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 def ormational feature in the Central Indian Basin. The purpose is to inve stigate what effects the deformation has had on the crust during the i ncipient rupture of oceanic lithosphere. The wide-angle seismic reflec tion/refraction profiles which are modelled in this paper come from tw o 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 upwarp ed and faulted basement ridge bounded by two undeformed troughs. Previ ous analyses of the refraction data gave conflicting accounts of veloc ity and thickness variations between trough and ridge structures. In t his paper, the refraction profiles are displayed in similar manner and both upsilon(s) and upsilon(p) structures are determined by forward m odelling using 1-D reflectivity synthetics. Results indicate a typical ly mature oceanic upper crust for all profiles (upsilon(p) = 4.5-6.3 k m s(-1); depth = 0-1.2 km; sigma = 0.35-0.27) above a transitional cru st (upsilon(p) = 6.3-6.8 km s(-1); depth = 1.2-2.6 km; sigma = 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 (upsilon(p) = 6.7 km s(-1); thickness = 1. 7 km; sigma = 0.26) and layer 3B (upsilon(p) = 7.1-7.4 km s(-1); thick ness = 2.0-2.5 km; sigma = 0.25); (ii) on the ridge crest, upsilon(p) is increased to 7.2 km s(-1) in layer 3A (sigma = 0.26) but reduced to 6.6 +/- 0.2 km s(-1) in layer 3B (sigma = 0.24-0.30), forming a disti nct low-velocity zone in both upsilon(p) and upsilon(s). The crustal t hickness is reduced progressively at its base from 6.2 +/- 0.3 km to 5 .4 +/- 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 cru st does not thicken beneath the basement ridge crest, as has previousl y been suggested. The crustal structure is, therefore, more consistent with lithospheric deformation by buckling than by boudinage. The crus t beneath the ridge is altered primarily by the development of a low-v elocity and low-density zone within the high-velocity lower crust of s eismic layer 3B. A possible primary origin for the LVZ is by serpentin ization 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 thinnin g under compression. Penetration of water to deep crustal levels withi n this region is consistent with recent deep multichannel reflection i mages of reverse faults which penetrate into or through the lower crus t. It is also consistent with fluid flow along faults within the sedim ent which have perturbed the surface geothermal gradients. This proces s could explain the discrepancy between the high regionally averaged h eat flow and normal basement depths when compared to standard lithosph eric thermal models.