Late Mesozoic intracontinental reactivation of the southern Altai, Central Asia

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The Altai orogenic belt is a main constituent of the Central Asian Orogenic Belt, and serves as a crucial site for studying strain propagation from the Meso-Cenozoic plate margins to the Eurasian interior. The ranges of the Altai Mountains have undergone multiple reactivation events during the Mesozoic and Cenozoic, but the full extent of these events is not yet fully understood. To constrain the thermo-tectonic history of the southern Altai orogenic belt of Northwest China, apatite fission-track (AFT) and apatite (U-Th)/He (AHe) thermochronological methods were used to study 29 pre-Mesozoic basement rocks from several key localities, including the Altay, Xibodu, Fuyun, and Qinghe regions. Late Jurassic to Early Cretaceous AFT and AHe ages were found in the low-elevation Xibodu region, which has been characterized by slow-to-moderate rock cooling since the Jurassic. However, rock samples from all other regions investigated are characterized by comparable Late Cretaceous AFT and AHe ages. Inverse thermal history modeling results reveal moderate-to-rapid upper crustal cooling in the mid- to Late Cretaceous (ca. 110−70 Ma), which is interpreted to be related to distant plate-margin processes (e.g., the Cimmerian collisions). These findings, combined with previously published data, indicate that Late Cretaceous exhumation was widespread in the western Altai orogenic belt, including in the Chinese and Siberian (Russian) parts of the Altai region. As in many other areas of Central Asia, no Cenozoic low-temperature thermochronological signal was detected in this study. We propose that Cenozoic deformation indeed occurred in the southern Altai, but the magnitude of associated denudation was insufficient to have replaced the Cretaceous cooling signals.