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| Titel |
Time constraints on post-rift evolution of the Southwest Indian passive margin from ⁴⁰Ar-³⁹Ar dating of supergene K-Mn oxides |
| VerfasserIn |
Nicolas Bonnet, Nicolas Arnaud, Anicet Beauvais, Dominique Chardon |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250083654
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| Zusammenfassung |
| The high-elevation passive margin of Southwest India is marked by the Western Ghats
escarpment, which separates the coastal domain from the low-relief East-dipping Mysore
plateau. The escarpment has evolved from the Seychelles rifting at ~ 63 Ma following the
Deccan traps volcanic event at ~ 65-63 Ma. This escarpment results from differential erosion
processes across the passive margin, the rate and timing of which depend upon
whether the margin has evolved according to a model of downwarped or rising flank
topography.
We explore the post-rift evolution of the South Indian passive margin through the
characterisation of stepped relicts of lateritic paleosurfaces across that margin, and notably by
40Ar-39Ar dating of in-situ formed K-Mn oxides in supergene Mn-ore deposits carried by
these paleosurfaces. The genesis and maturation of Mn-ore deposits are generally linked to
progressive weathering processes of the paleosurfaces, which expose them. Dating of K-Mn
oxides thus document the timing of these processes [1], and potentially the ages of the
altered paleosurface. Moreover, the elevation differences between successive lateritic
paleosurfaces of different ages may provide denudation rates for the considered time
spans.
Previous work (e.g., [2]) and our own field investigations, allow identifying three main
lateritic paleosurfaces on the plateau at altitude ranges of 1000-900 m (S2), 900-800 m (S3)
and 800-700 m (S3d), and a lower paleosurface in the coastal domain at 150-50 m
(S4).
K-Mn oxides (cryptomelane) were sampled in Mn ore deposits from different paleosurfaces,
particularly in the coastal area around Goa on S4 and in Sandur and Shimoga Mn-ore
deposits exposed on S2 and S3. The 40Ar-39Ar ages obtained from carefully characterised
mineralogical assemblages range from ~ 26 to ~ 36 Ma in the Sandur Mn-ore deposit
indicating intense lateritic weathering processes at the Eocene-Oligocene transition
underneath paleosurface S2. Similar ages of ~ 24 and ~ 32 Ma are obtained in two Shimoga
Mn ore deposits carried by S3 and S2, respectively. A younger age (~ 21 Ma) is also
obtained in a Goa deposit carried by S4. These first results suggest that the Western
Ghats passive margin escarpment was established at the latest by early Miocene and
that at least part of the inland Mysore plateau morphogenesis was achieved at that
time.
[1] Beauvais A. et al., Journal of Geophysical Research 113, F04007, 2008.
[2] Gunnell, Y., Basin Research 10, 281-310, 1998. |
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