Bedrock structural control on catchment scale connectivity and alluvial fan processes, High Atlas Mountains, Morocco.

Lithology is acknowledged to be an important internal catchment control on flow processes to adjacent alluvial fans. However, the role of inherited structural configurations (e.g.bedrock attitude) in catchment connectivity and sediment transport is rarely considered. We examine four young (,100-year-old) active tributary junction alluvial fan systems from the Dade`s Valley in the High Atlas of Morocco in terms of their catchment-scale connectivity, sediment transfer and resulting alluvial fan processes. The catchments occur on the same lithologies (limestones and interbedded mudstones), but experience different passive structural configurations (tilted and structurally thickened beds). The fan systems react differently to historical peak discharges (20–172 m3 s21). Catchments containing tectonically thickened limestone units develop slot canyons, which compartmentalize the catchment by acting as barriers to sediment transfer, encouraging lower sediment to water flows on the fans. Syn-dip catchments boost connectivity and sediment delivery from translational bedrock landslides as a result of steep channel gradients, encouraging higher sediment to water flows. By contrast, translational landslides in strike-oriented drainages disrupt longitudinal connectivity by constricting the valley width, while the gradients of the main channels are supressed by the attitude of the limestone beds, encouraging localised backfilling. This diminishes the sediment to water content of the resulting flows

Orogenic-orographic feedback and the rise of the Central Andes

The rise of large mountain ranges is considered to be driven by tectonics potentially coupled with climate driven-erosion, although the role of this coupling remains uncertain. The arid climate of the Central Andes allows us to strengthen our understanding of the relative roles of these processes in mountain range development globally. Here we compile estimates of exhumation, sedimentation, aridity and surface uplift across the Central Andes for the last 50 Ma. We aim to place constraints on the relative timing of rock uplift (displacement of rocks with respect to the geoid), exhumation (displacement of rocks with respect to the surface) and surface uplift (displacement of the earth's surface with respect to the geoid). We show that initial rock uplift of the Andes extends back at least 50 Myr. This rock uplift generated orographically driven precipitation on windward slopes leading to increased exhumation but limited preservation of surface uplift. Eastward propagation of the mountain range resulted in increasingly extreme orographic effects on the leeward side amplifying aridity, reducing exhumation and increasing preservation of surface uplift. Essentially, surface uplift shows a ∼5-10 Myr lag behind initial rock uplift as the Andes grow asymmetrically through time. We suggest that an eastward propagating pattern of exhumation, aridity and surface uplift with time, reconciles previous contradictory models of Andean uplift. One Sentence Summary: Uplift of the Central Andes is reconstructed over the last 50 Myr and the precise relationship between roles of tectonics and climate established

Using spatial patterns of fluvial incision to constrain continental-scale uplift in the Andes

This study was funded by BHP and the University of Brighton Rising Stars. The authors are grateful to Marit Van Zalinge and Masie Mather for discussions on the manuscript.Peer reviewedPostprin

Fluvial archives of NW African climate and tectonic evolution, Atlas Mountains, central Morocco

The Atlas mountains in Morocco are a natural laboratory at the junction between the Atlantic Ocean (passive margin), the Mediterranean (subduction) and the African Craton. Here, interactions between the mantle and lithosphere, crustal compression and uplift have been recorded in river terraces, alluvial fans, drainage patterns, river long profiles, and in wedge-top & foreland sediments. Limited work on terraces in one of the catchments crossing the south Atlas thrust front has shown rates of incision are low and have been sustained since the Pleistocene. Dating of terraces using Optically Stimulated Luminescence, together with field sedimentology, links the formation of terraces in the Dades River to 100 ka climate cycles. Studies of tributary fans and fan sediments in terraces suggest coupling of hillslopes, tributaries and trunk streams vary across glacial-interglacial cycles and is geologically controlled. River long profiles extracted across the southern Atlas Mountains contain knickzones (areas of increased steepness), resulting from tectonically driven uplift. We will use newly acquired high resolution DEM data together with field mapping and Optically Stimulated Luminescence dating to constrain river terrace formation in High Atlas catchments draining into the Ouarzazate foreland basin. These data will be used to constrain further, the regional tectonic and climatic controls on river terrace formation. Integrating the terrace records with the other fluvial archives will enable challenging questions on tectonic surface processes, source-to-sink sedimentology and intra-plate tectonics to be tackled

