Geomorphic Indices Research Articles

Appraisal of active tectonics in Hindu Kush: Insights from DEM derived geomorphic indices and drainage analysis

Landscapes in tectonically active Hindu Kush (NW Pakistan and NE Afghanistan) result from a complex integration of the effects of vertical and horizontal crustal block motions as well as erosion and deposition processes. Active tectonics in this region have greatly influenced the drainage system and geomorphic expressions. The study area is a junction of three important mountain ranges (Hindu Kush-Karakorum-Himalayas) and is thus an ideal natural laboratory to investigate the relative tectonic activity resulting from the India-Eurasia collision. We evaluate active tectonics using DEM derived drainage network and geomorphic indices hypsometric integral (HI), stream-length gradient (SL), fractal dimension (FD), basin asymmetry factor (AF), basin shape index (Bs), valley floor width to valley height ratio (Vf) and mountain front sinuosity (Smf).The results obtained from these indices were combined to yield an index of relative active tectonics (IRAT) using GIS. The average of the seven measured geomorphic indices was used to evaluate the distribution of relative tectonic activity in the study area. We defined four classes to define the degree of relative tectonic activity: class 1__very high (1.0 ≤ IRAT < 1.3); class 2__high (1.3 ≥ IRAT < 1.5); class 3—moderate (1.5 ≥ IRAT < 1.8); and class 4—low (1.8 ≥ IRAT). In view of the results, we conclude that this combined approach allows the identification of the highly deformed areas related to active tectonics. Landsat imagery and field observations also evidence the presence of active tectonics based on the deflected streams, deformed landforms, active mountain fronts and triangular facets. The indicative values of IRAT are consistent with the areas of known relative uplift rates, landforms and geology.

Tectonic versus climate influence on landscape evolution: A case study from the upper Spiti valley, NW Himalaya

We have undertaken structural, geomorphological, and morphometric analyses to investigate the role of tectonism and climate in the landscape evolution in the upper Spiti valley, NW Himalayas. Geomorphometric analyses coupled with field investigations reveal active tectonic deformation in the Spiti region. The calculated geomorphic indices (steepness, concavity and Hack) demonstrate uplift/subsidence along the Kaurik–Chango fault, whereas transverse topographic index (T-index) reveals basin tilting associated with active faulting near Hansa and Lingti valley. Investigation of well-dated Mane palaeolake sediments also provides evidence of regional tectonic instability. Four episodes (ca. 7.8, 7.4, 6.5 and 6.1calka) of neotectonic activity have been identified during the period of the lake's existence. We have also compiled data on the regional climate variability and compared it with the age of the Mane palaeo-landslide. Our results indicate that the landslide occurred towards the end of the early Holocene intensified monsoon phase and is located near an active fault. Our data on regional tectonic instability and the coincidences of modern and palaeo-landslides with zones of active deformation suggest that tectonism is an important factor governing landscape stability in the Spiti region.

A diachronic view of the net longshore sediment drift during the Late Holocene at the Jequitinhonha River delta, Brazil, using numerical modeling

Using numerical modeling, this work estimates sediment dispersion patterns caused by the incidence of waves in five distinct coastline contours of the Jequitinhonha River delta plain during the Late Holocene. For this study, a wave cliate odel based on the construction of ave refraction diagras relative to the current boundary conditions as defined and assumed to be valid for the five coastlines. Numerical modeling from the refraction diagrams was carried out considering the angle of incidence and wave height along the coastline. This work has allowed us to reproduce past and current patterns of net longshore sediment drift defined by a conceptual model developed from the integration of sediment cores, aerial photograph interpretation, C14 datings and geomorphic indicators of longshore drift. These results show that, on average, current wave conditions in the east-northeastern Brazilian region seem to have remained characteristically the same from approximately 5100 years BP until the present time. This type of information can be important when attempting to simulate possible future situations in terms of the long-term general behavior of the east-northeastern Brazilian coastal region.

