Metamorphism and Deformation on Subduction Interfaces: 1. Physical Framework

Abstract

A thermal and mechanical framework is presented for analysis of pressure‐temperature (P‐T) data and structural observations from high‐pressure‐low‐temperature (HPLT) terrains. P‐T data from 281 HPLT rocks exhibit two regimes separated at a pressure of ∼1.5 GPa, which corresponds to the modal maximum depth of thrust faulting in subduction zones. At pressures ≲1.5 GPa, interpreted as recording conditions on the plate interface, temperatures increase at about 350°C/GPa and are consistent with conditions calculated for shear stresses of ∼30–100 MPa on the interface. Such shear stresses are high enough to carry several kilometers' thickness of sediment at least to the base of the plate interface. Burial of material on plate interfaces occurs predominantly during large‐to‐great earthquakes; the exhumation phase involves contrasts in ascent rates of adjacent units, because of their differing buoyancies and strengths. In consequence, juxtaposition of unrelated rock types is expected to be ubiquitous, during both descent and ascent. The scarcity of temperatures higher than ∼650°C at pressures ≳1.5 GPa may reflect loss of material from the wedge‐slab interface by buoyant ascent. Exhumation of rocks in the subduction interface requires substantial reduction in shear stress, most plausibly by (near‐)cessation of subduction. During prograde metamorphism temperatures increase smoothly with depth in the plate interface, with almost isothermal compression in the wedge‐slab interface. Following cessation of subduction, rocks rising along the wedge‐slab interface are likely to heat slightly during decompression. Within the plate interface, temperatures drop following the cessation of shear heating, and rocks follow counter‐clockwise hairpin PT paths.

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

“…Because of the inverted temperature gradient within the interface, it is probable that deeper-therefore cooler and stronger-rocks remain when diapirs leave, but they are then brought rapidly into contact with hot wedge and may heat and form later generations of diapirs. At the cessation of subduction, however, there presumably remains a set of rocks that have experienced the same high pressures as the diapirs that have left, but have experienced somewhat lower temperatures; these would be the rocks lying along the T ∼ 550 ± 70°C array shown in the data compilation of the companion paper (England & Smye, 2022, Figure 1).…”

“…Figure 11 shows the variation of bulk-rock H 2 O with convergence rate and rock type. Because dependence of H 2 O content on convergence rate is small once V exceeds 10 mm/yr, and because fewer than 2% of present-day subduction segments have V ≲ 10 mm/yr (England & Smye, 2022;Heuret et al, 2017;Jarrard, 1986, Figure 6), we concentrate on the water-carrying potential for individual compositions with phase relations calculated with V = 60 mm/yr (median rate, England & Smye, 2022, Figure 6), plate age of 50 Myr, and at the subduction interface. Figure 11b summarizes the concentration of mineral-bound H 2 O as a function of P for the parameters on which we concentrate; compositions are presented in descending (from top-to-bottom) order of sub-arc (2.6 GPa) bulk rock H 2 O concentration.…”

“…Partitions for the majority of compositions are insensitive to convergence rate for V ≳ 20 mm/yr. Fewer than 5% of present‐day subduction segments have convergence rates below 20 mm/yr (England & Smye, 2022; Heuret et al., 2017; Jarrard, 1986, Figure 6); if the distribution of descent speeds in the past resembles that of the present, these observations imply that similar protoliths will undergo similar prograde mineralogical evolutions along the majority of subduction interfaces.…”

“…Here, we have assumed simple shear in the channel; see Molnar et al. (1998) for the relation between A and A ′ (also England & Smye, 2022; Appendix ).…”

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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.

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