Spreading rate dependence of the gravity structure of oceanic Spreading Rate Dependence of the Gravity Structure of Oceanic Transform Faults: Contrast Between Ultra-slow/Slow and Intermediate/Fast Slipping Systems

Decades of research and dozens of gravity studies at ultra-slow and slow-spreading mid-ocean ridges (MOR) have greatly shaped our understanding of crustal accretion processes and the role of transform fault offsets on the evolution of MOR spreading segments. Along the slow-spreading Mid-Atlantic Ridge (MAR) and ultra-slow spreading Southwest Indian Ridge (SWIR), calculated mantle Bouguer gravity anomalies appear as negative "bulls-eyes" over the center of ridge segments suggesting enhanced focused mantle upwelling at segment centers. Such focused upwelling is expected to be less important as spreading rate increases. Therefore, the crustal thickness of fast-spreading ridges was predicted to be relatively constant along axis. However, we recently documented crustal thickening at the end of intermediate and fast-spreading ridge segments and locally enhanced magmatic crustal accretion within intermediate and fast-slipping transform faults. Here, we calculate and compare residual mantle Bouguer anomalies (RMBA) for 19 transforms faults along the fast- spreading East Pacific Rise, the intermediate spreading Juan de Fuca and Southeast Indian Ridges, the slow- spreading MAR, and the ultra-slow spreading SWIR, with transform slip rates ranging from 1.3 to 15 cm/yr. Our findings indicate that global transform fault systems belong to one of two structural sub-groups: Ultra- slow/slow and intermediate/fast, separated by a slip rate of ~4 cm/yr. Our preferred interpretation for the origin of the RMBA patterns observed at ridge-transform systems of the intermediate/fast group is crustal thickening associated with hooked ridges and the overshooting of dikes beyond ridge segment ends. Furthermore, our findings support local magmatic upwelling and intrusion into the relatively thin and fractured lithosphere of the intermediate/fast-slipping transform domains at intra-transform spreading centers as a result of tectonic extension during episodes of plate motion reorganization.