Geodynamic constraints on the nature of the asthenosphere

对软流圈性质的地球动力学约束

• 批准号: 0911644
• 负责人: Paul Hall
• 金额: $ 17.85万
• 依托单位: Trustees of Boston University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-10-01 至 2012-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0911644&HistoricalAwards=false
• 关键词: Geodynamic; constraints; nature; asthenosphere

The Earth's rigid outer shell (the lithosphere) is broken into discrete units known as tectonic plates. These plates move about on the surface of the Earth in response to convection in the Earth's deep interior (the mantle). The motions of the plates ultimately give rise to phenomena such as earthquakes at the Earth's surface. The movement of the plates is made possible in part by a layer of relatively weak, easily deformable rock, known as the asthenosphere, which underlies the plates. While we have some appreciation of the important role the asthenosphere plays in accommodating the motion of tectonic plates, we do not yet understand what makes the rocks within the asthenosphere weak. A number of factors can influence the strength of rock, including its temperature, its composition (in particular the presence of volatile elements like water), and the presence of small amounts of magma within the rock. We will use computer simulations of convection in the asthenosphere beneath oceanic plates to determine which of these three mechanisms is responsible for the low viscosities in the asthenosphere. Distinguishing between these different mechanisms will improve our understanding of how and why plate tectonics developed on Earth but not on other terrestrial planets in the solar system such as Mars and Venus. It will also contribute to our understanding of how the Earth has evolved through time.The oceanic upper mantle is characterized by low seismic velocities, high seismic attenuation and high electrical conductivity at depths of ~80-200km. This corresponds roughly with the layer of relatively low viscosity within the upper mantle known as the asthenosphere. The close spatial correlation between these features suggests that a single mechanism may be responsible for all of them, and three competing explanations have been hypothesized: 1) variations in the physical properties of dry, melt-free peridotite with temperature and pressure; 2) the presence of volatiles (chiefly water); and 3) the presence of small degrees of partial melt. To date, geophysical observations alone have been unable to definitively identify which of these hypotheses is correct. However, it may be possible to gain further insight by considering their geodynamic implications. In particular, each proposed mechanism invokes a characteristic distribution of both volatiles and melts, which themselves influence the viscosity and density of the mantle, in addition to its seismic and electrical properties. Differences in viscosity and density structure will lead to differences in the development of convective instabilities at the base of the lithosphere, and thereby heat flow, bathymetry and seismic structure. We propose to test the three hypotheses regarding the nature of the asthenospheric mantle by developing regional-scale 3-D numerical models of mantle flow beneath oceanic plates that explicitly incorporate the effects of volatiles and melting. Results from the numerical experiments will be compared to a variety of geophysical and seismic observations to constrain the viability of each of the hypothesized mechanisms. In particular, model predicted temperature, composition and porosity will be mapped to seismic velocity and attenuation using empirical relationships derived from laboratory measurements, allowing direct comparison between the numerical experiments and global and regional seismic models. Ultimately the successful hypothesis will be identified as the one that best matches the constraints.

地球坚硬的外壳（岩石圈）被分解成离散的单元，称为构造板块。 这些板块在地球表面移动，以响应地球内部深处（地幔）的对流。板块的运动最终导致了地球表面的地震等现象。板块运动的部分原因是板块下面有一层相对较弱、容易变形的岩石，称为软流圈。虽然我们对软流圈在适应构造板块运动方面所起的重要作用有所了解，但我们还不了解是什么使软流圈内的岩石变弱。许多因素可以影响岩石的强度，包括它的温度，它的成分（特别是挥发性元素如水的存在），以及岩石中少量岩浆的存在。我们将使用计算机模拟海洋板块下的软流圈中的对流，以确定这三种机制中的哪一种是软流圈中低粘度的原因。 区分这些不同的机制将提高我们对板块构造如何以及为什么在地球上发展而不是在太阳系中的其他类地行星（如火星和金星）上发展的理解。 海洋上地幔在80- 200 km深度具有低地震速度、高地震衰减和高电导率的特征。 这大致对应于上地幔中粘度相对较低的层，称为软流圈。 这些特征之间的密切空间相关性表明，一个单一的机制可能是所有这些，和三个相互竞争的解释已被假设：1）在干燥的，熔体自由橄榄岩的物理性质与温度和压力的变化; 2）挥发物（主要是水）的存在;和3）存在的小程度的部分熔融。到目前为止，单靠地球物理观测还无法确定这些假设中哪一个是正确的。然而，通过考虑它们的地球动力学影响，也许可以获得进一步的见解。 特别是，每一个提出的机制调用的挥发分和熔体，这本身影响地幔的粘度和密度，除了其地震和电气性能的特征分布。 粘性和密度结构的差异将导致岩石圈底部对流不稳定性发展的差异，从而导致热流、测深和地震结构的差异。 我们建议测试三个假设的性质，软流圈地幔开发区域尺度的3-D数值模型的地幔流动下的海洋板块，明确纳入挥发分和熔融的影响。 数值实验的结果将与各种地球物理和地震观测进行比较，以限制每个假设机制的可行性。 特别是，模型预测的温度，成分和孔隙度将映射到地震速度和衰减使用实验室测量得出的经验关系，允许直接比较数值实验和全球和区域地震模型。 最终，成功的假设将被确定为最符合约束条件的假设。