Collaborative Research: Constraining Mantle Rheology, Mantle Flow, and Crust/Mantle Coupling Beneath New Zealand

大陆动力学中的一个关键问题是：“系统的流变学是什么？”岩石是通过扩散蠕变(应变率与应力成正比)还是位错蠕变(应力加倍使应变率增加约10倍)在韧性区域变形？是下地壳相对较强，地壳和地幔之间存在有效的应变耦合，还是像经典的“果冻三明治”模型那样较弱？上地幔是像预期的干燥橄榄岩那样坚固，还是由于挥发分含量高而变弱？上地幔中的变形是沿地壳断层下的剪切带定位的，还是像薄粘性薄片模型那样分布的？这是一个利用地震各向异性来测量地幔应变的项目，当与矿物物理相结合时，将约束变形机制，从而约束流变学。PI坚持认为新西兰的走滑系统是研究这些关系的最佳地点，因为信号大而简单，地表受地质、GPS和过去4500万年来已知的相对板块运动的约束。该项目具体涉及：1)部署30台海底地震仪；2)使用各种技术(剪切波分裂、面波频散、Pn和Sn走时以及接收器函数)测量地震各向异性；3)计算可能导致各向异性的地幔有限应变场，受新西兰相对板块运动和观测应变的约束，并考虑各种温度和变形机制的垂直和横向分布；以及4)结合实验室、理论和地震学对各向异性的约束，以限制位错蠕动发生的条件。

0409564HagerOne of the critical questions in Continental Dynamics is: "What is the rheology of the system?" Do rocks deform in the ductile regime by diffusion creep (with strain rate proportional to stress), or by dislocation creep (where doubling the stress increases strain rates ~10 times)?. Is the lower crust relatively strong, with efficient coupling of strain between the crust and mantle, or weak, as in the classic "jelly sandwich" model? Is the upper mantle strong, as expected for dry peridotite, or weak, due to high volatile content? Is deformation in the upper mantle localized along shear zones beneath crustal faults, or distributed, as in thin viscous sheet models?This is a project that will use seismic anisotropy to measure strain in the mantle and that when combined with mineral physics, to constrain deformation mechanisms and therefore to constrain rheology. The PIs maintain that the strike-slip system in New Zealand is the best place to study these relationships because the signal is large and simple with constraints at the surface provided by geology, GPS, and known relative plate motions for the last 45 million years. Specifically the project involves: 1) Deployment of 30 Ocean Bottom Seismographs; 2) Measurement of seismic anisotropy using a variety of techniques (shear-wave splitting, surface wave dispersion, Pn and Sn travel times, and receiver functions); 3) Calculations of mantle finite strain fields that might be responsible for anisotropy, constrained by relative plate motions and observed strain in New Zealand and considering a variety of vertical and lateral distributions of temperature and deformation mechanisms; and 4) Combination of laboratory, theoretical and seismological constraints on anisotropy to bound the conditions under which dislocation creep occurs.