Crustal architecture and deep structure of the Ninetyeast Ridge hotspot trail from active-source ocean bottom seismology

Abstract

Summary A 550‐km‐long transect across the Ninetyeast Ridge, a major Indian ocean hotspot trail, provided seismic refraction and wide‐angle reflection data recorded on 60 ocean bottom instruments. About 24 000 crustal and 15 000 upper mantle arrivals have been picked and used to derive an image of the hotspot track. Two approaches have been chosen: (i) a first‐arrival tomographic inversion yielding crustal properties; and (ii) forward modelling of mantle phases revealing the structure at the crust–mantle boundary region and of the uppermost mantle. Away from the volcanic edifice, seismic recordings show the typical phases from oceanic crust, that is, two crustal refraction branches (Pg), a wide‐angle reflection from the crust–mantle boundary (PmP) and a wave group turning within the upper mantle (Pn). Approaching the edifice, three additional phases have been detected. We interpret these arrivals as a wide‐angle reflection from the base of material trapped under the pre‐hotspot crust (Pm2P) and as a wide‐angle reflection (PnP) and its associated refraction branch (PN) from a layered upper mantle. The resulting models indicate normal oceanic crust to the west and east of the edifice. Crustal thickness averages 6.5–7 km. Wide‐angle reflections from both the pre‐hotspot and the post‐hotspot crust–mantle boundary suggest that the crust under the ridge has been bent downwards by loading the lithosphere, and hotspot volcanism has underplated the pre‐existing crust with material characterized by seismic velocities intermediate between those of mafic lower crustal and ultramafic upper mantle rocks (7.5–7.6 km s−1). In total, the crust is up to ≈ 24 km thick. The ratio between the volume of subcrustal plutonism forming the underplate and extrusive and intrusive volcanism forming the edifice is about 0.7. An important observation is that underplating continued to the east under the Wharton Basin. During the shield‐building phase, however, Ninetyeast Ridge was located adjacent to the Broken Ridge and was subsequently pulled apart along a transform fault boundary. Therefore, underplating eastwards of the fracture zone separating the edifice from the Wharton Basin suggests that prolonged crustal growth by subcrustal plutonism occurred over millions of years after the major shield‐building stage. This fact, however, requires mantle flow along the fossil hotspot trail. The occurrence of PnP and PN arrivals is probably associated with a layered and anisotropic upper mantle due to the preferential alignment of olivine crystals and may have formed by rising plume material which spread away under the base of the lithosphere.

Lithosphere structure and upper mantle characteristics below the Bay of Bengal

“…As the volcanism leading to the emplacement of the 85°E and Ninetyeast ridges in the BOB are older (> 80 Ma), the swells associated with the initial volcanism will have subsided. Earlier derived crustal models show the presence of thicker crust with and without underplating below these ridges (Detrick and Watts, 1979;Mukhopadhyay and Krishna, 1995;Subrahmanyam et al, 1999;Grevemeyer et al, 2001;Krishna et al, 2001b;Krishna, 2003;Subrahmanyam et al, 2008;Radhakrishna et al, 2010). The present analysis revealed that both ridges, in general, are earlier investigators for the ridge south of 2ºN revealed that this part of the ridge had evolved due to frequent ridge jumps in the vicinity of the Kerguelen hot spot during the rapid northward migration of the Wharton spreading ridge (Royer et al, 1991;Krishna et al, 1999;Sager et al, 2010;Krishna et al, 2012).…”

“…7B) within the flexural waveband (0.01 < k < 0.04). The choice of f equal to 0.5 is based on the earlier studies over the Ninetyeast Ridge in the southern part of the present study area (Grevemeyer and Flueh, 2000;Grevemeyer et al, 2001;. The Te values obtained from the admittance and coherence analysis along the 85 o E and Ninetyeast ridges are given in Table 1.…”

“…7B) in these blocks. As stated earlier, for the coherence analysis, we considered a value of 0.5 for the loading ratio f as the modeled f values elsewhere along the Ninetyeast Ridge range from 0.5 -0.7 (Grevemeyer and Flueh, 2000;Grevemeyer et al, 2001) and 0.3 -0.6 . The present Te values obtained from both 85°E and Ninetyeast ridges are combined with the Te estimates published for the 85ºE Ridge (Sreejith et al, 2011) and for the Ninetyeast Ridge Te values (Watts et al, 2006), however, our estimates are better constrained due to the availability of regional seismic data for the ridge north of 10 o N which is progressively buried below the Bengal Fan sediments.…”

Seismic constraints on the processes and consequences of secondary igneous evolution of Pacific oceanic lithosphere

“…We determine the seismic velocity structure of the downgoing plate along Line E using P-wave travel time tomography. Our preferred model from the tomographic inversion indicates that widespread magmatic activity has resulted in significant crustal thickening beneath seamounts, likely via crustal underplating [e.g., Caress et al, 1995;Grevemeyer et al, 2001]. Thickening of oceanic Layers 2 and 3 along our line seems to imply two distinct modes of melt transport within the lithosphere, either broadly pooling at the Moho (lowflux) or channeling through the crust to form extrusive additions (high-flux).…”

Seamounts

“…The base of the crust forms a marked density contrast, so it is likely that melts generated at depth will accumulate near the base of the crust, where they either underplate the crust or intrude it as sills. High-velocity, lowercrustal rocks are an integral part of large seamounts, including the Hawaiian chain (Watts et al, 1985), Ninetyeast Ridge (Grevemeyer et al, 2001), and Louisville seamounts (Contreras-Reyes et al, 2010). …”