West-facing Scarps Research Articles

Neotectonic, morphotectonic and paleoseismologic analysis of the Las Chacras Fault System, Sierras Pampeanas Occidentales, San Juan, Argentina

Las Chacras Fault System is a Quaternary structure located in the western piedmont of Sierra de la Huerta in the Sierras Pampeanas Occidentales, in west-central Argentina. This fault system is subsidiary of the Valle Fértil Megafault System, the NW-striking master structure that affects the western flank of the Valle Fértil - La Huerta ranges. The Las Chacras Fault System comprise several sections including the Pan de Azúcar fault, which presents the most conspicuous evidence of tectonic Quaternary activity. The Pan de Azúcar fault affects three Quaternary alluvial surfaces (Q1, Q2 and Q3) along 26 km in the western piedmont of Sierra de La Huerta, where three natural exposures allowed us to observe Triassic rocks thrust over Quaternary deposits. The observation of different kinematic markers and the relative position of the units affected by the fault, allow us to define this fault as a reverse fault. This structure trends 325° and dips 25° NE. With a DGPS we carried out detailed topographic profiles perpendicular to the average strike of the fault, along interfluves and rivers that cross the structure, for the purpose of establishing the relationship between the topography and the slope of the terrain. This analysis indicates that Quaternary activity of the fault has played an essential role in controlling the piedmont geomorphology, generating west-facing scarps and drainage anomalies such as rivers with convex Hack profiles. Paleoseismic analyses indicate that the fault has moved after 18.360 ± 2990 yr BP. Furthermore, we calculated a net displacement for the Pan de Azúcar fault of ∼50 ± 0.2 m since the Late Pleistocene to Holocene, yielding a shortening rate and a slip rate of ∼0.16 ± 0.1 mm/yr. Thus, the Pan de Azúcar fault is a potential seismogenic source, which could produce a moderate earthquake between Mw = 6.4 and 7.0, with a recurrence interval between 2600 and 6900 years.

Comparison between scarp and dip-slope rivers of the Cotswold Hills, UK

The Cotswold Hills, southwest UK, are properly described as a cuesta, with a steep, west-facing scarp slope and a plateau-like dip slope. Drainage reflects this surface morphology, with most rivers flowing southeast along topographic and stratigraphic dip. Here, we compare two superficially highly similar rivers – the Frome and Churn – whose sources are nearly coincident, but whose behaviour dramatically diverges thereafter. We examine longitudinal profiles, channel steepness, predicted discharge, and valley shapes, using digital topographic data. River discharge and water hardness/temperature values were obtained at seven sites on the Churn and nine on the Frome over a two-year sampling campaign, delineated into summer and winter phases. Nearly 100 borehole records were interrogated from the two catchments in order to assess groundwater level variations. The Frome, flowing west against regional dips, develops a steep course and has carved a deep and wide valley that exposes the full sequence of Cotswold Jurassic stratigraphy. On the other hand, discharge and channel gradients are lower for the dip-slope Churn, whose valley exposes less stratigraphy and fewer springs. Our measurements of river water hardness and temperature suggest that a greater proportion of groundwater flows into the Frome, regulating discharge and maintaining baseflow over summer. We suggest that flank uplift of the Cotswolds is at least part of the reason for the higher incision rates of the River Frome, leading to its intersecting a greater number of highly transmissive fractures that contribute to its discharge. In turn, the increased discharge could positively impact local incision rates.

