http://www.cnr.it/ontology/cnr/individuo/prodotto/ID48874

Oceanic broad multifault transform plate boundaries (Articolo in rivista)

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Oceanic broad multifault transform plate boundaries (Articolo in rivista) (literal)

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Ligi M.(1), Bonatti E.(1,2), Gasperini L.(1), Poliakov A.N.B. (3) (2002)
Oceanic broad multifault transform plate boundaries
in Geology (Boulder Colo.)
(literal)

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According to Wilsons classic definition and subsequent studies, oceanic ridge-transform plate boundaries are characterized by a single, few-kilometers-wide strike-slip deformation zone. This contrasts with major continental strike-slip zones where strike-slip motion is distributed in wide complex zones of deformation. We describe here oceanic transform boundaries that, contrary to Wilsons model, are expressed by complex, wide multifault zones. We also present numerical models that help determine what factors cause the development of one or the other type of oceanic transform boundary. Whether an oceanic transform boundary follows a single narrow strike-slip fault, as in most oceanic transforms, or a multiple broad boundary, depends ultimately on the thermal structure below the transform zone, i.e., on the age offset (length of offset combined with spreading rate). Our double transform model explains the migration of the Romanche principal transform boundary from the northern to the southern valley implying that the lithosphere between the two boundaries shifted from the South American to the African plate, thus reversing its sense of motion. We conclude that two classes of oceanic transform boundaries exist: simple narrow boundaries, as in Wilsons original model; and complex, multifault boundaries, which develop where thick and cold lithosphere is affected. Multifault oceanic boundaries have geometrical similarities to some continental strike-slip zones. (literal)

Note

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1. ISMAR-CNR, Sezione di Geologia Marina, Bologna. 2. Univ. La Sapienza, Roma. 3. CNRS, Montpellier, France. (literal)

Titolo

Abstract

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-kmlong, 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. (literal)

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