Volcanic Plateau Research Articles

Silica Microphorms in Soils Affected by Hydrothermal Activity on the Yellowstone Volcanic Plateau, USA

Silica Microphorms in Soils Affected by Hydrothermal Activity on the Yellowstone Volcanic Plateau, USA

Plant Life-Forms and Environmental Filtering during Primary Succession on Loose Volcanic Substrata (Kamchatka, Russia)

Peculiarities of the forest vegetation dynamics and the main factors and mechanisms of environmental filtering were studied on loose deposits of the Tolbachinskii Dol Volcanic Plateau (Kamchatka) by means of plant life-form analysis. It was found that, in the earlier stages of plant succession, polycarpic herbs, acrocarpous mosses, and fruticulose lichens dominated. Forest communities of 35 years old differed from the old-growth ones by the proportions of plant life-forms and the total coverage. Environmental filtering is manifested by the most successful development of plants having special morphological adaptations of belowground systems to loose unstable substrata.

Criteria and tools for determining drainage divide stability

Watersheds are the fundamental organizing units in landscapes and thus the controls on drainage divide location and mobility are an essential facet of landscape evolution. Additionally, many common topographic analyses fundamentally assume that river network topology and divide locations are largely static, allowing channel profile form to be interpreted in terms of spatio-temporal patterns of rock uplift rate relative to base level, climate, or rock properties. Recently however, it has been suggested that drainage divides are more mobile than previously thought and that divide mobility, and resulting changes in drainage area, could potentially confound interpretations of river profiles. Ultimately, reliable metrics are needed to diagnose the mobility of divides as part of routine landscape analyses. One such recently proposed metric is cross-divide contrasts in χ, a proxy for steady-state channel elevation, but cross-divide contrasts in a number of topographic metrics show promise. Here we use a series of landscape evolution simulations in which we induce divide mobility under different conditions to test the utility of a suite of topographic metrics of divide mobility and for comparison with natural examples in the eastern Greater Caucasus Mountains, the Kars Volcanic Plateau, and the western San Bernadino Mountains. Specifically, we test cross-divide contrasts in mean gradient, mean local relief, channel bed elevation, and χ all measured at, or averaged upstream of, a reference drainage area. Our results highlight that cross-divide contrasts in χ only faithfully reflect current divide mobility when uplift, rock erodibility, climate, and catchment outlet elevation are uniform across both river networks on either side of the divide, otherwise a χ-anomaly only indicates a possible future divide instability. The other metrics appear to be more reliable representations of current divide motion, but in natural landscapes, only cross-divide contrasts in mean gradient and local relief appear to consistently provide useful information. Multiple divide metrics should be considered simultaneously and across-divide values of all metrics examined quantitatively as visual assessment is not sufficiently reliable in many cases. We provide a series of Matlab tools built using TopoToolbox to facilitate routine analysis.

Material composition of the basalt-trachyte series of the early Devonian of the Saralin graben-rift

The article clarifies the structural-tectonic position of the Saralin graben. It is determined that, along with the Balyksinsky graben located to the south and the Goryachegorsky volcanic plateau to the north, they are the connecting structural links between the Kuznetsk-Alatau alkaline province and the adjacent Minusinsk trough. In the early Devonian, the alkaline province was formed as a vaulted-block structure (the “shoulder” of the rift), and the Minusinsk deflection as a depression with the dominant volcanism in it. The boundary between these positive and negative structures was the deep Balyksinsko-Saralinsky fault.In the stratigraphic section of the graben, the lower molassoid part (Ustkundustylskaya stratum), the middle trachybasaltoid stratum (Bazarskay) and the upper problematic Ashpanian stratum are distinguished. In addition to analcime basalts, a large extrusive-subvolcanic dome-shaped construction of trachytes is considered.By petrographic and material composition, volcanites of graben are represented by the dominant basalt-trachyte series in the composition of basanites, trachybasalts, trachyandesites, trachytes. The rocks of the basalt-andesibasalt-andesite series are limited. Foidites are rare.Geochemical data indicate the genetic relationship of the volcanics of the studied series. Geochemically, they are clearly specialized in Sc, Ti, Zn, Zr, Th, U, Mn and P. According to the content of some of these elements, the rocks may have an industrial potential.In geodynamic and genetic plans, graben and its volcanics were formed during plume-tectonic processes involving mantle plume, enriched mantle, and recycling processes of these formations with the substance of the consolidated PR-PZ1 cortex.

