Tectonic Consequences of Lower Crustal Convective Instability

研究人员将使用二维有限元模型研究压力和温度相关的密度和粘度对致密下地壳镁铁质和超镁铁质累积层对流不稳定性发展的影响。以前的工作表明，假设下地壳和地幔的密度和粘度恒定，莫霍面温度必须是“热”的（例如，在1 GPa时为600 ℃），以便致密的下地壳变得对流不稳定，并在短时间尺度（约1000万年）沉入地幔。可变的密度和粘度将允许研究人员考虑（1）在不稳定性发展期间地壳物质的粘性夹带和（2）随时间变化的地壳地热。他们提出，（1）可能在被动边缘的俯冲起始中起重要作用，在被动边缘，缓慢生长的下地壳根对板块边缘施加负浮力。他们进一步提出，（2）可以解释青藏高原的地形和岩浆演化，在那里，一个最初致密，稳定的下地壳根变得对流不稳定，并下沉到地幔的辐射加热降低下地壳和岩石圈的粘度。

Matthew Jull EAR-0087706 The investigators will study the effect of pressure and temperature-dependent density and viscosity on the development of convective instabilities in dense lower-crustal mafic and ultramafic cumulates using a two-dimensional finite element model. Previous work has shown that with an assumption of constant density and viscosity for the lower crust and mantle, Moho temperatures must be ``hot'' (e.g. 600 C at 1 GPa) in order for dense lower crust to become convectively unstable and sink into the mantle on short timescales (~10 Myr). Variable density and viscosity will allow the investigators to consider (1) viscous entrainment of crustal material during the development of an instability and (2) time-varying crustal geotherms. They propose that (1) could play an important role in initiating subduction at passive margins, where a slowly growing lower crustal root exerts a negative buoyancy force on the plate margin. They further propose that (2) can explain the topographic and magmatic evolution of the Tibetan Plateau, where an initially dense, stable, lower crustal root becomes convectively unstable and sinks into the mantle as radiogenic heating decreases the viscosity of the lower crust and lithosphere.