NEOGENE LEFT-LATERAL SHEARING OF THE CENTRAL PART OF MEXICO: THE WESTERN TERMINATION OF THE POLOCHIC-MOTAGUA FAULT SYSTEM?

The left-lateral strike-slip Polochic-Motagua fault system constitutes the boundary between the North America and the Caribbean plates. It is generally assumed that this system extends to the west to a triple junction point with the Cocos plate boundary. However, GPS data shows a westward decrease of the movement (Barrier, oral com., 2004). How is the deformation at the western termination of the Polochic-Motagua system accommodated? Guzmán-Speziale and Meneses-Rochas (2000) suggest that the motion between North America and the Caribbean dies out at the northwestern end of the strike-slip faults system of southeastern Mexico. On the basis of new field data collected in the central part of Mexico, we suggest that the motion between the two plates is distributed since the Miocene within a broad shear zone that extends from the Chiapas to central Mexico. In the San Luis Potosí area, we have made a detailed fault slip analysis on N000° grabens and N140° normal faults (described here as the Zacatecas-Zimapan fault system, ZZFS) that affect lower Oligocene volcanic rocks. Fault plane slickensides observed along the ZZFS indicate a left-lateral transtensional motion while field observations made along the flanks of the San Luis Potosi graben evidence right-lateral transtensional motion. Both transtensive systems have accommodated a post-27 My left-lateral shearing. Other data collected southeastward indicate that the ZZFS extends along a left-lateral shear zone that affects Pliocene Trans-Mexican volcanic rocks. At present the main seismic active zone in Central Mexico is the Trans Mexican Volcanic Belt (TMVB) that is also affected by left-lateral transtensive deformation. We conclude that central Mexico was affected during the Neogene by left-lateral shearing. During the Miocene and Pliocene the ZZFS accommodated a part of the deformation linked to the displacement of the Chortis block along the southern edge of Mexico. At present the motion between the two plates does not die out in southern Mexico but is accommodated along a 1000 km broad shear zone that extends from the Chiapas to the TMVB.

References : Guzmán-Speziale M., Meneses-Rocha J.J., 2000, The North America-Caribbean plate boundary west of the Motagua-Polochic fault system: a fault jog in southeastern Mexico. Journal of South American Sciences 13, p. 459–468.