Éditeur(s) : HAL CCSD
Résumé : International audience
The history of the peneplain in East Sayan was studied using apatite fission-track analysis (AFTA). This method is suitable for determining the formation time of the erosional surface and estimating its denudation rate. The largest known relic of the peneplanation surface in this area is the Oka Plateau, separated from the Kropotkin Ridge by the Oka-Jombolok fault. The AFTA shows that the peneplain on the Oka Plateau formed in the Late Jurassic-Early Cretaceous. This peneplain is much younger than the erosional surfaces that persist today in the Tien Shan, Gobi Altai, and Mongolian Altai (Early Jurassic). However, it is older than the peneplain on the Chulyshman Plateau, Altai (Late Cretaceous), suggesting asynchronous formation of the ancient peneplain in Central Asia. The similar exhumation histories of samples from the Oka Plateau and Kropotkin Ridge indicate that these morphotectonic structures developed from Jurassic to late Miocene as a single block, which underwent continuous slow denudation at an average rate of 0.0175 mm/yr. Active tectonic processes in the Late Miocene caused the destruction of the peneplanation surface and its partial uplifting to different altitudes. The rate of Pliocene-Quaternary vertical movements along the Oka-Jombolok fault is roughly estimated at 0.046-0.080 mm/yr, which is several times higher than the denudation rate in this area. During the Pliocene-Quaternary, the Oka Plateau has not undergone any significant morphologic changes owing to its intermediate position between the summit plain and datum surface of East Sayan and to its partial shielding by basaltic lavas.
Russian Geology and Geophysics

insu-00839215
https://hal-insu.archives-ouvertes.fr/insu-00839215
DOI : 10.1016/j.rgg.2013.06.004

Éditeur(s) : HAL CCSD Geological Society of America
Résumé : In Gobi Altay and Altay, Mongolia, several flat surfaces, worn through basement rocks and uplifted during the ongoing tectonic episode to a similar altitude of 4000 m, suggests disruption of a single large-scale surface. New thermochronology and field data show that the plateau surfaces represent uplifted parts of an ancient peneplain that formed during Jurassic time. The Gobi Altay and Altay flattopped massifs are tectonically and geomorphologically unique. Their preservation for ~150 m.y. implies that no further tectonic movements occurred before the onset of the last deformation episode, 5 ± 3 m.y. ago. It also suggests that very low erosion rates were maintained by a dry climate over millions of years.
ISSN: 0091-7613

hal-00406436
https://hal.archives-ouvertes.fr/hal-00406436
DOI : 10.1130/G23758A.1

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
We use digital elevation models and satellite imagery data completed with field observations to develop an integrated morphotectonic analysis of the anomalous topography of the Central Cordillera of Patagonia between 46 degrees S and 48 degrees S, in a region overlying subducted segments of the Chile Spreading Ridge. This region shows a number of peculiar features, not expected in a normal subduction-related belt located away from a buried spreading center. First, it is characterized by overall short-wavelength topography and highly contrasted reliefs, typical of deeply incised young mountain belts, but surprisingly, also by an average very low elevation of similar to 900 m. Second, the Cordillera lacks central high reliefs; instead, there is a central, depressed domain with average low slope gradients including the deepest point of the study area (300 m below sea level in the bottom of Lake General Carrera/Buenos Aires). This depressed area is flanked by western and eastern regions of higher elevation having opposite general slope dips. To the east, the central Patagonian Cordillera is now separated from the flat plains of the Pampean domain (representing its former piedmont) and several elevated basaltic plateaus (mesetas) by 160 degrees N-180 degrees N trending morphotectonic corridors and the western border scarps of the mesetas. These scarps correspond to neotectonic features controlled by recent vertical faults, mostly active after 3 Ma. Remnants of a former peneplain, the Avellanos surface, are widespread within the entire Cordilleran region. From geological evidence, this peneplain formed between 15 and 4 Ma and was connected originally with the mesetas domain and now is dissected by these corridors. It is shown that a negative inversion related to extensional and strike-slip faulting occurred in the frontal region of the Cordillera at 3 Ma synchronously with the emplacement of magmatic products. Timing of the extensional deformation in central Patagonia is concomitant with the migration at depth of different segments of the South Chile Ridge, which entered the trench at 6 and 3 Ma, respectively, and the consequent opening of a slab window allows asthenospheric hot mantle to reach lithospheric regions. Finally, migration of segments of the Chile Spreading Ridge since 6 Ma triggered regional uplift of the Cordillera followed by limited extension in relation with increased thermal weakening of the crust.
ISSN: 0278-7407

hal-00486334
https://hal.archives-ouvertes.fr/hal-00486334
DOI : 10.1029/2009TC002453

Éditeur(s) : HAL CCSD Elsevier
Résumé : URL de l'article : http://www.sciencedirect.com/science/article/B6WPN-4XY3KDK-2/2/fd8508af63d82e29b4f9829c65194e26
The impact of climatic changes that occurred during the last glacial maximum and the Holocene on vegetation changes in the Amazon Basin and the Guiana Shield are still widely debated. The aim of our study was to investigate whether major changes in vegetation (i.e. transitions between rainforests and C4 savannas) occurred in northern French Guiana during the Holocene. We measured variations in the δ13C of soil organic matter at eight sites now occupied by forest or savannah. The forest sites were selected to cover two regions (forest refugia and peneplains) which are thought to have experienced different intensities of disturbance during the latest Pleistocene and the Holocene. We found that none of the forest sites underwent major disturbances during the Holocene, i.e. they were not replaced by C4 savannahs or C4 forest savannahs for long periods. Our results thus suggest that tropical rainforests in northern French Guiana were resilient to drier climatic conditions during the Holocene. Nevertheless, geographical and vertical variations in the 13C of SOM were compatible with minor changes in vegetation, variations in soil processes or in soil physical properties.
ISSN: 0033-5894

halshs-00551758
https://halshs.archives-ouvertes.fr/halshs-00551758
DOI : 10.1016/j.yqres.2009.11.007