Syn‐deformational migmatites and magmatic‐arc metamorphism in the Xolapa Complex, southern Mexico

Abstract

The Xolapa Complex (XC) is the largest plutonic and metamorphic mid-crustal basement unit in Mexico and represents an ancient continental magmatic-arc. A complete range from metatexite to diatexite migmatitic structures has been produced during a single high-grade metamorphic event. However, structural relics reveal the existence of early Cpx + Pl + Qtz ± Opx and Grt + Opx + Pl + Qtz ± Cpx pre-migmatitic metamorphic assemblages. Field relationships and microstructural observations allow us to constrain five pre-, syn-and post-migmatitic deformational phases. It is argued that migmatitic structures and minor anatectic granites were developed during ductile recumbent folding and shear structures related to the D2-D3 phases. Late post-migmatitic ductile-brittle deformation is evidenced by the development of NNE trending transpressional thrusting (D4), and E-W left-lateral mylonitic shear zones (D5). Biotite-breakdown melting in felsic rocks and amphibole-breakdown melting in mafic rocks, as well as geothermobarometric results, indicate that metamorphism took place at temperatures from 830 to 900°C and pressures ranging from ‡6.3 to 9.5 kbar. Late migmatitic assemblages equilibrated in the highest temperature range along a clockwise P-T path. The relationships between the large diversity of migmatitic structures and the progressive production of melt suggest that feedback relations prevailed as a time-marker during a contractional regime. Deformation, metamorphism, and plutonism of the XC show that this terrane evolved as a north-east-verging thrust system with synkinematic metamorphism and partial melting, during the Late Cretaceous -Palaeogene. The tectonothermal history of XC is analogous to a Cordilleran metamorphic magmatic-arc formed in an accretionary tectonic framework. This new model provides constraints on the exhumation mechanism and thermal evolution of southern Mexico.

Earthquake Swarms and Slow Slip on a Sliver Fault in the Mexican Subduction Zone

“…In the central region where hypocenters are best constrained, the seismicity extends for over 20 km vertically, well beyond the depth uncertainty of 7 km (19). The horizontal trend of the swarms corresponds to a refined northern boundary of the Xolapa terrane based on the extent of the core complex (36), aeromagnetic data (20) and topographic information (22). GPS data indicate there are transient a-SSEs across this terrane boundary associated with SSEs and afterslip.…”

“…We did not solve for source locations of SSEs along the plate interface, as a catalog of SSEs in Oaxaca was recently compiled (53) utilizing the same cGPS stations. To better interpret the seismicity and GPS motions, we also reexamined the configuration of the northern boundary of the Xolapa terrane based on the extent of the core complex (36) , aeromagnetic data (20), and topographic information (22). See Supporting Information section S1 for a more detailed discussion of the methods used in this study.…”

“…In fact, the Xolapa terrane overlies the band of intense seismicity and the Xolapa-Mixteco boundary is consistent with its inland extent. The Xolapa terrane consists of the highly deformed Xolapa Metamorphic Complex, which by some have been proposed to be of accretionary origin, and un-deformed plutonic intrusions (35,36). Some studies have proposed that faults and shear zones occur along portions of the northern boundary of the Xolapa terrane (22,35,(37)(38)(39).…”

Oligocene‐Miocene back‐thrusting in southern Mexico linked to the rapid subduction erosion of a large forearc block

“…The Xolapa Complex is a plutonic and metamorphic mid‐crustal basement unit generally thought to represent a Jurassic through Cretaceous continental magmatic arc (JCMA) [ Herrmann et al , 1994]. Peak metamorphism in the Xolapa Complex produced extensive migmatitization at temperatures of ∼830–900°C and pressures of ∼630–950 MPa [ Corona ‐ Chavez et al , 2006] between ∼160–90 Ma and 40–25 Ma [ Corona‐Chavez et al , 2006; Morán ‐ Zenteno et al , 2007]. The Mixteca, Oaxaca, and Juarez Terranes consist broadly of Paleozoic, Proterozoic and Mesozoic rocks, respectively [ Campa and Coney , 1983; Keppie , 2004], on which Jurassic red beds and Cretaceous limestones have been deposited [ Anderson and Schmidt , 1983; Freydier et al , 1997; Filkorn , 2003].…”

Activity of crustal faults and the Xolapa sliver motion in Guerrero–Oaxaca forearc of Mexico, from seismic data

“…This sliver geologically corresponds to the Xolapa complex and is composed by a sequence of high-grade metasedimentary and metaigneous rocks that are frequently intruded by both deformed and undeformed plutonic rocks. It is suggested that the Xolapa terrane is allochthonous, and was accreted to the continental margin of southern Mexico during Mesozoic-Tertiary (e.g., Corona-Chávez et al 2006;Pérez-Gutiérrez et al 2009). Present work analyses the slip partitioning phenomenon observed in oblique subduction zones applying the model of McCaffrey (1992) to the dataset of shallow thrust events on the subduction interface with M w > 3.6 for the case of Mexico using regional and teleseismic data.…”