Transpression and transtension are strike-slip deformations that deviate from simple shear because of a component of, respectively, shortening or extension orthogonal to the deformation zone. These three-dimensional non-coaxial strains develop principally in response to obliquely convergent or divergent relative motions across plate boundary and other crustal deformation zones at various scales. The basic constant-volume strain model with a vertical stretch can be modified to allow for volume change, lateral stretch, an oblique simple shear component, heterogeneous strain and steady-state transpression and transtension. The more sophisticated triclinic models may be more realistic but their mathematical complexity may limit their general application when interpreting geological examples. Most transpression zones generate flattening (k < 1) and transtension zones constrictional (k > 1) finite strains, although exceptions can occur in certain situations. Relative plate motion vectors, instantaneous strain (or stress) axes and finite strain axes are all oblique to one another in transpression and transtension zones. Kinematic partitioning of non-coaxial strike-slip and coaxial strains appears to be a characteristic feature of many such zones, especially where the far-field (plate) displacement direction is markedly oblique (<20 ~ to the plate or deformation zone boundary. Complex foliation, lineation and other structural patterns are also expected in such settings, resulting from switching or progressive rotation of finite strain axes. The variation in style and kinematic linkage of transpressional and transtensional structures at different crustal depths is poorly understood at present but may be of central importance to understanding the relationship between deformation in the lithospheric mantle and crust. Existing analyses of obliquely convergent and divergent zones highlight the importance of kinematic boundary conditions and imply that stress may be of secondary importance in controlling the dynamics of deformation in the crust and lithosphere.
Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract 16Recent friction experiments carried out under upper crustal P-T conditions have shown that 17 microstructures typical of high temperature creep develop in the slip zone of experimental 18 faults. These mechanisms are more commonly thought to control aseismic viscous flow and 19 shear zone strength in the lower crust/upper mantle. In this study, displacement-controlled 20 experiments have been performed on carbonate gouges at seismic slip rates (1 ms -1 ), to 21 investigate whether they may also control the frictional strength of seismic faults at the higher 22 strain rates attained in the brittle crust. At relatively low displacements (< 1cm) and 23 temperatures (≤ 100 °C), brittle fracturing and cataclasis produce shear localisation and grain 24 size reduction in a thin slip zone (150 µm). With increasing displacement (up to 15 cm) and 25 2 temperatures (T up to 600 °C), due to frictional heating, intracrystalline plasticity mechanisms 26 start to accommodate intragranular strain in the slip zone, and play a key role in producing 27 nanoscale subgrains (≤ 100 nm). With further displacement and temperature rise, the onset of 28 weakening coincides with the formation in the slip zone of equiaxial, nanograin aggregates 29 exhibiting polygonal grain boundaries, no shape or crystal preferred orientation and low 30 dislocation densities, possibly due to high temperature (> 900 °C) grain boundary sliding 31 (GBS) deformation mechanisms. The observed micro-textures are strikingly similar to those 32 predicted by theoretical studies, and those observed during experiments on metals and fine-33 grained carbonates, where superplastic behaviour has been inferred. To a first approximation, 34 the measured drop in strength is in agreement with our flow stress calculations, suggesting 35 that strain could be accommodated more efficiently by these mechanisms within the weaker 36 bulk slip zone, rather than by frictional sliding along the main slip surfaces in the slip zone. 37Frictionally induced, grainsize-sensitive GBS deformation mechanisms can thus account for 38 the self-lubrication and dynamic weakening of carbonate faults during earthquake propagation 39 in nature. 40
A seismically active low-angle normal fault is recognized at depth in the Northern Apennines, Italy, where recent exhumation has also exposed ancient examples at the surface, notably the Zuccale fault on Elba. Field-based and microstructural studies of the Zuccale fault reveal that an initial phase of pervasive cataclasis increased fault zone permeability, promoting influx of CO 2 -rich hydrous fluids. This triggered low-grade alteration and the onset of stress-induced dissolution–precipitation processes (e.g. pressure solution) as the dominant grain-scale deformation process in the pre-existing cataclasites leading to shear localization and the formation of a narrow foliated fault core dominated by fine-grained hydrous mineral phases. These rocks exhibit ductile deformation textures very similar to those formed during pressure-solution-accommodated ‘frictional–viscous’ creep in experimental fault rock analogues. The presence of multiple hydrofracture sets also points to the local attainment of fluid overpressures following development of the foliated fault core, which significantly enhanced the sealing capacity of the fault zone. A slip model for low-angle normal faults in the Apennines is proposed in which aseismic frictional–viscous creep occurs on a weak, slow-moving (slip rate <1 mm a −1 ) fault, interspersed with small seismic ruptures caused by cyclic hydrofracturing events. Our findings are potentially applicable to other examples of low-angle normal faults in many tectonic settings.
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