Axial Summit Trough
KeywordsMidocean Ridge Hydrothermal Vent Spreading Center Eruptive Fissure Ridge Axis
An axial summit trough (AST) is a narrow trough or volcanically modified graben that develops at the crest of a midocean ridge and typically is the locus of volcanic and hydrothermal activity.
At the summit of magmatically robust midocean ridges (MORs), a narrow trough may develop that marks the location of the divergent plate boundary and is the locus of most magmatic and hydrothermal activity (Macdonald and Fox, 1988). This axial summit trough (AST) is typically less than 500 m in width and 50 m in depth and may be discontinuous along the ridge in terms of its width, depth, strike, and continuity. ASTs are common along fast- and intermediate-spreading rate ridges (e.g., East Pacific Rise, Juan de Fuca Ridge, Galapagos Spreading Center), and magma-rich, inflated portions of the slow-spreading Mid-Atlantic Ridge and Lau Basin back-arc spreading center.
Development of the Axial Summit Trough
In addition to hosting the majority of eruptive fissures, the AST hosts the bulk of hydrothermal vents along the ridge crest. High- and low-temperature hydrothermal venting occurs within the trough, commonly colocated with volcanic fissures as well as along the walls of the AST (e.g., Haymon et al., 1991; Fornari et al., 1998). The association of hydrothermal vents with the AST reflects the location of the graben directly over the shallowest axial melt lenses along the ridge. High permeability in the vertical direction reflects the presence of steeply dipping faults and dikes within what is the weakest and thinnest portion of the ocean crust.
The AST as a Record of Volcanic-Tectonic History
Discontinuities in the AST occur in the form of abrupt changes in the depth and/or width of the trough, physical breaks in the continuity of the trough, or changes in trough orientation. These discontinuities, referred to as devals (deviations in axial linearity) (Langmuir et al., 1986), reflect the finest degree of segmentation of the ridge axis (Macdonald et al., 1988; Haymon et al., 1991; White et al., 2000; Haymon and White, 2004; White et al., 2006) and in many cases correlate with other indicators of segmentation such as lava geochemistry, ridge-crest water depth, presence and continuity of the subridge melt lenses, and volcanic deposition processes (Langmuir et al., 1986; Carbotte et al., 2000; Soule et al., 2007, 2009; Smith et al., 2001).
The axial summit trough is a common feature along the ridge crest of fast- and intermediate-spreading rate ridges and is present along slower-spreading but magma-rich ridges. The trough forms from deformation due to shallowly intruded dikes, with deformation accumulating over many years, but the graben dimensions are modified by volcanic deposition from eruptive fissures located within the AST to degrees depending on the frequency and effusion rates of eruptions. The AST, where present, hosts the majority hydrothermal vents along the ridge axis due to its proximity to crustal heat sources (e.g., magma chambers) and anisotropic permeability within the sheeted dikes and tectonically disrupted upper crust. Discontinuities within the AST are coincident with discontinuities in other ridge-crest properties such as depth, axial magma chamber continuity and melt content, and lava geochemistry.
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