New geophysical constraints on a failed subduction initiation: The structure and potential evolution of the Gagua Ridge and Huatung Basin

Abstract

We modeled the velocity structure of the Huatung Basin and Gagua Ridge using offshore wideangle seismic data along four E-W transects. These transects are accompanied by several multichannel seismic reflection (MCS) profiles that highlight the shallow deformation in this area east of Taiwan. Although it is agreed that the Gagua Ridge was the product of a transient compressional episode in the past, relatively few data have been collected that reveal the deeper structure resulting from this enigmatic process. The velocity models show evidence for normal, to thin, oceanic crustal thicknesses in the Huatung Basin and West Philippine Basin. Moho reflections from the associated MCS profiles confirm the thickness observed in the velocity models. The velocity models indicate significant crustal thickening associated with the Gagua Ridge, to 12-18 km along its entire length. Most importantly, the two central velocity models also show a significant asymmetry in the crustal thickening, suggesting a westward underthrusting of >20 km of WPB oceanic crust beneath that of the Huatung Basin. This geometry is extremely unexpected given interpretations that indicate the Huatung Basin could be significantly older than the West Philippine Basin (Early Cretaceous versus Eocene). Our observations, along with recent geophysical data concerning the age of the Huatung Basin, indicate that the Gagua Ridge was the result of a failed subduction event during the Miocene that may have existed simultaneously and for a short time, competed with the Manila subduction zone to the west in accommodating convergence between the Eurasia and Philippine Sea plates. In this scenario, the present-day Gagua Ridge represents a snapshot of a failed subduction initiation preserved in the geologic record.

Philippine Sea Plate inception, evolution, and consumption with special emphasis on the early stages of Izu-Bonin-Mariana subduction

“…3). Transpression along the Gagua Ridge has been also proposed by Eakin et al (2015) but during the lower Miocene rather than the Oligocene.…”

“…The Gagua Ridge thus represents failed subduction along a former fracture zone or transform boundary between two oceanic basins. That episode of short-lived subduction occurred in the Oligocene according to Deschamps and Lallemand (2002), whereas Eakin et al (2015) favor a younger Miocene age.…”

“…Estimates vary from 131 to 119 Ma (Deschamps et al 2000) to 52-43 Ma for the northern part and Early Cretaceous for the southern part (Sibuet et al 2002), to 44-33 Ma (Hilde and Lee, 1984), 42-33 Ma (Doo et al 2015), or even ∼ 30-15 Ma (Kuo et al 2009). Further details and an interesting discussion about this controversy are available in Eakin et al (2015). East of the WPB lies a north-south elongated (∼2500 km long by ∼200-600 km wide) pair of oceanic basins called Shikoku and Parece Vela.…”

“…On the west side of the ridge, magnetic lineations of the WPB are highly oblique, so the Gagua Ridge is a non-transform discontinuity (NTD). Velocity and gravity models across the ridge suggest an episode of westward underthrusting of the WPB oceanic crust beneath the Huatung Basin (Deschamps et al 1998;Eakin et al 2015). The Gagua Ridge thus represents failed subduction along a former fracture zone or transform boundary between two oceanic basins.…”

Geochemistry and Geochronology of the Accreted Mafic Rocks From the Hengchun Peninsula, Southern Taiwan: Origin and Tectonic Implications

“…To the west of the Kaoping Slope, the northernmost Manila Trench is poorly defined and turns to the leading edge of the deformation of the collision prism (C. Y. Huang et al, ). The Huatung‐Philippine Sea Plate, which is located to the east of the Hengchun Peninsula, is further divided into the Huatung Basin (early Cretaceous, 131–119 Ma) and the West Philippine Basin (early Eocene to early Oligocene, 55–33/30 Ma; Deschamps & Lallemand, ; Deschamps et al, ) by the Gagua Ridge (Figure b), which is a strike‐slip feature that has accommodated the opening of the West Philippine Basin and been uplifted due to the compressive episode of a failed subduction event occurred along the western margin of the West Philippine Basin during the late Eocene‐early Oligocene (Deschamps et al, ; Deschamps & Lallemand, ; Eakin et al, ).…”