Folding of granite and Cretaceous exhumation associated with regional-scale flexural slip folding and ridge subduction, Kitakami zone, northeast Japan Soichi Osozawa a,⇑ , Chin-Ho Tsai b , John Wakabayashi c a Department of Earth Sciences, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan b Petrology and GeoMicroanalysis Lab, College of Environmental Studies, National Dong Hwa University—Meilun Campus, 123 Hua-his Rd., Hualien 970, Taiwan c Department of Earth and Environmental Sciences, California State University, Fresno, CA 93740, USA article info Article history: Available online 6 June 2012 Keywords: Kitakami granitic pluton Aplite marker Asymmetric fold Axial planar foliation Biotite Aureole Flexural slip anticline Tilted unconformity Extrusion Exhumation Adakitic magmatism Ridge subduction abstract The early Cretaceous granitic plutons intrude the Kitakami zone, northeast Japan, whose southern and northern regions consist of forearc basin and accretionary complex rocks, respectively. All country rock of the Kitakami zone exhibit prominent pressure-solution cleavage and associated folds formed during shortening with a small component of sinistral shear, whereas most plutons show only igneous textures. The Kesengawa granite and some other plutons, however, have foliations that cut the pluton boundaries and are continuous with those observed in surrounding sedimentary rocks. We document a tectonic fold with axial planar foliation in part of the Kesengawa granite. The metamorphic minerals associated with the contact aureole and country rocks of the Kesengawa and other deformed plutons indicate an increase in metamorphic grade toward the pluton showing that deformation of the pluton and country rock took place as the plutons cooled. The Kesengawa pluton and country rocks of the southern Kitakami zone are deformed into regional scale upright folds with parasitic asymmetric folds that verge toward regional anticlinal axes whereas regional scale folds in the northern Kitakami zone are overturned and verge to the east. The Kitakami basement is not bounded by normal or reverse faults, so the style of regional exhu- mation does not resemble the upright or inclined extrusion noted in other regions, nor is the exhumation associated with extensional doming. Instead, the vergence of parasitic folds toward the regional fold hinges indicates flexural slip deformation at least in the late stages of exhumation and exhumation occurred in the cores of regional scale anticlines. The regional shortening of accretionary prism, forearc basin, and older forearc basement was associated with intrusion of adakitic plutons that thermally weak- ened the forearc basin and enhanced the deformation and exhumation. The adakitic magmatism and forearc shortening resulted from subduction of the buoyant Izanagi–Kula ridge, a regional event known as the Oshima orogeny. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Many structural studies of granitic plutons focus on their emplacement mechanisms, with particular emphasis on those that explain how to create sufficient space for the pluton. Most studies have attributed fabrics within granitic rocks to magmatic pro- cesses, and have not considered the possibility of solid state folding of granite. For example, a granitic pluton in the Qinling orogenic belt of China exhibits an internal foliation, lineation, and fold axial planes oblique to the regional tectonic trends interpreted as mag- matic fabrics rather than subsolidus tectonic ones (Wang et al., 2000). Sant’Ovaia et al. (2010) postulated that the fabric in a Vari- scan granite in Portugal formed by magmatic process and post- dated the ductile deformation in the surrounding country rock. A granite in the French Central Massif has a laccolithic shape, and apparently filled a void opened by flexural slip of an anticlinal up- right fold, an emplacement mechanism that merges tectonic and magmatic process (Roig and Faure, 1998), but not a tectonic history wherein granitic deformation occurred after crystallization. Sierra Nevada (California) granitic plutons appear to have been emplaced along strike-slip fault zones, but the dominant fabric within them is considered magmatic (McNulty et al., 2000; Titus et al., 2005), although contemporaneous transpressional structures deform wall rocks (Sharp et al., 2000) and ductile transpressional shear zones locally cut the granite (Saint Blanquat and Tikoff, 1997). Folding of granitic rocks is seldom reported, an exception being kilome- ter-scale kink folds mapped in the Sierra Nevada of California (Pac- hell et al., 2003). In this paper we present data bearing on the mechanisms of so- lid-state deformation of the Kesengawa granite (Figs. 1 and 2), one of the granitic plutons that intrude the Kitakami zone of northeast Japan. In an earlier study, Ikeda (1984) examined the structures of the Kesengawa granite and wallrocks and concluded that foliations 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.05.023 ⇑ Corresponding author. E-mail address: osozawa@m.tohoku.ac.jp (S. Osozawa). Journal of Asian Earth Sciences 59 (2012) 85–98 Contents lists available at SciVerse ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes