Cenozoic evolution of the Yilan–Yitong Graben in NE China: An example of graben formation controlled by pre-existing structures

Abstract

The NE-trending Yilan–Yitong Graben (YYG) occurs along the Yilan–Yitong Fault Zone in the northern segment of the Tan-Lu Fault Zone, northeastern China. It shows an unusually narrow, straight shape in map view with a length of ca. 600km and width of <25km. From northeast to southwest, the YYG is composed of the Tangyuan Graben bounded by one main fault, and the Fangzheng, Shulan, and Yitong grabens bounded by two main faults. The graben boundary faults strike NE–SW, whereas the graben interiors are dominated by E–W normal faults. The graben architectures and depocenters were controlled mainly by the NE-striking faults, but were also affected by some major E–W normal faults. Integration of our structural data with oil exploration results demonstrate four stages of Cenozoic evolution for the YYG: (1) Paleogene rifting under N–S extension; (2) compression-induced inversion at the end of the Paleogene; (3) Neogene subsidence under a weak extensional setting; and (4) Quaternary E–W compression. The NE-striking graben boundary faults behaved as normal dextral faults during the Paleogene due to oblique extension. It is proposed that the YYG formed through reactivation of the pre-existing Yilan–Yitong strike-slip fault zone under Paleogene N–S extension and that it represents an oblique extensional graben rather than an orthogonal extensional graben, a pull-apart basin, or transtensional graben as previously proposed. The narrow, straight shape of the YYG results from reactivation of the straight strike-slip fault zone under oblique extension.