Constraints on the Central Indian Basin thermal structure from heat flow, seismicity and bathymetry

Abstract

The lithosphere beneath the Central Indian Basin is characterized by high heat flow, widespread deformation of sediment and acoustic basement, and unusually high seismicity. The high heat flow suggests that temperatures in the lithosphere may be higher than expected for its age. We explore the constraints on the temperature distribution with depth by combining heat flow, bathymetry, and seismicity data. The high heat flow (~30 mW m −2 greater than expected) constraints the near-surface temperature distribution. The bathymetric depths (on average those expected for these lithospheric ages after corrections for sediment loads) constrain the integral of the temperature with depth. Since the depths of oceanic intraplate earthquakes appear to be limited by an isotherm (~ 750°C), the maximum earthquake depth (40 km) constrains the minimum depth of deformation and the maximum temperature there. The present thermal structure is investigated by examining different models matching these three observational constraints. First, the possibility is examined that the high heat flow results from anomalous lithosphere with a basal temperature or thickness different from typical oceanic lithosphere. In this case the temperature constraints from the earthquake depths restrict the additional heat flux to less than 20 mW m −2. However, since the heat flux anomaly requires plate thicknesses substantially thinner than usually assumed, it appears that at most 10 mW m −2 can be added by this mechanism. Second, the possibility is examined that the extra flux results from reheating the bottom portion of the lithosphere to asthenospheric temperatures. Substantial reheating 30–40 km below the surface is required to match the high heat flow. Such reheating should produce $ ̃ 1 km of uplift, whereas the average basement depth is no shallower than expected for its age. Thus, significant deep lithospheric reheating cannot be widespread over the deformed region. Third, the effects of a temperature perturbation within the lithospheric column and the resulting change in surface heat flux with time are examined. A temperature perturbation at shallow depths could produce the present heat flow anomaly, but no evidence for shallow intrusion or other mechanism for such an effect exists. It is concluded that, despite the heat flow anomaly, the lack of an average bathymetric anomaly and the observation of seismicity to a depth of 40 km indicate that lithospheric temperatures in the Central Indian Basin are not significantly different from those expected for its age.