Inset Terraces Research Articles

The impact of Wallachian settlement on relief and alluvia composition in small valleys of the Carpathian Mts. (Czech Republic)

The impact of the Wallachian settlement on small mountain catchments has been studied at the example of Škorňanský and Hartisov Streams (Moravskoslezské Beskydy Mts, Western Outer Carpathians). Between the 16th and the 19th centuries the Wallachian people introduced pastoral management originating from the woodless Balkans to the forested mid-altitude mountain area under study. This area was subjected to rapid settlement, intense grazing in forests and slash-and-burn deforestation. To assess the geomorphic effects of Wallachian activity in the catchments studied, grain size, structure, organic matter content, and the plant macrofossil composition of the alluvia forming terrace levels in the mouths of the Škorňanský and Hartisov Streams were analysed. Radiocarbon dating, 137Cs and 210Pb analyses and dendrochronological dating were also applied to determine the age of the alluvia studied. The results were compared with land-use and land-cover changes recorded on archival maps and pictures, and with population changes in the study area. It was found that the two successive trends of catchment settlement and later depopulation have led to the development of inset terraces on the outlet fans of the Škorňanský and Hartisov Streams. In the study catchments both settlement/deforestation and depopulation/land abandonment occurred very rapidly which also caused abrupt geomorphic changes. Before human disturbance, fluvial erosion and the redeposition of older Pleistocene deposits predominated. As a result coarse, gravel alluvia were deposited within stream channels. The stage of massive overbank accumulation of fine-grained alluvia was a reflection of intensified delivery of slope material into valley floors from catchments deforested by the Wallachians. The later stage of vertical incision of channels resulted from the decrease of sediment supply from hillslopes to river channels during catchment depopulation and afforestation.

Middle Pleistocene glaciation, alluvial fan development and sea-level changes in the Bay of Kotor, Montenegro

Abstract A coarse-grained alluvial fan at Lipci in the Bay of Kotor, western Montenegro, was deposited in the Middle Pleistocene by a high-energy, steep gradient proglacial stream draining an outlet glacier on the Orjen massif. The fan apex is currently about 50 m above sea-level, but the majority ( c. 60%) of this landform now lies offshore. Field mapping, sedimentological analysis and uranium-series dating were combined with a marine bathymetric survey and seismic profiling to explore the morphology and history of the entire fan complex. The Lipci fan was deposited on the margin of a large polje downstream of moraines that formed during the Middle Pleistocene (Marine Isotope Stage 12). The sea-level may have been more than 120 m lower than present during the glacial stages of the Middle Pleistocene. The sediments on the terrestrial portion of the fan are strongly cemented by secondary calcite and the oldest uranium-series ages show that the fan was deposited before 320 ka. These ages are consistent with a larger uranium-series dataset ( n =39) from other glacial and glacio-fluvial formations surrounding Mount Orjen. Seismic profiling of the submerged portion of the fan in the Bay of Kotor shows well-preserved palaeochannels with inset terraces. The Lipci fan is unusual because even its distal segments are well preserved after exposure to multiple post-Marine Isotope Stage 12 regression–transgression cycles. This is probably due to the strong cementation of the fan sediments and its sheltered location in the Bay of Kotor.

Repeated Folding during Late Holocene Earthquakes on the La Cal Thrust Fault near Mendoza City (Argentina)

In 1861, one of the most destructive earthquakes in the history of Argentina destroyed the city of Mendoza (currently 1 million inhabitants). The magnitude M S∼7.0 earthquake is inferred to have occurred on the 31‐km‐long La Cal thrust fault, which extends from Mendoza to the north, where it offsets an alluvial fan and small inset terraces along a well‐preserved fault scarp. A trench excavated on a terrace that is vertically offset by ∼2.5 m exposes two main stratigraphic units separated by an erosional unconformity. The coarse‐grained upper unit is deformed by three east‐vergent folds ( F 1– F 3). Retrodeformation of these folds yields total displacements of ∼2.0 m, ∼2.4 m, and ∼0.5 m on the underlying fault splays, respectively. The displacement of ∼2.0 m recorded by fold F 1 is interpreted as the result of the fault rupture that caused the 1861 earthquake. F 2 and F 3 were presumably generated during the penultimate event with an inferred magnitude of M w∼7.0, although formation during two distinct ruptures cannot be excluded. Finite‐element modeling shows that coseismic folding above the tip of a blind thrust fault is a physically plausible mechanism to generate these folds. A published luminescence age of 770±76 years, which is interpreted to date the formation of the deformed terrace, indicates that the two (or possibly three) scarp‐forming events occurred during the last ∼800 years. The fine‐grained sediments below the erosional unconformity—that contain evidence for at least one older earthquake—are dated at ∼12 kyr. Our results indicate that elastic strain energy, which is accumulating at the front of the Precordillera today as shown by Global Positioning System (GPS) data, was repeatedly released during earthquakes on the La Cal fault in the past. Hence, the La Cal thrust fault poses a serious threat to the city of Mendoza.

