The spreading rate dependence of three‐dimensional mid‐ocean ridge gravity structure

Abstract

We analyze over 1300 km of high resolution along‐axis gravity profiles at ridges with half‐spreading rates ranging from 1.2 to 5.5 cm/yr. The results show consistently higher along‐axis gradients of mantle Bouguer anomaly at the slow‐spreading Mid‐Atlantic Ridge (MAR) (0.3–1.2 mgal/km) than at the intermediate‐ to fast‐spreading Cocos‐Nazca Ridge and East Pacific Rise (EPR) (0.1–0.2 mgal/km). The regional peak‐to‐trough amplitude of mantle Bouguer anomaly is also greater along the MAR (30–60 mgal) than the Cocos‐Nazca Ridge and the EPR (10–20 mgal). With increasing spreading rate, the regional peak‐to‐trough amplitude of axial seafloor depth decreases from 1000–1700 m to 200–700 m. 3‐D numerical experiments suggest that mantle contributions to the gravity can be significant only near large‐offset transforms. At the more commonly observed non‐transform offsets, gravity anomalies will reflect crustal thickness variations. The along‐axis gravity data thus indicate that the amplitude of along‐axis crustal thickness variation decreases with increasing spreading rate. We propose that this spreading rate dependent crustal accretion style may originate in the mantle: finite‐amplitude mantle upwelling is intrinsically plume‐like (3‐D) beneath a slow‐spreading ridge but more sheet‐like (2‐D) beneath a fast‐spreading ridge. Such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate. Small amplitude 3‐D upwellings may occur at a fast‐spreading ridge, but their effects on crustal thickness variations will be significantly reduced by along‐axis melt flows along a persistent low‐viscosity crustal magma chamber. In contrast, the large crustal thickness variations due to 3‐D mantle upwellings will be maintained at a slow‐spreading ridge because less along‐axis melt flows can occur in the colder and more rigid crust there.