Abstract
Author(s): Clague, DA; Paduan, JB; Caress, DW; McClain, J; Zierenberg, RA | Abstract: The northernmost segment of the Gorda mid-ocean ridge is the site of a small-volume eruption in 1996 and the persistent off-axis Sea Cliff hydrothermal vent field. To better understand the geologic setting and formation of these features, 1-m resolution bathymetric mapping using autonomous underwater vehicles was completed in 2016. The mapped region covers 35 km2 and 15.6 km of the volcanic axis from south of the 1996 lava flows, and a cross section for ∼4.5 km perpendicular to the axis, that extends beyond the Sea Cliff hydrothermal vent field. A proposed 1996 flow ∼7 km south of previously mapped flows is an artifact from a poor pre-eruption survey. The 1996 flows consist of three discrete steep hummocky mounds of pillows and syneruptive talus. The Sea Cliff hydrothermal field is located a few km north of the narrowest, shallowest section of the ridge segment, 2.6 km east of the center of the neovolcanic zone, and ∼370 m above the average depth of the axial graben on the largest offset ridge-parallel fault. No evidence supports the prior hypothesis that the site is located where two fault systems intersect. The axial graben is asymmetrical with larger fault offsets on the east side. The ridge axis below the hydrothermal field and to the south toward the 1996 flows is constructed dominantly of hummocky flows of pillow basalt, many unusually steep-sided, with syneruptive talus at the base of their steep slopes. Three channelized flows ponded between steep hummocky flows, and then partially drained. Some low-eruption-rate hummocky flows and high-eruption-rate channelized flows have nearly identical compositions, supporting the idea that eruption rates on mid-ocean ridges vary because of different dike widths. Four volcanic structures with volumes between 0.18 and 0.25 km3 occur in the axial graben south of the 1996 flows. Two are flat-topped cones, another is a 1.5-km diameter inflated hummocky flow with 7 pit craters that demonstrate that the flow had a molten interior during growth. The fourth voluminous structure is a steep ridge with abundant syneruptive talus on its lower slopes. The North Gorda segment is an end-member, structurally and volcanically, compared with other Pacific intermediate-rate spreading ridges.
Highlights
Construction of the oceanic crust requires frequent eruptions along the 65,000 km-long mid-ocean ridge system
Compositions of glasses collected during cruises in 1983, 1985, 1986, 2000, 2002, 2004, and 2005 (Davis and Clague, 1987, 1990; Rubin et al, 1998; Clague et al, 2003, 2009; Davis et al, 2008) are mostly published. These papers discuss the petrogenesis of the lavas in detail, whereas here we focus on combining the compositional data with mapped flow morphology to evaluate emplacement processes
The contacts are clearly identifiable with black lava flows (Figure 4A) draped on older flows covered with sediment and hosting extensive animal communities
Summary
Construction of the oceanic crust requires frequent eruptions along the 65,000 km-long mid-ocean ridge system. Answers to these and other questions about crustal formation and modification require detailed observations and sampling at a range of ridge segments spreading at different rates and with different axial morphologies Most such studies have been done at faster spreading segments of the East Pacific Rise (e.g., White et al, 2000, 2002; Fornari et al, 2004; Soule et al, 2005, 2007, 2009; Fundis et al, 2010; Waters et al, 2013; Le Saout et al, 2014) or the slow spreading Mid-Atlantic Ridge (MAR) (e.g., Ballard and van Andel, 1977; Smith and Cann, 1992, 1993; Bideau et al, 1998; Yeo et al, 2012; Yeo and Searle, 2013). Similar high-resolution studies are published for some intermediate-spreading ridges in the Pacific that range from full-spreading rates of 4.9 to 6.0 cm/yr, including the southern Juan de Fuca Ridge (Stakes et al, 2006), the Galapagos Spreading Center (Colman et al, 2012), the Endeavor Segment of the Juan de Fuca Ridge (Clague et al, 2014), and the northernmost segment of the East Pacific Rise named Alarcon Rise (Clague et al, 2018a)
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