Regional Landscape Evolution Research Articles

The Roman to medieval landscape transformation at Aardenburg (southern The Netherlands) based on palynology and diatom analysis

The region of Bruges (Flanders, Belgium) was an economic and cultural centre during the Late Middle Ages because it was connected to the North Sea via a large tidal inlet called the Zwin, along which smaller towns developed. One of these towns was Aardenburg (Zeeland, The Netherlands), which developed from a Roman castellum to become one of the important medieval towns in the hinterland of Bruges. Unfortunately, archaeological, historical and (palaeo)environmental data about the Roman to medieval evolution of the Zwin area are scarce. However, the continuous occupation of Aardenburg during the first millennium AD and its location on a Pleistocene sand ridge bordering the coastal plain provide a unique opportunity to investigate the natural dynamics of the coastal evolution and the interplay with the human impact that shaped the Zwin region before its heyday. The variable character of the depositional environments on the dynamic coastal plain makes local environmental studies essential for understanding larger patterns, which were previously reduced to a simplistic outdated model over the entire region. Palynological and diatom assemblages of two sequences in Aardenburg allow the reconstruction of the terrestrial and aquatic palaeoenvironments from the Middle Holocene to the high Middle Ages. The results reveal landscape evolution in relation to woodlands, peat bogs, coastal environments and human presence, consistent with previous regional landscape evolution. A distinct increase in marine influence during the Roman and early medieval periods, when human activities played a significant role, was followed by an increase in inland indicators from the high medieval period onwards. This evolution is in accordance with the expansion of the town and the reclamation of the coastal landscape as it evolved to become a cultural centre in the Late Middle Ages.

Onset of dune construction based on archaeological evidence, White Sands, New Mexico

AbstractThe White Sands dune field is the largest gypsum dune system in the world, derived from deflation of paleo-Lake Otero deposits. Understanding the timing of initial dune construction, and therefore lake deflation, is critical for understanding regional landscape evolution, including the history of lake desiccation. The onset of dune construction is currently estimated at ~8000 to 6500 cal yr BP, but numerical age control is limited. Archaeological evidence reported here indicates two older phases of gypsum dune construction. An archaeological site draped over a parabolic dune south of the main dune body contains artifacts dating to >12,200 cal yr BP, providing an upper age limit for the landform. Another site buried within a remnant of the main dune field yielded six statistically identical radiocarbon dates averaging ~8770 cal yr BP. The initial phase of terminal Pleistocene deflation and parabolic dune construction was perhaps localized but correlates with a period of regional aridity. Barchans and crescentic ridges comprising the main dune body developed in the Early Holocene in response to elevated salinity in local ground water and extensive exposures of gypsum available for deflation, likely due to aridity.

Holocene landscape evolution and its interaction with human activities in the southern piedmont of Taihang Mountain, Central China.

Piedmont zones have been witnessing intensive human activities since ancient times. However, it remains unclear when it comes to the environmental mechanism for early humans exploiting piedmont zones. Here we present a case study about the interactions between early human activities and landscape evolution in the piedmont of Taihang Mountain, an area with prominent ecological and cultural significance in Chinese history. Based on chronological and pollen analyses, we reconstruct the regional landscape evolution in the Fengtougang (FTG) site of southern Taihang Mountain during the Holocene. The results show that the area has been dominated by terrestrial plants since the late Longshan culture (4000 BP), including a large number of Pinus, Artemisia, Spiraea, and Gramineae, a few Cattails, and some other aquatic herbs. During the early history (4000-2000 BP), there is a combination of Pinus, Artemisia, Spiraea, Compositae, and Selaginella Chinensis, with a few aquatic plants. Since the late history (500 BP), the Chinese selaginella, Pinus, Selaginella, and Sedge families dominate, with no aquatic plant pollen found. Combining the detailed geoarchaeological survey, grain size analysis, and magnetic susceptibility analysis, we demonstrate that there should be a landscape of extensive floodplain during the early-middle Holocene (10000-4000 BP). During the late Longshan culture (about 4000 BP), the study area should be dominated by a landscape of sparse forest grassland with interlacing rivers and lakes. With river downcutting and watercourse fixation since the late Longshan culture, the flooded area massively shrinks, providing suitable habitat for human settlement. From then on, human activities begin to move to the study area on a large scale, followed by continuous cultural development and thriving early civilization.

