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1.
The NW Iberian Massif is part of an ancient basement that has been considered a seismically stable area with no outstanding Cenozoic tectonics. However, recent seismic activity revealed the need for better knowledge of the Cenozoic structures in the area. Because of the lack of Mesozoic deposits and the scarcity of Cenozoic sediments, as well as the intense deformation of the Pre-Mesozoic Variscan basement, it is difficult to study the Cenozoic tectonic structures. In this work, the combination of detailed structural mapping and study of geomorphological markers in the Variscan basement has allowed recognition of Cenozoic tectonic structures, kinematics and processes that otherwise would not have been identified. The identified structures have been gathered into three groups: a) NE–SW-trending strike-slip faults, mainly sinistral, b) NNW-vergent thrusts that uplift the Caurel Mountains and Galaico-Leoneses Mountains, and c) E–W and ENE–WSW thrusts that uplift the Ancares Mountains in a pop-up structure. The structures cut the Pre-Cenozoic erosion surface and affect the drainage network that shows patterns characteristic of tectonic activity. The three groups of structures define sectors with different relief showing a strong link between geomorphological elements and tectonic structures. The intense drainage reorganisations observed in the area and the deformation of Miocene–Pliocene deposits, point out to a significant Late Miocene tectonic activity in the region. Thus, the Cenozoic tectonic activity in the northwest of the Iberian Peninsula takes place during an extensive period of time which started with the episodes of compression in the Cantabrian Margin and it is identified nowadays by the recent seismic activity recorded in the north-western Iberian Peninsula (1995 and 1997 Lugo events). The seismicity is related to the Cenozoic structures identified in the area, which move under the present SE–NW horizontal maximum compression and coincide with the proposed seismogenic faults.  相似文献   

2.
Relationships between tectonic framework and gravity-driven phenomena have been investigated in an area of the Central Apennines (Italy) characterised by high relief. The north–south, half-dome shaped Maiella anticline lies in the easternmost part of the Apennine fold-and-thrust belt. Its backlimb is bordered by the Caramanico Fault, a normal fault with a maximum downthrown of about 3.5 km that separates the western slope of the Maiella Massif from the Caramanico Valley. The southwestern Maiella area is affected by deep-seated gravitational slope deformation indicated by major double crest lines, down-hill and up-hill facing scarps, a pattern of crossing trenches, bulging at the base of slopes and the presence of different types of landslide and talus slope deposits.The onset and development of deep-seated gravitational slope deformations and the location of Quaternary, massive rockslope failures have been strongly influenced by the structural framework and tectonic pattern of the anticline. Deep-seated gravitational slope deformation at Mt. Macellaro–Mt. Amaro ridge has developed along the Maiella western, reverse slope in correspondence with the anticline axial culmination; it is bordered at the rear by a NNW–SSE oriented, dextral, strike-slip fault zone and has an E–W direction of rock mass deformation. Closer to the southern plunging area of the anticline, gravity-driven phenomena show instead a N–S and NW–SE direction, influenced by bedding attitude.3D topographic models illustrate the relationship between deep-seated gravitational slope deformation and massive rockslope failures. The Campo di Giove rock avalanche, a huge Quaternary failure event, was the result of an instantaneous collapse on a mountaine slope affected by a long-term gravity-driven deformation.  相似文献   

