Palaeomagnetism of Middle Triassic red bedded cherts from southwest Japan: equatorial palaeolatitude of primary magnetization and widespread secondary magnetization

Abstract

SUMMARY Middle Triassic (Anisian) red bedded cherts were collected from the Ajiro area in Kyushu Island, southwest Japan, for a palaeomagnetic study. Thermal demagnetization showed five distinct remanent magnetization components from the cherts. The lowest temperature component below 200 °C (component A) is a present‐day viscous overprint. The second‐ (up to 420 °C, component B) and third‐demagnetized (up to 580 °C, component C) components are secondary magnetizations of thermoviscous and chemical origin, respectively, both of which reside in magnetite. The fourth‐removed component (component D) was demagnetized at 630 °C and is a secondary magnetization carried by haematite. Components B and C are considered to be a result of widespread remagnetization in southwest Japan during the Cretaceous; the basis of this consideration is that red chert remagnetizations from an area about 600 km away from the present study area, are of the same origins with unblocking spectra similar to those observed in this study. The last‐removed component during thermal demagnetization (up to 695 °C, component E) is a primary magnetization carried by haematite. The directions of this component, after correction for the inclination flattening, yielded a mean palaeolatitude of 2°S at the time of deposition in the Middle Triassic. The Ajiro cherts accreted as part of an accretionary complex to the eastern margin of the South China Block during the Middle Jurassic and were then translated northwards from the continental margin to their present position through a margin‐parallel translation process owing to the oblique convergence of the oceanic plate.

Osmium isotope evidence for a large Late Triassic impact event

“…The Sakahogi cherts accumulated at higher latitudes (16.9°±10.2°N or S) than the Enoura cherts (2.1°±5.2°N or S) during the Middle Triassic, as deduced from paleomagnetic data2021, and this indicates that the depositional sites of the two cherts were separated by more than a 1,000 km. At this time, an oceanic plate on which the Enoura Triassic cherts were deposited moved towards the continental margin of East Asia and was accreted during the Middle Jurassic at a lower latitude than the Sakahogi cherts20. We analysed nine chert and four claystone samples from the Enoura section (Supplementary Table S1).…”

“…Paleomagnetic analysis of these Triassic bedded cherts, located 600 km from the Sakahogi section, suggests that the depositional setting of these cherts was along the paleo-equator in the Middle Triassic20. The Sakahogi cherts accumulated at higher latitudes (16.9°±10.2°N or S) than the Enoura cherts (2.1°±5.2°N or S) during the Middle Triassic, as deduced from paleomagnetic data2021, and this indicates that the depositional sites of the two cherts were separated by more than a 1,000 km. At this time, an oceanic plate on which the Enoura Triassic cherts were deposited moved towards the continental margin of East Asia and was accreted during the Middle Jurassic at a lower latitude than the Sakahogi cherts20.…”

“…600 km from the Sakahogi section (Supplementary Figs S1–S4). These bedded cherts are considered to be deep-sea sediments that accumulated in a pelagic, open ocean setting within the paleo-Pacific Ocean (Panthalassa)181920. We reconstruct a high-resolution paleo-seawater Os isotope profile for the middle Norian.…”

Cyclostratigraphic examination of Middle Triassic (Anisian) bedded chert in the Chichibu Belt from Tsukumi area, eastern Kyushu, Japan

“…The dominant cycles in a bed number series of thickness variations in the Middle Triassic chert beds are approximately -, , , , , and beds cycles, which partly differ from those of the Inuyama area. Ikeda et al, 2010;Ikeda andTada, 2014 , Fischer, 1976;Hori et al, 1993;, 1994-7000 ATS Ikeda et al, 2010Ikeda, 2013;Ikeda andTada, 2013, 2014;Ikeda and Hori, 2014 Ogg, 2014 -1 2 Hori et al, 1993;Ikeda et al, 2010;Ikeda and Tada, 2014 -, , -Nishi, 1994Ogawa et al, 1996 3-10 km - 1994Takahashi et al, 1998Onoue et al, 2011;Uno et al, 2012 Fig. 1 A: Lithostratigraphy and radiolarian zones (RZ) identified in the study section (Sugiyama, 1997).…”

Astronomical cycles recorded in the sedimentary rhythms of deep-sea bedded chert and its significance on biogeochemical cycle

“…The cycle can be subdivided into two subcycles involving organic matter (left-hand side) and silicate weathering and carbonate deposition (right-hand side). 2 Hori et al, 1993;De Wever et al, 2015Winguth and Winguth, 2012De Wever et al, 2014Zhang et al, 2015;Ikeda et al, 2016Winckler et al, 2008Woodard et al, 2011Ikeda et al, 2010 1 mm 1994 SiO 2 /Al 2 O 3 K 2 O/Al 2 O 3 Ti 2 O/Al 2 O 3 10 Sugitani and Mimura, 1996;, 10 Grocke et al, 2010Sakuma et al, 2012;Sato et al, 2015;Ikeda et al, 2018, Abe-Ouchi et al, 2013, Winguth and Winguth, 2012Ando et al, 2001Uno et al, 2012Uno et al, , 2015; , --Newark Kent et al, 2017Guandao Li et al, 2016Takemura, 2007Hori et al, 2011 Re-Os 2 1000 , Fischer and Arthur, 1977…”