Drainage Density and Its Controlling Factors on the Eastern Margin of the Qinghai–Tibet Plateau

The drainage density (Dd) is an important index to show fluvial geomorphology. The study on Dd is helpful to understand the evolution of the whole hydrological and geomorphic process. Based on the Shuttle Radar Topography Mission 90-m digital elevation model, the drainage network of basins along the eastern margin of the Qinghai–Tibet Plateau is extracted using a terrain morphology-based method in ArcGIS 10.3, and Dd is calculated. The spatial characteristics of Dd are analyzed, and the relationship between Dd and its influencing factors, e.g., the topography, precipitation, and vegetation coverage, is explored. Our results show that terrains with a plan curvature ≥3 can represent the channels in the study area. Dd ranges from 2.5 to 0.1 km/km2, increases first, and then decreases from north to south on the eastern margin of the Qinghai–Tibet Plateau. Dd decreases with increasing average slope and average local relief. On the low-relief planation surfaces, Dd increases with increasing altitude, while on the rugged mountainous above planation surfaces, Dd decreases rapidly with increasing altitude. Dd first increased and then decreased with increasing mean annual precipitation (MAP) and normalized difference vegetation index (NDVI), and Dd reaches a maximum in the West Qinling Mountains with a semi-arid environment, indicating that Dd in different climatic regions of the eastern margin of the Qinghai–Tibet Plateau was mainly controlled by precipitation and vegetation.

Geomorphology and Neotectonics of Southwestern Crimea

—The area of southwestern Crimea includes the ending of the Crimean Mountains that arose during the neotectonic activation at the place of the Cretaceous–Paleogene denudation plain and the adjacent shallow-water carbonate sedimentation basin. The Crimean Mountains are one of the links of the Alpine–Himalayan orogenic belt formed during the collision of the Eurasian, African, and Indo–Australian plates. Their area includes late Cenozoic marine terraces of the complete Mediterranean series and a staircase of Neogene, Paleogene and Cretaceous planation surfaces over them. The planation surfaces of different ages resulted from the successive lowering of the World Ocean level. Their subsequent deformations make it possible to outline the area of the neotectonic uplifting and determine its parameters. The main mechanism of the neotectonic activation was the thrust of the East Black Sea microplate under the Scythian one and the formation of a ramp fold structure. The amplitude of the neotectonic uplifting of southwestern Crimea for the past 2 Myr varies from 0 to 800 m, i.e., is up to 0.04 mm/year. The recent neotectonic structure of the area is formed by the northern flank of the ramp fold; it is a monocline of NW dip consisting of “keys” of NW strike separated by the latest faults with vertical displacements of 10 to 120 m. The uplifting of the area and the lowering of the World Ocean level led to a widespread of denudation surfaces. Their good preservation makes it possible to refine the sequence of neotectonic events, whose first pulses reached the study area in the Oligocene, and the main activation phase began in the Pliocene.

BRAZILIAN RELIEF, PLANATION SURFACES AND MORPHOLOGICAL LEVELS

Este artigo sobre relevo, superfícies erosivas e níveis morfológicos é um produto síntese, fruto de pesquisa sistemática e mapeamento desenvolvido nas últimas duas décadas no Brasil e também por meio de pesquisa privada do autor. Seus objetivos são desenvolver uma análise crítica dos mais importantes estudos realizados no Brasil relacionados às superfícies erosivas e, ao mesmo tempo, estabelecer novas interpretações sobre fatos dessa natureza com o auxílio dos conhecimentos adquiridos nos últimos anos. Desta forma, tenta-se mostrar que a interpretação com correlações automáticas entre os níveis morfológicos topográficos e as superfícies erosivas são altamente duvidosos, sendo necessária a adoção de outros parâmetros para as análises. Os exemplares localizam-se nas regiões Sudeste, Centro-Oeste e Minas Gerais, pois podem ser encontradas superfícies niveladas, principalmente ao longo das divisórias das grandes bacias de drenagem do Brasil, ou ainda nas bordas niveladas de bacias sedimentares como Paraná, Parecis e Parnaíba. Além disso, as pesquisas desenvolvidas na Bacia do Alto Paraguai-Cuiabá confirmam um fato que já havia sido observado em outras áreas. Observou-se que naquela região os níveis morfológicos são decorrentes de diversos fatores de caráter estrutural, como superfícies erosivas, idades residuais ou diferenciais, níveis estruturais condicionados, níveis produzidos por erosão diferencial, níveis produzidos por efeito tectônico e níveis produzidos por deposição recente.

Denudation history of the French Massif Central: new insights from thermochronology, basement-basin cross-sections and semi-automated planation surfaces mapping