Stratigraphy of the Arriaga Palaeolithic sites. Implications for the geomorphological evolution recorded by thickened fluvial sequences within the Manzanares River valley (Madrid Neogene Basin, Central Spain)

The Arriaga Palaeolithic sites, located within the Middle–Late Pleistocene thickened terrace (TCMZ: +18–22m) of the Manzanares River valley (Madrid, Central Spain), were subject to intensive archaeological and palaeontological prospecting during the 1980s. Compilation of documents from these old excavations, together with new geoarchaeological, sedimentological, pedological and geophysical data, allow us to locate the morpho-stratigraphic position of the analysed sites within the overall stratigraphy of the TCMZ. This thickened terrace comprises two main fluvial sequences (Lower and Upper) topped by a thick (2.5–5m) alluvial–colluvial formation. The fluvial sequences are stacked in the study site located in the lowermost reach of the valley, but display complex inset relationships upstream, where they are individualized in two different terrace levels at +18–22 and +12–15m. Terrace thickening was primarily controlled by synsedimentary subsidence caused by dissolution of the evaporitic substratum and locally influenced and backfed by tectonic activity. The regional analysis of the dated (TL and OSL) fluvial sequences containing Palaeolithic sites within the TCMZ, together with new TL dates provided in this study, indicate that the three sedimentary sequences in the TCMZ are time-transgressive valley-fill bodies. Terrace thickening started before the Last Interglacial Period (MIS 6 or older) and continued during whole MIS 5 (lower fluvial sequence) and MIS 4 (upper fluvial sequence) reaching the MIS 3 (top alluvial formation), the latter characterized by the accumulation of alluvial–colluvial sequences derived from the main tributaries and valley slopes. The TCMZ records the Middle–Late Pleistocene boundary, but also the transition between the Lower and Middle Palaeolithic periods during the Late MIS 5 (ca. 96 to 74ka). The studied Arriaga sites contain evolved Lower Palaeolithic industry (evolved Acheulean techno-complexes) and warm faunal assemblages located within the Lower fluvial sequence, but apparently well constrained Middle Palaeolithic sites are placed within the Upper fluvial sequence at other upstream locations. Deposition of the thickened alluvium was mainly controlled by the upstream advance of dissolution-induced subsidence phenomena, blurring the impact of Late Pleistocene climatic cycles and producing time-transgressive longitudinal valley-fill bodies (i.e. sedimentary sequences). Late Quaternary climatic changes only seem to control the incision/aggradation cycles after the termination of the TCMZ from the Late MIS 3. Dates related to the development of younger inset terraces indicate that they are apparently linked with cold Heinrich events H4 to H1. These younger inset terraces yield cold faunal assemblages and abundant Middle Palaeolithic “Mousterian” assemblages.

