Sedimentary and tectonic evolution of the southern Qiangtang basin: Implications for the Lhasa‐Qiangtang collision timing

Abstract

The Mesozoic stratigraphic record of the southern Qiangtang basin in central Tibet records the evolution and closure of the Bangong‐Nujiang ocean to the south. The Jurassic succession includes Toarcian‐Aalenian shallow‐marine limestones (Quse Formation), Aalenian‐Bajocian feldspatho‐litho‐quartzose to feldspatho‐quartzo‐lithic sandstones (shallow‐marine Sewa Formation and deep‐sea Gaaco Formation), and Bathonian outer platform to shoal limestones (Buqu Formation). This succession is truncated by an angular unconformity, overlain by upper Bathonian to lower Callovian fan‐delta conglomerates and litho‐quartzose to quartzo‐lithic sandstones (Biluoco Formation) and Callovian shoal to outer platform limestones (Suowa Formation). Sandstone petrography coupled with detrital‐zircon U‐Pb and Hf isotope analysis indicate that the Sewa and Gaaco formations contain intermediate to felsic volcanic detritus and youngest detrital zircons (183–170 Ma) with εHf(t) ranging widely from +13 to −25, pointing to continental‐arc provenance from igneous rocks with mixed mantle and continental‐crust contributions. An arc‐trench system thus developed toward the end of the Early Jurassic, with the southern Qiangtang basin representing the fore‐arc basin. Above the angular unconformity, the Biluoco Formation documents a change to dominant sedimentary detritus including old detrital zircons (mainly >500 Ma ages in the lower part of the unit) with age spectra similar to those from Paleozoic strata in the central Qiangtang area. A major tectonic event with intense folding and thrusting thus took place in late Bathonian time (166 ± 1 Ma), when the Qiangtang block collided with another microcontinental block possibly the Lhasa block.

Comment on “Sedimentary and Tectonic Evolution of the Southern Qiangtang Basin: Implications for the Lhasa‐Qiangtang Collision Timing” by A. Ma et al.

“…As mentioned above, Late Jurassic‐Middle Cretaceous ophiolite and marine sedimentary rocks along/across the Bangong suture zone and a Late Jurassic‐Middle Cretaceous magmatic arc along the southern Qiangtang margin as well as paleomagnetic data indicate that the Bangong Meso‐Tethyan Ocean did not close until ~100–90 Ma. Therefore, we disagree of the late Bathonian collision model of Ma et al (), because their model is inconsistent with the geologic facts. We attribute the late Bathonian southern Qiangtang compression orogeny to the Bangong Meso‐Tethys oceanic subduction beneath the Qiangtang block.…”

“…We attribute the late Bathonian southern Qiangtang compression orogeny to the Bangong Meso‐Tethys oceanic subduction beneath the Qiangtang block. Ma et al (, section 7.4.1) noted such probability but did not elaborate, only based on the paucity of synkinematic magmatic clastic materials in the Biluoco Formation, that is, essentially the absence of synkinematic magmatism. However, on the contrary, the cessation of the arc‐related magmatism just is a signal of orogenesis caused by subduction of oceanic highs (ridge, plateau, or seamount), because subduction of oceanic highs will usually result in choke of subduction zone and pause of the subduction (e.g., Mann & Taira, ; Thorkelson et al, ).…”

“…Unfortunately, however, extensive exposure of Late Jurassic‐Middle Cretaceous ophiolite fragments (150–104 Ma) and marine sedimentary rocks along the Bangong suture zone indicate that the Bangong Meso‐Tethys Ocean did not close until the Late Cretaceous (~90 Ma), which is consistent with a Late Jurassic‐Middle Cretaceous magmatic arc in the southern Qiangtang terrane as well as paleomagnetic data. The late Bathonian orogenic event in the southern Qiantang area observed by Ma et al () can be explained by subduction of oceanic highs in the Bangong Meso‐Tethyan Ocean. Therefore, we disagree their late Bathonian Lhasa‐Qiangtang collision model and state the facts as follows.…”

“…Recently, Ma et al () proposed that the Lhasa‐Qiangtang collision commenced in late Bathonian time (~166 Ma), based on the observation of an angular unconformity that truncated the Bathonian Buqu Formation from the late Bathonian‐early Callovian Biluoco Formation and the recognition of siliciclastic provenance switch from arc igneous source to uplifted orogenic source in the Biluoco‐Qixiangco area of the southern Qiangtang basin. Unfortunately, however, extensive exposure of Late Jurassic‐Middle Cretaceous ophiolite fragments (150–104 Ma) and marine sedimentary rocks along the Bangong suture zone indicate that the Bangong Meso‐Tethys Ocean did not close until the Late Cretaceous (~90 Ma), which is consistent with a Late Jurassic‐Middle Cretaceous magmatic arc in the southern Qiangtang terrane as well as paleomagnetic data.…”

Palaeomagnetism and detrital zircon U–Pb geochronology of Cretaceous redbeds from central Tibet and tectonic implications

“…The angular unconformity and the transition from marine to terrestrial deposits are interpreted as the result of collision between the Lhasa and Qiangtang terranes (Kapp et al, ; Li et al, ; Li, He et al, ; Zhang et al, ; Zhang et al, ). The age and palaeolatitude of the terrestrial Abushan Formation in the Qiangtang terrane (Cheng & Xu, ; Li et al, ; Ma et al, ; Wang et al, ; Wang & Yi, ; Zhang et al, ; Zhao, Li, & Ye, ) provide critical information of dating the Lhasa–Qiangtang collision and determining the magnitude of intra‐Asian convergence. However, there are major difficulties: (a) the Cretaceous palaeomagnetic data from the Qiangtang terrane are scarce and partially remagnetizated (Ran et al, ; Yan et al, ); (b) lack of a robust age of the Abushan Formation; and (c) the possible inclination shallowing effects of redbeds.…”