Early Paleozoic Tectonic and Thermomechanical Evolution of Ultrahigh-Pressure (UHP) Metamorphic Rocks in the Northern Tibetan Plateau, Northwest China

Basalts and picrites from a plume-type ophiolite in the South Qilian Accretionary Belt, Qilian Orogen: Accretion of a Cambrian Oceanic Plateau?

“…The North Qilian Accretionary Belt records globally one of the earliest "cold" oceanic subduction zones, in response to the closure of the ancient Qilian Ocean between the Alashan and Qilian-Qaidam blocks during the Early Paleozoic (Wu et al, 1993;Song et al, 2006Song et al, , 2007Song et al, , 2009Song et al, , 2013Zhang et al, 2007;Yin et al, 2007;Xiao et al, 2009). It consists of Precambrian basement, Early Paleozoic subduction-related rock associations (ophiolite complexes, high-pressure/ low-temperature metamorphic rocks, arc-related volcanic and intrusive rocks),…”

The mountains that triggered the Late Neoproterozoic increase in oxygen: The Second Great Oxidation Event

“…Furthermore, outcrops of ultra high-pressure rocks are normally small and the pressures they yield require them to come from depths greater than 90 km, which places them within the mantle, well below the likely base of thickened crust (Yin et al, 2007). They are found in association with subduction zones or continent-continent collisions and are interpreted to be fragments of continental crust that have been dragged into the mantle and returned to the crust in buoyancy driven diapirs (Hall and Kincaid, 2001;Yin et al, 2007) or through channel flow along the subduction zone (Mancktelow, 1995). Furthermore, the ultra-highpressure rocks in the European Alps formed before the start of the collision between the African and Eurasian plates (Rubatto and Hermann, 2001).…”

Proceedings of the 25th Himalaya-Karakoram-Tibet Workshop

“…We present a crustal P-wave velocity structure model along the in-line wide-angle seismic profile, and find crustal thickness thins from about 52 km under the southwest end of the profile to about 49 km at the northeast end of the profile, which is consistent with the results from previous wide-angle seismic profiling by Liu and others (2006) and . Notable features of the crustal velocity model include: (1) the Kunlun (or the western Qingling block) and the Qaidam block are clearly distinguished, with the northern margin fault of the western Qingling Mountains as the boundary between them; the crust of these two tectonic blocks is thickened both in the upper and lower crust, and their crusts can be divided into upper and lower crust with thickness about 27 km and 22-26 km, respectively; (2) Pwave velocity is very low for the whole crust, especially in the lower crust (6.3-6.6 km/s) beneath the Qilian terrain; (3) there is one lower-velocity layer (about 5.7 km/s) with thickness of 7-8 km in the upper crust beneath the south segment of the profile, and a 6.2-6.3 km/s higher-velocity layer with thickness of about 5 km in the upper crust beneath the north segment of the profile; (3) the Southern Qilian fault that bounds the early Paleozoic North Qaidam UHP metamorphic belt is of crustal scale, implying continental collision of the Qaidam block with the Qilian terrane that seems inconsistent with the paired subduction model of Yang and others (2002), but may support a transition from oceanic subduction (along the Northern Qilian fault) to continental collision (along the Southern Qilian fault belt) as suggested by Song and others(2006); (4) a lower-velocity zone in the depth range 25-30 km beneath the Qilian terrain is inferred to be a zone of partial melt that may be related to the thermomechanical evolution of UHP metamorphic rocks or diapiric flow of subducted continental crust (Yin and others, 2007). Analysis of individual seismic shots suggests that strong seismic reflections from the lower crust are weak or absent on in-line shot gathers, suggesting a transparent lower crust beneath the Qilian tectonic block, but in contrast well-developed reflections are seen on the off-line shot gathers.…”