The lack of syncollisional sediments in the eastern Yarlung Zangbo suture zone (YZSZ) has impeded our understanding of the India-Asia collision, particularly as to whether the collision was synchronous or diachronous. In this paper, we present evidence for the vestige sedimentary archives of the initial India-Asia collision retained in the previously proposed "Cretaceous Langxian Mélange" in the eastern YZSZ, southern Tibet. Field investigations of marbles and intercalated meta-siliciclastic rocks, together with analyses of petrography, cathodoluminescence image, geochronology, trace elements, and Nd isotopes show that (1) marbles occur as thin-bedded to massive individual layers consistently dipping to the south at 50-80°, (2) marble and intercalated meta-siliciclastic rocks exhibit the same deformational (foliations S 0 and S 1 ) and metamorphic (green-schist facies) history with nearly same detrital-zircon age spectra and ε Nd (0) values, (3) protoliths of the meta-sedimentary rocks were sourced from both India and Asia and deposited during the early Eocene, and (4) protoliths of the marbles were probably precipitated in a marine environment. These lines of evidence suggest that the previously proposed mélange is indeed not the Cretaceous mélange, but the vestige of syncollisional sedimentary strata that was originally deposited in situ in a syncollisional foreland basin. The estimated collisional age of prior to~56 Ma in the eastern YZSZ is almost consistent with the 60-56 Ma age estimated from the western-central YZSZ and Himalaya, indicating a (quasi-) synchronous initial India-Asia collision at~60-56 Ma.
Abstract. Climatic oscillations have been developed through the (Early)
Jurassic from marine sedimentary archives but remain unclear from
terrestrial records. This work presents investigation of climate-sensitive
sediments and carbon and oxygen isotope analyses of lacustrine and pedogenic
carbonates for the Early Jurassic Ziliujing Formation taken from the Basin in southwestern China. Sedimentary and stable isotope proxies
manifest that an overall secular (semi)arid climate dominated the Sichuan
Basin
during the Early Jurassic, except for the Hettangian. This climate pattern is
similar to the arid climate in the Colorado Plateau region in western North
America but is distinct from the relatively warm and humid climate in northern China
and at high latitudes in the Southern Hemisphere. The estimated atmospheric CO2
concentration (pCO2) from carbon isotopes of pedogenic carbonates shows
a range of 980–2610 ppmv (∼3.5–10 times the pre-industrial
value) with a mean of 1660 ppmv. Three phases of pCO2 (the
Sinemurian 1500–2000 ppmv, the Pliensbachian 1000–1500 ppmv, and the early
Toarcian 1094–2610 ppmv) and two events of pCO2 rapidly falling by
∼1000–1300 ppmv are observed, illustrating the pCO2
perturbation in the Early Jurassic. The perturbation of pCO2 is
compatible with seawater temperature and carbon cycle from the coeval marine
sediments, suggesting a positive feedback of climate to pCO2 through the Early Jurassic.
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