Mantle £ow patterns at the Neyriz Paleo-spreading center, Iran Alireza Nadimi à Geology Department, Payame Noor University of Isfahan, Kohandej, 81465-617 Isfahan, Iran Received 4 April 2002; received in revised form 2 July 2002; accepted 23 July 2002 Abstract The mantle peridotites of Neyriz record two successive episodes of plastic deformations; the first one related to the igneous accretion of the lithosphere and the second one developed during the first stage of the emplacement of the peridotites. These two events have been distinguished on the basis of microstructural criteria. The diapiric pattern, particularly relevant to the mantle process beneath spreading ridges, features vertical flow lines and elliptic flow plane trajectories in a pipe and extends along the ridge axis about 5 km. These structures rotate to horizontal and diverge in every direction in a narrow transition zone, a few hundred meters thick, below the Moho discontinuity. Such a diapiric pattern has been recognized in a few places along the Neyriz paleo-ridge. A large amount of magma passed through these mantle diapirs that were probably the main zones feeding the overlying magma chamber. The most common pattern features very regular structures over several kilometers along the strike of the paleo-ridge: the flow plane dips away from the ridge axis, and the flow line is parallel to the spreading direction. This flow pattern is frozen during the gradual accretion of the lithospheric mantle away from the ridge in a steady-state spreading regime. A shear-sense inversion at just below the Moho is commonly observed, pointing to forced asthenospheric flow. The reconstructed orientation of the Neyriz paleo-spreading center is 105‡, compatible with the geometry and orientation of harzburgite foliations and lineations and sheeted dikes. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: Iran; ophiolite; peridotites; mantle; diapirs; spreading centers 1. Introduction During the formation of oceanic lithosphere, the asthenosphere rises to a shallow level beneath mid-oceanic ridges (e.g., [1,2]), involving two dis- tinct processes: crust formation from mantle melts and accretion of the residual lithospheric mantle. The greater the depth, the less these phe- nomena are understood. Geophysical studies and observations from submersibles are beginning to clarify the structure of active ridges and the mech- anisms of crust formation [3,4]. However, the ac- tual £ow structure in the underlying mantle and the mechanisms of melt extraction from perido- tites are poorly known due to the lack of direct observation [5]. Early in the development of plate tectonics, the great homogeneity of the structure and composi- 0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII:S0012-821X(02)00856-7 * Fax: +98-311-738-1002. E-mail address: anadimi@armanisp.net (A. Nadimi). Earth and Planetary Science Letters 203 (2002) 93^104 www.elsevier.com/locate/epsl