Institute of Geophysics, Victoria University, Salamanca Road, Wellington 6140, New Zealand.
Institute of Geological and Nuclear Sciences, 1 Fairway Drive, Lower Hutt 5010, New Zealand.
Nature. 2015 Feb 5;518(7537):85-8. doi: 10.1038/nature14146.
Plate tectonics successfully describes the surface of Earth as a mosaic of moving lithospheric plates. But it is not clear what happens at the base of the plates, the lithosphere-asthenosphere boundary (LAB). The LAB has been well imaged with converted teleseismic waves, whose 10-40-kilometre wavelength controls the structural resolution. Here we use explosion-generated seismic waves (of about 0.5-kilometre wavelength) to form a high-resolution image for the base of an oceanic plate that is subducting beneath North Island, New Zealand. Our 80-kilometre-wide image is based on P-wave reflections and shows an approximately 15° dipping, abrupt, seismic wave-speed transition (less than 1 kilometre thick) at a depth of about 100 kilometres. The boundary is parallel to the top of the plate and seismic attributes indicate a P-wave speed decrease of at least 8 ± 3 per cent across it. A parallel reflection event approximately 10 kilometres deeper shows that the decrease in P-wave speed is confined to a channel at the base of the plate, which we interpret as a sheared zone of ponded partial melts or volatiles. This is independent, high-resolution evidence for a low-viscosity channel at the LAB that decouples plates from mantle flow beneath, and allows plate tectonics to work.
板块构造成功地将地球表面描述为一个由移动的岩石圈板块组成的镶嵌体。但目前尚不清楚板块底部的岩石圈-软流圈边界 (LAB) 会发生什么情况。转换远震波已经很好地对 LAB 进行了成像，其 10-40 公里的波长控制着结构分辨率。在这里，我们使用爆炸产生的地震波（约 0.5 公里的波长）对新西兰北岛下方俯冲的大洋板块底部形成了高分辨率图像。我们的 80 公里宽的图像基于 P 波反射，显示出在约 100 公里深处深度约为 15°的倾斜、突然的地震波速转换（小于 1 公里厚）。边界与板块顶部平行，地震属性表明穿过该边界的 P 波速度至少下降了 8±3%。大约 10 公里深的平行反射事件表明，P 波速度的下降仅限于板块底部的一个通道，我们将其解释为部分熔融或挥发物堆积的剪切带。这是 LAB 处低粘度通道的独立、高分辨率证据，它使板块与下方地幔流解耦，从而使板块构造得以发挥作用。
