Low-relief Surfaces Research Articles

Interpreting Topographic Map Evidence Related to Northeast Nebraska Barbed Tributaries and Drainage Routes, USA

Northeast Nebraska barbed tributaries include north-oriented streams flowing to the south-oriented Missouri River and south-oriented streams flowing to the north-oriented Missouri River tributaries. Detailed topographic maps were used to determine how these northeast Nebraska drainage routes originated. A giant south-oriented supra-glacial melt water river is interpreted to have sliced an ice-walled and bedrock-floored canyon into a decaying ice sheet’s surface where eastern South Dakota’s east-facing Missouri Escarpment and west-facing Prairie Coteau escarpment are now located and to have flowed from that canyon’s mouth across northeast Nebraska while South Dakota’s north-facing Pine Ridge Escarpment is interpreted to be the south wall of a large east-oriented valley that was eroded headward across immense southeast-oriented ice-marginal melt water floods which had originally flowed across northeast Nebraska. Prior to Missouri River valley headward erosion these two different immense melt water floods created and then flowed across a low relief and low gradient northeast Nebraska topographic surface. Present day northeast Nebraska topography developed when the deep south-oriented Missouri River valley and its south-oriented tributary valleys eroded headward into this low relief and low gradient topographic surface. As the deep Missouri River valley eroded headward it beheaded shallow south-oriented flood flow channels supplying water to new and actively eroding south-oriented Missouri River tributary valleys and water on north ends of the beheaded channels reversed flow direction to move toward the much deeper Missouri River valley. Water still moving in south directions adjacent to these reversed flow channels was then captured leading to development of south-oriented tributaries to the north-oriented streams.

Permafrost and lakes control river isotope composition across a boreal Arctic transect in the Western Siberian lowlands

The Western Siberian Lowlands (WSL) store large quantities of organic carbon that will be exposed and mobilized by the thawing of permafrost. The fate of mobilized carbon, however, is not well understood, partly because of inadequate knowledge of hydrological controls in the region which has a vast low-relief surface area, extensive lake and wetland coverage and gradually increasing permafrost influence. We used stable water isotopes to improve our understanding of dominant landscape controls on the hydrology of the WSL. We sampled rivers along a 1700 km South–North transect from permafrost-free to continuous permafrost repeatedly over three years, and derived isotope proxies for catchment hydrological responsiveness and connectivity. We found correlations between the isotope proxies and catchment characteristics, suggesting that lakes and wetlands are intimately connected to rivers, and that permafrost increases the responsiveness of the catchment to rainfall and snowmelt events, reducing catchment mean transit times. Our work provides rare isotope-based field evidence that permafrost and lakes/wetlands influence hydrological pathways across a wide range of spatial scales (10–105 km2) and permafrost coverage (0%–70%). This has important implications, because both permafrost extent and lake/wetland coverage are affected by permafrost thaw in the changing climate. Changes in these hydrological landscape controls are likely to alter carbon export and emission via inland waters, which may be of global significance.

Variations of Lateral Bedrock Erosion Rates Control Planation of Uplifting Folds in the Foreland of the Tian Shan, NW China

AbstractFluvial planation surfaces, such as straths, commonly serve as recorders of climatic and tectonic changes and are formed by the lateral erosion of rivers, a process that remains poorly understood. Here we present a study of kilometer‐wide, fluvially eroded, low‐relief surfaces on rapidly uplifting folds in the foreland of the southwestern Tian Shan. A combination of field work, digital elevation model analysis, and dating of fluvial deposits reveals that despite an arid climate and rapid average rock‐uplift rates of 1–3 mm/yr, rivers cut extensive (>1–2 km wide) surfaces with typical height variations of <6 m over periods of >2–6 kyr. The extent of this “beveling” varies in space and time, such that different beveling episodes affect individual structures. Between times of planation, beveled surfaces are abandoned, incised, and deformed across the folds. In a challenge to models that link strath cutting and abandonment primarily to changes in river incision rates, we demonstrate that lateral erosion rates of antecedent streams crossing the folds have to vary by more than 1 order of magnitude to explain the creation of beveled platforms in the past and their incision at the present day. These variations do not appear to covary with climate variability and might be caused by relatively small (much less than an order of magnitude) changes in sediment or water fluxes. It remains uncertain in which settings variations in lateral bedrock erosion rates predominate over changes in vertical erosion rates. Therefore, when studying fluvial planation and strath terraces, variability of both lateral and vertical erosion rates should be considered.

