Slope-break Knickpoints Research Articles

Geomorphic evolution of Sutlej valley catchment in Western Himalayas: Imprint of surface processes in modulating fluvial erosion

The Sutlej River catchment in the tectonically active western Himalayas has several geomorphic signatures of river piracy and drainage reorganization across the Upper Sutlej-Zhada basin upstream. The accelerated growth of the Leo-Pargil Horst across the Kaurik Chango-fault (KC-fault) Zone and affected regional tectonics triggered enormous incisions, causing southward migration of the present Sutlej River, which is highly infested by bedrock landslides and debris flows and evolves as a very steep valley catchment. In order to understand the changing regional landscape and identify the tectonic perturbation over the channels, we evaluate the slope-break knickpoints in the Sutlej River profile. Furthermore, we model the presence of an elevated low-relief relict landscape in the Upper Sutlej-Zhada basin and the rapid incising lower Sutlej valley catchment using the Celerity model. The observation suggests that major knickpoints at ~2000–3000 m elevation comprise the fluvial erosion process, while higher elevation knickpoints recommend the hillslope process. In order to understand how fluvial erosion and surface process imprints have changed the landscape over time, we spatially correlate terrestrial datasets, such as rates of denudation and exhumation ages, with topographic metrics and climate variables. We observe a strong correlation between the pattern of denudation rates and the conditioning factors that modulate the surface processes. Increased channel erosion, high stream power, topographic metrics, and the occurrence of bedrock landslides over a regional structural disruption exhibit a positive spatial relationship, but their role in regional drainage capture, tectonic uplift, and the gradation process remains poorly understood.

River profile and relict landscape analysis reveal the Cenozoic geomorphic evolution of the Nihewan Basin in North China

The Nihewan Basin, renowned for the numerous world-class Paleolithic sites and diverse mammalian fauna preserved in its sedimentary sequence, is a key area for studying early human evolution in East Asia. Despite extensive studies on the fluvial-lacustrine basin sediments, the evolution and formation process and mechanism of the Nihewan Basin and Nihewan Paleolake remain unclear. Here we assess the spatial pattern and recent history of rock uplift of the Liuleng Shan, one of the main bedrock mountain ranges encompassing the Nihewan Basin, using river profile and topographic analysis. The transient topography of the Liuleng Shan is characterized by two generations of elevated non-lithological slope-break knickpoints that delimit the uplifted Dianziliang and Tangxian relict landscape surfaces, suggesting two episodes of accelerated rock uplift during the Cenozoic. We link the first phase of rapid rock uplift to the initiation of the central and northeastern portions of the North Liuleng Shan Fault (NLSF) during the opening of the Shanxi Graben System in the Late Miocene, implying the initiation of the Nihewan Basin at this time. We propose that the dextral transtensional southwestern portion of the NLSF formed and propagated northeastward at around 4.2 Ma, and was responsible for the second phase of surface uplift. This phase of deformation also terminated the Tangxian planation process and led to the further subsidence of the Nihewan Basin. Accordingly, drainage rearrangement and fault activity hydrologically closed the Nihewan Basin, resulting in the development of the Nihewan Paleolake. Our findings outline a timeline for the geomorphic evolution of the Nihewan Basin and Nihewan Paleolake, highlighting the dominant role of tectonics in their formation.

Structural and geomorphological constraints on the activity of the Sparta Fault (Greece)

The ancient city of Sparti (Greece) suffered extensive damage from an estimated M w 7.2 earthquake in 464 bce , resulting in c. 20 000 fatalities, but questions remain about the short- ( c. 10 4 years) and long-term (10 5 –10 6 years) activity of this important structure. This paper presents new structural data and fluvial geomorphological analysis from the Sparta Fault and, in particular, considers the less well-known northern fault segment. A new topographic profile on the well-developed post-glacial fault scarp from the northern strand indicates a 7.53 m offset over the last c. 15 ka, suggesting a throw-rate of c. 0.5 mm a −1 . The longitudinal profiles of rivers flowing across the fault allow the elucidation of longer term fault activity. Along the strike of the fault, rivers exhibit up to two slope-break knickpoints, which decrease in height from south to north. These knickpoints are interpreted to have formed as a result of the initiation of faulting and a subsequent slip-rate acceleration. The post-glacial fault scarp and fluvial geomorphology both indicate that the entire fault is active and has an asymmetrical throw profile that results in the highest slip-rate in the south.

