Mexico is composed of tectonostratigraphic terranes of Laurentian, Gondwanan, and Pacific provenance that were amalgamated during the Late Paleozoic formation of Pangea and during Mesozoic-Cenozoic accretion and breakup (Fig. 1). The first comprehensive terrane analysis was by Campa and Coney (1983), followed by Sedlock et al. (1993). Further refinements of the terranes and their boundaries have been proposed by Centeno-García et al. (1993, 2000, 2003), Talavera-Mendoza and Guerrero-Suastegui (2000), and Freydier et al. (2000). Recently, Keppie (2004) analyzed the tectonic evolution of these terranes._x000D_ From west to east, southern Mexico comprises (Fig. 1) the following terranes: Guerrero (Nahuatl), Xolapa (Chatino), Mixteca, Oaxaca (Zapoteco), Juarez (Cuicateco), and Maya (the names in parenthesis are those used by Sedlock et al., 1993). The approximately 1 Ga Oaxacan Complex underlying the Oaxaca (Zapoteco) terrane has been traced into northern Mexico (Fig. 2), and Ortega-Gutiérrez et al. (1995) have proposed the existence of the composite Oaxaquia terrane, which extends beneath Mesozoic and Cenozoic rocks from the Ouachita Front into southern Mexico, and possibly into the Chortís block of Honduras (Keppie, 2004) covering an area of ca. 1 000 000 km2. _x000D_ The Oaxaca Fault is a major Tertiary fault located along the western boundary of the 10-15 km wide, polyphase mylonitic, Juarez shear zone that forms the boundary between the Oaxaca (Zapoteco, Oaxaquia) and Juarez (Cuicateco) terranes (Campa and Coney, 1983; Sedlock et al., 1993; Alaniz-Alvarez et al., 1994) (Figs. 1, and 3). _x000D_ The Oaxaca Fault has a mean orientation of N10°W and dips steeply towards the west. _x000D_ It is made up of a series of parallel to slightly (<25°) oblique discontinuous faults, the corresponding sectors of which are named (from north to south): Tehuacán, Coxcatlán, Dominguillo Teotitlan, Jayacatlán, and Etla (Centeno-García, 1990). This terrane boundary has a complex history: (i) pre-Jurassic, E-vergent thrusting; (ii) Jurassic dextral mylonitic shearing; (iii) Late Cretaceous E-vergent thrusting; and (iv) normal faulting was superimposed on the terrane boundary during Cenozoic times (Alaniz-Alvarez et al., 1996; Alaniz-Alvarez and Nieto-Samaniego, 1997)._x000D_ The crust in southern México has been subjected to seismic refraction, and seismological and magnetotelluric (MT) studies (Nuñez-Cornu, 1988; Nava et al., 1989; Arzate et al., 1993; GEOLIMEX, 1999; Spranger, 1994, Jording et al., 2000; Joedicke et al., 2006) (Fig. 3). _x000D_ The seismic data indicate that the thickness of the crust varies from 23.5-19 km along the coast of Oaxaca State to 47.5-48 km beneath Oaxaca City (Nava et al., 1989; Nuñez-Cornú, 1988; GEOLIMEX, 1999; Spranger, 1994). Below the Oaxacan Complex, the lower part of the upper crust is characterized by higher seismic velocities (Valdés et al., 1994). A regional gravity profile (Mena et al., 1995) along the GEOLIMEX (Mexican Lithospheric Geotraverse) traverse was interpreted to indicate a common lower Grenvillian crust beneath both the Oaxaca and Juarez terranes with contrasting densities, i.e., 2.75 and 2.57 gr/cm3, respectively (Ramírez-Ruiz, 1994). Using seismic constraints, Mena et al. (1995) interpreted the gravity data in terms of a shallow, ca. 5 km thick cover of volcano-sedimentary rocks resting on more dense Precambrian metamorphic rocks. _x000D_ Although MT studies have imaged major crustal and mantle structures beneath the study area, the Oaxaca Fault is not apparent, probably because station coverage is sparse (Arzate et al., 1993; Jording et al., 2000; Joedicke et al., 2006). For example, along the GEOLIMEX traverse the nearest MT stations are located ca. 10 km to the southwest and northeast of the Oaxaca Fault. An earlier MT profile (Arzate et al., 1993) is approximately parallel to the Oaxaca Fault, and has only one station in the Juarez terrane. Nevertheless, magnetotelluric studies (Arzate et al., 1993; Jording et al., 2000) indicate a difference in the electrical conductivity across the Oaxaca Fault. Thus, the E-polarization pseudosection of the MT GEOLIMEX profile shows that most of the Oaxaca terrane has a shallow resisitivity layer that dips to the east and disappears near the Oaxaca Fault. In contrast, most of the Juarez terrane has higher resistivities. Several features on the B-polarization pseudosection also suggest a difference in