トレンチ調査からみた十日町盆地東縁断層の変位様式と活動履歴

The Eastern Marginal Fault of the Tokamachi Basin is located on the right bank of the Shinano River, the longest river in Japan, in the Niigata area, within the Neogene fold and thrust belt of central Japan. The activity of this fault is partly responsible for the formation of the Tokamachi tectonic basin. The fault zone is composed of several subparallel fault branches that strike N-S to NNW-SSE and deformed late Pleistocene to Holocene fluvial terraces. These fault branches are characterized by west-facing scarps with some subsidiary east-facing scarps to the east of the basin. Although the destructive 2004 and 2007 earthquakes occurred in the northern part of this fold and thrust belt, no historical rupture has been recorded in the Tokamachi Basin. To obtain paleoseismic records in this tectonically formed basin, we excavated four trenches across different fault branches. The Banba South, Banba North, and Shinmiya trenches are located across the west-facing scarps, and the Miyakuri trench is on an east-facing scarp. Fault exposures and deformed terrace deposits are present in these four trenches. These results confirm that these geomorphic scarps are indeed produced by faulting. At the Banba S trench, three fault traces that have nearly horizontal to very shallow dipping fault planes with upthrown side on the east are present, and show clear evidence of the latest event at ca. 3,500-3,100 yrs BP (BC 1,965-1,630 to BC 1,505-1,145). A penultimate event possibly occurred at after ca.11,000 yrs BP. (BC 11,810-9,800), although its exact timing is not determined. At the Banba N trench, the terrace deposits of ca. 5,300 yrs BP (BC 4,225-3,965) are deformed. This suggests the age of deformation is younger, and probably coincides with the latest event at the Banba S trench. At the Shinmiya trench, the late Pleistocene terrace is divided into two parts by a flexural scarp, probably produced by a blind reverse fault. The scarp is formed prior to 14,000 yrs BP (BC 14,710-13,700). In addition, a younger event is recognized to have occurred between 9,400 and 8,900 yrs BP (BC 8,555-8385 to BC 8,000-7,910). In contrast to those trenches on the west-facing scarps, trench logs at Miyakuri, located on an east-facing scarp, show possibly two events with relatively steep fault planes during the last 32,000 yrs. The latest activity at Miyakuri occurred between 9,500 and 7,400 yrs BP (BC 8,955 to BC 6,365-6,045), but the age of the penultimate event cannot be precisely determined. However, we suggest that the faulting interval at Miyakuri is longer than that of other fault branches. In summary, from this study, we can identify at least three paleoseismic events, namely, I, II, and III, and a possible event IV for the Eastern Marginal Fault of the Tokamachi Basin. Event I is clearly identified at Banba S and N trench. Three faults probably moved simultaneously by Event II. Two other events are only recognized locally. Faulting on the west-facing scarp is more frequent and has a high slip rate, reaching nearly 1 m/ky.

Geology of Salt Lake City, Utah, United States of America

Geology of Salt Lake City, Utah, United States of America

Effect of height and orientation (microclimate) on geomorphic degradation rates and processes, late-glacial terrace scarps in central Idaho

Terrace scarps can serve as a nearly ideal natural laboratory for the study of the evolution of slopes. This paper examines the effects of scarp size (height) and orientation (microclimate) by keeping constant variables such as age, lithology, and regional climate. If a scarp degrades as a closed system, and downslope movement is directly proportional to surface gradient , the evolution of the scarp is modeled by the diffusion equation. For a group of scarps of same age and known starting angle, the diffusion-equation model predicts the relation between maximum scarp angle (𝛉) and scarp height ( h ). Late Pleistocene terrace scarps now as steep as 33.25°, as well as measured angles of repose for sand and gravel, require a starting angle as steep as 33.5°. For latest Pleistocene Idaho and Utah scarps, as h increases, 𝛉 is gentler (more degraded) than modeled by the diffusion equation with a constant rate coefficient. The degradation-rate coefficient ( c ) increases tenfold with scarp height; it should not change with scarp height if downslope movement is solely determined by surface gradient (to the first power). Soil wash appears to be responsible for this departure from the diffusion-equation model, for transport rate by soil wash is a function of scarp size (height). South-facing scarps are less vegetated and more degraded than north-facing scarps. For scarps 2 m high, the degradation rate ( c *) on S-facing scarps is 2 times that on N-facing scarps; for 10-m scarps, it is 5 times. The observed dependence of the rate coefficient c * on scarp height can be removed by normalizing c * to values for west-facing scarps of the same height. The residual c * values calculated by this method correlate well with differences in incident solar radiation resulting from the different scarp orientations and maximum gradients. This correlation demonstrates the importance of orientation on slope processes and their rates through the differences in freeze-thaw cycles, soil moisture, and vegetative cover. Scarp morphology may be used to estimate age, if one accounts for the effects of climate and for scarp height, orientation, and lithology. For example, using the dated Bonneville shoreline scarps for calibration and comparing only scarps of equal height, we estimate the Drum Mountains fault scarps to be 9,000 yr old. This age is about twice that produced by previous diffusion-equation calculations that have not accounted for the height as we have here, but it is the same as independent geologic estimates of their age.

