Abstract: Enkai fault zone is one of the important fault zones along the coast area of southeast China. The fault zone with the length of about 160km is mainly constituted by three approximately parallel NE faults (F₁ and F₂ incline to NW and F₃ incline to SE). In Tertiary the faults moved in a way of normal faulting. However, at the end of Tertiary or early Quaternary, the faults turned to be compressive ones by WNW-ESE strong horizontal force, resulting in compressive deformation along the whole fault zone. Probably because of weakening of the WNW-ESE compressive stress from middle Pleistocene and because of weakening tendency of compressive stress from the south to the north along the southeastern coast of China, compressive stress in the south segments of the fault zone is strong enough to make the faults move in a way of reverse faulting and form reverse faulting landform while compressive stress in the north segments of the fault zone is so weak compared to isostacy force as to make the fault blocks restart isostacy uplift, move in a way of normal faulting and form normal faulting landform. Because of the approximate equilibrium between isostacy and horizontal compressive force in the middle segments of the fault zone, the faults have kept relatively steady state, resulting in weak contrast landform. In about late Pleistocene, a series of NW faults in the fault zone came into being or reactivated. On one hand, these NW faults, due to small angle between their strike and main compressing stress axis, have relatively small friction stress on the fault surfaces so that they could be moved more easily and often offset NE faults, forming obstacles to the strike slip of NE faults. On the other hand, the movements of NW faults release some strain energy so that they weaken compressive stress on the NE fault surfaces. Both of them reduce the activity of NE faults.

Key words: Enkai fault zone, active fault, regional stress field

摘要： 恩开断裂带是华南沿海地区重要的断裂带之一，长约160km，由3条大致平行展布的北东向断裂组成，其中苍城—海陵断裂(F₁)、均安断裂(F₂)倾向北西，鹤城—金鸡断裂(F₃)倾向南东。这3条断裂在第三纪主要作张性正断层运动。约在上新世末或第四纪初，断裂带受到北西西—南东东方向的强烈挤压，北东向断裂转为压性活动。然而，约在中更新世开始，挤压力逐渐减弱，加之，华南沿海地区水平挤压力有从南往北减弱的趋势，断裂带南段，挤压力仍足以使断裂上盘逆冲上升，形成逆断层地貌；继续往北，水平挤压力与重力大小相近，两者平衡使断层两盘处于动态稳定之中，差异升降弱；再往北，水平挤压力更弱，重力均衡使断裂再次发生正断层运动。约在晚更新世开始，区内的一系列北西向断裂形成或复活，由于与主压应力方向夹角小，断裂面摩擦力小而比北东向断裂更易发生错动，使北东向断裂多处被切。北西向断裂的活动一方面限制了北东向断裂的平移，另一方面释放了部分应变能，最终使北东向断裂的活动性降低。

关键词: 恩开断裂带, 活动断裂, 区域应力场