Geological records of the Lhasa-Qiangtang and Indo-Asian collisions in the Nima area of central Tibet

Abstract

A geological and geochronologic investigation of the Nima area along the Jurassic-Early Cretaceous Bangong suture of central Tibet (~32°N, ~87°E) provides well-dated records of contractional deformation and sedimentation during mid-Cretaceous and mid-Tertiary time. Jurassic to Lower Cretaceous (≤125 Ma) marine sedimentary rocks were transposed, intruded by granitoids, and uplifted above sea level by ca. 118 Ma, the age of the oldest nonmarine strata documented. Younger nonmarine Cretaceous rocks include ca. 110-106 Ma volcanic-bearing strata and Cenomanian red beds and conglomerates. The Jurassic-Cretaceous rocks are unconformably overlain by up to 4000 m of Upper Oligocene to Lower Miocene lacustrine, nearshore lacustrine, and fl uvial red-bed deposits. Paleocurrent directions, growth stratal relationships, and a structural restoration of the basin show that Cretaceous-Tertiary nonmarine deposition was coeval with mainly S-directed thrusting in the northern part of the Nima area and Ndirected thrusting along the southern margin of the basin. The structural restoration suggests >58 km (>47%) of N-S shortening following Early Cretaceous ocean closure and ~25 km shortening (~28%) of Nima basin strata since 26 Ma. Cretaceous magmatism and syncontractional basin development are attributed to northward low-angle subduction of the Neotethyan oceanic lithosphere and Lhasa-Qiangtang continental collision, respectively. Tertiary syncontractional basin development in the Nima area was coeval with that along the Bangong suture in westernmost Tibet and the Indus-Yarlung suture in southern Tibet, suggesting simultaneous, renewed contraction along these sutures during the Oligocene-Miocene. This suture-zone reactivation immediately predated major displacement within the Himalayan Main Central thrust system shear zone, raising the possibility that Tertiary shortening in Tibet and the Himalayas may be interpretable in the context of a mechanically linked, composite orogenic system.

Constraints on the early uplift history of the Tibetan Plateau

“…These folded deposits are unconformably overlain by Upper Cretaceous and Paleogene nonmarine deposits, as imaged by recent seismic profiling in the region (SI Fig. 11 in SI Appendix) and mapped in outcrop (11,42). This structuralstratigraphic relationship indicates that crustal shortening, thickening, and surface uplift were active in both the Qiangtang and Lhasa terranes well before the Early Eocene (10).…”

Crustal–lithospheric structure and continental extrusion of Tibet

“…Crustal thickening, folding and thrusting along the Bangong suture across central Tibet occurred during Early Cretaceous times. Geological mapping along the Qiangtang terrane shows mid-Cretaceous volcanic flows and continental red beds unconformably overlying upper Palaeozoic and Triassic-Jurassic rocks and early Mesozoic blueschist-bearing mélange (Kapp et al 2005(Kapp et al , 2007a. Geological evidence shows that west and central Tibet must have been above sea level since the mid-Cretaceous.…”

“…There is an increasing body of evidence that substantial crustal thickening and, by inference, surface uplift of Tibet may have occurred prior to the India-Asia collision (England & Searle 1986;Searle 1995;Murphy et al 1997;Kapp et al 2005Kapp et al , 2007aSpurlin et al 2005). Crustal thickening, folding and thrusting along the Bangong suture across central Tibet occurred during Early Cretaceous times.…”

Tectonic evolution and high-pressure rock exhumation in the Qiangtang terrane, central Tibet

“…Jurassic sediments, if once present, could have been removed during formation of the central Qiangtang Culmination. This domal structure may have formed as early as the Jurassic (Kapp et al, 2005) or during compressional stages resulting from the closure of the Meso-and Neo-Tethys during the Cretaceous and Paleogene respectively (Kapp et al, 2007;Xu et al, 2014). Minor terrestrial sediments, mostly conglomerates in fault-bounded basins, were deposited during the Cretaceous and Tertiary.…”