The Exhumation of Subducted Oceanic‐Derived Eclogites: Insights From Phase Equilibrium and Thermomechanical Modeling

Abstract

The dynamical evolution and exhumation mechanisms of oceanic‐derived eclogites are controversial conundrums of oceanic subduction zones. The previous studies indicated that density is the primary factor controlling the exhumation of oceanic rocks. To explore their density evolution, we systematically investigate the phase relations and densities of different rock types in oceanic crust, including mid ocean ridge basalt (MORB), serpentinite, and global subducting sediments (GLOSS). According to the density of eclogites, these currently exposed natural eclogites can be classified into two categories: the self‐exhumation of eclogites (ρMORB < ρMantle) and the carried exhumation of eclogites (ρMORB > ρMantle). The depth limit for an exhumation of oceanic‐derived eclogites solely driven by their own buoyancies is 100–110 km, and it increases with the lithospheric thickness of the overriding plate. The parameters of carried‐exhumation, that is, KGLOSS and KSerp, are defined in order to quantitatively evaluate the assistance ability of GLOSS and serpentinites for carrying the denser eclogites. KGLOSS is mainly controlled by pressure, whereas KSerp is dominantly affected by temperature. Using 2‐D thermomechanical models, we demonstrate that the presences of low‐density, low‐viscosity GLOSS and seafloor serpentinites are the prerequisites for the exhumation of oceanic‐derived eclogites. Our results show that oceanic‐derived eclogites should be stalled and exhumed slowly at the Moho and Conrad discontinuities (named Moho/Conrad stagnation). We propose that oceanic‐derived eclogites should undergo a two‐stage exhumation generally, that is, early fast exhumation driven by buoyancy at mantle levels, and final exposure to surface actuated by tectonic exhumation facilitated by divergence between upper plate and accretionary wedge or by rollback of lower plate.

Metamorphic Facies and Deformation Fabrics Diagnostic of Subduction: Insights From 2D Numerical Models

“…The geodynamic evolution of subduction systems has been thoroughly analyzed and described by many authors (e.g., Arcay et al., 2005; Dai et al., 2018; Gerya, 2011; Gerya & Stöckhert, 2006; Marotta & Spalla, 2007; Regorda et al., 2017; Roda et al., 2012; Stern & Gerya, 2018; Wang et al., 2019). Here, we will describe only the main features of the thermal evolution of the different models, to highlight the impact of the subduction velocity on the predicted P‐T conditions in different parts of the subduction system.…”

“…Following the pioneering works carried out in the seventies (e.g., England & Richardson, 1977; Oxburgh & Turcotte, 1970, 1971; Toksöz & Bird, 1977) several key advances have been made in numerical modeling methods that have improved the understanding of the thermo‐mechanical evolution of the oceanic and continental lithosphere during oceanic subduction and continental collision (e.g., Billen, 2008; Cloos, 1982, 1983, 1993; Cloos & Shreve, 1988a, 1988b; England & Thompson, 1984; Gerya & Stöckhert, 2006; Gerya et al., 2002; Luoni et al., 2020; Magni et al., 2014; Marotta & Spalla, 2007; Peacock, 1989, 1990b; Roda et al., 2011, 2012; Thompson, 1981; van Hunen & Allen, 2011). In particular, several models have shown the importance of dehydration‐hydration reactions (e.g., Arcay et al., 2005; Faccenda et al., 2009; Faccenda & Mancktelow, 2010; Guillot et al., 2001; Meda et al., 2010; Peacock, 1990a; Quinquis & Buiter, 2014; Rupke et al., 2004; Wang et al., 2019) on the mantle wedge dynamics, with the activation of short wavelength convective cells consequent to the serpentinization and the associated decrease in viscosity (Gerya et al., 2002; Hebert et al., 2009; Hirth & Kohlstedt, 2003; Honda & Saito, 2003; Meda et al., 2010; Regorda et al., 2017; Roda et al., 2010).…”

High‐P granulites of the Songshugou area (Qinling Orogen, east‐central China): Petrography, phase relations, and U/Pb zircon geochronology

“…The granulites do not show evidence of eclogite facies metamorphism, and they may have formed due to foundering of a crustal root underneath an active arc or due to subduction erosion. The convergence of the P – T paths below ~710°C, ~0.9 GPa (≲34 km depth) observed in the Songshugou area marks the beginning of exhumation stalling (Bader et al, 2019), which typically happens at the Moho, above which buoyancy can no longer drive exhumation (Wang, Zhang, Li, Li, & Bader, 2019). The P – T data of the Songshugou area therefore are inconsistent with significantly thickened crust, as it occurs underneath collisional orogens; rather, they imply a crustal thickness similar to the ~35 km of the present‐day Japanese arc system (e.g.…”

Joint Geodynamic‐Geophysical Inversion Suggests Passive Subduction and Accretion of the Ontong Java Plateau

“…We also consider the prograde metamorphism within the subducting plate. The data we adopted come from a previous study on this phase transformation (Y. Wang et al., 2019). On the basis of a layered structure, we further defined a young oceanic crust, island arc and oceanic plateau, together with convergence velocities (see Text S3 in Supporting Information ).…”