Catchment changes in response to tectonics and climate: using river terraces and DEM data in the southern High Atlas Mountains (Morocco)

Tectonics and climate drive the generation and transport of sediment in mountain rivers as these evolve over time. On a glacial-interglacial scale, in particular catchment reorganisation and catchment incision dynamics control these processes, and affect fan deposition in sedimentary basins. The Atlas Mountains in Morocco exhibit ongoing catchment reorganisation and an abundance of river terraces recording glacial fluvial aggradation and interglacial-glacial incisional periods, opening up insight into the processes behind catchment evolution over geological timescales. Topography and river profiles across drainage divides are similar in a stable divide, and if they are unequal they indicate active catchment reorganisation. When reorganisation occurs, it results in irregularities in river long profiles and changes in river valley erosion. River strath terraces are formed by transition between valley widening and downcutting of terraces in response to local divergence of sediment-transport capacity 3. Consequently, they record changes in catchments due to river capture, climate and tectonics. The presence of river terraces enables catchment processes over time to be investigated. A combination of remote sensing and field mapping and logging was completed in May 2018. River terraces have been mapped with newly released high resolution DEM data in the southern High Atlas in Morocco, and additional surveying was done in the field. Geomorphological indices suggest river catchment capture is a key control on the development of drainage networks. River long profiles suggest tectonic controls have also influenced landscape development over the last few million years. Logging of terrace sediments together with high-resolution sampling for OSL dating enables these catchment-wide effects to be compared with paleo-hydrological and sediment transport characteristics of the fluvial system. The combination of geomorphological DEM and sedimentological field data enables us to explore drivers of catchment change, and will contribute to the wider understanding of fluvial system response to climate and tectonic controls, and to its transport into the sedimentary record

Rock strength and structural controls on fluvial erodibility: Implications for drainage divide mobility in a collisional mountain belt

Numerical model simulations and experiments have suggested that when migration of the main drainage divide occurs in a mountain belt, it can lead to the rearrangement of river catchments, rejuvenation of topography, and changes in erosion rates and sediment flux. We assess the progressive mobility of the drainage divide in three lithologically and structurally distinct groups of bedrock in the High Atlas (NW Africa). The geological age of bedrock and its associated tectonic architecture in the mountain belt increases from east to west in the study area, allowing us to track both variations in rock strength and structural configuration which influence drainage mobility during erosion through an exhuming mountain belt. Collection of field derived measurements of rock strength using a Schmidt hammer and computer based extraction of river channel steepness permit estimations of contrasts in fluvial erodibilities of rock types. The resulting difference in fluvial erodibility between the weakest and the strongest lithological unit is up to two orders of magnitude. Published evidence of geomorphic mobility of the drainage divide indicates that such a range in erodibilities in horizontal stratigraphy of the sedimentary cover may lead to changes in erosion rates as rivers erode through strata, leading to drainage divide migration. In contrast, we show that the faulted and folded metamorphic sedimentary rocks in the centre of the mountain belt coincide with a stable drainage divide. Finally, where the strong igneous rocks of the crystalline basement are exposed after erosion of the covering meta-sediments, there is a decrease in fluvial erodibility of up to a factor of three, where the drainage divide is mobile towards the centre of the exposed crystalline basement. The mobility of the drainage divide in response to erosion through rock-types and their structural configuration in a mountain belt has implications for the perception of autogenic dynamism of drainage networks and fluvial erosion in mountain belts, and the interpretation of the geomorphology and downstream stratigraphy.</p

