Late Cenozoic tectonics offshore western Mexico and its relation to the structure and volcanic activity in the western Trans-Mexican Volcanic Belt

Abstract

The calculated convergence rate indicates that tectonics has been tensional in western Mexico for the last 3 m.y. This deformation style is expressed in the formation of rift structures in the western Trans-Mexican Volcanic Belt [TMVB]. The obtained values for the convergence rate predict less tensional tectonics (tending to be neutral) in the southeastern part of the interacting zone Rivera-North America, but more tensional in the northwestern part through time. On the other hand, a qualitative analysis of the spreading rates shows that they were systematically faster before ó.5 Ma than after. Subsequently, after testing different kinematic parameters, it was concluded that spreading rate of the Pacific-Rivera rise strongly influences the convergence rate. Therefore, a positive correlation between the evolution of spreading rate of the Pacific-Rivera rise and geology on land can be made. This correlation shows that relatively fast spreading rate coincides with monogenetic calc-alkaline volcanism in the western TMVB at least since 10 m.y. ago. Nevertheless, this kind of volcanism has continued up to the present. The ·spreading rate became much slower (and thcrefQre convergence rate became slower) during the period ó.5 Ma-3.5 Ma, coinciding with widespread extensional tectonics, explosive volcanism and lacustrine sedimentation in the Colima, TepicZacoalco and Chapala rifts, and between 4.ó Ma and 3.9 Ma with alkaline volcanism in the Colima and Tcpic-Zacoalco rifts. A slight increase in the spreading rate after 3.5 Ma coincides with a decrease in volcanic activity (mainly alkaline). During 1.7 Ma-0.7 Ma, the spreading rate slowed down again coinciding with continuous normal faulting in the Colima, Tepic-Zacoalco and Chapa! a rifts, and in part with :mother period of alkaline volcanism (1.4 Ma-0.2 Ma) in the Colima and Tcpic-Zacoalco rifts. After 0.7 Ma, the spreading rate became slightly faster. A genetic relationship of the content of incompatible elements in volcanic rocks of the region to convergence rate was found, implying that higher subduction rate supplies more accreted sediments in the mantle wedge. This is used to explain geochemical differences among volcanic rocks along the volcanic front of the western TMYB.