Abstract
AbstractWe present a high‐resolution 2‐D P‐wave velocity model from a 225‐km‐long active seismic profile, collected over ~60–75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three nontransform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two‐layer velocity structure with a high gradient Layer 2 (~0.9 s−1) above a lower gradient Layer 3 (0.2 s−1). Here, PmP coincides with the 7.5 km s−1 contour, and velocity increases to >7.8 km s−1 within 1 km below. We interpret these segments as magmatically robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between the upper and lower crust and PmP shallower than the 7.5 km s−1 contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. While velocity‐depth profiles fit within previous envelopes for slow‐spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments, we make the first wide‐angle seismic identifications of buried oceanic core complexes in mature (>20 Ma) Atlantic Ocean crust. They have a ~20‐km‐wide “domal” morphology with shallow basement and increased upper crustal velocities. We interpret their midcrustal seismic velocity inversions as alteration and rock‐type assemblage contrasts across crustal‐scale detachment faults.
Highlights
Oceanic crust is the site of major heat and mass flux exchange between the solid Earth and the oceans and atmosphere
While there is a small mismatch at the Marathon fracture zone (FZ), which may be due to uncertainties in the potential field data, overall, the resolved PmP reflection shallows toward each of these discontinuities over tens of kilometers
We have found two distinct seismic structures that are consistent with the two modes of oceanic crustal accretion observed at the modern Mid‐ Atlantic Ridge (MAR)
Summary
Oceanic crust is the site of major heat and mass flux exchange between the solid Earth and the oceans and atmosphere. Active‐source seismic experiments showed a distinct two‐layer structure, which was inferred to hold true for most oceanic settings (e.g., White et al, 1992). The upper igneous crust (Layer 2) is typically 1–2 km thick and is identified by a high P‐wave velocity gradient (~1.0 s−1), while the lower crust (Layer 3) is 4–6 km thick and has a higher‐velocity (>6.4 km s−1) but significantly lower‐velocity gradient (~0.1 s−1). The seismic structure was interpreted as an upper crust of extrusive basaltic flows and dolerite dikes, overlying a gabbroic lower crust (commonly referred to as the “Penrose model,” Anonymous, 1972). An issue that remains much debated is the interpretation of the PmP reflection and whether this represents the base of the petrological crust or a serpentinization front due to infiltration of water and alteration of olivine (e.g., Minshull et al, 1998). While some modifications were made, the broad concept of a unified 1‐D velocity structure of oceanic crust was upheld
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