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Geology; January 2002; v. 30; no. 1; p. 11-14; DOI: 10.1130/0091-7613(2002)030<0011:OBMTPB>2.0.CO;2
© 2002 Geological Society of America
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Oceanic broad multifault transform plate boundaries

Marco Ligi*,1, Enrico Bonatti*,2, Luca Gasperini*,3 and Alexei N.B. Poliakov*,4

1 Istituto di Geologia Marina, Consiglio Nazionale Ricerche, Via Gobetti 101, 40129 Bologna, Italy
2 Dipartimento Scienze della Terra, Università Roma La Sapienza, 00185 Rome, Italy, and Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA
3 Istituto di Geologia Marina, Consiglio Nazionale Ricerche, Via Gobetti 101, 40129 Bologna, Italy
4 Laboratoire de Geophysique, Centre National Recherche Scientifique, Université Montpellier II, 34095 Montpellier, France

Oceanic transform plate boundaries consist of a single, narrow (a few kilometers wide) strike-slip seismic zone offsetting two mid-ocean ridge segments. However, we define here a new class of oceanic transform boundaries, with broad complex multifault zones of deformation, similar to some continental strike-slip systems. Examples are the 750-km- long, 120-km-wide Andrew Bain transform on the Southwest Indian Ridge, and the Romanche transform, where the Mid-Atlantic Ridge is offset by a lens-shaped, ~900-km- long, ~100-km-wide sliver of deformed lithosphere bound by two major transform valleys. One of the valleys is seismically highly active and constitutes the present-day principal transform boundary. However, strike-slip seismic events also occur in the second valley and elsewhere in the deformed zone. Some of these events may be triggered by earthquakes from the principal boundary. Numerical modeling predicts the development of wide multiple transform boundaries when the age offset is above a threshold value of ~30 m.y., i.e., in extra-long (>500 km) slow-slip transforms. Multiple boundaries develop so that strike-slip ruptures avoid very thick and strong lithosphere.

Key Words: transform plate boundary • suboceanic lithosphere • numerical modeling • Romanche transform • Andrew Bain transform






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