The linkage of volcanic and tectonic segmentation on fast-spreading mid-ocean ridges: Constraints on the spatial and temporal extent of volcanic systems from the distribution of volcanic edifices and lava flow morphology at the East Pacific Rise

The volcanic processes generating new ocean crust at the East Pacific Rise systematically vary along the rise crest at a ∼20 km length scale that correlates to the fine-scale tectonic segmentation of the ridge. Along the ridge crest, changes in volcanic structures, from basaltic lava domes to collapsed lava lakes, revealed by near-seafloor sonar (120 kHz DSL-120 side-scan) and visual (Argo and Argo II) mapping of the rise crest characterize the variation in volcanic processes. Basaltic lava domes, on average 20 m high and 200 m diameter, and pillow lava flows are most abundant near ridge discontinuities. In contrast, collapsed lava lake troughs and sheet lava flows are most abundant near the centers of fine-scale ridge segments. The linkage of volcanic effusion rate and tectonic segmentation suggests that fine-scale ridge segments comprise individual volcanic systems. This linkage is documented for both the southern East Pacific Rise (Chapter 3, in Journal of Geophysical Research, V.105, pg. 23519–23536, 2000) and the northern East Pacific Rise (Chapter 4, to be submitted to Journal of Geophysical Research ). The association of low effusion rate eruptions, pillow lava domes, and tectonic discontinuities is dramatically demonstrated in the geologic record preserved on the ridge flanks. Volcanic mound fields identified from near-surface 12 kHz side-scan sonar data are dense, extensive clusters of pillow lava domes. Most volcanic mound fields are found in the zones of discordant seafloor fabric created by the migration of overlapping spreading centers along the rise (Chapter 5, submitted to Journal of Geophysical Research , Feb. 2001). Also on the ridge flanks, randomly distributed isolated off-axis volcanoes and lava flow fields form on crust <0.2 Ma in age. The zone of off-axis volcanism is a function of crustal age, not distance off-axis, and appears to be independent of spreading rate, suggesting that lithospheric thickness controls the extent of small volume off-axis eruptions. The distribution of off-axis volcanic edifices varies in cumulative volume and average size corresponding to the large-scale (>100 km) segmentation of the rise crest (Chapter 2, in Journal of Geophysical Research, V.103, 30371–30384, 1998).