Rationale for the SloMo project

The aim of SloMo (shorthand for “The nature of the lower crust and Moho at slower spreading ridges”) is simple: to drill as deeply as possible through Layer 3 in order to set the stage for the first ever penetration of the crust/mantle boundary (SloMo-Leg 2). Thus SloMo will test the hypothesis that the Moho, at least at slow and ultraslow ridges, represents an alteration boundary rather than the igneous crust–mantle transition. In doing so it will also be able to address key questions, including determining how magnetic reversal boundaries are expressed in the lower crust, assessing the role of the lower crust and shallow mantle in the global carbon cycle, and constraining the extent and nature of life at deep levels within the ocean lithosphere.

Deep drilling on Atlantis Bank will also provide an important step toward the long-term objective of drilling a total crustal penetration in fast-spreading ocean crust in the Pacific Ocean by providing critically needed experience in engineering a deep hole in lower crustal rocks.

SloMo is not simply about drilling the crust/mantle boundary but is as much about the journey on the way there. By recovering a near-complete section of the igneous lower crust and crust–mantle transition at Atlantis Bank, we will be able to address many of the important questions posed in the IODP Science Plan’s Challenge 9, developed further in Scientific objectives. At slow- and ultraslow-spreading ridges, the lower crust uniquely preserves that critical link where magmatic and tectonic processes directly reflect plate dynamics, melt input, and the pattern of mantle flow. We now understand that at such ridges a substantial portion of plate spreading is accommodated in the lower crust by tectonic extension due to faulting and, in places, ductile deformation. Large-offset “detachment” normal faults exert strong control on melt distribution and transport in the lower crust and delivery to the seafloor. This is in marked contrast to fast-spreading ridges, where it is accepted that the crust principally undergoes magmatic accretion by the injection of melt into the lower crust, diking, and eruption of magmas on the seafloor; thus, rollover and corner flow by ductile flow accompanying mantle upwelling and plate spreading is believed largely limited to the mantle. However, at slower spreading ridges, which are cooler and support only ephemeral magma chambers, the lower crust can potentially support a shear stress. As a consequence, with lower rates of magma supply, and colder, stronger lithosphere formed directly beneath the ridge axis, slower spreading ridges have very different morphologies and crustal architectures. Thus, as stated in Challenge 9, a full picture of crustal architecture and accretion can only be drawn if both fast- and slow-spreading environments are addressed.