Magma transport beneath mid-ocean ridges

<p>Melt transport beneath the lithosphere is elusive. With a distinct viscosity and density from the surrounding mantle, magmatic melt moves on a different time scale as the surrounding mantle. To resolve the temporal scale necessary to accurately capture melt transport in the mantle, the model simulations become numerically expensive quickly. Recent computational advances make possible two-phase numerical explorations to understand magma transport in the mantle. We review results from a suite of two-phase models applied to the mid-ocean ridges, where we varied half-spreading rate and intrinsic mantle permeability using new openly available models, with the goal of understanding melt focusing beneath mid-ocean ridges and its relevance to the lithosphere-asthenosphere boundary (LAB). Here, we highlight the importance of viscosities for the melt focusing mechanisms. In addition, magmatic porosity waves that are a natural consequence of these two-phase flow formulations. We show that these waves could explain long-period temporal variations in the seafloor bathymetry at the Southeast Indian Ridge.</p>

Large-scale asymmetry in thickness of crustal accretion at the Southeast Indian Ridge due to deep mantle anomalies

Hot mantle plumes and ancient cold slabs have been observed beneath modern mid-ocean ridges, but their specific and detailed effects on mid-ocean ridge crustal accretion are poorly understood. The oceanic lithosphere beneath the Southeast Indian Ocean displays unique morphological, geophysical, and geochemical characteristics, which may reflect the influence of both mantle anomalies and upwelling plumes on seafloor spreading. In this study, we combined gravity-derived oceanic crustal thickness with plate tectonic reconstructions to investigate patterns of asymmetry in thickness of crust accreted at the Southeast Indian Ridge over the last 50 m.y. Our results reveal several distinct features: (1) small-scale, short-lived asymmetries in the thickness of crustal accretion of up to 0.75 km are alternatively distributed on the southern and northern flanks of the 90°−120°E Southeast Indian Ridge segment. These can be explained by variations in mantle depletion or mantle temperature. (2) Two large-scale, long-lived (duration of ∼50 m.y.) asymmetries in crustal accretion of >2.5 km are observed around the Kerguelen Plateau and Balleny Islands, which we attribute to excess crust from the off-axis Kerguelen and Balleny mantle plumes. (3) Two large-scale, long-lived (duration of ∼50 m.y.) asymmetries in crustal accretion of 0.75−2.5 km are observed on the northern flank of the westernmost (70°−80°E) Southeast Indian Ridge and the southern flank of the eastern (120°−140°E) Southeast Indian Ridge segment, respectively. We attribute these to asymmetry in mantle temperature of up to 20−53 °C. We suggest these asymmetric temperatures across the Southeast Indian Ridge are associated with the foundered lithospheric fragments of the Indian Craton triggered by the African Large Low-Shear-Velocity Province during the breakup of Gondwanaland and an intraplate subducted slab of the Paleo-Tethys Ocean, respectively. The remnant craton fragments and subducted oceanic slab may have moved north in concert with the northward-migrating Southeast Indian Ridge beginning at 50 m.y. ago.

Supplemental Material: Large-scale asymmetry in thickness of crustal accretion at the Southeast Indian Ridge due to deep mantle anomalies

Figures S1 and S2, including the previous Euler Poles of Australia relative to Antarctica and the results of the asymmetric anomaly contributed by different factors; Table S1: The parameters of the Euler Poles we used in this study. Dataset (grid format) of our resulted crustal thickness.

Supplemental Material: Large-scale asymmetry in thickness of crustal accretion at the Southeast Indian Ridge due to deep mantle anomalies

Figures S1 and S2, including the previous Euler Poles of Australia relative to Antarctica and the results of the asymmetric anomaly contributed by different factors; Table S1: The parameters of the Euler Poles we used in this study. Dataset (grid format) of our resulted crustal thickness.

The 4 December 2015 Mw 7.1 Normal-Faulting Antarctic Plate Earthquake and Its Seismotectonic Implications

ABSTRACT Oceanic plate seismicity is generally dominated by normal and strike-slip faulting associated with active spreading ridges and transform faults. Fossil structural fabrics inherited from spreading ridges also host earthquakes. The Indian Oceanic plate, considered quite active seismically, has hosted earthquakes both on its active and fossil fault systems. The 4 December 2015 Mw 7.1 normal-faulting earthquake, located ∼700 km south of the southeast Indian ridge in the southern Indian Ocean, is a rarity due to its location away from the ridge, lack of association with any mapped faults and its focal depth close to the 800°C isotherm. We present results of teleseismic body-wave inversion that suggest that the earthquake occurred on a north-northwest–south-southeast-striking normal fault at a depth of 34 km. The rupture propagated at 2.7 km/s with compact slip over an area of 48×48 km2 around the hypocenter. Our analysis of the background tectonics suggests that our chosen fault plane is in the same direction as the mapped normal faults on the eastern flanks of the Kerguelen plateau. We propose that these buried normal faults, possibly the relics of the ancient rifting might have been reactivated, leading to the 2015 midplate earthquake.

Distribution, morphology and magnetic characteristics of near-axis seamounts along the KR1, the easternmost segment of the Australian-Antarctic Ridge

<p>The Australian-Antarctic Ridge (AAR) is the spreading boundary between the Australian and the Antarctic plates, and extended from the Southeast Indian Ridge (SEIR) to the Macquarie Triple Junction (MTJ) of Australian-Antarctic-Pacific plates. The KR1 is the easternmost segment of AAR, with a quite large variation in axial morphology. In this study, we identified 3-volcanic seamount chains aligned linearly parallel to the spreading direction of the KR1. The spatial distribution, morphology and summit types for the isolated volcanic structures composing the seamount chains were determined. Eastern seamount chain has the 3-isolated volcanoes which are significantly steep and located at a considerable distance away from the ridge-axis. The central seamount chain is morphologically connected to the ridge-axis, and relatively small and less isolated compared to the other seamount chains. Western seamount chain shows a massive volcanic eruption with significantly large volcanic structures. In usual, a seamount is formed on pre-existing seafloor, and the observed magnetic signal of the seamount is correspond to that of the underlying seafloor when the seamount formation occurs nearly simultaneously with the formation of the underlying seafloor. However, if the observed magnetic anomalies of the seamount have a large misfit or/and reversed geomagnetic polarities with respect to the modeled magnetic anomalies of the underlying seafloor, it implies that there is a sufficiently large temporal gap between the formations of the seamount and the underlying seafloor. Applying this assumption, we forced the relatively younger geomagnetic history to move into the seamount having such misfits, and finally reduced the misfits. As a result, our magnetic model for the seamount chains shows that the isolated volcanoes were mostly originated from off-axis volcanisms of 7~20 km, and have a time span of less than ~600 kyrs to build. In particular, it is assumed that the seamount formations were intensively active during four-periods of 0.3~0.8 Ma, 0.9~1.1 Ma, 1.6~2.1 Ma and 2.2~2.7 Ma.</p>