Kinematic model of triclinic transtension zonesThe case of the Boa Fé shear zone (Ossa-Morena Zone, Iberian Variscan Massif)

1. Carlos Fernández ¹
2. Manuel Díaz-Azpiroz ²
3. M. Francisco Pereira ³
4. Martim Chichorro ⁴
5. Dyanna Czeck ⁵

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More publications in: Geotemas (Madrid)

Abstract

A kinematic model of triclinic transtension with oblique coaxial shortening is presented in this work. The main parameters controlling the model are the simple-shear obliquity angle (), the angle between the infinitesimal shortening axis of the coaxial flow component and the true-dip line of the shear zone boundary (), and the kinematic vorticity number (Wk). It includes, as special cases, previous models of monoclinic and triclinic transtension. Our theoretical results allow predicting the shape of the finite strain ellipsoid and the orientation of the principal finite strain axes for the distinct combinations of the controlling parameters and increasing finite strain intensity. Testing of the plano-linear fabric of the sinistral transtensional Boa Fé shear zone (Ossa-Morena Zone, Iberian Variscan belt) against the model yielded a satisfactory fit for a horizontal to moderately SE-plunging simple-shear direction (=0 to -60º), coaxial shortening near the true-dip of the shear zone boundary (10º>>170º), and a pure-shear dominated flow (Wk<0.81). Measurement of the shape of the finite strain ellipsoid is needed in this and other natural shear zones to rigorously check them against the theoretical model presented here.

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