Fine-scale shallow velocity structure of the Rainbow massif constrained by downward continued multichannel seismic data
Abstract
The Rainbow non-transform offset at the Mid-Atlantic Ridge (36° 14'N) exhibits a complex morphology that includes a core complex with active and fossil hydrothermal fields. Previous seismic imaging with multichannel seismic reflection data found the presence of small intrusion bodies beneath the Rainbow massif at ~2-10 km depth. A velocity model derived from wide-angle ocean bottom seismic data shows that the Rainbow massif is capped by low velocities representing near-surface alteration zones, due to the fluid circulation driven by the heat from the intrusive bodies. In this study, we analyze multichannel seismic data, acquired with an 8-km-long hydrophone streamer during cruise MGL1309, with a combination of downward continuation and traveltime tomography methods to investigate the fine-scale structure of the upper 1-2 km beneath the seafloor around the Rainbow area. A preliminary velocity model along a 35-km-long profile crossing the Rainbow Massif over two fossil hydrothermal fields hosted in serpentinized peridotite shows an asymmetry across the axial valley. On the western outside corner crust, the velocity model is relatively uniform with a structure suggesting a volcanic crust: the shallowest 500-800 m has a high-gradient layer 2A where the velocity quickly increases from 2.5 km/s to 4.5 km/s, with a low-gradient layer lying below. On the Rainbow massif velocities increase from 3-3.5 km/s at the seafloor to ~6 km/s at ~1.5 km depth bsf with a relatively uniform gradient, suggesting fracturing and alteration effects. To the east of the Rainbow massif, the shallow structure beneath the fossil Pot of Gold detachment fault surface is dominated by larger high-velocity anomalies that may correspond to exhumed gabbroic rocks. The ongoing study will analyze the rest of the multichannel seismic profiles to constrain in detail the shallow 3-D variations in structure, lithology, and extent of alteration.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T13I0285J
- Keywords:
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- 3035 Midocean ridge processes;
- MARINE GEOLOGY AND GEOPHYSICS;
- 8034 Rheology and friction of fault zones;
- STRUCTURAL GEOLOGY;
- 8135 Hydrothermal systems;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS