Controls on the rheological properties of peridotite at a palaeosubduction interface: A transect across the base of the Oman–UAE ophiolite

Abstract

Studies of experimentally deformed rocks and small-scale natural shear zones have demonstrated that volumetrically minor phases can control strain localisation by limiting grain growth and promoting grain-size sensitive deformation mechanisms. These small-scale studies are often used to infer a critical role for minor phases in the development of plate boundaries. However, the role of minor phases in strain localisation at an actual plate boundary remains to be tested by direct observation. In order to test the hypothesis that minor phases control strain localisation at plate boundaries, we conducted microstructural analyses of peridotite samples collected along a ~1 km transect across the base of the Oman-United Arab Emirates (UAE) ophiolite. The base of the ophiolite is marked by the Semail thrust, which represents the now exhumed contact between subducted oceanic crust and the overlying mantle wedge. As such, the base of the ophiolite provides the opportunity to directly examine a former plate boundary. Our results demonstrate that the mean olivine grain size is inversely proportional to the abundance of minor phases (primarily orthopyroxene, as well as clinopyroxene, hornblende, and spinel), consistent with suppression of grain growth by grain-boundary pinning. Our results also reveal that mean olivine grain size is proportional to CPO strength (both of which generally decrease towards the metamorphic sole), suggesting that the fraction of strain produced by different deformation mechanisms varied spatially. Experimentally-derived flow laws indicate that under the inferred deformation conditions, the viscosity of olivine was grain-size sensitive. As such, grain size, and thereby the abundance of minor phases, influenced viscosity during subduction-related deformation along the base of the mantle wedge. We calculate an order of magnitude decrease in the viscosity of olivine towards the base of the ophiolite, which suggests strain was localized near the subduction interface. Our data indicate that this rheological weakening was primarily the result of more abundant minor phases near the base of the ophiolite. Our interpretations are consistent with those of previous studies on experimentally deformed rocks and smaller-scale natural shear zones that indicate minor phases can exert the primary control on strain localisation. However, our study demonstrates for the first time that minor phases can control strain localisation at the scales relevant to a major plate boundary.

Fluid‐Assisted Deformation and Strain Localization in the Cooling Mantle Wedge of a Young Subduction Zone (Semail Ophiolite)

“…Grain sizes of basal peridotites indeed follow a Zener pinning relationship (Figure S2; e.g., Hansen & Warren, ; Herwegh et al, ; Tasaka & Hiraga, ; Tasaka et al, ). However, in contrast to Ambrose et al (), we show that grain size reduction in (proto)mylonites is not associated with an increase in minor phase proportion (Figure e). This indicates that the initial olivine grain size reduction, and so peridotite weakening, is not a direct consequence of initial differences in minor phase abundance.…”

“…This suggests that olivine CPO developed by dislocation creep, by the dominant activation of the (010)[100], and the (001)[100] slip systems, respectively. The progressive decrease in olivine CPO strength with decreasing grain size (Figure ), a correlation also observed by Ambrose et al () in the basal part of the mantle in the Kwar Fakkan massif, therefore indicates a decreasing contribution of dislocation creep to olivine deformation during shearing. In ultramylonites and UMB, the very fine grain sizes and CPO randomization of olivine imply that olivine deformation no longer dominantly occurs by dislocation creep but rather by a GSS creep process.…”

“…As in those studies, we observe that grain size reduction is associated with a phase mixing process. As noted by Ambrose et al (), this phase mixing process and the increase of minor phases observed in ultramylonites/UMB likely inhibited olivine grain growth and pinned olivine grain size in the GSS creep domain (e.g., Evans et al, ). Grain sizes of basal peridotites indeed follow a Zener pinning relationship (Figure S2; e.g., Hansen & Warren, ; Herwegh et al, ; Tasaka & Hiraga, ; Tasaka et al, ).…”

“…In (proto)mylonites, subduction fluid/peridotite interaction led to a drastic grain size reduction by dissolution of millimetric grains and precipitation of polymineralic aggregates composed of grains <300 μm (Figure ). This grain size is small enough in the conditions of banded unit deformation (Ambrose et al, ) to trigger a switch in the dominant deformation mechanism from dislocation to wet diffusion creep of olivine. This fluid‐induced change in olivine dominant deformation provides an efficient weakening mechanism, enabling strain localization in the 200‐ to 500‐m‐thick banded unit at 900–800 °C (Braun et al, ; Drury et al, ; Karato & Wu, ; Newman et al, ; Précigout & Gueydan, ; Vissers et al, ; Warren & Hirth, ), that is, in (proto)mylonites produced by interaction with the subduction fluids compared to less deformed (dry) porphyroclastic tectonites lenses and overlying peridotites (Figure ).…”

“…This HT mantle is also crosscut by vertical mylonitic shear zones interpreted as developing coevally with the banded unit during the LT event (e.g., Boudier et al, ). In contrast to the basal banded unit, which has been studied only very recently from a rheological point of view in the Kwar Fakkan massif (Ambrose et al, ), the vertical shear zones have been thoroughly characterized by microstructural studies (Linckens, Herwegh, & Müntener, , Linckens, Herwegh, Müntener, & Mercolli, ; Michibayashi et al, , ; Michibayashi & Mainprice, ). The temperature of the LT deformation event, affecting high‐temperature (~1200 °C) porphyroclastic tectonites, has been estimated at ~900–800 °C for the (proto) mylonitic deformation and down to ~700 °C for the ultramylonitic deformation (Linckens, Herwegh, & Müntener, ).…”