Geometric disequilibrium of river basins produces long-lived transient landscapes

Although equilibrium has long been considered the attractor state for landscapes, the time required to reach equilibrium or even the possibility of reaching equilibrium is still debated. Using 10Be-based catchment-averaged denudation rates, topographic analysis, and analysis of the basin topology and geometry, including its area-channel length scaling relationship, we show that an ancient postorogenic dome on the North American Craton, the Ozark dome, is not in a state of equilibrium. The persistent state of disequilibrium on the Ozark dome is characterized by nonuniform erosion rates that vary by a factor of three, asymmetric drainage divides, and evidence for drainage rearrangement via stream capture. We find that planform geometric disequilibrium of river basins and drainage area exchange between adjoining basins can hold river networks in a disequilibrium state for potentially hundreds of million years and that, when sustained over time, erosion rate differences associated with drainage area exchange can lead to transient events such as stream capture and production of relief in the form of elevated, low-relief surfaces. Our results suggest that landscapes with slowly moving drainage divides might not reach equilibrium, and that river basin dynamics may contribute to setting the large-scale morphology of old cratonic landscapes.

Does slab-window opening cause uplift of the overriding plate? A case study from the Gulf of California

Complete subduction of an oceanic plate results in slab-window opening. A key uncertainty in this process is whether the higher heat flux and asthenospheric upwelling conventionally associated with slab-window opening generate a detectable topographic signature in the overriding plate. We focus on the Baja California Peninsula, which incorporates the western margin of the Gulf of California rift. The topography and tectonics of the rift flank along the peninsula are strongly bimodal. North of the Puertecitos accommodation zone, the primary drainage divide attains a mean elevation of ca. 1600m above sea level (asl), above an asthenospheric slab-window opened by Pacific-Farallon spreading ridge subduction along this section of the trench at ca. 17–15Ma. To the south, mean topography decreases abruptly to ca. 800m asl (excluding the structurally distinct Los Cabos block at the southern tip of the peninsula), above fragments of the oceanic Farallon slab which stalled following slab tear-off at ca. 15–14Ma. Along the peninsula, a low-relief surface established atop Miocene subduction-related volcaniclastic units has been incised by a west-draining canyon network in response to uplift. These canyons exhibit cut-and-fill relationships with widespread post-subduction lavas. Here, we utilise LANDSAT and digital elevation model (DEM) data, integrated with previously published K-Ar and 40Ar/39Ar lava crystallisation ages, to constrain the onset of rift flank uplift to ca. 9–5Ma later than slab-window formation in the north and ca. 11–10Ma later in the south. These greatly exceed response time estimates of ca. 2Ma or less for uplift triggered by slab-window opening. Instead, uplift timing of the high-elevation northern region is consistent with lower-lithospheric erosion driven by rift-related convective upwelling. To the south, stalled slab fragments likely inhibited convective return flow, preventing lithospheric erosion and limiting uplift to the isostatic response to crustal unloading during rifting.

