Mechanism of vertical crustal deformation in the Liupan Shan tectonic belt, northeastern Tibetan Plateau: A numerical study

As the forefront of the intracontinental extension of the Tibetan Plateau, the deformation mechanismof the Liupan Shan tectonic belt is crucial for understanding the growth of the plateau expansion. Based on thetwo models derived from geophysics data, we construct a series of two-dimensional visco-elastoplastic finiteelement models across the Liupan Shan tectonic belt, to investigate the influence of lateral crustal rheology variations on vertical crustal deformation. The modeled results show that the two-sided collision between thenortheastern Tibetan Plateau and the Ordos block causes significant vertical deformation between the blocks, butfails to produce the narrow uplift of Liupan Shan that corresponds to the actual topography. On the contrary,when a regional tectonic accretion wedge in the mid-lower crust between them is considered, the modeled upliftis consistent with the actual topographic gradient of the Liupan Shan tectonic belt. The elevation differencebetween the two blocks is approximately 500 m. Meanwhile, the modeled vertical deformation rate is about 1 similar to 2 mm center dot a(-1), consistent with the leveling rates, suggesting that the pre-existing tectonic accretion wedge plays animportant role in the regional vertical crustal deformation. However, when assuming a low viscosity coefficientin the mid-lower crust of the northeastern Tibetan Plateau, the whole plateau uplifts, rather than the localized upliftof the Liupan Shan tectonic belt, suggesting the predominant present-day crustal-scale coupling deformation on thenortheastern Tibetan Plateau. The deformation of the Liupan Shan tectonic belt is characterized by crustal shorteningand thickening under pure shear, in conjunction with a localized low viscosity coefficient in the mid-lower crust.Contrasting the topographic features of the Longmen Shan fault zone on the eastern margin of the plateau, wepropose that the rheological structure variations in the mid-lower crust control the vertical deformation pattern atthe forefront of plateau collision and expansion. Our results shed light on understanding the intracontinentalextension model and lithosphere deformation mechanism of the northeastern Tibetan Plateau