Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

Catherine M Mottram,S Gupta,T W Argles,M S A Horstwood,N B W Harris,C J Warren,R R Parrish

doi:10.1144/jgs2013-064

Abstract

Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the India–Asia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and U–Pb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c . 800–1000 Ma, 1500–1700 Ma and 2300–2500 Ma and an ϵ Nd(0) signature of −18.3 to −12.1, and is intruded by c . 800 Ma and c . 500–600 Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c . 1800–1900 Ma, with older populations spanning 1900–3600 Ma, and an ϵ Nd(0) signature of −27.7 to −23.4, intruded by c . 1830 Ma granites. The data reveal a c . 5 km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones. Supplementary material: Zircon U–Pb geochronological data, whole-rock Sm–Nd isotopic data, sample locations, photomicrographs of sample thin sections, zircon CL images, and detailed analytical conditions are available at www.geolsoc.org.uk/SUP18704 .

Highlights

Crustal thickening in major orogenic belts is often achieved by packages of rock being thrust upon one another along major thrust faults
The Lingtse gneiss samples from the Sikkim Himalaya analysed in this study provide a U–Pb zircon age cluster within error between 1834 ± 37 Ma and 1853 ± 19 Ma (Fig. 4) and may be age-correlated with other Lesser Himalayan granite gneisses across the Himalaya (Goswami et al 2009; for a summary of ages, see table 1 of Kohn et al 2010)
The Sikkim Himalaya exposes a window into a well-preserved mid-crustal thrust zone formed during the Himalayan orogeny

Summary

Introduction

Crustal thickening in major orogenic belts is often achieved by packages of rock being thrust upon one another along major thrust faults. Thrust faults form ductile shear zones and the amount of displacement along these structures is probably much larger than can be evaluated by strain analysis of the exposed rock. The Sikkim Himalaya provides a uniquely preserved window into the mid-crustal levels of one of the largest ductile shear zones on Earth. This study illustrates how isotope geochemistry and geochronology can be used to investigate major orogenic structures, affected by hundreds of kilometres of relative displacement and ductile deformation, to provide a unique perspective on the hanging wall–footwall relationships

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Discussion

Conclusion