Structural control on Meso-Cenozoic tectonic reactivation and denudation in the Siberian Altai: Insights from multi-method thermochronometry

The Siberian Altai forms part of the Central Asian Orogenic Belt and currently is an active intracontinental mountain belt. This Siberian Altai orogen was shaped during multiple phases of intense mountain building and associated denudation throughout the Mesozoic and Cenozoic. We performed a multi-method thermochronometric study on the Altai basement in Siberia in order to distinguish these denudation episodes and to place them in an absolute time-frame. Our data shows that inherited suture zones, such as the Charysh-Terekta-Ulagan-Sayan suture, played a dominant role during this episodic deformation in the Altai. Titanite fission track results reveal Late Triassic-Early Jurassic cooling, which is best expressed in intrusions close to the suture zone. Apatite fission track (AFT) results reveal a second phase of steady basement cooling during the Late Jurassic and Cretaceous. In the suture, AFT and additional apatite (U-Th-Sm)/He (AHe) data reveal a consistent Late Cretaceous-Early Palaeogene phase of fast cooling. We interpret the Mesozoic cooling signals as far-field effects of the Mongol-Okhotsk orogeny. The intense Late Cretaceous-Palaeogene basement cooling signal in the suture zone can be explained as a result of fault-reactivation during the collapse of this orogen and the subsequent re-organization of crustal blocks. During most of the Palaeogene and Early Neogene, basement cooling stagnated, indicating tectonic quiescence. Neogene AHe ages for the Kurai Range, directly adjacent to the suture may reflect renewed rapid cooling as a result of denudation of the present-day Altai mountains. We interpret this most recent cooling as an indirect consequence of stress-propagation from the India-Eurasia collision into the CAOB interior, which thus reached as far as southern Siberia. This study further demonstrates the importance of thermochrological studies on intrusions along major fault-zones in order to date and distinguish fault-reactivation episodes.