LARAMIDE AND NEOGENE SHALLOW SUBDUCTION IN MEXICO: CONSTRAINTS AND CONTRASTS

Shallow subduction is believed to cause widespread deformation and uplift of the upper plate. Shallow subduction is thought to be responsible for the Laramide orogeny and also characterize a segment of the Cocos plate presently subducting beneath southern Mexico. I show that the Laramide deformation in Mexico is unrelated to shallow subduction whereas the Cocos flat slab did not produced contractile deformation and is unrelated to the high elevation of the central-eastern Trans-Mexican Volcanic Belt (TMVB). In southern Mexico Laramide deformation migrated from west to east from Campanian to mid-Eocene. During the same period arc magmatism was almost continuous to the west of the deformation front and migrated eastward in a similar way. Erosion of the western continental edge also occurred at this time. Arc magmatism is incompatible with a flat slab geometry thus I conclude that the Laramide deformation was not driven by shallow subduction but rather by a period of accelerated Farallon-North America convergence that produced the closing of a back-arc basin and subduction erosion at the continental margin. The present shallow subduction is documented by seismological and MT data indicating that to the south of the central-eastern TMVB the Cocos plate is in direct contact with the North America crust. Seismological and gravity data also show that beneath the TMVB the upper mantle has a relatively low density and high temperature, incompatible with the presence of a slab. Shallow subduction began in mid-late Miocene (15-10 Ma), when arc volcanism migrated away from the trench and show an adakitic signature. The central-eastern TMVB has the highest elevation of the whole arc but no contractile deformation is observed in the region underlain by the flat slab. Thermal modeling indicates that a blueschist facies mineralogy in the flat part of the subducting plate produces a weak coupling between the two plates and control the occurrence of aseismic transient slip earthquakes. Low coupling explains why flat subduction did not produce shortening. Geologic and geomorphologic observations indicate that uplift of the central-eastern TMVB occurred after ~9 Ma. We relate this uplift to the inflow of asthenosphere in the mantle wedge following an eastward propagation of a slab detachment event occurred in late Miocene times.