ANOMALOUS MANTLE WEDGE IN SOUTHERN MEXICO (CHIAPAS): OBSERVATIOBAL CONSTRAINTS AND NUMERICAL MODELS

In southern México the subducting Cocos slab drastically changes its geometry: from a flat slab in Central México to a ~ 45° dip angle beneath Chiapas. Also, the currently active volcanic arc, the modern Chiapanecan volcanic arc, is oblique and situated far inland from the Middle America trench, where the slab depth is ~ 200 km. In contrast, the Central America volcanic arc is parallel to the Middle America trench and the slab depth is ~ 100 km. A 2D steady-state thermo-mechanical model explains the calc-alkaline volcanism by high temperature (~ 1300° C) in the mantle wedge just beneath the Central America volcanic arc and strong dehydration (~ 5 wt.%) of the Cocos slab. In contrast, the thermal model for the modern Chiapanecan volcanic arc shows high P-T conditions beneath the coast where the extinct Miocene Chiapanecan arc is present, and is therefore unable to offer a reasonable explanation for the origin of the modern Chiapanecan volcanic arc. We propose a model in which the origin of the modern Chiapanecan volcanic arc is related to the space-time evolution of the Cocos slab in Central México. The initiation of flat subduction in Central México in the middle Miocene would have generated a hot mantle wedge inflow from NW to SE, generating the new modern Chiapanecan volcanic arc. The position and the distinct K-alkaline volcanism at El Chichón volcano are proposed to be related to the arrival of the highly serpentinized Tehuantepec Ridge beneath modern Chiapanecan volcanic arc. The deserpentinization of Tehuantepec Ridge would have released significant amounts of water into the overlying mantle at higher depths (150-200 km), therefore favoring vigorous melting of the mantle wedge and probably of the slab.