周向应力在地壳运动中的作用

地壳运动的驱动力一直存在争议。目前虽然提出了很多假说,但这些假说所描述的驱动力数量级均较小,不足以推动地壳运动;另外,大量实际地应力测量表明,水平主压应力在三个地应力分量中最大,被看作地壳“异常”压力,其机理也没有统一的认识。因此,有必要弄清楚地壳运动的动力来源是什么及为什么会出现这种水平应力占主导的现象。受背斜构造或石拱桥的侧向支撑的启发,通过地球模型受力分析得出,地壳作为球壳在自重下会相互挤压,在圆周方向产生很强的周向应力。周向应力大于重力,且由重力派生,和实测的地应力特征是一致的。推测该应力在20 km深处约为900 MPa,足以驱动板块运动(>500 MPa)。因软流圈是可流动的,其上面的岩石圈只要存在薄弱带,该应力就会释放,板块之间从而产生相对运动。整个洋壳和拱桥类似,在该力的作用下,会在俯冲带处下插至陆壳深部,俯冲带就是岩石圈的薄弱区,它因此会承担部分甚至全部洋壳的重量。最后提出,没有独立于重力的、可独立起作用的构造力或碰撞力,周向应力是地壳运动的唯一具有足够数量级的驱动力。

Role of circumferential-direction stress in crustal movement

It is still controversial what drives crustal movement. Although there are many competing theories, the magnitude of the driving force described in these theories is too small to drive crustal movement. In addition, many in-situ stress measurements showed that the horizontal principal compressive stress is the largest of the three in-situ stress components and considered as the crustal “anomaly” pressure, however, there is no unified mechanistic explanation for this observation. Therefore, it is necessary to find out what the source of the driving force is and why such horizontal stress dominants. Inspired by anticlinal structures and stone arch bridge that are supported by lateral force, and through stress analysis of the Earth model, we concluded that the crust, as a spherical shell, can experience lateral extrusion from its own weight, which acts as circumferential-direction stress. Consistent with the in-situ measured stress characteristics, this circumferential-direction stress is derived from and greater than gravity. We estimated that the stress is about 900 MPa at 20 km depth, enough for driving plate movement (needs at least 500 MPa). Because, as asthenosphere can flow, stress will be released as long as there is a weak zone in the lithosphere above it, resulting in relative motion between plates. The whole oceanic crust behaves like an arch bridge: under circumferential-direction stress, it can insert into the deep part of the continental crust at the subduction zone, the weak area of the lithosphere, so the subduction zone bears part or even all of the weight of seafloor. Finally, we proposed that there is no structural force or collision force independent of gravity that can work independently, the circumferential-direction stress is the only driving force strong enough for crustal movement.