3.
Pre-volcanic structure of the basement influences volcanism distribution and avalanche generation in volcanic edifices. Therefore, systematic studies of basement structure below volcanic chains are necessary to understand the deformation effects observed in the surface and vice versa. Based on a compilation of pre-existing data, interpretation of aerial photographs and satellite images, and a collection of structural data we analyzed morphological and structural features of the Cofre de Perote–Pico de Orizaba (CP–PO) volcanic chain and its basement. We have identified three sets of regional lineaments that are related to basement trends. (1) NW 55° SE fractures are parallel to anticline folds observed in Cretaceous rocks that originated during Laramide shortening. These folds present an abrupt morphology observed only in the eastern flank but that is likely to continue below the volcanic chain. (2) NE 55° SW fractures are parallel to normal faults at the basement. We infer that these basement faults confine the CP–PO chain within a stepped graben with a total normal displacement of about 400 m. These faults have been active through time since they have affected volcanic deposits and induced the emplacement of monogenetic vents. Notably, lineaments of monogenetic vents concentrate where the basement is relatively shallow. (3) Another set of faults, oriented N–S, has been observed affecting the scarce basement outcrops at the western flank of the chain covered by lacustrine deposits. Lineaments measured in the volcanic edifice of Pico de Orizaba correlate with the regional trends.In particular, the NE 55° SW alignment of monogenetic vents and fractures at Pico de Orizaba suggest that the same dike trend exists within the volcanic edifice. A normal fault with similar orientation was documented at the NE continuation of an alignment crossing the volcanic edifice along the Jamapa canyon. In the absence of magmatic activity related to collapses, the displacement of NE 55° SW faults represents a potential triggering mechanism for generating avalanches at Pico de Orizaba volcano. Instability is enhanced by the presence of N–S trending fractures crossing the entire volcanic edifice and E–W fractures affecting only the present day cone. We conclude that mechanical instability of the volcanic chain is influenced by the basement structure heterogeneity, but further detailed studies are necessary at individual volcanoes to evaluate their effects on volcano deformation.  相似文献   

4.
We present a study on the impact of litho-structural setting and neotectonic activity on meso- and macro-scale relief production in Alpine areas. The topography of the high alpine Triglav Lakes Valley, NW Slovenia, was studied by means of detailed mapping and stratigraphic study of the valley. The Triglav Lakes Valley is characterised by a generally asymmetric transverse (E–W) profile: a very steep eastern slope, a relatively flat valley and a relatively gentle western slope. On the transverse profile the valley floor is essentially flat, gently dipping towards the east. In the longitudinal cross-section, however, the valley floor is marked by sharply-defined fault blocks extending in a W–E to NW–SE direction. Additionally, the highest block (elevations  2100 m) is in the northern part of the valley, the lowest (elevations  1600 m) in the southern part of the valley. Our research shows that the Triglav Lakes Valley directly represents the topographic expression of Paleogene–Neogene thrusting and faulting, having recorded the following geomorphologic evolutionary stages: 1. an Oligocene to early Miocene W-vergent thrusting phase, with steep W-facing slopes of the eastern part of the valley directly representing the thrusting front; and 2. a Neogene-to-present strike–slip faulting in NNE–SSW direction with two bifurcating right-lateral strike–slip systems. We show that the Triglav Lakes Valley almost perfectly mimics the wedge-shaped damage zone located between these faults.  相似文献   

5.
Reconnaissance level geomorphological observations in the northern part of Evia (Euboea) Island, suggest that a major topographic feature, the 17 km long and 15 km wide Nileas depression (NDpr), corresponds to a previously undetected graben structure, bounded by fault zones of ENE–WSW to NE–SW general strike. These fault zones have been active in the Quaternary, since they affect the Neogene deposits of the Limni–Histiaia basin. They strike transverse to the NW–SE active fault zones that bound northern Evia in the specific area and are characterised along most of their length by subtle geomorphic signatures in areas of extensive forest cover and poor exposure.The NDpr was formed during the Early–Middle Quaternary, after the deposition of the Neogene basin fill. During the Middle–Late Quaternary, the NW–SE fault zones that bound northern Evia have been the main active structures, truncating and uplifting the NDpr to a perched position in relation to the northern Gulf of Evia graben and the submarine basin on the Aegean side of the island. The present-day morphology of the NDpr, with an interior (floor) comprised of Middle Pleistocene erosional surfaces extensively dissected by drainages, was shaped by erosion during this uplift. Judging from their geomorphic signatures, the fault zones that bound the NDpr must have been characterised by low or very low rates of activity during the Late Quaternary. Yet, that they may still be accommodating strain today is suggested by moderate earthquakes that have been recorded within the NDpr.The fault zone at the SE flank of the NDpr (Prokopi–Pelion fault zone) may be very important in terms of earthquake segmentation of the active NW–SE Dirfys fault zone that controls the Aegean coast of northern Evia, given that the intersection between the two presents striking morpho-structural similarities with the intersection of two fault zones with the same directions on the mainland (the Atalanti and Hyampolis fault zones), which is known to have acted as a barrier to the propagation of the Atalanti earthquake ruptures in 1894.  相似文献   

