Low velocity layers in the subcrustal lithosphere beneath the Deccan Traps region of western India

Abstract

Travel times and relative amplitude modeling of a seismic record section from the Koyna DSS profile in the Deccan Traps region has yielded a velocity model for the Indian continental subcrustal lithosphere. The observed record section, assembled with a reduction velocity of 6 km s −1 and using digitized data from analog deep seismic sounding (DSS) recorders, reveals a prominent wave group with an apparent velocity similar to but about 3 s after the reflected P phase from the Moho (PMP), at a reduced time of 7.5 to 6.5 s and recording distance range from 70 to 95 km. We interpret this strong amplitude phase as the reflection from an interface in the subcrustal lithosphere. Comparison of synthetic seismograms for a range of velocity models with the observed record section shows that the observed phase under investigation cannot be explained either by a reflected phase from a subcrustal velocity discontinuity with a single velocity increase or by the P-to-S converted phase (PMS) from the Moho. Subcrustal velocity models with a single low velocity layer (LVL) do not seem to give a satisfactory fit to the amplitude observations on the field record section. We prefer a subcrustal velocity model with alternating LVL (at least two with a velocity of 7.0–7.4 km s −1) separated by a thin high-velocity layer. In this model, the synthetic amplitudes of the reflected phase from the top of the deeper LVL at 56 km depth well match those of the observed reflection phase. It is inferred that the continental subcrustal lithosphere in this region of the Indian shield has a lamellar structure with significant vertical variation of structural and mechanical properties. The alternating LVL, occurring at relatively shallow depths below the Moho (velocity decreasing from 8.3 to 7.4 km s −1), may be associated with zones of weakness and lower viscosity. The LVL in the subcrustal lithosphere, as well as the LVL inferred in the upper and the lower crust (Krishna et al., 1989) together with the observed depth distribution of seismicity in this region, suggest a well-defined rheological stratification of the continental lithosphere with varying material properties.