How erosional efficiency modulates landscape response to drainage reorganization: New empirical evidence from the Andes

The fluvial network in active mountain ranges is considered to evolve from longitudinal to transverse-dominated drainage systems. This hypothesis links to the forelandward migration of the orogen, while crustal thickening modifies the range's flank slopes, enhancing erosional efficiency. We probe such a proposition with new empirical observations from the Colombian Andes by studying the recent capture of the longitudinal Suárez and Chicamocha Rivers by the transverse Sogamoso River. Firstly, by integrating published low-temperature thermochronology and quantitative topographic metrics like χ, we found that rejuvenated rock uplift in the Cocuy and Yariguies ranges delays the transition from longitudinal to transverse-dominated drainage systems by compensating the across-divide contrasts in erosion. To further constrain the spatiotemporal response of the Suárez and Chicamocha Rivers after the discrete capture, we computed sediment-gauge erosion rates in conjunction with rainfall-normalized channel steepness and Gilbert metrics. We observe stark contrasts in erosion rates between the Chicamocha (maximum of 148.22 ± 44.86 m/Myr) and Suárez Rivers (maximum of 71.9 ± 21.5 m/Myr). Higher erosion rates in the former are expressed with widely distributed slope-break knickpoints that migrated further upstream with catchment expansion at the expense of the less efficiently erosive Suárez River basin. We interpret these erosional differences as a lithological control on the pace of topographic response to an external perturbation (discrete capture), given low-lying sedimentary rocks delay the erosional response of the Suárez River basin. Our research therefore emphasizes the role that lithology poses in keeping the landscape in an unsteady topographic condition.

Two-staged uplift of the southeast margin of the Tibetan plateau revealed by river longitudinal profiles

The Jinsha River, upstream of the Yangtze River, flows through the southeastern margin of the plateau, providing an opportunity to study the plateau uplift relative to the Sichuan basin through fluvial geomorphology. Eight tributaries of the Jinsha River located at the transition boundary from the plateau to the basin were analyzed for the feature of longitudinal profile and its evolution. The spatial distribution of slope-break knickpoints in the catchment was determined by slope-area analysis and integration analysis. Further, the magnitude and timing of the base level lowering (i.e., uplift of the Tibetan Plateau) were estimated by reconstructing the river longitudinal profile. The results show that there are two levels of slope-break knickpoints, with average elevations of 2865 m and 1523 m, corresponding to base levels relatively lowering of 1400 m and 1100 m, respectively. With reference to the long-term downcutting rate of the Jinsha River, the timings of these two phases base level lowering and uplifts are estimated to be ∼21–24 and ∼7–13 Ma, respectively. These are close to the timing of two uplifts revealed by inversion modeling of river longitudinal profiles. Therefore, the uplift of the studied region on the Tibetan Plateau can be divided into two stages, the early stage (∼20 Ma) uplift from 1500 to 2900 m and the late stage (∼10 Ma) uplift from 2900 to 4000 m, which further supports the staged outside uplift of the Tibetan Plateau.

Recent tectonic activity in Central Alborz Mountain, Iran: morphometric analysis and knickpoint identification

