Tall volcanic edifices on MOR spreading plate boundaries: Review and synthesis
|Location||International Geological Congress,oslo 2008|
|Author||Vogt, Peter۱; Michael, Peter۲|
|Holding Date||20 September 2008|
Along the Mid-Oceanic Ridge (MOR), Tall Volcanic Edifices (TAE), rising more than 1 km above adjacent MOR axes, are widely scattered along slow (S= 15-40 mm/a) and ultraslow (< 15 mm/a) spreading plate boundaries, and on the intermediate (60 mm/a) Juan de Fuca Ridge (JDFR). A few axial highs are ultramafic massifs, but most are volcanoes occupying MOR segment centers, reflecting locally robust magmatism. While a unified TAE theory is handicapped by data paucity, our global synthesis suggests that TAE formation is favored by 1) locally anomalous upper mantle source composition (MAR, 33 S); 2) orthogonal spreading segments within a regionally oblique MOR (SWIR, 11, 15, and 64 E; MAR, 48 N); 3) "Hotspots" (Iceland, Azores, JDFR); 4) triple junctions (MAR, 14 and 37 N); 5) other processes (Gakkel Ridge (GR); 69 E). TAE sites along slow/ultraslow ridges may locally behave seismically like fast ones, generating ’magmatic’ low-magnitude seismic swarms, vs. higher magnitude, tectonically generated teleseisms. Tectonic dismemberment of axial edifices impedes deciphering the evolution of TAE-forming mantle anomalies. The longest igneous trace from a present TAE is the 800 km seamount line (0-33 Ma) extending northwest from JDFRs Axial Seamount. On slow ridges, no off-axial trace connected to a present TAE is known beyond 1-2 Ma, although the MAR, 33 S TAE has a geochemical trace to 7 Ma crust, its MOR segment lengthening over time. "Split halves" of former TAEs, rafted from the axis, occur on the MAR and JDFR, their axis-facing slopes exposing edifice interiors. In studying the MAR, 48 N TAE, with its split volcano ’halves’, we simulate fault restoration techniques to ’reassemble’ the former TAE. Off-axial TAE traces imply intermittent volcano generation every 0.2-0.9 Ma, comparable to 0.1-4.0 magmato-tectonic "quasi-periods" inferred to generate abyssal hills. Axial volcano height may be isostatically limited by plate thickness (neglecting flexure), roughly proportional to S exp (-1/2). The latter relation is modified because 1) plate thickness does not exceed crustal thickness for typical < 60 mm/a spreading, and for slower ridges tapping melt-rich mantle; and 2) crustal thickness except at TAEs vanishes along magma-starved ultraslow ridges. (1) limits TAE heights to 300-600 m for typical 6 km thick crust; and (2) accounts for a 3 km high TAE on the ultraslow GR if the subjacent lithosphere (serpentinized ultramafics) is 9-11 km thick. While several TAEs (MAR, 37 N and 33 S) have erupted enriched (EMORB) basalts, others (GR, 69 E) have not. We propose that 1) at sufficiently slow spreading, some TAEs reflect passive melting of depleted mantle, with permeability-focused magma ascent; 2) intermittently created TAEs play a significant role in generating bands of shallower topography and thicker crust along slow-spreading ridges; and 3) at faster rates, up to at least 60 mm/a, magma excess from anomalous mantle can create TAEs.