Relict Landscape Evolution and Fault Reactivation in the Eastern Tianshan: Insights from the Harlik Mountains

Abstract. Relict low-relief surfaces, formed during tectonically quiescent periods, are prevalent within the active mountain ranges of Central Asia, but the timing and processes of their formation within the Mesozoic-Cenozoic tectonic context remain poorly understood. In the Harlik Mountains of the easternmost Tianshan, extensive low-relief surfaces are preserved. Terrain analysis and structural interpretations based on DEM data reveal that these surfaces are segmented by WNW-ESE striking faults, which experienced an initial phase of right-lateral transtensional movement followed by left-lateral strike-slip reactivation. Apatite fission-track (AFT) thermochronology of samples from both relict surfaces and fault zones yields AFT ages ranging from ~110 to ~100 Ma for the relict surfaces, while samples from fault zones record ages of 90–70 Ma. Thermal modeling of these samples indicates a period of moderate cooling in the mid-late Early Cretaceous, followed by a prolonged slow cooling phase for the relict surfaces. In contrast, fault zones show rapid cooling during the 90–70 Ma interval. Integrating these data with previous findings, we suggest that the mid-late Early Cretaceous cooling event corresponds to extensional collapse following the Mongol-Okhotsk orogeny. This period of cooling, enhanced by humid climate conditions, likely promoted erosion and relief reduction, fostering the development of low-relief surfaces. Subsequently, the region experienced right-lateral transtensional faulting at 90–70 Ma, linked to continued extensional tectonics from the orogenic collapse. Late-Cretaceous faulting segmented the area without generating significant topographic contrasts across the relict surfaces. During the Oligocene (~30 Ma), far-field tectonic effects from the India-Eurasia collision initiated a new uplift phase that reactivated boundary and internal faults in a left-lateral sense. This phase marked the end of relief reduction, as surrounding basins began receiving sediments, and resulted in the uplift, dissection, and tilting of low-relief surfaces, ultimately contributing to the formation of the modern Eastern Tianshan Mountains.