Éditeur(s) : HAL CCSD American Geophysical Union (AGU) American Geophysical Union
Résumé : International audience
GPS data reveal that the Brahmaputra Valley has broken from the Indian Plate and rotates clockwise relative to India about a point a few hundred kilometers west of the Shillong Plateau. The GPS velocity vectors define two distinct blocks separated by the Kopili fault upon which 2–3 mm/yr of dextral slip is observed: the Shillong block between longitudes 89 and 93°E rotating clockwise at 1.15°/Myr and the Assam block from 93.5°E to 97°E rotating at ≈1.13°/Myr. These two blocks are more than 120 km wide in a north-south sense, but they extend locally a similar distance beneath the Himalaya and Tibet. A result of these rotations is that convergence across the Himalaya east of Sikkim decreases in velocity eastward from 18 to ≈12 mm/yr and convergence between the Shillong Plateau and Bangladesh across the Dauki fault increases from 3 mm/yr in the west to >8 mm/yr in the east. This fast convergence rate is inconsistent with inferred geological uplift rates on the plateau (if a 45°N dip is assumed for the Dauki fault) unless clockwise rotation of the Shillong block has increased substantially in the past 4–8 Myr. Such acceleration is consistent with the reported recent slowing in the convergence rate across the Bhutan Himalaya. The current slip potential near Bhutan, based on present-day convergence rates and assuming no great earthquake since 1713 A.D., is now ~5.4 m, similar to the slip reported from alluvial terraces that offsets across the Main Himalayan Thrust and sufficient to sustain a Mw ≥ 8.0 earthquake in this area.
ISSN: 0148-0227

hal-01115084
https://hal.archives-ouvertes.fr/hal-01115084
DOI : 10.1002/2014JB011196

Éditeur(s) : HAL CCSD Geological Society of America
Résumé : International audience
The post-rift denudation history of high-elevation divergent continentalmargins is central to deciphering source-to-sink systems acrosssuch margins and to unraveling their topographic evolution. We performed40Ar-39Ar dating of supergene K-Mn oxides from supergenemanganese ore deposits in order to constrain the age of in situ formedlaterites on both the lowland and highland sides of the Western GhatsEscarpment of Peninsular India. Documentation of laterites as oldas 53 Ma on the highland and 47 Ma at the foot of the escarpmentshows that the escarpment stabilized before 47 Ma (possibly 60 Ma).The topographic setting of the dated weathering mantles also constrainsdenudation of the lowland and the highland since ca. 50 Mato maximums of 5 m/m.y. and 15 m/m.y., respectively. Our resultschallenge denudation rates derived from apatite fission track thermochronologyby documenting the topography and relief longevityof the southwest Indian margin and its hinterland. The results precludelarge-magnitude uplift of Peninsular India after the Eocene,and more generally argue against late Neogene uplift and topographicrejuvenation of continental shields.
ISSN: 0091-7613

ird-01419633
http://hal.ird.fr/ird-01419633
http://hal.ird.fr/ird-01419633/document
http://hal.ird.fr/ird-01419633/file/GEOLOGY-S-15-00851_HAL.pdf
DOI : 10.1130/G37303.1

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Since Deccan Traps extrusion ~ 65 Ma ago, thick weathering mantles have developed over Peninsular India on both the western coastal lowland and adjacent plateau separated by the Western Ghats Escarpment. Manganiferous lateritic profiles formed by supergene weathering of Late Archean manganiferous protores are exposed on paleolandsurface remnants on both sides of the escarpment. Petrological and geochemical characterizations of samples from those Mn lateritic profiles allowed identifying cryptomelane (K-Mn oxide) dated by 40Ar/39Ar geochronology. The ages obtained document major weathering periods, ca. 53-50 Ma, and ca. 37-23 Ma in the highland, and ca. 47-45 Ma, ca. 24-19 Ma and discrete weathering pulses at ~ 9 Ma and ~ 2.5 Ma in the lowland. Old ages of the highland (53-50 Ma) and the lowland (47-45 Ma) indicate synchronous lateritic weathering across the escarpment at a time the peninsula started to drift across the equatorial belt. Intense weathering periods at ca. 53-45 and ca. 37-23 Ma are interpreted to reflect the Early Eocene climatic optimum and the onset of Asian monsoon regimes, respectively. The ages further indicate that most of the dissection of the highland must have taken place after ~ 23 Ma, whereas the lowland was weakly incised essentially after ~ 19 Ma. Our results also document divergent erosion and weathering histories of the lowland and the highland after the Eocene, suggesting installation of a dual climatic regime across the Western Ghats escarpment.
ISSN: 0009-2541

ird-01419899
http://hal.ird.fr/ird-01419899
http://hal.ird.fr/ird-01419899/document
http://hal.ird.fr/ird-01419899/file/CHEMGE9048R2.va.pdf
DOI : 10.1016/j.chemgeo.2016.04.018