Assessment of morphotectonic properties of Mahan Tigrani watershed

Article history: Received March 25, 2011 Received in Revised form June, 27, 2011 Accepted 28 June 2011 Available online 28 June 2011 Tectonic geomorphology can be explained as the study of landforms produced by tectonic processes, or the application of geomorphic principles to the suggestion of tectonic problems. Tigrani's watershed is located in north part of hillside. Nayband Fault sub branch pass from the east of zone and kuhbanan fault from the north of zone supplies an appropriate theme for survey tectonic activities. For access to this aim geomorphic indexes contain Stream Length Gradient, Drainage Basin Shape Ratio, Ratio of Valley-floor with to Valley Height, Topographic Symmetry, Mountain Front Sinuosity and Asymmetry Factor with use of topographic maps, DEM, Arc GIS and Global mapper softwares have gained. The results of research which calculated with Iat index show west sub basin have medium tectonic activity (Iat=2), and east sub basin have high tectonic activity(Iat=1.5). Overall the results show that assessment of tectonic activities in Tigrani watershed this basin is active because of tectonic movements. © 2011 Growing Science Ltd. All rights reserved.

Geomorphologic assessment of relative tectonic activity in the Maharlou Lake Basin, Zagros Mountains of Iran

Spatial differences of Quaternary deformation and intensity of tectonic activity are assessed through a detailed quantitative geomorphic study of the fault‐generated mountain fronts and alluvial/fluvial systems around the Maharlou Lake Basin in the Zagros Fold–Thrust Belt of Iran. The Maharlou Lake Basin is defined as an approximately northwest–southeast trending, linear, topographic depression located in the central Zagros Mountains of Iran. The lake is located in a tectonically active area delineated by the Ghareh and Maharlou faults. Combined geomorphic and morphometric data reveal differences between the Ghareh and Maharlou mountain front faults indicating different levels of tectonic activity along each mountain front. Geomorphic indices show a relatively high degree of tectonic activity along the Ghareh Mountain Front in the southwest, in contrast with less tectonic activity along the Ahmadi Mountain Front northeast of the lake which is consistent with field evidence and seismotectonic data for the study area. A ramp valley tectonic setting is proposed to explain the tectonosedimentary evolution of the lake. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Morphotectonics of the Concud Fault (Iberian Chain, Spain): Comparing Geomorphic and Geologic Indices of Activity of an Intraplate Extensional Fault

The results of geomorphic analysis of the Concud fault-generated mountain front (central Iberian Chain, Spain) are introduced into classifications of fault activity proposed by previous authors, and compared with slip rates calculated from geologic markers. The Concud fault is an extensional structure active since the mid Pliocene times. It gives rise to a 60 to 120 m high mountain front, where footwall rocks belonging to the Triassic and Jurassic (north-western sector) and Miocene (south-eastern sector) crop out. Conspicuous triangular facets are preserved on Jurassic rocks of the central sector, while short, generally non-incised alluvial fans make the piedmont. The value of the Mountain-front sinuosity index is Smf = 1.24 for the whole mountain front (1.17 and 1.32, respectively, for both segments showing distinct footwall lithology), as obtained by the most conservative procedure. Average valley floor width/height ratios calculated for seventeen gullies crossing the fault are Vf = 0.30 (250 m upstream from the fault trace) and Vf = 0.22 (500 m upstream). These geomorphic indices, together with qualitative features of the escarpment and piedmont landscape, indicate ‘moderate’ to ‘rapid’ fault activity. The range of slip rates estimated from such morphotectonic classification (0.03 to 0.5 mm/y) encloses the range calculated from offset Late Pliocene and Pleistocene stratigraphic markers (0.07 to 0.33 mm/y). Nevertheless, the highest potential slip rate (0.5 mm/y) clearly represents an overestimate: the mountain front could give the impression of an anomalously high level of activity owing to episodic rejuvenation caused by base level drop.

