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Finite Strain Field Interferences: Dome/Shortening.
The concept of Finite Strain Field Interferences (FSFI) is relevant to situation where plate boundary forces (external forces) overlap with body forces (internal forces). This concept should not be confused with the that of Finite Strain Field Superimposition (FSFS) best examplified by fold superposition. FSFI involves not only a spatial superimposition of fabrics and structures, but also a superimposition in time. The best example of FSFI involves the emplacement of granitic pluton in a contexte of shortening or shearing. Shortening or shearing produces a regional finite strain fields which interfer with the local finite strain field that develop around plutons.
The sketches illustrate the interference dome/coaxial shortening. In the absence of regional tectonics, the finite strain field induced by the emplacement of plutons depends on the expansion geometry (cf. Slide 31).
In a context in shortening , the direction of maximum expansion corresponds to the direction of lengthening. Plutons tend to develop an elliptic shape with a long axis parallel to X. The pluton's flanks are parallel to the regional XY plane therefore there is coaxiality between the local (pluton-related) and regional (tectonic-related) strain fields, and consequently amplification of the fabric. At the tips of the pluton, the local and regional strain field are perpendicular. The characteristic of the finite strain field will depend on the regional and local strain rates, as well as the rheology of both the pluton and the host rock. Indeed if the regional fabric develop faster compared to the local fabric, then regional fabric may be folded by the expansion of the pluton. In contrast if the local fabric develop faster, one may expect the fabric at the tip of the pluton to be kinked by the regional fabric. If the strain rates are similar foliation triple junctions (constriction) can develop in the host rock and in the pluton. Alternatively is the pluton act as a rigid body, pressure shadows (blue triangle) may develop in the host rock.