Determining the origin and changing shape of landscape‐scale rock formations with three‐dimensional modelling: The Borologa rock shelters, Kimberley region, Australia

AbstractArchaeologists often wonder how and when rock shelters formed, yet their origins and antiquity are almost never systematically investigated. Here we present a new method to determine how and when individual boulders and rock shelters came to lie in their present landscape settings. We do so through 3D laser (LiDAR) mapping, illustrating the method by example of the Borologa Aboriginal site complex in the Kimberley region of northwestern Australia. Through a combination of geomorphological study and high‐resolution 3D modelling, individual blocks of rock are refitted and repositioned t680their originating cliff‐line. Preliminary cosmogenic nuclide ages on exposed vertical cliff faces and associated detached boulders above the Borologa archaeological sites signal very slow detachment rates for the mass movements of large blocks down the Drysdale Valley slopes, suggesting relative landscape stability over hundreds of thousands of years (predating the arrival of people). These findings offer hitherto unknown details of the pace of regional landscape evolution and move us toward a better understanding of patterns of human occupation in a context of relatively stable rock outcrops both within the sites and across the region.

Rock slope failure in the Western Alps: A first comprehensive inventory and spatial analysis

Rock slope failure (RSF) occurs in different contexts but is typically reported either as (i) single-category inventories or (ii) single-site geotechnical monographs. Few studies have sought to evaluate the spatial incidence of all modes of RSF conjointly, and to infer scenarios of regional landscape evolution from observed patterns of cumulative rock slope overstressing. Here we present the results of a systematic inventory of rock avalanches, rockfalls, rockslides, and gravitational rock slope deformations in the Western Alps (France, Italy, Switzerland) conducted using satellite imagery made available in Google Earth as a detection tool, and aided by preliminary ground-truth checks. The inventory totals 1416 montane RSFs, impacting 9.1% of the study area. Underpinned by GIS tools, the study further examines the spatial distribution of RSF with consideration for (i) predisposing factors (typically: lithology, geological structure), and (ii) preparatory factors (geomorphological process regimes that drives a given slope segment to the point of failure). The latter encompass slower variables (e.g., long-term crustal stress regime, cumulative residence time above equilibrium line altitudes) and faster variables (e.g., short-span glacier-related stresses, permafrost thaw, seismicity). RSF density patterns helped to define seven RSF super-hotspots (large diversity of RSF modes, up to 50% of displaced rock masses/unit area), which define the most intensely overstressed areas of the Western Alps. These super-hotspots occur at sites where highly dynamic, thick, warm-based glaciers above the equilibrium line either intersected (middle Maurienne) or followed the strike of (middle Isère) N-S bands of highly susceptible lithologies and structures during the Quaternary. The widespread incidence of rock slope deformation (cumulative area: 1760 km2, i.e., nearly 3 times the total of the other three RSF categories combined) appears further correlated with the low tectonic activity of the orogen and with its areas dominated by an extensional tectonic regime west of the Penninic Frontal Thrust. This contrasts with seismically active orogens, e.g., New Zealand's Southern Alps, where rock slope deformation is scarce compared to rock avalanches and shallow landslides.

Migrating Transverse Escarpments in Strike Valleys on the Colorado Plateau

AbstractErosion in strike valleys separating parallel cuesta escarpments controls the evolution of staircase‐like landscapes developed in layered rocks. We explore an important erosional process in strike valleys developed in the Mancos Formation of central Utah, USA. These shale‐floored valleys are segmented by transverse (strike‐perpendicular) scarps, which we call migrating transverse escarpments (MTEs). The MTEs separate axial stream networks that join larger transverse rivers from gullies that drain across adjacent sandstone layers. The MTEs mark a state of transience within the strike valleys as the axial stream networks expand along strike and adjust the gullies to a new base level at the transverse rivers. Here, we describe MTE morphology, consider the geomorphic processes that control their evolution, discuss their impacts on regional landscape evolution using field relationships, and explore how variable geologic parameters may affect their behavior using numerical simulations. Our simulation results suggest that fluvial pour points through the adjacent hard layers serve as internal base levels that modulate MTE evolution. Once a stable axial stream network forms, new MTEs cannot form until the channel network reroutes through the hard layers. This likely occurs over glacial‐interglacial timescales as debris deposition reroutes channel networks and variable hydrologic conditions accelerate transverse river incision. However, our results also suggest that MTEs can form autogenically as downdip strike valley retreat causes the lower hard layer to migrate underneath the axial stream. Our findings highlight the influence of lithologically controlled base levels on landscape evolution, particularly in regions of variable layered rocks.