Structural and Seismic Stratigrapic study in the Center of the Magdalena Shelf in the Western Margin of Baja California Based on Seismic Reflection Data

The Magdalena Shelf is a shallow, low-relief surface located along the Baja California Pacific margin. As part of a forearc basin, the shelf was a convergent margin setting before the oblique divergent plate boundary formed in the Gulf of California at 12 Ma. It is thought that since 12–8 Ma, this basin has been a transtensional or strike–slip basin. To constrain the geometry, structural characteristics and some stratigraphic relationships, an active-source, seismic-reflection study was carried out in the central part of the shelf. As a result, the analyzed data show faults, basins and unconformities. Two out of four observed basins are clearly controlled by the Santa Margarita and San Lazaro faults that dip ~40° NE; a third basin is controlled by the Tosco-Abreojos fault. These three basins are part of the deformation zone that is associated with the Tosco-Abreojos fault system. The Iray-Margarita basin, on the other hand, is a fourth basin located at the northeast sector of the study area. An additional feature observed is a stepover lying between the overlapping ends of the Santa Margarita and San Lazaro faults. Small faults oriented sub-parallel to the above major faults are present, mainly throughout the western sector of the study area. Some of those minor faults cut through the seafloor indicating recent tectonic activity. Santa Margarita, San Lazaro and Tosco-Abreojos are also the names given to half-grabens controlled by the active faults that have the same names. The first two basins are affected by many more small faults in comparison with what we see in the third basin. Tectonically, this means that those two basins are the more active in the area of study. In all four basins, the upper seismic sequence consists of sediments controlled by faults of Neogene age. We found that the Iray-Santa Margarita basin is the deepest of all four basins (beyond the resolution of the data, >5 km), and lack of minor faults there indicates that the basin is not tectonically active. Two unconformities are present in the region, unconformity-1 of Miocene age and unconformity-2 of Paleocene-Eocene and Cretaceous ages. Unconformity-1 is present in the entire region, while Unconformity-2 is present mainly in the Iray-Magdalena and San Lazaro basins. In the Iray-Magdalena basin the sedimentary sequences show an uplift that took place in late Jurassic time along its western portion. We also see that moderate compressive deformation increases gradually from west to east. A chaotic-reflector oriented NW–SE is observed with an irregular shape in all regions. It shows very superficial in some places, sub-parallel to the major faults and in association with the subduction complex. Seismic activity reported for the region showed a strong correlation with the fault plane of the Santa Margarita fault, indicating that the Tosco-Abreojos deformation zone is ~90 km wide in the area. Finally, the Santa Margarita fault should be considered in future hazard-risk assessments because of their proximity to Puerto San Carlos. As it has been reported, local seismic events (M < 4) have generated alarm in the local populace, as well as soil liquefaction and minor damage to structures in the region.

Geomorphology and topography of relict surfaces: the influence of inherited crustal structure in the northern Scandinavian Mountains

Low-relief surfaces in northern Norway are mapped and analysed to explore (1) whether surfaces were once continuous and possibly correlative, recording an ancient, relict landscape, (2) whether the distribution of (now disrupted) surfaces reveals a tectonic history related to rifting of the North Atlantic and (3) how topography changes across the transition between the North Atlantic and Barents Sea margins. Elevation contours on surfaces, a smoothed fit to mean elevations and histograms of elevation show three distinct, coast-parallel zones onshore northern Norway, and a different pattern in coastal Lofoten–Vesterålen. A sharp transition from continuous surfaces to lower, discretely stepped, discontinuous surfaces is located where the crust thins rapidly to the NW from &gt;39 to &lt;25 km. In margin-parallel transects, elevations remain high for &gt;100 km north of the Senja Fracture Zone and then decrease gradually further NE. The spatial pattern suggests that a continuous, low-relief, relatively low-elevation surface formed in late Mesozoic time and was preserved despite modification by Cenozoic faulting, uplift and slow erosion. Scandinavia's present-day topographic envelope reflects a crustal strength profile set up during earlier hyperextension and maintained by slow Neogene erosion rather than resulting from opening of the North Atlantic or caused by glacial erosion. Supplementary material: A map of polygons of all surface heights within the study area and data on the location of median centres of polygons, assigned attributes and selected zonal statistics are available at https://doi.org/10.6084/m9.figshare.c.3312960

Macro‐ and micro‐sedimentology of a modern melt‐out till – Matanuska Glacier, Alaska, USA

