Low‐angle midcrustal ductile shear zones and the related microseismic activity recorded below regions of active extension are seen here as two consequences of strain localization. The feldspar‐to‐mica reaction which occurs once feldspar grains are fractured is the destabilizing mechanism selected to explain the strain localization. The model problem considered to substantiate these claims is solved by numerical means and combines the simple shear due to the rigid gliding of the upper crust (at the velocity of V s ) and the stretch resulting from the extension of the whole crust (at the velocity V e ). The rheological model accounts for dislocation creep of quartz, feldspar, and mica, the feldspar‐to‐mica reaction, and its prerequisite, which is the feldspar fracturing detected by the Mohr‐Coulomb criterion. The one‐dimensional (1‐D) solution, which constrains shear bands to be horizontal, shows the depth partitioning in deformation mode between the simple shear of the low‐viscosity deep crust and the stretching of the highly viscous midcrust. Strain localization occurs during rapid increase of the shearing velocity V s , corresponding to low values of the velocity ratio V e / V s . The 2‐D solution (for V e / V s = 10 −3 ) reveals the development of a periodic system of extensional shear bands, dipping at 30° toward the shearing direction at a depth of 12 to 14 km. Shear bands are formed after less than half a million years at the base of the reaction zone defined by the region where feldspar‐to‐mica reaction is completed. Shear bands do not propagate to greater depths because the pressure prevents the feldspar from fracturing and thus the reaction to occur. The periodic system of shear bands defines a midcrustal flat weakened zone within which the equivalent shear stress is enhanced by at least a factor of three at the shear band tips. Brittle fracture could thus occur within the midcrustal flat weakened zone, explaining therefore the microseismicity monitored at these depths in regions of active extension.
This chapter contains sections titled: Overview, Theme 1: What are The Controls on Fluid-Rock Chemical Interaction in and Adjacent to Fault Zones?, Theme 2: How Does Fluid Flow Change Before, During, and After Earthquakes?, Theme 3: What are the Magnitudes of Fluid Flux Throughout the Lithosphere in Different Tectonic Environments?, Summary, References
Abstract Lawsonite pseudomorphs are used to identify and distinguish the kinematic records of subduction and exhumation in blueschist‐facies rocks from Syros (Cyclades; Greece). Lawsonite is a hydrous mineral that crystallizes at high‐pressure and low‐temperature conditions. During decompression, lawsonite is typically pseudomorphed by an aggregate dominated by epidote and paragonite. Such aggregates are easily deformable and if deformation occurs after the lawsonite breakdown, the pseudomorphs are difficult to distinguish from the matrix. The preservation of the lawsonite crystal shape, despite complete retrogression, indicates therefore that the host blueschist rock has not been affected by penetrative deformation during exhumation, thus providing indication of strain‐free conditions. Therefore, tracking the lawsonite growth and destabilization along the P–T path followed by the rocks during a subduction/exhumation cycle provides information about the subduction/exhumation‐related deformation. Using microstructural observations and P–T pseudosections calculated with thermocalc , it is inferred that top‐to‐the‐south sense of shear preserved in lawsonite pseudomorph‐bearing blueschists on Syros occurred during the prograde metamorphic path within the lawsonite stability field, and is therefore associated with subduction. On the contrary, the deformation with a top‐to‐the‐north sense of shear is observed in surrounding rocks, where lawsonite pseudomorphs are deformed or apparently lacking. This deformation occurred after the lawsonite breakdown during exhumation. At the regional scale, exhumation‐related deformation is heterogeneous, allowing the preservation of lawsonite pseudomorphs in significant volumes of blueschists of the central and southern Cyclades. It is argued that such successive shearing deformation events with opposite senses more likely correspond to an exhumation process driven by slab rollback, in which subduction and exhumation are not synchronous.
