Lithological control on erosional dynamics in a tectonically inactive mountain belt (Anti-Atlas, Morocco)

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The Atlas-Meseta orographic system: open questions

The Atlas-Meseta orographic system is characterzed by a complex, long-term geological history with several tectonic events as described above. A large number of studies have deciphered these phases through time. For example, the main deformation and exhumation events have been constrained by means of low-temperature thermochronology data, especially for the High Atlas and Anti-Atlas. The High Atlas is characterized by a complex cooling history, with a more recent peak of exhumation in the late Miocene in response to collisional processes. Conversely, in the Anti-Atlas, the estimates of exhumation are rather low since the late Cretaceous (Sehrt et al., 2018; Charton et al., 2020; Lanari et al., 2020a). Crustal shortening in the High Atlas and Middle Atlas domains is thought to be limited to moderate, with estimates ranging between 12 and 25% (Gomez et al., 1998; Beauchamp et al., 1999; Teixell et al., 2003; Arboleya et al., 2004; Domènech et al., 2016; Fekkak et al., 2018; Lanari et al., 2020b). This is consistent with geophysical data showing that crustal thickening is limited, and hence isostasy can not explain the modern Atlas-Meseta topography (e.g., Seber et al., 1996; Rimi et al., 1999; Ayarza et al., 2005; Missenard et al., 2006; Miller and Becker, 2014; Bezada et al., 2014; Miller et al., 2015). Moreover, the lithosphere astenoshpere boundary is unexpectedly shallow and this agrees with petrological data from the Sioua, Anti-Atlas and Middle Atlas domains indicating an alkaline affinity related to a sublithospheric mantle most likely caused by an asthenospheric uprise (El Azzouzi et al., 1999; De Beer et al., 2000; Missenard et al., 2006; Duggen et al., 2009; El Azzouzi et al., 2010).
The timing, cinematics and dynamics of the Cenozoic topographic evolution and the large scale of topographic resurgence of the entire orographic system is still a matter of debate. Neverthelss, geological evidence such as uplifted Messinian marine deposits in the Skoura Basin, shallowing upward trend of marine sediments in the Guercif Basin and the transient state of river network of the Atlas-Mesea domains, attest for a recent phase of topographic rejuvenation (Fig. 1.2; Krijgsman et al., 1999; Babault et a., 2008; Barbero et al., 2010; Pastor et al., 2015; Stokes et al., 2017).
Key and debated issues associated with this phase of uplift are the different contribution of short and long wavelength processes and hence the amount of uplift related to tectonic and mantle dynamic forces. To evaluate this dynamic component, we performed a geomorphic analysis using a combined approach of traditional and innovative techniques (e.g., cosmogenic nuclides, linear inverse method, knickpoints, longitudinal profiles, -plot, celerity model, swath profiles) in the slowly eroding and exhuming domains of the Atlas-Meseta system. Particularly, we focused on the Anti-Atlas Mountains and Western Moroccan Meseta, which represent weakly deformed regions, where local tectonic and climatic conditions have been stable over geological time, thus providing the possibility to quantify the timing, magnitudes and rates of mantle-driven uplift.

