The growth of oceanic plates at mid-ocean ridges, crustal accretion, occurs by a combination of magmatic and tectonic processes. Magmatic processes along ridges spreading at fast, intermediate and slow rates, continually add volcanic material to a centrally located spreading axis. This creates a narrow band of young volcanic rocks. However, at ridges spreading at ultraslow rates, diminished volcanism allows entire blocks of mantle to spread on the sea floor by tectonic processes. Remote imaging has advanced our observational understanding of crustal accretion, but temporal constraints are required to quantitatively understand ultraslow-spreading ridge construction. Here, we use U-series eruption ages of volcanic rocks collected from the ultraslow-spreading Southwest Indian Ridge. Unexpectedly, we find young volcanic eruption ages that are broadly dispersed throughout the rift valley, indicating that crustal accretion of young volcanic rocks is not confined to a narrow central spreading axis. As areas of young volcanism are observed close to distinct fault surfaces, we propose that the widely dispersed volcanism may result from magma rising along faults. Our results indicate that axial-centric spreading models may not accurately describe crustal accretion at ultraslow-spreading ridges, prompting the re-evaluation of these models.
Bibliographical noteFunding Information:
The analyses and interpretations reported here were made possible by the initial scientific vision and persistence of H. Dick and J. Lin, and the hard work of the KNR162-7 and VAN7 expedition participants. An official review from J. Maclennan vastly improved the clarity and focus of this manuscript. We would also like to thank K. Rubin, M. Behn, J. Morgan, A. Soule, C. Waters and P. Kelemen for informal reviews and fruitful discussions. Interactions during the early stages of this project with D. Smith, H. Schouten and S. Escrig provided useful feedback. This work was supported by the following NSF grants: NSF-OCE 9907630, NSF-OCE 0137325, NSF-OCE 0623838, NSF-OCE 0526905 and NSF-OCE 0527053N.