Deformation of Late Miocene Plutons along the Tonalá Shear Zone: A Relict Plate Boundary in the Eastern Tehuantepec Region

The Maya Block is part of the North America plate, and it is bordered by the Polochic-Motagua fault system. The Chiapas massif, a Late Permian plutonic complex is the most conspicuous feature in the western Maya Block. Late Miocene granitoides (U-Pb SHRIMP ages ca. 10 Ma) cropout along the western slopes of the massif, and it is intensely deformed by the WNW striking, ~120 km long, Tonalá shear zone (TSZ). The plutons are elongated parallel to the TSZ, indicating syn- or post-tectonic modification. The TSZ is characterized by mylonites with subhorizontal lineation, and left-lateral, strike-slip kinematic indicators. A collection of 30 paleomagnetic sites in the Miocene plutons along the TSZ yields well-defined magnetizations residing in a cubic magnetic phase. The overall mean (Dec=0.1°-Inc=39.3°; k=13.9, alpha95=10.3°; n=20 sites) is discordant when compared to the North America expected direction suggesting northward regional tilt of the sampling area (12.3°+/-13.8°). However, angular dispersion expressed primarily within the declination data is much larger than can be explained by paleosecular variation. The regional distribution of paleomagnetic sites explains this observation. NW of the shear zone, and apparently unaffected by it, sites near the town of Tepanatepec yield a mean Dec=352.9°-Inc=41.8°, k=36.2, alpha95=12.9°; n=5 (of statistically concordant declination). In turn, sites within the shear zone: (1) near the town of Arriaga, yield a mean Dec=15.8° - Inc=38.1°; k=16.0, alpha95=13.3°; n=9 sites (rotated in a clockwise sense); and, (2) sites near the town of Tonalá yield a mean Dec=329.6°-Inc=38.1°; k=14.4, alpha95=17.9°; n=5 sites (rotated in an anticlockwise sense). We have previously interpreted the TZS as a relict plate boundary that accommodated displacement of the Chortis blocks prior to being attached to the Caribbean plate. The observation of clockwise and anticlockwise rotations of Miocene plutons can be explained by a significant component of post emplacement transpression along the TSZ.