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| Titel |
Evolution of the Southwest Indian continental divergent margin: Constraints from 40Ar-39Ar dating of lateritic paleolandsurfaces |
| VerfasserIn |
Nicolas Bonnet, Anicet Beauvais, Dominique Chardon, Nicolas Arnaud |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250109470
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| Publikation (Nr.) |
 EGU/EGU2015-9377.pdf |
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| Zusammenfassung |
| The western continental passive margin of Peninsular India is marked by the Western Ghats
escarpment, which separates a coastal lowland from an East-dipping highland plateau
and is carved both into the 63-Ma old Deccan traps and their Archean basement.
Previous studies suggested establishment of the escarpment by differential erosion
across an elevated rift shoulder, and thermochronologic models predicted escarpment
formation from higher denudation in the coastal lowland than on the plateau until ~ 50
Ma.
We provided complementary time constraints on the evolution of the passive margin by
40Ar-39Ar dating of supergene K-Mn oxides (cryptomelane) sampled in lateritic formations
exposed on paleosurfaces, which are preserved as relicts on both sides of the escarpment.
Three main lateritic paleosurfaces were identified in the highland at altitude ranges of
1200-1000 m (S1), 1000-900 m (S2) and 850-600 m (S3), and a lower paleosurface in the
lowland at 150-50 m (S4).
All the 40Ar-39Ar ages obtained on either side of the escarpment document major
weathering periods for each paleosurface: 53 to 45 Ma (S1-S4) synchronously with the
bauxitic weathering, 40 to 32 Ma (S2), 30 to 23 Ma (S3), and 24 to 19 Ma (S4). These ages
indicate that most of the incision and dissection of plateau landsurfaces S1, S2, and S3 must
therefore have taken place after 45, 32 and 23 Ma respectively, while the coastal lowland
surface S4 was incised after 19 Ma.
Preservation of laterites as old as 47 Ma in the coastal lowland implies that the
escarpment already existed in the Mid-Eocene while intense bauxitic weathering was taking
place on both sides of the escarpment. The ages obtained in the lowland are also indicative of
limited erosion (~ 4 m Ma-1) at the foot of the escarpment since 45 Ma, and particularly low
incipient incision of the lowland (~ 5 m Ma-1) since 19 Ma. Ages obtained on the highland
plateau indicate further Neogene denudation inland but at less than 15 m Ma-1
since 45 Ma, and incision lower than 6 m Ma-1 since 23 Ma. Limited erosion in
the coastal lowland contrasts with Late Neogene increase in clastic fluxes on the
offshore margin, which may therefore be attributed to erosion of the shelf edge
or material imported from the Indus fan. Our results attest to the antiquity of the
first-order relief and topography of the high-elevation margin of Southwest India and to
a divergent erosion pattern on either side of its escarpment since at least 47 Ma. |
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