Jurassic to Neogene Quantitative Crustal Thickness Estimates in Southern Tibet

Abstract

Recent empirical calibrations of Sr/Y and La/Yb from intermediate igneous rocks as proxies of crustal thickness yield discrepancies when applied to high ratios from thick crust. We recalibrated Sr/Y and La/Yb as proxies of crustal thickness and applied them to the Gangdese Mountains in southern Tibet. Crustal thickness at 180-170 Ma decreased from 36 to 30 km, consistent with Jurassic backarc extension and ophiolite formation along the southern Asian margin during Neo-Tethys slab rollback. Available data preclude detailed estimates between 170 and 100 Ma and tentatively suggest ~55 km thick crust at ca. 135 Ma. Crustal thinning between 90 and 65 Ma is consistent with a phase of Neo-Tethys slab rollback that rifted a portion of the southern Gangdese arc (the Xigaze arc) from the southern Asian margin. Following the continental collision between India and Asia, crustal thickness increased by ~40 km at ~1.3 mm/a between 60 and 30 Ma to near modern crustal thickness, before the onset of Miocene east-west extension. Sustained thick crust in the Neogene suggests the onset and later acceleration of extension in southern Tibet together with ductile lower crustal flow works to balance the ongoing mass addition of under-thrusting Indian crust and maintain isostatic equilibrium.

Chemical Mohometry: Assessing Crustal Thickness of Ancient Orogens Using Geochemical and Isotopic Data

“…In thinner crusts, where the effect of MgO variations is smaller, the differences between new and old calibrations are negligible. In thicker crusts, as the effect of MgO increases, predictions of the new calibrations for the primitive and evolved compositions diverge from the Profeta et al (2015) and Sundell et al (2021) estimates.…”

“…In thinner crusts, where the effect of MgO variations is smaller, the differences between new and old calibrations are negligible. In thicker crusts, as the effect of MgO increases, predictions of the new calibrations for the primitive and evolved compositions diverge from theProfeta et al (2015) andSundell et al (2021) estimates. For La/Yb = 25, for example, the new formulation predicts a clear increase in Moho depths from 52 to 58 km as MgO increases from 1 to 6 wt%, whereas both older models yield Moho depths of ca.…”

“…To provide a comprehensive quantitative overview of the elevation-dependent accuracy of individual mohometers, an RMSE of calibration residuals specific to each predefined elevation bin is reported in Profeta et al, 2015;Sundell et al, 2021); the calibrations designed for collisional orogens (Hu et al, 2017) are also shown for reference. In spite of different approaches and number of datapoints involved in the calibrations, La/Yb-based arc mohometers generally agree (Figure 22a).…”

“…All the complexities aside, these mohometers were validated in most places investigated. Sundell et al (2021) reassessed the Sr/Y and La/Yb mohometers of J. B. and Profeta et al (2015) and proposed an expression that combines the two via multiple linear regressions (Figure 6): Profeta et al, 2015).…”

“…Our review of the literature focusing on the covariations between the composition of magmatic rocks and crustal thickness in orogenic areas leads us to three observations that may help formulate better mohometers. Though numerous correlations have been identified over the past decades between chemical parameters in magmatic rocks and crustal thickness of orogenic areas, most paleocrustal thickness calculations still rely only on a few ratios (La/Yb, Sr/Y and, to a lesser extent, Ce/Y) that often produce discrepant results. The problem of such discrepancies can be mitigated by using mohometers in combination (Sundell et al., 2021, discussed above), which suggests that crustal thickness estimates could be further improved by increasing the number of simultaneously applicable mohometers. While other composition–crustal thickness correlations have been used to address questions related to arc petrogenesis and crustal evolution, they either were never quantified or were not designed—and thus are rather difficult to be employed—to estimate paleoarc thicknesses as is the case for Rb/Sr (Dhuime et al., 2015) or SiO 2 , CaO, FeO t , etc.…”