Journal of the Virtual Explorer

A high-strain zone has been identified on top of the Bazar ophiolite. Structural and microstructural data demonstrate a general top-to-the S sense of shear. As a consequence, there is no kinetic relationship with those faults that bound the internal imbricates of the Bazar unit, which present a general top-to-the N and NE sense of shear (e.g. Díaz García, 1990). The limited number of outcrops prevented us to find cross-cuttingg relationships between those accidents, so the presented results alone cannot decipher the structural sequence. However, regional evidences in other units of the allochthonous complexes suggest a sequence of W-E-directed thrust system followed by a SE-directed out-of-sequence thrust system (Martínez Catalán et al., 2002). These include another ophiolitic unit (Careón unit; Díaz García et al. 1999; Gómez Barreiro et al., 2010a), the Upper units (Gómez Barreiro et al., 2007a), and some sectors of the the Basal units (Gómez Barreiro, et al., 2010b; Díez Fernández et al. 2012).

As a regional conclusion, we suggest that the Bazar Shear zone represents a part of the out-of-sequence thrust system which carried the Ophiolitic units and the Upper allochthon over the Basal allochthon and Parautochthon (Martínez Catalán et al., 2009). The meaning of the NE kinematics found within the Bazar unit will be examined in a future work, but they could be related to the in-sequence thrust system, identified in other sectors of the allochthonous complexes (Díez Fernández and Martínez Catalán, 2012)

Across the deformation gradient in the Bazar shear zone, distinct metamorphic changes occur. The relation between mineral phases and microstructures suggests that reactions were synkinematic with the activity of the Bazar shear zone. While more quantitative and extense work is needed, we could speculate about the qualitative meaning of those changes. Petrographic and previous chemical data suggest a retrogressive evolution from amphibolite facies to greenschist facies conditions. We are awared about the upscaling limitation, a common problem when extrapolation from small scale experiments to nature is pursued. In consequence our results should be considered preliminary.

Microstructural analysis reveals a brittle behaviour in pre-kinematic amphibole (brown amphibole am1) (Fig. 10), but not in synkinematic phases (am2, epidote albite). This is supported by the abundance of intragrain microcracks and the fragmentary character (sharp boundaries, “domino-like”) of most am1 porphyroclasts. On the other hand grain size distribution with a low frecuency of large grains and a high frequency of small ones is coherent with the activity of cataclastic processes (Passchier and Trouw, 1996).

These features are not present in metabasites outside the shear zone, and are found in an incipient stage of development in relics of HT-amphibolites located across the shear zone. Apparently synkinematic phases accommodate this behaviour by ductile deformation, which include the envelope of am1 clasts by mylonitic and ultramylonitic layers, and the filling of gaps between the am1 fragments with am2 fibres. Those fibers sometimes track the separation of the fragments or simply depict a normal angular relation to crack planes and envelope am1-rich domains (protomylonitic) with continuous mylonitic/utramylonitic layers.

Moreover, truncation of am1 porphyroclasts mineral zoning, in those faces parallel to the foliation, and development of am2 strain fringers in the oposite faces is a common feature in these mylonites. These findings could be interpreted as the removal of amphibole am1 by dissolution, along crystal faces perpendicular to the shortening direction (foliation plane), and precipitation of synkinematic amphibole am2, parallel to the tensile direction (mineral lineation). These observations have been interpreted to reflect stress-induced grain-boundary diffusive mass-transfer or pressure solution (Durney, 1972; Robin, 1978; Raj, 1982; Lehner, 1990; Wheeler, 1992, Whinch and Yi, 2002).

The dissolution-precipitation creep is an effective mechanism to develop (or amplify) shape fabric (SPO), and, potentially, crystallographic preferred orientation or texture (e.g. Hippertt, 1994; Stallard and Shelley, 1995; Bons and den Brok, 2000; Stokes et al., 2012). Growth and dissolution rates are anysotropic in many minerals. Growth is faster along c-axis in amphibole and quartz (Anh et al., 1991; Stallard and Shelley, 1995). The role of the brittle mechanisms documented in these mylonites could be important at the initial stages of deformation, by reducing the grain-size of the phases and increasing the grain boundary surface in the aggregate (Table 1, Fig. 10). All these features are compatible with the presence of a fluid phase, which promotes also synkinematic metamorphic reactions, at the grain boundary network. The combination of those processes as a cooperative group, in the deformation of metabasites, has already been proposed in the literature (e.g. Brodie and Rutter, 1985; Nyman et al. 1992; Lafrance and Vernon, 1993; Berger and Stünitz, 1996 ; Imon et al., 2004).

