceleration of subsidence rates in the Pyrenean retro-foreland since
the Eocene (Desegaulx and Brunet, 1990; Desegaulx et al., 1990). In any
case, the impact of varying subsidence rate on megafan growth and
abandonment remains to be evaluated (e.g. Dingle et al., 2016).
Autogenic incisionTime and space scales
The autogenic entrenchment happens around 15.57 Myr, which, within the
framework of the Lannemezan megafan evolution, is consistent with the
fan-building phase encompassing the early middle Miocene to Pliocene and
incision taking place during the Quaternary (since 300 ka; Mouchené et
al., 2017).
At 15.3 Myr (end of the building phase), the mean elevation in the
mountain range is about 1460 m, with a maximum elevation at 3160 m near the
southern border, which is consistent with the northern flank of the
Pyrenees. In the foreland, a maximum elevation of 950 m is reached at the
fan apex in the model, which is higher than the current elevation of the
Lannemezan megafan apex (∼ 660 m), but the mean elevation of
the foreland is around 270 m, comparable to that of the Lannemezan megafan.
Megafan shape and dimensions (area) agree between our model and the
Lannemezan megafan. At this point, the watershed of the main feeding river
is about 1100 km2, which is quite large when compared to the current
Neste watershed (∼ 750 km2). The scale of the vertical
entrenchment of the river is similar in the model and in the Lannemezan case
(∼ 100–150 m near the apex).
Mechanism and necessary conditions
For Pepin et al. (2010), autogenic entrenchment of an alluvial fan occurs
only if (i) progradation is limited by the open boundary with fixed
elevation and (ii) the transport threshold (critical shear stress) is
significant. Nicholas et al. (2009) also suggested that declining
aggradation in the fan results from increasing fan area during progradation
(building phase), and incision is triggered by the lack of accommodation
space when boundary conditions are reached. In nature, some incised fans are
linked to powerful transverse rivers (Milana and Ruzycki, 1999;
Dühnforth et al., 2007, 2008) but the causality is not proven and
external forcing is demonstrated in some cases (e.g. Dühnforth et al.,
2008).
In our model, entrenchment naturally occurs, but it happens long after the
moment when sediments reach the model boundaries (Figs. 3 and 4). This is
different from Pepin et al. (2010) since autogenic
entrenchment occurred precisely when the sediment reached the free border in their experiments.
We suspect that this difference comes from the lateral erosion included in
our modelling and absent in the simulations of Pepin et al. (2010). Lateral
erosion limits the incision by fostering lateral migration and channel
widening. Although our modelling seems more realistic, the comparison of
this prediction with natural settings is not straightforward because
boundary conditions are likely to change over time (e.g. Harvey, 2002).
Consistent with the findings of Pepin et al. (2010) and Nicholas and Quine (2007),
all our models that predict autogenic entrenchment use a significant
critical shear stress (entrainment threshold). We suspect this threshold to
control part of the incision magnitude and the delay between the moment when
sediment reaches the free border and the moment when incision occurs. This
aspect should be further evaluated by varying the critical shear stress in
other experiments.
Van Dijk et al. (2009) proposed that aggradation on fans allows a critical
slope to be reached, triggering the incision. However, in the tilting
experiments (Experiment 5), the fan slope reaches greater values than in the
default experiment at the time of entrenchment, but permanent entrenchment
does not occur. Therefore, attainment of a critical slope does not appear to
constitute a threshold for entrenchment. One explanation may be that tilting
fosters erosion in the mountain, with a larger incoming sediment discharge
entering the foreland, which prevents incision from growing.
Incision pattern
In the model, permanent entrenchment results from (limited) incision near
the apex by the feeding river and major headward incision of a stream from
the foot of the fan until both ends meet to define a continuously entrenched
pathway (Fig. 4). In the case of the Lannemezan megafan, we cannot provide
evidence to support or disprove this mechanism, but the drainage pattern at
the apex of the Lannemezan megafan resembles the model (Fig. 12a and b). We
could therefore envisage the following scenario:
A preexisting river Ariège–lower Garonne River flowing through the
foreland to the east of the megafan, while the Neste river feeds the
Lannemezan megafan through a distributive deposition pattern (Fig. 12c).
A tributary of the ancestral Ariège–Garonne River retreats headward
toward the apex of the megafan (Fig. 12d).
Headward incision of the tributary toward the west leads to sequential
capture of (a) the upper Garonne and (b) the upper Neste, abandonment of the
fan, and rapid incision and terrace formation (preferably on the left bank of
the now merged Neste–Garonne River due to southward river migration; Fig. 12e).
