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From faults to flow: How do shear zones change at different depths?
In geology, a shear zone is a thin layer of strongly deformed crust or upper mantle, often formed by two walls of rock sliding past each other. In the upper crust, the rock appears brittle and shear zones exist in the form of faults. And in the lower crust and upper mantle, extreme pressure and temperature conditions make rocks ductile, able to slowly deform without breaking, like a blacksmith manipulating hot metal. Here the shear zone is wider and the ductile rock slowly flows to accommodate the relative motion between the rock walls on either side.
Shear zones run through different depth ranges, and the various rock types and structural features associated with them also change.
A shear zone can be considered a region of strong deformation, possessing a high strain rate compared to the surrounding rock, and is characterized by a length-to-width ratio greater than 5:1. Shear zones form a range of geological structures ranging from brittle shear zones (or faults) to brittle-ductile shear zones (or semi-brittle shear zones), to ductile-brittle to ductile shear zones. In a brittle shear zone, the deformation is concentrated on a narrow fault plane, whereas in a ductile shear zone, the deformation spreads through a wider area, with the deformation state changing from one side of the wall to the other.
This continuous change in structural geometry reflects the different deformation mechanisms dominant in the crust, ranging from brittle (fracture) to ductile (flow) with increasing depth.
The brittle-semibrittle transition is not depth-specific but occurs over a range of depths, the so-called alternating zone, where brittle fracture and plastic flow coexist. The reason for this phenomenon is due to the often heterogeneous composition of rocks, with different minerals responding differently to applied stress (for example, under stress, quartz plastically deforms before breaking, whereas feldspar does so relatively later). Therefore, mineral composition, particle size and previous structure will influence different rheological responses.
In addition, purely physical factors also influence transition depth, including geothermal gradients, ambient temperature, confinement and fluid pressures, bulk strain rates, and stress field directions. In Scholz's model (using Southern California's geothermal as a reference), the brittle-semibrittle transition begins at a depth of about 11 kilometers, when ambient temperatures are about 300°C. The next alternating zone extends to a depth of about 16 kilometers and has a temperature of about 360°C. Below this, only ductile shear zones exist.
Seismic zones are areas associated with the brittle domain, and large earthquakes sometimes damage alternating zones and even deeper plastic zones.
Deformation in shear zones results in the formation of characteristic textures and mineral assemblages that reflect the prevailing pressure-temperature conditions, flow type, direction of motion, and deformation history. Thus, shear zones are considered important structures in unraveling the history of specific strata. Starting from the Earth's surface, the following rock types are commonly found in shear zones: non-cohesive fault rocks (such as fault soils, fault hornbeams, and foliation soils); cohesive fault rocks such as Jingpo hornbeams. Rocks (former Galvainite, Galvainite, and Super Galvainite); glassy pseudo-lavas.
The width of the shear zone varies from the particle scale to the kilometer scale. Crustal-scale shear zones (ultra-large shear zones) can reach 10 kilometers in width and therefore show large displacements of tens to hundreds of kilometers. Brittle shear zones (faults) generally widen with increasing depth and displacement.
Because shear bands are characterized by concentration of strain, some degree of strain softening must occur, allowing the affected parent material to deform more plastically.
This softening phenomenon can be achieved through particle size reduction, geometric softening, reaction softening and fluid-related softening. Furthermore, for an object to become more ductile (quasi-plastic) and undergo continuous deformation (flow) without cracking, the following deformation mechanisms (particle scale) must be considered:
- Diffusion crawling (various types)
- Dislocation crawling (various types)
- Dynamic recrystallization pressure dissolution process
- Particle boundary sliding (superplasticity) and particle boundary area reduction
Due to the deep penetration of shear zones, they are common in all metamorphic phases. Brittle shear zones are almost ubiquitous in the upper crust, whereas ductile shear zones originate from greenschist phase conditions and are therefore restricted to metamorphic regions. Shear zones may occur in the following geotectonic settings:
- Turn environment - vertically downward: boundary slip zone
- Compressional environment - low-angle perturbation folds; subduction zones; pushing plates
- Extensional environment - low angle metamorphic core complex separation
The existence of shear zones is not limited by rock type or geological age. They usually do not exist in isolation, but form a fractal-scale connected network that reflects the dominant direction of underlying motion at that time.
The importance of shear zones is that they are major areas of weakness in the Earth's crust, sometimes extending into the upper mantle. They can be persistent features and often show evidence of multiple covering events. Materials can be transported up and down them, the most important of which is circulating water and dissolved ions, which can metamorphose host rocks and even re-fertilize mantle materials. Shear zones may also host economically viable mineralization, such as important gold deposits in Precambrian strata.
How important are shear zones to human activities in understanding changes in the Earth's interior and their further applications, both from a scientific and economic perspective?
