THE DYNAMIC HISTORY OF THE TRANS-MEXICAN VOLCANIC BELT AND THE MEXICO SUBDUCTION ZONE

Due to its geochemical and geophysical peculiarity, the Trans-Mexican Volcanic Belt (TMVB) has spurred a large spectrum of models for its origin, which include mantle plumes, an ocean ridge jump, intra-continental rifting, and lithospheric delamination. However, when geochemical and geophysical data are integrated into a robust geologic frame, the TMVB can be seen as the result of the interaction of the Neogene Cocos-Rivera subduction system with the structural and compositionally heterogeneous southern edge of the North America lithosphere. Some important features of the present Mexico subduction system are: 1) a central and eastern region where the Cocos plate subduct horizontally up to 250 km from the trench, plunge at ~75° just before the volcanic front, and is truncated at ~450 km of depth; 2) a western region where the the Rivera slab dips at ~30° for 160 km then plunge at ~70° before the volcanic front and disappear below 350 km; 3) absence of lithospheric mantle throughout the TMVB and the region to the south; 4) a <5 km thick low viscosity zone, interpreted as a remnant of the paleo mantle wedge just above the flat slab region; 5) a diffuse low velocity anomaly beneath the TMVB.

The geologic and geochemical history of the TMVB point to a three stage evolution: 1) from ~20 to 10 Ma the initial andesitic arc migrated away from the trench showing progressively drier melting mechanism and slab melt as final stage. This is consistent with a progressive flattening of the slab until ~11-9 Ma; 2) starting at ~11 in the west, a pulse of mafic volcanism (calc-alkaline, tholeiitic and OIB-like) migrated eastward reaching the Gulf of Mexico by the end of Miocene. This is interpreted as the evidence of a lateral propagation of a slab tear that allowed infiltration of low viscosity asthenosphere, widening of the mantle wedge and producing local gravitational instabilities and dripping of the continental lithosphere; 3) since then the volcanic front started to move trenchward, with a marked phase of silicic volcanism shortly after stage two, local emplacement of enriched alkaline basalt since 5 Ma and formation of important intra-arc extensional fault systems in the western and central TMVB. These features are related to the rollback of the leading edge of the slab, enhancing asthenophere flux into the mantle wedge and crustal melting.