Valley Relief Research Articles

Valley Relief of the Northeastern Part of Terra Cimmeria on Mars

Valley Relief of the Northeastern Part of Terra Cimmeria on Mars

Lithologic Controls on Geomorphic Evolution of the Central Western Ghats: An Example from the Aghnashini Catchment, Karnataka, India

Abstract In spite of the prolonged exposure and denudation (for >50 Ma), the catchments of the Western Ghats escarpment depict significant relief and rugged topography along the Indian passive margin. In this study, an endeavour has been made to comprehend the function of lithology in landscape dynamics of the Aghanashini catchment through quantitative geomorphology. Longitudinal profiles of this catchment demonstrate disequilibrium through anomalous high stream gradient index and normalized steepness index (ksn) that are related to topographic breaks. Correlation between lithology types and ksn inferred predominant lithologic control on drainage disequilibrium. Transformed coordinate (χ-plot) of the longitudinal profile explained the knickpoint dynamics in an extensive manner where the outcomes suggested river capture process, formation of steep gorge, and series of hanging waterfalls due to sudden change in boundary conditions. This study indicates strong lithologic control under the influence of intense precipitation on shifting knickpoints throughout the catchment area forcing to maintain the valley relief and hillslope angles responsible for rugged and high relief topography of Aghnashini catchment of the Western Ghats.

Global Trends of Axial Relief and Faulting at Plate Spreading Centers Imply Discrete Magmatic Events

AbstractObserved variations in across‐axis topographic relief and faulting style at spreading centers have been challenging to explain. Axial highs are seen at fast‐spreading centers, while valleys occur for slow‐spreading centers. Fault offsets range from tens of meters at fast‐spreading ridges to tens of kilometers at some slow‐spreading ridges. Models that fit the axial relief fail to produce observed fault patterns, while models that fit the fault patterns fail to produce observed variations in axial relief. A recent mechanical analysis (Liu & Buck, 2018, https://doi.org/10.1016/j.epsl.2018.03.045) suggests that including the effect of many discrete diking events can result in a gradual change in axial relief with crustal thicknesses. To compare this mechanical model directly with observations requires us to couple it with a two‐dimensional thermal model. This allows us to estimate the axial lithospheric thickness consistently as a function of the spreading rate and crustal thickness. For thinner axial lithosphere the model predicts an axial high with relief supported by low‐density material beneath the axial lithosphere. For axial lithospheric thickness between approximately one half and approximately three fourths of the crustal thickness, the axial depth decreases with magma supply increase. For thicker axial lithosphere the axial valley relief is controlled by axial brittle lithospheric thickness and near‐axis lithospheric geometry. We compared model predictions to data by compiling observations on axial relief and faulting mode for all spreading centers where seismic crustal thickness has been measured. Good fit to the data is obtained for model parameters giving dike widths in the axial lithosphere close to a meter.

The relative efficiency and influence of glacial and fluvial erosion on Tibetan Plateau landscapes

Driven by climate change, periods of dominating glacial and fluvial erosion have both sculpted mountain ranges during the Quaternary. To understand the evolution of mountain landforms, there is considerable interest in knowing which erosion agent leads to more incision, how glacial thermal regime impacts erosion and landscape development, and whether glacial erosion can result in increased relief. To investigate the relative efficiency of glacial and fluvial erosion, and the influence of glacial thermal regime on topography, we compare two different mountain ranges–the Qionglai Shan (monsoonal temperate glaciation) and Lenglongling (subcontinental glaciation) in western China. We use a 1-D model to construct presumptive fluvial longitudinal profiles if glaciation had not occurred, and compare these with the actual glacial profiles to obtain relative incision rates of fluvial and glacial erosion. Results demonstrate that glacial incision is smaller than fluvial incision in Lenglongling and larger than fluvial incision in the Qionglai Shan, which may be influenced by different tectonics and glacier types. Topographic analysis of glacial and fluvial valleys with similar drainage areas show that glacial erosion results in valley widening, local valley relief increase induced by isostatic uplift, and slope gradient increase in Lenglongling. However, there is no local valley relief and slope increase observed in the Qionglai Shan, which may reflect slope process responses to threshold hillslope limiting the height above valley bottoms.