Dating inset terraces and offset fans along the Dehshir Fault (Iran) combining cosmogenic and OSL methods

SUMMARY Be and 36 Cl cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of offset terraces have been performed to constrain the long-term slip rate of the Dehshir fault. Analysis of cosmogenic 10 Be and 36 Cl in 73 surface cobbles and 27 near-surface amalgamscollectedfrominsetterracesdemonstratestheoccurrenceofalowdenudationrateof 1mM a −1 and of a significant and variable inheritance from exposure prior to the aggradation of these alluvial terraces. The significant concentrations of cosmogenic nuclides measured in the cobbles collected within the riverbeds correspond to 72 ± 20 ka of inheritance. The mean CREageofthesurfacesamplescollectedontheolderterraceT3is469 ±88kabuttheanalysis of the distribution of 10 Be concentration in the near-surface samples discard ages older than 412 ka. The mean CRE age of the surface samples collected on terrace T2 is 175 ± 62 ka but the 10 Be depth profile discard ages older than 107 ka. For each terrace, there is a statistical outlier with a younger age of 49.9 ± 3.3 and 235.5 ± 35.4 ka on T2 and T3, respectively. The late sediments aggraded before the abandonment of T2 and inset levels, T1 b and T1a, yielded OSL ages of, respectively, 26.9 ± 1.3, 21.9 ± 1.5 and 10.0 ± 0.6 ka. For a given terrace, the OSL ages, where available, provide ages that are systematically younger than the CRE ages. These discrepancies between the CRE and OSL ages exemplify the variability of the inheritance and indicate the youngest cobble on a terrace, that minimizes the inheritance, is the most appropriate CRE age for approaching that of terrace abandonment. However, the upper bound on the age of abandonment of a terrace that is young with respect to the amount of inheritance is best estimated by the OSL dating of the terrace material. For such terraces, the CRE measurements are complementary of OSL dating and can be used to unravel the complex historyofweatheringandtransportinthecatchmentofdesertalluvialfans.Thiscomprehensive set of dating is combined with morphological offsets ranging from 12 ± 2t o 380± 20 m to demonstrate the Dehshir fault slips at a rate in the range 0.9 mm yr −1 –1.5 mm yr −1 .T he variable inheritance exemplified here may have significant implications for CRE dating in arid endorheic plateaus such as Tibet and Altiplano.

10Be exposure ages of ancient desert pavements reveal Quaternary evolution of the Dead Sea drainage basin and rift margin tilting

Early to late Pleistocene 10Be exposure ages of abandoned surfaces in the Negev desert reveal the regional drainage evolution history and its relationship with the subsidence of the western margin of the Dead Sea Rift. The dated desert paved surfaces have developed over originally westward-flowing rivers, which were abandoned by early Pleistocene and whose relicts are now progressively tilted towards the rift axis. The slow and non-destructive subsidence coupled with extreme hyperaridity enabled the preservation of these ancient surfaces along some of the main water divides in the Negev, nearly irrespective of their distance from the rift axis. Constraints on the tilting history are obtained from analyzing the spatial pattern of the exposure ages, suggesting subsidence rates as low as 120–300 m Ma –1 in the southern Arava Valley since the late Pliocene. It is shown that the transition from the Pliocene to current drainage pattern occurred over a short period during the early Pleistocene, and that the governing fluvial response that followed the delineation of current basins is represented by a continuous spectrum of ages of inset terraces.

Timing and controls on late Quaternary landscape development along the eastern Sierra El Mayor range front in northern Baja California, Mexico

