The Langfang—Gu’an hollow is tectonically a second—order tectonic unit in the western part of the Bohai Bay basin and in the northern part of the central Hebei depression．It is bounded on the west by the Daxing fault and on the east by the Niudong—Hexiwu fault．It extends in a NE direction for about 70—80 km，covering an area of about 2800 km2．Its development was con— troHed by the Daxing fau．1t and the PaleOgene Occurs as a listric hollow．An Eocene—Oligocene pelite bed up to over one thousand meters thick is developed in the area，thus forming the Gu'an— Beisifa and Caojiawu—Liuquan pelite updoming belts．A complex graben fracture system is devel— oped at the top of the pelite structure．The typical structural style of the hollow is an extensional block association with ihe basement involved。but an analysis of the deformation features of the cover structure indicates that this area underwent compression in the terminal Paleolzene．