Abstract
The Central Asian Orogenic Belt (CAOB) is key to understanding the Paleozoic–Mesozoic geodynamics of Eurasian continent. The geological structure of the Middle-to-Late Paleozoic rock units in the North Mongolia–West Transbaikal region is critical in revealing development process of CAOB. The region is largely comprised of rocks from the continental affinity and accretionary complexes which form the Sayan-Baikal (SB) and Hangai-Daur (HD) belts. This paper describes the lithology, stratigraphy, geological structure, and U-Pb age of the rocks in the Züünharaa area, which is located within the Haraa terrane of the HD belt in Mongolia. We identified a regional low-angle southward-dipping thrust in this area. The tectonic implication of the low-angle south-dipping thrust is discussed within the North Mongolia–West Transbaikal region. The study area exposes metamorphosed clastic rocks of the Haraa Group intruded by Ordovician–Silurian granitic rocks, Devonian felsic volcanic rocks of the Ulaan Öndör Formation, and Visean clastic rocks of the Örmögtei Formation in ascending order. The Haraa Group, granitic rock, and Ulaan Öndör Formation are cut by the low-angle southward-dipping thrust throughout this area. The rocks along the thrust are fractured to form cataclasite zone up to ~40 m wide. The thrust includes granite–rhyolite clast of ~450–420 Ma, and is unconformably covered by Visean Örmögtei Formation. Therefore, thrusting occurred after Ordovician–Silurian and before Visean. Late Paleozoic low-angle southward-dipping thrusts, similar to the present study, are widely distributed in the Haraa terrane of the Hangai-Daur belt and in terranes of the Sayan-Baikal belt. Whereas, the contemporaneous southeast-verging composite folds and northward-dipping thrusts are developed in the accretionary complexes, which are exposed at south of the Haraa terrane. These contrasting structures suggest a couple of “landward-verging” and “oceanward-verging” structures and may correspond to the “doubly vergent asymmetric structure” of Alpine-type compressional orogen.
Highlights
The Altaid Collage (e.g., Sengör et al 1993; Sengör and Natal’in 1996) or the Central Asian Orogenic Belt (CAOB; e.g., Janh et al 2000; Xiao et al 2003; Windley et al 2007) is one of the largest Paleo-Mesozoic orogenic belts in the world (Fig. 1a)
The granitic rocks had U-Pb ages ranging from 561 to 424 Ma (Figs. 2, 10) and coincide with the U-Pb ages of the Boroogol plutonic rock complex dated at 460–440 Ma (e.g., Kröner et al 2007; Hou et al 2010)
Magmatic zircon from the porphyritic granite–rhyolite clast in the cataclasite shows ages of ~450–420 Ma, and detrital zircons of ~450 Ma were reported from the Haraa Group (Bussien et al 2011; Kröner et al 2007; Kelty et al 2008)
Summary
The Altaid Collage (e.g., Sengör et al 1993; Sengör and Natal’in 1996) or the Central Asian Orogenic Belt (CAOB; e.g., Janh et al 2000; Xiao et al 2003; Windley et al 2007) is one of the largest Paleo-Mesozoic orogenic belts in the world (Fig. 1a). The North Mongolia–West Transbaikal region (Fig. 1a, b), which lies north of the Main Mongolian Lineament (MML; Badarch et al 2002; Tomurtogoo 2006; Kröner et al 2007; Windley et al 2007), is one of the significant portions for Paleozoic–Mesozoic tectonics of the CAOB in the Northeast Asia This region is comprised of rocks from the Middle Devonian Sayan-Baikal and Middle–Late Triassic Hangai-Daur belts (Fig. 1b, Petrov et al 2014). The bedding and structural planes in the Zag and Tsetserleg terranes generally trend northwest and steeply dip north- or southward, west of the HD belt (e.g., Badarch et al 2002; Buchan et al 2001; Purevjav and Roser 2013; Tsukada et al 2013), whereas those in the Haraa, Asralt Hairhan, Ulaanbaatar, Daur, and Aga terranes trend northeast and steeply dip northward in the east (e.g., Zorin 1999; Kurihara et al 2009; Takeuchi et al 2012). The clastic rocks of the Örmögtei Formation unconformably overlie the granitic rock with basal conglomerate (Figs. 2a, b, 3d)
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