Seismic structure and magmatic construction of crust at the ultraslow‐spreading Southwest Indian Ridge at 50°28'E

Abstract

We present a three‐dimensional crustal structure of a magmatically robust segment of the ultraslow‐spreading Southwest Indian Ridge at 50°28'E based on tomographic inversions of an ocean bottom seismometer data set. Our results show an upper crustal low‐velocity band in the axial zone, which is attributed to increased porosities due to active extensions, leading to anisotropy in the upper crust with a fast direction subperpendicular to the spreading direction. In the lower crust, the results reveal a round‐shaped low‐velocity anomaly at the segment center, indicative of high temperatures and/or a small amount of melt, suggestive of the presence of an axial magma chamber. At the midcrustal depth, an along‐axis asymmetry is observed with respect to the segment center. While a small low‐velocity anomaly indicates lateral magma redistribution toward the western segment end, the deep‐penetrating low velocities and high velocity gradients toward the eastern end suggest that the crust is colder and contains a thicker fractured layer. This asymmetry occurs very close to the axial magma chamber (<5 km) and seems to be related to the fact that the oblique‐spreading domain at the eastern end offsets the ridge axis by a larger distance than that at the western end. We suggest that an along‐axis deep‐penetrating hydrothermal circulation develops on the east side of the axial magma chamber, in response to the rapid change from orthogonal‐ to oblique‐spreading domains and cools the crust.

Lithospheric Structure and Tectonic Processes Constrained by Microearthquake Activity at the Central Ultraslow‐Spreading Southwest Indian Ridge (49.2° to 50.8°E)

“…The crustal structure in the study region is well recovered (Figure ), and the primary features are similar to those identified in previous studies (Jian, Chen, et al, ; Jian, Singh, et al, ; Li et al, ; Niu et al, ; Zhao et al, ), which supports the reliability of our results. Although we provide a brief description of our velocity model below, the main crustal anomalies have been interpreted in detail by Zhao et al () and Jian, Chen, et al (); therefore, we largely focus on using the 3‐D model to relocate earthquakes.…”

“…Recently, Zhao et al () and Jian, Chen, et al () conducted three‐dimensional (3‐D) P wave tomography of Segments 28 and 27, respectively. However, the two models, which are obtained using different methods, are difficult to merge to cover our study region and all available shot lines are not used in both studies.…”

Seismic structure and segmentation of the axial valley of the Mid‐Cayman Spreading Center

“…Unlike at fast spreading centers, magmatism may be more intermittent on slow and ultraslow spreading centers [ Phipps Morgan and Chen , ]. Nonetheless, wide‐angle seismic data have previously shown LVZs in the crust of the slow spreading Mid‐Atlantic Ridge [ Navin et al ., ; Seher et al ., ] and the ultraslow spreading Southwest Indian Ridge [ Li et al ., ; Jian et al ., ]. In their study of Lucky Strike volcano, Singh et al .…”

“…[] created a seismic reflection image of an axial magma chamber (AMC) near the top of the LVZ [ Seher et al ., ; Arnulf et al ., ], showing that the seismic velocity anomalies are indeed associated with melts. The LVZ depth of 2.5–3.5 km beneath the MCSC seafloor is comparable to the mentioned results from other slow and ultraslow spreading centers, though the LVZ lies deeper (3.5–4.5 km) on the ultraslow spreading robust spreading segment at 50°E on the Southwest Indian Ridge [ Jian et al ., ]. The 0.8–1.6 km AMC depth of the fast spreading East Pacific Rise is much smaller in comparison (Figure ).…”

“…Dashed blue lines: Evidence from wide‐angle seismic data for AMCs or LVZs in the oceanic crust. CRR = Costa Rica Rift [ Mutter et al ., ]; EGSC and WGSC = East and West Galapagos Spreading Center [ Blacic et al ., ]; JdF = Juan de Fuca Ridge [ Canales et al ., ; Van Ark et al ., ; Arnulf et al ., ]; LB = Lau Basin [ Turner et al ., ; Jacobs et al ., ]; LS = Lucky Strike volcano [ Singh et al ., ]; MCSC = Mid‐Cayman Spreading Center (this study); RR = Reykjanes Ridge [ Navin et al ., ]; SWIR = Robust spreading segment at 50°E on the Southwest Indian Ridge [ Li et al ., ; Jian et al ., ]; NEPR = Northern East Pacific Rise [ Harding et al ., ; Carbotte et al ., ]; SEPR = Southern East Pacific Rise [ Hussenoeder et al ., ; Hooft et al ., ; Tolstoy et al ., ]; 35°N = Mid‐Atlantic Ridge 35°N [ Magde et al ., ]. Dashed black line: Predicted depth of a stable AMC [ Phipps Morgan and Chen , ].…”