Cosmogenic nuclides: an overview

In 1912 the Austrian physicist Victor Hess discovered the cosmic radiations, and in the 1934 A.V. Grosse suggested that they could produce radioactive nuclides at the surface of the Earth, thus generating “cosmic radio-elements”. At the same time, Domenico Pacini, a professor at the University of Bari, made a series of measurements to determine the variation in the speed of discharge of an electroscope (and thus the intensity of the radiation) while the electroscope was immersed in a box in the sea near the Naval Academy in the Bay of Livorno. Pacini discovered that the discharge of the oscilloscope was significantly slower than at the surface. From that time, scientists started to think that this interaction between cosmic particles and rock materials could be used to solve geomorphic problems. Despite the efforts, until the mid-1980s the cosmogenic nuclide application remained just a theory without any natural appliction because available analytical instrumentation was not capable to measure the concentrations of most cosmogenic nuclides producted at the Earth-surface. In the meantime, a spatial model for production rates in the atmosphere was verified empiritically by Lal et al. (1960) at high elevation at 51°N latitude, providing the possibility to normalized production rates on the surface. A key step in the history has been played by the development of AMS (accelerator mass spectrometry) in the early 1980s, which allow to measure the isotopic ratios (10Be/9Be and 26Al/27Al), despite the presence of isobars. After that, the method has been refined by the development of a more reliably cosmogenic nuclide production scaling model that incorporate different physical processes, such as influence of geomagnetic field, temporal spatial variation in the paleo-atmosphere and surface elevation for different latitudes and altitudes of sampling sites (Lal, 1991; Stone et al., 2000; Gosse and Phillips, 2001).
Hence, the production of cosmogenic nuclides on Earth results from nuclear reactions initiated by primary and secondary energetic ray particles, by spallation process, that interact with atmosphere and surface rock. Spallation-produced nucleons, which in turn induce spallation in other target nuclides, producing a nuclear cascade composed of secondary nucleons (protons and neutrons) and mesons (kaons and muons) (Gosse and Phillips, 2001; Dunai, 2010). The most used cosmogenic isotopes such as 10Be, 26Al, 36Cl, 14C, 3He and 21Ne allow addressing a wide range of geomorphologic problems due to their surface production in commonly occurring minerals (for example quartz, in case of 10Be, 26Al; e.g., Gosse and Phillips, 2001; Dunai et al., 2010).
However, there are unstable and stable cosmogenic nuclides (Table 1.1). Assuming that the production rate is constant with time, and that a given target is continuously exposed, the concentration of “stable” cosmogenic nuclides increases monotonously at a constant rate (Fig. 1.5). Cosmogenic radionuclides decay with time and after 5-6 half-life, they can be considered extinct (Dunai et al., 2010). The cosmogenic nuclides become a widely used tool in geomorphology, in order to: (i) dating the exposure ages of geomorphic surfaces, (ii) burial dating, (iii) estimates of erosion/denudation rates (constrainingn uplift rates), (iv) study soil dynamics and sedimentary processes of fluvial catchments.
Figure 1.5. Accumulation of cosmogenic nuclides in a non-eroding surface. Radioactive nuclides approach a secular equilibrium between production and decay after 2–3 half-lives. The concentration of stable nuclides increases continuously (from Dunai, 2010).
In the thesis, we used the 10Be concentratin in river-borne sediments to estimate the basin-wide denudation rates.
The nuclides can be used to infer denudation rates because their production rates within a mineral grain depend on their proximity to the surface, as then production rates decrease exponentially with the depth in rock (60 cm in rock of density 2.6 g cm3). When studying basin-wide denudation rates the cosmogenic nuclide concentrations reflect how quickly the overlying mass went away (Granger et al., 2014). Thus, the concentration of cosmogenic nuclides is inversely proportional to the denudation rate (Lal, 1991).
The basin-wide denudation rates are usually calculated considering the three different categories of particles responsible for the in-situ production of 10Be (Braucher et al., 2011).
Where C(x,t) corresponds to the nuclide concentration as a function of depth x (g/cm2), e (g/cm2/yr) represents the denudation rate and t (yr) the exposure time. Pn, Pus, Puf and m, us, uf are the production rates and attenuation lengths of neutrons, slow muons and fast muons, respectively, and is the radioactive decay constant of 10Be and is the rock density.
To derive production rates several scaling and correction factors have to be considered. Not only the production rates change as a function of latitude and altitude, but also it depends on the local factors (topographic shielding), such as sloping surfaces of exposure and vicinity of topographic irregularities (Gosse and Phillips, 2001; Dunai et al., 2010).
Furthermore, the calculation of 10Be-derived denudation rates requires several assumptions, such as:sediments are supplied at the rate that is proportional to the denudation rate, quartz (in case of 10Be, 26Al) is evenly distributed throughout the catchment and that the analyzed cosmogenic nuclide was absent before the rock approached the surface (Fig. 1.6). An additional constrain in case of unstable cosmogenic nuclides is that denudation is fast enough to neglet the radioactive decay. This is not an issue with stable nuclides.
Figure 1.6. Schematich sketch of an eroding landscape and catchment dyanamics (from Granger et al., 2014).
However, some assumptions may not be representative for real case studies. For example, if it is likely true that the sampled sand in the active channels might be representative for the whole catchment, it is however important to consider that surface processes, such as landslides, can perturbate the supply system, and thus deliver an overwhelming load of sediment that can biases the estimates of basin-wide denudation rate (Fig. 1.6). Another important point to discuss is the timescale, which is equivalent to the time necessary to lower the landscape by 60 cm. However, an acceleration in denudation rate due to and increase in uplift or a climate change (more erodible conditions) will perturb the response time of cosmogenic nuclide concentration (“damped response time”). Despite these complications and assumptions, the method has been used over the past 30 years to estimate basin-wide denudation rates. The measurement of in-situ 10Be produced in quartz is the most used method to infer the basin-wide denudation. An impressive amount of works has been done in active and inactive regions to quantify denudation rates (e.g., Ouimet et al., 2009; Miller et al., 2013; Derrieux et al., 2014; Olivetti et al., 2016). A first conclusion from these studies indicates that usually a primary role in controlling the erosional dynamics of a landscape is played by climate and topographic steepening (only few studies suggest the main role of bedrock erodibility even in active ranges, i.e., Molliex et al., 2017; Zondervan et al., 2020), which is caused by rock uplift and drives river incision and hillslope erosion, in the active regions (e.g., Adams et al., 2020). Instead, in the quiescent areas, where tectonic and climate forces are stable and do not vary significantly in space, landscape dynamics seem mainly control by lithology and spatial variation of bedrock erodibility (e.g., Scharf et al., 2013; Piefer et al., 2021).