The macro‐ and micro‐sedimentology of a supraglacial melt‐out till forming at the Matanuska Glacier was examined in relationship to the properties of the stratified basal zone ice and debris from which it is originating. In situ melting of the basal ice has produced a laminated to bedded diamicton consisting mainly of silt. Macroscopic properties include: discontinuous laminae and beds; lenses of sand, silt aggregates and open‐work gravel; deformed and elongate clasts of clay; widely dispersed pebbles and cobbles, those that are prolate usually with their long axes subparallel to parallel to the bedding. Evidence for deformation is absent except for localized bending of beds over or under rock clasts. Microscopic properties are a unique element of this work and include: discontinuous lineations; silt to granule size laminae; prolate coarse sand and rock fragments commonly with their long axis subparallel to bedding; subangular to subrounded irregular shaped clay clasts often appearing as bands; sorted and unsorted silt to granule size horizons, sometimes disrupted by pore‐water pathways. Limited deformation occurs around rock clasts and thicker parts of lamina. This study shows that in situ melting of debris‐rich basal ice can produce a laminated and bedded diamicton that inherits and thereby preserves stratified basal ice properties. Production and preservation of supraglacial melt‐out till require in situ melting of a stagnant, debris‐rich basal ice source with a low relief surface that becomes buried by a thick, stable, insulating cover of ice‐marginal sediment. Also required are a slow melt rate and adequate drainage to minimize pore‐water pressures in the till and overlying sediment cover to maintain stability and uninterrupted deposition. Many modern and ancient hummocky moraines down glacier of subglacial overdeepenings probably meet these process criteria and their common occurrence suggests that both modern and pre‐modern supraglacial melt‐out tills may be more common than previously thought.

Peneplanation and lithosphere dynamics in the Pyrenees

The morphology of the Pyrenees is characterized by the presence of high-elevation, low-relief surfaces. The origin of these Lower-Miocene surfaces is still debated. Two major interpretations have been proposed, both assuming that these surfaces are remnants of a single composite planation surface. The first interpretation proposes that this surface corresponds to a peneplain developed near sea level before the Late Miocene, subsequently uplifted and dissected. The present-day Pyrenees is therefore supposed to rise from the Late Miocene. In the second interpretation, the rise of the efficient base level of the chain induced the progressive inhibition of erosion and the smoothing of the relief before the Late Miocene, resulting in a highly elevated peneplain. According to this latter interpretation, the high elevation of the low-relief surfaces does not equate to post-orogenic uplift. We test these two interpretations by investigating, among other considerations, the relation between the elevation of the planation surface remnants and the deep structure of the chain. We find that (1) the isostatic compensation of the dissected Pyrenean planation surface by crustal thickening and (2) the absence of thinning of the lithosphere mantle below the chain favors the second interpretation.

The periglacial engine of mountain erosion – Part 2: Modelling large-scale landscape evolution

Abstract. There is growing recognition of strong periglacial control on bedrock erosion in mountain landscapes, including the shaping of low-relief surfaces at high elevations (summit flats). But, as yet, the hypothesis that frost action was crucial to the assumed Late Cenozoic rise in erosion rates remains compelling and untested. Here we present a landscape evolution model incorporating two key periglacial processes – regolith production via frost cracking and sediment transport via frost creep – which together are harnessed to variations in temperature and the evolving thickness of sediment cover. Our computational experiments time-integrate the contribution of frost action to shaping mountain topography over million-year timescales, with the primary and highly reproducible outcome being the development of flattish or gently convex summit flats. A simple scaling of temperature to marine δ18O records spanning the past 14 Myr indicates that the highest summit flats in mid- to high-latitude mountains may have formed via frost action prior to the Quaternary. We suggest that deep cooling in the Quaternary accelerated mechanical weathering globally by significantly expanding the area subject to frost. Further, the inclusion of subglacial erosion alongside periglacial processes in our computational experiments points to alpine glaciers increasing the long-term efficiency of frost-driven erosion by steepening hillslopes.

