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The 900-km-long Yushu-Ganzi-Xianshuihe fault system that separates the Bayan Har and Qiangtang/Chuandian blocks is a primary tectonic structure in central eastern Tibet. Being one of the world's most seismically active fault systems, it hosted 12 M > 7 earthquakes since the 18th century, and its future seismic risk has been the subject of extensive research over the past two decades. Previous studies have suggested that the western segment of the fault system differs largely from its eastern segment in strain accumulation and release, but the origin of such difference remains unknown. Here we use Sentinel-1 data to produce a complete map of present-day deformation for the entire fault system, in order to explore the along-strike variation in fault slip behaviour. Based on the InSAR deformation map, we solve for fault kinematic parameters including slip rate, locking depth and dip angle using a Bayesian Markov Chain Monte Carlo inversion approach. Slip rate on the Dangjiang-Yushu-Ganzi fault is consistently ∼5–6 mm/yr, only half of that on the Xianshuihe fault. This rate difference originates from nonuniform deformation mechanism within the Bayan Har block, with the western part of the block deforming more rigidly than the eastern part. Both historical and modern earthquake catalogues show that the western fault segment has only recorded M > 7 earthquakes whereas the eastern segment has seen many small-to-moderate ruptures in addition to large earthquakes. These observations imply that the western fault segment may be more structurally mature than the eastern segment, which agrees with the long-term fault propagation as indicated by geochronological data and fault geometrical complexities. We calculate the seismic moment deficits along the fault system and find that several segments, e.g. the Dangjiang fault and the Manigange segment of the Ganzi fault, have accumulated a large seismic moment (equates to an M w > 7 earthquake) since the last major event. • We explore lateral variation in slip behaviour along the YS-GZ-XSH fault system. • Slip rate on the YS-GZ fault is 5-6 mm/yr, only half of that on the XSH fault. • Rate variation results from nonuniform deformation within the Bayan Har block. • The western YS-GZ-XSH fault segment is more mature than its eastern segment. • Some segments have accumulated a large seismic moment that equates to Mw>7. [ABSTRACT FROM AUTHOR]

The Haiyuan fault zone is an important tectonic boundary and strong seismic activity belt in northeastern Tibet, but no major earthquake has occurred in the past ∼100 years, since the Haiyuan M8.5 event in 1920. The current state of strain accumulation and seismic potential along the fault zone have attracted significant attention. In this study, we obtained the interseismic deformation field along the Haiyuan fault zone using Envisat/ASAR data in the period 2003–2010, and inverted fault kinematic parameters including the long-term slip rate, locking degree and slip deficit distribution based on InSAR and GPS individually and jointly. The results show that there is near-surface creep in the Laohushan segment of about 19 km. The locking degree changes significantly along the strike with the western part reaching 17 km and the eastern part of 3–7 km. The long-term slip rate gradually decreases from west 4.7 mm/yr to east 2.0 mm/yr. As such, there is large strain accumulation along the western part of the fault and shallow creep along the Laohushan segment; while in the eastern section, the degree of strain accumulation is low, which suggests the rupture segments of the 1920 earthquake may have been not completely relocked. [ABSTRACT FROM AUTHOR]

