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The effect of damage on the brittle deformation of mafic and ultramafic rocks has been investigated by performing triaxial deformation experiments on thermally cracked and intact rock samples. The investigation was performed by recording the axial and lateral strains during deformation while simultaneously capturing the ultrasonic velocity, and electrical resistivity. While the peak strength is presumably controlled by the stiff intrinsic fractures, the crack opening mode also showed critical effects on the attained peak strength. The pore pressure distribution showed an apparent control over the dynamic Young's modulus as the ratio between the dynamic and static modulus of thermally cracked rocks is significantly higher than that of intact rocks. The compliant nature and the higher inelastic volumetric strain of the thermally cracked samples further indicated a possible explanation to the steep dipping plates and the taller topographic heights at the trench outer rise systems of old subduction zones. Plain Language Summary: The presence of fractures strongly influences the physical properties of rocks and not many experiments have been performed to understand the effect those fractures have on the deformation of mafic and ultramafic rocks. We find that, to address the deformation of the subduction zones, it is essential to perform experiments on such rocks under fully saturated conditions. As the incoming, relatively intact, oceanic plate sinks at the convergent boundaries, the plate bends significantly and creates a large number of faults. Therefore, the physical properties of the rock are subject to change and the way that rock deforms changes compared to an undamaged rock. However, so far, no research has been conducted to understand this change fully. Therefore, by performing experiments in the laboratory, we have attempted to understand how damage affects the physical properties and the deformation. By such experiments, we have identified that when fractures are present, the oceanic rocks show strong volume expansions before break by fracture and alter the physical properties. The results we have obtained here by gathering velocity, electrical resistivity, and strain, at the same time, can be related to the geophysical and topographic observations of the trench outer‐rise systems. Key Points: The effect of damage on brittle deformation of mafic and ultramafic rocks has been addressed via experimentsThe ultrasonic velocity, electrical resistivity, and strain are measured simultaneously during deformation of rocksPresence of cracks strongly influence deformation and physical properties which are comparable to outer rise geophysical observations [ABSTRACT FROM AUTHOR]

Geophysical surveys of the oceanic crust indicate that hydrothermal circulation universally occurs in seismic layer 2, which results in a low seismic velocity and high VP/VS due to the occurrence of cracks. However, the anomalously low VP/VS observed at the layer 2/3 transition cannot be explained by the crack model, because the effective medium theory predicts an increase in VP/VS due to crack development. In this study, we present the first evidence that shows the low VP/VS in the oceanic crust is caused by epidotization due to upward fluid flow in hydrothermal systems. Simultaneous measurements of elastic wave velocity and porosity of epidosites collected by the Oman Drilling Project show that quartz precipitation and spheroidal pores results in low VP/VS, in contrast to diabases that contain thin cracks. The presence of spheroidal pores in epidosites is supported by CT imaging, and is consistent with predictions from the effective medium model. Plain Language Summary: The low velocity and high VP/VS structure in the upper oceanic crust ("seismic layer 2") is generally associated with the development of fluid‐filled thin cracks. However, the anomalously low VP/VS ratios that have been observed in the layer 2/3 transition cannot be explained by the classical thin crack model. We provide the first direct evidence of low VP/VS ratios in epidotized rocks that are associated with the occurrence of spheroidal pores and quartz precipitation. Our theoretical calculations show that both the crack geometry and alteration minerals are required to explain the anomalously low VP/VS ratios in the transition zone. Knowledge of these unique physical properties of epidotized rocks will facilitate the detection of fluid upwelling zones in the oceanic crust, which are possibly linked to the hydrothermal ore resources. Key Points: Epidosites are characterized by low VP/VS due to the presence of spheroidal poresLow VP/VS anomaly in the layer 2/3 transition can be explained by alterations and pore development during epidotizationThe unique characteristics of epidosites may be useful to detect the fluid upwelling zones in the oceanic crust [ABSTRACT FROM AUTHOR]

Permeability profiles in the crust‐mantle sequences of the Samail ophiolite were constructed based on onboard measurements of the electrical resistivity of cores recovered during the Oman Drilling Project. For each sample, we measured dry and brine‐saturated resistivity during the description campaign on the drilling vessel Chikyu. Owing to the conductive brine in the pore space, wet resistivity is systematically lower than dry resistivity. The difference between dry and wet resistivity is attributed to the movement of dissolved ions in brine that occupies the pore space. We applied effective medium theory to calculate the volume fraction of pores that contribute to electrical transport. Using an empirical cubic law between transport porosity and permeability, we constructed permeability profiles for the crust‐mantle transition zone and the serpentinized mantle sections in the Samail ophiolite. The results indicate that (1) the gabbro sequence has a markedly lower permeability than the underlying mantle sequence; (2) serpentinized dunites have higher permeability than serpentinized harzburgites; and (3) discrete sample permeability is correlated with ultrasonic velocity, suggesting that the permeability variations predominately reflect crack density and geometry. Plain Language Summary: Aqueous fluids that circulated beneath the seafloor play an important role in heat transfer, chemical exchange, and microbial activity in the oceanic lithosphere. The Oman Drilling Project was successful in obtaining continuous drill cores through the crust‐mantle sequences in the Samail ophiolite, where the paleo‐oceanic basement was thrust onto the continental crust in the Late Cretaceous. In this study, we constructed profiles of permeability across the crust‐mantle sections using the effective medium theory and resistivity data, which provide insights into fluid circulation in the oceanic lithosphere. The results indicate higher permeability in the uppermost mantle sequence between the crust and the underlying mantle, suggesting that present‐day fluid transfer is predominant at the crust‐mantle boundary. Although the fluid flow and chemical reactions are likely coupled, the high permeability could promote the transformation of crust‐mantle materials and hence the recycling of water into the mantle. Application of this technique to forthcoming deep drilling projects through the Mohorovičić discontinuity (Moho) and into the upper mantle may provide insights into the permeability structure and fluid circulation system in the oceanic lithosphere. Key Points: Brine‐saturated resistivity is systematically lower than dry resistivity, with the differences attributed to the volume fraction of poresPermeability through the crust‐mantle sequences was modeled using the effective medium theory and resistivity dataThe dunite sequence is characterized by higher permeability than the overlying gabbro and underlying harzburgite sequences [ABSTRACT FROM AUTHOR]