Alborz, as part of the Alpine-Himalayan orogeny belt, has been the result of the convergence of the Central Iranian and Eurasian plate since the late Triassic. The study area located in Central Alborz encompasses different Quaternary faults. Thus, the present study aimed to evaluate the effect of faults on river landforms through morphotectonic indices and the causes of knickpoints and knickzones in the Bedrock Rivers in part of Central Alborz. To this end, six morphometric indices such as longitudinal gradient (SL), hierarchical anomaly (Δa), the bifurcation index (R), hypsometric integral and hypsometric curve (Hi-Hc), relative relief (Bh), and basin shape (Bs) were calculated and analyzed in 38 sub-basins of the area. Finally, the area was divided into extremely high, high, medium, and low tectonic activity areas using the relative tectonic activity (IAT). Normalized steepness and concavity indices along with the longitudinal profiles of the streams were obtained using Matlab and Arc GIS software. Based on the findings, the existence of slope-break knickpoints at the mountain piedmont at the outlet of large rivers through active faults demonstrated that most of these landforms were created by movements along the main faults such as Khazar, Nusha, Soheil, Deylaman, Kashachal, representing high tectonic activity in the central and southern parts of the study area. However, the values of these indices were extremely low and indicated low tectonic activity in the northern parts located in the embayment part of the Caspian Sea. Finally, the findings revealed that these classes correspond well to the areas with landforms such as V-shaped and narrow valleys, gorges, deflected and offset streams, deformed Neogene, and Quaternary units that indicate the post-Neogene activity.

Patterns of recent deformation of the western Maracaibo block, northern Colombia and western Venezuela, based on integration of geomorphic indices with regional geology

The Maracaibo block is a triangular, continental tectonic terrane that includes two isolated Andean ranges of northern Colombia and western Venezuela: the Sierra de Santa Marta Massif (SSMM; maximum elevation 5700 m) in the west and the Perija Range (PR; 3600 m) to the east. The Cesar-Rancheria Basin (CRB) is an intermontane basin that separates the two ranges. To establish patterns of recent deformation of this elevated region and to infer its tectonic mechanism, we have integrated the following results: (1) analysis of 350 stream profiles and calculations of geomorphic indices, including stream length-gradient (SL) index, ratio of valley-floor width to valley height (VF), and hysometric curves for 20 watersheds in both ranges and (2) interpretation of three seismic reflection profiles within the CRB and adjacent areas. We determine that the northeastern part of the SSMM is tectonically quiescent based on its concave stream profiles, low geomorphic indices, and few vertical-step knickpoints. In comparison, we find that the central, southern, and eastern parts of the SSMM show tectonic uplift and recent fault control based on slope-break knickpoints and values in steepness and geomorphic indices with possible additional controls from lithologies of varying erosional resistance. Correlations between steepness, SL indices, slope-break knickpoints, and topographic elevations of the SSMM and central PR all indicate recent deformation of these areas. We use seismic reflection profiles from the eastern part of the CRB to confirm the existence of late Quaternary faulting and folding in these geomorphologically active areas. We propose that active, southeastward shallow (approximately 10°–15°) subduction of the Caribbean plate along the base of the South American continental crust produces active crustal deformation within the southern and eastern SSMM. The central PR and eastern CRB are also being deformed by active strike-slip faults.

Geomorphic Response to Differential Uplift: River Long Profiles and Knickpoints From Guadalcanal and Makira (Solomon Islands)

Knickpoints have long been recognised as key geomorphic features that can be used to reveal the landscape evolution of a region. In particular, mobile knickpoints resulting from relative base-level fall record a landscape in the process of change and can encode information about the timing and rate of landscape response. Here, digital elevation model analysis is undertaken to; a) identify topographic lineaments related to active faulting, and b) extract geomorphic metrics and document associated knickpoints for rivers on Guadalcanal and Makira (San Cristobal) part of the Solomon Island chain. These islands have been experiencing uplift of up to 2 mm/yr since at least the mid Holocene on the upper (Pacific) plate of the San Cristobal Trench of the Solomon Island Forearc. For Guadalcanal, 23 out of 53 studied rivers exhibit slope-break knickpoints, characteristic of base-level fall, and 27 new topographic lineaments with ~E-W orientation are identified. By contrast, on Makira 14 of 41 studied rivers have slope-break knickpoints, where the rivers are steeper below the knickpoint than above. In addition, 76 new lineaments are inferred, trending NE-SW and likely to be extensional faults. For both Guadalcanal and Makira there is a good correlation between knickpoint elevation/catchment area and distance upstream from the sea, and a weak correlation between relief and knickpoint elevation. There are no clear relationships between the knickpoints and the new topographic lineaments. These data indicate that both islands are undergoing active river incision caused by regional tectonic uplift along an active subduction zone. On Makira, river steepness (ksn) scales with uplift, and K, coefficient of erosion, is in the range 1 x 10-5 – 7 x 10-6 m0.1yr-1, while K can be estimated as 1 x 10-5 – 5 x 10-8 m0.1yr-1 for Guadalcanal. Calculation of K for steady-state rivers demonstrates a rock strength control on the fluvial response and highlights the importance of lithology on river evolution. Furthermore, the distinct landscape response of the two islands supports the hypothesis that there are different arc segments present along the Solomon Arc and suggests that the Holocene uplift rates for Guadalcanal may not be representative of long-term uplift.