Geomorphology on geologic timescales: Evolution of the late Cenozoic Pacific paleosurface in Northern Chile and Southern Peru

publisher: Elsevier articletitle: Geomorphology on geologic timescales: Evolution of the late Cenozoic Pacific paleosurface in Northern Chile and Southern Peru journaltitle: Earth-Science Reviews articlelink: http://dx.doi.org/10.1016/j.earscirev.2017.04.004 content_type: article copyright: © 2017 Elsevier B.V. All rights reserved

Thinking about a limited future enhances the positivity of younger and older adults’ recall: support for socioemotional selectivity theory

Compared with younger adults, older adults have a relative preference to attend to and remember positive over negative information. This is known as the “positivity effect,” and researchers have typically evoked socioemotional selectivity theory to explain it. According to socioemotional selectivity theory, as people get older they begin to perceive their time left in life as more limited. These reduced time horizons prompt older adults to prioritize achieving emotional gratification and thus exhibit increased positivity in attention and recall. Although this is the most commonly cited explanation of the positivity effect, there is currently a lack of clear experimental evidence demonstrating a link between time horizons and positivity. The goal of the current research was to address this issue. In two separate experiments, we asked participants to complete a writing activity, which directed them to think of time as being either limited or expansive (Experiments 1 and 2) or did not orient them to think about time in a particular manner (Experiment 2). Participants were then shown a series of emotional pictures, which they subsequently tried to recall. Results from both studies showed that regardless of chronological age, thinking about a limited future enhanced the relative positivity of participants’ recall. Furthermore, the results of Experiment 2 showed that this effect was not driven by changes in mood. Thus, the fact that older adults’ recall is typically more positive than younger adults’ recall may index naturally shifting time horizons and goals with age

Constraining a model of punctuated river incision for Quaternary strath terrace formation

In the small fraction of Earth's surface with the highest erosion rates such as the Alps and Himalayas quantifying rates of incision, rock uplift and inferring climatic controls on the landscape can be relatively straightforward once the ages of river terraces cut in bedrock (strath terraces) are constrained. However, in many mid to lower relief settings that are more typical of mountain belts worldwide, periods of net river incision and riverbed lowering are relatively short (punctuated), interrupted by long periods of sediment aggradation or stasis. We define a conceptual model of punctuated river incision and strath terrace formation for the calculation of incision and rock uplift rates, and recommend strategies for geochronological sampling and interpretation. An approach using OSL dating of terrace gravels allows us to constrain a detailed ~150 kyr history of punctuated river incision and strath terrace formation spanning two stratigraphic landform levels in the High Atlas Mountains (NW Africa). Extensive preservation and exposure of strath-top gravels, within a post-orogenic setting unaffected by eustatic influences, enables the derivation of rates of base-level fall, integrated over periods of strath-top aggradation and incision, that are consistent with independently constrained regional rock uplift rates. Combining a punctuated river incision model with our well-constrained terrace formation history allows us to demonstrate how assumptions concerning Quaternary river incision and aggradation can lead to the problematic Sadler Effect, an apparent dependence of incision rates on measured time interval. Subsequently, we demonstrate that an approach to reinterpreting previously published data using the punctuated incision model, even when combined with limited terrace age data, results in more consistent conclusions about rates of river incision, rock uplift and base-level lowering across the mountain belt. Our recommendations for sampling strategies to constrain rock uplift rates require samples to be taken just above the strath surface, and in addition towards the top of the deposit for river incision rates. In a setting with punctuated river incision and strath terrace formation, both rock uplift and incision rates require burial dates, as exclusive use of abandonment ages will not yield constraints on accurate rates of rock uplift or incision. Furthermore, we find that only with multiple along-stream locations and multiple burial dates in each terrace deposit, could a reliable climatic signal be extracted: this signal would not have shown up in terrace abandonment ages such as those derived from cosmogenic exposure dates. The demonstrated effects of assumptions about strath terrace formation, and the recommended approaches for sampling and interpretation, have implications for those attempting to constrain palaeoclimatic, tectonic, and geomorphic histories from strath terrace records in regions exhibiting punctuated river incision