Rheological controls on the emplacement of extremely high-grade ignimbrites
Rheological controls on the emplacement of extremely high-grade ignimbrites
复制标题
对极高品位熔结岩就位的流变控制
DOI：
10.1130/g34519.1
复制
发表时间：
2013
期刊：
影响因子：
5.8
通讯作者：
A. Whittington
中科院分区：
文献类型：
--
作者：
G. Robert;G. Andrews;Jiyang Ye;A. Whittington
许多大的爆炸性喷发产生了极高品位的熔岩状焊接熔岩，其体积通常为101-103千米。然而，对这些不寻常沉积物的物理性质以及它们的迁移和沉积机制的了解还不完整。爱达荷州(美国西部)类熔岩和流变学的Grey‘s Landing火成岩提供了丰富的野外证据，支持在火山碎屑密度流和矿床之间的界面上形成一条2米厚的瞬变亚水平韧性剪切带，所有堆积的火山碎屑物质都通过该剪切带。在这里，我们使用流变实验和热力学模拟相结合的方法来检验同沉积剪切带模型。结果表明，共沉积焊接和延性流动可以在一个非常有限的温度-应变速率空间内实现，较高的定位温度(≥850°C)有利于快速变形。由于考虑了应变加热，韧性变形的范围显著扩大，这使得剪切带内的温度持续上升高达250°C，并有助于解释熔岩相的巨大范围和在极高品位的点火沸石中记录的强烈流变学。对流变性熔岩中应变加热的认识表明，大的火山碎屑密度流可能在热的基底上传播，潜在地比密度流本身更热。
Extremely high-grade, lava-like welded ignimbrites are produced by many large explosive eruptions with volumes typically 101–103 km3. However, understanding of the physical properties of these unusual deposits, and their transport and depositional mechanisms, is incomplete. The lava-like and rheomorphic Grey’s Landing ignimbrite, Idaho (western United States), provides abundant field evidence supporting the upward migration of a transient, <2-m-thick, sub-horizontal ductile shear zone at the interface between the pyroclastic density current and the deposit, through which all of the aggrading pyroclastic material passed. Here we use a combination of rheological experiments and thermo-mechanical modeling to test the syndepositional shear zone model. We show that syndepositional welding and ductile flow are achievable within a very restricted field of likely temperature–strain rate space, where rapid deformation is favored by higher emplacement temperatures (≥850 °C). The field of ductile deformation is broadened significantly by accounting for strain heating, which permits a sustained temperature increase of up to 250 °C within the shear zone and helps to explain the enormous extents of lava-like lithofacies and the intense rheomorphism recorded in extremely high-grade ignimbrites. Recognition of strain heating within rheomorphic ignimbrites suggests that large pyroclastic density currents may travel over a hot substrate, potentially hotter than the density current itself.
影响因子：
4.9
作者：
G. Andrews;M. Branney
通讯作者：
G. Andrews;M. Branney