The Yarlung suture mélange, Lopu Range, southern Tibet: Provenance of sandstone blocks and transition from oceanic subduction to continental collision

Abstract

With the aim of better understanding the history of ocean closure and suturing between India and Asia, we conducted a geologic investigation of a siliciclastic matrix tectonic mélange within the western Yarlung suture zone of southern Tibet (Lopu Range region, ~50 km northwest of Saga). The siliciclastic matrix mélange includes abundant blocks of ocean plate stratigraphy and sparse blocks of sandstone. Metapelite and metabasite blocks in the mélange exhibit lower greenschist facies mineral assemblages, indicating that they were not deeply subducted. We obtained detrital zircon U-Pb geochronologic and sandstone petrographic data from sandstone blocks in the mélange and sandstone beds from Tethyan Himalaya strata exposed to the south of the suture. The sandstones from both units are all similar in U-Pb detrital zircon age spectra and 15 petrography to the nearby Tethyan Cretaceous-Paleocene Sangdanlin section, which records the 16 earliest appearance (at ~59 Ma) of arc-affinity strata deposited conformably on Indian-affinity strata. Two Paleocene sandstones, one of which is a schistose block incorporated in the siliciclastic matrix mélange, yielded indistinguishable maximum depositional ages of ~59 Ma. Mesozoic Asian-affinity sandstone blocks previously documented in the siliciclastic matrix mélange 200-500 km along strike to the east are notably absent in the Lopu Range region. We documented a gradational transition in structural style from the block-in-matrix mélange in the northeast to the south-vergent Tethyan thrust belt in the southwest. Blocks of Tethyan Himalaya strata increase in 2 size and the volumetric proportion of matrix decreases from northeast to southwest. We conclude 24 that no arc-affinity sandstone blocks were incorporated into the subduction complex until India-Asia collision at ~59 Ma when the Xigaze forearc basin became overfilled and Tethyan Himalaya strata entered the trench. As collision progressed, there was a gradual transition in structural style from block-in-matrix mélange formation to imbricate-style thrust belt formation.

Discovery of Upper Cretaceous Neo-Tethyan trench deposits in south Tibet (Luogangcuo Formation)

“…In alternative, a trench-slope-basin setting may be envisaged for the Luogangcuo Formation. Previous studies, however, have suggested that the Transhimalayan subduction complex was rather small without large ridges or thrust slices before the onset of the initial India-Asia collision (An et al, 2017;Metcalf and Kapp, 2017), and therefore insufficiently wide and prominent to host a trench-slope basin on top. Most important, the Luogangcuo Formation lies in fault contact with the Xiukang Mélange.…”

“…In the Xigaze forearc basin, the middle-lower Ngamring Formation contains detrital zircons with typical Gangdese arc signatures (Mesozoic ages and positive εHf(t) values), whereas the upper Ngamring and Padana formations yield numerous zircon grains with either pre-Mesozoic or Mesozoic ages and positive or negative εHf(t) values, testifying to provenance from both Gangdese arc and central Lhasa terrane (Wu et al, 2010;An et al, 2014;Orme et al, 2015;Orme and Laskowski, 2016). Adjacent to the Luogangcuo Formation, the Xiukang Mélange contains numerous blocks of: (i) Upper Cretaceous to Eocene lithic-rich volcaniclastic sandstones derived from the Gangdese arc and central Lhasa terrane, (ii) pre-Cretaceous quartzose sandstones derived from the Indian subcontinent; and, (iii) Middle Triassic to Lower Cretaceous chert or Middle Permian to Upper Cretaceous limestone derived either from the distal Indian margin or the Neo-Tethyan oceanic domain (Shen et al, 2003a(Shen et al, , 2003bCai et al, 2012;An et al, 2017;Metcalf and Kapp, 2017).…”

“…1B), chaotic units with blocks embedded in siliciclastic matrix have been named variously in the literature (e.g., Xiukang Mélange after XBGMR, 1979; wildflysch with exotic blocks after Tapponnier et al, 1981). These mélange units, traditionally interpreted as representing the Neo-Tethyan subduction system (Tapponnier et al, 1981;Searle et al, 1987;Cai et al, 2012), have been recently documented to have developed at least in part during the initial Paleocene-Eocene stage of the India-Asia collision (An et al, 2017;Metcalf and Kapp, 2017). The Xiukang Mélange is composed of exotic blocks of sandstone, limestone, chert, and basalt set in a deformed matrix of Upper Triassic-Lower Aptian abyssal chert, siliceous mudstone, and locally turbidites (Cai et al, 2012).…”

Discovery of Vestige Sedimentary Archives of the India‐Asia Collision in the Eastern Yarlung Zangbo Suture Zone

“…The latter two formations are considered as the syncollisional sedimentary records (e.g., Ding et al, 2005; Hu, Wang et al, 2016; Jia et al, 2005; Orme et al, 2015; Wan et al, 2001). The sandy‐muddy mélange subzone of the YZSZ outcrops east to Zedong and contains abundant syncollisional sedimentary records (e.g., An et al, 2017; Burg & Chen, 1984; Ding et al, 2005; Metcalf & Kapp, 2017, 2019). Within the subzone, the syncollisional sediments are characterized by the Paleocene radiolarian cherts and sandstones in Saga area (e.g., DeCelles et al, 2014; Ding et al, 2005; Hu et al, 2015) and by the Zongzhuo mélange dominated by olistoliths in Gyangze area (e.g., Cai et al, 2011; Li et al, 2005; Wu et al, 1977; Wu et al, 2014).…”

“…Indeed, sedimentary records have been widely used to study the collisional process in the western and central YZSZ (Figure 1). Up to date, the initial collision age has been precisely, to some degree, constrained to ~56–60 Ma in the western (Critelli & Garzanti, 1994; Gaetani & Garzanti, 1991; Garzanti et al, 1987; Najman et al, 2017) and central YZSZ (e.g., DeCelles et al, 2014; Hu et al, 2015; Hu, Wang et al, 2016; Metcalf & Kapp, 2017; Zhang et al, 2012).…”