Generation of talc in the mantle wedge and its role in subduction dynamics in central Mexico

- We provide seismic constraints that can explain flat subduction dynamics in Mexico.
- We compare seismic observations with petrological results to examine the role of water.
- We suggest that talc is produced in the mantle wedge of the subduction system.
- We suggest talc played critical role in the dynamics during slab flattening process.

Geophysical evidence shows the presence of low-seismic velocity material at the surface of slabs in subduction zones. In the central Mexican subduction zone this appears as a thin (∼4 km) low-velocity zone that absorbs nearly all of the strain. The P-to-S velocity ratio as a function of S wave velocity distinguishes among the various candidate hydrous (low-strength) minerals; the thin layer in the flat-slab region is most consistent with a layer showing enrichment in talc overlying normal MORB-like gabbro. Based on available thermodynamic data for equilibria for talc, its generation at the trench is nearly impossible, and hence we propose it originates from the mantle wedge during the slab flattening process coupled with trench rollback. The evolution of this low-strength zone has important implications for the dynamics of the slab-flattening process as well as the geochemistry of the mantle wedge and arc in central Mexico.