The Cleft revealed: Geologic, magnetic, and morphologic evidence for construction of upper oceanic crust along the southern Juan de Fuca Ridge

Abstract

[1] The geology and structure of the Cleft Segment of the Southern Juan de Fuca Ridge (JdFR) have been examined using high-resolution mapping systems, observations by remotely operated vehicle (ROV), ROV-mounted magnetometer, and the geochemical analysis of recovered lavas. Bathymetric mapping using multibeam (EM300) coupled with in situ observations that focused on near-axis and flank regions provides a detailed picture of 0 to 400 ka upper crust created at the southern terminus of the JdFR. A total of 53 rock cores and 276 precisely located rock or glass samples were collected during three cruises that included sixteen ROV dives. Our observations of the seafloor during these dives suggest that many of the unfaulted and extensive lava flows that comprise and/or cap the prominent ridges that flank the axial valley emanate from ridge parallel faults and fissures that formed in the highly tectonized zone that forms the walls of the axial valley. The geochemically evolved and heterogeneous nature of these near-axis and flank eruptions is consistent with an origin within the cooler distal edges of a crustal magma chamber or mush zone. In contrast, the most recent axial eruptions are more primitive (higher MgO), chemically homogeneous lobate, sheet, and massive flows that generate a distinct magnetic high over the axial valley. We suggest that the syntectonic capping volcanics observed off-axis were erupted from near-axis and flank fissures and created a thickened extrusive layer as suggested by the magnetic and seismic data. This model suggests that many of the lavas that comprise the elevated ridges that bound the axial valley of the Cleft Segment were erupted during the collapse of a magmatic cycle not during the robust phase that established a new magmatic cycle.

Crust and Lithospheric Structure - Seismic Structure of Mid-Ocean Ridges

“…At the axis of fast-and intermediate-spreading ridges, seismic layer 2A is consistently thin but thickens rapidly within 1-2 km of the ridge axis ( Figure 13), presumably due to a rapid buildup of the extrusive basalts on the flanks of the ridge (e.g., Harding et al, 1993;Hooft et al, 1996;Kent et al, 1994;Stakes et al, 2005). At the axis of fast-and intermediate-spreading ridges, seismic layer 2A is consistently thin but thickens rapidly within 1-2 km of the ridge axis ( Figure 13), presumably due to a rapid buildup of the extrusive basalts on the flanks of the ridge (e.g., Harding et al, 1993;Hooft et al, 1996;Kent et al, 1994;Stakes et al, 2005).…”

Volcanic Eruptions in the Deep Sea

“…In particular, seafloor studies at various MOR sites have provided information about eruption styles, sizes, and frequencies over a broad range of dominantly effusive volcanoes (e.g., Bryan and Moore, 1977;Ballard et al, 1979Ballard et al, , 1982Sinton et al, 2002Sinton et al, , 2010White et al, 2002;Fornari et al, 2004;Soule et al, 2005Soule et al, , 2009Stakes et al, 2006;Escartín et al, 2007).…”

“…ridges (e.g., Stakes et al, 2006). tions (e.g., Rubin et al, 2001;Sinton et al, 2002;Bergmanis et al, 2007;Goss et al, 2010).…”

Geochemical and age constraints on the formation of the Gorda Escarpment and Mendocino Ridge of the Mendocino transform fault in the NE Pacific

“…Comparative data of an andesite pillow basalt from the southernmost Juan de Fuca ridge-transform intersection (Cleft-Blanco system; Stakes et al, 2006) are presented in Figure 5. The highly fractionated basalts, andesites, and dacites from the southernmost Cleft segment at its intersection with the Blanco transform fault are all related by extended crystal fractionation and cooling from a common source (Stakes et al, , 2006Perfi t et al, 2003;Cotsonika et al, 2005). The abundance of E-MORBs and alkali basalts, the crossing REE patterns, and high Ce/Yb populations for the Mendocino transform fault, however, suggest that magma may have been derived from melting of a deeper, more heterogeneous, source to variable extents.…”