Kinematics and strain distribution in an orogen-scale shear zone: Insights from structural analyses and magnetic fabrics in the Gavarnie thrust, Pyrenees

Abstract

the pole of the S, C or C' planes and k max axes are parallel to the transport direction and related to ductile S-C structures. Furthermore, the Pj-T changes across the shear zone characterize strain variations: larger Pj and T are found in the basal, most deformed part of the shear zone, and lower values are found where the interaction between Alpine and Variscan-related petrofabrics is stronger. We also interpret the reactivation of Variscan inherited fabrics within the Alpine shear zone. In spite of the heterogeneous strain, markers indicate a common, top-to-the-South (N190E) Alpine transport direction, which contrasts with the strong obliquity of the genetically-related structures developed in the Southern Mesozoic sedimentary cover. In this sense, our data suggest a complete decoupling between basement and cover units during the Alpine compression.

Magnetic Fabric Signature Within a Thrust Imbricate; an Analog Modeling Approach

“…The change from oblate to a more prolate fabric with a lower degree in anisotropy is consistently observed in the vicinity of a thrusts in the models and is common in nature as deformation increases and localizes toward a structure/thrust (Hirt et al., 2004; Kligfield et al., 1981). In general, changes in magnetic fabric, where magnetic foliation is parallel to thrust surface and show low degrees of anisotropy, are in agreement with laboratory experiments (Borradaile & Alford, 1988; Borradaile & Puumala, 1989; Housen et al., 1993; Schöfisch et al., 2021), numerical models (Housen et al., 1993), and field observations (e.g., Averbuch, et al., 1992; Greve et al., 2020; Hirt et al., 2004; Housen et al., 1996; Marcén et al., 2018; Parés and van der Pluijm, 2002, 2004).…”

“…For example, comparisons of the resulting LPS fabric away from thrusts and kinkzones can be made with AMS data from marine clays in Greece and Taiwan (Kissel et al., 1986), mudstones in Taiwan (Lee et al., 1990), sandstones in Zagros mountains (Bakhtari et al., 1998), and different locations in the Pyrenees (Averbuch et al., 1992; Parés and van der Plujim, 2002; Sans et al., 2003). Furthermore, the “thrust‐induced” fabric in our models with magnetic foliation parallel to the thrust is also observed in the Pyrenees (e.g., Averbuch et al., 1992; Marcén et al., 2018), Appalachians (Hirt et al., 2004; Parés and van der Pluijm, 2002, 2004), and the Hikurangi subduction margin (Greve et al., 2020). It can be argued that the procedure of sampling the models impacts the magnetic fabric (e.g., pushing the plastic cube into the model compare to Snowball et al.…”

“…Furthermore, a trend of decreasing degree of anisotropy is observed toward a thrust or kinkzone and the presence of prolate fabric increases in areas closer to a thrust (Figure 3c). Similar changes of the magnetic fabric toward a thrust are identified from, for example, the French Alps (Kligfield et al., 1981), Corbières in the Pyrenees (Averbuch et al., 1992), the Central Appalachian FTB (Hirt et al., 2004), the Gavarnie thrust in the Pyrenees (Marcén et al., 2018), the Barbados Accretionary Prism (Housen et al., 1996), and the Hikurangi Subduction Margin in New Zealand (Greve et al., 2020). It is worth mentioning that the first four studies above show the effect of magnetic mineral reprecipitation, and the latter two studies show the additional effect of fluid on the magnetic fabric.…”

“…As strain, especially LPS, is non‐uniformly distributed throughout a model (Koyi, 1995; Mulugeta & Koyi, 1992), notably with depth (Koyi et al., 2003), the variation in strain will also be reflected in the magnetic fabric prior to thrusting and can be inherited in overprinting (e.g., Ferré et al., 2014; Marcén et al., 2018; Parés and van der Pluijm, 2002). The models reported here also show a change in strain with depth (Figures 2d–2e, and 2f), especially due to decreasing displacement along the thrusts at deeper levels (Figure 2).…”

“…This means that thrusting was less efficient in realigning the grains at depth, therefore, the compaction‐induced fabric is inherited with little thrust overprinting. An inherited fabric, resulting from a previous deformation phase along thrusts are known from for example, the Gavarnie thrust in the Pyrenees (Marcén et al., 2018), where an intersection lineation prior the Alpine deformation is inherited during reactivation of the thrust and a low degree of anisotropy is observed at the reworked Variscan fabric. During overprinting of a fabric by a “thrust‐induced fabric,” the principal axes can have different “starting” orientations that influence the finite fabric after thrusting.…”