TECTONIC EVOLUTION OF THE CENTRAL ANNAPURNA RANGE, NEPALESE HIMALAYAS

The metamorphic core of the Himalayan orogen, or Greater Himalayan seq uence, is a northward tapering prism bound at the bottom by a N dippin g family of thrust faults (the Main Central thrust system) and at the top by a N dipping family of normal faults (the South Tibetan detachme nt system). Research in the central Annapurna Range of Nepal demonstra tes a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within th e metamorphic core throughout much of the Early Miocene. The Main Cent ral thrust system is represented here by a 2- to 3-km-thick zone of hi gh strain that developed during two or more episodes of movement. Most of its displacement was concentrated along the Chomrong thrust, a sha rp, late-metamorphic discontinuity that places middle amphibolite faci es rocks of the Greater Himalayan sequence on top of lower amphibolite facies rocks of the Lesser Himalayan sequence. The earliest demonstra ble movement on this thrust system occurred similar to 22.5 Ma; the mo st recent movement may be as young as Pliocene. The oldest element of the South Tibetan detachment system in this area is the Deorali detach ment, which appears to have been active at the same time as the earlie st shortening structures of the Main Central thrust system. Fabrics re lated to the Deorali detachment are disrupted by a previously unrecogn ized, SW vergent, thrust structure, the Modi Khola shear zone. The eff ect of this structure, which is constrained to be between 22.5 and 18. 5 Ma, was to shorten rock packages that had been extended previously d uring movement on the Deorali detachment. Transition back to a local e xtensional regime after 18.5 Ma was marked by development of the Machh upuchhare detachment and related splays. Geologic evidence for rapid, two-way transitions between contraction and extension in the Annapurna Range indicates that extensional deformation in convergent settings d oes not only represent gravitational collapse at the end of an orogeni c cycle; it also appears to be an important factor in mountain range d evelopment.