Re-evaluation of the activity of the Thoen Fault in the Lampang Basin, northern Thailand, based on geomorphology and geochronology

We applied remote sensing techniques and geomorphic index analysis to a study of the NE-SW-striking Thoen Fault, Lampang Basin, northern Thailand. Morphotectonic landforms, formed by normal faulting in the basin, include fault scarps, triangular facets, wine-glass canyons, and a linear mountain front. Along the Thoen Fault, the stream length gradient index records steeper slopes near the mountain front; the index values are possibly related to a normal fault system. Moreover, we obtained low values of the ratio of the valley floor width to valley height (0.44–2.75), and of mountain-front sinuosity (1.11–1.82) along various segments of the fault. These geomorphic indices suggest tectonic activity involving dip-slip displacement on faults. Although the geomorphology and geomorphic indices in the study area indicate active normal faulting, sedimentary units exposed in a trench at Ban Don Fai show no evidence of recent fault movement. In Ban Don Fai trench No. 2, accelerator mass spectrometry radiocarbon (AMS) ages and optically stimulated luminescence (OSL) ages indicate that deposition of the lowest exposed sedimentary unit occurred between 960 and 910 years ago. Therefore, the most recent movement upon the Ban Don Fai segment of the Thoen Fault occurred more than 960 years ago.

Morphotectonic Analysis in the Ghezel Ozan River Basin, NW Iran

Morphotectonic analysis by the use of geomorphic indices serves as a tool of recognition in regions with active tectonic deformation. Landforms in active deformation area are created from interaction of tectonic and surficial processes. One of the most important landforms in ground are rivers that are very sensitive to tectonic movements especially uplift and tilting. Thus based on analysis of the rivers and related drainage networks by the use of geomorphic indices we will be able to attain valuable information about tectonic history of the area. In this article, determine of tectonic movements bye the use of geomorphic indices is surveyed in the Ghezel Ozan River basin between 2007 and 2010. After segmentation of the Ghezel Ozan River and preparation of digital elevation model (DEM) amounts of geomorphic indices per segments is separately attained. The attained amounts show that different segments of the Ghezel Ozan River vary from each other regarding the amount of tectonic activity and tectonic movements increase from west to east and also, the amount of tectonic tilting is negligible in a lot of the Ghezel Ozan River segments. This situation is completely in agreement with the trend of the seismicity of the area.

Transpressional tectonics in the Marrakech High Atlas: Insight by the geomorphic evolution of drainage basins

The Ouzzelarh Massif extends across the Marrakech High Atlas (MHA) and forms the highest elevated mountain belt. To better understand the evolution of collision-related topography, we present the results of a geomorphological study in which elevation changes generated by reactivated pre-Alpine (Variscan and Triassic–Jurassic) faults drive a landscape evolution model. We aim to evaluate the relationship between the geometry of the drainage network and the main fault systems in this region. New insight into geomorphological changes in drainage patterns and related landforms is based on geological fieldwork combined with DEM analysis. To quantitatively measure landscape features we used several classical geomorphic indices (spacing ratio, hypsometric curves and integral, stream frequency drainage, stream length-gradient). The Ouzzelarh Massif is bounded to the north by the Tizi N'Test Fault Zone (TTFZ) and to the south by the Sour Fault Zone (SFZ). These faults delimit a pop-up structure. By using the above geomorphic parameters, we ascertained that the Ouzzelarh Massif is affected by a high spatial variability of uplift. The actual landscape of the Ouzzelarh Massif reveals remnants of an uplifted ancient erosional surface and the heterogeneity of exposed rocks in the range explaining the possibility that the topographic asymmetry between north and south flanks is due to differences in lithology-controlled resistance to erosion. Drainage, topography and fault pattern all concur to show uplifted rhomboidal-shaped blocks. It exhibits high stream frequency drainage and uplift in separate tectonically-uplifted blocks such as Jebel Toubkal which is characterized by asymmetric drainage basins.