Along-strike variations in recent tectonic activity in the Santander Massif: New insights on landscape evolution in the Northern Andes

The integration of drainage analysis and topographic metrics provides excellent tools to assess the Quaternary tectonic activity modeling landscape evolution in evolving orogens. Furthermore, detecting active structures through the geomorphological analysis of landscapes helps to identify potentially seismogenic structures that could impact the socio-economic conditions in developing countries. Therefore, this paper presents a tectonic geomorphology study conducted on the Santander Massif (SM, Colombian Eastern Cordillera) and surrounding areas. The SM has structures with reported Quaternary activity such as the Bucaramanga, Morro Negro-Las Mercedes, and Chitagá faults, which summed to a poorly-constrained paleosismological history with significant shallow events is the prime incentive of this work. Our study is based on the acquisition of topographic data through swath profiles, local relief, slope variability, filtered topography, minimum bulk erosion, which along with geomorphic indices like the normalized concavity steepness (ksn), the hypsometric integral (HI), the ratio valley floor width to valley height (Vf) and normalized river profiles provided new insights on the recent landscape evolution in this part of the Northern Andes.We compared published apatite fission tracks (AFT) data for the study area with the uplift pattern deciphered from the geomorphic indices to detect the consistency in exhumation and denudation processes both in the long and short-term. Consequently, the central SM, as well as the faulted block between the Bucaramanga and Guamalito faults, evidence a highly-incised landscape with local relief values exceeding the 1500 m, then, responding with a sharp incision to recent surface uplift. Interestingly, this uplift pattern in central SM matches with published AFT data and is related to Neogene exhumation events controlled by secondary structures like the Suratá Fault. We attribute this exhumation event, and accelerated denudation rates within the SM, to the influence exerted by the “collision” between the SM and the Pamplona Indenter, leading to the unroofing of basement rocks at the deformation front (e.g., the Vetas High) and the topographic building of the range.For the upthrown block (hanging wall) of the Bucaramanga Fault, the occurrence of poorly-graded river profiles with an average concavity factor of 12.01, as well as irregular hypsometric curves with convex lower reaches, reinforce the hypothesis that late Cenozoic topographic rejuvenation in the SM was also forced by the tectonic activity of the Bucaramanga Fault. We expect empirical uplift rates >0.08 mm/yr (very high to high tectonic activity) for the mountain front encompassed between La Esperanza and Bucaramanga localities. We state this assumption since the mountain front delimited by the structure mentioned above is quite straight, it preserves remarkable morphostructural features and displays V-shaped valleys with an average Vf index of 0.38. This uplift rate is in good agreement with recent thermochronological exhumation rates (0.1 to > 0.3 mm/yr) published by Siravo et al. (2019).Higher uplift rates associated with the Bucaramanga Fault were also constrained with the erosion proxy index, i.e., the ksn index. High ksn values (above 128) were observed following a linear pattern, which in turn seems controlled by the main trace of the Bucaramanga Fault. For instance, the river system draining the western piedmont of the SM, near Aguachica, records slope-break knickpoints that we interpret as evidence of an incisional wave migrating upstream and adjusting a late Neogene uplift event associated with the Bucaramanga Fault. Conversely, the northern SM presents variable tectonic activity with the inner part, east of the Guamalito Fault, presenting low tectonic activity with little surface uplift and dominant strike-slip kinematics. We recognize the Ocaña-Ábrego zone as a relict landscape with well-preserved non-consolidated deposits that may record the initial exhumation and denudational pulses of the SM during the Andean Orogeny.

