Upper crustal seismic structure along the Southeast Indian Ridge: Evolution from 0 to 550 ka and variation with axial morphology

Abstract

The seismic structure of uppermost crust evolves after crustal formation with precipitation of alteration minerals during ridge‐flank hydrothermal circulation. However, key parameters of crustal evolution including depth extent and rates of change in crustal properties, and factors contributing to this evolution remain poorly understood. Here, long‐offset multichannel seismic data are used to study the evolution of seismic layer 2A and uppermost 2B from 0 to 550 ka at three segments of the intermediate spreading rate Southeast Indian Ridge. The segments differ in on‐axis morphology and structure with crustal magma bodies imaged at axial high and rifted high segments P1 and P2, but not at axial valley segment S1 and marked differences in thermal conditions within the upper crust are inferred. One‐dimensional travel time modeling of common midpoint supergathers is used to determine the thickness and velocity of layer 2A and velocity of uppermost 2B. At all three segments, layer 2A velocities are higher in 550 ka crust than on‐axis (by 7–14%) with the largest increases at segments P1 and P2. Velocities increase more rapidly (by 125 ka) at P1 with spatial variations in velocity gradients linked to location of the underlying crustal magma body. We attribute these differences in crustal evolution to higher rates of fluid flow and temperatures of reaction at these ridge segments where crustal magma bodies are present. Layer 2A thickens off‐axis at segments P1 and P2 but not at S1; both off‐axis volcanic thickening and downward propagation of a cracking front linked to the vigor of axial hydrothermal activity could contribute to these differences. In zero‐age crust, layer 2B velocities are significantly lower at segments P1 and P2 than S1 (5.0, 5.4, and 5.8 km/s respectively), whereas similar velocities are measured off‐axis at all segments (5.7–5.9 km/s). Lower on‐axis 2B velocities at segments P1 and P2 can be partly attributed to thinner layer 2A, with lower overburden pressures leading to higher porosities in shallowest 2B. However, other factors must also contribute. Likely candidates include subaxial deformation due to magmatic processes and enhanced cracking with axial hydrothermal activity at these segments with crustal magma bodies.