Interactions between hotspots and the Southwest Indian Ridge during the last 90 Ma: Implications on the formation of oceanic plateaus and intra-plate seamounts

Abstract

This study investigates the relationship between the hotspot-ridge interaction and the formation of oceanic plateaus and seamounts in the Southwest Indian Ocean. We first calculated the relative distance between the Southwest Indian Ridge (SWIR) and relevant hotspots on the basis of models of plate reconstruction, and then calculated the corresponding excess magmatic anomalies of the hotspots on the basis of residual bathymetry and Airy isostasy. The results reveal that the activities of the Marion hotspot can be divided into three main phases: interaction with the paleo-Rodrigues triple junction (73.6-68.5 Ma), interaction with the SWIR (68.5-42.7 Ma), and intra-plate volcanism (42.7-0 Ma). These three phases correspond to the formation of the eastern, central, and western parts of the Del Cano Rise, respectively. The magnitude and apparent periodicity of the magmatic volume flux of the Marion hotspot appear to be dominated by the hotspot-ridge distance. The periodicity of the Marion hotspot is about 25 Ma, which is much longer than that of the Hawaii and Iceland hotspots (about 15 Ma). Southwest Indian Ocean, hotspot-ridge interaction, magmatic volume flux, oceanic plateaus, seamounts Citation:Zhang T, Lin J, Gao J Y. Interactions between hotspots and the Southwest Indian Ridge during the last 90 Ma: Implications on the formation of oceanic plateaus and intra-plate seamounts.Numerous seamounts are generated at plate boundaries by the volcanic and tectonic activities of mid-ocean ridges and transform faults [1]. Correspondingly, hotspots are mainly observed as intra-plate volcanism [2]. With the interaction of mid-ocean ridges and hotspots, the intensity of hotspot activity is amplified, resulting in the formation of broad plateaus [3]. These plateaus and seamounts (or seamount chains) substantially reflect the intensity of magmatism and the tracks of hotspots [2,[4][5][6][7], and record the ages, locations and magmatic volume of seamounts created by hotspots. It would be useful to study the time variation and periodicity of the intensity of hotspot activity and the interaction of hotspots, mid-ocean ridges and transform faults [4,5,8,9]. Radioactive dating of rocks can be adopted to determine directly the ages of seamounts, and the thickness of the oceanic crust (layer 2A to layer 3) can be regarded as a first-order measure of the mantle magmatic volume [10]. However, very few rock samples have been taken and there has been little determination of the crustal thickness from seismic velocity profiles of ocean bottom experiments. In many areas that have been little studied, the mentioned methods cannot be used to calculate hotspot tracks and

Thin crust as evidence for depleted mantle supporting the Marion Rise

“…Even assuming large temperature anomalies, however, it has been noted that the Hawaiian Swell, possibly the largest 9 thermal plume on the planet, has a peak amplitude of only ~1.4 km 43 while Ito et al 44 modeling the Iceland Swell found little over 1 km supported by mantle temperature variations of 180°C, with the rest being an isostatic response to crustal thickness. Thus, it is very unlikely that the geriatric Marion Plume could account for the 2.1-km depth anomaly along the Marion Rise.Mantle composition gradients along rises can explain their gravity signal, which when modeled assuming uniform mantle composition, predicts 8 to 10 km of crust on the Marion and Azores Rises 14,45 . Varying serpentinization depth is not a likely alternative as serpentine is unlikely to be stable to 9 or 10 km depth beneath a ridge.…”

“…Notes: a) Marion -Zhang et al 14 ; b) Iceland& Azores -Wang et al 5 ; Galapagos -Canales et al 6 ; Marion -Zhang et al 14 . c) Maximum and minimum elevations are measured from shallowest point on axis, or midpoint of Iceland plateau to the rift valley axis point where average depth stabilizes or blocking transform.…”

“…Modeling using sea surface gravity gives 2-4 km in the eastern SWIR, but around 6 to 12 km north of Marion Island 14 . Six to 12 km of igneous crust where mantle peridotites are abundantly exposed, however, seems unlikely.…”

“…: 18,19 ). Using this approach, the 1 st author surveyed a ridge segment at 53°E where low basalt Na 8 20 , refractory peridotite, gravity 14 , and the mantle wedge hypothesis suggest 4-8 km of crust. Instead of thick crust, however, this found nearly amagmatic spreading, supporting our interpretation of sampling elsewhere on the ridge.…”

“…Mantle composition gradients along rises can explain their gravity signal, which when modeled assuming uniform mantle composition, predicts 8 to 10 km of crust on the Marion and Azores Rises 14,45 . Varying serpentinization depth is not a likely alternative as serpentine is unlikely to be stable to 9 or 10 km depth beneath a ridge.…”