Variations in crustal structure related to intraplate deformation: evidence from seismic refraction and gravity profiles in the Central Indian Basin

In this paper, an analysis of seismic refraction and gravity profiles is presented to define the crustal structure across a ridge-trough deformational feature in the Central Indian Basin. The purpose is to investigate what effects the deformation has had on the crust during the incipient rupture of oceanic lithosphere. The wide-angle seismic reflection/refraction profiles which are modelled in this paper come from two previously reported data sets (three R/V Conrad profiles and two R/V Mendeleev profiles) located south of the ODP Leg 116 drilling sites, where N-S reflection profiles clearly delineate a symmetrically upwarped and faulted basement ridge bounded by two undeformed troughs. Previous analyses of the refraction data gave conflicting accounts of velocity and thickness variations between trough and ridge structures. In this paper, the refraction profiles are displayed in similar manner and both v<inf>s</inf></it> and v<inf>p</inf></it> structures are determined by forward modelling using 1-D reflectivity synthetics. Results indicate a typically mature oceanic upper crust for all profiles (v<inf>p</inf></it> = 4.5–6.3 km s-1; depth=0–1.2 km; σ=0.35–0.27) above a transitional crust (v<inf>p</inf></it> = 6.3–6.8 km s-1; depth=1.2–2.6 km; σ=0.25). In the middle and lower crust, there are major changes between trough and ridge profiles: (i) within the basement troughs, there is a sharp transition between layer 3A (v<inf>p</inf></it> = 6.7 km s-1; thickness=1.7 km; σ=0.26) and layer 3B (v<inf>p</inf></it> = 7.1–7.4 km s-1; thickness=2.0–2.5 km; σ=0.25); (ii) on the ridge crest, v<inf>p</inf></it> is increased to 7.2 km s-1 in layer 3A (σ=0.26) but reduced to 6.6 μM 0.2 km s-1 in layer 3B (σ=0.24–0.30), forming a distinct low-velocity zone in both v<inf>p</inf></it> and v<inf>s</inf></it>. The crustal thickness is reduced progressively at its base from 6.2 μM 0.3 km to 5.4 μM 0.2 km. A 2-D gravity model across the ridge-trough structure, based on the 1-D seismic velocity-depth models, is consistent with an observed anomaly of 40–45 mGal. These results demonstrate that the crust does not thicken beneath the basement ridge crest, as has previously been suggested. The crustal structure is, therefore, more consistent with lithospheric deformation by buckling than by boudinage. The crust beneath the ridge is altered primarily by the development of a low-velocity and low-density zone within the high-velocity lower crust of seismic layer 3B. A possible primary origin for the LVZ is by serpentinization at low temperatures of olivine clasts within the mafic gabbro of layer 3B. Asymmetric folding of the lower crust or lateral flow of the more viscous serpentine might explain the possible crustal thinning under compression. Penetration of water to deep crustal levels within this region is consistent with recent deep multichannel reflection images of reverse faults which penetrate into or through the lower crust. It is also consistent with fluid flow along faults within the sediment which have perturbed the surface geothermal gradients. This process could explain the discrepancy between the high regionally averaged heat flow and normal basement depths when compared to standard lithospheric thermal models.