Seamounts, knolls and petit‐spot monogenetic volcanoes on the subducting Pacific Plate

Abstract

On the western part of the Paci¢c Plate most seamounts formed during the Cretaceous period in the so -called West Paci¢c Seamount Province (WPSP). On the northwestern part of the same plate, the Joban and Japanese Seamount Trail (JJST) are also composed of Early Cretaceous seamounts. However, two new groups of knolls were recently discovered during multibeam surveys on the Paci¢c Plate along the Japan Trench. One group consists of circular knolls that are £at-topped in shape and correspond to eruptive ages of approximately 75 Ma.The other group consists of irregularly shaped knolls, also called petit-spot volcanoes, that are found on the outer-rise systems of the subducting Paci¢c Plate.These petit-spots seem much younger and available age data suggest that they only formed in the last few million years. Acoustic re£ective data, which are simultaneously obtained with bathymetrical data, are a most powerful tool to distinguish the petit-spots from the Cretaceous edi¢ces in the WPSP and JJST. In this paper, we present the results of an exploratory search for these new kind of petit-spot volcanoes along the trenches in the Paci¢c Ocean, with an emphasis on the Japan and Tonga trenches.The sizes of these irregularly shaped petit-spot volcanoes are several orders of magnitude less than the Cretaceous seamounts and circular knolls, yet they appear to be ubiquitous on the ocean £oor, in particular, where incipient melts in the asthenosphere can be squeezed out by tectonic forces.

Outer-rise normal fault development and influence on near-trench décollement propagation along the Japan Trench, off Tohoku

“…We find a range of sediment thickness of 0 to 600 m along the incoming Pacific Plate. Thinning in the southeast is largely found near potential 'petite spot' volcanoes ( Figure 1A; Site C of Hirano et al 2008) and is likely related to the formation of these small cones, or to erosion around them. Elsewhere, thickness is generally related to initial deposition or reworking of the sediment.…”

“…The preexisting fabric appears to limit certain fault orientations and promote others, as active fault strikes coincide with the orientations of fracture zones and abyssal hill fabric (Nakanishi 2011). The southeast region of the survey area is slightly more complex due to potential petite spot volcanism (Site C of Hirano et al 2008) making it difficult to determine the strike of basement-cutting faults because of more relief in the topography and regional sediment thinning (Figure 1). Fault interactions on the incoming Pacific Plate produce a heavily deformed and spatially complex fault pattern just seaward of the trench.…”

Petit-spot lava fields off the central Chile trench induced by plate flexure

“…Two dredges were also undertaken during the cruise in order to sample rocks from the tiny knolls (dredge D08) and the fault escarpment that truncates the volcanic edifices (dredge D10). At the knoll and escarpment, the acoustic reflectivity is more than 10 times higher than the reflective values of the surrounding abyssal plain, indicating that the lava flows are exposed at the seafloor (Hirano et al, 2008) (Fig. 3).…”

Seamounts

“…The term "guyot" can be used for seamounts having a truncated cone shape with a flat summit produced by erosion at sea level (Hess, 1946), development of carbonate reefs (e.g., Flood, 1999), or partial collapse due to caldera formation (e.g., Batiza et al, 1984). Seamounts <1,000 m tall are sometimes referred to as "knolls" (e.g., Hirano et al, 2008). "Petit spots" are a newly discovered subset of sea knolls confined to the bulge of subducting oceanic plates of oceanic plates seaward of deep-sea trenches (Hirano et al, 2006).…”