Penrose on a diet: magma-lite crustal accretion at Masirah, a slow-spread, ‘true’ MOR-ophiolite in southeast Oman

Abstract:

Current models for crustal accretion at slow-spreading ridges hold that, as melt supply varies in space and time, the mode of crustal accretion alternates between two styles: either generation of ‘classic’ magmatically-robust crust, or detachment faulting and oceanic core complex formation. However, reconstructing lithospheric architecture from seafloor observations remains challenging, and other configurations may be possible. Ophiolites provide 3D insights into lithospheric architecture, but many (most?) originate near subduction zones, hampering direct comparisons to ‘true’ MOR ocean lithosphere. An important exception is Masirah, an island off the SE coast of Oman, which preserves a 600 km² ophiolite that is shown to be a rare, if not unique, exhumed fragment of true 'open-ocean' lithosphere. It formed 150 Myr ago in an uncontroversial tectonic setting at a slow-spreading ridge during opening of the proto-Indian Ocean [1]. It thus offers a rare opportunity to study a fragment of slow-spreading ocean lithosphere, in 3D, from the peridotites of the upper mantle to the basaltic lavas at the palaeo-seafloor.

Detailed field observations have allowed a reconstruction of the lithospheric architecture. Notably, whereas all of the classical ophiolite lithostratigraphic units are present on Masirah, the magmatic ocean crust is unusually thin (<2.5 km, compared to 'standard' 6-7 km-thick crust), despite overlying a harzburgitic, apparently refractory mantle section. In particular, the gabbroic lower crust is extremely thin (150-500 m). The lower-to-upper crust transition is ill-defined and characterised by gabbros (and sometimes mantle peridotites) intruded over broad areas by a sub-parallel dyke swarm that passes upward into a well-developed sheeted dyke complex. The presence of a sheeted dyke complex and absence of evidence for early large-offset and/or rotational faults indicate that the thin crust is the result of magmatic accretion under melt-poor conditions rather than tectonic thinning through detachment faulting. These findings suggest that a reduced melt supply does not necessarily result in detachment faulting and show that variation in crustal architecture at slow-spreading mid-ocean ridges may be greater than previously thought.

1. Peters, Tj. Geol Soc Am Spec Pap 349, 525–536 (2000)