A Geophysical Model for the Origin of Volcano Vent Clusters in a Colorado Plateau Volcanic Field

AbstractVariation in spatial density of Quaternary volcanic vents, and the occurrence of vent clusters, correlates with boundaries in Proterozoic crust in the Springerville volcanic field (SVF), Arizona, USA. Inverse modeling using 538 gravity measurements shows that vent clusters correlate with gradients in the gravity field due to lateral variation in crustal density. These lateral discontinuities in the crustal density can be explained by boundaries in the North American crust formed during Proterozoic accretion. Spatial density of volcanic vents is low in regions of high‐density Proterozoic crust, high in areas of relatively low density Proterozoic crust, and is greatest adjacent to crustal boundaries. Vent alignments parallel these boundaries. We have developed 2‐D and 3‐D numerical models of magma ascent through the crust to simulate long‐term, average magma migration that led to the development of vent clusters in the SVF, assuming that a viscous fluid flow through a porous media is statistically equivalent to magma migration averaged over geological time in the full field scale. The location and flux from the uniform magma source region are boundary conditions of the model. Changes in model diffusivity, associated with changes in the bulk properties of the lithosphere, can simulate preferential magma migration paths and alter estimated magma flux at the surface, implying that large‐scale crustal structures, such as inherited tectonic block boundaries, influence magma ascent and clustering of volcanic vents. Probabilistic models of volcanic hazard for distributed volcanic fields can be improved by identifying crustal structures and assessing their impact on volcano distribution with the use of numerical models.

Evaluation of the evolving stress field of the Yellowstone volcanic plateau, 1988 to 2010, from earthquake first-motion inversions

Although the last rhyolite eruption occurred around 70ka ago, the silicic Yellowstone volcanic field is still considered active due to high hydrothermal and seismic activity and possible recent magma intrusions. Geodetic measurements document complex deformation patterns in crustal strain and seismic activity likewise reveal spatial and temporal variations in the stress field. We use earthquake data recorded between 1988 and 2010 to investigate these variations and their possible causes in more detail. Earthquake relocations and a set of 369 well-constrained, double-couple, focal mechanism solutions were computed. Events were grouped according to location and time to investigate trends in faulting. The majority of the events have normal-faulting solutions, subordinate strike-slip kinematics, and very rarely, reverse motions. The dominant direction of extension throughout the 0.64Ma Yellowstone caldera is nearly ENE, consistent with the perpendicular direction of alignments of volcanic vents within the caldera, but our study also reveals spatial and temporal variations. Stress-field solutions for different areas and time periods were calculated from earthquake focal mechanism inversion. A well-resolved rotation of σ3 was found, from NNE-SSW near the Hebgen Lake fault zone, to ENE-WSW near Norris Junction. In particular, the σ3 direction changed throughout the years around Norris Geyser Basin, from being ENE-WSW, as calculated in the study by Waite and Smith (2004), to NNE-SSW, while the other σ3 directions are mostly unchanged over time. The presence of “chocolate tablet” structures, with two sets of nearly perpendicular normal faults, was identified in many stages of the deformation history both in the Norris Geyser Basin area and inside the caldera.

