This paper provides an overview of research on core from Oman Drilling Project Hole BT1B and the surrounding area, plus new data and calculations, constraining processes in the Tethyan subduction zone beneath the Samail ophiolite. The area is underlain by gently dipping, broadly folded layers of allochthonous Hawasina pelagic sediments, the metamorphic sole of the Samail ophiolite, and Banded Unit peridotites at the base of the Samail mantle section. Despite reactivation of some faults during uplift of the Jebel Akdar and Saih Hatat domes, the area preserves the tectonic “stratigraphy” of the Cretaceous subduction zone. Gently dipping listvenite bands, parallel to peridotite banding and to contacts between the peridotite and the metamorphic sole, replace peridotite at and near the basal thrust. Listvenites formed at less than 200°C and (poorly constrained) depths of 25 to 40 km by reaction with CO2-rich, aqueous fluids migrating from greater depths, derived from devolatilization of subducting sediments analogous to clastic sediments in the Hawasina Formation, at 400-500°. Such processes could form important reservoirs for subducted CO2. Listvenite formation was accompanied by ductile deformation of serpentinites and listvenites – perhaps facilitated by fluid-rock reaction – in a process that could lead to aseismic subduction in some regions. Addition of H2O and CO2 to the mantle wedge, forming serpentinites and listvenites, caused large increases in the solid mass and volume of the rocks. This may have been accommodated by fractures formed as a result of volume changes, perhaps mainly at a serpentinization front.

Nearly all frictional interfaces strengthen as the logarithm of time when sliding at ultra-low speeds. Observations of also logarithmic-in-time growth of interfacial contact area under such conditions has led to constitutive models which assume that this frictional strengthening results from purely time-dependent, and slip-insensitive, contact area growth. The main laboratory support for such strengthening has traditionally been derived from increases in friction during ‘load-point hold’ experiments, wherein a sliding interface is allowed to gradually self-relax down to sub-nanometric slip rates. In contrast , following step decreases in the shear loading rate, friction is widely reported to increase over a characteristic slip scale, independent of the magnitude of the slip-rate decrease-a signature of slip-dependent strengthening. To investigate this apparent contradiction, we subjected granite samples to a series of step decreases in shear rate of up to 3.5 orders of magnitude, and load-point holds of up to 10,000 s, such that both protocols accessed the phenomenologi-cal regime traditionally inferred to demonstrate time-dependent fric-tional strengthening. When modeling the resultant data, which probe interfacial slip rates ranging from 3 μm/s to less than 10^-5 μm/s, we found that constitutive models where low slip-rate friction evolution mimics log-time contact area growth require parameters that differ by orders of magnitude across the different experiments. In contrast, an alternative constitutive model in which friction evolves only with interfacial slip fits most of the data well with nearly identical parameters. This leads to the surprising conclusion that frictional strengthening is dominantly slip dependent even at sub-nanometric slip rates.

We evaluated the mineral fractions of listvenite (completely carbonated peridotite) in Hole BT1B drilled by the ICDP Oman Drilling Project from 3D X-ray Computed Tomography (XCT) images. Total >250,000 XCT images from continuous ~200 m listvenite core samples were analyzed. Histograms of the intensity of X-ray attenuation (XCT number) of each XCT core-slice image were fitted assuming that the CT histogram is composed of magnesite, quartz, and dolomite peaks. These mineral peaks were confirmed by comparison of XCT numbers with chemical mapping data obtained using an XRF core scanner. In most core sections, XCT data indicate that listvenite matrix is composed of magnesite and quartz, consistent with discrete XRD and XRF data. Veins are composed mostly of dolomite. The mean abundance of dolomite in listvenite from BT1B is 11 vol.%, whereas that in core sections within 15 m of the basal thrust is >50 vol.%, suggesting that the basal thrust acted as a pathway for Ca- and CO fraction than that of Oman peridotite (39:60:1), indicating enrichment of Si during carbonation. P- and S-wave velocities and density of listvenite is close to that of peridotite while higher than that of serpentinites. These results suggest that limited material transfer during carbonation and hydration of the Oman Ophiolite, except for Si, Ca, CO2 and H2O, but potential as an overlooked carrier of CO2 into the deep of Earth’s interior.