The Role of Strain Partitioning During the Mesozoic–Cenozoic Reactivation in Exhuming and Preserving the Worldclass Late Paleozoic Almalyk Porphyry Copper–Gold District, Tien Shan, Uzbekistan

Abstract Porphyry Cu systems are most commonly preserved in Cenozoic magmatic arcs of the Circum‐Pacific belt. However, Paleozoic magmatic arcs in the Tien Shan of western Central Asia host some of the world's largest porphyry Cu–Au accumulations. The limited abundance of Paleozoic porphyry Cu–Au systems is suggested to be controlled by post‐mineralization exhumation and erosion. As a result, quantifying the magnitude of exhumation remains critical in the development and exploration of Paleozoic porphyry Cu–Au systems. Hereinto addressing the role of exhumation of porphyry preservation, we report 19 new zircon and apatite fission track (∼213–154 and 177–133 Ma) and apatite (U–Th)/He (∼124–90 Ma) ages from the worldclass late Paleozoic Almalyk porphyry Cu–Au district (46 Mt Cu and 5,700 t Au) in the western Tien Shan, Uzbekistan. Integrated with published geo‐ and thermochronological data sets, we quantify the cooling and exhumation of the Almalyk district. We identify three phases of cooling, including Late Triassic–Early Jurassic (∼1.9°C/Ma), Late Jurassic–Early Cretaceous (∼4.1–17.8°C/Ma), and Late Cretaceous to the present day (∼0.6–0.8°C/Ma). The first two phases of cooling events are associated with accelerated exhumation (from ∼0.08 mm/yr to 0.16–0.71 mm/yr), due to accretion and associated shortening of the Qiangtang and Lhasa blocks onto the southern Eurasian margin, respectively. From the late Cretaceous until present day erosion rates have remained relatively low (∼0.02–0.05 mm/yr). Together, these data suggest that burial by the Upper Carboniferous–Lower Permian volcano‐sedimentary rocks and slow exhumation from the Late Cretaceous until present are critical factors in the preservation of the Almalyk district. The exhumation history was correlated with crustal shortening and associated strain accommodated by strike‐slip reactivation of the Talas–Fergana Fault and folding and thrusting in the Northern Pamir–Southern Tien Shan contact during the Mesozoic–Cenozoic. We suggest that strain partitioning by major fault zones during the Cenozoic has been key in the preservation of the Almalyk district.

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