福州盆地活动断裂的探槽研究

STUDY ON ACTIVE FAULTS IN FUZHOU BASIN THROUGH TRENCHING

在前人对福州盆地及其周缘活动断裂 1/ 1万填图研究的基础上 ,通过室内航卫片判读、野外详细调查等方法 ,经过详细论证 ,在福州盆地及周缘 6条活动断裂上选择了 11个认为可能是断层通过的位置 ,进行了探槽开挖。大部分探槽只揭露到了基岩中的断层 ,有些探槽在开挖深度内既没有揭露到基岩 ,在第四系中也没有揭露到断层。对于没有揭露到断层的探槽 ,认为断层没有错断开挖深度内的第四纪地层 ,以探槽最底部的地层年代作为断层最新活动时代的上限。有些探槽揭露到了基岩中的断层 ,这些断层向上没有错断第四系 ,就以上覆第四系底部的时代作为断层最新活动时代的上限。只有在沿桐口 -洪山桥断裂开挖的探槽中 ,揭露出 2条断层 ,它们向上都错断了相同层位的第四纪地层 ,并被更新的第四系所覆盖。根据测年结果 ,这些断裂不是全新世活动断裂 ,其中 ,闽侯 -南屿断裂可能为晚更新世活动断裂 ,其它断裂晚更新世以来都已不活动。

On the basis of 1/10,000 active fault mapping in Fuzhou Basin, interpretation of aerial photos and detailed field investigation, 11 sites on six faults in the basin were selected for exploratory trenching. The main results of trenching are as follows: (1) Two trenches were dug across the Hutou-Miaopu Fault. One of the trenches reveals a fault plane, which is developed in lava bed and covered by Quaternary strata with an age of 75±6ka B.P. The other trench reveals that a series of joints are developed in tuff bed, but they do not offset the overlying strata of 119±10ka in age. These facts may indicate that the fault had ceased moving since late Pleistocene. (2) Two trenches were excavated across the Gushan Fault. No fault plane can be identified in one of the trenches, but the second trench reveals a minor fault plane, which is developed in granite and does not offset the overlying strata of 100±9ka in age. It can be concluded, therefore,that the fault had also ceased moving since late Pleistocene. (3) Two trenches were excavated across the Wuhushan Fault. In both trenches, the relationships between the volcanic rocks and Quaternary deposits, as well as the fault planes in volcanic rocks and overlying Quaternary deposits are well exposed. A series of minor fault planes developed in granitic rocks are well exposed in one trench, and these minor faults do not offset the overlying strata of 98±8.3ka in age. It is postulated, therefore,that the fault might have ceased moving since the early stage of late Pleistocene. (4) The micro-gemorphological expression of the Minhou-Nanyu Fault can be distinctly traced along the fault strand. A fault trough is observed at Heshangsi Temple, where two trenches were excavated to identify the two faults on the eastern and western sides of the trough. The trench logs show that the fault planes that are developed in granitic rocks do not offset the overlying Quaternary deposits of 29±2ka in age. Obviously, the Minhou-Nanyu Fault had ceased moving since the late stage of late Pleistocene. (5)One trench was excavated across the Tongkou-Hongshanqiao Fault. The trench log reveals two fault planes, which dislocate the same bed of Quaternary strata and are covered by the younger bed. The age of offset bed is dated to be 193±16ka B.P, and that of the overlying bed is 100±9ka. It is suggested, therefore, that the fault had also ceased moving since late Pleistocene. (6)A 4-5 meters deep trench was excavated across the Bayishuiku-Shanggan buried fault. Geophysical prospecting data suggested that the fault probably passes the trenching site. However, no fault is found in the trench, and the Quaternary deposits here are nearly horizontal. The age of bottom deposits is dated to be 39±3ka B.P. It seems that the Bayishuiku-Shanggan Fault had ceased its activity since the late stage of late Pleistocene. The afore-mentioned conclusions are merely drawn from the results of trenching, and more detailed study is needed in the future.