Foreland basin sedimentary strata are key records of orogenic development, but identifying detrital thermochronometers of appropriate temperature sensitivity to detect provenance shifts can be challenging. Here, we utilize the medium-temperature apatite U-Pb system as a detrital thermochronometer. With a closure temperature range of ∼375–550 °C, it is a system sensitive to the typical peak temperatures attained by the amphibolite-eclogite cores of many magma-poor Phanerozoic orogens, while apatite itself is a common accessory mineral that is virtually ubiquitous in clastic rocks.

We present apatite U-Pb data from the Oligocene–Miocene Barrême and Valensole basins of the Alpine foreland in SE France, together with data from the rutile U-Pb, apatite fission-track (AFT), and zircon U-Pb systems. At ca. 26–25 Ma, an abrupt cessation of detrital apatites yielding Alpine U-Pb ages derived from the eclogite-facies core of the Western Alps records a major westward migration of the primary drainage divide of the orogen. This shift, not visible in AFT and zircon U-Pb data, was synchronous with uplift and exhumation of the Argentera external basement massif. The drainage connection from the foreland to the eclogite-facies core of the Western Alps was restored by ca. 13 Ma and likely as early as ca. 16 Ma, and the primary drainage divide location has likely remained stable since this time. The synchronicity of late Oligocene–early Miocene uplift and initial exhumation of the external basement massifs around the western Alpine arc suggests that a major westward drainage divide migration occurred throughout the Western Alps at that time.

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