Volcanic Morphology of the East Pacific Rise Crest 9 degree 49'-52': Implications for volcanic emplacement processes at fast-spreading mid-ocean ridges

• Published in: Marine geophysical researches Vol. 21; no. 1-2; pp. 23 - 41
• Main Authors: Kurras, G J, nari, D J, Edwards, M H, Perfit, M R, Smith, M C
• Format: Journal Article
• Language: English
• Published: 01.02.2000
• Subjects: Marine; ISE, East Pacific Rise
• ISSN: 0025-3235
• DOI: 10.1023/A:1004792202764

Deep sea photographs were collected for several camera-tow transects along and across the axis at the East Pacific Rise crest between 9 degree 49' and 9 degree 52' N, covering terrain out to 2 km from the ridge axis. The objective of the surveys was to utilize fine-scale morphology and imagery of seafloor volcanic terrain to aid in interpreting eruptive history and lava emplacement processes along this fast-spreading mid-ocean ridge. The area surveyed corresponds to the region over which seismic layer 2A, believed to correspond to the extrusive oceanic layer, attains full thickness. The photographic data are used to identify the different eruptive styles occurring along the ridge crest, map the distribution of the different morphologies, constrain the relative proportions of the three main morphologies and discuss the implications of these results. Morphologic distributions of lava for the area investigated are 66% lobate lava, 20% sheet lava, 10% pillow lava, and 4% transitional morphologies between the other three main types. There are variations in inferred relative lava ages among the different morphological types that do not conform to a simple increase in age versus distance relationship from the spreading axis, suggesting a model in which off-axis transport and volcanism contribute to the accumulation of the extrusive layer. Analysis of the data suggests this ridge crest has experienced three distinctly different types of volcanic emplacement processes: (1) axial summit eruptions within a similar to 1 km wide zone centered on the axial summit collapse trough (ASCT); (2) off-axis transport of lava erupted at or near the ASCT through channelized surface flows; and (3) off-axis eruptions and local constructional volcanism at distances of similar to 0.5-1.5 km from the axis. Major element analyses of basaltic glasses from lavas collected by Alvin, rock corer and dredging in this area indicate that the most recent magmatic event associated with the present ASCT erupted relatively homogeneous and mafic (>8.25 weight percent wt.% MgO) basalts compared to older, off-axis lavas which tend to be more chemically evolved (Perfit and Chadwick, 1998; Perfit and Fornari, unpublished data). The more primitive lavas have a more extensive distribution within and east of the ASCT. More evolved basalts (MgO <8.0wt.%) are concentrated in a broad area a few kilometers east of the axis, and in an oval-shaped area south of 9 degree 50' N, west of the ASCT. Transitional and enriched (T- and E-) mid-ocean ridge basalts exist in relatively small areas (<1 km super(2)) on the crestal plateau and correlate with scarps or fissures where pillow lavas were erupted. Mafic lavas in this area are primarily related to the youngest magmatic events.