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Geology; March 2006; v. 34; no. 3; p. 209-212; DOI: 10.1130/G21969.1
© 2006 Geological Society of America
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Rift topography linked to magmatism at the intermediate spreading Juan de Fuca Ridge

Suzanne M. Carbotte1, Robert S. Detrick2, Alistair Harding3, Juan Pablo Canales4, Jeffrey Babcock5, Graham Kent5, Emily Van Ark6, Mladen Nedimovic7 and John Diebold7

1 Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA
2 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
3 Scripps Institution of Oceanography, La Jolla, California 92093, USA
4 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
5 Scripps Institution of Oceanography, La Jolla, California 92093, USA
6 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
7 Lamont-Doherty Earth Observatory, Palisades, New York 10964, USA

New seismic observations of crustal structure along the Juan de Fuca Ridge indicate that the axial rift topography reflects magma-induced deformation rather than alternating phases of magmatism and tectonic extension, as previously proposed. Contrary to predictions of the episodic models, crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2.1–2.6 km. The shallow rift valley or axial graben associated with each Juan de Fuca segment is ~50–200 m deep and 1–8 km wide and is well correlated with a magma body in the subsurface. Analysis of graben dimensions (height and width) shows that the axial graben narrows and graben height diminishes where the magma body disappears, rather than deepening and broadening, as expected for rift topography due to tectonic extension. We propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor. In this model an evolving axial topography results from feedbacks between the rheology of the crust above a magma sill and dike intrusion, rather than episodic magma delivery from the mantle.

Key Words: Juan de Fuca Ridge • magma chambers • dikes • faulting • spreading centers






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