Late Cretaceous Ridge Reorganization, Microplate Formation, and the Evolution of the Rio Grande Rise – Walvis Ridge Hot Spot Twins, South Atlantic Ocean

Abstract

The Rio Grande Rise (RGR) and the Walvis Ridge (WR) make an odd pair. Both are large igneous provinces (LIPs) apparently erupted at the same hot spot during the opening of the South Atlantic Ocean (Morgan, 1971, 1981, 1983). The conjugate LIPs formed because the hot spot erupted at the Mid-Atlantic Ridge (MAR) during the Late Cretaceous, emplacing volcanic edifices on both the South American and African plates (Kumar, 1979; O'Connor & Duncan, 1990). Despite the similar circumstances, the two features have Abstract Rio Grande Rise (RGR) and Walvis Ridge (WR) are South Atlantic large igneous provinces (LIPs), formed on the South American and African plates, respectively, mainly by volcanism from a hot spot erupting at the Mid-Atlantic Ridge (MAR) during the Late Cretaceous. Both display morphologic complexities that imply their tectonic evolution is incompletely understood. We studied bathymetry, gravity, and vertical gravity gradient maps derived from satellite altimetry to trace faults providing indications of seafloor spreading directions and changes. We also examined magnetic anomalies for time constraint and reflection seismic data for structural information. Abyssal hill fabric and magnetic anomaly data indicate that the area between RGR and WR was anomalous between anomalies C34 (83.6 Ma) and C30 (66.4 Ma) owing to reorganization of a right-lateral transform on the MAR. This event began ∼92 Ma as the transform shifted south to form multiple, short-offset right-lateral transforms, with the reorganization extending through anomaly C34 and ending before anomaly C30. Anomalous spacing of magnetic anomalies and discordant fault fabric indicate that a microplate formed with a core of Cretaceous Quiet Zone seafloor. As the MAR jumped eastward, this microplate was captured by the South American plate and now resides mostly in a basin between the main RGR plateau and a related ridge to the east (East Rio Grande Rise). The microplate is ringed by igneous massifs, implying a link with volcanism. The results presented here indicate that these two LIPs had a complex Late Cretaceous history that belies simple hot spot models. Plain Language Summary Rio Grande Rise (RGR) and Walvis Ridge (WR) are large volcanic mountains formed on the South American and African plates, respectively, as the South Atlantic Ocean opened during Cretaceous and Cenozoic time. They are considered to be similar to the classic Hawaiian-Emperor hot spot seamount chain and to represent motion of the two plates over a single mantle hot spot, once located at the Mid-Atlantic Ridge (MAR). Complex morphologies of these two igneous provinces imply that this simple model is an oversimplification. We examine geophysical evidence for Late Cretaceous time, mainly between magnetic anomalies C34 (83.6 Ma) to C30 (66.4 Ma). During this period the MAR reorganized, causing complex tectonics between RGR and WR. A long-offset, transform fault broke into a number of lesser offset transforms with several ridge jumps, beginning at ∼92 Ma and ending before anomaly C30. ...

Expedition 391 Preliminary Report: Walvis Ridge Hotspot

“…The current explanation for these troughs is a failed Eocene rifting event (Camboa and Rabinowitz, 1984;Mohriak et al, 2010), which occurred in a midplate setting and has no clear tectonic cause. A similar rift has been observed at Valdivia Bank (Jimenez Garcia, 2017;Sager et al, 2021), but its origin is still unknown. Notably, similar rifts are observed at other LIPs such as the Manihiki Plateau (Danger Island and Suvorov Troughs) (Nakanishi et al, 2015).…”

“…In contrast to many Pacific seamount tracks, Atlantic seamount chains are diffuse and sometimes imply different rates of motion (e.g., O'Connor et al, 1999). Some chains, such as Ninetyeast Ridge and Walvis Ridge, have complex morphologies, containing ridge-like structures, plateaus, multiple tracks, and large offsets, which indicates that their shapes are partly a result of interactions with plate boundaries (e.g., Expedition 324 Scientists, 2010;Sager, 2021;Krishna et al, 2012;Jokat, 2015a, 2015b;Hoernle et al, 2016). In addition, there is evidence that large igneous provinces (LIPs) and mid-ocean ridges can interact over tens of millions of years (Krishna et al, 2012;Sager et al, 2016Sager et al, , 2019Sager et al, , 2021, suggesting that plume and mid-ocean ridge convection may be linked (Whittaker et al, 2015).…”

“…While the plume was beneath the Mid-Atlantic Ridge during the Late Cretaceous, eruptions occurred on both the South American and African plates, forming Rio Grande Rise and Walvis Ridge (O'Connor and Duncan, 1990;Rohde et al, 2013b). Although the TGW chain appears as a quasilinear ridge and seamount chain, LIP eruptions formed two ocean plateau-size edifices, Valdivia Bank (400 km × 550 km) and Rio Grande Rise Massif (500 km × 950 km), at ~80-90 Ma (Rohde et al, 2013b;Homrighausen et al, 2019;Sager et al, 2021). The hotspot and the Mid-Atlantic Ridge separated at ~60-70 Ma, and the hotspot erupted on the African plate thereafter.…”

“…Results from a recent study of Rio Grande Rise-Walvis Ridge tectonics, based on tectonic fabric in satellite gravity and magnetic data (Sager et al, 2015(Sager et al, , 2021Thoram et al, 2019), reveals tectonic anomalies associated with the formation of the two large igneous provinces. extra (older) material between the two.…”

“…Both the TGW chain (EMI-type) and the HIMU seamounts continuing into Africa display remarkably linear age progressions, offset by 30-40 Ma, that are interpreted as reflecting two distinct plumes derived from distinct portions of the large low-shear velocity province (LLSVP) at the base of the lower mantle (Homrighausen et al, 2018a(Homrighausen et al, , 2019(Homrighausen et al, , 2020. Moreover, the Valdivia Bank Plateau apparently formed on the boundary of a microplate, which may have affected its development along with that of its twin, Rio Grande Rise on the South American plate (Sager et al, 2021).…”