Soil distribution and soil properties in the subalpine region of Kazbegi; Greater Caucasus; Georgia: Soil quality rating of agricultural soils

Abstract Soils of the alpine ecosystem of Kazbegi region were investigated according to the Muencheberg Soil Quality Rating (M-SQR). Most limiting factors are climate as well as steepness, while the low nutrient supply and soil acidity can be tackled by adequate fertilization and liming practice. Inorganic or organic pollution were not detected. Soils on sediment fans as well as glacial sediments, mostly Cambisols (Humic) , are characterized by a low to moderate yield potential while high-yield soils, mostly Cambic Umbrisols , can be found on volcanic plateaus. A common element of all soils is the high humus content. Actually, most of them are used only for pasture, due to poor accessibility. Soils on fluvial deposits, mostly Fluvisols , show a very high range of M-SQR-scores. Altogether, the soils of the study area have the actually untapped potential to optimize the basic supply of the local population as well as tourism also by cultivation of cereals. Nevertheless, variety trials on different soil forming substrates as well as erosion control are major preconditions for successful implementation of new cropping systems in the Kazbegi region. Furthermore, particularly rare soils, e.g. Cambisols on Tephra, should be protected.

Rejuvenation of dry paleochannels in arid regions in NE Africa: a geological and geomorphological study

Although the River Nile Basin receives annually ca. 1600 billion cubic meters of rainfall, yet some countries within the Basin are suffering much from lack of water. The great changes in the physiography of the Nile Basin are well displayed on its many high mountains, mostly basement rocks that are overlain by clastic sediments and capped by volcanics in eastern and western Sudan. The central part of the Nile Basin is nearly flat including volcanics in the Bayuda Mountains and volcanic cones and plateaus in southwestern Egypt. The high mountains bordering the Nile Basin range in elevation from 3300 to 4600 m.a.s.l. in the Ethiopian volcanic plateau in the east to ca. 3070 m.a.s.l. in the western Gebel Marra, and 1310 m.a.s.l. in the Ennedi Mountains in northwestern Sudan. In central Sudan, the Nile Valley rises approximately 200–300 m.a.s.l. In Egypt, the River Nile is bounded by the Red Sea Mountains in the east, assuming ca. 1000–2600 m.a.s.l., mostly of basement rocks, which are covered to the north of Aswan by Phanerozoic sediments sloping to the west, passing by the Nile Valley and continuing through the Western Desert. The Phanerozoic cover on both sides of the Nile is known as the Eastern and Western Limestone Plateaus. These plateaus assume elevations varying from 300 to 350 m.a.s.l. near the eastern bank of the Nile to 400–500 m.a.s.l. south Luxor at Esna and west of Aswan. The nearly flat Sahara west of the Nile Valley rises gradually westward until it reaches Gebel Uweinat in the triple junction between Egypt, Sudan, and Libya. Gebel Uweinat has an elevation of 1900 m.a.s.l. sloping northward towards the Gilf Kebir Plateau, which is 1100 m.a.s.l. The high mountains and plateaus in the southern and western Egypt slope gradually northward where the Qattara Depression is located near the Mediterranean coast. The depression is −134 m.b.s.l., which is the lowest natural point in Africa. All these physiographic features in Sudan and Egypt are related to (i) the separation of South America from Africa, which started in the Late Paleozoic and continued up to the Cretaceous, giving rise to several generally E–W-oriented tectonic features inside Africa, (ii) the uplift of the Red Sea Mountains and their continuation inside Africa resulted in the East African Rift System (EARS), (iii) the Guinea–Nubia Lineament crossing Africa from the Atlantic to the Red Sea where many havoc trends, mostly E–W-trending faults, and uplifted basement features pierce the overlying sediments, (iv) parallel and longitudinal structures associated with volcanic plateaus and cones extend from west Sudan (Gebel Marra) to Ethiopian Plateau, passing by volcanics and plume features in between and the basins in east Africa were subjected to wrench related inversions, and (v) the Sudd linear E–W area stretching more than 1000 km between Gebel Marra in the west, passing by South Sudan and reaching southwestern Ethiopia. Here, fluviatile and subsurface waters led to ponds, lakes, and wet areas that are hard to exploit. The impact of these features led to the present south to north River Nile, but passing by many changes in the direction of its many tributaries and slope reversal of some of the major extinct rivers, either sectors of the main Nile or the rivers once flowed into the main river. The paleoclimatic changes during the Quaternary period: wet and dry have a great effect on the physiographic features and slope reversal of the Nile Basin drainage system.

