Variations in the P-T-t of deformation in a crustal-scale shear zone in metagranite
Variations in the P-T-t of deformation in a crustal-scale shear zone in metagranite
复制标题
变花岗岩地壳尺度剪切带变形 P-T-t 的变化
DOI：
10.1029/2020gc009384
复制
发表时间：
2020
影响因子：
3.5
通讯作者：
Platt, J.P.
中科院分区：
文献类型：
--
作者：
Cawood, T.K.;Platt, J.P.
地壳尺度剪切带的变形发生在一系列压力-温度-时间（P-T-t）条件下，这既是因为它们可能是垂直广泛的结构，同时影响从下地壳到地表的物质，也因为任何特定体积的岩石变形的条件随着时间的推移而演变，因为该物质是由断层活动平流输送的。提取此类 P-T-t 记录具有挑战性，因为结构可能会因渐进变形而被套印。此外，特别是花岗岩可能缺乏符合传统变质岩石学和岩石年代学的同运动矿物组合。我们通过研究阿尔卑斯山中部的正常意义辛普伦剪切带（SSZ）来克服这些挑战，其中挖出的下盘中的应变局部化导致剪切带逐渐变窄，导致从远至下盘的高T剪切到与上盘相邻的低T剪切的分区。石英中钛和多晶硅中硅温压计产生变形 P-T 条件，因为两者都是同步运动重置的，尽管剪切变花岗岩缺乏典型的石油测年仪，但我们通过将计算的变形温度与已发布的热年代学年龄进行比较来估计变形时间。暴露的 SSZ 下盘保留了折返过程中逆行变形的证据，从约 24.5 Ma 时略低于角闪岩相条件（约 490°C，6.7 kbar）到约 11.5 Ma 时较低的绿片岩相条件（约 305°C，1.5 kbar），随后发生脆性断层作用。我们的估计值落在由逆行延性变形的最大和最小条件的独立约束提供的 P-T-t 括号内，并且与估计 P-T 的替代方法相当好。
Deformation in crustal‐scale shear zones occurs over a range of pressure‐temperature‐time (P‐T‐t) conditions, both because they may be vertically extensive structures that simultaneously affect material from the lower crust to the surface, and because the conditions at which any specific volume of rock is deformed evolve over time, as that material is advected by fault activity. Extracting such P‐T‐t records is challenging, because structures may be overprinted by progressive deformation. In addition, granitic rocks, in particular, may lack syn‐kinematic mineral assemblages amenable to traditional metamorphic petrology and petrochronology. We overcome these challenges by studying the normal‐sense Simplon Shear Zone (SSZ) in the central Alps, where strain localization in the exhuming footwall caused progressive narrowing of the shear zone, resulting in a zonation from high‐T shearing preserved far into the footwall, to low‐T shearing adjacent to the hanging wall. The Ti‐in‐quartz and Si‐in‐phengite thermobarometers yield deformation P‐T conditions, as both were reset syn‐kinematically, and although the sheared metagranites lack typical petrochronometers, we estimate the timing of deformation by comparing our calculated deformation temperatures to published thermochronological ages. The exposed SSZ footwall preserves evidence for retrograde deformation during exhumation, from just below amphibolite‐facies conditions (∼490°C, 6.7 kbar) at ∼24.5 Ma, to lower greenschist‐facies conditions (∼305°C, 1.5 kbar) at ∼11.5 Ma, with subsequent slip taken up by brittle faulting. Our estimates fall within the P‐T‐t brackets provided by independent constraints on the maximum and minimum conditions of retrograde ductile deformation, and compare reasonably well to alternative approaches for estimating P‐T.