Crustal Accretion and Mantle Geodynamics at Microplates: Constraints from Gravity Analysis and Numerical Modeling

• Main Author: Ames, Katherine
• Format: Text
• Language: unknown
• Published: ODU Digital Commons 2014
• Subjects: Microplates; Continental crust; Mantle plumes; Geodynamics; South Pacific Ocean; Geology; Tectonics and Structure; Antarc*; Antarctic
• Online Access: https://digitalcommons.odu.edu/oeas_etds/343; https://doi.org/10.25777/cydb-w518; https://digitalcommons.odu.edu/context/oeas_etds/article/1328/viewcontent/Ames_CrustalAccretion_2014_Redacted.pdf

This study investigates crustal accretion and mantle geodynamics at microplates using mantle Bouguer anomaly (MBA) gravity calculations and exploratory numerical models. The Easter and Juan Fernandez microplates are located in the eastern Pacific Ocean along the Pacific, Nazca, and Antarctic plate boundaries. Both microplates formed 3-5 Ma and they are currently rotating clockwise at 15° myr-1 and 9° myr-1,respectively [ e.g., Searle et al., 1993]. The study area also encompasses the Easter /Salas y Gomez mantle plume located near the Easter microplate. Both microplates show a difference in average MBA between their west and east ridges, with the west ridges having more negative MBA values than the eastern ones. This is consistent with the east ridges rifting into cooler, older lithosphere. The thermal effect of the Easter/Salas y Gomez plume calculated by comparing MBA along the Easter microplate east and west rifts was minimal (~10°C); however, the cooler lithosphere on the east ridge may mask the hotspot gravity signal. Several finite element numerical models were run to investigate the interaction between a microplate and a mantle plume. In the models, an off-axis plume was assigned one of three locations around the microplate as well as one of two values of excess temperature. The models predict thicker lithosphere on the east ridge of the microplate than on the west, consistent with the MBA data. In all of the models the plume is advected in the equivalent of a southward direction, along the clockwise rotating microplate. This southward plume flow is inconsistent with isotopic data indicating that Easter/Salas y Gomez has northward preferential flow [e.g., Haase, 2002]. Based on studies of large-scale mantle flow in the eastern Pacific [e.g., Buck et al., 2009; Zhang et al., 2013], a northward component of differential lithosphere/asthenosphere velocity was added to the models. However, this addition was insufficient to cause the modeled plume to flow north along the east ridge of the micro plate. ...