Geophysical imaging of shallow degassing in a Yellowstone hydrothermal system

AbstractThe Yellowstone Plateau Volcanic Field, which hosts over 10,000 thermal features, is the world's largest active continental hydrothermal system, yet very little is known about the shallow “plumbing” system connecting hydrothermal reservoirs to surface features. Here we present the results of geophysical investigations of shallow hydrothermal degassing in Yellowstone. We measured electrical resistivity, compressional‐wave velocity from refraction data, and shear wave velocity from surface‐wave analysis to image shallow hydrothermal degassing to depths of 15–30 m. We find that resistivity helps identify fluid pathways and that Poisson's ratio shows good sensitivity to saturation variations, highlighting gas‐saturated areas and the local water table. Porosity and saturation predicted from rock physics modeling provide critical insight to estimate the fluid phase separation depth and understand the structure of hydrothermal systems. Finally, our results show that Poisson's ratio can effectively discriminate gas‐ from water‐saturated zones in hydrothermal systems.

Vertical tectonics at an active continental margin

Direct observations of vertical movements of the earth's surface are now possible with space-based GPS networks, and have applications to resources, hazards and tectonics. Here we present data on vertical movements of the Earth's surface in New Zealand, computed from the processing of GPS data collected between 2000 and 2015 by 189 permanent GPS stations. We map the geographical variation in vertical rates and show how these variations are explicable within a tectonic framework of subduction, volcanic activity and slow slip earthquakes. Subsidence of >3 mm/yr is observed along southeastern North Island and is interpreted to be due to the locked segment of the Hikurangi subduction zone. Uplift of 1–3 mm/yr further north along the margin of the eastern North Island is interpreted as being due to the plate interface being unlocked and underplating of sediment on the subduction thrust. The Volcanic Plateau of the central North Island is being uplifted at about 1 mm/yr, which can be explained by basaltic melts being injected in the active mantle-wedge at a rate of ∼6 mm/yr. Within the Central Volcanic Region there is a 250 km2 area that subsided between 2005 and 2012 at a rate of up to 14 mm/yr. Time series from the stations located within and near the zone of subsidence show a strong link between subsidence, adjacent uplift and local earthquake swarms.

Focal mechanisms and stress variations in the Caucasus and Northeast Turkey from constraints of regional waveforms

The continental collision between Arabia and Eurasia created large strike-slip faults in Turkey as well as mountains in the Caucasus and the volcanic plateau between them. In this study, we use regional waveforms of a new seismic array deployed between 2008 and 2012 to constrain the focal mechanisms and depths of small to moderate sized earthquakes occurring in the western part of the Central Caucasus and northeast Turkey. The distribution of aftershocks and the twelve focal mechanisms involved in the sequence of the 2009 earthquake in Racha are clearly a reactivation of a deeper segment of the 1991 M7 Racha rupture zone. The deeper segment is not well connected to the shallower décollement separating the basement and sedimentary basin. The earthquakes we determined in northeastern Turkey and southern Georgia are related to the strike-slip fault system. We further combined all of the reliably determined focal mechanisms over the last 30years to investigate the current stress status of the crust in three areas: Racha in the western Greater Caucasus, Javakheti near the Lesser Caucasus and in Northeast Turkey. Our results show that the directions of maximum compressional stress consistently fall within −2 to 14°N throughout the entire study region. This appears to be controlled by the continental collision. Nonetheless, the minimum compression switches from vertical (in the Greater Caucasus) to the east-west direction (in northeastern Turkey), due to the westward extrusion of the Anatolia block, which is driven partly by the Hellenic subduction. The transition of the stress field is close to the Javakheti volcanic plateau in the Lesser Caucasus, where the relative magnitude between the principal stresses appears to be strongly variable.