Timing and rate of exhumation along the Litang fault system, implication for fault reorganization in Southeast Tibet

The Litang fault system that crosses the Litang Plateau, a low relief surface at high elevation (~4200–4800 m above sea level) that is not affected by regional incision, provides the opportunity to study exhumation related to tectonics in the SE Tibetan Plateau independently of regional erosion. Combining apatite and zircon fission track with apatite (U-Th)/He thermochronologic data, we constrain the cooling history of the Litang fault system footwall along two transects. Apatite fission track ages range from 4 to 16 Ma, AHe ages from 2 to 6 Ma, and one zircon fission track age is ~99 Ma. These data imply a tectonic quiet period sustained since at least 100 Ma with a slow denudation rate of ~0.03 km/Ma, interrupted at 7 to 5 Ma by exhumation at a rate between 0.59 and 0.99 km/Ma. We relate that faster exhumation to the onset of motion along the left-lateral/normal Litang fault system. That onset is linked to a Lower Miocene important kinematic reorganization between the Xianshuihe and the Red River faults, with the eastward propagation of the Xianshuihe fault along the Xiaojiang fault system and the formation of the Zhongdian fault. Such strike-slip faults allow the sliding to the east of a wide continental block, with the Litang fault system accommodating differential motion between rigid blocks. The regional evolution appears to be guided by the strike-slip faults, with different phases of deformation, which appears more in agreement with an “hidden plate-tectonic” model rather than with a “lower channel flow” model.

In situ low-relief landscape formation as a result ofriver network disruption.

Landscapes on Earth retain a record of the tectonic, environmental and climatic history under which they formed. Landscapes tend towards an equilibrium in which rivers attain a stable grade that balances the tectonic production of elevation and with hillslopes that attain a gradient steep enough to transport material to river channels. Equilibrium low-relief surfaces are typically found at low elevations, graded to sea level. However, there are many examples of high-elevation, low-relief surfaces, often referred to as relict landscapes, or as elevated peneplains. These do not grade to sea level and are typically interpreted as uplifted old landscapes, preserving former, more moderate tectonic conditions. Here we test this model of landscape evolution through digital topographic analysis of a set of purportedly relict landscapes on the southeastern margin of the Tibetan Plateau, one of the most geographically complex, climatically varied and biologically diverse regions of the world. We find that, in contrast to theory, the purported surfaces are not consistent with progressive establishment of a new, steeper, river grade, and therefore they cannot necessarily be interpreted as a remnant of an old, low relief surface. We propose an alternative model, supported by numerical experiments, in which tectonic deformation has disrupted the regional river network, leaving remnants of it isolated and starved of drainage area and thus unable to balance tectonic uplift. The implication is that the state of low relief with low erosion rate is developing in situ, rather than preserving past erosional conditions.

Identification of peneplains by multi‐parameter assessment of digital elevation models

ABSTRACTThe concept of peneplains has existed since the end of the nineteenth century. Typical peneplains are elevated geomorphological features with a low relief surface on top. They may be tilted due to tectonic activity or intersected by evolving erosion. Until now neither a standardized definition for peneplains, nor an established procedure to identify and quantify well preserved peneplains as prominent landforms existed. At present the global availability of homogeneous digital elevation models (DEMs) provides an accurate characterization of the morphology of the Earth surface. In this study a new, numerical, DEM‐based fuzzy‐logic approach is presented for the delineation of peneplains solely from a morphological perspective. The approach is based on a morphometric analysis of the 90‐arcsec Shuttle Radar Topography Mission (SRTM) DEM. Four critical parameters are employed which are implemented within a geographic information system (GIS). The parameters for the correct and unambiguous description of a ‘flat top mountain’ are: (i) slope, (ii) curvature, (iii) terrain ruggedness index, and (iv) relative height. The approach was developed using a test area in the central Tibetan Plateau, which is characterized by representative and well preserved peneplains and for which additional field data were collected. In order to verify the method, peneplains were delineated in different regions with various geological settings for which potential peneplains were already described in the literature. The results from the Appalachian Mountains, Andes, Massif Central, and New Zealand confirm the robustness of the proposed approach. Copyright © 2015 John Wiley &amp; Sons, Ltd.

