Kinematic Implications of New Paleomagnetic Data From the Northern Walker Lane, Western Nevada: Counterintuitive Anticlockwise Vertical-Axis Rotation in an Incipient Dextral Shear Zone
Abstract
The Walker Lane/eastern California shear zone is a major dextral fault system that splays from the San Andreas fault in southern California and shunts 20-25% of the Pacific-North America transform motion east of the Sierra Nevada block. In NW Nevada and NE California, the north end of the system has developed in the past 3-6 Ma and is propagating northwestward in the wake of the retreating Cascade arc. This rapidly evolving region, known as the northern Walker Lane (NWL), is one of the youngest parts of the Pacific-North America transform margin and therefore affords an opportunity to analyze the incipient development of a major strike-slip fault system. The NWL consists of kinematically linked NW-striking, left-stepping right-lateral faults, N-striking normal faults, and subordinate ENE-striking sinistral faults. Major dextral faults terminate in arrays of northerly striking normal faults in the western Great Basin. The en echelon left-stepping pattern of dextral faults is a curious geometry. Although small left steps on individual faults are associated with local shortening, the broad left steps between major dextral faults accommodate little, if any, shortening and are therefore unlike typical restraining bends. One possible model is that the left-stepping dextral faults are primary Riedel shears developing above a dextral shear zone at depth. The ENE-striking sinistral faults may be secondary, conjugate Riedel shears. Paleomagnetic data from the 25.1 Ma Nine Hill Tuff indicate slight anticlockwise rotation of blocks between the overlapping, NW-striking dextral faults. Our work has established a new reference direction (D=341, I=55, a95=4, 11 sites; 83 samples) for Nine Hill Tuff in the presumably unrotated Sierra Nevada. Data from the Nine Hill Tuff in the NWL suggest about 15 deg of anticlockwise rotation of the Virginia Mts, Dogskin Mt, and Seven Lakes Mt (D=326, I=52, a95=7, 12 sites) but negligible rotation of the Fort Sage Mts (D=342, I=55, a95=13, 5 sites). Slight anticlockwise rotation is also supported by paleomagnetic data from the 28.6 Ma tuff of Campbell Creek (D=191, I= -40 [2 sites] compared to reference direction of D=205, I= -43, a95=3 [10 sites in Sierra Nevada]) and the predominance of WNW-striking strata in these fault blocks, which contrasts with a more northerly striking regional norm. However, data from the Nine Hill Tuff do indicate 30-60 deg of clockwise rotation (D=16, I=47, a95=10, 6 sites) in narrow 2 km wide bands along major dextral faults in the NWL. In the transtensional setting of the NWL, the slight anticlockwise rotation may reflect coeval E-W to WNW regional extension and NW-directed dextral shear. In this model, extension induces a domino-like, map-view collapse and slight anticlockwise rotation of fault blocks between the left-stepping dextral faults. The anticlockwise rotation is opposite to the clockwise rotation typically found in dextral shear zones. Anticlockwise rotation may ultimately rotate Riedel shears toward the main shear zone at depth, thus facilitating eventual development of a through-going, upper-crustal strike-slip fault. Ironically, as the system matures and a through-going fault develops, the predominant sense of vertical-axis rotation may reverse and become compatible with the dextral sense of shear. Such complex kinematics may characterize incipient strike-slip fault systems in both transtensional and transpressional settings.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFMGP42A..08F
- Keywords:
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- 8110 Continental tectonics: general (0905);
- 8150 Plate boundary: general (3040);
- 7218 Lithosphere and upper mantle;
- 1206 Crustal movements: interplate (8155);
- 1525 Paleomagnetism applied to tectonics (regional;
- global)