Were a couple of kilometres of Palaeocene–Eocene strata ever deposited across most of the Sichuan basin? Reply

Abstract

Deng et al. (2015b) propose that a couple of kilometres of Palaeocene– Eocene strata were deposited across most of the Sichuan basin and later eroded, mainly based on their low-T thermochronological data. They further propose that such a geological process would influence the datum plane of our isostatic model and the Cenozoic elevation history of the Sichuan, Tarim and Qaidam basins. Indeed, we selected the modern state of Suining as the datum plane for all our isostatic calculations (Yu et al., 2015). It is a static datum plane rather than a dynamic datum plane; this approach is conventional (Zhang et al., 2010). Therefore, how Suining evolved during the Cenozoic does not influence the isostatic model and the palaeoelevation estimation of the Qaidam basin. We stress the lack of Cenozoic deposits and the lack of visible Cenozoic uplift in the Sichuan basin (Yu et al., 2015), but we do not rule out the possibility of erosion. The Sichuan basin may have experienced a certain amount of erosion during the Cenozoic. According to the isostatic model, the erosion of crust (including basement and stratigraphy) leads to elevation loss (Fig. 1). Endorheic basins accumulate sediments and increase in elevation, while exorheic basins, under certain conditions, discharge sediments by erosion and thereby decrease elevation. However, compared with the visible deposit thickness, the magnitude of erosion is more difficult to estimate. We do not adopt the low-T thermochronological data provided by Deng et al. (2009, 2013, 2015a,b), because their data and conclusion are contradictory and questionable for the central Sichuan basin. Deng et al. (2015b) suggest that ~1000–2000 m of Palaeogene strata were deposited across the Sichuan basin, and then eroded during the late Cenozoic. However, if such thick Palaeocene–Eocene strata had been deposited, a dramatic drop between 65 and 34 Ma should be seen on the t(time)–T(Temperature) paths of AFT (apatite fission track) thermalhistory modelling (Fig. 2). Regretfully, no such drop is seen in their articles (Deng et al., 2009, 2013, 2015b), and, on the contrary, the area records uplift since the late Cretaceous (Deng et al., 2015b). Furthermore, the Sichuan basin that we refer to is the central part, to the west of the Huaying Mountain and to the east of the Longquan Mountain. Most of the AFT thermal-history modelling results presented by Deng et al. (2009, 2013, 2015b) are distributed at the margins of the Sichuan basin (e.g. the Huaying Mountain, the Longmen Mountain’s piedmont). It is common sense that the exhumation rate of the surrounding mountains or basin-mountain transition zones is much faster than that of the central basin. Therefore, low-T thermochronological results from the basin margin cannot reveal the geological history of the central Sichuan basin. Only four AFT thermal-history modelling results (SLG01, SN01, M24-1, M242) in the central Sichuan basin are presented by Deng et al. (2009, 2013, 2015b). However, the single grain age of SLG01 does not pass the chisquared test (P (v) < 5%), so it is unsuitable for AFT thermal history modelling (Ketcham et al., 2000). P (v) of sample SN01 is only 8.6%. The cooling magnitude of the drilling sample M24-1 is limited since the late Cenozoic, and the region of good fit is broad (Deng et al., 2013). The modern temperature setting of the drilling sample M24-2 on the t–T