In contrast to mature mid-oceanic ridges, where magmatic activity is little affected by the slow accumulation of sediments, in young spreading centers (such as that of the Guaymas Basin in the Gulf of California) the basaltic magma of “great magmatic pulses” forms dikes and sills within the uppermost few hundred metres of soft sediments. In general, younger dikes and sills are injected next to or on top of the contact zone of the older ones. In this manner a distinctive sill-sediment complex is built up, the sediments of which are rather compacted and partly metamorphosed despite the low burial depth.

The thickness of this transitional zone between the sheeted dike complex (seismic layer 2) and younger sediment (seismic layer 1) is controlled chiefly by the rates of sedimentation and spreading. If the half-spreading rate is approximately one order of magnitude greater than the sedimentation rate, the sill-sediment complex can reach a thickness of only a few hundred metres, and the depth of the spreading trough remains approximately constant. Sedimentation rates approaching or surpassing the spreading rate cause filling up of the basins, which probably hampers the injection of magma into sediments. Sill-sediment complexes similar to those in the Gulf of California are also expected to occur at the passive margins of older oceanic basins as well as orogenic belts.

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