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TitleOn unusual conditions for the exhumation of subducted oceanic crustal rocks: How to make rocks hotter than models
AuthorWang, Y; Wang, KORCID logo; Zhang, L
SourceEarth and Planetary Science Letters 615, 118213, 2023 p. 1-13,
Alt SeriesNatural Resources Canada, Contribution Series 20220669
Mediapaper; digital; on-line
File formatpdf
Subjectsgeneral geology; subduction zones; subduction; metamorphic rocks; modelling
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2023 05 25
AbstractThe thermal structure of subduction zones controls many importance processes such as metamorphic devolatilization, arc magmatism, and seismicity. However, the thermal state of subducted oceanic slab predicted by subduction zone thermal models down to ~80 km depth is systematically cooler than inferred from exhumed metamorphic slab rocks, raising questions about the current understanding of subduction dynamics portrayed by these models. Upon synthesizing previous petrological studies and estimating slab ages of ancient subduction zones from metamorphic terranes, we think that exhumation of subducted oceanic metamorphic rocks is extremely rare and likely requires unusual processes that are not an integral component of normal subduction dynamics. We construct simple numerical scenarios to illustrate how the thermal regime under some unusual conditions at the beginning and ending stages of a subduction margin may deviate from the normal subduction process. At the beginning stage of subduction, a shallow maximum decoupling depth (MDD) between the slab and mantle wedge enables viscous mantle wedge flow to reach a shallow depth, bringing heat to make slab rocks warmer than in a mature subduction zone. At the ending stage of subduction, if subduction cessation is in the form of slab stalling and if the stalled slab is not immediately exhumed, the slab rocks can be warmed up by the heat from the warm mantle wedge and underlying asthenosphere. In either situation, the slab rocks can be much warmer than normal before they are exhumed by other dynamic processes. Observed exhumed rocks are not expected to represent the normal subduction process and should not be directly compared with thermal models that are designed for the normal process. To extract important information about general geodynamic processes from these rocks, we must first understand the processes these rocks went through.
Summary(Plain Language Summary, not published)
Study of earthquake processes in subduction zones requires the knowledge of the thermal regime. In addition to modern heat flow measurements, exhumed rocks from ancient subducting oceanic plate provide important calibration for models that are used to predict the thermal regime. However, recent compilation of exhumed rocks are found to be systematically warmer than what thermal models would predict, causing intense debates in the scientific community. In this work, we argue that the exhumed rocks represent unusual geological conditions and do not represent normal subduction processes that the thermal models are designed to simulate. We use numerical models do demonstrate how some of the unusual conditions can make the rocks warmer than usual. The exhumed rocks should not be directly compared with thermal models. To extract important information about general geodynamic processes from these rocks, we must first understand the processes these rocks went through.

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