Mechanics of oblique spreading and ridge segmentation

Abstract

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 ΔP/ΔS (where ΔP measures a magmatic overpressure within the crack and ΔS is the remote ‘tectonic tension’). The analogy suggests that high ΔP/ΔS tends to preserve the continuity of an oblique spreading axis, whereas low ΔP/ΔS 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 ( ΔP/ΔS> 20), a result compatible with the proximity to the Icelandic hot spot and with low ΔS anticipated in passive margins. Away from major hot spots, the Mid-Atlantic Ridge (MAR) with its passive margins typically has a value of ΔP/ΔS 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 ΔS) despite high magmatic activity. The inferred value of ΔP ≈ 0 in the Pacific, stable through space and time, is consistent with passive melt injection.