The Global Range of Subduction Zone Thermal Structures From Exhumed Blueschists and Eclogites: Rocks are Hotter than Models

Abstract:

The maximum-pressure P-T conditions (P_max-T) and prograde P-Tpaths of exhumed

subduction-related metamorphic rocks are compared to predictions of P-Tconditions from

computational thermal models of subduction systems. While the range of proposed models

encompasses most estimated P_max-Tconditions, models predict temperatures that are on average

colder than those recorded by exhumed rocks. In general, discrepancies are greatest for P_max< 2 GPa

where only a few of the highest-Tmodeled paths overlap typical petrologic observations and model

averages are 100-300 °C colder than average conditions recorded by rocks. Prograde P-Tpaths

similarly indicate warmer subduction than typical models. Both petrologic estimates and models have

inherent biases. Petrologic analysis may overestimate temperatures at P_maxwhere overprinting

occurs during exhumation, although P-Tpaths suggest that relatively warm conditions are experienced

by rocks on the prograde subduction path. Models may underestimate temperatures at depth by

neglecting shear heating, hydration reactions and fluid and rock advection. Our compilation and

comparison suggest that exhumed high-P rocks provide a more accurate constraint on P-Tconditions

within subduction zones, and that those conditions may closely represent the subduction geotherm.

While exhumation processes in subduction zones require closer petrologic scrutiny, the next

generation of models should more comprehensively incorporate all sources of heat. Subduction-zone

thermal structures from currently available models appear to be inaccurate, and this mismatch has

wide-reaching implications for our understanding of global geochemical cycles, the petrologic

structure of subduction zones, and fluid-rock interactions and seismicity within subduction zones.