ALBERT

Description: The debated mechanism and timing of formation of the Pamir orogenic salient provides an ideal case to combine paleomagnetic and metamorphic analyses. Opposing views argue for either oroclinal bending of the Pamir since the India-Asia collision or for an originally arcuate shape, which can be tested using paleomagnetism to estimate vertical-axis rotations. Furthermore, Pamir deformation can be dated and characterized by the analysis of a well-expressed regional metamorphism. However, paleomagnetism is seldom applied to metamorphic rocks such that the significance of their rock magnetic signal with respect to deformation remains poorly understood. We studied a 〉 2 km thick sequence of slightly metamorphosed Cretaceous-Paleogene volcanic and volcaniclastic rocks from the western Central Pamir Mountains using metamorphic petrology, geochronology, and paleomagnetic analyses. These rocks present a medium-grade metamorphism and have undergone fine grained recrystallisation of biotite due to NW-SE compression. 40Ar/39Ar and Rb/Sr ages (∼17 to 15 Ma) on whole rock and biotite, confirm that the greenschist facies metamorphism is related to the coeval exhumation of Pamir gneiss domes during the Middle Miocene. The rocks generally show homogenous magnetic properties. High field magnetic properties and SEM/EDS data show that the main magnetic remanent carrier is titano-hematite. Thermal demagnetizations yield complex characteristic remanent magnetization with site-mean directions obtained for only 14 of the 44 studied sites but scattered mainly within the foliation plane defined by the anisotropy of magnetic susceptibility (AMS). These rocks have also unusual magnetic characteristics where strong-field isothermal remanent magnetizations induce an AMS fabric, likely related to the specific composition and structure of the metamorphic titano-hematite but independent of the original anisotropy fabrics. Combined paleomagnetic and geochronological data from the Bartang volcaniclastics rocks enable us to identify a ∼ 17–15 Ma compressive deformation event contemporaneous with the emplacement of the gneiss domes and suggest no oroclinal bending since that event.