Complementary information comes from texture in the ultramilonitic domains (Fig. 12). The limited evidence of intracrystalline plasticity suggests that other mechanisms could be accounting for the strain accommodation in the matrix. The symmetry of pole figures reflects, to some extent, the combination of rotational and non-rotational components of the flow. In the case of amphibole (am2), pole figure patterns are compatible with the activity of the main slip system (100) [001] (Biermann and Van Roermund, 1983; Biermann, 1981; Skrotzki, 1992). However, an equivalent texture could be developed by rigid rotation of grains and/or slip on (110) cleavage plane, in a general context of dissolution-precipitation creep (e.g. Allison and La Tour, 1977; Babaie and La Tour, 1994). The strongly anisotropic crystal growth of amphibole may result in grains with very high shape ratios (SR) that tend to rotate toward a stable end orientation, close to the attractors of the fabric (lineation and foliation; Passchier, 1997, Mancktelow et al., 2002), resulting in a strong texture.

Albite, epidote and quartz appear typically segregated in a different layer than amphibole (Fig. 12). Diffusive mass-transfer could explain some observations, such as the shrinkage of larger plagioclase crystals into the epidote-plagioclase matrix (Fig. 10). However, the presence of some evidence of intracrystalline plasticity, points to the activation of some other mechanisms in these domains. Interestingly, texture of albite shows a very uncommon pattern, with (010) poles parallel to the lineation and [100] and [001] plotting close to the foliation pole. To our knowledge, [010] has never been identified as a slip direction in plagioclase (Gómez Barreiro et al., 2007c, 2010a, and references therein). Some similarities could be drawing with textures obtained by Heidelbach et al. (2000), after experimentally deformed fine-grained albite aggregates by solution precipitation creep. They found a strong texture, with the (010) poles tending to lie orthogonally to the compression direction. Another supporting argument comes from crystal-growth experiments and observations in nature, where the [010]-axis is generally considered the faster growth direction for plagioclase (e.g.: Muncill and Lasaga, 1987, Deer et al., 1967). In summary we suggest that the albite texture could be interpreted as the result of solution precipitation creep.

The measurement of epidote texture is not common in fabric studies. Our results are slightly monoclinic, and depict pole figure patterns similar to those reported in mid-low grade mylonites (Crampton, 1957), blueschists (Bezacier et al., 2010), and eclogites (Cao et al. 2011), with (010) poles parallel to the lineation and (001) (100) parallel to the foliation. There is an incomplete knowledge of epidote slip systems (e.g. Din et al., 2001). Bezacier et al. (2010) suggest that [010] has the shortest Burgers vectors, so it could be the easy slip direction in epidote. As a consequence, our textures could be interpreted as the result of dislocation glide on [010] (001) (100). However we cannot exclude other mechanisms as rigid rotation of grains as an alternative way to develop shuch a texture.

As a summary, deformation in the Bazar shear zone is likely to have occurred under amphibolite to greenschists facies conditions. Microstructures suggest a combination of simple and pure shear (stretching shear) dominating the flow. In addition, shape and crystallographic fabrics are compatible with the activity of several mechanisms such as cataclasis, solution transfer, passive rotation, and, to some extent, intracrystalline strain. While not necessarily coeval processes, brittle and ductile mechanisms, accounted for the strain accommodation in the rock, reflecting the mechanical contrast among phases like amphibole and plagioclase. The existence of a distinct texture in plagioclase developed under dissolution-precipitation creep is supported by our results. There is strong evidence about the importance of solution transfer creep transfer in mid-crust (wet) shear zones (e.g. Wincht and Yi, 2002; Stokes et al, 2012). Whether this mechanism dominated or not along a major tectonic contact like the Bazar shear zone, must be carefully evaluated if a proper rheological evolution is to be drawn.