<p>A founding ambition of scientific ocean drilling is to drill a MoHole that penetrates the entire ocean crust and into the upper mantle at a location representative of normal crustal accretion and evolution. This remains the only way to test many of our key ideas about how new crust forms at mid-ocean ridges, cools and ages through interactions with the oceans. The technical challenges of drilling such a deep hole limit potential locations to a small number of candidate regions, which need to be sufficiently old to be cool at Moho depths but shallow enough for riser drilling.</p><p>In December 2022 and January 2023 RRS James Cook expedition JC228 carried out the first site survey to collect complete seismic datasets in one of the candidate regions, the Guatemala Basin. We collected two grids and a long flowline profile of multichannel seismic reflection (MCS) data using a tuned airgun array of 5000 in<sup>3</sup> together with a 6 km hydrophone streamer. Airgun shots were simultaneously recorded on 52 ocean-bottom seismometers (OBS) deployed at 84 locations. Shot spacings of 150 m and 75 m were optimised for the different recordings. The survey samples crust formed between 19 and 21 Ma, at present-day water depths of 3200-3400 m, and is approximately along a flowline from the existing ODP/IODP Site 1256, where intact ocean crust has been drilled to the gabbros. Initial processing of the MCS data on board the ship shows a normal incidence reflection Moho that is variable in amplitude over distances of ~10 km, but is present at the intersections of several MCS profiles. Wide-angle PmP reflections on the OBS are clear across the region. There is obvious anisotropy in the Pn upper mantle refraction on the OBS, with a strong and high-velocity arrival along the flowline, and weaker and slower arrivals in the isochron direction at each grid. Overall, the initial observations are extremely promising for identification of multiple viable Mohole drilling locations.</p>

At passive continental margins, magmatism and tectonics are two important factors that control the fluid, thermal, and rheological conditions in the lithosphere, from continental rifting to break-up. As a result, different extension modes may occur in the continental crust, such as pure shear, simple shear, and depth independent stretching (Franke et al., 2011;Lister et al., 1986;McKenzie, 1978;Wernicke, 1981). Different architectures are also found in the continent-ocean transition zone (COT), such as wide domains of exhumed continental mantle (e.g., Dean et al., 2000;Grevemeyer et al., 2022) versus rapid transition from continental to oceanic crust (e.g., Larsen et al., 2018). After break-up, the crustal thickness in oceanic basins may vary widely depending on magma