Graduate Thesis Or Dissertation
 

The termination of the Basin and Range Province into a clockwise rotating region of transtension and volcanism, central Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qr46r331w

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  • Normal faults characterizing extensional provinces may terminate along-strike at regions of zero extension, at zones of transform faults, or at triple junctions. Termination of the Basin and Range extensional province in southeastern Oregon is thought to occur by right-lateral transform motion distributed across the Brothers Fault zone (BFZ) in central Oregon (Lawrence, 1976). New field mapping across the transition from the Hart Mountain fault, a Basin and Range normal fault, into the BFZ suggests a more complex model for the northern termination of Basin and Range extension. Topographic relief along the Hart Mountain fault decreases to zero approaching the BFZ. Northwest trending BFZ faults simultaneously increase from 5 m of relief to a maximum of 107 m to the northwest away from the zone of transition. Field data indicate that predominantly dip-slip separation with little apparent strike-separation characterize BFZ faults. Independent models of fault slip direction and style imply, predominantly dip-slip motion for the Hart Mountain faults, oblique slip for the BFZ, and an east-west regional extension direction. Cross-sections estimate the BFZ has recorded 63 m (+ 10 m) of extension, the overlap region between the Basin and Range and the BFZ has recorded 224 m (+ 10 m) of extension, and the Hart Mountain system has recorded 157 m (+ 10 m) of extension. Faults at the transition between the BFZ and the Basin and Range accumulate extension from both the BFZ and the Hart Mountain system, suggesting a kinematic link between the two. Two episodes of deformation are suggested. Prior to 5.68 Ma, 161 m (+ 20 m) of extension accumulated, and an additional 63 (+ 20 m) of extension occurred after 5.68 Ma. Cross-section restorations imply the BFZ has slipped independently of the NWBR since 5.68 Ma. Deformation after 5.68 Ma roughly correlates with periods of basaltic magmatism in the BFZ, suggesting a possible link between volcanism and the two-phase extensional history in the study area. A model for the interaction between the BFZ and the Basin and Range must be compatible with the active deformational field of North America in the Pacific Northwest. Multiple data sets demonstrate that clockwise rotation characterizes the velocity field of the northern Basin and Range extensional province. I propose a model where the BFZ defines a small circle about a pole of rotation in northeastern Oregon, characterized by oblique opening and periodic magmatism, linked to the Basin and Range by horsetail fractures associated with the northward propagating Basin and Range faults. The structural style and temporal development of both the BFZ and the major Basin and Range faults are forming in response to the clockwise rotation of the Oregon coastal block to the west. This model unites observations from this study area, the active tectonics, and northern Basin and Range magmatism.
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