New insight into the South Tibetan detachment system: Not a single progressive deformation

Abstract

[1] Low-angle normal faults (LANF), typically regarded as accommodating crustal or lithospheric extension, may also form during lithospheric shortening. The best-studied system of syn-contractional LANFs is the South Tibetan detachment system, a network of low-angle normal sense faults and shear zones that formed coevally with and parallel to south-vergent thrusts during lithospheric shortening accompanying development of the Himalayan orogen. In the eastern Himalaya, there are several across-strike exposures of the South Tibetan detachment system. We present new structural and thermometry data from the eastern Himalaya that demonstrate that the South Tibetan detachment system cannot have formed as a single progressive structure. We characterize and distinguish two distinct structural and tectonic components within the currently recognized system: (1) an extensive diffuse, sheared layer that formed the boundary between strong upper crust and weak, southward-flowing middle crust, and (2) a network of brittle-ductile LANFs that locally exhume, partly excise and overprint the earlier mylonite zone at the topographic break between the Himalayan orogen and the Tibetan plateau. The sheared layer, not a LANF, formed the boundary between upper and middle crust during ductile flow of the middle crust and is extensively exposed in the Himalaya at the base of klippen of upper crustal rocks preserved in Bhutan, along the crest of the Himalaya where it has been excised and exhumed by the brittle-ductile extrusion LANFs, and bounding the cores of the North Himalayan gneiss domes.

Large normal-sense displacement on the South Tibetan fault system in the eastern Himalaya

“…The lower unit, named the Chekha Formation, comprises metapelite, pelitic gneiss, augen gneiss, quartzite, and calc-silicate intruded by numerous leucogranite sills and dikes (Gansser, 1983;Grujic et al, 2002;Kellett et al, 2009;Kellett and Grujic, 2012), and has been correlated regionally with the Everest Series and North Col Formation of eastern Nepal, the Annapurna Yellow Formation of central Nepal, and the Haimanta Group of northwestern India Gleeson and Godin, 2006;Chambers et al, 2009). Above this is a unit of recumbently folded marbles that is devoid of the leucogranite intrusions present beneath.…”

“…However, only a few have mapped this contact as a detachment in Bhutan (Edwards et al, 1996;Hollister and Grujic, 2006), while most have identifi ed it as conformable (Gansser, 1983;Carosi et al, 2006;Kellett and Grujic, 2012). Instead, it is the lower contact between the Chekha Formation and underlying Greater Himalayan sequence gneisses and metapelites that is usually mapped as the basal STFS detachment, even though no discrete shear zone or structural discordance has been described (Grujic et al, 2002;Carosi et al, 2006;Kellett et al, 2009;Kellett and Grujic, 2012).…”

The South Tibetan detachment system facilitates ultra rapid cooling of granulite‐facies rocks in Sikkim Himalaya

“…[] and this work. TSS – Tethyan sedimentary sequence, CHf – Chekha formation, GHS – Greater Himalayan sequence, STDi – inner STD system, STDo – outer STD system [as described in Kellett and Grujic , ]. SK55‐SK74 samples are described in this paper.…”

Synconvergence flow inside and at the margin of orogenic plateaus: Lithospheric‐scale experimental approach

“…In details, the exhumation process was probably more complex, not fully synchronous along the range, and likely of lesser amplitude than that simulated in Beaumont et al . [] and in our experiments, especially considering the lateral structural/rheological heterogeneity of the lower crust [ Long and McQuarrie , ; Rey et al ., ; Kellett and Grujic , ].…”