Constructing the crust along the Galapagos Spreading Center 91.3°–95.5°W: Correlation of seismic layer 2A with axial magma lens and topographic characteristics

Abstract

[1] Multichannel seismic reflection data are used to infer crustal accretion processes along the intermediate spreading Galapagos Spreading Center. East of 92.5°W, we image a magma lens beneath the ridge axis that is relatively shallow (1.0-2.5 km below the seafloor) and narrow ($0.5-1.5 km, cross-axis width). We also image a thin seismic layer 2A (0.24-0.42 km) that thickens away from the ridge axis by as much as 150%. West of 92.7°W, the magma lens is deeper (2.5-4.5 km) and wider (0.7-2.4 km), and layer 2A is thicker (0.36-0.66 km) and thickens off axis by <40%. The positive correlation between layer 2A thickness and magma lens depth supports the interpretation of layer 2A as the extrusive volcanic layer with thickness controlled by the pressure on the magma lens and its ability to push magma to the surface. Our findings also suggest that narrower magma lenses focus diking close the ridge axis such that lava flowing away from the ridge axis will blanket older flows and thicken the extrusive crust off axis. Flow of lava away from the ridge axis is probably promoted by the slope of the axial bathymetric high, which is largest east of 92.5°W. West of $94°W the ''transitional'' axial morphology lacks a prominent bathymetric high and layer 2A no longer thickens off axis. We detect no magma lens west of 94.7°W where a small axial valley appears. The above changes can be linked to the westward decrease in the magma and heat flux associated with the fading influence of the Galapagos hot spot on the Galapagos Spreading Center.

Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observations

“…MCS data have been acquired along the ridge axis from c. 958W to 91820 ′ W and reveal a crustal magma body beneath most of the ridge axis east of 94820 ′ W ( Fig. 7g; Detrick et al 2002;Blacic et al 2004). Although there is a 15 km-long break in the continuity of the AML at the 93815 ′ W OSC, this discontinuity is not a significant boundary in magma lens properties as might be expected if regional upwelling centres fed the adjoining second-order segments.…”

“…Notably, a major change in the depth and width of the AML coincides with a much smaller offset at 92840 ′ W (see below) and not the second-order 93815 ′ W discontinuity ( Fig. 7g; Blacic et al 2004). …”

“…The available seismic data from the western GSC show breaks in continuity and changes in the depth of the AML at several of these small offsets (93853 ′ W, 92840 ′ W, 91833 ′ W; Fig. 7g; Detrick et al 2002;Blacic et al 2004), suggestive of separate magmatic systems beneath the adjacent segments. Furthermore, some of these small offsets also mark major regional boundaries in other ridge properties.…”

“…An abrupt step in both the depth (0.6s twoway travel time or c. 1.5 km; Fig. 7g) and width (from 0.5-1.5 km east of 92840 ′ W to 0.7-2.4 km to the west) of the AML Blacic et al 2004) also occurs at this discontinuity, along with a transition in the nature of seafloorvolcanic relief (Colman et al 2012). Similarly, along the SEIR a small offset of ,200 m at 103835 ′ E coincides with an abrupt transition in ridge morphology from rifted axial highs to shallow rift valleys (Figs 5 & 6) and a c. 10 mGal step in the regional MBA gradient ( Fig.…”

A 1.5 Ma record of plume-ridge interaction at the Western Galápagos Spreading Center (91°40′–92°00′W)

“…The ridge crest at 92°W is characterized by axial high morphology, cut by a 10-40 m deep axial summit graben and is inferred to be associated with high magma supply enhanced by presumable influx of Galápagos plume material Detrick et al, 2002;Sinton et al, 2003;Cushman et al, 2004;Colman et al, 2012). Beneath the spreading axis, a melt lens has been seismically imaged at ~1.6 km depth (Blacic et al, 2004). Colman et al (2012) and McClinton et al (2013;2015) provide a detailed geological map of this High Magma Supply (HMS) area along with the geochemical characterization of individual lava units.…”