Paleozoic Geodynamics and Architecture of the Southern Part of the Mongolian Altai Zone

Abstract

The Mongolian Altai Zone of the Central Asian Orogenic Belt has been traditionally interpreted as a mosaic of Paleozoic magmatic arcs, back‐arcs, and Precambrian continental terranes. In order to define its architecture and its tectonic evolution, three domains previously interpreted as terranes were investigated. The findings show that the Northern and Central domains are formed by a metamorphic sequence characterized by Barrovian S1 fabric transposed by recumbent folds and dominant sub‐horizontal amphibolite facies S2 schistosity. The latter is associated with the intrusions of late Devonian syntectonic granite sheets and anatexis in the south. The Southern domain is formed by early Permian migmatites and anatectic granites separated from the metamorphic envelope by amphibolite to green‐schist facies D3 shear zone cross‐cutting S2 fabrics. All domains have been reworked by E‐W upright folds associated with axial‐planar greenschist facies cleavage, reflecting the final mid‐Permian to Triassic D4 shortening. Lithological, geochemical, and U‐Pb zircon analyses of metasediments of all domains indicate that they are formed by Ordovician mature quartzite derived from Precambrian basement intruded by Cambrian‐Ordovician continental arc and Silurian immature graywacke which originated through erosion of an oceanic arc. Altogether, the whole sequence represents a fore‐arc basin in front of a migrating arc affected by thickening and late Devonian extension. The Southern domain is interpreted as an early Permian core complex amplified by mid‐Permian to Triassic compression. The apparent “terrane” architecture of the Mongol Altai Zone originated due to Devonian and Permian heterogeneous reworking of a giant Ordovician to Silurian fore‐arc basin.

Modes and geometry of crustal-scale detachment folding in hot orogens—Insights from physical modeling

“…At the same time, the degree of vertical melt transfer is decreasing with progressively increasing distance from the indentor, and the maximum melt content is spatially distributed in the form of horizontally elongated and gently arcuated pools ponding at the base of the models close to the backstop. These observations are in agreement with some structural and metamorphic patterns of the domes (Chandmann, Bugat, Tseel) developed in Mongol-Altai zone of the CAOB that consist of magmatic-migmatite rocks enveloped by low to medium-grade metasediments (Jiang et al, 2015;Lehmann et al, 2017;Sukhbaatar et al, 2022). Rotation during later stages of dome development, as displayed in our models, might be manifested in the asymmetric zonality of the domes in their horizontal section, with dominantly higher grade rock facies facing the indenter.…”

“…However, this zonality is visible in frame of one, polyphase-developed dome which corresponds to the oldest folds in our model (F1 and F2) that undergo both stages described in Figure 10A. Generally, this should be a trend typical for domes developed close to the indenter while domes formed in the foreland should be characterized by more symmetrical P-T-t zonality in map view [e.g., Lake Zone shown at Figure 10B or in profile 2 at Figure 4B in Sukhbaatar et al (2022)]. Additionally, younger domes (in the foreland of collision) might be associated with the presence of planar-subhorizontal fabrics in the mantling migmatites (with vergence from the axial plane of the dome) while older, amplified and squeezed domes may display dominantly steep structures in the mantling migmatites.…”

“…Rotation during later stages of dome development, as displayed in our models, might be manifested in the asymmetric zonality of the domes in their horizontal section, with dominantly higher grade rock facies facing the indenter. Such structural patterns can be seen in the S-N section through southern part of the Mongolian-Altai Zone, namely in the Tseel terrane (Sukhbaatar et al, 2022; Figure 10B). However, this zonality is visible in frame of one, polyphase-developed dome which corresponds to the oldest folds in our model (F1 and F2) that undergo both stages described in Figure 10A.…”

“…This scenario is similar to the tectonic evolution of the Lugian unit in the Bohemian Massif during Variscan orogeny (e.g., Figure 3 in Maierová et al, 2014) where the compressional exhumation was associated with buckling of the orogenic lower crust. Another example is the development of magmatic-metamorphic domes in the CAOB (Figure 9C) where intruded granitic masses are mantled by metasediments that grew in a compressional regime (e.g., Jiang et al, 2016;Lehmann et al, 2017;Sukhbaatar et al, 2022). At the maximum shortening rate, fold architecture is transformed to higher-amplitude, shorterwavelength folds with smaller content of melt.…”

“…These dome-fold structures were interpreted as large buckle folds of the lower anatectic felsic crust enveloped by medium and low grade metasedimentary units. Crustal scale folding was associated with the inflow of partially molten granitic masses from MOHO depths and their redistribution parallel to the axial planes of the large scale dome-shaped folds (Lehmann et al, 2017;Krýza et al, 2019;Sukhbaatar et al, 2022). Similarly, exhumation of mafic rocks from lower crustal levels can be achieved by the same mechanism of compression and detachment folding.…”