Axial Structure of Fast Spreading Mid-Ocean Ridges: Implications for Overlapping Spreading Centers

摘要

Studies of the structure of mid-ocean ridges and the processes involved at these accreting plate boundaries have played an important role in the development of plate tectonics models over the last two decades. During this time our concept of the accretion/spreading process has evolved from a two- dimensional, steady-state picture to a three dimensional, time-dependent view of the volcanism and rifting. Using the axial morphology of a fast spreading mid-ocean ridge as a constraint, isostatically balanced density structures, representing the central portion of a ridge segment, the off-axis region and the ridge tip at an overlapping spreading center, are determined for two end member models. One emphasizes the role of the subaxial asthenosphere in maintaining axial ridge topography. The other emphasizes the role of a crustal magma chamber. The results of mass balance calculations that are consistent with gravity data indicate that both models are viable in terms of their ability to support an axial ridge of the size observed along the East Pacific Rise (EPR). Isostatic support of a 300-400 m high axial ridge requires either subaxial asthenospheric melt contents of 10-25% or a 2.5-4.0 km thick magma chamber. Axial ridge volumes are shown to range from 1 X 106 to 5 X 106 m3/m along the EPR, with significant decreases at overlapping spreading centers. The volume of subaxial asthenospheric melt required to balance a ridge of a given volume is less than half that required in a crustal magma chamber where the density contrast between the melt and its surroundings is considerably less. Gravity and petrologic data suggest that a large magma chamber is unlikely to be maintained along the EPR. Although it is not presently possible to put strict limits on the maximum size of the axial magma chamber, there is some evidence that the shallower sections of the EPR axial ridge m.