Manon Carpenter, Sandra Piazolo, Tim Craig, Tim J. Wright
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引用次数: 0
Abstract
Understanding mid-crustal deformation is vital for determining the spatial and temporal distribution of strain localisation, with implications for upper-crust deformation including seismic hazard. Here, we conduct fieldwork and microstructural and minerochemical analyses on the amphibolite-facies, 100-m-wide Upper Badcall shear zone in northwest Scotland, which deforms initially anhydrous quartzofeldspathic gneiss and a mafic dyke. We show that with increasing strain, m-scale strain distribution and mineral chemistry become increasingly homogeneous, while hydrous phases and syntectonic quartz veins become more abundant. With increasing strain there is an overall increase in grain size, grain boundary alignment and shape preferred orientation in amphibole, plagioclase and quartz. Only amphibole and large grained quartz exhibit crystallographic preferred orientation in strained areas. Subtle microstructures that may be overlooked elsewhere, particularly in felsic gneiss, indicate dominant activity of dissolution–precipitation creep and equivalent rheological weakening in both mafic and felsic rocks. We propose that brittle fractures, now preserved as syntectonic quartz veins, allowed localised fluid-infiltration in previously anhydrous crust. This triggered local retrogressive reactions and introduced sufficient grain boundary fluid for deformation to favour dissolution–precipitation creep over dislocation creep. Our study suggests that dissolution–precipitation creep may be more dominant in mid- to lower-crustal localised zones of deformation than previously thought.
期刊介绍:
The Journal of Structural Geology publishes process-oriented investigations about structural geology using appropriate combinations of analog and digital field data, seismic reflection data, satellite-derived data, geometric analysis, kinematic analysis, laboratory experiments, computer visualizations, and analogue or numerical modelling on all scales. Contributions are encouraged to draw perspectives from rheology, rock mechanics, geophysics,metamorphism, sedimentology, petroleum geology, economic geology, geodynamics, planetary geology, tectonics and neotectonics to provide a more powerful understanding of deformation processes and systems. Given the visual nature of the discipline, supplementary materials that portray the data and analysis in 3-D or quasi 3-D manners, including the use of videos, and/or graphical abstracts can significantly strengthen the impact of contributions.