Geomorphological conditions of the location historical ironworks. A contribution to the research based on DEM analysis from LIDAR data

Abstract The aim of this study was to present the use of the natural elements of the relief of river valleys such as changes in the width of the valley bottom, landforms occurring in the bottom of the valley, differences in height of the valley terraces as favourable for the location of the dam partitioning the bottom of the valley and creating a water reservoir for the requirements of historic metallurgical centres. The research was carried out based on DEM analysis from LiDAR data. Features were chosen in river basins with a rich metallurgical legacy. Analysis of the location of the former ironworks was carried out using Surfer 12 software. Five centres were selected due to the fact that only these are the only centres suitable for research which have survived to this day. Using the shaded relief models and contour coloured maps absolute differences in height between valley levels and other forms of relief occurring in the valley were analyzed, as well as the distribution of individual terrain forms in the designated part of the valley and changes in the width of the valley bottom were analysed in the context of the location of former metallurgical centres. On the basis of the contours of the former water reservoir visible in the valley relief, and using a surface area measurement tool (Surfer software), the range of the area that the reservoir could cover was measured. On the basis of the results obtained, it can be seen that convenient geomorphological conditions were used for the placement of selected weirs and metallurgical ponds which facilitated the damming of the valley. Natural narrowing of the valley bottom, or dunes and hills directly adjacent to the valley floor, were utilised during the construction of the dam. The rivers on which the furnace ponds were constructed are relatively small watercourses, so the weirs created by the constructors are not impressive. Their height is generally in the range of about 2 to 3 metres and their length is from about 120 to 300 metres. Nevertheless, they were effective in allowing sufficient water retention and the creation of furnace ponds with a measured area of about 4.5 ha to about 25 ha.

Magmatic controls on axial relief and faulting at mid-ocean ridges

Previous models do not simultaneously reproduce the observed range of axial relief and fault patterns at plate spreading centers. We suggest that this failure is due to the approximation that magmatic dikes open continuously rather than in discrete events. During short – lived events, dikes open not only in the strong axial lithosphere but also some distance into the underlying weaker asthenosphere. Axial valley relief affects the partitioning of magma between the lithosphere and asthenosphere during diking events. The deeper the valley, the more magma goes into lithospheric dikes in each event and so the greater the average opening rate of those dikes. The long-term rate of lithospheric dike opening controls faulting rate and axial depth. The feedback between axial valley depth D and lithospheric dike opening rate allows us to analytically relate steady-state values of D to lithospheric thickness HL and crustal thickness HC. A two-dimensional model numerical model with a fixed axial lithospheric structure illustrates the analytic model implications for axial faulting. The predictions of this new model are broadly consistent with global and segment-scale trends of axial depth and fault patterns with HL and HC.

Landslides control the spatial and temporal variation of channel width in southern Taiwan: Implications for landscape evolution and cascading hazards in steep, tectonically active landscapes

AbstractIntense precipitation or seismic events can generate clustered mass movement processes across a landscape. These rare events have significant impacts on the landscape, however, the rarity of such events leads to uncertainty in how they impact the entire geomorphic system over a range of timescales. Taiwan is steep, tectonically active, and prone to landslide and debris flows, especially when exposed to heavy rainfall events. Typhoon Morakot made landfall in Taiwan in August of 2009, causing widespread landslides in southern Taiwan. The south to north trend in valley relief in southern Taiwan leads to spatial variability in landslide susceptibility providing an opportunity to infer the long‐term impact of such landslide events on channel morphology. We use pre‐ and post‐typhoon imagery to quantify the propagating impact of this event on channel width as the debris is routed through the landscape. The results show the importance of cascading hazards from landslides on landscape evolution based on patterns of channel width (both pre‐ and post‐typhoon) and hillslope gradients in 20 basins along strike in southern Taiwan. Prior to Typhoon Morakot, the river channels in the central part of the study area were about 3–10 times wider than the channels in the south. Following the typhoon, aggradation and widening was also a maximum in these central to northern basins where hillslope gradients and channel steepness is high, accentuating the pre‐typhoon pattern. The results further show that the narrowest channels are located where channel steepness is the lowest, an observation inconsistent with a detachment‐limited model for river evolution. We infer this pattern is indicative of a strong role of sediment supply, and associated landslide events, on long‐term channel evolution. These findings have implications across a range of spatial and temporal scales including understanding the cascade of hazards in steep landscapes and geomorphic interpretation of channel morphology. Copyright © 2018 John Wiley & Sons, Ltd.

