Instability of a lithospheric step beneath western North Island, New Zealand
Instability of a lithospheric step beneath western North Island, New Zealand
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DOI：
10.1130/g34028.1
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发表时间：
2013-04
期刊：
影响因子：
5.8
通讯作者：
T. Stern;G. Houseman;M. Salmon;L. Evans
中科院分区：
文献类型：
--
作者：
T. Stern;G. Houseman;M. Salmon;L. Evans
不同厚度的岩石圈经常因大陆转换边界上的运动而并列在一起。这种密度阶跃变化的边界可能会引发重力不稳定，因为在正常地幔岩石圈终止于密度较低的软流圈地幔的地方产生了侧向压力梯度。在这里，我们展示了对于岩石圈粘度的合理值，较厚的地幔岩石圈将滴入较低密度的软流圈的机制。随着地幔的变形，它也会逐渐增厚，然后减薄上覆的地壳，产生一种地形波，随着地幔岩石圈的移动而迁移。在新西兰北岛西部，地球物理数据确定了跨越塔拉纳基-鲁阿佩胡线的岩石圈陡峭台阶，地质观测提供了过去12英里内向南传播的隆起和沉降史。如果最上层岩石圈地幔的粘性为∼5×10 2 0Pa⋅S，则观测到的波由北向南迁移的速度(∼30 mm/yr)、波长(∼2 5 0 km)和幅度(∼±1 km)与地幔岩石圈的逐步剥离所引起的∼相一致，为这一基本地质过程提供了迄今最清楚的例子之一。
Lithospheres of different thicknesses are often juxtaposed by movement on a continental-transform boundary. Such a boundary with a step change in densities may trigger a gravitational instability as lateral pressure gradients are created where normal mantle lithosphere terminates against less dense asthenospheric mantle. Here we show, for plausible values of the lithospheric viscosity, a mechanism by which the thicker mantle lithosphere will drip off into the lower density asthenosphere. As the mantle deforms it also progressively thickens and then thins the overlying crust, creating a topographic wave that migrates in concert with the removal of mantle lithosphere. Within western North Island, New Zealand, geophysical data define a sharp lithospheric step across the Taranaki-Ruapehu line, and geological observations provide a history of uplift and subsidence that has propagated southward in the past 12 m.y. The rate of observed north to south migration of the wave (∼30 mm/yr), its wavelength (∼250 km), and amplitude (∼±1 km) are compatible with it being caused by progressive removal of mantle lithosphere, if the viscosity of the uppermost lithospheric mantle is ∼5 × 10 20 Pa⋅s, providing one of the clearest examples yet of this fundamental geological process.