Moho Beneath Tibet Based on a Joint Analysis of Gravity and Seismic Data

Abstract

We use the improved Parker‐Oldenburg's formulas that include a reference depth into the exponential term and employ the Gauss‐fast Fourier transform method to determine Moho depth beneath the Tibetan Plateau. The synthetic models demonstrate that the improved Parker's formula has high accuracy with the maximum absolute error less than 0.25 mGal compared to the analytical solution. Two inversion parameters, that is, the reference depth and the density contrast, are essential for the Moho estimation based on the gravity field, and they need to be determined in advance to obtain correct results. Therefore, the Moho estimates derived from existing seismic studies are used to reduce the nonuniqueness of the gravity inversion and to determine these parameters by searching for the maximum correlation between the gravity‐inverted and seismic‐derived Moho depths. Another critical issue is to remove beforehand the gravity effects of other factors, which affect the observed gravity field besides Moho variations. In addition to the topography, the gravity effects of the sedimentary layer and crystalline crust are removed based on existing crustal models, while the upper mantle impact is determined based on the seismic tomography model. The inversion results show that the Moho structure under the Tibetan plateau is very complex with the depths varying from about 30–40 km in the surrounding basins (e.g., Ganges basin, Sichuan basin, and Tarim basin) to 60–80 km within the plateau. This considerable difference up to 40 km on the Moho depth reveals the substantial uplift and thickening of the crust in the Tibetan Plateau. Furthermore, two visible “Moho depression belts” are observed within the plateau with the maximum Moho deepening along the Indus‐Tsangpo Suture and along the northern margin of Tibet bounding the Tarim basin with the relatively shallow Moho in central Tibet between them. The southern “belt” is likely formed in compressional environment, where the Indian plate underthrusts northward beneath the Tibetan Plateau, while the northern one could be formed by the southward underthrust of the Asian lithosphere beneath Tibet.

Two-step Gravity Inversion Reveals Variable Architecture of African Cratons

“…For the Phanerozoic domain, the seismic stations are predominantly located in mountainous regions (Atlas, Great Rift Valley, and Cameroon Volcanic Line), which are characterized by a deeper Moho that isostatically compensates the high topography. This coincides with a high density contrast across the Moho, which is also confirmed by gravity inversions in mountain ranges (e.g., Zhao et al, 2020). In the East African Rift System, the Moho depth is overestimated compared to both seismic constraints because of an unaccounted gravity signal of the upwelling mantle.…”

“…When possible, gravity data are corrected for the effect of sediments when applied to inversion for the Moho depth (e.g. Uieda and Barbosa, 2016;Zhao et al, 2020). However, the thickness of a large portion of the African sedimentary basins is unknown.…”

Lithospheric Density Structure and Effective Elastic Thickness Beneath Himalaya and Tibetan Plateau: Inference From the Integrated Analysis of Gravity, Geoid, and Topographic Data Incorporating Seismic Constraints

“…In the past, a variety of geophysical methods have been used to study the crust and lithospheric structure of the Himalaya and Tibet regions (Figure 1). Important among them are deep seismic studies (Galve et al, 2006; Haines et al, 2003), seismic tomography (Acton et al, 2010; Chen et al, 2017; Li et al, 2008; Li & Song, 2018; McNamara et al, 1997; Priestley et al, 2008), magnetotellurics (Unsworth et al, 2005; Wei et al, 2001), gravimetric studies (Braitenberg et al, 2000; Cattin et al, 2001; Hetényi et al, 2006, 2016; Jiang et al, 2004; Jiménez‐Munt et al, 2008; Jin et al, 1994, 1996; McKenzie et al, 2019; Ravikumar, Mishra, & Singh, 2013; Ravikumar, Mishra, Singh, Venkat Raju, et al, 2013; Robert et al, 2015; Shin et al, 2007; Tiwari et al, 2006, 2008; Tunini et al, 2016; Zhao et al, 2020), receiver function studies (Gilligan et al, 2015; Mitra et al, 2005; Nabelek et al, 2009; Priestley et al, 2019; Rai et al, 2006; Shi et al, 2015, 2016; Wittlinger et al, 2004; Zhao et al, 2010), and geothermics (Chung et al, 2005), which provided useful constraints in building the initial model. For constraining the shallow crustal structure, geological cross‐sections were adopted from Yin and Harrison (2000), Guillot et al (2003), Wittlinger et al (2004), and Searle (2010).…”

“…Yellow and black filled circles mark two different episodes of magmatism distributions (Chung et al, 2005(Chung et al, , 2009. Gravimetric studies of Himalayas and Tibetan plateau based on Airy isostasy and spectral analysis methods have mainly brought out large scale Moho undulations using ground gravity (Braitenberg et al, 2000(Braitenberg et al, , 2003He et al, 2014;Jin et al, 1994Jin et al, , 1996 and satellite-derived GRACE and GOCE data (Bagherbandi, 2011;Basuyau et al, 2013;McKenzie et al, 2019;Shin et al, 2007;Tenzer et al, 2015;Zhao et al, 2020). Studies based on the flexural modeling approach (Braitenberg et al, 2003;Cattin et al, 2001;Chen et al, 2015;Hetényi et al, 2006;Tiwari et al, 2006Tiwari et al, , 2008 have yielded lower values of effective elastic thickness (<40 km) over Tibetan plateau and higher values (>50 km) over the adjoining Indian shield suggesting thin and thick lithosphere, respectively.…”

Magnetic Structure and its Tectonic Implication Around Longmenshan Fault Zone Revealed by EMAG2v3

“…By contrast, the SGFB on the west side of LFZ exhibits high velocities in the upper crust but low velocities in the mid-lower crust, which is considered as mid-lower crustal flow channel and is in agreement with low resistivity in magnetotelluric studies (Bai et al, 2010;Wang et al, 2010;Wang et al, 2015bWang et al, , 2017aWang et al, 2018a). Due to the uplift of the Moho, SB has relatively high gravity anomalies, and SGFB show strong negative anomalies surrounded by the gravity gradient belt, suggesting that a density contrast exists cross LFZ (Zhang et al, 2009;Zhang et al, 2010;Zhao et al, 2020).…”