Discovery of a giant caldera in the Yingcheng Formation in the Xujiaweizi fault depression, northern Songliao basin

Abstract：The formation of a caldera is the most catastrophic volcanic phenomenon leading to the most voluminous pyroclastic eruptions. Therefore, to explore the evidence for the existence of a caldera is the key to the understanding of the characteristics and superimposition relationship of volcanic eruptions in the Lower Cretaceous Yingcheng Formation in the Xujiaweizi fault depression, northern Songliao basin. The Xudong caldera, filled with thick rhyolitic tuff and lava, is a large caldera, ellipse in form with an area of 17 km×10 km and a maximum subsidence depth of 3 km. Its trapdoor subsidence was caused by its underlying asymmetrical magma chamber or asymmetrical deflation of the magma chamber. In section, there is a pair of completely developed normal and reverse faults, indicating that the Xudong caldera experienced a complete process of sinking subsidence, and syn-subsidence volcanic eruption. The caldera is the product of lithosphere thinning and substantial magmatism in a continental rift setting. The N-S-elongated ellipse shape of the caldera was formed under the control of the NNW-trending structures.