UPLIFT HISTORY OF STEENS MOUNTAIN AND ONSET OF BASIN AND RANGE EXTENSION IN EASTERN OREGON

Steens Mountain is one of the most distinct topographic features in eastern Oregon. It has been described in the literature as a shield volcano, though it has clearly been affected by Basin and Range extension.

Here we report multiple constraints on the uplift of Steens Mountain. Apatite fission track data from the early Miocene Alvord Creek Formation outcropping at the base of Steens Mountain yield a pooled age of 10.5 +5.9/-3.8 (2s). These results indicate that the Alvord Creek Formation remained at greater than 100 degrees C until circa 10 Ma. The samples come from the footwall of the Alvord Fault, about 2 kilometers beneath the top of the Steens Basalts. Therefore, the AFT results require that cooling in the footwall of the Alvord Fault due to structural uplift of Steens Mountain postdates 10 Ma. This is consistent with unpublished mapping at Duck Butte (Milliard, 2010).

We also conducted a paleomagnetic fold-test on the 9.7 Ma Devine Canyon Tuff, which sits on the western flank of Steens Mountain. Results from this test indicate that the tuff was originally flat-lying and tilted by 5 degrees after deposition.

Our results indicate that motion along the Alvord Fault began ~10.5 Ma. However, significant topographic relief along the fault was not present until after 9.7 Ma. These results provide a constraint on Basin and Range extension in eastern Oregon largely in agreement with previous work (e.g. Johnson, 1996), and suggest that significant structural activity at Steens Mountain did not occur until after ~10 Ma.