the distribution of the electrical resistivity between the two terranes (Fig. 6). Detailed MT, and gravity and magnetic studies across the Oaxaca Fault was undertook in the framework of the first phase of this project(Arzate et al., 2007; Corbo-Camargo et al., 2008._x000D_ THe results obtained from the gravity and magnetic study (Campos-Enríquez et al, 2009) indicate the presence of a composite depression comprising three N-S sub-basins: the northern Etla and southern Zaachila sub-basins separated by the Atzompa sub-basin. The Etla sub-basin is bounded by the moderately E-dipping, Etla Fault and the more steeply W-dipping Oaxaca Fault, which together constitute a graben that continues southwards into the Atzompa graben. The deeper Zaachila sub-basin, south of Oaxaca city, is a wide V-shaped graben with a horst in the middle. The new gravity and magnetic data suggest that the Oaxaca-Juarez terrane boundary is displaced sinistrally ca. 20 km along the E-W Donají Fault, which defines the northern boundary of the Zaachila sub-basin. On the other hand, the Oaxaca Fault may either continue unbroken southwards along the western margin of the horst in the Zaachila sub-basin or be offset along with the terrane boundary. The sinistral movement may have taken place either during the Late Mesozoic-Early Cenozoic, Laramide Orogeny as a lateral ramp in the thrust plane or under Miocene-Pliocene, NE-SW extension. The former suggests that the Donají Fault is a transcurrent fault, whereas the latter implies that it is a transfer fault. The models imply that originally the suture was continuous south of the Donaji Fault and provide a constraint for the accretion of the Oaxaca and Juarez terranes. This conclusion, as well as the inferred horizontal movements are to be taken into account in the study of the evolution of this suture. _x000D_ The corresponding MT results succeded in imaging the structure of the Oaxaca Fault in the neighbouhood of Oaxaca City. The main facts observed are: 1) that the Oaxaca faults continues at depth southward of Oaxaca City, but 2)also that the Oaxaquian Complex continues eastward of Oaxaca City beneath the Juarez terrane. These results are compatible with the gravity and magnetic results._x000D_ Field work would be undertaken during the period october-december in the area of the Tehuacan Valley (northern portion of the Oaxaca Fault)._x000D_ The results obtained in the first phase of this project (PAPIIT IN116107) indicate that hypothesis was correct. The results are very interesting for the studies of the evolution of the suture of the Oaxaca_Juareaz suture._x000D_ The second phase of this project will be focused to further study the possible continuation of the Oaxaca Fault 1) south of the Valles Centrales (its possible continuation up to the Ocean Pacific), and north of the Tehuacan Valley (does it continue northewards beneath the Trans-Mexican Volcanic Belt. This project will be based in MT, gravity and magnetic studies._x000D_

Las contribuciones de la fase precedente de esta investigación son muy interesantes para estudios sobre la evolución de la sutura de los terrenos tectonoestratigráficos Oaxaca (Zapoteco) y Juárez (Cuicateco)._x000D_ Esperamos que igualmente las contribuciones de la segunda fase de esta investigación sean igualmente interesantes para los estudios tectónicos, tectonestratigráficos del sur de México._x000D_ Geográficamente, nos proponemos seguir estudiando las continuaciones tanto al norte, como al sur de la Falla Oaxaca._x000D_ En particular, en la porción norte, nos proponemos estudiar la eventual prolongación de la Falla Oaxaca hacia el norte del Valle de Tehuacan (y más allá, debajo de la cubierta volcánica del Eje Neovolcánico Trans-Mexicano). En particular, los resultados de esta parte permitirán definir la relación de la Falla Oaxaca, y del sistema volcánico Cofre de Perote-Citlaltepetl. También se tendrá información sobre la continuidad del Complejo Oaxaca hacia el norte del Valle de Tehuacan. Permitirá, también, estudiar la sutura de los complejos Oaxaca y Acatlan._x000D_ En la porción meridional de la Falla Oaxaca, nos proponemos realizar estudios geofísicos, más allá de los Valles Centrales, con el objeto, de delimitar hasta dónde se continua la Falla Oaxaca (así como la zona de sutura de los terrenos Oaxaca y Juárez)._x000D_ El objetivo es delimitar si la Falla Oaxaca puede ser trazada eventualmente hasta el Océano Pacífico._x000D_