The Sierra El Mayor–Sierra Cucapá mountain range in northern Baja separates the Colorado River delta from the Laguna Salada Basin and is important in terms of understanding the structural transition from the northern Gulf Extensional Province to the San Andreas system. One of the principal Quaternary landscape features of the eastern Sierra El Mayor–Sierra Cucapá is a prominent surface that slopes ~ 1–2° eastward towards the Colorado River Delta. Along the eastern side of the Sierra El Mayor and facing the Colorado River delta, but not along other piedmont mountain fronts that face the delta, this surface is terminated by a series of stepped terrace surfaces. The terrace surfaces are cut into fluvial–deltaic deposits from the Colorado River delta system and are capped by ~ 1 m thick gravels. Optically stimulated luminescence ages from interbedded coarse sands in the capping gravels of the upper surface and two of the inset terraces range from ~ 17 to 31 ka (average 24 ± 3 ka) but lack a systematic age trend from upper to lower surface. The underlying fluvial–deltaic deposits have older optically stimulated luminescence ages of 31 to 39 ka and suggest Colorado River delta deposition during the relatively stable interstadial of the Middle Wisconsinan substage (Marine Isotope Stage 3) when sea level was ca − 60 to − 80 m. Samples from depth-profiles in the capping gravels and from the top of the upper surface have a mean 10Be terrestrial cosmogenic nuclide age of 59 ± 5 ka. When corrected for inheritance, an average age of ~ 21 ka is computed for exposure of the capping gravels on the upper surface and inset terraces, which is in the range of the optically stimulated luminescence burial ages. Both the upper surface and inset terraces formed 17–30 ka at a transition time from dry to relatively wet climate when increased aggradation caused alluvial fan development on the upper surface. Regionally, these surfaces correlate with the Q2c–Q3a age deposits of the southwestern North America alluvial fan and terrace deposits. Sea level was also falling rapidly at this time, which may have caused the Colorado River to become entrenched near the range front thus allowing rapid response and down-cutting to form the inset terraces on the east side of the Sierra El Mayor facing the delta. Assuming the 34 ka delta deposits graded to a base level that was − 60 to − 80 m during Marine Isotope Stage 3, the computed surface uplift rate for the eastern side of the Sierra El Mayor is ~ 2.1–2.7 mm/yr. This uplift may be accommodated on a fault or fault system that is a southward extension of the Mexicali spreading and seismic zone that is buried beneath the Colorado River delta east of the Sierra El Mayor. The uplift rates from this study are consistent with geologic and geodetic data for the Sierra El Mayor and Sierra Cucapá, and account for much of the missing extensional strain across the area.

Topographic expression of active faults in the foothills of the Northern Apennines

Active faults that rupture the earth's surface leave an imprint on the topography that is recognized using a combination of geomorphic and geologic metrics including triangular facets, the shape of mountain fronts, the drainage network, and incised river valleys with inset terraces. We document the presence of a network of active, high-angle extensional faults, collectively embedded in the actively shortening mountain front of the Northern Apennines, that possess unique geomorphic expressions. We measure the strain rate for these structures and find that they have a constant throw-to-length ratio. We demonstrate the necessary and sufficient conditions for triangular facet development in the footwalls of these faults and argue that rock-type exerts the strongest control. The slip rates of these faults range from 0.1 to 0.3 mm/yr, which is similar to the average rate of river incision and mountain front unroofing determined by corollary studies. The faults are a near-surface manifestation of deeper crustal processes that are actively uplifting rocks and growing topography at a rate commensurate with surface processes that are eroding the mountain front to base level.

Transition from alluvial to fluvial systems in the Guadalentín Depression (SE Spain) during the Holocene: Lorca Fan versus Guadalentín River

The Lorca alluvial fan (SE Spain) constitutes an excellent example of transition from alluvial to fluvial sedimentary environments in a semiarid region. The fan body represents a large gently-sloping alluvial fan of mainly sandy–silty composition. It is presently dissected throughout its length by the Guadalentín River, which worked as the main fan channel and still operates as a torrential rambla-type channel responsible for historic large flood events. Three different outcrops are analysed at fan-apex, mid-fan and distal-fan locations, taking also into account the different inset terrace levels developed within the main fan trench at + 8–9 m, + 5–6 m, + 3–4 m and + 1–2 m above the present thalweg. Available radiocarbon data and archeological remains collected in fan sediments and inset terraces enable the construction of a preliminary hypothesis about the progressive incision of the fan body from pre-historical to historical times. Proximal fan aggradation took place during the Early Holocene before ~ 7900–7700 cal. yr. BP and continued until the Late Neolithic to Calcolithic Period (~ 5500–4200 cal. yr. BP) when the fan surface stabilised, and channel incision generated a first fanhead trench. Subsequent infilling of the fan trench led to the development of the upper terrace level (+ 8–9 m) which merged with the palustrine environments 23 km downstream of Lorca, feeding distal fan lobes during Late Bronze Age and Post-Roman times. The second terrace level (+ 5–6 m) is a cut & fill episode of about 5 m thickness developed from Roman times, and dated to ~ 1700 to 950 cal. yr. BP. The geometry and altimetry of the upper terrace level, together with the occurrence of different steps in the longitudinal profile of the river at mid to distal fan locations, strongly suggest that the incision of the ancient fan channel was assisted by intersection point headcut development ( sensu [Harvey, A.M., 1978. Dissected alluvial fans in southeast Spain. Catena 5, 177–211.]) at proximal-mid fan locations, but by headward erosion exerted by neighbouring drainage basins at distal palustrine environments. Therefore, the Lorca fan exemplifies the behaviour of large-sized fans in its transition from pure alluvial environments to fluvial ones, and its eventual incipient integration into a regional drainage network subject to a dynamic base-level during the historic period.