The 1000-km-long Haiyuan fault system on the northeastern edge of the Tibetan Plateau contributes to accommodating the deformation in response to the India/Asia collision. In spite of its importance, the kinematics of the fault including the geometry and along-strike slip rate have not been completely defined. In this study, we use synthetic aperture radar data acquired between 2014 and 2021 by Sentinel-1 satellites to investigate the present-day strain accumulation on the Haiyuan fault system. We produce a high-resolution velocity map for the ∼300,000 km2 Haiyuan region using the Small BAseline Subset method. Our new velocity fields reveal deformation patterns dominated by the eastward motion of Tibet relative to Alaxan and localised strain accumulation along the Haiyuan, Gulang and Xiangshan-Tianjingshan faults. The western ∼300-km-long section of the Haiyuan fault, which was previously unmapped, seems to follow Tuolaishan and terminate at Halahu. We compute the along-strike slip rate using a Bayesian Markov Chain Monte Carlo inversion approach, and find that the overall strike-slip rate along the Haiyuan fault system gradually increases from the western end (1.8±0.3 mm/yr close to Halahu) to the east (6.4±0.5 mm/yr before entering Liupanshan), and further east, it decreases from 6.4±0.5 mm/yr to 1.3±0.7 mm/yr. The Haiyuan fault absorbs most of the left-lateral strike-slip motion with a rate of ∼4.2±0.4 mm/yr, and the Gulang and Xiangshan-Tianjingshan faults take up a fraction of 2.2±0.6 mm/yr. We re-map the previously identified shallow creeping zone on the Laohushan segment for a length of 45 km, slightly larger than the previous estimate of 35 km. The average shallow creep rate, 3 mm/yr between 2014–2021, is consistent with the rate before 2007 (2–3 mm/yr), implying that the shallow creep is a steady behaviour. • We produce a high-resolution InSAR velocity map for the ∼300,000 km2 Haiyuan region. • The western section of the Haiyuan fault follows Tuolaishan and terminates at Halahu. • Strike-slip rate gradually increases from the western end (∼1.8 mm/yr) to the east. • We re-map the shallow creeping zone on the Laohushan segment for a length of 45 km. [ABSTRACT FROM AUTHOR]

Rodingite is produced by Ca-rich fluid metasomatism of mafic igneous rocks in oceanic lithosphere. Despite its importance for the composition of subduction zone magmas, its formation mechanism still remains obscure. To address this issue, we investigated the major-trace element and Mg–O–Sr–Nd isotope compositions of rodingites in serpentinites from the Xigaze ophiolite, southern Tibet. Coexisting gabbros and rodingites show consistently depleted Sr Nd isotope compositions, suggesting their formation in a mid-ocean ridge setting. The rodingites exhibit variable δ26Mg values from −0.61‰ to −0.14‰, with an average of −0.37‰ ± 0.28‰ (n = 19; 2 SD), mostly lower than those of −0.26‰ to −0.13‰ for the protolith gabbros. Compared to the relatively homogeneous δ18O values of 6.1‰–6.7‰ for the gabbros, the rodingites show highly variable δ18O values from 3.8‰ to 9.4‰. The rodingites can be subdivided into two groups based on mineral assemblages and formation conditions. Group I rodingite is prehnite-dominated and hydrogarnet-free, formed at temperatures of 200–300 °C, and has higher δ18O values of 8.1‰ ± 1.4‰ (n = 15) and δ26Mg values of −0.32‰ ± 0.19‰ (n = 12). Group II rodingite consists mainly of hydrogarnet and clinopyroxene, formed at temperatures of 300–400 °C, and has lower δ18O values of 5.4‰ ± 2.5‰ (n = 7) and δ26Mg values of −0.44‰ ± 0.35‰ (n = 7). Modeling of the fluid–rock interactions between the protolith gabbro and fluid was undertaken using the Mg O isotope compositions of the serpentinizing fluid inferred from the associated serpentinites. Based on the modeling results, we propose that the metasomatism of gabbros by Ca-rich, low-δ26Mg fluids produced during serpentinization would result in the Mg O isotope compositions of the rodingites. Such rodingites with low δ26Mg values would generate the geochemical heterogeneity in the mantle wedge overlying the subducting oceanic lithosphere and eventually give rise to Ca-rich, low-δ26Mg basaltic magmas. The results indicate that low δ26Mg values of basaltic rocks cannot be unambiguously ascribed to carbonated mantle sources. • The Xigaze rodingites show low δ26Mg and variable δ18O values. • The rodingites were formed through metasomatism of gabbros by serpentinizing fluids under seafloor. • The subduction of rodingites in subduction zones can greatly affect mantle Mg isotopic composition. [ABSTRACT FROM AUTHOR]