Tectonic Overpressures in High-Strain Zones: A Quantitative Investigation

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0003315JiangTectonic overpressure is an old, fundamental but still controversial problem in geology. Recent work clearly demonstrates that, in non-Andersonian stress regimes, the magnitude of tectonic overpressures is not limited by the rock strength. Ductile high-strain zones are non-Andersonian stress domains. Little is known of overpressures in such important tectonic environments. This investigation uses numerical and analytical modeling techniques to address two questions: (1) Can high overpressures be generated in high-strain zones, given realistic rheology and natural deformation conditions? (2) Can high overpressures,if generated, be dynamically maintained for significant geological times to affect metamorphic reactions? Regardless of whether the answers are positive or negative, the research results will have significant scientific merit. A negative answer to either question would place the now widely-practiced pressure-depth conversion on a rigorous basis. A positive answer to both questions implies that the pressure data from high-strain zone rocks cannot be converted to depths without qualification. Current tectonic interpretation of geothermobarometric data needs to be reconsidered. Since many high-pressure and ultrahigh-pressure rocks show evidence of a high-strain zone origin, a significant overpressure would imply that these rocks may have formed at depths considerably shallower than currently believed. A higher-than-lithostatic pressure gradient as a result of overpressure buildup is a possible driving force to be considered for fluid flow, melt extraction and migration, and pluton emplacement.