Last appearance of Homo erectus at Ngandong, Java, 117,000-108,000years ago.

Homo erectus is the founding early hominin species of Island Southeast Asia, and reached Java (Indonesia) more than 1.5million years ago1,2. Twelve H.erectus calvaria (skull caps) and two tibiae (lower leg bones) were discovered from a bone bed located about 20m above the Solo River at Ngandong (Central Java) between 1931 and 19333,4, and are of the youngest, most-advanced form of H.erectus5-8. Despite the importance of the Ngandong fossils, the relationship between the fossils, terrace fill and ages have been heavily debated9-14. Here, to resolve the age of the Ngandong evidence, we use Bayesian modelling of 52radiometric age estimates to establish-to our knowledge-the firstrobust chronology at regional, valley and local scales. We used uranium-series dating of speleothems to constrain regional landscape evolution; luminescence, 40argon/39argon (40Ar/39Ar) and uranium-series dating to constrain the sequence of terrace evolution; and applied uranium-series and uranium series-electron-spin resonance (US-ESR) dating to non-human fossils to directly date our re-excavation of Ngandong5,15. We show that at least by 500thousand years ago (ka) the Solo River was diverted into the Kendeng Hills, and that it formed the Solo terrace sequence between 316 and 31ka and the Ngandong terrace between about 140 and 92ka. Non-human fossils recovered during the re-excavation of Ngandong date to between 109 and 106ka (uranium-series minimum)16 and 134 and 118 ka (US-ESR), with modelled ages of 117 to 108thousand years (kyr) for the H.erectus bone bed, which accumulated during flood conditions3,17. These results negate the extreme ages that have been proposed for the site and solidify Ngandong as the last known occurrence of this long-lived species.

Loess in the mid-Atlantic region, USA

AbstractLoess is common in the Mid-Atlantic region of the United States south of the Late Wisconsinan glacial border particularly along rivers draining the glaciated areas of Pennsylvania, New Jersey, and New York. The broadest deposits occur on the flat landscapes of the Delmarva Peninsula in Maryland where two episodes of deposition have been identified. The earlier Miles Point Loess has a limited distribution and is buried by the more widespread Paw Paw Loess. OSL and 14C dates place deposition of the Miles Point Loess during MIS 3. The well developed paleosol formed in the Miles Point Loess acts as a stratigraphic marker. The Paw Paw Loess buries Clovis age cultural materials which date deposition to the end of the Pleistocene. Loess deposits and paleosols are critical in understanding regional landscape evolution, Late Pleistocene environments, and early North American cultural history. Mapping the extent of loess in the Mid-Atlantic using the Natural Resources Conservation Service’s gSSURGO database overrepresents loess in some areas and underrepresents in others.

Post-tectonic landscape evolution in NE Iberia using staircase terraces: Combined effects of uplift and climate

