The behaviour of the lithosphere on seismic to geologic time-scales and its implications for landscape evolution and mantle dynamics

岩石圈是我们生活的地球最外层坚硬的岩石，由许多大板块和几个较小的板块组成，这些板块相互之间以及下面的深地幔都在运动。根据板块构造理论，板块是刚性的，变形仅限于其边界。但是我们怎么知道板的刚性呢？主要的证据来自对岩石圈变形方式的研究，这些变形方式是对放置在岩石圈表面或底部的载荷作出反应的。这种载荷的例子包括地震、冰原的盛衰、火山的生长和衰亡以及沉积物的沉积、崩塌和滑动。虽然这些载荷是在一定的时间和空间尺度上施加的，因此只能提供岩石圈行为的“快照”，但它们为我们提供了一个有用的见解，让我们了解板块实际上是如何响应过去和有趣的未来载荷而变形的。先前对海底火山负荷的研究表明，岩石圈随着年龄的增长而冷却和下沉，其强度会增加。形成于年轻海底（即洋中脊嵴附近）的火山位于弱岩石圈上，而形成于老海底（即洋中脊侧翼）的火山位于强岩石圈上。然而，同样的研究表明，当火山对海底的特定热年龄进行负荷时，下面的岩石圈就会松弛，从而随着负荷年龄而减弱。因此，在加强岩石圈的热冷却和削弱岩石圈的负荷引起的应力松弛之间似乎存在一种竞争。岩石圈的强度是控制沉积盆地“构造”以及伸展、挤压和走滑断裂环境下发育的构造样式的基本参数。因此，我们建议在此汇编所有与岩石圈强度相关的现场和实验室观测结果，然后构建一个计算机模型，预测岩石圈如何响应地震（即短）到地质（即长）时间尺度的载荷。我们的模型将考虑由于冷却而增强和由于应力松弛而减弱的影响，对地质过程，特别是景观演化和地幔动力学具有重要意义。例如，在景观演化中，我们的目标是使用该模型来预测冰载荷和卸载近场发生的变形，在那里，先前的工作表明，岩石圈的强度在控制基岩几何形状方面起着主要作用，而基岩几何形状反过来又影响冰和融水体积的估计。当我们的初步模型显示岩石圈的强度可以通过基准面和陆架坡度的变化来影响河流的流向时，我们还将使用新模型来评估在冰期/间冰期旋回期间泥沙和水的装载和卸载对地形发展的影响。这项工作的重点是岩石圈以及它如何与上面的冰冻圈、水圈和大气以及下面的软流圈相互作用，这对社会和科学都有意义。例如，大负荷地区岩石圈的变形是一个重要的应力来源，它可以控制断层和地震的位置，夏威夷岛下面的情况似乎就是这样。此外，在冰、火山和沉积物装卸岩石圈期间和之后发生的地壳和地幔的垂直运动都是局部海平面变化的潜在因素，在评估全球海平面及其对过去和未来环境变化的影响时，需要考虑到这一点。

The lithosphere, which is the strong rocky outermost layer of the Earth on which we live, is made up of a number of large plates and several smaller ones which are in motion with respect to each other and the deep mantle below. According to plate tectonic theory, the plates are rigid and deformation is limited to their boundaries. But how do we know how rigid the plates are? The principal evidence has come from studies of the way the lithosphere deforms in response to loads that have been emplaced on its surface or base. Examples of such loads include earthquakes, the waxing and waning of ice sheets, the growth and decay of volcanoes and the deposition, slumping and sliding of sediment. While these loads have been applied over a range of temporal and spatial scales and so only provide a "snapshot" of lithosphere behaviour, they have provided us with a useful insight into how the plates might actually deform in response to past and, interestingly, future loads.Previous studies at submarine volcano loads suggest that as the lithosphere cools and subsides with age its strength increases. Volcanoes that form on young seafloor (i.e. on near a mid-ocean ridge crest) are emplaced on weak lithosphere while volcanoes emplaced on old seafloor (i.e. on a ridge flank) are emplaced on strong lithosphere. The same studies reveal, however, that when a volcano loads a particular thermal age of seafloor the underlying lithosphere relaxes such that it weakens with load age. There therefore appears to be a competition between thermal cooling which strengthens the lithosphere and a load-induced stress relaxation that weakens it. The strength of the lithosphere is a fundamental parameter that controls the "architecture" of sedimentary basins and the structural styles that develop in extensional, compressional and strike-slip faulting settings. We propose here therefore to compile all the available field and laboratory observations that relate to lithospheric strength and then construct a computer model that predicts how the lithosphere responds to loads on seismic (i.e. short) through geologic (i.e. long) time-scales. Our model, which will incorporate the effects of both strengthening due to cooling and weakening due to stress relaxation, has major implications for geological processes, especially landscape evolution and mantle dynamics. In landscape evolution, for example, we aim to use the model to predict the deformation that occurs in the near-field of ice loads and unloads where previous work has shown that the strength of the lithosphere plays a major role in controlling bedrock geometry, which in turn influences estimates of ice and melt-water volume. We will also use the new model to evaluate the effects of sediment and water loading and unloading on the development of topography during glacial/inter-glacial cycles when our preliminary models show that the strength of the lithosphere can influence the course of rivers through changes in base-level and shelf grade. The proposed work, by focussing on the lithosphere and how it interacts with the cryosphere, hydrosphere and atmosphere above and the asthenosphere below, is of societal as well as scientific interest. The deformation of the lithosphere in the region of large loads, for example, is an important source of stress which may control the location of faults and earthquakes, as appears to be the case beneath Hawaii Island. Moreover, the vertical motions of the crust and mantle that occur during and following loading and unloading of the lithosphere by ice, volcanoes and sediment are all potential contributors to local sea-level change that needs to be taken into account when assessing global sea-level and its impact on past and future environmental change.