Along‐axis variation in crustal thickness at the ultraslow spreading Southwest Indian Ridge (50°E) from a wide‐angle seismic experiment

Abstract

The Southwest Indian Ridge (SWIR) is characterized by an ultraslow spreading rate, thin crust, and extensive outcrops of serpentinized peridotite. Previous studies have used geochemical and geophysical data to suggest the presence of a thicker crust at the central and shallowest portions of the SWIR, from the Prince Edward (35 30 0 E) to the Gallieni (52 20 0 E) fracture zones. Here we present a new analysis of wide-angle seismic data along the ridge 49 17 0 E-50 49 0 E. Our main conclusions are as follows: (1) we find an oceanic layer 2 of roughly constant thickness and steep velocity gradient, underlain by a layer 3 with variable thickness and low velocity gradient; (2) the crustal thickness varies from 5 km beneath nontransform discontinuities (NTDs) up to 10 km beneath a segment center; (3) the melt supply is focused in segment centers despite a small NTD between adjacent segments; (4) the presence of a normal upper mantle velocity indicates that no serpentinization occurs beneath this thick crust. Our observation of thick crust at an ultraslow spreading ridge adds further complexity to relationships between crustal thickness and spreading rate, and supports previous suggestions that the extent of mantle melting is not a simple function of spreading rate, and that mantle temperature or chemistry (or both) must vary significantly along axis.

Isotopically enriched N‐MORB: A new geochemical signature of off‐axis plume‐ridge interaction—A case study at 50°28′E, Southwest Indian Ridge

“…Anomalously shallow ridge flanks were interpreted to indicate a dramatic increase in magma supply since 8–10 Ma [ Sauter et al ., , ] and that it is characterized by robust magmatism with abundant basalts at the seabed [ Sauter et al ., ], and without peridotites [ Zhou and Dick , ]. The only axial high exists along segment 27 (~50°28′E), a 72 km long segment where the axial depth is on average 1000 m shallower than nearby ridge segments (Figure a) and numerous volcanic edifices are present on both flanks of the axial valley, with relatively flat topography to the north and greater relief to the south (Figure b) [ Niu et al ., ]. Segment 27 shows a pronounced MBA low and a large negative anomalies of S wave velocities at depth [ Sauter et al ., ], with anomalously thick crust of up to 10 km at the segment center [ Niu et al ., ] and off axis within a 20 km distance [ Li et al ., ].…”

“…The only axial high exists along segment 27 (~50°28′E), a 72 km long segment where the axial depth is on average 1000 m shallower than nearby ridge segments (Figure a) and numerous volcanic edifices are present on both flanks of the axial valley, with relatively flat topography to the north and greater relief to the south (Figure b) [ Niu et al ., ]. Segment 27 shows a pronounced MBA low and a large negative anomalies of S wave velocities at depth [ Sauter et al ., ], with anomalously thick crust of up to 10 km at the segment center [ Niu et al ., ] and off axis within a 20 km distance [ Li et al ., ]. The thick oceanic crust indicates robust magma supply in this area, which has been inferred to be associated with the Crozet hotspot [ Sauter et al ., ; Zhang et al ., ].…”

“…There is a bathymetric high and a mantle Bouguer anomaly (MBA) low between the Indomed (IFZ) and Gallieni (GFZ) fracture zones (46°E and 52°20′E, Figure a) [ Georgen et al ., ]. At 50°28′E with the shallowest water depth and the only axial high along this region (Figure b), seismic studies indicate the thickest oceanic crust along ultraslow‐spreading ridges [ Carbotte et al ., ] up to 10 km thick [ Niu et al ., ] and off axis [ Li et al ., ], suggesting that robust magmatism at this particular location has lasted at least 3 Ma [ Li et al ., ]. Off‐axis topographic and gravimetric studies further extend such a melting anomaly to persist for 10 Ma [ Sauter et al ., ].…”

230Th/238U dating of hydrothermal sulfides from Duanqiao hydrothermal field, Southwest Indian Ridge

“…Such style of heat source can contribute to continuous hydrothermal activity for over 1000 years. Duanqiao field is located near the central volcano and there is a hot mantle and/or fertile melt beneath Duanqiao field (Tao et al 2012;Niu et al 2015). The crust thickness is 9.5 km, suggesting the existence of AMC (Axial Magma Chamber) which provides magma source to the field Mendel et al 2003).…”

“…Li et al (2015) used three-dimensional seismic tomography at 50°28 0 E and revealed a low-velocity anomaly (*0.6 km/s) which was accompanied by an unusually thick crust (*9.5 km). Niu et al (2015) also identified the thick crust (*10 km) beneath the segment center by wide-angle seismic reflection. Li et al (2013) reported the geochemical and molecular biological compositions of low-temperature hydrothermal deposits.…”