Extension and Volcanism During Propagation of the Northwest Basin and Range into Southeastern Oregon

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The spectacular corrugated landscape known as the Basin and Range Province in western North America is the product of extension of the North American Plate largely between the Colorado Plateau and the Sierra Nevada. Normal faults drop valley bottoms (the 'basins') relative to adjacent mountain peaks (the 'ranges'). A major goal of this proposal addresses the question: when did this spectacular region become a part of the North American landscape? In Nevada at approximately 39 degrees North, the province grew about 100 kilometers incrementally westward in width in three main increments, at 26 to 22 million years ago, approximately 15 to 14 million years ago, and 10 to 3 million years ago. The Walker Lane, a broad zone of strike-slip and oblique-slip faulting that lies along the western boundary of the Basin and Range Province, initiated after 26 million years ago and apparently propagated northward approaching the southern Cascade Range at 41 degrees North after 9 million years ago. Whereas the growth of the Basin and Range in east-west and north-south directions is relatively well known in Nevada, few data constrain when Basin and Range-style faulting "arrived" in Oregon. Grunder and Meigs are conducting a 3-year investigation into two key questions--When did the Basin and Range form in southeastern Oregon? And, what factors limited how far into Oregon it grew? They are able to answer these questions because the development of the basins and ranges was both preceded and accompanied by volcanic activity. Volcanic rocks are central to resolving the questions because they are easily dated using conventional isotopic dating techniques. The essential observation of this study is the relationship between Basin and Range normal faults and volcanic rocks. Timing of the initiation of extension at a given place can be identified, for example, depending on whether faults can be shown to be older, younger, or coeval with emplacement of a given volcanic unit. With these constraints, the investigators are able to determine when the Basin and Range propagated into southern Oregon, at what rate that expansion occurred, at what rate individual faults have grown, and whether the normal faulting was controlled by the pattern of volcanism or, conversely, whether the faults controlled where volcanism occurred. These data contribute to the ongoing debate about the role of volcanism in the development of Basin and Range-type provinces around the world. Moreover, they provide a new perspective on the rate of faulting, which helps constrain the seismic hazard posed by normal faults to the population centers of southeastern Oregon (including Klamath Falls, Lakeview, and Bend). Additional societal benefits include the training of graduate and undergraduate students at Oregon State University, broadening of participation of underrepresented groups in science, and integration of research results with ongoing geophysical studies in the Basin-Range, including EarthScope.