Encyclopedia of Marine Geosciences

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Axial Summit Trough

  • Samuel Adam SouleEmail author
  • Michael Perfit
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DOI: https://doi.org/10.1007/978-94-007-6644-0_3-2


Midocean Ridge Hydrothermal Vent Spreading Center Eruptive Fissure Ridge Axis 
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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

The AST is a volcanically modified tectonic graben. The graben forms in response to deformation above magmatic dikes originating at the axial magma lens located 1–3 km beneath the ridge crest (Chadwick and Embley, 1998; Fornari et al., 1998). Models of dike intrusion into a homogeneous elastic medium with physical properties similar to oceanic crust produce horizontal stresses at the seafloor, which reach their maximum at distances from the dike centerline of ~1.5 times the depth to the dike tip (Rubin and Pollard, 1988; Fig. 1). Slips along normal faults between these symmetric zones of dilation and the dike tip produce grabens with widths many times greater than depths. The typical depth and width of ASTs on ridges are larger than could be expected for the intrusion of a single 1-m wide dike, the nominal dike width on midocean ridges (Qin and Buck, 2008). Thus, it is assumed that the AST represents accumulated deformation over many dike intrusion cycles. In some instances, an AST may evolve into a much wider (~1–2 km) and deeper (~50–100 m) axial valley by this process (Carbotte et al., 2006; Soule et al., 2009). Other contributions to graben subsidence may include magma withdrawal from subridge melt lenses (Carbotte et al., 2003).
Fig. 1

Perspective view of the AST at the northern EPR with schematic representation of normal faults between the seafloor and intruding dike (After Soule et al., 2009)

The dimensions of the AST are established by tectonic processes but modified by volcanic processes (Fig. 2). Volcanic overprinting during a single eruptive event can range from complete infilling of the AST to minor narrowing and shallowing of the trough based on the style (e.g., low or high eruption rate) and frequency of eruptions (Chadwick and Embley, 1998; Fornari et al., 1998; Soule et al., 2009). At low eruption rates, pillow lavas can fill the trough and completely obscure the AST. At high eruption rates, lava filling the trough can drain back into eruptive fissures after the eruption has ceased. In the latter case, the solidified upper crust of the lava flow that filled the AST founders and leaves only thin fragmental remnants of the flow on the trough floor (Fornari et al., 1998). Some portions of the upper crust may remain intact, supported by lava pillars – hollow conduits composed of solidified lava – that form between the base and upper surface of the flow (Francheteau et al., 1979; Chadwick and Embley, 1998; Gregg et al., 2000; Chadwick, 2003). This can result in an apparent narrowing of the trough although the open space of the graben remains beneath a thin lid of solidified lava.
Fig. 2

Examples of tectonically defined graben (white) overprinted by recent volcanic deposition (gray) (Soule et al., 2009)

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).

As an example, the AST along the well-studied EPR between 9 °N and 10 °N varies in width from 20 to 300 m and depth from 2 to 20 m over 60 km of ridge length. The AST is divided into roughly eight segments that range in length from 1 to 25 km. The most pronounced change in AST properties occurs at ~9°44′N, coincident with the onset of significant shoaling in the ridge-crest depth (Fig. 3). In addition, this marks the southernmost extent of two recent eruptions in 1991–1992 and 2005–2006. The AST maintains average widths of ~180 and ~75 m to the north and south of this discontinuity, respectively. The greater width and depth of the AST across this discontinuity would suggest that a greater proportion of dikes reach the seafloor and erupt to the north resulting in greater magmatic overprinting. This is consistent with a break in the axial magma lens and magma compositions that become more mafic to the north of this discontinuity. Thus, it appears that the AST provides a visible record of the frequency of eruptions, which tend to infill a consistently growing graben.
Fig. 3

(a) Example of AST discontinuity in width (blue shading) along the East Pacific Rise (EPR) as imaged by 120-kHz side-scan sonar backscatter. The AST narrows north of 9°43.5 due to recent volcanic deposition. (b) Compilation of AST properties along the EPR shows that this transition in width (measured from side-scan sonar) persists for tens of kilometers north and south and is consistent with AST depth (measured from towed camera crossings) and ridge-crest depth (measured from ship-based bathymetry) as well as geophysical and geochemical indicators of melt supply (Soule et al., 2009)


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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© Springer Science+Business Media Dordrecht 2015

Authors and Affiliations

  1. 1.Woods Hole Oceanographic InstitutionWoods HoleUSA
  2. 2.Department of Geological SciencesUniversity of FloridaGainesvilleUSA