Quantifying the transient landscape response to active faulting using fluvial geomorphic analysis in the Qianhe Graben on the southwest margin of Ordos, China

River morphology has been widely used to record and track the transient landscape response to active faulting. Here we evaluate the landscape response to active faulting in the Qianhe Graben of southwest Ordos, China. In this region, it has been difficult to determine the activity of mapped faults because of the presence of thick Quaternary Loess; however, by analysing the presence and distribution of slope-break knickpoints in river longitudinal profiles, the ongoing tectonic uplift of the Qianhe Graben can be investigated. The alignment of vertical-step knickpoints gives a new insight into the location of an active fault on the southern margin of Qianhe Graben. Additionally, slope-break knickpoints, typical of fault controlled landscape change, were identified from 24 river longitudinal profiles that drain across normal faults along both graben margins. Along strike from north to south, the knickpoints varied systematically with relief, and the height of the knickpoints also decrease to the southeast. Indicating that the rate of motion on the faults, likely is greater in the northwest and decays southeastwards. The horizontal knickpoint retreat rates range from 0.3 to 27.3 mm/yr, constraining the landscape response time with fault initiation at 1.2–1.4 Myr. In comparison with other studies, the knickpoint recession triggered by base-level fall as a result of faulting is relatively lower than when the base-level fall is the result of sea-level fall potentially the result of different mechanism of retreat. Finally, the potential for earthquakes along the Taoyuan-Guichuansi Fault (TGF) before and after fault linkage was assessed, indicating the potential for earthquakes of Mw of 6.3–6.7 and 6.8–7.0, respectively. These observations not only suggest the knickpoints are recording fault evolution in Qianhe Graben, but also provide information on seismic hazard in this populous region.

Transient basin as indicator of tectonic expressions in bedrock landscape: Approach based on MATLAB geomorphic tool (Transient-profiler)

The tectonic and erosional processes sculpture the Earth's surface, producing a landscape that preserves the evolutionary history of rock uplift and incision. The bedrock channels act as an efficient gradation agent where the river profile adjusts to changing tectonic, climatic, or topological (e.g., river capture) boundary conditions, and develops a transient basin. A suite of slope-break knickpoints at comparable elevations in the channel profiles of the transient basin act as important markers. We developed a set of comprehensive and interactive MATLAB tools, called a Transient-profiler, to extract the geomorphic indices from the digital elevation model for a basin characterization and landscape evolution period. The paleo-channel reconstruction from the relict reach in the three studied transient drainage basins (Palar, Umiam, and Dibang) suggest a varying surface uplift from 514, 1246, and 1981 m with a respective relief gain of 158%, 230%, and 95%, respectively. We found that the horizontal knickpoint retreat rate is faster in the Himalayan thrust system as compared to the Shillong pop-up system, whereas the vertical knickpoint retreat rate is faster in the latter.

Uplift analysis on a pericratonic region: An example in the Sierras de Córdoba (29°–34°S), Argentina

The Sierras de Córdoba (Argentina) is a mountain belt located in the most distal (pericratonic) part of the South-Central Andean foreland, in the easternmost end of the Sierras Pampeanas (Pampean ranges) broken foreland. Although this region is considered an excellent example for settings where basement thrusting generates relief and intermontane basin systems, hundreds of kilometers away from the Cordilleran front, the driving mechanisms and main controls on topography are still poorly understood. Given that the landscape, and particularly the rivers, react to changes in climate and tectonics on timescales between 103 and 106 yrs, we used DEM-based morphometry (elevation patterns, local relief, and analysis of 242 longitudinal stream profiles) to evaluate the main controls on modern topography. We focus on the morphologies of the main streams and their tributaries using metrics of the channel concavity and normalized channel steepness index (ksn). The distribution of local relief, concavity and ksn values was compared with the present-day topography, tectonic structures, rock-type and modern rainfall patterns. The stream longitudinal profiles show homogeneous concavities and ksn background values, with some local anomalies. Local structures (east and west vergent thrusts) in the Sierras de Córdoba seem to be the most fundamental controls on the formation of these anomalies, slope-break knickpoints as well as deep incision (high local relief). This suggests localized and accelerated rock uplift rates, which produce localized increases in the gradient along the longitudinal stream profile. It is important to notice that the highest ksn values were reported on the eastern range flanks, coincident with the highest values of mean annual rainfall. However, high ksn values appear to be maintained even with greater rainfall. Our study provides new insights on the Neogene Sierra de Córdoba uplift evolution, which will assist future sampling and modelling (cosmogenic nuclides analysis, thermochronology and GPS analysis).

