A STANDARDIZED PROTOCOL TO TEST OBLIQUE TRANSPRESSION MODELS IN HETEROGENEOUS HIGH-STRAIN ZONES (THE TORCAL DE ANTEQUERA MASSIF, SOUTHERN SPAIN)

In the last years, progressively more complex kinematic models have been proposed to better describe natural high-strain zones, but discrepancies with nature are still appreciated, in part due to simplifications assumed by the models. Before increasing the complexity of these models to obtain a better approach to natural cases, it is important to test available models to ensure they reproduce high-strain zones accurately and to constrain the range of some variables to facilitate including new ones. To achieve this, the use of an objective and standardized procedure that minimizes subjectivity is needed. Mathematical modelling of flow within high-strain zones usually compares with ductile deformation. Nevertheless, understanding brittle heterogeneous high-strain zones developed in the upper crust is also important for a better knowledge of the processes related to active tectonics.

In this study, a heterogeneous dextral transpressive high-strain zone (The Torcal de Antequera Massif, TAM) at the Betic chain (southern Spain) is analysed. Kinematic estimations (orientation and shape of the finite strain ellipsoid and amount of finite strain) are obtained from fault-slip data, fold analysis and cross-sections. These are compared against a triclinic transpression model using a standardized protocol with four successive steps, in which different transpression parameters (φ, υ and W_k) are constrained.

The TAM presents two types of domains that have been analysed separately. Domains located at the boundaries of the TAM (Outer Domains, ODs) mainly show strike-slip structures, whereas the inner part of the massif (Inner Domain, ID) presents shortening structures oblique to the main boundaries (S = 0.2 – 0.3) and orthogonal normal faults. The extrusion angle (υ) is close to 0º in both domains. The ODs are compatible with a simple shear dominated transpression with φ = 5 – 15º, and W_k = 0.89 – 0.95. The ID probably represents a pure shear dominated transpression with φ = 10 – 20º, and W_k= 0.6 – 0.65. The resulting oblique convergence angle (α) between the far-field velocity vector and the azimuth of the TAM should be between 17º and 35º.

This procedure will help to constrain geometrical parameters of analogue models, which are crucial to unravel deformation patterns in heterogeneous upper crust high-strain zones.