The Crust and Upper Mantle of Central Mexico

Abstract

Summary Six strain and inertial seismographs at Queen Creek, Arizona (QC-AZ), recorded six modes of surface waves from a suite of 18 earthquakes near Chiapas, Mexico. The six group-velocity dispersion curves were used in a least-squares inversion to estimate the shear velocity structure of the upper 380km of crust and mantle in central Mexico. Multiple filter analysis of 108 seismograms produced group velocity dispersion curves slower than average continental paths. The range of the average deviation from the mean dispersion curve for the fundamental modes was 0.025 to 0-160 kni s-' for Rayleigh waves and 0.025-0.281 km s-l for Love waves. The inversion models have low velocities that correspond with a representative geotherm and petrologic P-Tdiagrams to indicate partial melting. The 4layer crust is 30 km thick with a high-temperature gradient LVZ in the granitic layer and with a LVZ in the lower 8 km of a basaltic layer resulting from a high geothermal gradient or from partial melting of water saturated rock. The mantle has a 4-8 km thick solid lid and a shallow low velocity zone. The lowest velocities correspond to 10-20 per cent partial melting. A sharp velocity gradient at 70-80 km probably results from both the phase change to garnet pyrolite and the lower extreme of partial water pressure with the disappearance of amphiboles from the host pyrolite. Based on the velocities, 5 per cent anhydrous melting extends to 260 km. From 300 to 380 km temperature gradients and crystal lattice instabilities prior to the olivine-spinel phase change produce another LVZ. A hypothesis is presented that the large volume of low density magma produces a regional vertical force that creates high Aat plateaus as found in central Mexico and the Colorado Plateau. The inversion models have a 7.5s S-wave residual relative to the Canadian Shield model CANSD in agreement with observed US station anomalies.