Résumé
Small mantle diapirs structurally defined and mapped in the southern part of the Oman ophiolite are associated with a thick Moho transition zone (MTZ) and copious wehrlitic intrusions in the overlying crust. The presence of an abundance of wehrlites in the crust has been used as a first indicator of focused melt delivery to the crust that is related to mantle diapirism. The second indicator of melt activity is the thickness of the MTZ, which has previously been shown to sharply decrease away from mapped diapirs. Both indicators converge, but the second allows a finer definition of the presumed upwelling areas. Throughout this 500-km-long ophiolite, three broad areas, thus defined, may represent the loci of mantle upwelling below the paleoridge at which the ophiolite originated. The northernmost area, 70 km long, is located in the United Arab Emirates part of the ophiolite; another area, 30 km long, is located in the northern Fizh massif in Oman. The southernmost area covers 130 km of a 150-km-long propagating-ridge segment that opened into very young lithosphere in the central and southern part of the ophiolite. In contrast with the situation of the central area, where the ophiolite represents a piece of lithosphere that was drifted away from the oceanic ridge before obduction, the spreading axis of the northern and southern propagating-ridge segments and thus the underlying mantle diapirs were incorporated into the ophiolite during its oceanic detachment. The small individual diapirs, 10-20 km across, which have been mapped within the propagating-ridge segment, represent distinct heads of a much larger mantle upwelling. If the criterion of MTZ thickness is used, we see that these three upwelling zones encompass one to three small structurally defined diapirs. In a preceding study, we showed that the gabbro section of the crust was significantly thinner above the small structurally defined diapirs than away from them. We extend the previous study with 17 new, more complete sections and confirm the earlier result, as well showing that it applies to the wider upwelling domains. The thickness of the gabbro section is, on average, 2.5 km above these domains versus 3.7 km away from the upwelling areas. The thickness of the sheeted-dike section may also be less above the upwellings (on average, I km thick there versus 1.2 km away from the upwellings). If to these thicknesses is added the 0.7 km estimate for the so-called Geotimes or V1 lavas directly associated with the ophiolite, the total thickness of the crust is only 4.2 km above the presumed mantle upwellings compared to 5.6 km away from these areas.