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Mantle Bouguer Anomaly Along an Ultra Slow-Spreading Ridge: Implications for Accretionary Processes and Comparison with Results from Central Mid-Atlantic Ridge

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Abstract

A three-dimensional analysis of gravity andbathymetry data has been achieved along the Southwest Indian Ridge (SWIR)between the Rodriguez Triple Junction (RTJ) and the Atlantis II transform,in order to define the morphological and geophysical expression ofsecond-order segmentation along an ultra slow-spreading ridge(spreading rate of 8 mm/yr), and to compare it with awell-studied section along a slow-spreading ridge (spreadingrate of 12.5 mm/yr): the Mid-Atlantic Ridge (MAR) between 28°and 31°30′ N.

Between the Atlantis II transform and theRTJ, the SWIR axis exhibits a deep axial valley with an ∼30°oblique trend relative to the north–south spreading direction. Onlythree transform faults offset the axis, so the obliquity has to beaccommodated by the second-order segmentation. Alongslow-spreading ridges such as the MAR, second-order segmentshave been defined as linear features perpendicular to the spreadingdirection, with a shallow axial valley floor at the segment midpoint,deepening to the segment ends, and are associated with Mantle BouguerAnomaly (MBA) lows. Along the SWIR, our gravity study reveals the presenceof circular MBA lows, but they are spaced further apart than expected. Thesegravity lows are systematically centred over narrow bathymetric highs, andinterpreted as the centres of spreading cells. However, along some obliquesections of the axis, the valley floor displays small topographicundulations, which can be interpreted as small accretionary segments frommorphological analysis, but as large discontinuity domains from thegeophysical data. Therefore, both bathymetry and MBA variations have to beused to define the second-order segmentation of an ultraslow-spreading ridge. This segmentation appears to be characterisedby short segments and large oblique discontinuity domains. Analysis of alongaxis bathymetric and gravimetric profiles exhibits three different sectionsthat can be related to the thermal structure of the lithosphere beneath theSWIR axis.

The comparison between characteristics of segmentationalong the SWIR and the MAR reveals two major differences: first, the poorcorrelation between MBA and bathymetry variations and second, the largerspacing and amplitude of MBA lows along the SWIR compared to the MAR. Theseobservations seem to be correlated with the spreading rate and the thermalstructure of the ridge. Therefore, the gravity signature of the segmentationand thus the accretionary processes appear to be very different: there areno distinct MBA lows on fast-spreading ridges, adjacent ones on slowspreading ridges and finally separate ones on ultra slow-spreadingridges. The main result of this study is to point out that 2nd ordersegmentation of an ultra slow-spreading ridge is characterised bywide discontinuity domains with very short accretionary segments, suggestingvery focused mantle upwelling, with a limited magma supply through a cold,thick lithosphere. We also emphasise the stronger influence of themechanical lithosphere on accretionary processes along an ultra slow-spreading ridge.

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  1. Céline Rommevaux-Jestin

    Present address: URA 736, CNRS, Univ. Pierre et Marie Curie, 4 Place Jussieu, 75252, Paris Cedex 05, France

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  1. CNRS, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252, Paris Cedex 05, France

    Céline Rommevaux-Jestin, Christine Deplus & Philippe Patriat

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Rommevaux-Jestin, C., Deplus, C. & Patriat, P. Mantle Bouguer Anomaly Along an Ultra Slow-Spreading Ridge: Implications for Accretionary Processes and Comparison with Results from Central Mid-Atlantic Ridge. Marine Geophysical Researches 19, 481–503 (1997). https://doi.org/10.1023/A:1004269003009

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