Pottery technology, settlement and landscape in Antofagasta de la Sierra (Catamarca, Argentina)

Abstract

Evolution of the biome of the Middle Siberian Trappean Plateau

The Middle Siberian volcanic plateau is characterized with respect to its paleoecological environments. In this area, volcanism commenced in the Severodvinian (Vishkilian) Age. The lower (tuffaceous) part of the plateau section (Lebedev deposits) was formed in the Vyatkian Age. Its middle (tufflava) part (Khungtukun deposits) is correlative with the Goudikeng Formation (interval of 65–111 m) in the Dalongkou type section of China and corresponds to the Quadrocladus pachyphyllus ... Echinolimnadia mattoxi Zone. The top of this sedimentary unit (111 m up from the base of the Goudikeng Formation) should be correlated with the base of the Otoceras Ecozone. The upper (lava) part of the plateau (Putorana deposits) corresponds to the interval of 111–210 m, belongs to the Elatocladus linearis ... Cornia vosini Ecozone, and is older than the Hindeodus parvus Zone, which is now considered as the basal unit of the Triassic marine sequences. The volcanic plateau section shows now evidence of any biotic crisis.

Primary plant successions of forest belt vegetation on the Tolbachinskii Dol volcanic plateau (Kamchatka)

Primary plant successions on volcanic deposits of the Tolbachinskii DoI Plateau (Central Kamchatka) were studied. The main factors determining the succession rate were revealed. The peculiarities of plant successions on lava flows and ash-scoria deposits differed significantly. Some common mechanisms for all volcanic regions of the world were revealed. The leading factors of plant succession on lava flows were the type of lava surface, the texture of lava, and the fine tephra accumulation rate. The main factors determining plant succession on ash-scoria deposits were wind and water erosion and the distance to the seed sources. The time of formation of a secondary permanent larch (Larix cajanderi Mayr.) forest should take about 1500-2000 years on lava flows and up to 1000 years (but not less than 300-500 years) on ash-scoria plains.

Magmatic activity during the breakup of Greenland-Eurasia and fluid-flow in Stappen High, SW Barents Sea

A great deal of magmatic activity was recorded along the northern Atlantic margins during the Cenozoic opening of the Norwegian- Greenland Seas. This work uses a recently acquired two-dimensional seismic reflection dataset to characterize magmatic rocks and fluid-flow features on the western margin of Stappen High. ‘Hard kicks’ are positive seismic high-amplitude anomalies related to igneous rocks in the subsurface while ’soft kicks’ are negative high-amplitude anomalies interpreted as fluid-related anomalies. The hard kicks are the most prominent anomalies in the study area, and they are distributed in a N-S zone that is coincident with the trend of the Stappen High. The hard kicks include extrusive and intrusive rocks such as lava flows, dykes, and volcanic sills. The sills have saucer, bowl, transgressive, tabular, and convex geometries. Extrusive igneous rocks and other deeply-seated volcanic sills are buried to a depth of greater than 3000 ms. Soft kicks are bright spots and enhanced reflections found close to angular unconformities, channels, clinoforms, and canyons. Hydrothermal vents and vertical clusters of soft-kicks/gas-charged sediments are evidence of vertically focused fluid-flow in the study area. This work finds that large-scale magmatic edifices recorded in other northern Atlantic margins are also present in the study area as volcanoes, volcanic plateaus and cascade-transgressive sills. Volcanic rocks in Stappen High were emplaced during the Late Eocene, Oligocene and Early Pliocene delimiting the areas into a southern part that is magma-rich and a northern hydrocarbon-rich province.