Hydromorphological alteration of a large Mediterranean river: Relative role of high and low flows on the evolution of riparian forests and channel morphology

AbstractThis paper evaluates the causes and effects of the hydrogeomorphological alteration of the central reach of the Ebro River (NE Spain). The Ebro River is one of the largest Mediterranean rivers. In this reach, it develops a meandering planform in a wide floodplain. Geographic Information System (GIS) analyses of historic aerial photographs, analysis of hydrologic data and measurement of various indicators linked to the fluvial morphology and the structure and distribution of the riparian vegetation led to the establishment of the prevailing processes in the dynamics of this river.Statistical analyses conducted on some of the main components of the flow regime, including floods, droughts and flow duration curves, showed a role for these components in river dynamics. Similarly, a thorough analysis of the evolution of the aforementioned indicators was performed to identify and measure the effects of the hydrological regulation of the river. These indicators were measured in 1927, 1956 and 2003 for a 106 km reach.The geomorphic dynamics of the Ebro River in its central reach reflect a remarkable tendency for stabilization and rigidification of the channel. The active river corridor has largely been modified, primarily in the second half of the twentieth century. The corridor lost a huge portion of its width and extension, the channel suffered an intense narrowing and the natural mobility of the meander train decayed proportionally. The structure and distribution of the riparian vegetation were completely transformed. The riparian forest lost its original function, behaving as a linear corridor and was notably continuous and very close to the channel thalweg. The vegetation colonized most of the previously active channel, contributing to the loss of the natural dynamics of the river.The hydrological analyses suggest that the large morphological modification of the river planform and the parallel alteration of the riparian forests are not to be seen as a consequence of a loss of the attributes of natural floods. On the contrary, these extreme hydrological events only generate slight alterations due to river regulation and are not capable of enhancing the aforementioned evolution. Nevertheless, a profound change in the attributes of the low (summer) flows was found. The modification of the low flows was studied through its relationship with the global evolution of the geomorphic indicators and the riparian forest indicators. The results show the relative role played by high and low flows in the evolution of the river dynamics. These results are used to propose a future scenario of ecohydrological management in the central reach of the Ebro River. This scenario is intended to improve its ecological status and recover, at least partially, its natural dynamics. Copyright © 2010 John Wiley &amp; Sons, Ltd.

The localized role of base level lowering on channel adjustment of tributary streams in the Trinity River basin downstream of Livingston Dam, Texas, USA

The purpose of this study is to describe and explain river channel cross-sectional change in tributary streams within the 60-km scour zone of the Trinity River, Texas, downstream of Livingston Dam. This study investigates changes within the tributary systems to base level changes triggered by incision, associated with scour and channel migration within the dammed trunk stream. The dam represents a precise moment and place of a system perturbation and therefore represents an opportunistic geomorphic experiment. Geomorphological effects of the lower Trinity River tributaries were investigated through three different types of data: resurveys of bridge cross sections, planform change as measured from aerial photographs, and field mapping of changes in channel geometry using fallen trees, undercut banks, and other geomorphic indicators of change. While field evidence of channel change was found at sites well upstream of tributary confluences with the Trinity, these were not consistent with responses to lower base level. Downcutting and lateral changes in response to lowered base level were confined to lower tributary reaches. This suggests distance decay and lag times in transmission of responses upstream and/or masking of dam-induced effects by other processes and controls. The most dynamic locations of geomorphic change are near the confluences of the tributaries with the Trinity River, where the response to dam-induced incision is strongly influenced by local trunk stream morphology, indicating a high degree of historical and geographical contingency.

Active tectonic assessment around Rudbar Lorestan dam site, High Zagros Belt (SW of Iran)

The aim of this study is to improve a method to develop an integrated quantitative geomorphic analysis of tectonic activity around the Rudbar Lorestan dam site and powerhouse in the High Zagros Belt (HZB), southwest Iran, using several tectonic geomorphic indexes including stream length–gradient index ( SL), drainage basin asymmetry ( Af), hypsometric integral ( Hi), ratio of valley floor width to valley height ( Vf), and transverse topographic symmetry factor ( T). Considering the results obtained from each of the above indexes, as combined and weighted by the “analytic hierarchy process” (AHP) and plotted on the “index map of relative active tectonics” ( Iat) by El Hamdouni et al. (2008), four classes are recognized in the study area; i.e. low, moderate, high, and very high, according to the rate of tectonic activity in the subbasins. Low to moderate intensities correspond to the northeast (along the Main Zagros Reverse Fault: MZRF) and southwest parts of the study area; whereas subbasins in the middle parts, along the Saravand–Baznavid strike-slip fault, are classified into high or very high intensities because of recent activity and the oblique-slip movement of the Saravand–Baznavid fault. The results show that the Rudbar Lorestan dam site and powerhouse are located in an active strike-slip fault zone and that a seismic hazard may occur along this zone in the future.