New insights into debris-flow hazards from an extraordinary event in the Colorado Front Range

Rainfall on 9–13 September 2013 triggered at least 1,138 debris flows in a 3430 km area of the Colorado Front Range. The historical record reveals that the occurrence of these flows over such a large area in the interior of North America is highly unusual. Rainfall that triggered the debris flows began after ~75 mm of antecedent rain had fallen, a relatively low amount compared to other parts of the United States. Most flows were triggered in response to two intense rainfall periods, one 12.5-hour-long period on 11–12 September, and one 8-hour-long period on 12 September. The maximum 10 min. intensities during these periods were 67 and 39 mm/hr. Ninety-five percent of flows initiated in canyons and on hogbacks at elevations lower than a widespread erosion surface of low slope and relief ( 25°), predominantly southand east-facing slopes with upslope contributing areas 3300 m. Areal concentrations of debris flows revealed that colluvial soils formed on sedimentary rocks were more susceptible to flows than soils on crystalline rocks. This event should serve as an alert to government authorities, emergency responders, and residents in the Front Range and other interior continental areas with steep slopes. Widespread debris flows in these areas occur infrequently but may pose a greater risk than in areas with shorter return periods, because the public is typically unprepared for them.

Paleogene Grand Canyon incompatible with Tertiary paleogeography and stratigraphy

The Hualapai Plateau in northwest Arizona, the location of the western Grand Canyon, contains an unusually lengthy Tertiary stratigraphic record dominated by fluvial deposition and extending from at least late Paleocene through late Miocene time. The thickest and oldest Tertiary sections are best exposed in a system of partially re-exhumed Laramide paleocanyons. The Paleogene drainage system was locally disrupted and ponded by Laramide monoclines. In pre-Oligocene time, extensive alluvial fans spread southward from the Shivwits Plateau scarp across the current location of the modern Colorado River gorge to the northern margin of the Laramide drainage system at Hindu Canyon. Locally derived, fluvial Buck and Doe Conglomerate subsequently filled the disrupted Paleogene channels, spilled out over the local interfluves, and formed an extensive aggradational surface of low relief by late Oligocene time. Early Miocene volcanism filled in much of the relict Laramide relief. Erosional recession of the adjacent Shivwits Plateau escarpment shifted the northern Hualapai Plateau margin 8 km northeastward after the Laramide drainage episode and before the incision by the modern Colorado River. Partially exhumed tributaries to the Hindu Canyon paleochannel and associated sedimentary deposits bordering the southern edge of the Grand Canyon gorge demonstrate that local surface runoff flowed south, away from the modern Grand Canyon location, during early Paleogene time. Headwardly eroding Colorado River tributaries exhumed, captured, and reversed the flow of these tributaries to the Laramide canyon, beginning in late Miocene or Pliocene time. The geomorphic and stratigraphic records show no evidence of, and provide no space for, incision of a Late Cretaceous–Paleogene ancestral precursor to the modern Colorado River gorge. Instead, all the field evidence clearly supports a late Miocene–Pliocene origin for integration of the western Grand Canyon on the central Hualapai Plateau with the upper Colorado River.

Stratigraphic landscape analysis, thermochronology and the episodic development of elevated, passive continental margins

