Asymmetrical to overturned folds of the Cagayan Valley, Northern Luzon, are interpreted as gravity structures which slid off the Middle Pleistocene Central Cordillera. Lateral compression appears to have played a secondary role. Beneath and around the edges of the north-striking Middle Miocene to Recent Cagayan Basin are remnants of a very differently oriented Early Miocene basin trending north-easterly parallel to the Palawan-Visayan·Sulu structural system which still prevails generally west of the Philippine Rift. Similar relict northeast structural trends
are found in pre-Middle Miocene rocks of central Luzon and southern Mindoro. It is suggested that this northeast structural grain was more general throughout the Philippines prior to the Middle Miocene, and the Mindanao Trench and Philippine Rift probably did not come into existence until the end of Early Miocene time in response to subcrustal convection currents and westward drift of the weakly linked Ryuku and Taiwan-Luzon arcs against and beneath Taiwan. Interference between early Miocene and later lines of folding in the northeastern half of the Philippines created a variety of complex surface structural patterns as well as subsurface
conditions of potential economic significance.