The amount of incision is already very important in the first time step
following the connection (∼ 100 m near the apex) and will only
be further increased by another 80 m near the apex. Downstream, the stream
erodes its right bank towards a strike-parallel pathway but does not incise
vertically (Fig. 4). This last characteristic resembles the lateral
migration of the Neste and Garonne rivers during their incision, evidenced
by the extensive alluvial terrace staircase left almost systematically on
their left banks. In our model, the sediments of the channel bed could
enhance the effect of lateral incision and inhibit further vertical incision
through their cover effect (see also Hancock and Anderson, 2002; Brocard and
van der Beek, 2006).
However, the terraces of the northern Pyrenean foreland prove that the
∼ 100 m incision of the Lannemezan megafan at its apex was
episodic, which contrasts with the pulse of incision predicted by the model.
Cosmogenic nuclide surface-exposure dating suggests that these incision
episodes in the northern Pyrenean foreland are linked to cold-to-warm
climatic transitions (Mouchené et al., 2017).
Impact of climate change
In the model with sinusoidal precipitation rates (Experiment 2b), humid
periods are characterized by erosion in the mountains and deposition in the
foreland (with episodic incision); both decrease in drier periods because
stream power decreases and less material is transported from the
mountains. The wet-to-dry transition corresponds to a decrease in sediment
input but also to a decrease in fluvial efficiency as the run-off nears zero,
which prevents incision.
In the northern Pyrenean foreland, incision and abandonment of alluvial
terraces has been linked to cold-to-warm climatic transitions (Mouchené
et al., 2017) where the rapid decrease in sediment flux and gradual
transitioning of the river to a single meandering thread, with a low
width / depth ratio, would encourage vertical incision (e.g. Hancock and
Anderson, 2002). Warm-to-cold transitions can also be associated with
incision because of the increase in run-off variability and decline in
vegetation that characterizes these periods; however, in nature, they are usually
more gradual than cold-to-warm transitions. During glacial (dry, cold)
periods, regolith is actively produced on hillslopes by efficient frost
cracking but it is mobilized only at the onset of the following interglacial
(wetter) period, when rainfall increases (e.g. Carretier et al., 1998). To
reproduce and further explore this effect, we would need to include a
climate (temperature)-dependant law for sediment production in the model.
In nature, incision is not always related to the return of wetter
conditions; Meyer et al. (1995) suggest that incision of the terraces in
their study site in northwestern Yellowstone National Park happens during
warmer, more drought-prone periods because of the infrequent floods scouring
the channel bed. Langston et al. (2015) recently modelled a similar pattern
of incision by applying more intense, longer duration precipitation events
during interglacial periods, but without changing the average precipitation
rate. Periglacial processes have also been suggested to be a key controlling
factor for erosion (e.g. Marshall et al., 2015; Dosseto and Schaller, 2016):
erosion is enhanced during cold periods in regions where they occur, whereas
it is enhanced during warmer periods in regions exempt of periglacial
processes. Mass wasting processes could be the main driver for erosion
increase during wet periods (e.g. Bookhagen et al., 2005), although their
relationship to other environmental parameters, such as vegetation cover,
remains disputed (e.g. Istanbulluoglu and Bras, 2005; Carretier et al.,
2013; Dosseto and Schaller, 2016). Our current model does not to take such
processes into account. Aggradation and incision thus seem to be controlled
by the variability in rainfall intensity and event duration but also by
temperature-dependent hillslope processes, rather than by mean
precipitation rate alone.
A number of studies have related terrace incision with climate changes (e.g.
Barnard et al., 2006; Bridgland and Westaway, 2008). This also seems to be
the case in the northern Pyrenean foreland, where terrace abandonment was
related to Quaternary climatic changes, although the model does not
reproduce this pattern (it does not produce terraces at all). Several
experiments suggest that the longer the foreland, the more it buffers the
effects of short-period variations (Métivier and Gaudemer, 1999; Babault
et al., 2005; Carretier and Lucazeau, 2005). Therefore, the effect of rapid climatic
changes could be dampened by the large dimensions of the foreland in our
model, preventing terrace formation. The lack of temperature-dependent
processes in our experiments (glacial erosion, temperature-dependent
regolith production) may also prevent terrace formation. Finally, the model
resolution could be insufficient to resolve alluvial terraces.
Uplift rate
In the experiment where uplift stops after 15.3 Myr (Experiment 4a), the
mountains erode at a rate of 0.19 mm yr-1, comparable to the highest
values obtained through estimation of basin-averaged erosion rates using
cosmogenic nuclides in river sands (0.01 to 0.16 mm yr-1; Mouchené,
2016). Uplift is thought to have significantly decreased in the Pyrenees
since the Miocene, with modern GPS-derived uplift rates being small (0.1 ± 0.2 mm yr-1
of differential uplift of the mountain belt with
respect to a regional reference frame; Nguyen et al., 2016). Our results
suggest that the Lannemezan megafan could have been built in a period of
reduced tectonic uplift. The evolution of the piedmont is very similar to
that of the default experiment (where uplift is maintained at
0.3 mm yr-1) except for the entrenchment that is refilled in experiment 4a.
