Reconciling Geophysical and Petrological Estimates of the Thermal Structure of Southern Tibet

Abstract

The thermal structure of the Tibetan plateau—the largest orogenic system on Earth—remains largely unknown. Numerous avenues provide fragmentary pressure/temperature information, both at the present (predominantly informed though geophysical observation) and on the evolution of the thermal structure over the recent past (combining petrological, geochemical, and geophysical observables). However, these individual constraints have proven hard to reconcile with each other. Here, we show that models for the simple underthrusting of India beneath southern Tibet are capable of matching all available constraints on its thermal structure, both at the present day and since the Miocene. Many parameters in such models remain poorly constrained, and we explore the various trade‐offs among the competing influences these parameters may have. However, three consistent features to such models emerge: (i) that present‐day geophysical observations require the presence of relatively cold underthrust Indian lithosphere beneath southern Tibet; (ii) that geochemical constraints require the removal of Indian mantle from beneath southern Tibet at some point during the early Miocene, although the mechanism of this removal, and whether it includes the removal of any crustal material, is not constrained by our models; and (iii) that the combination of the southern extent of Miocene mantle‐derived magmatism and the present‐day geophysical structure and earthquake distribution of southern Tibet require that the time‐averaged rate of underthrusting of India relative to central Tibet since the middle Miocene has been faster than it is at present.

Variations in the Seismogenic Thickness of East Africa

“…But, between Figures 2c, 2d, and 5, we do not see any significant increase in seismicity associated with proximity to the Moho, nor do we see an identifiable aseismic gap between any potential mantle seismicity and seismicity at shallower depths in the crust. Irrespective of whether seismicity extends slightly into the uppermost mantle, East Africa fits the global pattern of seismicity that is confined to a single continuous layer (except in cases far from thermal steady state, like the underthrusting of India beneath Tibet; see Craig et al, 2012Craig et al, , 2020, even when considering lateral variations that occur on the scale seen in Northern Tanzania (Figure 4).…”

“…In Figure 4b, we show a set of theoretical geotherms calculated with variable lithospheric thicknesses between 75 and 200 km, using the approach outlined in Craig et al (2012Craig et al ( , 2020. The crustal structure (both thickness and the distribution of radiogenic elements) in each case is kept the same, changing only the depth to the base of the lithosphere.…”

“…Some earthquakes in the northernmost Indian shield where it underthrusts Nepal and southern Tibet occur to a depth of 15–20 below the Moho (Schulte‐Pelkum et al., 2019). However, this is likely also to be the depth of the 600°C–650°C isotherm in the Indian lithospheric mantle, which is unusually cold because of the thick lithosphere and relatively thin Precambrian crust of north India (Craig et al., 2020; Priestley et al., 2008). The same explanation is likely for an earthquake at 65 km depth on the northern continental margin of Australia in the Arafura Sea (Sloan & Jackson, 2012).…”

Delamination in Tibet: Deriving constraints from the density of eclogite

“…The thickened lower crust undergoes "convective removal" due to gravitational instability, which is another type of delamination that occurred in Tibet from 25 Ma to 0 Ma (Chung et al, 2005;Nomade et al, 2004). The convective removal of the lithosphere during delamination corresponds to higher temperature conditions (Craig et al, 2020). In this circumstance, the density of Tibetan eclogite is 7.6%-11.6% denser than the surrounding peridotite at ~60 km (Fig.…”

Quantifying Water Diffusivity and Metamorphic Reaction Rates Within Mountain Belts, and Their Implications for the Rheology of Cratons

“…∼ 100-250 km north of the range-front in the present-day Himalayan-Tibetan orogen (e.g., Cattin & Avouac, 2000;Craig et al, 2020). As such, the Douglas Harbor structural window presents an ideal natural laboratory to study the effects of weakening cratons by water infiltration in a ductile mid-crustal setting.…”