Geochemical, Mineralogical, and Fluid Processes in the Brittle-Plastic Transition of Continental Crust

IF 2.9 2区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS Geochemistry Geophysics Geosystems Pub Date : 2024-08-20 DOI:10.1029/2024GC011446
Gary Axen
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Abstract

The brittle-plastic transition (BPT), the strongest part of the crust, is critical to continental geodynamics but is poorly understood relative to simpler crust above and below. It is typically represented as a depth transition from brittle/frictional to plastic/viscous deformation controlled by temperature and pressure. Footwalls of low-angle normal faults (LANFs) exhumed through the BPT provide rock records that challenge this view. Three well-studied LANF footwalls are reviewed. All record geochemical, mineralogical and fluid-related controls on embrittlement, not just monotonic P-T decrease. Two quartz-rich examples record embrittlement at unexpectedly high T (≥450–500°C) that was modulated by wetting characteristics of fluids. One had an inverted BPT: brittle fracture beneath contemporaneous mylonites. In another study, a brittle LANF grew from plastic mylonites due to mineralogic changes that strengthened parts, causing initial frictional slip and cataclasis on weak planes that ultimately linked. In all, geologically abrupt small-scale processes controlled behavior at kilometer scales. Similar processes likely affect other tectonic settings and seismic cycles. Such processes offer fertile research opportunities in continental geodynamics; they will be increasingly tractable as computational abilities improve. Adaptive, multi-scale approaches including the effects of fluid-rock geochemistry and mineralogical changes on rock strength and deformation are needed. Thoughtful modeling approaches may yield key insights into the positive and negative feedbacks that are likely. Discontinuous deformation is probably needed explicitly along with exploration of initial and boundary conditions.

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大陆地壳脆塑转变过程中的地球化学、矿物学和流体过程
脆塑转换(BPT)是地壳最坚固的部分,对大陆地球动力学至关重要,但相对于上下较简单的地壳而言,人们对它的了解却很少。它通常表现为由温度和压力控制的从脆性/摩擦变形到塑性/粘性变形的深度转变。通过 BPT 挖掘出的低角度正断层(LANF)底壁提供的岩石记录对这一观点提出了挑战。本文回顾了三个经过深入研究的 LANF 脚墙。它们都记录了对脆化的地球化学、矿物学和流体相关控制,而不仅仅是单调的 P-T 下降。两个富含石英的实例记录了在出乎意料的高温度(≥450-500°C)下的脆化,这种脆化受流体润湿特性的影响。其中一个有倒 BPT:同时代的岩浆岩下有脆性断裂。在另一项研究中,由于矿物学上的变化,塑性麦饭石中长出了脆性LANF,这些脆性LANF强化了某些部分,导致最初的摩擦滑移和最终相连的薄弱平面上的崩塌。总之,地质上突变的小尺度过程控制了千米尺度的行为。类似的过程可能会影响其他构造环境和地震周期。这些过程为大陆地球动力学提供了丰富的研究机会;随着计算能力的提高,这些过程将越来越容易理解。需要采用自适应的多尺度方法,包括流体-岩石地球化学和矿物学变化对岩石强度和变形的影响。深思熟虑的建模方法可能会对可能出现的正反馈产生重要影响。在探索初始条件和边界条件的同时,可能还需要明确不连续的变形。
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来源期刊
Geochemistry Geophysics Geosystems
Geochemistry Geophysics Geosystems 地学-地球化学与地球物理
CiteScore
5.90
自引率
11.40%
发文量
252
审稿时长
1 months
期刊介绍: Geochemistry, Geophysics, Geosystems (G3) publishes research papers on Earth and planetary processes with a focus on understanding the Earth as a system. Observational, experimental, and theoretical investigations of the solid Earth, hydrosphere, atmosphere, biosphere, and solar system at all spatial and temporal scales are welcome. Articles should be of broad interest, and interdisciplinary approaches are encouraged. Areas of interest for this peer-reviewed journal include, but are not limited to: The physics and chemistry of the Earth, including its structure, composition, physical properties, dynamics, and evolution Principles and applications of geochemical proxies to studies of Earth history The physical properties, composition, and temporal evolution of the Earth''s major reservoirs and the coupling between them The dynamics of geochemical and biogeochemical cycles at all spatial and temporal scales Physical and cosmochemical constraints on the composition, origin, and evolution of the Earth and other terrestrial planets The chemistry and physics of solar system materials that are relevant to the formation, evolution, and current state of the Earth and the planets Advances in modeling, observation, and experimentation that are of widespread interest in the geosciences.
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