Late Cretaceous Ridge Reorganization, Microplate Formation, and the Evolution of the Rio Grande Rise – Walvis Ridge Hot Spot Twins, South Atlantic Ocean. (16th March 2021)

• Record Type: Journal Article
• Authors: Sager, William W.; Thoram, Sriharsha; Engfer, Daniel W.; Koppers, Anthony A. P.; Class, Cornelia
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 3(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-03-16
• Subjects: abyssal hills -- gravity -- plate motion -- Rio Grande Rise -- South Atlantic -- Walvis Ridge; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2020GC009390
• Languages: English
• ISSNs: 1525-2027

Abstract

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. This reorganization caused the formation of a microplate between RGR and WR that was eventually incorporated into the South American plate by eastward ridge jumps. These findings imply that age progressions within these igneous provinces may not be linear (as expected for a simple hot spot) and were instead influenced by the ridge reorganization and microplate formation. Key Points: Late Cretaceous tectonic fault fabric and magnetic anomalies near Rio Grande Rise (RGR) and Walvis Ridge (WR) indicate complex tectonics Tectonic data imply that a microplate formed between RGR and WR The complex tectonics occurred as a reorganization of a long‐offset Mid‐Atlantic Ridge transform fault