Shear zones – A review Fossen, Haakon; Cavalcante, Geane Carolina G.
Earth-science reviews,
August 2017, 2017-08-00, Letnik:
171
Journal Article
Recenzirano
Strain in the lithosphere localizes into tabular zones known as shear zones that grow from small outcrop-size individual zones to large composite structures. Nucleation is related to distributed ...microscale flaws or mesoscale structures such as fractures and dikes, and they soon establish displacement profiles similar to faults. Also similar to faults, they grow in width and length primarily by segment linkage as they accumulate strain and displacement, and this process typically results in shear zone networks. Consequently, mature shear zones are heterogeneous and composite zones characterized by anastomosing patterns and local variations in thickness and finite strain. Kinematic vorticity estimates suggest that most shear zones deviate from simple shear, and even if subsimple shear may be a useful reference model in many cases, finite strain data indicate that many shear zones involve three-dimensional combinations of coaxial and non-coaxial deformation, such as transpression and transtension. Strain geometry and kinematic vorticity can vary significantly within shear zone networks, which makes it difficult to estimate the bulk deformation type for a composite shear zone or shear zone network. However, perhaps the most challenging aspect is that of progressive deformation, i.e. to what extent and how flow parameters change during deformation (non-steady state deformation), which needs to be addressed by a combination of detailed field observations and numerical modeling.
Deformation – Progressive or multiphase? Fossen, Haakon; Cavalcante, Geane Carolina G.; Pinheiro, Roberto Vizeu L. ...
Journal of structural geology,
August 2019, 2019-08-00, Letnik:
125
Journal Article
Recenzirano
Complexly deformed rocks that exhibit refolded folds and overprinting planar fabrics are often interpreted as the result of polyphase deformation and described in a scheme of deformation phases (D1, ...D2 … Dn). At the same time, a number of models have been presented over the last decades that explain how complex and overprinting structures can form by single-phase progressive deformation as strain accumulates. Strain generally accumulates heterogeneously in a deforming region, depending on strain intensity and geometry, flow parameters, rheology and lithology. The result is regions of different structural complexity, fabrics and fabric orientations that can be sorted into different structural facies. Such structural pattern should be mapped and presented before defining a scheme of deformation phases, and attempts should be made to explain them by a single structural-kinematic model, such as partitioned transpression. Only if this is not possible, or if independent geochronologic data reveals a more punctuated history, should we define an additional tectonic regime and deformation phase.
Multiphase and progressive deformation histories are not mutually exclusive, and it is important that both possibilities are considered when describing the deformation history of any complexly deformed region. When the concept of deformation phases is employed, the number of phases should be minimized and be related to major tectonic event of external character. Uncritically defining deformation phases can easily generate a complicated discrete deformation history with no link to tectonic reality.
The presence of melt during deformation produces a drastic change in the rheological behavior of the continental crust; rock strength is decreased even for melt fractions as low as ∼7%. At ...pressure/temperature conditions typical of the middle to lower crust, melt-bearing systems may play a critical role in the process of strain localization and in the overall strength of the continental lithosphere. In this contribution we focus on the role and dynamics of melt flow in two different mid-crustal settings formed during the Brasiliano orogeny: (i) a large-scale anatectic layer in an orthogonal collision belt, represented by the Carlos Chagas anatexite in southeastern Brazil, and (ii) a strike-slip setting, in which the Espinho Branco anatexite in the Patos shear zone (northeast Brazil) serves as an analogue. Both settings, located in eastern Brazil, are part of the Neoproterozoic tectonics that resulted in widespread partial melting, shear zone development and the exhumation of middle to lower crustal layers. These layers consist of compositionally heterogeneous anatexites, with variable former melt fractions and leucosome structures. The leucosomes usually form thick interconnected networks of magma that reflect a high melt content (>30%) during deformation. From a comparison of previous work based on detailed petrostructural and AMS studies of the anatexites exposed in these areas, we discuss the rheological implications caused by the accumulation of a large volume of melt “trapped” in mid-crustal levels, and by the efficient melt extraction along steep shear zones. Our analyses suggest that rocks undergoing partial melting along shear settings exhibit layers with contrasting competence, implying successive periods of weakening and strengthening. In contrast, regions where a large amount of magma accumulates lack clear evidence of competence contrast between layers, indicating that they experienced only one major stage of dramatic strength drop. This comparative analysis also suggests that the middle part of both belts contained large volumes of migmatites, attesting that the orogenic root was partially molten and encompassed more than 30% of granitic melt at the time of deformation.
