TIMING OF EXHUMATION OF THE WALLOWA MOUNTAINS, NE OREGON: A U-TH/HE THERMOCHRONOLOGIC STUDY

The Wallowa Mountains are a region of anomalously high modern topography within the Columbia River Basalt (CRB) plain. The Wallowas are bounded to the SW by normal faults that form the margin of the La Grande graben, to the S by the Pine Valley graben, and to the NE by faults that form part of the Olympic-Wallowa lineament. Each of these fault systems offsets the sub-CRB unconformity.

U-Th/He ages were determined for apatite from 24 samples and for zircon from 4 samples of the Cretaceous Wallowa batholith. The samples represent over 1.3 km of modern topographic relief. Depth of emplacement of the Wallowa batholith is estimated to be 6-10 km, placing the batholith below the closure temperature of both apatite and zircon. Therefore, these ages all reflect post-emplacement exhumation. Apatite ages range from 15-101 Ma and zircon ages range from 20 to 123 Ma. Zircon ages are 4 to 53 Ma older than the apatite ages from the same samples.

Overall, the ages do not correlate well with modern elevation, depth of emplacement or structural depth beneath the sub-CRB unconformity, but rather the ages form geographically distinct clusters. Apatite ages of ca. 100 Ma occur over a range of elevations in the NE Wallowas. In the rest of the Wallowas, the age clusters form local age-elevation transects with shallow age-elevation slopes. These age-elevation correlations are interpreted to represent an exhumed helium partial-retention zone (PRZ). The elevation of this exhumed PRZ gets systematically higher towards the highest part of the Wallowas.

Exhumation of most of the Wallowas must have occurred at or after 15 Ma, the youngest ages found in the exhumed PRZ, most likely associated with the normal faulting that produced the La Grande and Pine Valley grabens and with the inception of CRB magmatism. Exhumation of the NE Wallowas must have occurred earlier, possibly ca. 100 Ma.