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Motivations and descriptions of thesis chapters

The long-term tectonic evolution and the deformation style of the High Atlas and to a lesser extent the Middle Atlas are well described in literature as well as the geomorphic configurations of some domains of the Atlas-Meseta system. Nevertheless, there are still debated issues and open questions. For example, how can the Atlas-Meseta topography be so high without a significant crustal root? What are the rates and tha magnitude of the large wavelength surface uplift? How does tectonic deformation and large wavelength uplift interact? What are the main processes that causes Cenozoic topographic rejuvenation?
This Ph.D. project focuses on some of these questions, with the main goal of deciphering the mantle-driven contribution on uplift, for the domains of the Atlas-Meseta system that experienced limited tectonic deformation (Anti-Atlas and Western Meseta). To achieve this goal, I combined basin-wide and in situ denudation rates derived from cosmogenic nuclides with stream profiles, regional and basins geomorphic analysis using innovative and traditional geomorphic approaches. The results are subdivided into three manuscripts presented here in form of three chapters.
In chapter two, I characterized the topographic metrics and the erosion rates obtained from 10Be cosmogenic concentrations, of the uplifted relict landscapes of the Anti-Atlas domain to assess the main controlling factors on the erosional dynamics. This allowed quantifying the bedrock erodibility of different rocks exposed in the sampled catchments. Through a comparison between the Anti-Atlas and similar ancient, tectonically inactive belt, I concluded that lithology plays a major role in lowering and creating topographic relief, without any dependence on climate and mean annual precipitation. This chapter has been submitted to Earth and Planetary Science Letters.
The third chapter is focused on the long-term topographic evolution of the Anti-Atlas Mountains in order to decipher the mechanisms of uplift that were responsible for the Cenozic topographic resurgence. In particular, I extracted information from topography and channel networks for the entire landscape (i.e., upstream and downstream of the major non-lithological knickpoints) and calculated the timing, magnitude and rates of uplift during the Cenozoic. The results of this analysis suggest that the Anti-Atlas is characterized by an increase of surface uplift from 500 m, in the western sector of Anti-Atlas, to 1200 m in the Siroua Massif during the middle-late Miocene. The Anti-Atlas is only the result of deep-seated processes. Instead, Siroua Massif topography results from the contribution of different signals (long- and short-wavelength components), such as faulting activity and magma injection and deep-seated processes. Finally, the upwelling of hot asthenosphere seems the only possible mechanism to explain this long wavelength topographic swell. This chapter will be submitted to Tectonics.
The fourth chapter aimed to dechipere the long-term topographic evolution of the Western Moroccan Meseta, a quiescent tectonic region characterized by a transient adjustment of hillslope and channels in response to a Cenozoic increase in rock uplift rates. Here, I also quantified the erosional processes using the 10Be-derived denudation rates, and the magnitude of dynamic uplift through a detailed topographic analysis. The main results let me conclude that Western Moroccan Meseta is characterized by a transient state, as documented by the linear correlation between denudation rates and topographic and channel metrics. The results highlight the importance of considering uneven distribution of quartz-bearing rocks in the sampled catchments, which represents a critical issue for deriving basin-wide denudation rates. Finally, the estimates of surface uplift from the different domains of Atlas-Meseta system describe a large wavelength positive feature, with a highest uplift recorded by the uplifted Messinian marine deposits in the Middle Atlas domain.
Overall, this thesis provides new insights into the topographic evolution and the erosional dynamics of two tectonically inactive regions of Morocco such as the Anti-Atlas and the Western Meseta. Specifically, it demonstrates that the morphologic signatures of transience can be used to understand the landscape dynamics and to assess the temporal and spatial scales of tectonic perturbations.