6.
Johan M. Bonow   《Geomorphology》2005,72(1-4):106-127
Classifications of large-scale landscapes in Greenland have traditionally been based on type and intensity of glacial erosion, with the general idea that present landforms are mainly the result of erosion from ice sheets and glaciers. However, on southern Disko and in areas offshore in Disko Bugt, a basement surface has preserved remnants of weathered gneiss and pre-Paleocene landforms, recently exhumed from Paleocene basalt. Isolated hills and lineaments have been mapped in a digital terrain model and aerial photographs. Offshore have hills been mapped from seismic lines. The medium size bedrock forms on southern Disko as tors, clefts and roche moutonées have been studied in the field. Remnant saprolites were inventoried, sampled and analysed according to grain size and clay mineralogy. The basement surface retains saprolites up to 8 m thick in close relation to the cover rocks. The landforms in the basement rocks belong essentially to an etched surface only slightly remodelled by glacial erosion and, below the highest coastline, also by wave action. The outline of hills is governed by two lineament directions, ENE–WSW representing the schistocity of the gneiss and NW–SE fracture zones. These structures are thus interpreted to have been exploited by the deep weathering while the frequent N–S lineaments have not and thus might be younger. Main ice-flow has been from the NE and has resulted in plucking of SW facing lee sides, however the resulting bedrock forms are mainly controlled by structures and orientation of joints. The identification of re-exposed sub-Paleocene etch forms on Disko and the hills of similar size offshore, forming a hilly relief, have implications for identification of a hilly relief south of Disko Bugt, its relation to younger planation surfaces as well as for conclusions of uplift events.  相似文献   

7.
The Gödöllő Hills, a low-relief terrain within the Central Pannonian Basin in Hungary, is characterised by moderate tectonic deformation rates. Although typical tectonic landforms are not clearly recognisable in the study area, this paper succeeded in discriminating between tectonically controlled landforms and features shaped by fluvial erosion or deflation with no tectonic control.DEM-based morphometric parameters including elevation, slope and surface roughness, enabled the delineation of two NW–SE trending spearhead-shaped ridges separated by a wide rectilinear valley of the same strike. Although directional statistics suggested possible tectonic control of NW–SE striking landforms, precise morphometry completed with an analysis of subsurface structures rejected their tectonic preformation. Deflation plays a significant role in shaping the area, and the presence of two large-scale yardangs separated by a wind channel is proposed. In temperate-continental areas of Europe, no deflational landforms of such scale have been described so far, suggesting that Pleistocene wind power in periglacial areas was more significant than it was previously thought.Characteristic drainage patterns and longitudinal valley profiles enabled the recognition of areas probably affected by neotectonic deformation. A good agreement was observed between locations of Quaternary warping predicted by the morphometric study and subsurface structures revealed by the tectonic analysis. Zones of surface uplift and subsidence corresponded to anticlinal and synclinal hinges of fault-related folds. In low-relief and slowly-deforming areas, where exogenous forces may override tectonic deformation, only the integrated application of morphometric and subsurface-structural indications could assure correct interpretation of the origin of various landforms, while a morphometric study alone could have led to misinterpretation of some morphometric indices apparently suggesting tectonic preformation. On the other hand, the described morphological expression of subsurface structures could verify Quaternary age of the deformation.  相似文献   

