MIDDLE–LATE PALEOGENE TOPOGRAPHIC EVOLUTION IN SOUTHWESTERN MONTANA

We use 77 new stable hydrogen isotope data (dD) of Eocene–Miocene hydrated volcanic glass samples from southwestern Montana and western North Dakota to constrain the topographic history of the North American Cordillera. We interpret that the environmental water recorded in the hydrated glass could be both river/shallow ground water and precipitation depending on the depositional environment and sedimentation rate. In southwestern Montana, the change of dD values can be divided into four stages. During the early middle Eocene, the dD values show an abrupt 18‰ negative shift at ca. 47 Ma and a 36‰ positive shift at ca. 46 Ma. The negative shift most likely reflects drainage capture of a distal river from the high Cordillera hinterland, while the positive shift likely reflects another drainage reorganization that the distal river was replaced by local rivers. Both of these two drainage reorganizations were probably associated with southwestward rollback of the Farallon Plate. Between 46–33 Ma, the dD values show gradual decrease following global deep ocean d18O trend, reflecting regional cooling in response to global climate change and minimal topography change in the region. Between 33–29 Ma, the dD values show ~26‰ decrease, most likely reflecting another stage of surface uplift. If this interpretation is correct, the magnitude of this stage of uplift is ~0.9 km. Surface uplift during the early Oligocene is likely a result of both thermal and isostatic uplift associated with magmatic inflation and crustal thickening induced by magma body. Finally, during the late Oligocene, the dD values show 14‰ increase in southwestern Montana, while remain stable in North Dakota. Therefore, the dD gradient between southwestern Montana and North Dakota became smaller through time, which we interpret to be resulted from gradual surface lowering of southwestern Montana in association with regional extension.