Active tectonics in southern Xinjiang, China: Analysis of terrace riser and normal fault scarp degradation along the Hotan‐Qira Fault System

The northern piedmont of the western Kunlun mountains (Xinjiang, China) is marked at its easternmost extremity, south of the Hotan‐Qira oases, by a set of normal faults trending N50E for nearly 70 km. Conspicuous on Landsat and SPOT images, these faults follow the southeastern border of a deep flexural basin and may be related to the subsidence of the Tarim platform loaded by the western Kunlun northward overthrust. The Hotan‐Qira normal fault system vertically offsets the piedmont slope by 70 m. Highest fault scarps reach 20 m and often display evidence for recent reactivations about 2 m high. Successive stream entrenchments in uplifted footwalls have formed inset terraces. We have leveled topographic profiles across fault scarps and transverse abandoned terrace risers. The state of degradation of each terrace edge has been characterized by a degradation coefficient τ, derived by comparison with analytical erosion models. Edges of highest abandoned terraces yield a degradation coefficient of 33 ± 4 m2. Profiles of cumulative fault scarps have been analyzed in a similar way using synthetic profiles generated with a simple incremental fault scarp model. The analysis shows that (1) rate of fault slip remained essentially constant since the aggradation of the piedmont surface and (2) the occurrence of inset terraces was synchronous at all studied sites, suggesting a climate‐driven terrace formation. Observation of glacial and periglacial geomorphic features along the northern front of the western Kunlun range indicates that the Qira glaciofluvial fan emplaced after the last glacial maximum, during the retreat of the Kunlun glaciers (12–22 ka). The age of the most developed inset terrace in uplifted valleys is inferred to be 10 ± 3 ka, coeval with humid climate pulses of the last deglaciation. The mass diffusivity constant (k=τ/T, being time B.P.) in the Hotan region is determined to be 3.3 ± 1.4 m2/103 years, consistent with other estimates in similar climatic and geologic environments of western China. These results imply a minimum rate for the Tarim subsidence of 3.5 ± 2 mm/yr. If Western Kunlun overthrusts the Tarim platform on a crustal ramp dipping 40°–45° to the south, it would absorb at least 4.5 ± 3 mm/yr of convergence between western Tibet and Tarim.

Active thrusting and folding along the northern Tien Shan and Late Cenozoic rotation of the Tarim relative to Dzungaria and Kazakhstan