8.
The glacial buzzsaw hypothesis suggests that efficient erosion limits topographic elevations in extensively glaciated orogens. Studies to date have largely focussed on regions where large glaciers (tens of kilometres long) have been active. In light of recent studies emphasising the importance of lateral glacial erosion in lowering peaks and ridgelines, we examine the effectiveness of small glaciers in limiting topography under both relatively slow and rapid rock uplift conditions. Four ranges in the northern Basin and Range, Idaho, Montana, and Wyoming, USA, were chosen for this analysis. Estimates of maximum Pleistocene slip rates along normal faults bounding the Beaverhead–Bitterroot Mountains (~ 0.14 mm y− 1), Lemhi Range (~ 0.3 mm y− 1) and Lost River Range (~ 0.3 mm y− 1) are an order of magnitude lower than those on the Teton Fault (~ 2 mm y− 1). We compare the distribution of glacial erosion (estimated from cirque floor elevations and last glacial maximum (LGM) equilibrium line altitude (ELA) reconstructions) and fault slip rate with three metrics of topography in each range: the along-strike maximum elevation swath profile, hypsometry, and slope-elevation profiles. In the slowly uplifting Beaverhead–Bitterroot Mountains, and Lemhi and Lost River Ranges, trends in maximum elevation parallel ELAs, independent of variations in fault slip rate. Maximum elevations are offset ~ 500 m from LGM ELAs in the Lost River Range, Lemhi Range, and northern Beaverhead–Bitterroot Mountains, and by ~ 350 m in the southern Beaverhead–Bitterroot Mountains, where glacial extents were less. The offset between maximum topography and mean Quaternary ELAs, inferred from cirque floor elevations, is ~ 350 m in the Lost River and Lemhi Ranges, and 200–250 m in the Beaverhead–Bitterroot Mountains. Additionally, slope-elevation profiles are flattened and hypsometry profiles show a peak in surface areas close to the ELA in the Lemhi Range and Beaverhead–Bitterroot Mountains, suggesting that small glaciers efficiently limit topography. The situation in the Lost River Range is less clear as a glacial signature is not apparent in either slope-elevation profiles or the hypsometry. In the rapidly uplifting Teton Range, the distribution of ELAs appears superficially to correspond to maximum topography, hypsometry, and slope-elevations profiles, with regression lines on maximum elevations offset by ~ 700 and ~ 350 m from the LGM and mean Quaternary ELA respectively. However, Grand Teton and Mt. Moran represent high-elevation “Teflon Peaks” that appear impervious to glacial erosion, formed in the hard crystalline bedrock at the core of the range. Glacier size and drainage density, rock uplift rate, and bedrock lithology are all important considerations when assessing the ability of glaciers to limit mountain range topography. In the northern Basin and Range, it is only under exceptional circumstances in the Teton Range that small glaciers appear to be incapable of imposing a fully efficient glacial buzzsaw, emphasising that high peaks represent an important caveat to the glacial buzzsaw hypothesis.  相似文献   

9.
Staircases of strath terraces and strongly incised valleys are the most typical landscape features of Portuguese rivers. This paper examines the incision achieved during the late Cenozoic in an area crossed by the Tejo river between the border with Spain and the small town of Gavião. In the more upstream reach of this area, the Tejo crosses the Ródão tectonic depression, where four levels of terraces are distinguished. During the late Cenozoic fluvial incision stage, the Ródão depression underwent less uplift than the adjacent areas along the river. This is reflected by the greater thicknesses and spatial extent of the terraces; terrace genesis was promoted by impoundment of alluvium behind a quartzitic ridge and the local presence of a soft substratum. Outside this tectonic depression, the Tejo has a narrow valley incised in the Hercynian basement, with some straight reaches that probably correspond to NE–SW and NNW–SSE faults, the terraces being nearly absent. Geomorphological evidence of tectonic displacements affecting the Ródão dissected terrace remnants is described. Geochronological dating of the two younger and lower terrace levels of this depression suggests a time-averaged incision rate for the Tejo in the Ródão area, of ca. 1.0 m/ka over the last 60 thousand years. A clear discrepancy exists between this rate and the 0.1 m/ka estimated for the longer period since the end of the Pliocene. Although episodes of valley incision may be conditioned by climate and base-level changes, they may also have been controlled by local factors such as movement of small fault-bounded blocks, lithology and structure. Regional crustal uplift is considered to be the main control of the episodes of valley incision identified for this large, long-lived river. A model is proposed in which successive regional uplift events—tectonic phases—essentially determined the long periods of rapid river downcutting that were punctuated by short periods of lateral erosion and later by some aggradation, producing strath terraces.  相似文献   