Indonesian Landforms and Plate Tectonics

DOI: 10.17014/ijog.v5i3.103The horizontal configuration and vertical dimension of the landforms occurring in the tectonically unstable parts of Indonesia were resulted in the first place from plate tectonics. Most of them date from the Quaternary and endogenous forces are ongoing. Three major plates – the northward moving Indo-Australian Plate, the south-eastward moving SE-Asian Plate and the westward moving Pacific Plate - meet at a plate triple-junction situated in the south of New Guinea’s Bird’s Head. The narrow North-Moluccan plate is interposed between the Asia and Pacific. It tapers out northward in the Philippine Mobile Belt and is gradually disappearing. The greatest relief amplitudes occur near the plate boundaries: deep ocean trenches are associated with subduction zones and mountain ranges with collision belts. The landforms of the more stable areas of the plates date back to a more remote past and, where emerged, have a more subdued relief that is in the first place related to the resistance of the rocks to humid tropical weathering Rising mountain ranges and emerging island arcs are subjected to rapid humid-tropical river erosions and mass movements. The erosion products accumulate in adjacent sedimentary basins where their increasing weight causes subsidence by gravity and isostatic compensations. Living and raised coral reefs, volcanoes, and fault scarps are important geomorphic indicators of active plate tectonics. Compartmental faults may strongly affect island arcs stretching perpendicular to the plate movement. This is the case on Java. Transcurrent faults and related pull-apart basins are a leading factor where plates meet at an angle, such as on Sumatra. The most complicated situation exists near the triple-junction and in the Moluccas. Modern research methods, such as GPS measurements of plate movements and absolute dating of volcanic outbursts and raised coral reefs are important tools. The mega-landforms resulting from the collision of India with the Asian continent, around 50.0 my. ago, and the final collision of Australia with the Pacific, about 5.0 my. ago, also had an important impact on geomorphologic processes and the natural environment of SE-Asia through changes of the monsoonal wind system in the region and of the oceanic thermo-haline circulation in eastern Indonesia between the Pacific and the Indian ocean. In addition the landforms of the region were, of course, affected by the Quaternary global climatic fluctuations and sea level changes.

Tectonic geomorphology of the southern Sierra Nevada Mountains (California): Evidence for uplift and basin formation

Hypotheses regarding uplift of the Sierra Nevada Mountains (Sierra), California, USA, vary from a single slowly tilting block to rapid Pliocene and/or Quaternary uplift in the south exclusively. To test these hypotheses, we examined geomorphic indices of the western Sierra. We interpret longitudinal profiles of the larger westerly flowing rivers, mountain front sinuosity, valley floor-to-width to height ratio, and relief ratio to show that relative tectonic activity is greater in the southern Sierra near the Kern River Gorge fault. The Sierra range crest profile indicates that the increased tectonism is related to more recent uplift in the southern Sierra. Only westward migration of Basin and range extension is consistent with the locus of uplift in the southern Sierra. We hypothesize that the Sierra topography is the result of Pliocene delamination-related uplift in the central Sierra and post-Pliocene interaction of the San Andreas, Garlock, and Sierra Nevada Frontal fault zones.