ON THE LOCATION OF NATIVE VILLAGES ON AND AROUND THE ABERDARE RANGE, KENYA

The Aberdare Range, which lies north of Nairobi, the capital of Kenya, is composed of Tertiary (Neogene) volcanic rocks, such as basalt, phonolite, trachyte. Its westward slopes are steep, belonging to the main fault scarps of Eastern (Gregory) Rift Valley; while its eastward slopes are very gentle and dessected by many consequent valleys. On the interfluve ridges of a part of these eastward slopes, many native Kikuyu villages or hamlets are densely distributed. As shown in Fig. 1, villages are crowded on its eastern part at an altitude between 1, 300 and 2, 300 meters, and on its southeastern part between 1, 700 and 2, 200 meters, extraordinarily in contrast with surrounding areas. Villagers' lives chiefly depend on cultivation of subsistence crops, such as maize, pulses, potato and banana, and also on breeding of livestock such as cattle, sheep, goats and poultry (Fig. 2). The vegetation and landuse of treated area are represented by profiles in Fig. 3. The concentration of native population mentioned above is partly or chiefly the result of former colonial policy being kept by Britain, for instance, setting up of so-called ‘White Highlands’. On one hand, it is also important to consider the natural conditions which permit such a concentration. The subject of this article is just on this point. As easily noticed, climatic factors are concerned, of which precipitation must be the most effective one in such a tropical semi-arid region. Fig. 4 shows the monthly rainfall of selected stations: Karamaini in the southern part, Fort Hall in the middle part, and Nyeri in the northern part. Mean values show two maxima and two minima in a year round, though the absolute values tell irregularity or unreliability, which is also the case of annual value as shown in Fig. 5. Isohyets of mean annual rainfall are drawn in Fig. 6, using 110 stations which have data of over 20 years and supplementarily 63 stations which have data of under 10 years. The pattern of isohyets well indicates orographical precipitation on the eastward slopes facing the prevailing easterlies, and also tells that its northeastern part is the rain-shadow of Mount Kenya. The lower limit of location of native villages on the eastern and southeastern slopes coincides 1, 000 milimeters isohyet fairly well. Fig. 6 also contains isoplethes of annual rainfall reliability over 800 milimeters during 8 years, 1955 to 62, using the available 77 stations. And, the lower limit mentioned above seems to situate between 8/8 and 6/8. In the next place, the upper limit must be considered. It is generally set up politically, for example to reserve forests, but it may not be the case without considering natural environments. Here, temperature is to be regarded. There are few data available in the treated area. So, to see the relation between altitude and average annual temperature, Fig. 7 was made, which implies linear relation between them. The solid line in the figure tells the lapse-rate of about 0.62°C per 100 meters. The upper limit of native villages, that is about 2, 300 meters in altitude, is to have average annual temperature of 15°C, which seems to be near the critical point or boundary for Kikuyu's traditional way of life. Moreover, the author actually saw in 1964 and 1965 that clouds often covered the summit part of Aberdare Range even in so-called long dry season. So the deficiency of sunshine also seems to relate with the upper limit here. Now, the scarcity of native villages on the northeastern slopes can be explained by discrepancy between rainfall factor and altitudinal or temperature factor there, And, this is also the case of the Kinangop Plateau which situates below the steep west-facing scarps of Aberdare Range, and where pyroclastic porous soil and scarcity of water flow emphasize unfavorable conditions for traditional cultivation.

Reliefumkehr Durch Rumpfflächenbildung in Tanganyika

Click to increase image sizeClick to decrease image sizeAbstractThe large peneplain-region to the north, south and west of Masasi in southern Tanganyika is formed on crystalline rocks of the folded African Basement complex. At a distance of 150 km this peneplain slopes from west to east from 600 m above sea-level to between 350 and 100 m. Then it ends on the base of a zone of westward facing plateau-scarps. These have a socle of the Basement complex and are covered by horizontal sandstones, mainly Cretaceous. They form the plateaus of Rondo, Maconde and Mawia with hights of 700 to 800 m and separate the peneplain-region from the Indian Ocean. But major rivers as the Ruvuma and Lukuledi cross through the plateau-zone in deep (to 600 m) but large valleys.The west-facing scarps of the plateau-zone are not simple cuestas. This was already shown by v. Staff 1914. Here the African Basement complex plunges slowly eastwards to the Indian Ocean. The peneplain-region, though orographically of minor height, is placed on higher elevated geological fundament than the orographically high sandstone-plateaus. The west scarps of the sandstone-plateaus are filed up along a flexure-line, and have been developed to inversional relief. To attain this result the surface-erosion working in the peneplain-region must have been by far more effective here than the valley- erosion in the zone of the sandstone-plateaus. The reason of this fact may be found in the petrographic and climatic differences of the two regions.All erosional waste of the peneplain-region has surely been carried off by crossing the zone of the sandstone-plateaus in the valleys of the rivers Ruvuma and Lukuledi. So peneplanation has taken place in large distance from the coast, separated from it by a mountainous zone of considerable height, and it was done in rather high position above sea-level. Except the inselbergs there exist no distinct remnants of older reliefs in the peneplairegion. Peneplanation may have gone on here since very long time, but, as surface-erosion continues, all actual surfaces of this peneplain are recent. For hinting at this fact we can perhaps call the age of these surfaces persistently-recent or per-recent.