Gondwana dispersion and Asian accretion: Tectonic and palaeogeographic evolution of eastern Tethys

Phanerozoic tectonics of the South China Block: Key observations and controversies

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 fluvial 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 N-directed 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.

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

The geologic map pattern of the Qiangtang terrane in central Tibet defines a >600-km-long and up to 270-km-wide east-plunging structural culmination. It is characterized by early Mesozoic blueschist-bearing melange and upper Paleozoic strata in the core and mainly Triassic–Jurassic strata along the limbs. In the western Qiangtang terrane (∼84°E), the culmination is unconformably overlain by weakly deformed mid-Cretaceous volcanic flows and tuffs. Along the Bangong suture to the south (32°N, 84°E), mid-Cretaceous nonmarine red beds and volcanic rocks lie unconformably on Jurassic suture zone rocks. These relationships demonstrate that west-central Tibet was above sea level during the mid-Cretaceous and experienced significant denudation prior to mid-Cretaceous time. Growth of the Qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early Mesozoic melange and upper Paleozoic strata during the Early Cretaceous Lhasa-Qiangtang collision. The north-south width of the inferred thrust sheet provides a minimum slip estimate of ∼150 km at 84°E, decreasing eastward to ∼70 km at 87°E.

Paleogene deformation in the Qiangtang terrane is characterized by widely distributed, mainly north-dipping thrust faults that cut Eocene–Oligocene red beds and volcanic rocks in their footwall. Along the Bangong suture, the north-dipping Shiquanhe-Gaize-Amdo thrust system cuts 64 and 43 m.y. old volcanic tuffs in its footwall and accommodated >40 km of post–mid-Cretaceous shortening. The Tertiary south-dipping Gaize–Siling Co backthrust bounds the southern margin of the Bangong suture and marks the northernmost limit of mid-Cretaceous marine strata in central Tibet. Cretaceous deformation and denudation in central Tibet is attributed to northward underthrusting of the Lhasa terrane beneath the Qiangtang terrane along the Bangong suture. This model implies that (1) Cretaceous strata along the Bangong suture and in the northern Lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the Qiangtang terrane, and (2) the central Tibetan crust was thickened substantially prior to the Indo-Asian collision. Although its magnitude is poorly known, Tertiary shortening in the Qiangtang terrane is more prevalent than in the Lhasa terrane; this difference may be attributed to the presence of underthrust melange in the deeper central Tibetan crust, which would have made it weaker than the Lhasa terrane during the Indo-Asian collision.

Cretaceous-Tertiary shortening, basin development, and volcanism in central Tibet

Principles of Isotope Geology

Continental crust (density ~2.8 g·cm-3) resists subduction into the earth's mantle (~3.3 g·cm-3) because of buoyancy. However, more than 20 recognized ultrahigh-pressure (UHP) terranes have been documented; these occurrences demonstrate that not only is continental crust subducted to depths as great as 150 km, but also that some supracrustal rocks were then exhumed to the earth's surface. UHP terranes are composed of mainly supracrustal rocks that contain minor amounts of minerals such as coesite or diamond, indicative of P > 2.5 GPa. In general, quartzofeldspathic units are thoroughly back reacted, and only mafic eclogite lenses and boudins retain scattered UHP phases. These index minerals are restricted to micron-scale inclusions in chemically and mechanically resistant zircon, garnet, and a few other strong container minerals, and are difficult to identify by conventional petrologic studies. The continental rocks were subjected to UHP metamorphism at T ranging from ~700 to 950°C and P > 2.8 to 5.0 GPa,...

Global UHP metamorphism and continental subduction/collision: The himalayan model

Ultrahigh-pressure minerals and metamorphic terranes – The view from China

Convergent plate motion at ∼320–210 Ma generated the Tongbai-Dabie-Sulu (east-central China)-Imjingang-Gyeonggi (central Korea)-Renge-Suo (Southwestern Japan)-Sikhote-Alin orogen along the paleo-Pacific edge of cratonal Asia. This amalgamated belt reflects collision between the Sino-Korean and Yangtze cratons on the SW portion, and accretion of outboard oceanic arcs ± sialic fragments against the NE margin. Subducted Proterozoic-Paleozoic continental and oceanic crustal complexes underwent high- and ultrahigh-pressure metamorphism at low to moderate temperatures. Tectonic slices of sialic crust episodically disengaged from the downgoing plate and, driven by buoyancy, ascended rapidly to midcrustal levels from depths exceeding 90–200 km after continental collision in east-central China plus or minus Korea, and from ∼30–50 km after arrival of far-traveled oceanic terranes in SW Japan and the Russian Far East. On achieving neutral buoyancy and stalling out at 10–20 km depth, later doming, gravitation...

Permo-Triassic Collision, Subduction-Zone Metamorphism, and Tectonic Exhumation Along the East Asian Continental Margin

Very-low-temperature record of the subduction process: A review of worldwide lawsonite eclogites

Ophiolites and high-pressure (HP) metamorphic rocks are studied to test continuation of Paleozoic and early Mesozoic geological units from Japan to Primorye over the Japan Sea. The early Paleozoic ophiolites are present on both sides, and the late Paleozoic ophiolite of south-western Japan may also have its counterpart in Primorye. The Shaiginskiy HP schist and the associated Avdakimov gneiss in Primorye, both tectonically underlying the early Paleozoic ophiolitic complex, yield a 250-Ma phengite and hornblende K–Ar age, which is intermediate between those of the Renge (280–330 Ma) and Suo (170–220 Ma) blueschists in south-western Japan. This age also coincides with that of the coesite-bearing eclogites in the Sulu–Dabie suture in China and several medium-pressure metamorphic rocks in East Asia. On the basis of these results and other geological data, the authors propose the ‘Yaeyama promontory’ model for an eastward extension of the Sulu–Dabie suture. The collision suture warps southward into the Yellow Sea and detours around Korea, turns to the north at Ishigaki Island in the Yaeyama Archipelago of Ryukyu, where it changes into a subduction zone and further continues toward south-western Japan and Primorye. Most ophiolites from this area represent crust–mantle fragments of an island arc–back-arc basin system, and the repeated formation of ophiolite–blueschist associations may be due to the repetition of the Mariana-type non-accreting subduction and Nankai-type accreting subduction.

Tatsuki Tsujimori

Papers

Global UHP metamorphism and continental subduction/collision: The himalayan model

Ultrahigh-pressure minerals and metamorphic terranes – The view from China

Permo-Triassic Collision, Subduction-Zone Metamorphism, and Tectonic Exhumation Along the East Asian Continental Margin

Very-low-temperature record of the subduction process: A review of worldwide lawsonite eclogites

Paleozoic ophiolites and blueschists in Japan and Russian Primorye in the tectonic framework of East Asia: A synthesis