Quantitative analysis of relative tectonic activity in the Sarvestan area, central Zagros, Iran

Neotectonics is a major factor controlling landform development in tectonically active regions, and it has significantly affected fluvial systems and mountain-front landscapes in the Sarvestan area of the central Zagros, Iran. The area is located along the simply folded belt of the Zagros, and is an outcome of the SW–NE oriented tectonic conversion that initiated in the Late Cretaceous and strengthened during the Early Miocene due to the collision of the Arabian and Eurasian plates. To assess tectonic activities in the area, we analyzed geomorphic indices: the stream-gradient index ( SL), drainage basin asymmetry ( Af), hypsometric integral ( Hi), valley floor width–valley height ratio ( Vf), drainage basin shape ( Bs), and mountain-front sinuosity ( J). These indices were combined to yield the relative active tectonics index ( Iat) using geographic information systems (GIS). Based on Iat values, the study area was divided into four parts: Class 1 (very high relative tectonic activity, 1.0% in area); Class 2 (high, 20.0%); Class 3 (moderate, 67.0%), and Class 4 (low, 12.0%). The results are consistent with field observations on landforms and geology.

Active tectonics in the Sierra Nevada (Betic Cordillera, SE Spain): Insights from geomorphic indexes and drainage pattern analysis

The Sierra Nevada of the central Betic Cordillera is a 3000 m-high mountain range surrounded by Neogene–Quaternary sedimentary basins, having been uplifted since Late Miocene times. The southern and western mountain fronts of the Sierra Nevada are fault-bounded, while the northern one is an unconformity between the Neogene–Quaternary sediments of the Guadix–Baza basin and the metamorphic rocks of the Nevado–Filabride complex. We have carried out a geomorphic study by examining drainage patterns and characteristics of mountain fronts in order to reveal areal variations and styles of rock uplift. Mountain front sinuosity ( S mf), area–altitude relations (hypsometric curves), and valley floor entrenchment differ significantly between the northern, western, and southern mountain fronts. The lack of important faults along the northern Sierra Nevada mountain front, together with the elevated topographic position of the Guadix–Baza basin (average altitude is around 1100 m), points to similar uplift of both geomorphic units (sierra and basin) in a single large-scale crustal block. The asymmetry factors show systematic asymmetries at both sides of the Lanjarón River, probably due to the presence of an active NNE–SSW oriented antiform in the western Sierra Nevada. Finally, river profiles indicate maximal river entrenchment in the western part of the Sierra Nevada, probably related to the uplift of the footwall of the Padul–Nigüelas fault-system. Therefore, our geomorphic analysis suggests that the western part of the Sierra Nevada is tectonically active by means of a combination of normal faults along the mountain front and NNE–SSW oriented active folds, which, in turn, likely have a gravitational origin related to the exhumation of the footwall of the normal fault-system.

Efectos de la tectonica y el clima en la configuracion morfologica del relieve costero del norte de Chile

This work analyzes the morphological features of the western margin of the northern Chile Coastal Cordillera, characterized by the presence of a major Coastal Cliff. The main purpose of this research is to better understand the tectonic and climatic controls on the topography. The methodology used consists basically of a detailed study of the topography with quantitative geomorphic indexes such as incision grade, hypsometric curves and integrals, thalweg profiles and sinuosity index. These are applied to digital elevation models and satellite images. The main results show that the topography of the western margin of the Coastal Cordillera, exhibits an increasing intensity of the erosion with latitude. This increment is non uniform. The tectonic processes that produced coastal subsidence/uplift and fault activity, and the erosion of the paleotopography of Coastal Cordillera due to processes related with the construction of Coastal Cliff and increasing rainfall, modify the morphology of the coastal border of north Chile. The existence and conservation of the Coastal Cliff result from a non-equilibrium between the uplift and erosion rates. Both rates have one-two order magnitude difference, controlled by the extreme hyperarid climate of the Atacama Desert. These conditions prevailed at least for the last 2 m.yrs. The variations in the intensity of the erosion from north to south are interpreted as the result of the southward increase in precipitacion by one order of magnitude.

