Sub-axial deformation in oceanic lower crust: Insights from seismic reflection profiles in the Enderby Basin and comparison with the Oman ophiolite

Abstract

We analyzed high-quality seismic reflection profiles across the ocean-continent transition in the Enderby Basin between the Kerguelen Plateau and the Antarctic margin. There, we observe numerous high-amplitude dipping reflections in the lower oceanic crust which was accreted at a magmatic spreading center as testified by the almost uniform 6.4-7 km thick crust and its unfaulted, flat top basement. The deep reflections are rooting onto the Moho and are dipping both ridgeward and continentward. They occur in dense networks in mature oceanic crust as well as close to the continentward termination of oceanic crust and in the oceancontinent transition zone. The comparison with field observations in the Oman ophiolite suggests that these lower crustal dipping reflectors could correspond to syn-magmatic faults. In Oman, very high temperature (up to syn-magmatic), high temperature (sub-solidus plastic deformation) and low temperature (brittle) deformation coexist along the same fault over distances of a few hundred meters at Moho level. This very high temperature gradient may be explained by the sudden and intense interaction between crystallizing magmas and hydrothermal fluids induced by the episodic nucleation of faults in a context of continuous magmatic spreading. The igneous layering becomes extremely irregular compared to its monotonous sub-horizontal orientation away from the faults which, together with enhanced hydrothermal alteration restricted to the fault zones, might change the physical properties (velocity, density) and increase the reflectivity of syn-magmatic faults. We further speculate that these processes could explain the brightness of the lower crustal dipping reflectors observed in our seismic reflection data. Both the seismic reflection profiles of the Enderby Basin and the Oman ophiolite show evidence for syn-accretion tectonism at depth together with the systematic rotation of originally horizontal lava flows or originally vertical dikes, predating cessation of magmatic activity. This indicates ubiquitous deformation processes within the axial zone of magmatic spreading centers.

The Limpopo Magma‐Rich Transform Margin, South Mozambique: 1. Insights From Deep‐Structure Seismic Imaging

“…A clear series of dipping reflectors are also observed in the oceanic crust along MZ4 (Figure 7c, intra‐crustal reflections, highlighted in purple in the top panel). Similar features were observed in seismic profiles on other oceanic crusts (e.g., Bécel et al., 2015; Momoh et al., 2017; Sauter et al., 2021). These can be related to magmatic layering or shear zones that are crossed with a slight angle compared to the oceanic crust fabric.…”

Geochemical Characterization of the Oman Crust‐Mantle Transition Zone, OmanDP Holes CM1A and CM2B

“…Similar trend changes attributed to the focus of the percolation/migration of melts or HT fluids along faults have been observed for the Maqsad CMTZ dunites, with the same characteristic thickness of about 50 m (Rospabé, Benoit et al., 2019, Rospabé, Ceuleneer et al., 2020). Our results seem to confirm the significant impact of deep‐seated syn‐magmatic faults on the development of the crust‐mantle transition at the expense of the shallower mantle and the recorded whole rock chemical signatures, in addition to their impact on the formation of the lower oceanic crust (Abily et al., 2021; see also, 2011; Sauter et al., 2021). Such structural characters must have developed early and are not just restricted to the Maqsad area (e.g., Rospabé (2018) and Abily et al.…”

“…Our results seem to confirm the significant impact of deep-seated syn-magmatic faults on the development of the crust-mantle transition at the expense of the shallower mantle and the recorded whole rock chemical signatures, in addition to their impact on the formation of the lower oceanic crust (Abily et al, 2021;see also, 2011;Sauter et al, 2021). Such structural characters must have developed early and are not just restricted to the Maqsad area (e.g., Rospabé (2018) and Abily et al (2021) for other Oman areas and Sauter et al (2021) for present-day oceans).…”

The Limpopo Magma‐Rich Transform Margin, South Mozambique – 2: Implications for the Gondwana Breakup

“…The presence of OSDRs, CDRs and ODRs together above a high amplitude deepest reflector interpreted as the oceanic Moho is typical of oceanic crust (Karson, 2002; Mutter & Carton, 2013; Sauter et al., 2021). This interpretation fits well with the presence of magnetic anomalies (Mueller & Jokat, 2019) in some places (Figure 2).…”

“…In detail, the OSDRs may correspond to the progressive asymmetric subsidence of lava flows (Table 1, Karson, 2002) that could be associated locally with faults (subset 2 in Figure 8). Below, the abundance of ODRs may correspond to tilted grabbro whereas CDRs may be interpreted as syn-magmatic faults (Table 1, Bécel et al, 2015;Momoh et al, 2017Momoh et al, , 2020Sauter et al, 2021) which root into the oceanic Moho. Interestingly, these reflectors (OSDRs, CDRs and ODRs) and U5 seismic facies are primarily found in the over-thickened oceanic crust (deeper Moho) attesting to robust magma supply at some places.…”