10.
A large number of blowouts and playas occur in the marginal sectors of the aeolian deposits located in the southern sector of the Duero Depression (Tierra de Pinares) in Spain. The blowouts are relict landforms that were developed on sand sheets by deflation during dry periods with lower vegetation cover and a deeper water table. The studied blowouts form complexes of NW–SE and NNW–SSE elongated hollows with accompanying dunes up to 4 km long in the leeward margins. Some hollows host lakes or swampy areas related to a shallow water table. The dunes formed by NE–ENE winds show steep windward slopes and gentle leeward slopes. The studied playas, with prevalent NNW–SSE orientations, result from the aeolian excavation of terrace deposits and the underlying marly bedrock. It is probable that the formation of these depressions in an initial stage was related to deflation processes affected preferentially NNW–SSE sandy channels perpendicular to the dominant wind direction. The precipitation of salts in the playas generates aggregates of clay particles (peloids) that are easily removed by the wind. Once the bottom reached the substratum, the deepening of the depressions progressed by the deflation of particles produced by weathering of the argillaceous bedrock.  相似文献   

11.
In this paper, we apply current geological knowledge on faulting processes to digital processing of Digital Elevation Models (DEM) in order to pinpoint locations of active faults. The analysis is based on semiautomatic interpretation of 20- and 60-m DEM and their products (slope, shaded relief). In Northern–Eastern Attica, five normal fault segments were recognized on the 20-m DEM. All faults strike WNW–ESE. The faults are from west to east: Thriassion (THFS), Fili (FIFS), Afidnai (AFFS), Avlon (AVFS), and Pendeli (PEFS) and range in length from 10 to 20 km. All of them show geomorphic evidence for recent activity such as prominent range-front escarpments, V-shaped valleys, triangular facets, and tilted footwall areas. However, escarpment morphometry and footwall geometry reveal systematic differences between the “external” segments (PEFS, THFS, and AVFS) and the “internal” segments (AFFS and FIFS), which may be due to mechanical interaction among segments and/or preexisting topography. In addition, transects across all five escarpments show mean scarp slope angles of 22.1°±0.7° for both carbonate and metamorphic bedrock. The slope angle equation for the external segments shows asymptotic behaviour with increasing height. We make an empirical suggestion that slope angle is a function of the long-term fault slip rate which ranges between 0.13 and 0.3 mm/yr. The identified faults may rupture up to magnitude 6.4–6.6 earthquakes. The analysis of the 60-m DEM shows a difference in fault patterns between Western and Northern Attica, which is related to crustal rheology variations.  相似文献   

12.
Studies on denudation rates can provide insight into the influence of climate change, tectonics, and human activities on landscape evolution. Research performed in Central Italy has shown considerable spatial variability of denudation rates in the major river basins. These studies have focused mainly on the Tyrrhenian side of the Italian peninsula, where Plio-Pleistocene marine deposits filling NW–SE elongated sedimentary basins have been uplifted during the Quaternary up to several hundreds of meters above present sea level. Small sub-catchments developed on clays are affected by sharp- and/or rounded-edged badlands (i.e. calanchi and biancane), representing denudation “hot spots” in the present-day morphoclimatic framework.In this paper, we analyze the relationships between indirectly estimated denudation rates at the catchment scale and field monitoring data at the hillslope scale. We attempt to better understand and quantify all hillslope processes that contribute to seasonal variability of denudation, to help with predicting the net input from “hot spots” to the overall estimated sediment yield at the basin outlets. At the hillslope scale, we discuss, in particular, the variability of denudation rates at calanchi and biancane badlands as a function of their different morphoevolution.  相似文献   