Magmatic evolution of the Early Pliocene Etrüsk stratovolcano, Eastern Anatolian Collision Zone, Turkey

The Pliocene Etrüsk stratovolcano, located in the northeast of Lake Van (Eastern Anatolia; Turkey), is one of the important volcanic centres in the Eastern Anatolian collision zone. Mt. Etrüsk overlies a widespread volcanic plateau, consisting of basaltic and hawaiitic lavas formed by fissure eruptions between 4.9–4.5Ma. These basic lavas contain a phenocryst phase consisting of olivine, plagioclase and clinopyroxene. Trace element ratio diagrams imply that these basic magmas were generated from a mantle that contained a clear subduction component that is related to the subducted sediments rather than fluids or altered oceanic crust. Results of the melting models on the basaltic plateau lavas indicate that there was a marked variation both in the mantle source mineralogy (i.e. the ratio of garnet peridotite to spinel peridotite in the source varies between 60/40% and 40/60%) and the degree of melting (i.e. F between 0.8–4%). This can be explained by a model in which magmas were generated by partial melting of both metasomatised lithospheric and deeper asthenospheric mantle sources in an extensional setting in response to the partial delamination of the lithospheric mantle of Eastern Anatolia and then mixed with each other during Pliocene times.Central eruptions that formed the Etrüsk volcano lasted ~600kyr between 4.3–3.7Ma during Zanclean times. The estimated depth of the Etrüsk magma chamber is ~9–12km. The volcano erupted lavas with a rather narrow compositional range from latite to rhyolite, which are either transitional or mildly alkaline in character. The Etrüsk lavas contain plagioclase, clino- and orthopyroxene, biotite, K-feldspar and rarely, minor amounts of olivine and amphibole in the phenocryst phase. A composite chemo-stratigraphic section of the volcano and petrological models indicate that the evolved lavas of the Etrüsk volcano differentiated from a parental magma composition, which is similar to that of the most primitive plateau basalt lavas underlying the volcano, via the AFC process, and experienced at least two major magma replenishment episodes at 4.1Ma and 3.8Ma during the magma chamber evolution.

Interpretación geológica y geofísica de la Elevación de Río Grande, margen brasileño suroriental: tectónica extensional y rifting de las cortezas continental y oceánica

This paper discusses the geological and geophysical interpretation of rift structures in the region extending from the Rio Grande Rise, in the Southeastern Brazilian margin, towards the Cabo Frio High, which separates the Campos and Santos basins. We have analysed potential field data (gravity and magnetic) from the Argentine to the Brazilian oceanic basins and extending over the Pelotas, Santos and Campos basins. The Rio Grande Rise shows a relatively negative Bouguer anomaly in an area that corresponds to a major positive bathymetric feature between the Argentine and Brazil basins. North–south propagators related to the early spreading centres of the Atlantic Ocean are observed from Argentina towards the southern Santos Basin, which is characterized by an elevated basement topography relative to the Pelotas Basin. The region adjacent to the Florianopolis Fracture Zone between the Santos and Pelotas basins is also characterized by an elevated basement region aligned in an east–west direction, and locally it is marked by rift structures aligned along a NW–SE direction, forming a lineament or shear zone (Cruzeiro do Sul lineament) that extends from the Cabo Frio High towards the Rio Grande Rise, thus involving both continental and oceanic crusts. The Rio Grande Rise is associated with the east–west-trending fracture zones, which are characterized by several aligned magnetic anomalies in the southern Santos Basin. The Rio Grande Fracture Zone continues landward as the Sao Paulo Ridge, and extends towards the platform as the Florianopolis High. Oceanic propagators are identified from Argentina towards the Pelotas and Santos basins, and locally we observe rupturing of the salt layer by igneous intrusions or possibly by mantle exhumation. The Florianopolis (or Rio Grande) Fracture Zone is marked by an abrupt topographic offset separating the Pelotas Basin from the southern Santos Basin, and the associated volcanic belts limit the southernmost occurrence of the late Aptian evaporate sequence. The evaporite sequence in this segment of the continental margin shows remarkable layering of halite, anhydrite and carnalite. Conjugate to the Rio Grande Rise, the Walvis Ridge, offshore Namibia, is similarly a topographic high, but rift structures as observed in the Brazilian side are apparently unique in the South Atlantic. Alternative interpretations for the origin of the Rio Grande Rise include: a volcanic edifice or plateau rooted in the mantle; an intraplate shear zone affecting both continental and oceanic crust; an oceanic area of igneous over-productivity caused by a hotspot or a thermal anomaly in the mantle; a palaeo-spreading centre in the Cretaceous Atlantic Ocean; an area of excessive volcanic activity resulting from mantle differentiation due to adiabatic decompression; or perhaps an isolated remnant of continental crust left outboard of the Brazilian continental margin during the drifting process.

