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Clay Minerals; June 2001; v. 36; no. 2; p. 237-247
© 2001 Mineralogical Society of Great Britain and Ireland
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Research Paper

Synorogenic extension in the Tethyan Himalaya documented by structural studies and the Kübler index, Lachung La area, NW India

M. GIRARD*, P. THÉLIN and A. STECK

Institut de Minéralogie et Petrographie, Université de Lausanne, BFSH2, CH-1015 Lausanne, Switzerland

* E-mail: matthieu.girard{at}imp.unil.ch

(Received 1 December 1999; revised 22 May 2000)

Tectonic observations in the Tethyan Himalaya reveal an important extensional event that succeeds the emplacement of SW-verging nappes. A major thrust, called the Kum Tso Thrust, has been backfolded and reactivated by normal faulting associated with this event.

Measurements of the Kübler index, coupled with characterization of clay-size paragenesis show the effect of normal faulting on the regional metamorphic zonation and indicate that important extension zones, like the Sarchu-Lachung La Normal Fault Zone (SLFZ), exist within the Tethyan Himalaya. Diagenetic limestones from within the SLFZ are characterized by the occurrence of mixed-layered clay phases, kaolinite and an illite with a 001 peak >0.4 {Delta}°2{theta}. This zone is bordered by two anchizonal-to-epizonal zones, where illite peaks become narrower. Further to the NE the successive appearance of biotite, chloritoid, garnet and garnet-staurolite-kyanite assemblages testifies to an increase in metamorphic grade. The cataclastic samples from the normal faults contain kaolinite, smectite and a ‘broad’ illite, indicating that extension occurs under diagenetic conditions.

KEYWORDS: Tethyan Himalaya, Kübler index, India, mixed-layer clay phases






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