Onset Timing of Median Tectonic Line (MTL): Constraints from U–Pb Ages of Detrital Zircons from 3 Distinct Cretaceous Sandstone Units Adjacent to the Low-angle MTL in the Mikawa-Ono/Idaira Area, Central Japan

Abstract

In order to constrain the onset timing of the Median Tectonic Line in SW Japan, detrital zircon geochronology is examined for pre-Miocene undated sandstones at both sides of the subhorizontal MTL in the Mikawa-Ono area, central Japan, where the 3-D configuration of the primary MTL between the Ryoke and Sanbagawa belts is observed. In addition, another Cretaceous sandstone unit (the Idaira Formation) in the Chichibu belt, ca. 10 km to the south, is analyzed for comparison. The following new results were obtained. The undated sandstonedominant unit, previously called the Kawachi Formation or the Nanasato-Isshiki Formation as a whole is, in fact, composed of two distinct units with contrasting characteristics of distribution, rock type, and zircon age spectrum, although both are revealed to be of the Cretaceous. The one to the north of the subhorizontal MTL (in the Ryoke belt) is redefined as the Nanasato-Isshiki Formation sensu stricto, whereas the one to the south (in the Sanbagawa belt) is newly designated as the Rokutazawa Formation. The Lower Cretaceous Idaira Formation forms the third group with a distinct signature in the zircon age spectrum. The Nanasato-Isshiki Formation (together with the associated Atera-Nanataki conglomerate) and the Idaira Formation, overlying the Ryoke granitoids and Chichibu pre-Cretaceous accretionary complex, respectively, form autochthonous units. In contrast, the Rokutazawa Formation alone represents an allochthonous units sitting structurally on the Sanbagawa schists. Age spectra of detrital zircons suggest that both the Nanasato-Isshiki Formation and Atera-Nanataki conglomerate are roughly dated as the Late Cretaceous, and also that they are correlated with the Upper Cretaceous Izumi Group in the Shikoku and Kinki regions of SW Japan. On the other hand, the sandstone of the Rokutazawa

Syn‐Subduction Strike‐Slip Faults Shape an Accretionary Orogen and its Provenance Signatures: Insights From Sikhote‐Alin in NE Asia During the Late Jurassic to Early Cretaceous

“…Sample LK15-56 was collected from a sandstone in a Late Jurassic formation in the central part of the Samarka terrane Figure 4b). Also, previous low-angle thrusts were transformed to dextral strike-slip faults during the Paleogene, as examplified by the Median Tectonic Line in SW Japan (Hasegawa et al, 2019(Hasegawa et al, , 2020Isozaki et al, 2010).…”

“…But sinistral strike‐slip faulting was later reversed to dextral movement in the Cenozoic (Cheon et al., 2019; K. Liu, Zhang, Wilde, Liu, et al., 2017; K. Liu, Zhang, Xiao, et al., 2020; S. Zhang et al., 2018). Also, previous low‐angle thrusts were transformed to dextral strike‐slip faults during the Paleogene, as examplified by the Median Tectonic Line in SW Japan (Hasegawa et al., 2019, 2020; Isozaki et al., 2010).…”

“…The Tan‐Lu Fault and its northern branches in eastern China, the Central Sikhote‐Alin Fault in Sikhote‐Alin of the Russian Far East and the lateral slip fault systems in the Korean Peninsula (such as the Yangsan Fault system) are the major wrench faults of this system (Figure 2; Faure & Natal'in, 1992; Khanchuk, 2001; Lee, 1999; Natal'in, 1993; Şengör & Natal'in, 1996; Utkin, 2013). It is noted that although the Median Tectonic Line (MTL) in SW Japan was considered as an important component of this fault system (Khanchuk, 2001), this viewpoint is now challenged by new evidence, showing that the proto‐MTL formed as a low‐angle thrust and the modern MTL was initiated only in the Paleogene (Hasegawa et al., 2019, 2020; Isozaki, 1996; Isozaki et al., 2010). The Sikhote‐Alin orogenic belt in the Russian Far East mainly consists of Late Jurassic to Early Cretaceous accretionary prisms, fore‐arc basins and fragments of island arcs (Figures 2 and 3).…”