Abstract
AbstractA geophysical survey of the Ellice Basin, located in the south equatorial Pacific between the Ontong Java and Manihiki plateaus, revealed evidence for an extinct seafloor spreading system between the Pacific Plate and the Manihiki Plate. The spreading occurred during the Cretaceous Normal Superchron, the longest normal period of magnetic polarity from 121 to 83 Ma, and therefore lacks magnetic‐isochron‐derived age and plate motion constraints. Utilizing high‐resolution bathymetric data acquired on survey KM1609 during December 2016–January 2017, morphological and directional analyses were performed on the seafloor spreading fabric. Plate motion between the Pacific Plate and the Manihiki Plate during the Cretaceous Normal Superchron is described by three main stages of spreading. Stage 1 spreading was generally E‐W until a clockwise rotation of the spreading direction rotated transform faults by ~15° and lengthened spreading ridges to form Stage 2 zed pattern rhomboids. An offset between Stage 2 fracture zones evidences the presence of a complex and short‐lived Stage 3. Reconstructions, with respect to the Pacific Plate, are created for each of the three stages. Our reconstructions show that there was an earlier opening history prior to Stage 1 that ultimately reconstructs the Ontong Java and Manihiki plateaus; its quantification will await high‐resolution multibeam bathymetry data closer to the two plateaus.
Highlights
In order to provide additional age constraints on the Ellice Basin, we identified and dredged 16 targets during survey KM1609; suitable basalt samples will be 40Ar/39Ar age dated and reported in upcoming work by our colleagues at Oregon State University
A key difference observed in the Ellice Basin compared to other Pacific fracture zones is the elimination of fault strands accompanying a transtensional change in spreading direction resulting in longer and fewer spreading segments
The exact starting pseudo-isochrons for Stage 3 are difficult to characterize due to the proximity to the termination of spreading in Ellice Basin, though the total opening between them is constrained by the Stage 2 fracture zone offsets
Summary
The most concentrated time period of this increased hotspot activity and LIP emplacement was the Early Cretaceous (140-120 Ma; Müller et al, 2016). Following this period was the beginning of the long normal magnetic polarity period known as the Cretaceous Normal Superchron (CNS; 121-83 Ma, Granot et al, 2012). Rift structures on the northeast and southeast of the Manihiki Plateau suggest that additional LIP fragments were originally part of the super-plateau These were rafted onto the Farallon plate and the Phoenix plate, respectively, and have since been subducted beneath the South America and Antarctica (Viso et al, 2005).
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