The continental margin of West Greenland is similar in many respects to other elevated, passive continental margins (EPCMs) around the world. These margins are characterised by extensive regions of low relief at elevations of 1–2 kilometres above sea level sloping gently inland, with a much steeper, oceanward decline, often termed a 'Great Escarpment', terminating at a coastal plain. Recent studies, based on integration of geological, geomorphological and thermochronological evidence, have shown that the high topography of West Greenland was formed by differential uplift and dissection of an Oligo-Miocene peneplain since the late Miocene, many millions of years after continental break-up between Greenland and North America. In contrast, many studies of other EPCMs have proposed a different style of development in which the high plateaux and the steep, oceanward decline are regarded as a direct result of rifting and continental separation. Some studies assume that the elevated regions have remained high since break-up, with the high topography continuously renewed by isostasy. Others identify the elevated plains as remnants of pre-rift landscapes. Key to understanding the development of the West Greenland margin is a new approach to the study of landforms, stratigraphic landscape analysis, in which the low-relief, high-elevation plateaux at EPCMs are interpreted as uplifted peneplains: low-relief surfaces of large extent, cutting across bedrock of different age and resistance, and originally graded to sea level. Identification of different generations of peneplain (re-exposed and epigene) from regional mapping, combined with geological constraints and thermochronology, allows definition of the evolution leading to the formation of the modern-day topography. This approach is founded particularly on results from the South Swedish Dome, which document former sea levels as base levels for the formation of peneplains. These results support the view that peneplains grade towards base level, and that in the absence of other options (e.g. widespread resistant lithologies), the most likely base level is sea level. This is particularly so at continental margins due to their proximity to the adjacent ocean. Studies in which EPCMs are interpreted as related to rifting or break-up commonly favour histories involving continuous denudation of margins following rifting, and interpretation of thermochronology data in terms of monotonic cooling histories. However, in several regions, including southern Africa, south-east Australia and eastern Brazil, geological constraints demonstrate that such scenarios are inappropriate, and an episodic development involving post-breakup subsidence and burial followed later by uplift and denudation is more realistic. Such development is also indicated by the presence in sedimentary basins adjacent to many EPCMs of major erosional unconformities within the post-breakup sedimentary section which correlate with onshore denudation episodes. The nature of the processes responsible is not yet understood, but it seems likely that plate-scale forces are required in order to explain the regional extent of the effects involved. New geodynamic models are required to explain the episodic development of EPCMs, accommodating post-breakup subsidence and burial as well as subsequent uplift and denudation, long after break-up which created the characteristic, modern-day EPCM landscapes.

Glacial and periglacial buzzsaws: fitting mechanisms to metaphors

The buzzsaw hypothesis refers to the potential for glacial and periglacial processes to rapidly denude mountains at and above glacier Equilibrium Line Altitudes (ELAs), irrespective of uplift rates, rock type or pre-existing topography. Here the appropriateness of the buzzsaw metaphor is examined alongside questions of the links between glacial erosion and ELAs, and whether the glacial system can produce low-relief surfaces or limit summit heights. Plateau fragments in mountains on both active orogens and passive margins that have been cited as products of glacial and periglacial buzzsaw erosion instead generally represent dissected remnants of largely inherited, pre-glacial relief. Summit heights may correlate with ELAs but no causal link need be implied as summit erosion rates are low, cirque headwalls may not directly abut summits and, on passive margins, cirques are cut into pre-existing mountain topography. Any simple links between ELAs and glacial erosion break down on passive margins due to topographic forcing of ice-sheet growth, and to the km-scale vertical swaths through which ELAs have shifted through the Quaternary. Glaciers destroy rather than create low-relief rock surfaces through the innate tendency for ice flow to be faster, thicker and warmer along valleys. The glacial buzzsaw cuts down.

Selective glacial erosion on the Norwegian passive margin

Glaciated passive margins display dramatic fjord coasts, but also commonly retain plateau fragments inland. It has been proposed recently that such elevated, low-relief surfaces on the Norwegian margin are products of highly efficient and extensive glacial and periglacial erosion (the glacial buzzsaw) operating at equilibrium line altitudes (ELAs). We demonstrate here that glacial erosion has acted instead to dissect plateaus in western Norway. Low-relief surfaces are not generally spatially associated with cirques, and do not correlate regionally with modern and Last Glacial Maximum ELAs. Glacier dynamics require instead that glacial erosion is selective, with low-relief surfaces representing islands of limited Pleistocene erosion. Deep glacial erosion of the coast and inner shelf has provided huge volumes of sediment (70,000 km3), largely resolving apparent mismatches (65–100,000 km3) between fjord and valley volumes and Pliocene–Pleistocene sediment wedges offshore. Nonetheless, as Pleistocene glacial valleys and cirques are cut into preexisting mountain relief, tectonics rather than isostatic compensation for glacial erosion have been the main driver for late Cenozoic uplift on the Norwegian passive margin.

