Numerical Experiments as a Guide to Rates of Metamorphic Processes

Computational modeling of metamorphic terranes provides an insightful method to determine the P-T-t evolution as well as the timing and rates of processes active during the metamorphic cycle. During the past few decades, thermal modeling of metamorphic terranes has increased in sophistication as more complete physics is incorporated in the algorithms and numerical models more closely approximate natural systems. With these advances, high resolution 4-D information can be extracted and compared with data derived from other thermobarometric and geochronologic techniques.

A series of time-dependent 3D computational experiments of heat and mass transport for contact metamorphic systems with variable host rock and intrusion properties allows rates and timing of processes to be examined, in the context of a controlled environment, throughout the entire system's history. For example, calculations indicate that host rocks, with anisotropic permeability (K) of Kx,y 10^-15 Kz =10^-16m², allow convection. In this system, rocks 0.25km from the contact with a 15x3x3km tabular granitic intrusion at 12km depth, heat rapidly to the first thermal maximum (Tmax1 at ~ 20,000 yrs), cool to a local minimum (~100,000 yrs), and reheat through a second long-lived cycle (~ 0.0002^oC/yr) as warm fluids infiltrate, to Tmax2  ~450,000 yrs. With increasing distance from the contact, rocks heat more uniformly (<0.001^oC/yr), experience a single thermal maximum and prograde longer (~ 500,000 yrs.) Time integrated fluid flux reaches a maximum of  ~250,000 m³H₂O/m² rock. In contrast, rocks heated by conduction undergo a single, short rapid, thermal pulse, Tmax ~20,000 yrs.  These data suggest that modeling provides valuable new insights into rates of these dynamic processes, the timing of events during metamorphism and constrains scenarios for P-T-t evolution.