Orientation of joints and arrangement of solid inclusions in fibrous veins in the Shatsky Rise, NW Pacific: implications for crack‐seal mechanisms and stress fields

Abstract

The ocean floor is leaky because it has numerous faults, cracks and joints upon formation and during the process of seafloor spreading. In time, these fractures are often closed after ocean floor cooling, hydrothermal circulation, and vein filling. The crack-seal mechanism of fractures of the oceanic crust is thus important for understanding its kinematics, kinetics and evolution. Coring and log data from IODP Expedition 324 reveal that the Shatsky Rise, an oceanic plateau in the NW Pacific Ocean, developed abundant joints and veins, and some veins formed along previous joints. We use log data from the Formation Micro-scanner Scanner (FMS) to reconstruct the original dip and dip direction of these structural elements. Using FMS microstructural analyses, the dip directions of joints and arrangement of solid inclusions in fibrous veins were examined for Holes U1347A, U1348A and U1349A. We found two types of veins, non-fibrous and fibrous, based on their physical appearance and mineralogical composition. Common to all samples is a straight fibrous inclusion fabric, associated with bands oriented parallel to the vein wall and trails typically at high angles to the vein wall. Cross-cutting relationships between the bands and the straight fibrous inclusions imply that inclusion bands reflect simple crack-seal increments. In the veins, inclusion bands are a sufficient criterion to infer the crack-seal mechanism. Further evidence for solid inclusions formed by the classic crack-seal mechanism is given by inclusion bands in carbon crystals grown in basalts. During incorporation, solid inclusions can remain undeformed, depending on their orientation with respect to the opening and spreading direction of mid-oceanic ridges. Simple displacement fields within the veins are recorded by straight crystal fibres, which track the opening direction. Based on the arrangement of solid inclusions within the veins, we suggest that the veins grew continuously during post-tectonic vein formation. Solid inclusions formed by steady adhesion at the vein wall interface during crack sealing and growth of a few veins were driven by the force of crystallization and extension of mid-oceanic ridges. Based on these two lines of evidence, we conclude that the formation of the Tamu Massif is consistent with the seafloor spreading history revealed by magnetic lineations, possibly accompanied with an interaction to the mantle plume head. In contrast, the formation of the Ori Massif, off the mid-ocean ridge, has no obvious preferred stress field, deduced to be related to a mantle plume tail with interaction to the mid-oceanic ridge.

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