Ductile deformation within Upper Himalaya Crystalline Sequence and geological implications, in Nyalam area, Southern Tibet

Abstract

The South Tibet Detachment System (STDS) is a flat normal fault that separates the Upper Himalaya Crystalline Sequence (UHCS) below from the Tethyan Sedimentary Sequence (TSS) above. Timing of deformations related to the STDS is critical to understand the mechanism and evolution of the Himalaya collision zone. The Nyalam detachment (ND) (~86°E) locates in the middle portion of STDS (81°-89°E). Dating of deformed leucocratic dykes that are most probably syntectonic at different depth beneath the ND, allow us to constrain the timing of deformation. (1) Dyke T11N37 located ~3500 m structurally below the ND emplaced at 27.4± 0.2 Ma; (2) Dyke T11N32 located ~1400 m structurally below the ND emplaced at 22.0±0.3 Ma; (3) T11N25 located within the top to the north STD shear zone, ~150 m structurally below the ND, emplaced at 17.1±0.2 Ma. Combining ND footwall cooling history and T11N25 deformation temperature, we indicate a probable onset of top to the north deformation at ~16 Ma at this location. These results show an upward younging of the probable timing of onset of the deformation at different structural distance below the ND. We then propose a new model for deformation migration below the ND with deformation starting by pure shear deformation at depth prior to ~27.5 Ma that migrates upward at a rate of ~ 0.3 mm/a until ~18 Ma when deformation switches to top to the north shearing in the South Tibet Detachment shear zone (STDsz). As deformation on the ND stops at 14-13 Ma this would imply that significant top to the North motion would be limited to less than 5 Ma and would jeopardize the importance of lower channel flow.

New constraints on the timing of partial melting and deformation along the Nyalam section (central Himalaya): implications for extrusion models

“…U -Th/Pb isotopic data for the monazites and zircons were obtained by laser ablation inductively coupled plasma atomic emission spectrometry at the same laboratory. The analytical techniques and procedures were similar to those described by Leloup et al (2010), Boutonnet et al (2012) and Liu et al (2012). The detailed analytical techniques have been reported by Paquette & Tiepolo (2007 (Tera & Wasserburg 1972).…”

“…M. Wang et al 2013). Southeast of Ruji, foliation in the STDsz trends approximately N708, dips about 308 to the N, and lineation strikes approximately N358 (Burchfiel et al 1992;Liu et al 2012) (Fig. 2a).…”

“…In the upper part of the section, the STD, locally called the Nyalam Detachment, separates the metapelites and leucogranites of the UGHS from slightly metamorphosed Tethyan sediments. The metasediments and leucogranites of the UGHS are strongly foliated and exhibit a strong NE-SW stretching lineation in the c. 300 m thick STDsz (Liu et al 2012;Wang et al 2013), with top-to-the-NE shear criteria (Burchfiel et al 1992;Y. Wang et al 2006;Liu et al 2012;J.…”

“…1e), with an exhumation rate of c. 0.38 mm a 21 between 6 and 2 Ma (Wang et al 2010). (Liu et al 2012;Zhu et al 2002), satellite image interpretation and fieldwork. Projection is UTM45.…”

“…2f ). In the STDsz, all the shear criteria indicate a top-to-the-north motion (Burchfiel et al 1992;Wang et al 2006;Liu et al 2012), corresponding to normal faulting near Ruji and rightlateral shear near ZhaSongLe. That strike-slip motion can be interpreted either as a local ramp along the STD or as late tilting as a result of the NNW -SSE brittle faults that affect the area and offset the STD and the STDsz (see later).…”