Accretion and tectonic erosion processes revealed by the mode of occurrence and geochemistry of greenstones in the Cretaceous accretionary complexes of the Idonnappu Zone, southern central Hokkaido, Japan

Abstract

The Cretaceous accretionary complexes of the Idonnappu Zone in the Urakawa area are divided into five lithological units, four of which contain greenstone bodies. The Lower Cretaceous Naizawa Complex consists of two lithologic units. The Basaltic Unit (B‐Unit) is a large‐scale tectonic slab of greenstone, consisting of depleted tholeiite similar to that of the Lower Sorachi Ophiolite (basal forearc basin ophiolite) in the Sorachi‐Yezo Belt. The Mixed Unit of Naizawa Complex (MN‐Unit) contains oceanic island‐type alkaline greenstones which occur as slab‐like bodies and faulted blocks with tectonically dismembered trench‐fill sediments. Repeated alternations of the two units in the Naizawa Complex may have been formed by the collision of seamounts with forearc ophiolitic body (Lower Sorachi Ophiolite) in the trench. The Upper Cretaceous Horobetsugawa Complex structurally underlies the Naizawa Complex in its original configuration, and it also contains greenstone bodies. Greenstones in the MH‐Unit occur as blocks and sedimentary clasts in a clastic matrix, and exhibit depleted tholeiite and oceanic‐island alkaline basalt/tholeiite chemistry. This unit is interpreted as submarine slide and debris flow deposits. Greenstones in the PT‐Unit occur at the base of several chert‐clastic successions. Most of the greenstones are severely sheared and show normal‐type mid‐ocean ridge basalt composition. The PT‐Unit greenstones are considered to have been derived from abyssal basement peeled off during accretion. The different accretion mechanism of the greenstones in the Naizawa and Horobetsugawa complexes reflects temporal changes in subduction zone conditions. Seamount accretion and tectonic erosion were dominant in the Early Cretaceous, due to highly oblique subduction of the old oceanic crust and minimal sediment supply. Whereas, thick sediments with minor mid‐ocean ridge basalt and olistostrome accreted in the Late Cretaceous, due to near‐orthogonal subduction of young oceanic crust with voluminous sediment supply.

Geochemical modeling of sedimentary rocks in the central Hokkaido, Japan: Episodic deformation and subsequent confined basin-formation along the eastern Eurasian margin since the Cretaceous

“…However, sedimentological studies (Iijima, 1959(Iijima, , 1996 imply a confined N-S trending elongate Paleogene fluvial basin in the central Hokkaido on the basis of heavy mineral associations of sediments, which suggests uplift and erosion of the Kamuikotan ultramafic rocks to the east (trench-side). Such deformation of the 'forearc' does not accord with an active basal erosion regime, but may be related with development of trench slope break as a result of voluminous accretion of sediments during the Paleogene (Ueda et al, 2000).…”

“…It is noted that the vertical tilting just after sedimentation was not recognized through geologic studies, but has been clearly indicated on the basis of this geochemical modeling study. Synchronous increase in convergence rate of the IzanagiKula Plate (Ueda et al, 2000) based on relative plate motions after Engebretson et al (1985) implies regional tectonic event. However the Cretaceous deformation of the presumed Eurasian margin seems to occur in a confined part of the forearc basins, because the contemporaneous unit in the MITI Yubari borehole does not indicate such a drastic event.…”

Episodicity of Mesozoic terrane accretion along the Pacific margin of Gondwana: implications for superplume-plate interactions

“…2 (Kimura 1997;Kimura et al 1994). Some evidence for a continuation of more rapid accretion is suggested by more intense metamorphism of ophiolitic units accreted between Aptian and Cenomanian times (Kiyokawa 1992;Ueda et al 2000).…”

Mechanism of long-standing Cenozoic basin formation in central Hokkaido: an integrated basin study on an oblique convergent margin

“…The Hakobuchi Formation shows upward-coarsening facies successions formed by progradation of the inner shelf to delta plain systems (Ando 2003), and these were categorized to be outer to inner shelf, shoreface, subordinate estuary, incised valley, river-channel, back-marsh, and floodplain after Takashima et al (2004). Next, the forearc basin was probably uplifted in the early Paleocene as a result of voluminous accretion beneath the subduction zone (e.g., Ueda et al 2000). Burial history of the Hakobuchi Formation (Figure 4a) optimized on the basis of maturation profiles of Ro and T max (Figure 4b) within the upper part of the Yezo Group indicates an extremely high sedimentation rate (1.57 m/1,000 years), which seems to be unrealistic considering the abovementioned sedimentary facies.…”