Haean, South Korea: is it of volcanic or meteor impact origin?
A Korean here.
Haean Myeon in Gangwon-do has two theories of creation. One is Meteor impact and another is differential erosion. There were no meteor related evidence found at Haean so the erosion theory is more agreed upon at the moment.

Direct translation from introduction website for Hae An:


  According to the analysis of the granite of the punchbowl basin [nickname for Haean], it consists of 71% feldspar, 22% quartz and 5.7% mica. Feldspar is much more weathering than quartz and mica, and since it is rapidly clayed at room temperature, the lowland is eroded earlier than the current highland with high resistance to weathering, resulting in this unusual branching form.