Ageing and Evolution of Oceanic Crust Formed by Magma-Rich and Magma-Poor Spreading
Abstract
Oceanic crust evolves over time as a result of processes that alter its original fabric and permeability. As the fabric of the crust is controlled by its dominant mode of accretion, this has a significant influence on ageing. Studies at mid-ocean ridges have shown how highly variable the mode of crustal formation can be, from magmatic to tectonic extension-dominated, and how fluid circulation through the crust enables cooling, serpentinization, and porosity reduction.
Here we present results from the OSCAR study of crustal evolution at the intermediate spreading Costa Rica Rift (CRR), which comprises two 3D wide-angle (WA) surveys at a spreading ridge (0 Ma) and ~230 km off-axis (~6 Ma), linked by a 2D WA refraction and synthetic aperture (SA) profile, and ground-truthed against geophysical and lithology logs from borehole 504B. The integration of the models generated from these datasets shows that the crustal evolution at the CRR is episodic with regions of rougher, uplifted basement and thin sediment cover correlating with a slower layer 2 velocity, up to 0.5 km s-1 faster than at the ridge axis. Conversely, regions with a smoother basement and continuous sediment cover are characterised by a velocity up to 1.0-1.4 km s-1 faster than at the CRR. Zones of lower velocity also correlate with residual mantle Bouguer anomaly lows, reflecting regions of lower density in the lower crust or upper mantle. We show that phases of magmatic accretion occur during periods of faster asymmetric spreading at the CRR, while increased tectonic extension is observed during periods of slower spreading, and results in increased crustal permeability and fluid flow, which can be observed in S-wave velocity and Poisson's ratio models. Although it remains unclear what drives the observed changes in spreading at the CRR, crust forms through magmatic and tectonic modes of spreading. These mechanisms are not mutually exclusive and vary in dominance on either ridge flank at any time. Therefore, the term 'crust formed at an intermediate-spreading ridge' is not an accurate description of the underlying formation processes. Instead, it represents a temporally and spatially integrated view of crustal structure where variations in formation occur on the order of a few Myr.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T41B..05F
- Keywords:
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- 7218 Lithosphere;
- SEISMOLOGY;
- 7220 Oceanic crust;
- SEISMOLOGY;
- 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICS;
- 8416 Mid-oceanic ridge processes;
- VOLCANOLOGY