EVIDENCE FOR FAST MAGMA CHAMBER CONSTRUCTION: THE DEFORMED CARAPACE OF THE SAN JOSE TONALITE PLUTON, MEXICO

Field and theoretical studies indicate that strain rates associated with pluton emplacement can be much faster than rates associated with regional tectonic deformation. Rate estimates for emplacement-related deformation cover a huge range from 10^-3 s^-1 to 10^-12 s^-1, whereas regional tectonic strain rates are thought to lie in the range of 10^-13 s^-1 to 10^-15 s^-1. The problem is what microstructural evidence would reflect such rapid deformation, and under what conditions might it be preserved?

We present the case of the San Jose pluton in Baja California. The pluton contains two separate units - the northern and central units. The northern unit was emplaced first, and had partly crystallized before being intruded by the central unit tonalite. The contact between these two units indicates that they were juxtaposed as magmas, yet a sharp solid-state deformation gradient occurs within the margin of the northern unit. Rocks at the inner edge of this deformation gradient show an intrusive igneous microstructure, whereas rocks at the outer edge are mylonitic. Owing to the "post-kinematic" timing of the pluton, this solid-state deformation apparently occurred during emplacement of the central unit into the northern unit, as opposed to occurring during a later regional deformation overprint. If the solid-state deformation in the northern unit was caused by intrusion of the central unit, microstructures in the deformed rocks are directly linked to growth of the central chamber.

Evidence for melt-present deformation in these rocks suggests that they were deformed at temperatures near the wet tonalite solidus (~680° C). At these temperatures, plagioclase should deform largely by dislocation creep at tectonic strain rates. Instead, plagioclase underwent extensive brittle deformation. This, and other microstructural evidence, suggests that these rocks were deformed at relatively fast rates, and provides some confirmation that the central unit chamber was constructed very rapidly, perhaps in a matter of months to thousands of years. We suggest that preservation of subtle microstructures resulted from rapid drops in stress, strain rate and temperature following emplacement of the central unit.