Constraints on the rheology of the lower crust in a strike-slip plate boundary:evidence from the San Quintin xenoliths,Baja California,Mexico

Author:

van der Werf, Thomas  Chatzaras, Vasileios  Kriegsman, Leo Marcel  Kronenberg, Andreas  Tikoff, Basil  Drury, Martyn R.  

Journal:

SOLID EARTH

Issue Date:

2017

Abstract(summary):

The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene-Holocene San Quintin volcanic field of northern Baja California, Mexico. The San Quintin volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintin volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750-890 degrees C and pressures of 400-560MPa, corresponding to 15-22 km depth. A hot crustal geotherm of 40 degrees C km(-1) is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12-33 and 17MPa for plagioclase and olivine, respectively. The lower range of stresses (12-17MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress in-crease. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1-1.3 x 10(20) Pa.s for the high temperature conditions (890 degrees C) in the lower crust. Significantly lower viscosities in the range of 10(16)-10(19) Pa.s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7 x 10(19) Pa.s, which indicates the lack of an upper-mantle lid beneath northern Baja California. Our data show that during post-seismic transients, the upper mantle and the lower crust in the Pacific-Baja California plate boundary are characterized by similar and low differential stress. Transient viscosity of the lower crust is similar to the viscosity of the upper mantle.

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