Melting conditions in the modern Tibetan crust since the Miocene

Abstract

Abundant granitic rocks exposed in ancient mountain belts suggest that crustal melting plays a major role in orogenic processes. However, complex field relations and superposition of multiple tectonic events make it difficult to determine the role of melting in orogenesis. In contrast, geophysical measurements image present-day crustal conditions but cannot discriminate between partial melt and aqueous fluids. Here we connect pressure–temperature paths of Himalayan Miocene crustal rocks to the present-day conditions beneath the Tibetan plateau imaged with geophysical data. We use measurements of electrical conductivity to show that 4–16% water-rich melt is required to explain the crustal conductivity in the north-western Himalaya. In southern Tibet, higher melt fractions >30% reflect a crust that is either fluid-enriched (+1% H2O) or hotter (+100 °C) compared to the Miocene crust. These melt fractions are high enough for the partially molten rocks to be significantly weaker than the solid crust.

Eastward Dipping Style of the Underthrusting Indian Lithosphere Beneath the Tethyan Himalaya Illuminated by P and S Receiver Functions

“…Furthermore, the existence of the mid‐crustal LVZ is also suggested by the RF results beneath the northwest Himalaya (Hazarika et al., 2017) and along the Hi‐CLIMB profile at 85°E (Duputel et al., 2016). We suggest that the formation of this mid‐crustal LVZ observed in the Tethyan Himalaya could be due to the presence of crustal dehydration partial melting, which is justified by the conclusion drawn from a comparison between the experimental conductivity measurements of Himalayan Miocene metamorphic rocks and the present geophysical MT data (J. Y. Chen et al., 2018). The results of 3D thermomechanical modeling indicate that radioactive, shear and basal heating during crustal thickening provide important heat sources for generating such mid‐crustal partial melting (L. Chen et al., 2019).…”

Shaping the Surface Deformation of Central and South Tibetan Plateau: Insights From Magnetotelluric Array Data

“…Recent studies show that dehydration melting may result in melt water content of as high as 6%-8% (J. Chen et al, 2018) in the southern Tibetan Plateau. Conductivity models with laboratory constraints also predict a similarly high level of water content (7%-9%) in the lower crust (Comeau et al, 2016;Laumonier et al, 2014).…”

“…J. Chen et al (2018) developed a similar empirical model for the estimation of the melt/fluid conductivity using the Arrhenius equation. Figure 6a shows the calculated fluid conductivity from both the Guo et al (2018) and J.…”

“…Experimental constrains suggest bound H 2 O content of 5–6 wt.% (Patino Douce & Harris, 1998) or >6 wt.% (Hashim et al, 2013) for water dissolved in Himalayan granite magmas in the late Oligocene to Miocene times. Recent studies show that dehydration melting may result in melt water content of as high as 6%–8% (J. Chen et al, 2018) in the southern Tibetan Plateau. Conductivity models with laboratory constraints also predict a similarly high level of water content (7%–9%) in the lower crust (Comeau et al, 2016; Laumonier et al, 2014).…”

The Role of Crustal Buoyancy in the Generation and Emplacement of Magmatism During Continental Collision

“…Zircons originating from the Indian crust and found in magmatic rocks close to the suture in the Himalayan belt suggest that some magmatism may well be partially derived from subducted continental crust (Bouilhol et al, ), consistent with the Sr‐Nd‐Pb isotope systematics of magmatism further north (Guo et al, ). Missing compositional variations or small‐scale processes at shallower depth (e.g., faulting and shear heating) in our models prevent predictions of melting at shallow depths, such as crustal melting due to thickening of the overriding crust during collision (e.g., Chen et al, ; Wang et al, ).…”