Genthon Pierre, Join J. L., Jeanpert J. (2017). Differential weathering in ultramafic rocks of New Caledonia : the role of infiltration instability. Journal of Hydrology, 550, p. 268-278. ISSN 0022-1694.
Titre du document
Differential weathering in ultramafic rocks of New Caledonia : the role of infiltration instability
Journal of Hydrology, 2017,
550, p. 268-278 ISSN 0022-1694
Ultramafic rocks of New Caledonia present extensive pseudokarstic features: undulations of the base of the weathering profile, fractures enlarged by dissolution, dolines. Possible influence of the infiltrative instability on differential weathering of ultramafic rocks in tropical climate is investigated here, starting from the Tiebaghi case for which a groundwater model is available. Infiltration instability is governed by nondimensional numbers, i.e. the Damkohler and Peclet numbers, and by the permeability contrasts between the bedrock and the different layers of the weathering profile. These numbers are firstly estimated for the Tiebaghi case, and secondly a sensitivity study of the weathering style is achieved through numerical modeling. Computations are firstly carried out without the impervious laterite cover, and then the effect of this cover is considered. The characteristic time constant for weathering of peridotites is directly estimated at around 7 yrs from the thickness of the weathering front and the fluid velocity. The results show that a permeability contrast of at least 100 between the most pervious layer in the weathering front and bedrock and a diffusivity of chemical species lower than 2.5 x 10-8 m(2)/s are required to allow instability. The Tiebaghi case lies at the limit of instability and dissolution fingers develop in a time lapse of 1 Ma. However, this requires the infiltration of nearly 800 mm of rainfall yearly, which is not the case presently. The development of dissolution fingers is promoted by any initial pervious zone such as fracture concentration around a faulted zone and the characteristic time for instability development drops to 0.1-0.3 Ma. Extension of these results to other peridotite massifs in the tropical zone submitted to different temperatures and rainfalls is finally discussed.
Plan de classement
Hydrologie [062]
;
Géologie et formations superficielles [064]
