Tectonic evolution of the Tualatin basin, northwest Oregon, as revealed by inversion of gravity data
Tectonic evolution of the Tualatin basin, northwest Oregon, as revealed by inversion of gravity data
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重力数据反演揭示俄勒冈州西北部图拉丁盆地的构造演化
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发表时间：
2014
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通讯作者：
R. Blakely
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作者：
D. McPhee;V. Langenheim;R. Wells;R. Blakely
图拉丁盆地位于波特兰（美国俄勒冈州）以西，与普吉特-威拉米特低地沿线的 110 毫加仑重力低压重合。新的重力测量（n = 3000）揭示了三维（3-D）地下几何形状，表明早期发育为断层边界的拉分盆地。强大的西北向重力梯度与形成图拉丁盆地西南边界的盖尔斯溪断层重合。沿波特兰山东北边缘的断层和沿东南盆地边缘的东北走向的舍伍德断层也与重力梯度有关，但幅度较小。重力低反映了盆地充填物与构成Siletz地体的基底的镁菲结壳之间的较大密度对比。重力数据反演表明，图拉丁盆地深约 6 公里，因此比该盆地中新世哥伦巴河玄武岩群 (CRBG) 1 公里最大深度深 6 倍，这意味着该盆地包含数公里的低密度前 CRBG 沉积物，因此主要形成于 CRBG 侵位之前的 15 Ma 之前。盆地的形状以及沿西南和东北边缘平行、线性盆地边界断层的位置表明图拉丁盆地起源于拉开的菱形裂隙。图拉丁盆地的前 CRBG 延伸与海岸山脉其他地方记录的始新世晚期延伸事件一致。图拉丁盆地目前的褶皱和冲断几何形状是新近纪压缩的结果，叠加在祖先的拉分盆地之上。目前的 3-D 盆地几何形状可能意味着沿盆地边缘的地面震动更强，特别是沿大波特兰地区下方图拉丁盆地隐蔽的东北边缘。
The Tualatin basin, west of Portland (Oregon, USA), coincides with a 110 mGal gravity low along the Puget-Willamette lowland. New gravity measurements (n = 3000) reveal a three-dimensional (3-D) subsurface geometry suggesting early development as a fault-bounded pull-apart basin. A strong northwest-trending gravity gradient coincides with the Gales Creek fault, which forms the southwestern boundary of the Tualatin basin. Faults along the northeastern margin in the Portland Hills and the northeast-trending Sherwood fault along the southeastern basin margin are also associated with gravity gradients, but of smaller magnitude. The gravity low reflects the large density contrast between basin fill and the mafi ccrust of the Siletz terrane composing basement. Inversions of gravity data indicate that the Tualatin basin is ∼6 km deep, therefore 6 times deeper than the 1 km maximum depth of the Miocene Columba River Basalt Group (CRBG) in the basin, implying that the basin contains several kilometers of low-density pre-CRBG sediments and so formed primarily before the 15 Ma emplacement of the CRBG. The shape of the basin and the location of parallel, linear basin-bounding faults along the southwest and northeast margins suggest that the Tualatin basin originated as a pull-apart rhombochasm. Pre-CRBG extension in the Tualatin basin is consistent with an episode of late Eocene extension documented elsewhere in the Coast Ranges. The present fold and thrust geometry of the Tualatin basin, the result of Neogene compression, is superimposed on the ancestral pull-apart basin. The present 3-D basin geometry may imply stronger ground shaking along basin edges, particularly along the concealed northeast edge of the Tualatin basin beneath the greater Portland area.