Influencia de la glaciación en la red de drenaje: el ejemplo de los valles de Respina y Rebueno (Alto Porma, Cordillera Cantábrica, noroeste de España)

En el presente trabajo se aborda el estudio de dos valles en la cuenca del río Porma, en la vertiente meridional de la Cordillera Cantábrica, norte de España. El primero de ellos, el de Respina, presenta múltiples evidencias glaciares, como circos, morrenas, cubetas, till o bloques erráticos que demuestran la importancia que tuvo la glaciación en ese valle. El segundo, el de Rebueno, apenas cuenta con restos glaciares y están restringidos a la zona de cabecera. La comparación de los perfiles longitudinales y transversales de ambos valles, pone de manifiesto la importancia que tuvo la glaciación en la configuración del relieve actual del valle de Respina, contrastando con las formas fluviales generadas en el valle de Rebueno. En ambos se dieron procesos distintos que configuraron un relieve con importantes diferencias a pesar de su proximidad y similares características. Además, el trabajo aborda la influencia que la glaciación ha tenido en la configuración actual de la red de drenaje, aspecto que apenas ha sido analizado previamente, y que en este caso se concreta en el desvío del río Respina y la creación de un nuevo valle, como consecuencia de una potente acumulación morrénica procedente de la lengua glaciar del valle de Silván. Se trata de un ejemplo muy significativo de la importancia que tuvo la glaciación en la red fluvial.

Miocene development of alpine glacial relief in the Patagonian Andes, as revealed by low-temperature thermochronometry

Apatite thermochronometry and synthetic maps of ages and rates for thermochronometric data are used to estimate the timing of incision of valley relief in the Andes. Central Patagonia offers a unique location to study the feedbacks between long-term climate, topography, and erosion due to the high relief and well-resolved mid-latitude glacial history. New apatite (U–Th)/He ages from two vertical transects and two 4He/3He release spectra in the fjord network around 47°S reveal fast cooling (15–30 °C/Ma) from ∼10 to 5 Ma. Samples currently at the surface cooled below ∼35 °C by ∼5 Ma, indicating slow cooling and little erosion in those regions since 5 Ma. We show that these very low-temperature thermochronometric data are useful indicators of changes in topography, and insensitive to deep thermal processes, such as migration of the Chile triple junction. Map-based predictions of the thermochronometric signatures of disparate topographic scenarios show the distribution of sample data necessary to resolve the timing of relief change. Comparisons to predicted cooling ages and rates indicate that our new apatite He data are most consistent with a pulse of early glacial incision, with much of the observed valley relief in Patagonia carved between 10 and 5 Ma. Early onset of glaciation in Patagonia is supported by glacial till with bracketing ages of 7.4 and 5 Ma. We therefore conclude that the observed thermochronometric signal of fast cooling from 10 to 5 Ma is likely due to an increase in valley relief coinciding with these early glaciations in the Andes. In other glaciated areas at lower latitudes, studies have found a dramatic increase in valley relief at ∼1 Ma. This timing has generated the idea that incision of glacial valleys may be related to the mid-Pleistocene transition, when the global glacial cycle changed from 40 to 100 ka periods. Our results from a higher latitude indicate an alternative, that glacial valleys incised rapidly after the onset of alpine glaciation.

GEOMORPHOLOGIC ANALYSIS OF URBAN SYSTEM STABILITY

Within the inherited valleys of the Moscow, Yausa and Neglinnaya rivers the amount of distorted buildings is two times more than in the interstream area. The underground water level and the thickness of anthropogenic sediments are 3^1 m higher in the valleys. These data together with some other geologic-geomorphologic features indicate an increased instability of valley relief and its substratum.

Quantifying postglacial sediment storage at the mountain-belt scale

Sediment storage is an often-neglected term in sediment budgets, despite being the crucial link between rates of erosion and sediment yield. Mountain belts in particular host large valley fills that modulate fluxes of water and sediment, buffer the geomorphic coupling between hillslopes and river channels, reduce local valley relief, and protect bedrock from fluvial incision. Here we propose a region-growing algorithm based on a slope-gradient criterion to automatically extract areas of postglacial fluvial and lacustrine valley fills from digital topography at the mountain-belt scale. Applying this method to the European Alps, we find that the size-frequency relationship of 17,766 individual sediment storage units, expressed by either area or volume, follows a power law over four and five orders of magnitude with scaling exponents −1.77 and −1.71, respectively. We show that 90% of the area covered by sediment storage is in the lower quartile of the mountain belt9s elevation and below the median local relief. On average, low-gradient valley fill occupies 6% of the studied drainage basins. This fraction increases with basin size, likely reflecting the larger accommodation space of trunk valleys generated by multiple glacial-interglacial cycles. Comparison with sediment storage mapped in the Southern Alps, New Zealand, indicates that rates of uplift and precipitation control the extent and spatial distribution of sediment storage at the mountain-belt scale.