Table of contents :

Chapter 1. Intoduction
1.1. The Atlas-Meseta orographic system: a geologic overview
1.2. The Atlas-Meseta orographic system: open questions
1.3. Cosmogenic nuclides: an overview
1.4. Motivations and descriptions of thesis chapters
1.5. References
Chapter 2. Lithological control on erosional dynamics in a tectonically inactive mountain belt (Anti Atlas, Morocco)
2.1. Abstract
2.2. Introduction
2.3. Anti-Atlas and Siroua Massif
2.4. Methods
2.4.1. Stream profiles, network and topographic analysis
2.4.2. 10Be-derived denudation rates
2.4.3. Erosion rates from incised lava flows
2.5. Results
2.5.1. Regional topographic analysis
2.5.2. Denudation rates across time scales
2.5.3. Basin-scale topographic analysis
2.6. Discussion
2.6.1. Erosional steady state landscape
2.6.2. Lithological control on erosional dynamics
2.6.3. Quantification of the bedrock erodibility parameter (K)
2.6.4. Possible impact of climate on K values in tectonically inactive settings
2.7. Conclusions
2.8. Acknowledgments
2.9. References
Chapter 3. Large wavelength surface uplift in the Anti-Atlas and the Siroua Massif (Morocco): Insights into topographic rejuvenation of a tectonically inactive mou
3.1. Abstract
3.2. Introduction
3.3. Geological background
3.3.1. Geological setting
3.3.2. Geophysical and petrological data
3.4. Methods
3.4.1. River profile and topographic analysis
3.4.2. Knickpoints discretization and celerity model
3.4.3. River projections and linear inverse method
3.5. Results
3.5.1. Topographic analysis
3.5.2. River morphology
3.5.3. Magnitude of fluvial incision
3.5.4. River projection
3.5.5. Linear inverse method
3.5.6. Timescales of knickpoint migration
3.6. Discussion
3.6.1. Significance of the transient topography
3.6.2. Topographic evolution and surface uplift history of Anti-Atlas
3.6.3. Topographic evolution and surface uplift history of Siroua Massif
3.6.4. Causes of surface uplift and topographic expression of the Anti-Atlas and Siroua Massif
3.7. Conclusion
3.8. Acknowledgments
3.9. References
Chapter 4. Erosional dynamics and surface uplift in a rejuvenated landscape: Insights from the Western Moroccan Meseta
4.1. Abstract
4.2. Introduction
4.3. Geological setting
4.4. Methods
4.4.1. Topographic, stream profiles and knickpoints analysis
4.4.2. 10Be-derived denudation rates
4.5. Results
4.5.1. Topographic analysis
4.5.2. River morphology
4.5.4. 10Be-derived denudation rates
4.5.5. Basin-wide denudation rates versus topographic metrics
4.6. Discussion
4.6.1. Transient topography in Western Moroccan Meseta and Middle Atlas
4.6.2. Erosional dynamics in a rejuvenated landscape
4.6.3. Topographic evolution and surface uplift
4.7. Conclusion
4.8. Acknowledgments
4.9. References
Chapter 5. General conclusions

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