Marine Geophysical Researches 21: 243–257, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 243 Oceanic ridge off-axis deep structure in the Mansah region (Sumail massif, Oman ophiolite) D. Jousselin ∗ & A. Nicolas Laboratoire de Tectonophysique CNRS/UM2, Place E. Bataillon, 34095 Montpellier cedex 05, France ∗ Present address: Department of Geological sciences, University of Oregon, Eugene, OR 97401, USA Key words: diapir, mantle, Oman, ophiolite, seamount, structure Abstract Large scale structural mapping of the Oman ophiolite indicates that the Mansah area (Sumail massif) was a ridge off-axis region at the time the ophiolite was detached. This paper presents a detailed structural mapping of the region. We show that, as opposed to other off-axis areas, it contains plunging lineations, correlated with a thick Moho transition zone and chromite pods, indicative of a mantle diapir. However this diapir has a discontinuous structure, it is bounded by shear zones and types of diabases that are not found elsewhere in Oman; it also has a broken crust, strongly affected by hydrothermal alteration. This suggests that Mansah fossilized an off-axis diapir intruding a cooling lithosphere. It may be a good candidate to be the root of an off-axis seamount such as those found in the East Pacific, and may bring new insights into this particular volcanism which we are only beginning to explore. Introduction Structural mapping in the southern massifs of the Oman ophiolite has revealed four mantle diapirs crudely aligned along a NW–SE direction with a spac- ing of 10 to 30 km. From NW to SE, these are the Rustaq, Nakhl, Maqsad, and the Sayma diapirs. This alignment of diapirs is found in a 30 to 50 km broad region where dikes in the mantle and the sheeted dike complex are oriented NW–SE. The mapping also shows that this diapiric region is bounded at the NE and SW by regions where dikes trend NE–SW and are older than the NW–SE dikes (Ceuleneer, 1986, pp. 105 and 120 ; Reuber et al., 1991; Boudier et al., 1997) (Figure 1). From these observations it has been proposed that this part of the Oman ophio- lite represents a ridge segment, underlain by mantle diapirs, which was propagating in a NW–SE direc- tion, opening through an older lithosphere which has been accreted along a NE–SW direction (Nicolas and Boudier, 1995; Boudier et al., 1997; Nicolas et al., 2000a) (Figure 1). Two mapped diapirs (Batin in the Wadi Tayin massif and Mansah in the Sumail massif) are not aligned with the other four. Batin is well in- side the NE–SW domain, suggesting that it belongs to this domain, a conclusion supported by the dominant NE–SW trend of the pyroxene and gabbro dikes which cut it (Ildefonse et al., 1993). While the Batin diapir is located far from any crustal outcrop, the Mansah area offers good exposures that allow one to observe the interaction of the diapir with the crust. Consider- ing that the Mansah diapir is located at the northern limit between the most recent NW–SE ridge system and the older NE–SW system, we present a more ex- tensive mapping of the Mansah structure to clarify whether it represents a diapir feeding the older ridge or whether it corresponds to an off-axis diapir. Other hypotheses will also be considered. The region is now covered by 211 field stations of which 120 are in the mantle section (Figure 2). Several measurements were made at each station, and at least one oriented sample was taken to control the structural measurements on bleached samples and thin sections. If the diapirs were formed on-axis with respect to the Oman paleoridge, there should exist a struc- tural continuity at the crustal level and in the high- temperature (High-T) mantle-flow structure around