Which DEM is best for analyzing fluvial landscape development in mountainous terrains?

Regional studies of fluvial landforms and long-term (Quaternary) landscape development in remote mountain landscapes routinely use satellite-derived DEM data sets. The SRTM and ASTER DEMs are the most commonly utilised because of their longer availability, free cost, and ease of access. However, rapid technological developments mean that newer and higher resolution DEM data sets such as ALOS World 3D (AW3D) and TanDEM-X are being released to the scientific community. Geomorphologists are thus faced with an increasingly problematic challenge of selecting an appropriate DEM for their landscape analyses. Here, we test the application of four medium resolution DEM products (30m=SRTM, ASTER, AW3D; 12m=TanDEM-X) for qualitative and quantitative analysis of a fluvial mountain landscape using the Dades River catchment (High Atlas Mountains, Morocco). This landscape comprises significant DEM remote sensing challenges, notably a high mountain relief, steep slopes, and a deeply incised high sinuosity drainage network with narrow canyon/gorge reaches. Our goal was to see which DEM produced the most representative best fit drainage network and meaningful quantification. To achieve this, we used ArcGIS and Stream Profiler platforms to generate catchment hillshade and slope rasters and to extract drainage network, channel long profile and channel slope, and area data. TanDEM-X produces the clearest landscape representation but with channel routing errors in localised high relief areas. Thirty-metre DEMs are smoother and less detailed, but the AW3D shows the closest fit to the real drainage network configuration. The TanDEM-X elevation values are the closest to field-derived GPS measurements. Long profiles exhibit similar shapes but with minor differences in length, elevation, and the degree of noise/smoothing, with AW3D producing the best representation. Slope-area plots display similarly positioned slope-break knickpoints with modest differences in steepness and concavity indices, but again best represented by AW3D. Collectively, our study shows that despite the higher effective resolution of TanDEM-X (12m), the AW3D (30m) data performs strongly across all analyses suggesting that it currently offers the greatest potential for regional mountain geomorphological analyses.

Along-strike variation in catchment morphology and cosmogenic denudation rates reveal the pattern and history of footwall uplift, Main Gulf Escarpment, Baja California

Topography is expected to record tectonic, climatic, and rock strength controls on long-term denudation rates in active margins. We test this hypothesis in the Sierra San Pedro Martir, Mexico—the footwall of the normal-faulted, western margin of the Gulf of California rift system—by relating topographic metrics with 10 Be-derived catchment-averaged denudation rates. Denudation rates and topographic metrics record along-strike gradients in rock uplift relative to base level that increase asymmetrically from fault tips to maxima within the northern half of the range. Surface uplift of an Eocene erosional surface and slope-break knickpoints found at increasingly higher elevations in the northern segments of the Sierra San Pedro Martir fault system suggest that range asymmetry is due to a recent northward acceleration in the rate of rock uplift relative to base level. By characterizing the relationship between channel steepness and cosmogenic denudation rates, we extrapolate millennial-scale denudation rates to million-year time scales and estimate ages for the transient increase in rock uplift rates and the initial onset of normal faulting. We infer that the Sierra San Pedro Martir fault system initiated during the middle Miocene (ca. 16–14 Ma) in the center of the range and ca. 12–8 Ma near the fault tips. Recent increases in rock uplift rates during the late Pliocene (ca. 3–2 Ma) coincide with lithospheric rupture in the Delfin basins to the east and represent a westward migration of strain from hanging-wall detachments to the Sierra San Pedro Martir fault system. Age estimates are consistent with independent geologic constraints and show that pairing of carefully selected cosmogenic denudation rates with topographic analysis can be used to extract tectonic signals from topography over million-year time scales.

