| How the present-day northern Tibetan Plateau was involved in the plateau formation process under the context of the Cenozoic India-Eurasia collision has been disputed for decades.As the largest sedimentary basin within the Tibetan Plateau and enclosed by the Qiman Tagh-East Kunlun Shan and the Qilian Shan in the Cenozoic,the Qaidam Basin preserves a continuous and complete record that is key to addressing this issue.Herein we implement two-dimensional(2-D)subsidence analysis and related numerical modeling to investigate the subsidence and deformation mechanisms of the Cenozoic Qaidam Basin mainly based on seismic reflection data.The conclusions are summarized as follows.(1)The tectonic subsidence across the Qaidam Basin is the largest and reaches～4-6 kilometers near the center,but decreases gradually toward the margins during the Cenozoic,relative to the reference elevation of the basement in the early Cenozoic.And the tectonic subsidence during each depositional period displays a similar pattern,with the subsidence being generally largest near the center and decreasing gradually toward the margins.Avery good consistency is observed between the tectonic subsidence rates and the horizontal shortening rates across the basin,reflecting a close relationship between the tectonic subsidence and horizontal compression during the Cenozoic.(2)Two end-member elastic plate models are chosen to unravel the subsidence mechanism of the Qaidam Basin,including a vertical load model and a horizontal load model.The result of numerical modeling under vertical loads shows that,regardless of the values of the parameters used,the modeled tectonic subsidence along the studied sections in this scenario is the largest in the basin margins adjacent to the orogenic belts,and decreases gradually toward the basin interior,which fails to generate the observed tectonic subsidence pattern of the Qaidam Basin during the Cenozoic.The modeled tectonic subsidence under horizontal loads presents much better fits to the calculated tectonic subsidence,which forms lithospheric buckling with a wavelength of 200±50 km.With the largest values occurring in the center of the basin,the Qaidam Basin subsides as a syncline depression.(3)Combined with the Cenozoic deformation characteristics of the Qaidam Basin and its sedimentary relationship with the bordering orogenic belts,the subsidence analysis and mechanism modeling reveal that multiscale buckling occurs in the northern Tibetan Plateau.It includes lithospheric bucking with a wavelength of～200km and upper crust buckling with a wavelength of～20km.The former is likely under the control of the strength of the olivine-dominated uppermost mantle.In this buckling model,the Qaidam Basin subsided as a synclinal depression,whereas the adjacent Qiman Tagh-East Kunlun Shan and Qilian Shan uplifted as anticlines in the early Cenozoic.The parallel folds with nearly upright axial surfaces in the Qaidam Basin are interpreted to result from buckling of the upper crust with wavelengths of the～20 km.Basement-involved faults within and along the margins of the present-day Qaidam Basin were likely caused by instability of the buckling folds due to continuous horizontal force in the Neogene.These faults led to localization of strain in the surrounding mountains,as represented by intense uplift since the middle Miocene.Multiscale lithospheric buckling implies that the "jelly sandwich" rheological structure featuring a weak lower crust confined by a strong upper crust and uppermost mantle beneath the Qaidam Basin,which has been independently testified by geophysical observations.Our findings highlight that buckling due to horizontal forces in response to the far-field effect of the India-Eurasia collision,rather than gravitational loads of high mountains and under-thrusting of basins,plays a dominant role in the subsidence and deformation of the Qaidam Basin,as well as the tectonic evolution of the entire northern Tibetan Plateau. |
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