Combined Gravimetric-Seismic Moho Model of Tibet

Abstract

Substantial progress has been achieved over the last four decades to better understand a deep structure in the Himalayas and Tibet. Nevertheless, the remoteness of this part of the world still considerably limits the use of seismic data. A possible way to overcome this practical restriction partially is to use products from the Earth’s satellite observation systems. Global topographic data are provided by the Shuttle Radar Topography Mission (SRTM). Global gravitational models have been derived from observables delivered by the gravity-dedicated satellite missions, such as the Gravity Recovery and Climate Experiment (GRACE) and the Gravity field and steady-state Ocean Circulation Explorer (GOCE). Optimally, the topographic and gravity data should be combined with available results from tomographic surveys to interpret the lithospheric structure, including also a Moho relief. In this study, we use seismic, gravity, and topographic data to estimate the Moho depth under orogenic structures of the Himalayas and Tibet. The combined Moho model is computed based on solving the Vening Meinesz–Moritz (VMM) inverse problem of isostasy, while incorporating seismic data to constrain the gravimetric solution. The result of the combined gravimetric-seismic data analysis exhibits an anticipated more detailed structure of the Moho geometry when compared to the solution obtained merely from seismic data. This is especially evident over regions with sparse seismic data coverage. The newly-determined combined Moho model of Tibet shows a typical contrast between a thick crustal structure of orogenic formations compared to a thinner crust of continental basins. The Moho depth under most of the Himalayas and the Tibetan Plateau is typically within 60–70 km. The maximum Moho deepening of ~76 km occurs to the south of the Bangong-Nujiang suture under the Lhasa terrane. Local maxima of the Moho depth to ~74 km are also found beneath Taksha at the Karakoram fault. This Moho pattern generally agrees with the findings from existing gravimetric and seismic studies, but some inconsistencies are also identified and discussed in this study.

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

“…In the northwestern Tibet, around the Karakorum, the Moho deepens to 70–80 km in our result, which coincides with the receiver function results beneath the Pamir (Schneider et al, ) and the seismic‐derived Moho model (Stolk et al, ), while the Moho is not so deep according to Baranov et al (), Shin et al (), and Chen and Tenzer (). Compared to the seismic‐derived Moho model of Stolk et al (), we found that the pattern of our inverted Moho is much closer to the seismically derived map than of the previous results obtained from the gravity data (e.g., Baranov et al, ; Chen & Tenzer, ; Shin et al, ; Xu et al, ) as shown in Figure .…”

“…Noticeable differences also occur in the northwestern and central Tibet, as shown in Figure 9. For example, in the central Tibet, the visible shallow region is observed in the Lhasa Block and Qiangtang Block crossed by the Bangong-Nujiang Suture (BNS) in Shin et al (2015), Xu et al (2017), and our results, but this feature is not observed in the results of Chen and Tenzer (2017), Stolk et al (2013), and Baranov et al (2018). In the northwestern Tibet, around the Karakorum, the Moho deepens to 70-80 km in our result, which coincides with the receiver function results beneath the Pamir (Schneider et al, 2013) and the seismic-derived Moho model (Stolk et al, 2013), while the Moho is not so deep according to Baranov et al (2018), Shin et al (2015), and Chen and Tenzer (2017).…”

“…For example, in the central Tibet, the visible shallow region is observed in the Lhasa Block and Qiangtang Block crossed by the Bangong-Nujiang Suture (BNS) in Shin et al (2015), Xu et al (2017), and our results, but this feature is not observed in the results of Chen and Tenzer (2017), Stolk et al (2013), and Baranov et al (2018). In the northwestern Tibet, around the Karakorum, the Moho deepens to 70-80 km in our result, which coincides with the receiver function results beneath the Pamir (Schneider et al, 2013) and the seismic-derived Moho model (Stolk et al, 2013), while the Moho is not so deep according to Baranov et al (2018), Shin et al (2015), and Chen and Tenzer (2017). Compared to the seismic-derived Moho model of Stolk et al (2013), we found that the pattern of our inverted Moho is much closer to the seismically derived map than of the previous results obtained from the gravity data (e.g., Baranov et al, 2018;Chen & Tenzer, 2017;Shin et al, 2015;Xu et al, 2017) as shown in Figure 9.…”

“…Comparison of Moho models derived by (a) our results; (b) Stolk et al (); (c) Xu et al (); (d) Shin et al (); (e) Baranov et al (); and (f) Chen and Tenzer (). Note that there is no data in 70–75°E in Xu et al ().…”

“…To decrease the nonuniqueness, a joint inversion of gravity with other available geological and geophysical data is usually performed. In particular, seismic data are widely combined with the gravity data to investigate 3‐D density structure of the upper mantle (e.g., Deng et al, ; Kaban et al, , ; Kaban, El Khrepy, et al, ), and for the Moho depth inversion (e.g., Baranov et al, ; O'Donnell & Nyblade, ; Szwillus et al, ; Uieda & Barbosa, ).…”