论文部分内容阅读
穿过波特兰市区西部图拉丁山脉(波特兰山)的高分辨率重力测量,展现出以前根据地表地质学和航磁测绘识别出的断层的证据。本次重力测量是在1996年沿着长 4．5km的双孔隧道进行的,该隧道在当时的建设中及现在一直在波特兰市区与波特兰山以西的居民区之间通轻便火车。在隧道内的重力从西到东逐渐增加,这种情况反映了隧道位于西边的图拉丁盆地低密度沉积物与其东边高密度大部分隐蔽的始新世玄武岩之间。叠加在这一梯度之上的是几个阶梯状异常,我们将其解释为密度明显差异的岩石之间断续接触的证据。这些异常的最大值发生在西尔万克里克之下,这里存在一条以前测绘出的断层。另一异常发生在距西洞口1 200 m处,大约是隧道与航磁异常相交汇的部位,航磁异常与西尔万断层(原来称为奥特菲尔德断层)有关。以这些重力资料为依据的岩石学横剖面显示出阶梯状异常与陡倾逆断层相一致,虽然走滑位移也许同样重要。与上方地形洼地相对应的3个重力低异常可能反映存在着剪切带。几个中等地震(M≥ 3．5)1991年发生在隧道当前的位置,说明这些断层中的几条或波特兰山断层带的其他断层是地震活动断层。
High-resolution gravimetric measurements through the Tulatin Mountains (Portland Hills) in the western part of downtown Portland present evidence of faults previously identified from surface geology and aeromagnetic mapping. The gravimetric survey was conducted in 1996 along a 4.5-km-long two-hole tunnel that was constructed at the time and has now been portrayed between downtown Portland and residential areas west of Portland Hills train. The gradual increase of gravity in the tunnel from west to east reflects the low-density sediments of the Tulatin basin in the west of the tunnel and most of the concealed Eocene basalts with high density in the east. Superimposed on this gradient are several step-like anomalies, which we interpret as evidence of intermittent contact between rocks of markedly different density. The maximum of these anomalies occurred under Sylvain Creek where there is a previously mapped fault. Another anomaly occurred at a distance of 1 200 m from the west entrance, which was about the intersection of tunnel and aeromagnetic anomaly. The aeromagnetic anomaly was related to the Silvan fault (formerly the Ottfield fault). The petrological cross-section based on these gravity data shows that the staircase anomaly is consistent with the steep reverse thrust, although strike-slip displacement may be equally important. Three low gravitational anomalies corresponding to the topographic depressions above may reflect the presence of shear bands. Several moderate earthquakes (M ≥ 3.5) occurred at the current tunnel location in 1991, indicating that a few of these faults or other faults in the Portland Mountains fault zone are seismically active faults.