Thus, it appears that tectonic activity in the mountain belt does not
strongly influence incision dynamics in the foreland.
Flexural isostatic rebound
We attempted to simulate the effect of flexural isostatic rebound on the
incision pattern through tilting of the model. In the Alps, tilting of the
foreland appears related to isostatic rebound in response to accelerated
glacial erosion and possibly deep-seated geodynamic processes (Champagnac
et al., 2008). This pattern has not been demonstrated for the Pyrenees.
Although the simplistic approach we used does not reproduce the flexural
response to erosional unloading of the range in detail, the slope of the fan
topographic profile increases with time through this process, as suggested
for alpine fans by Champagnac et al. (2008). Quantification of this increase
in slope, although complicated by poor outcrop conditions, needs to be done
in the northern Pyrenean piedmont to compare with the slope angles obtained
in our model. In any case, tilting prevented permanent
entrenchment in the experiment so this mechanism cannot explain the abandonment of a foreland
megafan.
In the model, the topographic profiles merge downstream as a consequence of
tilting. The alluvial terraces along the northern Pyrenean rivers also merge
downstream and this pattern is also observed in the Alpine foreland.
However, this pattern does not necessarily relate to tilting of the megafan:
in other settings, this characteristic has been interpreted as a climatic
imprint on incision (Poisson and Avouac, 2004; Wobus et al., 2010; Pepin et
al., 2013). Thus, tilting does not appear to play a major role in the
abandonment of the Lannemezan megafan.
Conclusions
Numerical modelling of the evolution of a catchment–foreland system has
provided (i) new insight into the building and incision of a foreland megafan
and (ii) key elements to infer the driving forces in the natural evolution
of the remarkable Lannemezan megafan and its mountainous catchment in the
northwestern Pyrenees.
For a megafan to develop, the foreland must be large enough to provide
sufficient space for the fan to expand for a long period of time; a lack of
subsidence may help this process. The role of preexisting transverse rivers
flowing across the foreland seems to be critical in the building and
incision of the megafan. They rapidly capture the closest streams exiting
the range, which allows for a central mountainous stream to be singled out
and to provide for most of the foreland deposits stacked in the megafan. In
the northern Pyrenean foreland, the through-flowing Adour and
Garonne–Ariège rivers may have helped shaping the Lannemezan megafan:
the spacing of these preexisting major drainage axes controls the size of
the fan, limit its extension and efficiently evacuate water and
sediments out of the megafan. The megafan grows in response to the autogenic
oscillations between sheet-flow and channelized flow. These oscillations
trigger small incisions that are subsequently overfilled and rapid lateral
movement of the flow over the whole fan surface.
Permanent entrenchment of the Lannemezan megafan could thus be the result of
autogenic processes through (i) progressive headward incision of a stream
from the foot of the fan (not too far from the apex) and (ii) final and
rapid incision of the apex once this stream has captured the feeding river
at its mountainous outlet. No external forcing is needed to induce long-term
entrenchment on the order of magnitude observed in the field (100 m vertical
incision near the apex) but external factors cannot be ruled out. In
particular, on a shorter timescale, incision may have been influenced by
Quaternary climatic variations as suggested by the abandonment of terrace
staircases along the foreland rivers, incising the Lannemezan megafan.
Variations in precipitation rate alone do not appear to be sufficient to
produce these episodic incision and alluviation phases, and
temperature-dependent hillslope processes may also be involved. In contrast,
base-level changes, tectonic activity in the mountain range or tilting of
the foreland through flexural isostatic rebound appear to be unimportant factors
in the abandonment of the megafan.
Data availability
The source code is available upon request to Sébastien Carretier
(sebastien.carretier@get.omp.eu). The input files for the simulations described in this paper are available in
the GIT repository https://github.com/margauxmouchene/CIDRE_Lannemezan_input.
The authors declare that they have no conflict of interest.
Acknowledgements
This study was supported by French National Research Agency ANR (Project
PYRAMID, ANR-11-BS56-0031) and forms part of Margaux Mouchené's PhD thesis funded by the
French Ministry of Higher Education (MESR). ISTerre is part of Labex
OSUG@2020 (ANR10 LABX56). We thank Hugh Sinclair and an anonymous referee
for constructive comments that helped clarify the paper.
Edited by: S. Castelltort
Reviewed by: H. Sinclair and one anonymous referee
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