3 years ago

Revision of the Chinese Altai ‐ East Junggar terrane accretion model based on geophysical and geological constraints

Alexandra Guy, Karel Schulmann, Igor Soejono, Wenjiao Xiao

Potential field data analysis, geology and geochemistry are used to revise the terrane accretion model of the Chinese Altai and East Junggar. Major gradients of potential field data demarcate significant crustal structures and their continuity in depth. These data demonstrate that the distribution of geophysical anomalies does not match with terranes defined previously. Instead, heterogeneously developed NE/SW‐trending gravity and NW–SE magnetic anomalies in both units correlate with Devonian and Permian tectonometamorphic zones, respectively. Geophysical data also indicate a dense lower crust of the East Junggar confirmed by previous igneous petrology studies and seismic experiment. The northern tip of this dense crust coincides with the main gravity gradient beneath the Chinese Altai and a prominent magnetic high located above a NW/SE‐trending zone of Permian granulites. This zone forms a boundary between north‐ and south‐dipping gravity and magnetic anomalies that are interpreted as reflecting quasi‐symmetrical extrusion of the Chinese Altai crust. In contrast, all geophysical data show the absence of a prominent deep‐seated discontinuity which can be correlated with the Erqis Zone. These data are compared with corresponding datasets from southern Mongolia allowing the proposition of a new geodynamic model for the studied area. This model involves: 1) accretionary stage characterized by Late Devonian joint evolution of the East Junggar and Chinese Altai characterized by N–S trending orogenic fabrics and 2) Early Permian oroclinal bending associated with underthrusting of the dense Junggar basement beneath the Chinese Altai and the development of NW–SE trending deformation zones in both units.

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