Mechanics of oblique spreading and ridge segmentation

Mid-ocean ridges display a variety of plan view geometries (planforms) that correlate with the tectonic setting. A mechanical analysis is proposed to rationalize the variety of planforms of mid-ocean ridges at various tectonic settings. We model spreading centers as fluid-filled cracks, and find the variation of segment orientation with Delta P/Delta S (where Delta P measures a magmatic overpressure within the crack and Delta S is the remote 'tectonic tension'). The analogy suggests that high Delta P/Delta S tends to preserve the continuity of an oblique spreading axis, whereas low Delta P/Delta S prompts segmentation. It follows that a planform of the spreading center is an indicator for the forces driving melt injection. The results are in qualitative agreement with principal geological observations. For instance, the concordant, oblique, and continuous Reykjanes Ridge reflects pressurized magma emplacement (Delta P/Delta S > 20), a result compatible with the proximity to the Icelandic hot spot and with low Delta S anticipated in passive margins. Away from major hot spots, the Mid-Atlantic Ridge (MAR) with its passive margins typically has a value of Delta P/Delta S of around unity. The values change widely through space and time, locally becoming negative (amagmatic spreading). This spatial and temporal variability in MAR is consistent with dynamic melt injection. Conversely, in orthogonal-discordant-segmented axes of the Pacific, magma emplacement is dominated by slab pull (high Delta S) despite high magmatic activity. The inferred value of Delta P approximate to 0 in the Pacific, stable through space and time, is consistent with passive melt injection.