River incision into bedrock resulting from the combined effects of tectonic uplift and climate governs long-term regional landscape evolution. We determined spatial and temporal patterns of post-orogenic stream incision from a sequence of well-preserved staircase terraces developed over the last 1Ma in the Central Pyrenees and its southern foreland Ebro basin (NE Spain). Extensive remnants of ten vertically separated terraces (Qt1 to Qt10, from oldest to youngest) were mapped along 170km of the Cinca River valley, transverse to the Pyrenean mountain belt. Multiple outcrops appear in the upper reach of the valley (Ainsa sector, 50km from headwaters) as well as in the lower reach (Albalate sector, 125km from headwaters). Fluvial incision into bedrock was calculated using (i) differentially corrected GPS measurements of the altitude of straths and (ii) numerical dating of alluvial sediments from the lower terraces (Qt5 to Qt9) by Optically Stimulated Luminescence, previously reported by Lewis et al. (2009), and supplemented with new dates for the upper terraces (Qt1, Qt2 and Qt3) based on paleomagnetism and supported by soil development. Considering altitude differences and the elapsed time between successive well preserved terrace couples (Qt3–Qt7, Qt7–Qt9 and Qt9-Active channel), mean bedrock incision rates ranged from 0.76 to 0.38m ka−1, at the upper reach of the valley (Ainsa section), and from 0.61 to 0.20mka−1, at the lower reach (Albalate section). River incision along the valley produced vertically separated, near-parallel longitudinal terrace profiles evidencing a rapid near-uniform regional uplift as response to (i) the tectonic lithospheric thickening in NE Iberia and (ii) the erosional download rebound related to the Ebro basin exorheism. Moreover, a subtle upstream divergence of strath profiles may have been a consequence of an increase in uplift rate toward the head of the valley. Additionally, incision rates changed over time as indicate results from the lower reach (Albalate section); the maximum rate was 1.48mka−1 between Qt7 (61ka) and Qt8 (47ka), and the minimum rate was 0.11mka−1 between Qt3 (401ka) and Qt5 (178ka). The highest incision rates were produced after the Marine Isotope Stage 4 most likely in response to (i) an increased snowmelt discharge during the subsequent deglaciation related to the last maximum advance of glaciers in the southern Pyrenees, and (ii) a limited width of the valley after Qt7 formation, resulting from the deactivation of the westward river migration. Therefore, incision rates over the last 1Ma in the Cinca River valley were basically controlled by near-uniform bedrock uplift, in the context of climate variability. The results reported in this study represent significant data on fluvial incision in NE Iberia, and provide an assessment of the regional post-tectonic landscape evolution.

Holocene landscape evolution of an estuarine wetland in relation to its human occupation and exploitation: Waasland Scheldt polders, northern Belgium

AbstractThis paper describes the landscape evolution of the Waasland Scheldt polders in the north of Belgium from the Late Glacial – early Holocene to the present time, and the effects of this changing landscape on the human settlement. The regional landscape evolution has been visualised in a series of palaeogeographical maps for successive time frames. Two different map series were produced: a series of Holocene palaeogeographical reconstructions (11,000–950 cal BP) based on geotechnical, geological and archaeological data, and a series of post-Medieval landscape reconstructions (16th- to 19th-century) based on historical maps, land registers and soil data. Additional palaeoenvironmental information from fossil pollen and plant remains allowed reconstruction of the vegetation and wetland changes, particularly for the middle to late Holocene. Peat growth was the main key to understanding the landscape evolution of the Waasland Scheldt polders. Whereas the landscape evolution during the Holocene was mainly sea-level driven, the transformation of the landscape during the last millennium was largely due to human interventions.

Late Pleistocene river migrations in response to thrust belt advance and sediment-flux steering — The Kura River (southern Caucasus)

One reaction of rivers toward allogenic triggers is the large-scale river channel migration in the form of avulsions or progressive lateral migrations (combing) that are widespread phenomena around the world during the late Quaternary. Because they potentially cause significant human and economic losses and significantly change geomorphic processes in the affected regions, a deeper knowledge about causes and rates is essential and furthermore helps to identify the dominant drivers of regional landscape evolution during different periods. One possible cause for river channel migrations is sediment-flux steering, i.e. the shift of rivers in sedimentary basins against a tectonically driven trend caused by transverse sediment discharge. During the last 30years, sediment-flux steering has been investigated by field and experimental studies in extensional half-grabens with generally small-sized transverse catchments and/or volcaniclastic sedimentation.This study presents geomorphologic, geochronologic, and heavy mineral analyses together with complementary tectonomorphometric and earthquake data to investigate late Quaternary channel migrations of the Kura River in the southern foreland basin of the Greater Caucasus, a region where the late Quaternary landscape evolution is rather fragmentarily understood so far. Special emphasis of this study is given to the interplay between axial river flow and transverse sediment supply leading to sediment-flux steering. Large-scale migrations of the course of the Kura River during the late Quaternary reflect the interplay between tectonic processes leading to the southwestward advance of the Kura Fold-and-Thrust-Belt and climatically-triggered sediment-flux steering caused by aggradation phases of transverse rivers with comparatively large catchment areas in the Lesser Caucasus. During generally warmer periods such as the Holocene with fluvial incision and low sediment supply from the transverse rivers, the main Kura River could follow its tectonically driven trend toward the southwest. In contrast, during generally colder periods such as the upper late Pleistocene, sediment-flux steering caused by aggradation of the transverse rivers forced the main Kura River to migrate >10km against that tectonically induced trend toward the northeast. Apart from giving information about the main drivers of the late Quaternary landscape evolution in this part of the southern Caucasus region, this study also helps to understand the cause of a permanent threat of settlements and the loss of fertile agricultural land in the intensively labored Marneuli Depression of southern Georgia.

