High sensitivity of ocean ridge thermal structure to changes in magma supply: the Galapagos Spreading Center

We explore the physical mechanisms for the observed apparent sensitivity of ridge axis topography and crustal magma systems to small changes in magma supply at intermediate spreading rates. Numerical experiments were carried out to simulate crustal temperature structure of the Galapagos Spreading Center, which spreads at intermediate spreading rates with its various sections appear to have been influenced to different degrees by the nearby Galapagos hotspot. Model results show a strong 'threshold' effect: as the crustal thickness decreases from 7.4 km at the 92degreesW area with an axial high westward to 6.0 km at the 94degreesW area with a transitional topography, the depth to the top of a magma lens is calculated to increase from 1.7 to 2.5 km. In contrast, at the 97degreesW area, where crustal thickness is only 5.6 km and a rift valley is present, the model results predict no steady-state magma lens in the crust. These model calculations provide a simple physical explanation for the recent observations along the Galapagos Spreading Center, where abrupt changes in both magma lens and axial morphology occur within a short distance but crustal thickness changes only modestly. Results of this investigation illustrate the critical importance of hotspots in influencing mid-ocean ridge crustal thickness and the associated changes in thermal structure, especially for ridges that spread at the sensitive range of intermediate spreading rates. (C) 2004 Elsevier B.V. All rights reserved.