Magma Chamber Pressure Controls Along Axis Variations in Crustal Thickness, Axial Depth and Axial Relief at Plate Spreading Centers

Abstract:

Multibeam bathymetry and Mantle Bouguer gravity anomalies (MBA) reveal systematic variations in the gradient of along axis variations in crustal thickness, axial topography and morphology on the scale of plate spreading segments. Previous ideas for these variations focus on either mantle melting processes or crustal level magma diking processes, but no hypothesis has been shown to quantitatively explain observations.

We build on the magmatic dike ideas and assume that magma pools at segment centered magma chambers for intermediate to ultra-slow spreading ridges. Magmatic dikes are taken to transport magma from the central magma chamber to build the crust along a segment. The magma chamber and dike are regarded as a closed system with the volume gained by dike opening equal to volume lost in magma chamber. The magma pressure P is taken to decrease linearly with the volume V of magma going into dike and dP/dV is the key variable in our model. The pressure to drive magma to flow into crust-forming dike is taken to be proportional to the gradient of topography along the axis of a segment. This topography is taken to be related to two things: (1) isostatic topography due to crustal thickness and (2) the deepening of the axis due to across-axis valley relief. We use the recently published model of Liu and Buck (2018) to relate lithospheric thickness and crustal thickness to axial valley relief. For given values of dP/dV, crustal thickness at the segment center and axial lithospheric thickness (for simplicity taken to be constant along the axis), our model predicts the along-axis variations in topography, crustal thickness and across-axis topographic relief.

We find an excellent fit between model predictions and observations for all 12 segments we considered from intermediate to ultra-slow spreading ridges. The key result is that the magma chamber pressure-volume relation, dP/dV is strongly dependent on and inversely correlated with spreading rate. Because dP/dV should be inversely related to magma chamber size it makes sense that fasted spreading centers should have larger magma chambers. Places with very thick crust, like parts of the Reykjanes Ridge, appear to have anomalously large magma chambers for their spreading rate.