Traces of dammed paleolake activity in the main valley relief and fill in the south of the Irkutsk amphitheater

Thick sand-argillaceous deposits filling the paleorelief roughness are widespread in the south of the Irkutsk amphitheater. The origin of these deposits is still debatable: Some researchers relate them to eolian processes, and others consider them sediments of a dammed paleolake. A series of sand-argillaceous sections was studied in order to refine the genesis of deposits in the area of possible flooding. Results of these studies and laboratory analyses of the deposits argue for their lacustrine genesis and a drastic change in sedimentation conditions during the early formation of the paleolake. The tectonically induced instantaneous Badarma landsliding to the Angara River valley is assumed to have caused the damming of the main rivers in the study area.

Relative size of fluvial and glaciated valleys in central Idaho

Quantitative comparisons of the morphometry of glaciated and fluvial valleys in central Idaho were used to investigate the differences in valley relief and width in otherwise similar geologic and geomorphic settings. The local relief, width, and cross-sectional area of valleys were measured using GIS software to extract information from USGS digital elevation models. Hillslope gradients were also measured using GIS software. Power-law relationships for local valley relief, width, and cross-sectional area as a function of drainage area were developed. Local valley relief in glaciated valleys relates to drainage area with a power-law exponent similar to fluvial valleys, but glaciated valleys are deeper for a given drainage area. Local valley width in glaciated valleys is greater than in fluvial valleys, but the exponent of the power-law relationship to drainage area is similar in both valley types. Local valley cross-sectional area in glaciated valleys increases with drainage area with a power-law exponent similar to fluvial valleys, however, glacial valleys have roughly 80% greater cross-sectional area. Steep valley walls in glaciated basins increase the potential for bedrock landsliding relative to fluvial basins. Both the Olympic Mountains of Washington and valleys in central Idaho show relationships in which glaciated valleys are up to 30% deeper than fluvial valleys despite differences in lithology, tectonic setting, and climate.

Effect of lithospheric geometry on rift valley relief

Continental and mid‐oceanic rifts commonly show a rift valley bounded by normal faults and uplifted shoulders. In this paper we numerically model the evolution of rift valley relief in a notched brittle lithosphere. Results show that the rift valley has a maximum valley relief, which is controlled by the competition between isostasy and lithospheric geometry. If the axial lithosphere is much thinner than the flanking lithosphere, the isostatic force will break outward dipping faults on the axis and stop the deepening of the rift valley. We call this the axial failure mode. On the other hand, if the thickness of axial lithosphere is close to that of the flanks, flexural bending will break new faults on the flanks and stop the valley deepening. We call this the flanking failure mode. Triggering of either mode gives the maximum valley depth, and the selection is controlled by the interplay between isostasy and lithospheric geometry. On the basis of these mechanisms we propose a scaling equation that relates the maximum valley depth to the geometry (flanking thickness, axial thickness, and notch width) and strength (friction angle and cohesion) of the lithosphere. The model explains along‐axis variation of valley relief at mid‐oceanic ridges by changes of the axial lithosphere thickness.

Climate and other factors in the development of river and interfluve profiles in Bhutan, Eastern Himalayas

The longitudinal profiles of the main N–S aligned rivers and the crests of the interfluve mountain ranges of Bhutan have been plotted against latitude. The river profiles are highly variable, even between branches of the same system. The main rivers in Eastern Bhutan are antecedent and rise in Tibet. They have irregular concave bed profiles in deep steeply sided valleys. The rivers further west rise on the southern slopes of the High Himalaya. They have stepped profiles with steep concave sections in gorges through the southern mountains and one or more concave sections upstream, separated by knickpoints. All of the N–S interfluve ranges rise steeply from the piedmont. Some then dip to major passes before again rising irregularly northwards to the High Himalaya, whilst others continue to climb northwards as irregular monoclines. The combination of various types of river and interfluve profiles creates a range of valley forms. The heterogeneity means that it is not possible to generalise about a typical Bhutanese river, interfluve or valley relief profile. There is no indication that the rivers of Bhutan have more knickpoints than those of the Central and Western Himalayas. Rainfall and runoff data, soils and natural vegetation have been examined for indications of significantly drier conditions in eastern Bhutan. The rainfall data show an eastwards decrease in the southern foothills, probably due to the rainshadow cast by the Meghalaya Plateau to the south, but mean annual totals are about or above three metres throughout, and the whole of this zone has a wet climate. There is no marked E–W climatic trend in the drier interior of Bhutan. We attribute the general topographic structure of Bhutan, and the variability of river and interfluve profiles and valley forms more to tectonic factors than to climatic variation.