Effect of <i>Serratia entomophila</i> and diazinon applied with seed against grass grub populations on the North Island volcanic plateau

The bacterial biocontrol agent Serratia entomophila and the insecticide diazinon were applied as separate granular formulations with ryegrass seed and compared with a seedonly control treatment on three pastures of different ages and composition on the North Island volcanic plateau In the first 2 years diazinon and S entomophila significantly reduced healthy grass grub populations compared with the control However by the third year populations in the diazinon treatments had recovered and were significantly higher than in S entomophila or control plots Grass grub populations were reduced by disease outbreaks after S entomophila was applied which infected >40 of grass grub larvae in the treated plots in year two Bacterial extraction from soil a year after application confirmed establishment and persistence of S entomophila in treated plots Visual positive pasture growth responses were noted in both the S entomophila and diazinontreated plots

Late Pleistocene glaciations in southern East Sayan and detection of MIS 2 terminal moraines based on beryllium (10Be) dating of glacier complexes

Analysis and summary of publications on southern East Sayan, eastern Tuva, and northern Mongolia have shown that the late Pleistocene glaciation covered a large area and had a complicated dynamics of glacier advance and retreat. Starting with MIS 5, the Todza Basin and, partly, the Oka Plateau, Azas Volcanic Plateau, Mondy Basin, and river valleys in southern East Sayan were periodically covered with ice. The thickness of ice in the eastern Todza Basin was 700 m, on the Azas Volcanic Plateau it reached 300–600 m, and in the valleys of southern East Sayan it is estimated as 700–800 m. The thickness of ice in the Mondy Basin was 300–350 m.Geological and geomorphological studies and isotope surface exposure dating (10Be method) of boulders from terminal moraine complexes have provided evidence for extensive MIS 2 glacier advance in the Mondy Basin and in the Sentsa, Jombolok, and Sailag river valleys (southern East Sayan). The average age of exposure for three groups of samples is 14, 16, and 22 ka.

The Cenozoic volcanic province of Tibesti (Sahara of Chad): major units, chronology, and structural features

Using both field relationships and some absolute ages, the sequence of volcanic units in the Cenozoic Tibesti Volcanic Province (TVP) (Chad) is established as follows: (1) plateau volcanism, between at least 17 and 8 Ma, consisting of flood basalts and silicic lava plugs, with intercalated ignimbritic sheets in the upper basalt succession increasing in amount upwards. Ages decrease from NE to SW, following the migration of the small NW-SE flexures concentrating the feeding dike swarms; (2) Late Miocene large central composite volcanoes exhibiting diverse and original structures. Some of them (Tarso Toon, Ehi Oye, and Tarso Yega) are located along a major NNE fault, representing the main tectonic direction in Tibesti since Precambrian times; (3) construction of three large ignimbritic volcanoes, associated with significant updoming of the basement, ending with the collapse of large calderas: Voon (about 5–7 Ma), Emi Koussi (2.4–1.33 Ma), and Yirrigue (0.43 Ma); (4) basaltic activity, starting at about 5–7 Ma, and essentially consisting of cinder cones and associated lava flows (Tarso Toh, Tarso Ahon, and Tarso Emi Chi); and (5) final volcanic activity represented by post-Yirrigue caldera activity in the Tarso Tousside Volcanic Complex, and especially Ehi Tousside (the only active volcano in Tibesti), plus Ehi Timi and Ehi Mousgou volcanoes, similar to Ehi Tousside. The two tectonic directions controlling some volcanic features of the province correspond to the major old lithospheric structures delimiting the volcanic province, namely, the great NW-SE Tassilian flexure to the SW and a major NE-NNE fault zone to the E. Unusual conditions of uplift and erosion in the TVP enable exceptional exposure of the internal structure of its volcanoes.