2. Geological setting
The 80 km2 Cerro de Costilla complex (Fig. 1) is located in the Sierra San Pedro Mártir region of the Jurassic to Cretaceous Peninsular Ranges batholith in northern Baja California, México (e.g. Gastil et al., 1975; Johnson et al., 1999, 2003; Melis, 2006). From margins to core, the complex consists of (1) a nearly complete outer ring of tonalite, (2) a partial ring of deformed and partially melted metasedimentary and meta-igneous rocks, with zones of variably mingled gabbro, diorite and tonalite, (3) a partial inner ring of tonalite, and (4) a core of coarse-grained gabbro. On the basis of its distinctive geometry, Johnson et al. (2002) described the Cerro de Costilla as a magmatic ring complex, a conclusion supported by more extensive mapping (Melis, 2006). The mylonitic transition described here occurs in the outer tonalite ring; so the following discussion will focus on that unit.
The outer ring is composed of coarse-grained hornblende-biotite tonalite with abundant (~1%) microgranitoid enclaves. Johnson et al. (1999, 2003) reported a SHRIMP U/Pb age for the tonalite of 103 ± 1.0 Ma. The pluton was emplaced at pressures of 3-4 kbar (Melis, 2006, unpublished Al-in-hornblende data). The tonalite and the rest of the magmatic complex show a variably-developed magmatic foliation defined by alignment of plagioclase, hornblende and biotite grains. The tonalite cuts migmatitic gneisses in the hanging wall of the Main Martir Thrust, and so provides a younger limit to the relative age of the main movement on the thrust (Johnson et al., 1999). During thrusting, a prominent north- to northeast-dipping foliation developed in both the hanging wall and footwall rocks. In the hanging wall, this foliation development was accompanied by partial melting, evidence of which is preserved as migmatite. A later tectonic foliation locally cuts the hanging wall rocks immediately to the south and southeast of the Cerro de Costilla complex. It is a steeply north- to northeast-dipping S/C fabric (Simpson and Schmid, 1983) that consistently shows normal (north side down) displacement, indicating post-thrusting extension of the hanging wall. It cuts into the margin of the outer tonalite ring of the complex in one place on the southern end, resulting in the strain and fabric gradient examined here. Deformation temperatures, as reported below, are approximately 475 ± 50°C. In the following section, we discuss initiation and progressive development of foliation formed in this strain and fabric gradient, focusing on the role played by biotite.