Overview of lithology and structure of a mantle section of the Oman Ophiolite (BA active alteration sites, Oman Drilling Project): an integrated field mapping and boreholes imaging study.

The mantle section in the Oman Ophiolite was drilled at the BA sites along the axis of Wadi Laywani (Oman Drilling Project Phase 2). The BA sites are located in a complex structural zone with disturbance of the foliation and lineation of mantle rocks and intrusions, that were interpreted as evidence of an aborted off-axis mantle diapir (Nicolas et al, Marine Geophys. Res. 2000). Recovered lithologies consist of discordant to concordant dunites in a dominantly harzburgite mantle section. The mantle section show evidence of extensive melt-rock interaction with abundant gabbro and clinopyroxenite dikes. It underwent intense alteration, dominated by serpentinization and characterized by the occurrence of carbonate veins in the upper 100m of the boreholes (Noël et al, 2018; Kelemen and Matter, 2008).

Here, we present an integrated field mapping and borehole imaging study to investigate this complex zone. Surface observation and structural measurements in a dunite - harzburgite environnement, intruded by gabbro and clinopyroxenite dikes, are reported in a 2D map and a 3D model. Observations and analyses of images from the optical Borehole Imager provide information on the true geographical orientation of veins, dikes and fractures in boreholes 300-400 m deep. These structural data can be combined with orientations measured in outcrop.

First observations and analyses of the cores drilled at the BA sites during Oman Drilling Project Phase 2 highlight localization of deformation in veins and dikes, and fractures associated with harzburgite-dunite contacts ( Hole BA1B). These new data provide 3-D observations from surface to depth, allowing new insights into the tectonic and petrogenetic mechanisms driving formation of this complex zone.