The role of waterfalls and knickzones in controlling the style and pace of landscape adjustment in the western San Gabriel Mountains, California

Bedrock rivers set the pace of landscape adjustment to tectonic and climatic forcing by transmitting signals of base-level change upstream through the channel network and ultimately to hillslopes. River incision is typically modeled as a monotonic function of bed shear stress or stream power, modulated by sediment tools and cover effects, but these models do not apply in channels with steep or vertical bedrock reaches due to changes in flow dynamics, hydraulic geometry, and bed cover. Here, we investigate how such knickzones (oversteepened channel reaches often containing waterfalls) influence the propagation of slope-break knickpoints that separate relict from adjusting topography, and thus the response times of landscapes to external forcing. We use a conceptual long-profile model to explore the consequences of waterfalls and knickzones on channel response and compare predictions to light detection and ranging (LiDAR) topography, field observations, and cosmogenic radionuclide data from Big Tujunga Creek, a 300 km2 watershed in the San Gabriel Mountains, California. Three prominent knickzones along Big Tujunga Creek, characterized by numerous waterfalls, show contrasting behavior. For the upper knickzone, waterfalls align with bands of harder rock exposed on adjacent hillslopes, and between waterfalls, the channel is mantled by large (>2 m) boulders, indicating knickzone retreat is slow compared to predictions of slope-break knickpoint retreat from stream-power models, enhancing the preservation of an upstream relict landscape. The middle knickzone shows evidence for both fast and slow knickzone retreat, as well as significant deviations from predictions of uniform tributary knickpoint elevations derived from stream-power models. The lower knickzone is characterized by a waterfall and knickzone within an incised inner gorge that provide evidence of rapid retreat relative to background channel incision. Overall, we find a pattern of decreasing knickzone and waterfall retreat rate with distance upstream of the range front, beyond decreases predicted by simple area-dependent celerity models. Our results highlight that waterfalls and knickzones can both enhance and inhibit landscape adjustment, leading to divergent controls on the pace of landscape evolution.

Transient fluvial incision as an indicator of active faulting and Plio-Quaternary uplift of the Moroccan High Atlas

One of the challenges facing Earth Scientists is to determine the extent to which geomorphic features can be used to extract tectonic signals from landscapes. Here, we quantitatively analyse the long profiles of rivers that drain southwards across the South Atlas Fault (SAF), a thrust fault that forms the southern margin of the High Atlas Mountains in Morocco, to derive new data on the Late Cenozoic activity of this fault system. River long profiles were extracted for 32 major rivers flowing southwards into the Ouarzazate Basin. Of these, eleven exhibit concave-up river profiles with a mean concavity of 0.64 and normalized steepness indices in the range 47.5–219.0m0.9. By contrast, 21 rivers exhibit at least one knickpoint upstream of the thrust front. Knickpoint height varies from 100 to 1300m, with calculated incision at the range bounding fault ranging from 80 to 900m, despite the drainage areas upstream of the knickpoint ranging over several orders of magnitude. In map view, knickpoint locations generally plot along sub-parallel lines and there are no obvious relationships with lithological units for knickpoints exhibiting slope-break morphology. Channel reaches below slope-break knickpoints have higher mean concavities (0.76) than above the knickpoint indicative. This observation combined with a lithological or river-capture origin for the knickpoints having been ruled out suggests that an increase in uplift rate along a planar fault zone during the Plio-Quaternary caused the initiation of the transient response (i.e., knickpoint formation) to a change in base-level observed in the river profiles. This uplift event can be correlated to the convective removal of the lithospheric root to the Atlas Mountains resulting in the anomalously high topography at the present day.