The Lomagundi-Jatuli Event (LJE) is one of the largest and earliest positive carbon isotope excursions preserving δ 13 C carb values between +5 and +16‰ in Paleoproterozoic carbonates worldwide. However, the duration, amplitude and patterns of these excursions remain poorly constrained. The 2.14-1.83 Ga Hutuo Group in the North China Craton is a >10 km thick volcano-sedimentary sequence, including >5 km thick well-preserved carbonates that were deposited in supra-tidal to sub-tidal environments. CO isotopic and elemental analyses of 152 least altered samples of the carbonates revealed a three-stage δ 13 C evolution. It began with an exclusively positive δ 13 C carb (+1.3 to + 3.4‰) stage in the ~ 2.1 Ga carbonate in the Dashiling and Qingshicun Formations, followed by a transition from positive values to oscillating positive and negative values in ~3000 m thick carbonates of the Wenshan, Hebiancun, Jianancun, and Daguandong Formations, and end with exclusively negative δ 13 C carb values preserved in > 500m thick dolostones of the Huaiyincun and Beidaxing Formations. It appears that much of the LJE, particularly those extremely positive δ C carb signals, was not recorded in the Hutuo carbonates. The exclusively positive δ 13 C carb values (+1.3 to + 3.4‰) preserved in the lower formations likely correspond to the end of the LJE, whereas the subsequent two stages reflect the aftermath of the LJE and the onset of Shunga-Francevillian Event (SFE). The present data point to an increased influence of oxygen on the carbon cycle from the Doucun to the Dongye Subgroups and demonstrate that the termination of the LJE in the North China Craton is nearly simultaneous with those in Fennoscandia and South Africa.

A modern tectonic stress field for the northeastern margin of Tibetan Plateau was determined using linear inversion. Focal mechanism solutions and the depths of 54 earthquakes from 2009 to 2017 were obtained from broadband seismic waveforms. We derived the tectonic stress field using the SATSI (Spatial and Temporal Stress Inversion) software based on the damped linear inversion method. The stress tensor structures are primarily indicative of strike-slip and thrust faulting; their distributions are controlled by the West Qinling and Haiyuan faults. The Longxi Block is surrounded by two faults dominated by thrust faulting; the regions in the periphery of the faults, (e.g., West Qingling Fault Zone and Liupanshan Basin) are primarily characterized by strike-slip faulting. However, normal faulting has developed in the Haiyuan-Zhongwei Belt of Liupanshan Basin, indicating that the latest tectonic regime is an NNW-SSE extension instead of a strike-slip fault with a thrust component, as has been previously suggested. This anomalous stress mechanism reflects the combined effects of block rotation and local extension resulting from movement along strike-slip faults. The West Qinling Fault is dominated by thrust faulting instead of strike-slip faulting. The Haiyuan-Liupanshan and Niushoushan-Luoshan faults are predominantly strike-slip faulting. The directions of the maximum stress axes (σ1) in both Liupanshan Basin and the Longxi Block are ENE-WSW down to Liupanshan in the south and West Qinling Fault Zone, where the stress axes gradually rotate clockwise to E-W. The modern tectonic stress field implies that regional stress originates from far-field effects of Indian and Eurasian plate collision. K E Y W O R D S focal mechanism solutions, Liupanshan Basin, northeastern margin of Tibetan Plateau, tectonic stress field, West Qinling Fault Zone 1 | INTRODUCTION The leading edge of growth and expansion of the Tibetan Plateau is along the northeastern margin, which is a modern geomorphologic and tectonic feature formed by ordered extension of the plateau during its gradual formation (

Abstract. The slip rates of active faults in the northeastern Tibetan Plateau (NETP) require clarification to understand the lateral expansion of the Tibetan Plateau and assess the seismic hazards in this region. To obtain the continuous slip rates of active faults at the NETP, we constructed a three-dimensional (3D) numerical geomechanics model that includes a complex 3D fault system. The model also accounts for the physical rock properties, gravity fields, fault friction coefficients, initial stress, and boundary conditions. Following this, we present the long-term kinematics of NETP based on the horizontal and vertical velocities and fault slip rates acquired from the model. The fault kinematic characteristics indicate that the Laohushan, middle–southern Liupanshan, and Guguan–Baoji faults, as well as the junction area of the Maxianshan and Zhuanglanghe faults, are potential hazard areas for strong earthquakes. However, as these faults are currently in the stress accumulation stage, they are unlikely to cause a strong earthquake in the short term. In contrast, it is likely that the Jinqiangshan–Maomaoshan fault will generate a earthquake with a surface-wave magnitude (MS) of 7.1–7.3 in the coming decades. In addition, the velocity profiles across the NETP imply that the plate rotation is the primary deformation mechanism of the NETP even though the intra-block straining and faulting are non-negligible.

Abstract. The slip rates of active faults at the northeastern margin of the Tibetan Plateau (NETP) must be clarified to understand the lateral expansion of the Tibetan Plateau and assess the seismic hazards in this region. To obtain the continuous slip rates of active faults at the NETP, we constructed a three-dimensional geomechanics-numerical model of the NETP. The model explains the fault systems, topographic undulations, and crustal stratigraphy of the study area. It also accounts for the physical rock properties, gravity fields, fault friction coefficients, initial crustal stresses, and boundary conditions. The horizontal and vertical crustal velocities and slip rates of active faults in the study area were obtained from simulations using the aforementioned model. The results were then validated against independent geographic datasets. Based on the analysis of the fault kinematics in the study area, the Laohushan, middle–southern Liupanshan, and Guguan–Baoji faults, as well as the locked fault zone at the junction of the Maxianshan and Zhuanglanghe faults, represent potential hazard areas for strong earthquakes. However, as these faults are currently in the stress accumulation stage, they are unlikely to cause a major earthquake in the short term. In contrast, it is likely that the Jinqiangshan–Maomaoshan fault will generate a ~MS7.0 earthquake in the coming decades. Based on the analysis of several profiles across the NETP, the deformations at the NETP are continuous in the Bayan Har and Qaidam blocks, as well as in the block-like in Qilian Block, particularly around the Haiyuan Fault.

Petrography, zircon cathodoluminescence, and laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon dating were performed on the Paleoproterozoic khondalite series and the intrusive altered diabase dykes developed in the northeastern Helanshan region were analyzed. The results showed that most of the zircons in the khondalite series were detrital zircons with oscillatory zoning and a high Th/U ratio, with few metamorphic zircons having a low Th/U ratio. The 207Pb/206Pb age of the detrital zircons ranged from 3131–1980 Ma, which constrained the protolith age of the Helanshan khondalite series to after 1980 Ma. The age of the metamorphic zircons indicated two age groups as follows: 1965–1921 Ma and 1876–1820 Ma. Besides, the age of altered diabase dykes was 1865–1850 Ma. In combination with previous studies, these new metamorphic ages indicated that the metamorphic events in the northeastern Helanshan region involved the collision followed by post-collisional extension and exhumation between the Yinshan Block to the north and the Ordos Block to the south. Moreover, the 1965–1921 Ma group represented the period of the collision between the Yinshan Block and the Ordos Block and the subsequent post-collisional extension event, whereas 1876–1820 Ma indicated the period of the exhumation stage.