L’histoire des activités agricoles et des paysages révélée par un outil en plein essor : l’ADN sédimentaire lacustre

Here we present the results from a bourgeoning tool, the analyses of lake sediment DNA applied to the reconstruction of agriculture and landscape histories (plant and mammal DNA) in the French Alps. As the tool is pioneer, the first part of the manuscript is focused on the study of the conditions to archive the DNA in lake sediments and on the reliability of the data. Results suggest that lakes submitted to high detrital inputs are more favourable to the record of DNA coming from the catchment. Moreover, we show that DNA and pollen analyses are complementary. Combining these two approaches, we can distinguish the local from the more regional landscape evolution. The reconstruction of local landscapes and the access to the composition of livestock farming are the main advantages of the lake sediment DNA tool. With this innovative tool, we can reconstruct a detailed picture of the alpine agriculture dynamic in space and time, as well as study their effects on the environment.

Quaternary fluvial archives and landscape evolution: a global synthesis

Late Cenozoic (and especially Quaternary) fluvial deposits and related landforms provide valuable information about landscape evolution, not just in terms of changing drainage patterns but also documenting changes in topography and relief. Recently compiled records from river systems worldwide have shed much light on this subject, particularly records of terrace sequences, although other types of fluvial archive can be equally informative. Terraces are especially valuable if they can be dated with reference to biostratigraphy, geochronology or by other means. The various data accumulated support the hypothesis that the incision observed from river terraces has been a response to progressive uplift during the Late Cenozoic. This has not occurred everywhere, however. Stacked fluvial sequences have formed in subsiding depocentres and have greater potential for surviving to become part of the longer-term geological record. More enigmatic are regions in the ancient cores of continents (cratons), which show little indication of sustained uplift or subsidence, with fluvial deposits of various ages occurring within a restricted range of elevation with respect to the valley floor. In areas of dynamic crust that were glaciated during the Last Glacial Maximum, post-glacial river valleys are typically incised and often terraced in a similar way to valleys on post-Precambrian crust elsewhere, although the terraces and gorges in these systems are very much younger (∼15ka) and therefore the processes have been considerably more rapid. This paper is illustrated with various case-study examples of these different types of archives and discusses the implications of each for regional landscape evolution.

Tectonic beheading of fluvial valleys in the Maestrat grabens (eastern Spain): Insights into slip rates of Pleistocene extensional faults

Interaction between faulting and landscape evolution in regions of active tectonics allows us to use subtle geomorphological markers for estimating fault slip rates. Geomorphic features of two valleys connected with the bottom of the Alcalà de Xivert graben, at the Maestrat graben system (eastern Spain), suggest that they correspond to the lowest segments of ancient valleys whose original heads were located at the axis of the neighbouring Irta range. They were beheaded owing to displacement of the Torreblanca and Irta faults during a period of active extensional faulting in Middle Pleistocene times. These faults produced a negative inversion of the relief, sinking a narrow graben (the Ametler graben) at the middle of the Irta range whose alluvial infill buried the midsegments of the beheaded valleys. This hypothesis has been tested by applying two geomorphic indices, the Stream-gradient index (SL) and the Valley width/height ratio (Vf), as well as by considering surface and subsoil information about the sedimentary infill of the Ametler graben. From this evolutionary model, after reconstructing hypothetical longitudinal profiles of the ancient rivers, and taking into account a new absolute age obtained for the alluvial infill of the Ametler graben, slip rates at the northern segment of the Torreblanca fault have been approached. The throw rate has been constrained between a minimum of 0.04–0.07mm/year for the last 1.9 to 2.6Ma, and a maximum of 0.26–0.30mm/year for the last 253.3±18.0ky. These values are comparable to those averaged on other active faults in the central-eastern Iberian Chain for the overall Late Pliocene–Pleistocene times. Nevertheless, the Torreblanca fault shows exceptionally high activity within the context of the Maestrat and Catalonian grabens, which can explain its deep imprint in landscape evolution.

