Spatial variations in isostatic compensation mechanisms of the Ninetyeast Ridge and their tectonic significance

Abstract

[1] The Ninetyeast Ridge (NER), one of the longest linear volcanic features on the Earth, offers an excellent opportunity of understanding the isostatic response to the interactions of mantle plume with the migrating mid-ocean ridge. Bathymetry, geoid, and gravity (shipborne and satellite) data along 72 closely spaced transects and 17 overlapping grids on the NER are analyzed and modeled to determine the effective elastic thickness (Te) beneath the entire ridge. The results of 2-D and 3-D flexural modeling of the NER show large spatial variations in Te values ranging from 4 to 35 km, suggesting that the ridge was compensated along its length by different isostatic mechanisms. The southern (south of 22°S latitude) and northern (north of 2°N latitude) parts of the NER have Te values of >10 and >23 km, respectively, revealing that the southern part was emplaced on a lithosphere of intermediate strength possibly on flank of the Indian plate, whereas the northern part was emplaced in an intraplate setting. In contrast, in the central part of the NER (between latitudes 22°S and 2°N), highly variable Te values (4-22 km) are estimated. The scattered Te values in the central NER suggest that this part may have evolved due to the occurrence of frequent ridge jumps caused by the interaction of Kerguelen hot spot with rapid northward migration of the Wharton spreading ridge. Residual Mantle Bouguer Anomaly (RMBA) map of the NER and adjacent basins reveals that the entire length of the NER is associated with a significant negative anomaly up to 200 mGal, indicating the presence of thickened crust or less dense mantle beneath the ridge. 3-D crustal thickness map of the NER, generated by inversion of the RMBA data, shows a thick crust ranging from 15 to 19 km. The present study clearly shows that NER possesses a highly segmented isostatic pattern with the occurrence of subcrustal underplating or subsurface loading.

Magma production rate along the Ninetyeast Ridge and its relationship to Indian plate motion and Kerguelen hot spot activity

“…The plate velocity slowed down abruptly by about 30% at around 63 Ma followed by a phase of relatively constant velocity for about 10 Myr up to 52 Ma. This phase is largely associated with relatively smaller number of southward ridge jumps [ Krishna et al ., ], less variable Te values [ Sreejith and Krishna , ], and increasing magma production rates (Figures a–c). The peak in magma production rate at 57 Ma may probably have coincided with another episode of ridge‐hot spot interaction produced by a major southward ridge jump as indicated by progressively increasing Te values between 8°S and 18°S.…”

“…In an alternative approach, Van Ark and Lin [] utilized crustal thickness data for computation of magma volume flux along the Hawaiian hot spot track. As the NER track possess highly segmented isostatic compensation (Te = 4–35 km) together with underplated volcanic material beneath the crust [ Sreejith and Krishna , ], we used RMBA derived crustal thickness data of the ridge including the underplated rocks, for the magma volume computations. A reference crustal thickness of 6 km was subtracted from the crustal thickness data of the NER, and the excess crustal thickness was calculated.…”

“…[] and divided it by age difference of the ridge from north to south (from 85 to 35 Ma). In the present study we have computed the volume flux using crustal thickness data of the ridge [ Sreejith and Krishna , ]. An important observation in our result is the extremely low magma production of about 2 m 3 /s at ~62 Ma, flanked by high production rates of about 12 m 3 /s and 15 m 3 /s.…”

“…(b) Variations in the Indian plate motion as indicated by changes in spreading rates along a synthetic flow line across the India‐Antarctica Ridge [ Cande and Stegman , ]. (c) Plot showing Te values along the NER from 28°S to 8°N derived from 2‐D and 3‐D flexural modeling [ Sreejith and Krishna , ]. The lower and upper bounds of Te estimates using 2‐D and 3‐D methods are indicated by gray shading and error bars.…”

“…We have utilized satellite-derived bathymetry [Smith and Sandwell, 1997], free-air gravity [Sandwell and Smith, 2009], sediment thickness [Levchenko et al, 1993;Curray, 1994;Divins, 2009], and seafloor age [Müller et al, 2008] data from the NER and adjacent oceanic basins. Then we computed the Residual Mantle Bouguer Anomaly (RMBA) data for the NER using free-air gravity anomaly data and inverted to generate a crustal thickness map (for computational details, refer to Sreejith and Krishna [2013]). The derived crustal thickness data are, in turn, used for the calculation of magma production rates along the ridge track and utilized for understanding the spatial and temporal behavior of the Kerguelen hot spot and its possible relationship with Indian plate motion.…”