Sur le mecanisme de formation de la schistosite dans l'himalaya

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Abstract

After recalling briefly the major geological characters of the Nepal Himalayas and the global evolution of the Himalayan chain we stress the importance of three dominant and remarkably constant tectonic features in this chain: (1) a huge overthrust is seen everywhere to entail a visible throw of more than 100 km; (2) a shallow dipping cleavage develops over an observable thickness in excess of 15 km; and (3) in the underthrusting slab the stretching lineation keeps a similar direction constantly perpendicular to the local trend of the chain.

We suggest that these features, especially the cleavage, can be explained in terms of a simple mechanical model, namely simple shear on a crustal scale. This clearcut case could be used as a model for the origin of flattish cleavages in many orogens. After discussing other modes of formation for such cleavages (diapiric intrusion, extension) we show that the Himalayan model could apply to other chains.

Résumé

Apre`s avoir situe´la chaiˆne de l'Himalaya dans son cadre ge´ne´ral et rappele´les grands traits de sone´volution ge´ologique au niveau du Ne´pal, nous montrons que cette chaiˆne est essentiellement caracte´rise´e, au point de vue structural, par: (1) tre´s grand chevauchement, ayant une fle`che visible de plus de 100 km; (2) le de´veloppement sur unee´paisseur visible de plus de 15 km de schistosite´a`pendage faible, et (3) l'existence, sous le chevauchement, d'une tre`s nette line´ation d'allongement ayant une direction proche de Nord-Sud, c'est-a`-dire perpendiculairea`la chaiˆne.

Dans ces conditions il apparait que le me´canisme de formation de la schistosite´corresponda`du cisaillement simple se produisanta`lh¸elle de l'e´corce.

Cet exemple assez clair conduita`se demander si l'on peut interpre´ter les schistosite´s subhorizontales de beaucoup de chaines suivant ce mode`le himalayen. Apr`es avoir examine´les autresde formation de schistosite´a`pendage faible (distension et diapirisme) il apparait que le mode`le himalayen doiteˆtre assez facilea`tester. Il semble qu'il se rencontre dans plusieurs autres chaines.

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