<p>Documenting surface uplift of basement areas is challenging, usually due to large gaps in the sedimentary record. In order to address this issue for the French Massif Central, we here investigate its denudation history through an integrated study that involves planation surface mapping, Apatite Fission-Track (AFT) Analysis and basement to basin cross-sections.</p><p>First, Planation surfaces were identified using a semi-automated fuzzy classification of pixels based on relationships between DEM derivatives (slope, curvature, ruggedness and incision) and field-recognized training samples.  Then, their different generations and age ranges were discriminated from hypsometry, fault partitioning and relationships with dated sedimentary and/or volcanic remnants, providing constraints on basement exhumation. Afterwards, integrating the previous planation surface analysis, geological cross-sections were produced from the Massif Central basement to the surrounding basins (Aquitaine Basin and Paris Basin). These sections provide local thicknesses estimates of the missing sedimentary cover over basement domains. Theses local thicknesses and exhumation phases were finally used as constraints to produce a thermal history modelling and a denudation map of different areas of the French Massif Central estimated from AFT inversion.</p><p>Our results show different burial and exhumation patterns with i) a main burial of its western parts (Limousin, Rouergue) during Jurassic times followed by an important regional denudation (1 to 2 km of missing cover and crystallized basement) during the early Cretaceous and ii) an Upper Cretaceous burial of its northeastern parts (Morvan, Forez) followed by an uppermost Cretaceous to Paleogene exhumation (<1 km of missing cover and crystallized basement). This further illustrates the different behavior of each units of the Massif Central during the Mesozoic to Cenozoic times. These results will ultimately be discussed and placed back into the western European deformation framework.</p><p> </p><p>(This work is founded and carried out in the framework of the BRGM-TOTAL project Source-to-Sink)</p>

Young Uplift at the Eastern End of the Alps. Evidence for Uplift Unrelated to the Inversion of the Pannonian Basin?

<p>The eastern end of the Alps features a series of low relief surfaces at elevations up to 2500 m. These surfaces have long been known to reflect uplifted planation surfaces that have not yet been dissected by fluvial processes and thus preserve a strong geomorphic disequilibrium. While their age would present a good handle on the age of surface uplift in the Eastern Alps, these surfaces are barely dated and their age is only indirectly inferred to reflect the Miocene and Pliocene uplift history. Recent geomorphological cosmogenic nucleide-based studies have shown that these surfaces may record up to 1000 m of surface uplift in the last 5 Ma. Such a distinct uplift event in the recent past is surprising and needs to be interpreted. Interestingly, this time frame appears not to be accompanied by crustal shortening and the standard hypothesis about the inversion of the Pannonian Basin as the underlying cause needs to be questioned. In order to get a better handle on the nature of this young uplift event and its overriding driver it is crucial to understand its spatial extent. However, much of the Eastern Alps was glaciated in the Pleistocene and currently several studies suggest that elevated low-relief landscapes were shaped by the glacial buzz-saw, instead of interpreting them in terms of fluvial prematurity of recently uplifted planation surfaces. The models of glacial erosion versus fluvial prematurity as the formation agent of the low-relief surfaces can be discerned if it can be shown that the surfaces formed prior to the Pleistocene. Here we report of a currently operating research project in which we employ cosmogenic nucleide burial dating on a substantial part of the entire Eastern Alps to derive the age of these surfaces. We use the burial age of siliceous sediments in caves formed at the phreatic-vadose transition as a proxy. Correlation of cave levels with low-relief surfaces and their mapping in the field is an integral part of the project.</p><p> </p>

First proofs of preservation of a Mesozoic paleorelief in Southeast Africa: Insights from the (U-Th)/He dating of iron oxides from Malawian duricrusts

<p>Approximately 70% of the emerged relief on the Earth is characterized by erosional low-gradient topography also known as planation surfaces (PS). Many geomorphologists defend the idea that some of these surfaces could be relics of old reliefs uplifted and preserved from erosion for tens of millions years. Some of the highest PS of Southeast Africa (> 2000 m) were considered by King (1962) as remnants of an ante-Cretaceous paleorelief called “Gondwana Surface”. Specifically, the Nyika Plateau (Northern Malawi, 2200 m) is one of the largest potential relics of the “Gondwana Surface” in Southeastern Africa. This PS overlooks the stripped etchplain of the Malawian Plateau, a potential Late Cretaceous PS about 1200 m of elevation.</p><p>However, the preservation of such ancient reliefs is controversial, particularly under a tropical wet-dry climate. Doubts about the ages of these PS exist mainly due to the lack of a precise chronology of these objects on a continental scale. In detail, African PS are often covered by preserved or partly eroded tropical weathering covers such as unconsolidated laterites and/or duricrusts. Under these climatic conditions, lateritic duricrusts can be preserved for millions of years and thus contain several generations of iron oxides witnesses of past local paleoenvironment and geodynamic evolution. In order to understand the formation and preservation of the Southeast African highest PS and date them, we decided to apply (U-Th)/He dating of iron oxides on selected duricrust samples. The exploration of the Nyika Plateau allowed the discovery of an outcropping duricrust and a depositional area of eroded duricrust blocks from different origins. We study duricrust samples from these two areas in order to find some clues about the plateau antiquity and to improve our knowledge about the local paleoclimatic and geodynamic history.</p><p>Samples from the in situ duricrust levels, outcropping on the plateau, are polygenic and are formed by three main types of zones: preserved and degraded hematite-rich zones, that are considered to correspond to the initial generation of iron oxides, and a goethitic matrix. The preserved hematites have a Mesozoic (U-Th)/He ages, whereas the goethite-rich matrix of this duricrust formed during the Quaternary. The degraded hematite-rich parts, also rich in quartz, have more dispersed ages ranging from the Mesozoic to the Tertiary. In the detrital accumulation zone, blocks from a similar duricrust were found as well as blocks of another type of duricrusts: a pisolithic one rich in goethite. This last type of duricrust was eroded from a more recent duricrust level, as their iron oxides have Late Tertiary/Quaternary ages. These dating proved the Nyika Plateau relative stability since the Mesozoic period, confirming that duricrusting of reliefs in tropical area can also protect old emerged landscapes from total erosion.</p><p>King L.C. (1962) Morphology of Earth, Oliver and Boyld, Edinburg.</p><p> </p>