INTERSEISMIC STRESS TRANSIENTS REVEALED BY FORWARD MODELING OF GARNET GROWTH-DIFFUSION PROFILES

Metamorphic reactions are thought to take place at pressures near lithostatic. Extremely fast rates of compression or decompression, however, are inconsistent with the lithostatic model and require alternative explanations. Eclogite in a Taconic thrust slice in Northwest Connecticut records an eclogite facies pressure-temperature (P-T) anomaly superimposed on the regional amphibolite facies background (Chu et al. 2016, Lithos). The eclogites crop out in a band of migmatized paragneiss bordered by steep drops in grade to amphibolite facies over 10-50 m length scales. The garnet compositional maps display a non-concentric zoning pattern in the core; the garnet core is surrounded by discontinuous growth zones. Pseudosection analysis shows that two of the growth zones correspond to a T peak (~ 760 °C at 9 kbar) followed by a P peak (~ 14.5 kbar at 710 °C).

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.