Termination of the northwestern Basin and Range province into a clockwise rotating region of transtension and volcanism, southeast Oregon

• Published in: Journal of structural geology 2012-06, Vol.39, p.52-65
• Main Authors: Trench, David, Meigs, Andrew, Grunder, Anita
• Format: Article
• Language: English
• Subjects: Along-strike growth of extensional provinces; Brothers fault zone; Normal-fault tip propagation; Northwest Basin and Range
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/j.jsg.2012.03.007

New data indicate that northeast-directed extensional faulting characterizes slip across the Brothers fault zone (BFZ), which marks the northern limit of the northwestern Basin and Range (NWBR) extensional province in southeastern Oregon. Structural separation across individual north-northeast striking NWBR faults decreases to zero south of the BFZ. Field relationships and cross-sections demonstrate limited kinematic linkage and independent evolution of the two fault systems since ∼7 Ma. West-directed extension accumulated on NWBR faults at 0.01 mm/yr and lengthened northward after 7.05 Ma. BFZ faults accumulated northeast-directed extension at rates of 0.01 mm/yr since 5.68 Ma. Deformation coincides with periods of heightened basaltic magmatism in the High Lava Plains, implying that volcanism weakened the crust and promoted extension in the BFZ. In a new model, we reconcile the observed northward diminishing rate and clockwise motion of the modern NWBR deformation field with regional geology. The BFZ defines a small circle about the pole of rotation and separates a stable block to the NE from the extending region to the south. Faults to the south are growing northward, consistent with the northward decrease in rate and magnitude of extension in the NWBR. ► New constraints on the kinematics of faults in the northwest Basin and Range and the Brothers fault zone. ► Quantify timing and rate of extension across Basin and Range and Brothers fault zone. ► Quantify kinematic relationship between Basin and Range and Brothers fault zone. ► Relate structural history to volcanic history of High Lava Plains volcanic province. ► New model for backarc deformation reconciles present GPS data and structural history.