Résumé
Over the past 30 years, the boundary between seismic layers 2 and 3 in modern oceanic crust has been successively explained by changes in porosity, fracturing, lithology, and metamorphic grade. This transition was most recently proposed to lie within the sheeted-dike complex of a deep oceanic borehole (Hole 504B of the Deep Sea Drilling Project and the Ocean Drilling Program). Both models that promote this interpretation rule out the lithologic hypothesis, which suggests that the boundary between seismic layers 2 and 3 corresponds to the downward transition from sheeted dikes to gabbro. Downhole measurements in the same hole reveal a gradual change in stress regime within the sheeted-dike complex (1.3 to 1.7 km into basement), confirming the previous findings and allowing the proposal of a common cause to explain some of the earlier hypotheses. From compressional above to strike slip below the seismic transition, this stress change is proposed to induce a depth-dependent reopening of cracks and fractures. With a nearly constant porosity throughout the sheeted-dike complex, this variable reopening of cracks can have a significant impact on acoustic and hydraulic properties of the crust and hence on the mode and nature of fluid circulation and crustal alteration. Horizontal fluid movement and heat transfer are favored in the upper part of the crust under a compressional regime, whereas vertical mining of hot fluids and associated fluid-rock interactions are enhanced at the base of the dolerites under a strike-slip regime. As the crust ages and subsides, the transition between these two regimes (where the vertical load equals the minimum horizontal stress) migrates upward, which provides an explanation for the upward migration of the alteration front in dikes with time. More generally, this result indicates that seismic methods might allow one to map subsurface changes of the stress field with depth in the upper oceanic crust.