Valley incision by debris flows: Evidence of a topographic signature

The sculpture of valleys by flowing water is widely recognized, and simplified models of incision by this process (e.g., the stream power law) are the basis for most recent landscape evolution models. Under steady state conditions a stream power law predicts that channel slope varies as an inverse power law of drainage area. Using both contour maps and laser altimetry, we find that this inverse power law rarely extends to slopes greater than ∼0.03 to 0.10, values below which debris flows rarely travel. Instead, with decreasing drainage area the rate of increase in slope declines, leading to a curved relationship on a log‐log plot of slope against drainage area. Fieldwork in the western United States and Taiwan indicates that debris flow incision of bedrock valley floors tends to terminate upstream of where strath terraces begin and where area‐slope data follow fluvial power laws. These observations lead us to propose that the steeper portions of unglaciated valley networks of landscapes steep enough to produce mass failures are predominately cut by debris flows, whose topographic signature is an area‐slope plot that curves in log‐log space. This matters greatly as valleys with curved area‐slope plots are both extensive by length (>80% of large steepland basins) and comprise large fractions of main stem valley relief (25–100%). As a consequence, valleys carved by debris flows, not rivers, bound most hillslopes in unglaciated steeplands. Debris flow scour of these valleys appears to limit the height of some mountains to substantially lower elevations than river incision laws would predict, an effect absent in current landscape evolution models. We anticipate that an understanding of debris flow incision, for which we currently lack even an empirical expression, would substantially change model results and inferences drawn about linkages between landscape morphology and tectonics, climate, and geology.

City of Los Angeles Tests 48 Year Old T-Lock Protected Pipe and 72 Year Old Tile Lined Pipe

City of Los Angeles Tests 48 Year Old T-Lock Protected Pipe and 72 Year Old Tile Lined PipeThe City of Los Angeles has removed a section of pipe from the La Cienega / San Ferndando Valley Relief Sewer after 48 years of service and performed testing on it to determine how its current condition. This pipe was protected by T-Lock and has been continually in service. Testing included compressive strength of the concrete, tensile strength and pull out resistance of the reinforcing steel,...Author(s)Keith HanksS. HallAmeron InternationalR.T. HaugSamara Ali-AhmadAnsar MustafaSourceProceedings of the Water Environment FederationSubjectSession 29 - Collection Systems Symposium: Using Today's Tools to Effectively Maintain Your SewersDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2002ISSN1938-6478SICI1938-6478(20020101)2002:15L.752;1-DOI10.2175/193864702784247530Volume / Issue2002 / 15Content sourceWEFTECFirst / last page(s)752 - 770Copyright2002Word count177