The influence of surface and tectonic processes on landscape evolution of the Iberian Chain (Spain): Quantitative geomorphological analysis and geochronology

In tectonically active areas, the landscape response to tectonic forcing is described and possibly quantified by regional topographic and hydrographic features as well as by spatial variation in rates of surface processes. We investigated the recent landscape evolution of the Iberian Chain (NE Spain), an intraplate thrust-belt formed in Cenozoic times and characterized by a dome-shaped topography. In its central sector the landscape is dominated by low relief surfaces, Late Neogene (?) in age, presently standing at an average altitude of 1300m. A recent regional uplift controlled the organization of the present fluvial network and dissection of the landscape. In this framework we investigated the geomorphic responses to tectonic forcing by the calculation of morphometric parameters, focusing on topography (map of local relief, swath profiles) and hydrography (basin hypsometric curve and integral, basin asymmetry factor, river longitudinal profiles and relative indices), and using SRTM DEM. The results of morphometric analysis have been coupled with radiometric uranium-series dating of calcareous tufas lying on fluvial strath terraces. The obtained ages allow the estimation of incision rate along the High Tajo and Martín rivers. Our results indicate that uplift and rock-type erodibility are the main factors influencing landscape evolution of the study area. The incision rates are very similar throughout the central sector of the range, indicating that, despite subtle local variation, the rivers are responding to a main tectonic input: the regional uplift. In conclusion, the Iberian Chain landscape is in a transient state in response to a recent dome-like uplift. Indeed, the fluvial processes that weakly incised this landscape at a rate of ~0.6mm/yr are approaching a radial pattern. On the basis of geological and geomorphic constraints, we hypothesize that the uplift started around or after 3Ma.

Cenozoic erosion and flexural isostasy of Scandinavia

The presence of Cenozoic deposits along the Norwegian Atlantic margin required extensive erosion of the Scandinavian Mountains in a generally cooling climate from the Oligocene to the present. The volume of the deposits implies that the transfer of mass from the inland area to the offshore shelf induced isostatic displacements on a kilometer scale. However, except for glacial excavation of the deep fjords, little is known about the distribution of Cenozoic inland erosion. A long-lasting paradigm incorporates remnants of peneplains at high elevation and assumes very little Cenozoic erosion on these surfaces through time. This scenario has recently been challenged by quantitative geomorphological studies indicating that the matrix of Cenozoic sediments deposited offshore must have been sourced from these surfaces. An alternative explanation for the present-day high-elevation low-relief surfaces is therefore that they evolved throughout the Cenozoic because of glacial and periglacial erosion processes that are known to vary strongly with altitude. Here we explore the implications of the latter scenario by reconstructing a pre-Cenozoic fluvial landscape without elevated low-relief surfaces. We use the present-day offshore sediment volumes for constraining the total Cenozoic erosion, and we find that a likely pre-Cenozoic fluvial landscape is only in few places more than 1km higher than today. The rock mass of the offshore sediments is generally used for filling the fjords created during the Quaternary glaciations and for restoring concave river profiles from sea level to the peaks. Our reconstruction is based on a fluvial landscape algorithm and considers the isostatic response to the transfer of rock mass – from the basins onto the onshore area. A comparison between the reconstructed and the present-day topography demonstrates that offshore tilting of pre-Cenozoic strata can be partly explained by flexural isostatic compensation in response to the Cenozoic erosion and deposition. Locations of future thermochronometry studies for testing Scandinavian landscape evolution models are suggested based on temperature estimates of the present-day surface buried beneath the erosion products restored from the offshore basins.