Paleoaltimetry of intracontinental mountains: The uplift history of the Wallowa Mountains (Eastern Oregon, USA) as constrained by triple oxygen and hydrogen isotope analysis of meteoric hydrothermal systems

Abstract

We here report widespread low-δD and triple oxygen isotope depletions around dikes of the Columbia River Basalt Large Igneous Province (CRB), attributed to dike-groundwater interaction. CRB dikes exposed in the uplifted, currently 2–3 km-high Wallowa Mountain granites yield δD values down to −166 ‰ and δ¹⁸O down to 0–2 ‰. At the time of dike emplacement (∼16 Ma), the Wallowa area was <1 km in elevation, and the dikes were first altered by reconstructed low-altitude Miocene groundwater with δ¹⁸O of −7 to −10 ‰. However, the extremely low δD value of reconstructed meteoric water (−175 ‰), comparable to modern ice caps, suggests that the Wallowas had already attained high elevations before D/H closure but after ¹⁸O/¹⁷O/¹⁶O closure. Oxygen isotope exchange persisted for ∼0.8 Ma until closure (375–175 °C), while hydrogen exchange continued for another ∼2 Ma, until 200–78 °C at ∼13 Ma. Thus, the uplift of the Wallowa block began just ∼0.8 Ma after CRB eruptions, likely driven by eclogitic root delamination. Our data track this uplift and cooling of the CRB dike-intruded Wallowa crustal block. Applying published δD lapse rates, a − 175 ‰ δD_mw value implies elevations up to 4.5 km with ice cover. The estimated 3.5 km elevation gain corresponds to rapid uplift rates of 0.12–0.35 cm/yr from 15.7 to 13.1 Ma. Afterward, uplift ceased, lithospheric cooling and erosion reduced elevation, exposing 1–2 km deep CRB dikes. These results establish altered basalt groundmass as a novel paleoclimate and paleoaltimetry proxy, when it represents the last thermal episode.