Overprint of secondary Du folding in the Sambagawa metamorphic belt, SW Japan: Implications for strain ellipsoids and Paleogene tectonics of the east-Eurasian margin

IF 1 4区 地球科学 Q4 GEOSCIENCES, MULTIDISCIPLINARY Island Arc Pub Date : 2022-09-13 DOI:10.1111/iar.12463
Mutsuki Aoya
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Abstract

Two contrasting results of strain analyses, constriction and flattening, are recognized in the Sambagawa high-pressure/temperature metamorphic belt, SW Japan. An unverified proposal to account for this situation is that the constrictional strain ellipsoids develop only in areas where there is strong overprinting by a secondary Du-phase folding after a penetrative Ds-phase deformation. Field studies in the Hibihara district, central Shikoku, which is located between a southern constrictional region and a northern flattening region, reveal there is a map-scale contrast in the effect of Du: outcrop-scale Du upright folds are common in the southern region while they are rare in the northern region. Field measurements show that overall orientation of Ds strain is characterized by E–W stretching and vertical shortening, while that of Du strain is characterized by N–S shortening and vertical extension. The shortening caused by Du in the southern high-Du-strain regions estimated by the fold-curve tracing method are down to about 70%–56% and by removing this Du shortening most of the constrictional strain ellipsoids are restored back to the flattening field with only a few exceptions, suggesting that pre-Du strain states of the Sambagawa metamorphic rocks were dominantly of the flattening type. These results support the previous ideas that invoke differences in the strength of Du overprinting for the two different types of strain ellipsoids observed in the Sambagawa belt. From a tectonic point of view, the semi-penetrative occurrence of Du folds throughout the Sambagawa belt, which stretches about 800 km in SW Japan, suggests that the Du phase can be related to some ancient plate movement. The significant shortening subnormal to the orogen that characterizes Du may reflect a shift to orogen-subnormal subduction of the Pacific plate beneath the Eurasian plate at around 60 Ma.

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日本西南三巴川变质带次生杜褶皱叠印:对东欧亚边缘应变椭球体和古近系构造的启示
在日本西南部的三巴川高压/高温变质带中,发现了两种截然相反的应变分析结果:收缩和扁化。一种未经证实的解释这种情况的建议是,收缩应变椭球只在穿透性ds相变形后二次du相折叠产生强烈套印的区域发展。在位于南部收缩区和北部平坦区之间的日本中部日比原地区进行的野外研究表明,杜作用在地图尺度上存在差异:南部地区露头规模的杜直立褶皱较为常见,而北部地区则较为少见。现场测量结果表明,Ds应变总体取向表现为东西向拉伸和垂直方向缩短,Du应变总体取向表现为南北向缩短和垂直方向延长。褶皱曲线示踪法估计,在南部高应变区,由于Du的作用,收缩应变椭球的缩短率降至70% ~ 56%左右,除少数例外,大部分收缩应变椭球恢复到平坦场,说明三巴川变质岩在Du前的应变状态主要为平坦型。这些结果支持了先前的观点,即在三巴川带观测到的两种不同类型的应变椭球的杜套印强度存在差异。从构造学的观点来看,在日本西南部绵延约800公里的三巴川带中,杜褶皱的半穿透性存在表明,杜期可能与某些古代板块运动有关。杜氏构造向造山带次正向的显著缩短可能反映了60 Ma左右太平洋板块在欧亚板块下向造山带次正向俯冲的转变。
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来源期刊
Island Arc
Island Arc 地学-地球科学综合
CiteScore
2.90
自引率
26.70%
发文量
32
审稿时长
>12 weeks
期刊介绍: Island Arc is the official journal of the Geological Society of Japan. This journal focuses on the structure, dynamics and evolution of convergent plate boundaries, including trenches, volcanic arcs, subducting plates, and both accretionary and collisional orogens in modern and ancient settings. The Journal also opens to other key geological processes and features of broad interest such as oceanic basins, mid-ocean ridges, hot spots, continental cratons, and their surfaces and roots. Papers that discuss the interaction between solid earth, atmosphere, and bodies of water are also welcome. Articles of immediate importance to other researchers, either by virtue of their new data, results or ideas are given priority publication. Island Arc publishes peer-reviewed articles and reviews. Original scientific articles, of a maximum length of 15 printed pages, are published promptly with a standard publication time from submission of 3 months. All articles are peer reviewed by at least two research experts in the field of the submitted paper.
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