13.
The Gohpur–Ganga section is located southwest of Itanagar, India. The study area and its adjacent regions lie between the Main Boundary Thrust (MBT) and the Himalayan Front Fault (HFF) within the Sub-Himalaya of the Eastern Himalaya. The Senkhi stream, draining from the north, passes through the MBT and exhibits local meandering as it approaches the study area. Here, five levels of terraces are observed on the eastern part, whereas only four levels of terraces are observed on the western part. The Senkhi and Dokhoso streams show unpaired terraces consisting of very poorly sorted riverbed materials lacking stratification, indicating tectonic activity during deposition. Crude imbrications are also observed on the terrace deposits. A wind gap from an earlier active channel is observed at latitude 27°04′42.4″ N and longitude 93°35′22.4″ E at the height of about 35 m from the present active channel of Senkhi stream. Linear arrangements of ponds trending northeast–southwest on the western side of the study section may represent the paleochannel of Dokhoso stream meeting the Senkhi stream abruptly through this gap earlier. Major lineament trends are observed along NNE–SSW, NE–SW and ENE–WSW direction. The Gohpur–Ganga section is on Quaternary deposits, resting over the Siwaliks with angular contact. Climatic changes of Pleistocene–Holocene times seem to have affected the sedimentation pattern of this part of the Sub-Himalaya, in association with proximal tectonism associated with active tectonic activities, which uplifted the Quaternary deposits. Older and younger terrace deposits seem to mark the Pleistocene–Holocene boundary in the study area with the older terraces showing a well-oxidized and semi-consolidated nature compared to the unoxidized nature of the younger terraces.  相似文献   

14.
This study investigates the origin and regional tectonic implications of high-altitude Plio (?)–Quaternary fluvial deposits developed over the Bozdağ horst which is an important structural element within the horst–graben system of western Anatolia, Turkey.A total of 23 deposits occur near the modern drainage divide comprising fluvial to occasionally lacustrine deposits. The deposits are all elongated in N–S direction with a width / length ratio of 1 / 10. The largest of them is of 13 km in length with a maximum observable thickness of about 100–110 m. Morphological, lithological, deformational characteristics of these deposits and the drainage system of the area all suggest that the deposits were formed due to uplift and southward tilting of the Bozdağ horst. This tilting which is estimated as 1.2° to 2.2° caused accumulation of the stream load along channels flowing from south to north. All the deposits were later dissected by the same streams with the exception of one deposit which still preserves its original lake form. These deposits are of Quaternary age, which corresponds to the latest N–S directed extensional tectonic phase in the region.  相似文献   

15.
A geomorphological study focussing on slope instability and landslide susceptibility modelling was performed on a 278 km2 area in the Nalón River Basin (Central Coalfield, NW Spain). The methodology of the study includes: 1) geomorphological mapping at both 1:5000 and 1:25,000 scales based on air-photo interpretation and field work; 2) Digital Terrain Model (DTM) creation and overlay of geomorphological and DTM layers in a Geographical Information System (GIS); and 3) statistical treatment of variables using SPSS and development of a logistic regression model. A total of 603 mass movements including earth flow and debris flow were inventoried and were classified into two groups according to their size. This study focuses on the first group with small mass movements (100 to 101 m in size), which often cause damage to infrastructures and even victims. The detected conditioning factors of these landslides are lithology (soils and colluviums), vegetation (pasture) and topography. DTM analyses show that high instabilities are linked to slopes with NE and SW orientations, curvature values between − 6 and − 0.7, and slope values from 16° to 30°. Bedrock lithology (Carboniferous sandstone and siltstone), presence of Quaternary soils and sediments, vegetation, and the topographical factors were used to develop a landslide susceptibility model using the logistic regression method. Application of “zoom method” allows us to accurately detect small mass movements using a 5-m grid cell data even if geomorphological mapping is done at a 1:25,000 scale.  相似文献   

16.
River adjustment and incision in the Sabarmati basin, Gujarat, India have been examined at a site near Mahudi. Towards this, the morphostratigraphy and depositional chronometry of the middle alluvial plains were investigated. The upper fluvial sequence, along with the overlying aeolian sand and riverbed scroll plains, provide clues to the evolution of the present Sabarmati River. Sedimentological analyses of the upper fluvial sequence indicate its deposition by a meandering river system during what is believed to be a persistent wetter phase. Luminescence chronology bracketed this sequence to between 54 and 30 ka, which corresponds to Oxygen Isotope Stage-3, during which the SW monsoon was enhanced. The overlying aeolian sand has been dated to 12 ka, indicating that dune accretion occurred simultaneously with the strengthening of the SW monsoon during the Early Holocene. Adjustment of the Sabarmati along a N–S transect is placed around 12 ka and the incision is bracketed between 12 and 4.5 ka. River adjustment could have been tectonic; however, the incision was facilitated by the availability of continuous flow in the river caused by the SW monsoon. The basin experienced two tectonic events at about 3 and 0.3 ka, as demonstrated by the morphology of the scroll plains.  相似文献   

17.
Jon D. Pelletier   《Geomorphology》2009,105(3-4):322-333
Ripples and transverse dunes in areas of abundant sand supply increase in height and spacing as a function of time, grain size, and excess shear velocity. How and why each of these factors influence ripple and transverse dune size, however, is not precisely known. In this paper, the controls on the height and spacing of ripples and transverse dunes in areas of abundant sand supply are investigated using a numerical model for the formation of eolian bedforms from an initially flat surface. This bedform evolution model combines the basic elements of Werner's [Werner, B.T., 1995. Eolian dunes: Computer simulations and attractor interpretation. Geology 23, 1107–1110.] cellular automaton model of dune formation with a model for boundary layer flow over complex topography. Particular attention is paid to the relationship between bed shear stress and slope on the windward (stoss) side of evolving bedforms. Nonlinear boundary layer model results indicate that bed shear stresses on stoss slopes increase with increasing slope angle up to approximately 20°, then decrease with increasing slope angle as backpressure effects become limiting. In the bedform evolution model, the linear boundary layer flow model of Jackson and Hunt [Jackson, P.S., Hunt, J.C.R., 1975. Turbulent wind flow over a low hill. Quarterly Journal of the Royal Meteorological Society 101, 929–955.], generalized to 3D, is modified to include the nonlinear relationship between bed shear stress and slope. Bed shear stresses predicted by the modified Jackson and Hunt flow model are then used to predict rates of erosion and deposition iteratively through time within a mass-conservative framework similar to Werner [Werner, B.T., 1995. Eolian dunes: Computer simulations and attractor interpretation. Geology 23, 1107–1110.]. Beginning with a flat bed, the model forms ripples that grow in height and spacing until a dynamic steady-state condition is achieved in which bedforms migrate downwind without further growth. The steady-state ripple spacing predicted by this model is approximately 3000 times greater than the aerodynamic roughness length of the initially flat surface, which is a function of grain size and excess shear velocity. Once steady-state ripples form, they become the dominant roughness element of the surface. The increase in roughness associated with ripple formation triggers the same bedform instability that created ripples, causing dunes to form at a larger scale. In this way, the numerical model of this paper suggests that ripples and dunes are genetically linked. Transverse dunes in this model have a steady-state height and spacing that is controlled by the effective roughness length of the rippled surface, which is shown to be on the order of 500 times greater than the original roughness length, but varies significantly with the details of ripple morphology. The model predictions for ripple and dune spacing and their controlling variables are consistent with field measurements from the published literature. The model of this paper provides a preliminary process-based understanding of the granulometric control of ripples and dunes in areas of abundant sand supply and unidirectional prevailing winds, and it argues for a genetic linkage between ripples and dunes via a scaling relationship between eolian bedform size and the aerodynamic roughness length.  相似文献   