Engineering Geological Mapping in the Basaltic Cuestas, São Paulo State, Brazil

Some difficulties of performing an engineering geological mapping task in sites where residual and transported sandy soils are widespread are discussed in this paper. The study area is located in the municipality of Analândia in the southeast of Brazil. In this region landscape evolution is dominated by scarp retreat and relief inversion processes. As transported and residual soils have very similar grain size distributions, the results of geotechnical tests performed on both kinds of soils are very similar. Understanding the regional landscape evolution was essential for performing the mapping task. The method used combined aspects of engineering geological mapping based on synthetic and analytical approaches and it is strongly supported by field observations. The expected engineering behaviors of the mapped units are presented. Some of the units correspond to colluviums that occupy hilltops, a geomorphological position were they would not be expected. Moreover, the “stone lines” are common features in the area and their identification clearly defines the unconsolidated material as allochtonous. However, they are not always located at the bottom of the colluvial deposits, and sometimes they are not present at all or show lateral discontinuity. Some other characteristics are distinctive of these widespread deposits.

A thin predominantly cold-based Late Miocene East Antarctic ice sheet inferred from glaciovolcanic sequences in northern Victoria Land, Antarctica

Glaciovolcanic sequences are widespread in northern Victoria Land and their interpretation enables estimates of past ice thicknesses and other defining parameters of the coeval ice sheet to be made with considerable certainty. The terrestrial sequences are mainly Late Miocene in age (c. 12–5 Ma) and conform to five main types: aa lava-fed deltas, volcanic sheet-like sequences, a glaciolacustrine sequence, scoria cones and tuff cones. Apart from the environmentally undiagnostic pyroclastic cones, the volcanic sequences were erupted in association with a glacial cover that was typically, and possibly never much more than, a few hundred metres thick (< 300 m). Despite gaps in the record, no evidence was observed for prolonged subaerial (i.e. fully ice-free) eruptive conditions, suggesting either that ice-free conditions did not occur or else were of such limited areal extent or short duration that they left no record. These observations are consistent with the presence of a thin persistent Late Miocene ice dome or icefield draping the pre-Miocene topography in northern Victoria Land for the period, although it may have been confluent with the greater East Antarctic Ice Sheet similar to conditions present today. The presence of glacially emplaced sequences right down to present datum suggests that the Transantarctic Mountains in the region were already uplifted to their current elevation prior to the volcanism. The high terrain would have helped the ice sheet to establish and persist. The glacial thermal regime varied from wet-based and dynamic, to cold-based (frozen to its bed) and presumably relatively stable. A major transition from wet-based ice to cold-based ice might have occurred between 9.7 and 7.5 Ma consistent with published interpretations of the regional landscape evolution. Alternatively, the glacial thermal regime was dominantly polar since the earliest eruptions but with several temporary changes to a wet-based (sub-polar or temperate) more dynamic regime.

Relief evolution of the Quadrilatero Ferrifero (Minas Gerais, Brazil) by means of (10Be) cosmogenic nuclei

This paper investigates the evolution of the relief within the so-called Iron Quadrangle (Quadrilatero Ferrifero) in Minas Gerais, Brazil, by means of quantification of erosion processes, which affected the principal rock types of the region. The method used is based on measurement of in-situ produced cosmogenic 10Be concentration in fluvial sediments. The results indicate that the regional landscape evolution is controlled by lithotypes: the most resistant areas correspond to substrates developed on itabirites and quartzites (long-term erosion rates between 0.29 to 2.35 m/My) and, whereas the most fragile ones developed on schist-phyllites (long-term erosion rates between 7.95 to 11.82 m/My) and granite-gneisses (long-term erosion rate at 12.92 m/My).

