The frictional velocity dependence and healing behavior of subduction fault zones play key roles in the nucleation of stick-slip instabilities at convergent margins. Diagenetic to low-grade metamorphic processes such as pressure solution are proposed to be responsible for the change in frictional properties of fault materials along plate interfaces; pressure solution also likely contributes to the acceleration of healing according to previous studies. Here, we report velocity-step experiments using rocks collected from ancient subduction fault zones, the Lower Mugi and Makimine mélanges of the Cretaceous Shimanto belt. The two mélanges preserve paleotemperature records corresponding to the updip and downdip limits of the seismogenic zone and deformation recording a lower versus higher degree of pressure solution. Our data show that the Lower Mugi mélange sample exhibits velocity-weakening to velocity-neutral behavior under low normal stress, and the Makimine mélange sample shows velocity-strengthening behavior under high normal stress. This is consistent with the slip behavior observed at the depths they have been subducted to along the plate interface. We also perform a series of slide-hold-slide experiments under different hydrothermal conditions using the Lower Mugi mélange sample to evaluate the role of pressure solution in fault healing and its dependency on temperature. The results show that healing rates increase in tests operated at higher temperatures. The microstructures related to pressure solution found in the postexperimental gouges support the idea that the elevated healing rate can be related to pressure solution.

Studies of the high-pressure (HP) As Sifah eclogites in the NE part of the Saih Hatat window, Oman, have used different radiometric dating results (Ar/Ar, Sm-Nd vs. U-Pb, Rb-Sr) to interpret disparate tectonic models for the timing, geometry, and cause of continental subduction – including its association with the Samail Ophiolite. To determine the absolute timing of continental subduction, we coupled geochronological and geochemical analyses of major (garnet) and accessory phases (zircon, rutile) from the highest-grade metamorphic rocks in the Saih Hatat (As Sifah eclogites). Early Permian (283.8+/-0.7 Ma) tuffaceous zircon cores are consistent with earlier interpretations that the As Sifah rocks were sourced from a distal portion of the Arabian continental margin. Data from a range of bulk compositions, metamorphic assemblages, and rock textures consistently suggest a single metamorphic event, with garnet growth starting by ~81 Ma and ceasing by ~77 Ma, with slight but consistent offsets in the timing of metamorphic (re)crystallization between different lithologies. These new data confirm previous structural, metamorphic, and geochronological interpretations for continental HP metamorphism in a single NE-dipping subduction zone beneath the already obducted Samail Ophiolite; there is no robust evidence for a ~110 Ma event or a continental-ward dipping subduction zone. Combined with literature constraints, our data suggest that the As Sifah unit was subducted and exhumed relatively slowly (~5 mm/yr) compared to other continental high-pressure settings - likely associated with the dragging to mantle depths by a mafic root, followed by long residence in the lower to middle crust.