Continental plate collision, P-T-t-z conditions and unstable vs. stable plate dynamics: Insights from thermo-mechanical modelling

摘要

There was much debate recently on the mechanisms of continental convergence and related pressure-temperature (P—T) conditions, both in modeling and petrologic community. Depending on the mechanisms of convergence (subduction, collision, folding or RT instability) one can argue about the possibility of large-scale deviations of pressure and temperature in the accretion prism and below it, from "reference" (i.e. lithostatic) conditions commonly used for petrologic reconstructions of evolution of exhumed metamorphic rocks. These deviations can be caused, for example, by tectonic overpressure that, as suggested in some studies, may be responsible for formation of UHP (Ultra High Pressure) rocks. However, overpressure in exhumation zones can be built only in specific collision scenarios associated with plate coupling (pure shear, folding). In this study, we analyze conditions that define various mechanisms of convergence, and consequently, of exhumation. These mechanisms can be represented as a superposition of (1) simple shear (subduction), (2) pure shear (collision), (3) folding and (4) Rayleigh-Taylor instability: We study these scenarios using a thermo-mechanical model that accounts for brittle—elastic-ductile rheology, surface processes, and metamorphic changes. It appears that stable, "oceanic-type" subduction may occur in the case of cold lithospheres (T_(Moho) < 550 °C) and relatively high convergence rates (>3-5 cm/yr). Depending on the lower-crustal rheology (strong or weak), either the whole (upper and lower) crust or only the lower crust can be involved in subduction. In case of weak metamorphic rheologies, phase changes improve chances for stable subduction. Pure shear becomes a dominant mechanism when T_(Moho) > 550 °C or convergence rates are lower than 3 cm/yr. Large-scale folding is favored in case of T_(Moho)=500-650 °C and is more effective in the case of mechanical coupling between crust and mantle (e.g., strong lower crust). Gravitational (Rayleigh-Taylor) instabilities overcome other mechanisms for very high values of T_(Moho) (>800 °C) and may lead to development of subvertical "cold spots." However, it is reasonable to assume that in most cases continental collision is initiated at oceanic subduction rate, which is rarely slower than 5 cm/yr. This rate is sufficient to drive continental subduction during the first several Myr of collision. In this case, the subduction channel is characterized by nearly lithostatic pressure conditions. Large-scale zones of tectonic overpressure may be built outside the channel but do not affect the exhumed rocks. Overpressure may be built inside the channel in the short moment of its closure. We suggest that most continental orogenic belts could have started their formation from continental subduction. This evokes small tectonic overpressures and thus deep origin of the UHP rocks that may be brought to the surface via the suggested multi-level mechanism of exhumation.