We have studied geometries and rates of late Cenozoic thrust faulting and folding along the northern piedmont of the Tien Shan mountain belt, West of Urumqi, where the M= 8.3 Manas earthquake occurred on December 23, 1906. The northern range of the Tien Shan, rising above 5000 m, overthrusts a flexural foredeep, filled with up to 11,000 m of sediment, of the Dzungarian basement. Our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front, within a 200‐km‐long zone of Neogene‐Quaternary anticlines. Fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range. In all the valleys, the scarps offset vertically the highest terrace surface by the same amount (10.2±0.7 m). Inferring an early Holocene age (10±2 kyr) for this terrace, which is continuous with the largest recent fans of the piedmont, yields a rate of vertical throw of 1.0±0.3mm/yr on the main active thrust at the surface. A quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5.5±2.5 m2/kyr. A rather fresh surface scarp, 0.8±0.15 m high, that is unlikely to result from shallow earthquakes with 6 < M < 7 in the last 230 years, is visible at the extremities of the main fold zone. We associate this scarp with the 1906 Manas earthquake and infer that a structure comprising a deep basement ramp under the range, gently dipping flats in the foreland, and shallow ramps responsible for the formation of the active, fault propagation anticlines could have been activated by that earthquake. If so, the return period of a 1906 type event would be 850 ±380 years. The small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth (≈⅔) is taken up by incremental folding near the surface. Comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising Quaternary ranges such as the San Gabriel mountains in California or the Mont Blanc‐Aar massifs in the Alps. We estimate the finite Cenozoic shortening of the folded Dzungarian sediments to be of the order of 30 km and the Cenozoic shortening rate to have been 3 ± 1.5 mm/yr. Assuming comparable shortening along the Tarim piedmont and minor additional active thrusting within the mountain belt, we infer the rate of shortening across the Tien Shan to be at least 6 ± 3 mm/yr at the longitude of Manas (≈85.5°E). A total shortening of 125±30 km is estimated from crustal thickening, assuming local Airy isostatic equilibrium. Under the same assumption, serial N‐S sections imply that Cenozoic shortening across the belt increases westwards to 203±50 km at the longitude of Kashgar (≈ 76 °E), as reflected by the westward increase of the width of the belt. This strain gradient implies a clockwise rotation of Tarim relative to Dzungaria and Kazakhstan of 7±2.5° around a pole located near the eastern extremity of the Tien Shan, west of Hami (≈96°E, 43.5°N), comparable to that revealed by paleomagnetism between Tarim and Dzungaria (8.6° ± 8.7°). A 6 mm/yr rate of shortening at the longitude of Manas would imply a rate of rotation of 0.45°/m.y. and would be consistent with a shortening rate of 12 mm/yr north of Kashgar. Taking such values to be representative of Late Cenozoic rates would place the onset of reactivation of the Tien Shan by the India‐Asia collision in the early to middle Miocene (16 +22/−9 m.y.), in accord with the existence of particularly thick late Neogene and Quaternary deposits. Such reactivation would thus have started much later than the collision, roughly at the time of the great mid‐Miocene changes in tectonic regimes, denudation and sedimentation rates observed in southeast Asia, the Himalayas and the Bay of Bengal, and of the correlative rapid change in seawater Sr isotopic ratio (20 to 15 Ma). Like these other changes, the rise of the Tien Shan might be a distant consequence of the end of Indochina's escape.

ASYNCHRONOUS TERRACE DEVELOPMENT IN DEGRADING BRAIDED CHANNELS

Terrace remnants are commonly used to reconstruct longitudinal profiles of rivers and floodplains, and to establish temporal correlations of events in fluvial systems. In most cases, it is assumed that the terrace remnants represent time-equivalent surfaces. Our observations of terrace formation in flume experiments and in a degrading braided river, Ash Creek, Arizona, suggest that this assumption is not always valid. Degradation resulted from a reduction in upstream sediment delivery to braided channels. In both the flume and Ash Creek, degradation in the upstream reach produced a number of inset terraces, while the production of sediment in the degrading reach simultaneously caused further aggradation downstream. Thus, stratigraphically lower surfaces in the upstream reaches are temporally equivalent to higher surfaces in downstream reaches. The downstream progression of the wave of incision produced more terraces upstream than downstream, and terrace surfaces could not be correlated on the basis of rel...

Bordure orientale du massif du Mont Dore jusqu'en Limagne. Le volcanisme holocène de la région de Besse-en-Chandesse. Formations péri-montdoriennes de Champeix et Perrier

The different types of projections sent out by the puys of Montchal, of Montcineyre and the Pavin lake-crater are described and the areas that they cover are delimited. Notably the relations between the trachytic nuée surrounding the Pavin and the pumices in the vicinity of Montcineyre have been established. The projections can be dated by means of paleosols that they overlie. Lastly, the maar of les Costes furnishes a recapitulative tephrochronological section, thanks to the superposition of the following formations and intercalated paleosols bedded basaltic tuffs of the maar properly speaking, basaltic lapilli of Montcineyre (or Montchal) and the trachytic nuée of Lake Pavin. In the region of Champeix can be observed pumiceous alluvium of Middle Pleistocene age, the base of which presents a morainic character ; these were probably contemporary with the eruptions of the Adventif Massif. The route leading from the Perrier cemetery to the summit of the plateau offers a complete section of the Perner infilling : vestiges of inset terraces, the basalt of the Roca-Neyra, the Perrier conglomerate consisting of mud flows intercalated with beds of fluvial sands and pebbles, lying locally on volcano-sedimentary strata.