Large rockslides in the Southern Central Andes of Chile (32–34.5°S): Tectonic control and significance for Quaternary landscape evolution

The distribution and age of large (> 0.1 km 2) Pliocene to recent rockslides in the Chilean Cordillera Principal (32–34.5 S), the Southern Central Andes, has been analyzed to determine the rockslide triggering mechanisms and impact on regional landscape evolution. Most of the rockslides appear in the western Cordillera Principal and cluster along major geological structures. Variographic analyses show spatial correlation between rockslides, geological structures and shallow seismicity. A relative chronosequence was calibrated with existing 14C and 40Ar/ 39Ar dates and new cosmogenic nuclide exposure ages for selected rockslides. Rockslide-induced sediment yield was estimated with empirical relations for rockslide area distributions. Throughout the Quaternary, rockslides have delivered sediment to streams at rates equivalent to denudation rates of 0.10 ±0.06 mm a − 1 , while estimates using short term (20 a) seismicity records are 0.3 − 0.2 + 0.6 mm a − 1 . The estimates of sediment transfer and the spatial distribution of rockslides reflect a landscape in which tectonic and geological controls on denudation are more significant than climate.

Impact of rock uplift on rates of late Cenozoic Rocky Mountain river incision

[1] The high relief of the modern Rocky Mountain landscape formed in the late Cenozoic by downcutting of a fluvial network that links a series of easily eroded sedimentary basins across relatively resistant crystalline cores of adjacent ranges. Using a numerical model of fluvial erosion and the flexural isostatic response to the associated unloading, we first calculate the expected pattern and pace of incision caused by rock uplift related to migration of the Yellowstone hot spot and to growth of the northern portion of the Rio Grande rift. Calculated incision rates are <60 m/Myr, and total depth of erosion of sedimentary basins is <300 m, well below the long-term incision rates and amounts of erosion interpreted from the geologic record. Broad-scale tilting of the region toward the east, accomplished by a gradient in rock uplift of ∼1 km along the north-south axis of the central Rockies, declining to zero 1000 km to the east, can account for the additional erosion needed to match observations. In each modeling scenario, stream incision is nonsteady, with rock uplift outpacing erosion for <1 Myr in perimeter basins and 1–5 Myr in interior basins. Three factors dominate the spatial and temporal pattern of regional landscape evolution: (1) the time since uplift began, (2) the uplift pattern, and (3) the distribution of relatively resistant bedrock within the region. Our results suggest that the spatial variability in late Cenozoic exhumation can be explained by a long-lived transience in the stream network response to these various late Cenozoic geophysical events.

Sensitivity of channel profiles to precipitation properties in mountain ranges

The stream power erosion law, which describes the erosion rate as a function of channel discharge and gradient, has often been used for modeling landscape evolution in regions dominated by fluvial processes. However, most previous studies utilizing the stream power erosion law simply use drainage area as a surrogate for channel discharge. Despite its convenience this simplification has important shortcomings. Specifically, it ignores the effects of precipitation properties on channel discharge and hence erosion rate, and it ignores the interactions between mountain ranges and precipitation properties. By using the stream power erosion law together with the geomorphoclimatic instantaneous unit hydrograph we provide a method for linking the landscape evolution and precipitation properties directly. Our results demonstrate that the channel profile is sensitive not only to the total precipitation but also to precipitation properties like the rainfall frequency, intensity, duration, and their distribution in space. The channel profile is most sensitive to the variation of rainfall intensity and less sensitive to rainfall frequency and duration. Shorter and more intense rainfall could lead to significantly higher erosion rate and flatter channel profiles compared to longer and less intense rainfall. The spatial variation of precipitation can also influence the evolution of channel profile. Even if the total precipitation remains spatially homogeneous, different spatial behavior of rainfall intensity and rainfall duration may lead to different steady state river profiles. The channel profile tends to be flatter under the conditions of increasing rainfall intensity and decreasing rainfall duration with elevation and vice versa.