Amagmatic Accretionary Segments, Ultraslow Spreading, and How Continents Break

The evolution of non-volcanic rifted margins is key to understanding continental breakup and the early evolution of some of the world's most productive hydrocarbon basins. However, the early stages of such basement rifting are largely constrained by limited observations on ancient heavily sedimented margins such as Newfoundland and Iberia. A modern analogue, however, exists at ultraslow spreading ocean ridges.

Ultraslow spreading ridges (<20 mm/yr) are ~30% of the global ridge system (e.g. Gakkel, Southwest Indian, and Knipovitch Ridges). They have unique tectonics, unusual magma chemistry, and amagmatic accretionary ridge segments. Amagmatic accretionary segments are the 4th class of plate boundary structure, and, we believe, the defining tectonic feature of early continental breakup. They form at effective spreading rates <12 mm/yr, assume any orientation to spreading, and replace transform faults and magmatic segments. At amagmatic segments the earth splits apart with the mantle emplaced directly to the seafloor, and great slabs of peridotite uplifted into the rift mountains to either side. A thick conductive lid suppresses mantle melting, and magmatic segments form only at widely spaced intervals, with only scattered volcanics in between. Amagmatic segments link with the magmatic segments to form curvilinear plate boundaries, rather than the step-like morphology that forms at faster spreading ridges. These are all key features of non-volcanic rifted margins; explaining, for example, the presence of mantle peridotites emplaced simultaneously on both the Newfoundland and Iberian Margins in the Jurassic and Cretaceous.

Miocene Lena Trough represents a new mid-ocean rift plate boundary representing the final event in the separation of the North American and Eurasian continents. Recent mapping and sampling of Lena Trough confirms that it is both oblique and amagmatic, showing that the initiation of seafloor spreading at a non-volcanic rifted continental margin follows the same pattern as ultraslow spreading ridges.