Evidence for surface uplift of the Atlas Mountains and the surrounding peripheral plateaux: Combining apatite fission-track results and geomorphic indicators in the Western Moroccan Meseta (coastal Variscan Paleozoic basement)

This work represents an initial attempt to link the evolution of the topography in relation to the general tectonic framework of western Morocco. For this purpose, in a section of the Western Moroccan Meseta different tools are combined in order to attain the general objective. Apatite fission-track (AFT) data of granitic rocks of the Rabat–Khenifra area give ages around 200 Ma with track length distributions which are compatible with the thermal models already established for the area. An inverse correlation between AFT ages and elevation is observed which is compatible with previous models indicating northward tilting of the whole Western Moroccan Meseta which is younger than 20–25 Ma. In order to test this possibility a detailed analysis of the topography at different scales in the Western Moroccan Meseta has been performed. Results indicate that two open folds with different amplitudes are recognized and that the one with wider wavelength could correspond to a lithospheric fold as previously stated by other authors on the basis of independent geological arguments. The northward tilting proposed based on the AFT data agrees with the results obtained in the analysis of the topography which reinforces the presence of a very open fold with a wavelength of 200–300 km in the north-western limb of the Western Moroccan Meseta.

Amplifiers, filters and geomorphic responses to climate change in Kentucky rivers

Efforts to predict responses to climate change and to interpret modern or paleoclimate indicators are influenced by several levels of potential amplifiers, which increase or exaggerate climate impacts, and/or filters, which reduce or mute impacts. With respect to geomorphic responses and indicators, climate forcings are partly mediated by ecological, hydrological, and other processes which may amplify or filter impacts on surface processes and landforms. Then, geomorphic responses themselves may be threshold-dominated or dynamically unstable, producing disproportionately large and long-lived responses to climate changes or disturbances. Or, responses may be dynamically stable, whereby resistance or resilience of geomorphic systems minimizes the effects of changes. Thus a given geomorphic response to climate could represent (at least) two levels of amplification and/or filtering. An example is given for three fluvial systems in Kentucky, U.S.A, the Kentucky, Green, and Big South Fork Rivers. Climate impacts in the early Quaternary were amplified by glacially-driven reorganization of the ancestral Ohio River system to the North, and by dynamical instability in the down-cutting response of rivers incising plateau surfaces. Effects of more recent climate changes, however, have been filtered to varying extents. Using alluvial terraces as an example, the study rivers show distinctly different responses to climate forcings. The lower Green River has extensive, well-developed terraces recording several episodes of aggradation and downcutting, while the Big South Fork River has no alluvial terraces. The Kentucky River is intermediate, with limited preservation of relatively recent terraces. The differences can be explained in terms of differences among the rivers in (1) filtering effects of constraints on fluvial responses imposed by strongly incised, steep-walled bedrock controlled valleys; and (2) amplifier effects of periodic damming of lower river reaches by glaciofluvial outwash.

Efectos de la tectónica y el clima en la configuración morfológica del relieve costero del norte de Chile

Tectonic and climatic effects in the morphologic configuration of the coastal relief of northern Chile. This work analyzes the morphological features of the western margin of the northern Chile Coastal Cordillera, character- ized by the presence of a major Coastal Cliff. The main purpose of this research is to better understand the tectonic and climatic controls on the topography. The methodology used consists basically of a detailed study of the topography with quantitative geomorphic indexes such as incision grade, hypsometric curves and integrals, thalweg profiles and sinuosity index. These are applied to digital elevation models and satellite images. The main results show that the topography of the western margin of the Coastal Cordillera, exhibits an increasing intensity of the erosion with latitude. This increment is non uniform. The tectonic processes that produced coastal subsidence/uplift and fault activity, and the erosion of the paleotopography of Coastal Cordillera due to processes related with the construction of the Coastal Cliff and increasing rainfall, modify the morphology of the coastal border of northern Chile. The existence and conservation of the Coastal Cliff results from the existance of non-equilibrium between the uplift and erosion rates. Both rates have one-two order magnitude difference, controlled by the extreme hyperarid climate of the Atacama Desert. These conditions prevailed at least for the last 2 m.yrs. The variations in the intensity of the erosion from north to south are interpreted as the result of the southward increase in precipitacion by one order of magnitude.