Metamorphism and Deformation on Subduction Interfaces: 2. Petrological and Tectonic Implications

Abstract

In the companion paper (England & Smye, 2022), we lay out a framework for determining distributions of pressure, temperature, deviatoric stress, and strain rate on subduction interfaces. The purpose of the present paper is to use that framework to investigate the records of subduction zone processes that are preserved in rocks of high-pressure-low-temperature (HPLT) terrains. Among the questions we shall address are: What ranges of mineral assemblages form on the interface for different protoliths, under differing parameters of subduction? How does metamorphism affect the physical properties of the subduction interface, particularly its density, water content and strength? Which process or processes determine the upper limits to temperature and pressure that are recorded by HPLT rocks? How do the evolution of metamorphic phases, of density, and of strength of the interface influence the transport of rocks and volatiles to depth along the interface-in particular to beneath the volcanic arcs, and deeper? How does the presence of differing lithologies on the plate interface influence the generation of earthquakes?Previous studies that addressed these questions generally considered a limited range of protoliths, and often neglected dissipative heating, which affects temperatures on the plate interface by some hundreds of degrees (e.g.,

Metamorphism and Deformation on Subduction Interfaces: 1. Physical Framework

“…Some of these paths (e.g., J.‐L. Li et al., 2016) appear to record a few kilobars of approximately isothermal decompression, followed by a P‐T‐t path that is consistent with later heating and exhumation within thickened crust, as we discuss in the companion paper (Smye & England, 2022). Others, however (e.g., Blanco‐Quintero et al., 2011), are consistent with the circulation that might be expected within the plate interface if a rapid increase in sediment influx were to exceed the capacity of the interface (Shreve & Cloos, 1986, Figure 3).…”

“…These estimated shear stresses imply that the effective viscosity of interfaces in which the relative plate motion is accommodated by distributed flow, must be much higher (≳10 19 Pa s: tens of MPa divided by strain rates of ∼10 −14 to ∼10 −11 s −1 , Section 5.1.1), than assumed in some previous analyses (∼10 17 Pa s (e.g., Shreve & Cloos, 1986, Section 5.3)). In the companion paper (Smye & England, 2022) we discuss the conditions under which different rock types within the interface are capable of sustaining these stresses.…”

“…We illustrate the principal features of this behavior by evaluating the expression for capacity (Equation C26) using the rheological parameters of wet quartzite and aragonite. We emphasize that these calculations are purely illustrative, they are uncertain by at least a factor of ten-and far greater than that if the influence of dissolution-precipitation creep is considered (Smye & England, 2022, Section 4 and Figure 13). Figure 11 shows that interface capacity of rocks having the rheological properties of wet quartzite (Hirth et al, 2001) drop from ∼10 km at 300°C to of order 1 km at 800°C.…”

“…Furthermore at temperatures of several hundred degrees (Figure 1), pseudotachylytes and other features associated with rapid slip may be erased swiftly (e.g., Fondriest et al., 2020; Kirkpatrick & Rowe, 2013; Kirkpatrick et al., 2009). In the companion paper (Smye & England, 2022) we discuss which of the lithologies present in the plate interface are likely to strain slowly enough under ambient conditions that the relative motion across the interface would likely be seismic.…”

“…Section 6 investigates the P-T-time (P-T-t) and P-T-deformation paths that rocks follow on the subduction interface. In a companion paper (Smye & England, 2022) we use this framework to calculate the mineral parageneses that would be generated for sedimentary and mafic rock types that are likely to be found on subduction interfaces, to constrain their rheological parameters and other physical properties, and to relate observations of HPLT terrains to processes occurring on present-day subduction interfaces.…”