18.
Zones of distributed faulting with narrow (2–3 km) across‐strike spacing form a common structural style within rifts, especially in accommodation zones, and contrast with crustal‐scale half‐grabens, where strain is localised on normal faults spaced 10–30 km apart. These contrasting styles are likely to have a significant impact on geomorphic development, sediment routing and the stratigraphic record. Perachora Peninsula, in the eastern part of the active Corinth Rift, Greece, is one such zone of distributed faulting. We analyse the topography and drainage networks developed around these closely spaced normal faults, and compare our results with published studies from crustal‐scale half‐grabens. We subdivide the Perachora Peninsula into a series of drainage domains and examine the tectono‐geomorphic evolution of three domains that best represent the range of topographic characteristics, base levels and drainage network styles. We interpret that the perched, endorheic nature of the Asprokampos domain developed due to uplift and backtilt on offshore faults. The Pisia West domain, which drains the valley between the Skinos and Pisia Faults and responds to a perched base level, is interpreted to have experienced a complex base‐level history with episodic connections to sea level. The Skinos Relay domain drains to sea level, lying on the relay ramp between the closely spaced Kamarissa and Skinos Faults. Here, interaction between the displacement fields associated with each of the closely spaced faults controls the rate and style of landscape evolution. In contrast to crustal‐scale half‐grabens, observations from Perachora Peninsula suggest that zones of distributed faulting may be characterised by: (i) perched, internal sediment sinks at different elevations, responding to multiple base levels; (ii) minimal fault‐transverse sediment transport; (iii) interaction of uplift and subsidence fields associated with closely spaced faults, which modulate the rate and style of landscape response; and (iv) complex erosion and sedimentation histories, the evidence for which may have low preservation potential in the stratigraphic record.  相似文献   

19.
《Basin Research》2018,30(Z1):382-400
High‐resolution acoustic and seismic data acquired 100 km offshore Cape São Vicente, image with unprecedented detail one of the largest active reverse faults of the SW Iberian Margin, the Horseshoe Fault (HF). The HF region is an area seismogenically active, source of the largest magnitude instrumental and historical earthquake (Mw > 6) occurred in the SW Iberian Margin. The HF corresponds to a N40 trending, 110 km long, and NW‐verging active thrust that affects the whole sedimentary sequence and reaches up to the seafloor, generating a relief of more than 1 km. The along‐strike structural variability as well as fault trend suggests that the HF is composed by three main sub‐segments: North (N25), Central (N50) and South (N45). Swath‐bathymetry, TOBI sidescan sonar backscatter and parametric echosounder TOPAS profiles reveal the surface morphology of the HF block, characterized by several, steep (20°) small scarps located on the hangingwall, and a succession of mass transport deposits (i.e. turbidites) on its footwall, located in the Horseshoe Abyssal Plain. A succession of pre‐stack depth‐migrated multichannel seismic reflection profiles across the HF and neighbouring areas allowed us to constrain their seismo‐stratigraphy, structural geometry, tectono‐sedimentary evolution from Upper Jurassic to present‐day, and to calculate their fault parameters. Finally, on the basis of segment length, surface fault area and seismogenic depth we evaluated the seismic potential of the HF, which in the worst‐case scenario may generate an earthquake of magnitude Mw 7.8 ± 0.1. Thus, considering the tectonic behaviour and near‐shore location, the HF should be recognized in seismic and tsunami hazard assessment models of Western Europe and North Africa.  相似文献   

20.
The Pakuashan anticline is uniquely suited for study of the forward and lateral growth of fault-related folds. The Pakuashan ridge development arises from the late Quaternary uplift of the most external thrust zone of the western foothills of Taiwan. From Kaoshiung to Taichung, recent and active westward thrusting occurs at the front of the foothills. The Pakuashan anticline, trending N 150°E in the northern part to N 000° in the southern part, has been active throughout the Quaternary period. This activity is marked by geological structures, tectonic geomorphology and seismicity. A multisource and multiscale approach to study of the continental collision setting has been undertaken to combine tectonics, sedimentology and geomorphology. Studies of fracture patterns allow identification of two main features of stress orientations: a WNW/ESE compression direction, and E–W and N–S extension directions. Quantitative geomorphic parameters have been used to define the morphotectonic evolution and to infer tectonic style along the mountain front. Geomorphic evidence provides significant information on the processes that govern lateral propagation of an active anticline. Quaternary terraces are uplifted, tilted and folded over the Pakuashan ridge. Drainage systems in areas of active compression give information on the thrust zone structures and their development. Steep drainage and high local relief indicate that the Pakuashan anticline forms a well-defined zone of high uplift, especially in the southern part. The two main controls on drainage in that area are rock strength in the hanging wall and propagation of the deformation towards the south.  相似文献   

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