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Solid Earth An interactive open-access journal of the European Geosciences Union
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Volume 8, issue 6
Solid Earth, 8, 1211–1239, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: Analysis of deformation microstructures and mechanisms on...

Solid Earth, 8, 1211–1239, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 21 Dec 2017

Research article | 21 Dec 2017

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

Thomas van der Werf1, Vasileios Chatzaras1,2, Leo Marcel Kriegsman1,3, Andreas Kronenberg4, Basil Tikoff2, and Martyn R. Drury1 Thomas van der Werf et al.
  • 1Department of Earth Sciences, Utrecht University, Utrecht, the Netherlands
  • 2Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin, USA
  • 3Department of Research & Education, Naturalis Biodiversity Center, Leiden, the Netherlands
  • 4Department of Geology and Geophysics, Texas A&M University, College Station, Texas, USA

Abstract. 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 Quintín volcanic field of northern Baja California, Mexico. The San Quintín 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 Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750–890 °C and pressures of 400–560 MPa, corresponding to 15–22 km depth. A hot crustal geotherm of 40 ° 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 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12–17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1–1.3×1020 Pa ⋅ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016–1019 Pa ⋅ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 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.

Publications Copernicus
Short summary
The strength of Earth's lower crust affects the cycle of earthquakes in tectonic plate boundaries. To understand the mechanical properties of the lower crust beneath northern Baja California, Mexico, we studied rocks, which were transferred to the surface during the eruption of Quaternary volcanoes. The lower crust is strongly deformed, hot, and dry. During transient events of deformation, the lower crust is weak, and its strength is similar to the strength of the upper mantle.
The strength of Earth's lower crust affects the cycle of earthquakes in tectonic plate...