Numerical simulations of glacial-valley longitudinal profile evolution

Research Article| November 01, 2000 Numerical simulations of glacial-valley longitudinal profile evolution K.R. MacGregor; K.R. MacGregor 1Department of Earth Sciences and Center for the Study of Imaging and Dynamics of the Earth, University of California, Santa Cruz, California 95064, USA Search for other works by this author on: GSW Google Scholar R.S. Anderson; R.S. Anderson 1Department of Earth Sciences and Center for the Study of Imaging and Dynamics of the Earth, University of California, Santa Cruz, California 95064, USA Search for other works by this author on: GSW Google Scholar S.P. Anderson; S.P. Anderson 1Department of Earth Sciences and Center for the Study of Imaging and Dynamics of the Earth, University of California, Santa Cruz, California 95064, USA Search for other works by this author on: GSW Google Scholar E.D. Waddington E.D. Waddington 2Geophysics Program, University of Washington, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Author and Article Information K.R. MacGregor 1Department of Earth Sciences and Center for the Study of Imaging and Dynamics of the Earth, University of California, Santa Cruz, California 95064, USA R.S. Anderson 1Department of Earth Sciences and Center for the Study of Imaging and Dynamics of the Earth, University of California, Santa Cruz, California 95064, USA S.P. Anderson 1Department of Earth Sciences and Center for the Study of Imaging and Dynamics of the Earth, University of California, Santa Cruz, California 95064, USA E.D. Waddington 2Geophysics Program, University of Washington, Seattle, Washington 98195, USA Publisher: Geological Society of America Received: 04 May 2000 Revision Received: 17 Aug 2000 Accepted: 25 Aug 2000 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2000) 28 (11): 1031–1034. https://doi.org/10.1130/0091-7613(2000)28<1031:NSOGLP>2.0.CO;2 Article history Received: 04 May 2000 Revision Received: 17 Aug 2000 Accepted: 25 Aug 2000 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation K.R. MacGregor, R.S. Anderson, S.P. Anderson, E.D. Waddington; Numerical simulations of glacial-valley longitudinal profile evolution. Geology 2000;; 28 (11): 1031–1034. doi: https://doi.org/10.1130/0091-7613(2000)28<1031:NSOGLP>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Glaciers shape alpine landscapes. They broaden valley bottoms, enhance local valley relief, generate multiple steps, overdeepen valley floors, and cause tributary valleys to hang. These distinctive glacial signatures result from 104–105 yr of erosion, during which swings in climate drive advances and retreats of alpine glaciers. We use a numerical model of glacial erosion to explore the development of the longitudinal profiles of glaciated valleys. The model is driven by the past 400 k.y. of variable climate. Because both sliding speed, which dictates abrasion rate, and water-pressure fluctuations, which strongly modulate quarrying rate, should peak at the equilibrium-line altitude (ELA), we expect the locus of most rapid erosion to follow the transient ELA. Simulations of a single glacial valley show rapid flattening of the longitudinal profile. Inclusion of a tributary glacier creates a step immediately downvalley of the tributary junction that persists over multiple glaciations and commonly leaves the tributary valley hanging. Steps and overdeepenings result from an increase in ice discharge immediately below the tributary junction, which is accommodated primarily by increased ice thickness and hence sliding rate. The size of the step increases with the ratio of tributary to trunk ice discharge, while the height of a hanging valley reflects the difference in the time-integrated ice discharge in tributary and trunk valleys and therefore increases as the discharge ratio decreases. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Late Quaternary intra-continental river palaeohydrology and polycyclic terrace formation: the example of south Siberian river valleys

Abstract The subject of intra-continental river regime and related polycyclic terrace formation has been investigated in south and central Siberia. The intra-continental river regime in the past has differed from the regimes of regions with both marine dominated climates and marine/continental transitional climates. The main feature of the intra-continental paleohydrological river regime was extremely high increases in river level. These increases in level were caused by periodic increases of river discharge during the Pleistocene and Holocene which occurred during periods of valley relief instability caused by climate cooling. The simultaneous accumulation of alluvium on surfaces of different heights occurred during high floods and caused the formation of polycyclic terraces. These are the type terraces of the intra-continental regions.

A global analysis of mid-ocean ridge axial topography

SUMMARY Current views of mid-ocean ridges are strongly influenced by extensive mapping of the Mid-Atlantic Ridge and East Pacific Rise. The global picture of the mid-ocean ridge system, particularly in the sparsely surveyed Southern Oceans, is still based primarily on underway bathymetry profiles collected over the past 40 years. This study presents a quantitative analysis of global mid-ocean ridge morphology based on 156 of these underway bathymetric profiles, thereby allowing commonly recognized features such as axial valleys and axial ridges to be compared on a global basis. An Empirical Orthogonal Function (EOF) analysis is used to separate deterministic and stochastic components of axial morphology and to quantify the dependence of each on parameters such as spreading rate and axial depth. It is found that approximately 50 per cent of the variance in axial morphology may be described as a linear combination of five independent symmetric and anti-symmetric modes; the remainder is considered stochastic. Maximum axial valley relief decreases with spreading rate for rates less than 80 mm yr-1 while axial ridge relief remains relatively constant for all rates greater than 50 mm yr-1. The stochastic component of the axial morphology, referred to as bathymetric roughness, also decreases with spreading rate for rates less than 80 mm yr-1 and remains relatively constant at higher rates. Although both axial valley relief and bathymetric roughness near the ridge axis show a similar spreading rate dependence, they are weakly correlated at slow spreading centres. The distinct differences in morphologic variability of fast and slow spreading ridges may result from the episodicity of magmatic heat input which controls the lithospheric rheology at slow spreading ridges. These observations support the notion of a critical threshold separating two dynamically distinct modes of lithospheric accretion on mid-ocean ridges.