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

Abstract

S U M M A R YIn 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 u, and up structures are determined by forward modelling using 1-D reflectivity synthetics. Results indicate a typically mature oceanic upper crust for all profiles (up = 4.5-6.3 km s-'; depth = 0-1.2 km; u = 0.35-0.27) above a transitional crust (up = 6.3-6.8 km s-'; depth = 1.2-2.6 km; u = 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 (up = 6.7 km s-'; thickness = 1.7 km; u = 0.26) and layer 3B (up = 7.1-7.4 km s-'; thickness = 2.0-2.5 km; u = 0.25); (ii) on the ridge crest, up is increased to 7.2 km s-' in layer 3A ( a = 0.26) but reduced to 6.6 f 0.2 km s-' in layer 3B (u = 0.24-0.30), forming a distinct low-velocity zone in both up and us. The crustal thickness is reduced progressively at its base from 6.2 f 0.3 km to 5.4 f 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...

Seismic evidence of a two-layer lithospheric deformation in the Indian Ocean

“…In the CIB, extensive thrust faults have been imaged cutting through the crust, some of them extending into the mantle 41,42 , which are suggested to be due to the active intra-plate deformation in the CIB. There too, not all the mantle faults extend into the crust and subsequently to the sediments, indicating that these faults are either older or associated with the original oceanic fabric.…”

Oceanic intraplate faulting as a pathway for deep hydration of the lithosphere: Perspectives from the Caribbean

“…They fall into two populations: small (<100 m), vertical throw faults spaced, on average, 3 km apart, and large, vertical throw faults spaced ~20 km apart . These faults extend 8-15 km below the surface of the oceanic crust (Delescluse and Chamot-Rooke, 2008), ~2-9 km into the mantle, and are associated with pervasive serpentinization to ~20 km beneath the surface of the oceanic crust (Louden, 1995;, or to ~14 km into the mantle.…”

Formation of diapiric structure in the deformation zone, central Indian Ocean: A model from gravity and seismic reflection data

“…Two seismic refraction stations located near the diapiric structure discussed in this paper display the higher velocities (7.6 to 7.7 km/s) in the lower crust (Curray et al 1982). Louden (1995) has observed a low velocity zone (6.5 to 6.8 km/s) within the high velocity lower crust of layer 3B and attributed its origin to serpentinization of olivine clasts within the gabbro. These higher velocities near the base of layer 3 are indicative of serpentinized peridotites (Li Pichon et al 1972).…”

“…Free-air gravity anomalies across the basement trough and diapiric structure are modeled with the consideration of seismic constraints such as sediment thickness, basement trend (figure 4), general thickness of layer 2, layer 3, and proposed serpentinite layer (Curray et al 1982;Lobkovsky et al 1986;Leger et al 1987;Neprochnov et al 1988;Leger and Louden 1990;Louden 1995;Neprochnov et al 1998). Densities ranging from 1.03 to 3.35 gm/cc are used for water column, sediments, various crustal layers, and mantle rocks.…”