INTERSEISMIC STRESS TRANSIENTS REVEALED BY FORWARD MODELING OF GARNET GROWTH-DIFFUSION PROFILES
We forward model coupled phase equilibria and garnet diffusion to fit the observed garnet profiles and test the likely P-T-t paths using a Monte Carlo-type approach. The 2-D chemical map is modeled as an off-center section of a trapezohedral-trisoctahedron that approaches the spherical diffusion model. Sharp compositional contrasts between the garnet zones are little modified by diffusion, requiring that the high-temperature stage (> 700 °C) lasted only ~103 years. The simulation also demonstrates that a ~ 4 kbar pressure increase after the temperature peak is necessary to reproduce the garnet zoning. Remarkably, this post-peak-T compression (9 kbar → 14.5 kbar) lasted only ~500 years.
Such ultra-fast compression, if due to burial along a lithostatic pressure gradient, requires a descent rate >30 m/yr, orders of magnitude faster than long-term tectonic rates. Local overpressure due to partial melting in a confined volume by transient shear heating could explain the ultra-fast compression without necessitating burial to great depth (Vrijmoed et al. 2009, Eur. J. Mineral.). Dynamic simulation (Kelemen & Hirth, 2007, Nature) shows that shear heating events can occur with a period of 200-250 years in convergent zones, and the thermally-activated rapid stress drops can be associated with episodic earthquake cycles at intermediate depth. The garnet zonation documents a similar timescale and, thus, may reflect ancient interseismic stress transients in the orogenic belt.