Origin and tectonic evolution of the Caroline Ridge and the Sorol Trough, western tropical Pacific, from admittance and a tectonic modeling analysis

Abstract An independent Caroline Sea plate, surrounded by prominent seismotectonic features, is said to exist in the western tropical Pacific; the Caroline Sea plate is a transient feature, existing only when the Caroline–Philippine Sea plate boundary is jumping to the Caroline–Pacific boundary. The origin and the tectonic evolution of the Caroline Ridge–Sorol Trough region, which forms the northern part of the Caroline–Pacific plate boundary, are examined using a standard admittance analysis, a tectonic modeling study, and additional direct gravity and magnetic analyses. The observed admittance function is fitted well by shallow Airy model in the ∼250> λ >125 km (∼0.025 k −1 ) wavelength region, whose average crustal thickness t c is ∼20 km, and also by thin plate flexure model of isostasy within the diagnostic waveband of ∼250> λ >80 km (∼0.025 k −1 ), with an effective elastic thickness T e of ∼5–7 km. A simple interpretation of the observed admittance function is that the local Airy model of isostasy is the main support mechanism in the study region for the longer wavelength, more fundamental topography corresponding to that of the Caroline Ridge. The Caroline Ridge is inferred to have formed in a near-ridge tectonic setting, on top of oceanic lithosphere which was young, thin, and weak at the time of emplacement. The tectonic model study, after an explanation for rifting along the Mendana Fracture Zone in the Nazca plate, suggests that the Sorol Trough formed in the 17–7 Ma interval, splitting the Caroline Ridge into two longitudinal halves. The rifting along the Sorol Trough, which took place in the manner described in Weissel and Karner (1989. Flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension. J. Geophys. Res. 94, 13,919-13,950) (necking of unbroken lithosphere), post-dated the formation of the Caroline Ridge by ∼10–15 m.y., during which time T e in the region increased to ∼5–10 km. The rifting (necking) along the Sorol Trough represents a second loading event in the region; the combined gravity effects of the rifting as well as the increase in the T e in the region, from essentially nil to ∼5–10 km, is reflected in the agreement between the observed admittance and the regional model with a ∼5–7 km T e in the wavelength region λ >80 km. These results are consistent with the findings and the suggestions of the previous investigators in the region in that (1) the Caroline Ridge was constructed in the Late Oligocene by hot spot volcanism on top of Oligocene seafloor of the Caroline Sea back-arc basin, (2) the Eauripik Rise, genetically related to the Caroline Ridge, formed on young and weak Caroline Basin lithosphere, and is supported by Airy-manner crustal thickening, and that (3) the lithosphere of the Caroline Ridge was young at the time of rifting (necking) along the Sorol Trough. On the basis of the current results and the findings of the previous workers, a tentative, hypothetical rheological model for the crustal density structure of the Caroline Ridge–Sorol Trough is also given. The Caroline Sea plate appears to be bounded in the north and south by E–W-trending boundaries that deform diffusely, while more normal, narrow, N–S-trending boundaries exhibiting more localized, albeit slow, deformation bound the plate in the east and west.