Paleomagnetic results from the Seling Co Basin of central Tibet: constraints on V-shaped conjugate strike-slip fault formation

Conjugate strike-slip faults are widespread features throughout the Alpine-Himalayan collision zone. They often exhibit V-shapes in map view and trend 60-75° from the maximum compressive-stress (σ1). Andersonian fault mechanics, however, predict faults to form X-shaped at ~30° from σ1. Consequently, V-shaped conjugate faults have been thought to initiate at ~30° to σ1, and subsequently rotate into their current orientation through continued shortening. A new Paired General Shear Zone (Yin and Taylor, 2011) model or a model with a ductily deforming strong upper mantle may both explain development of faults in their modern orientations, predicting no rotation. Strike-slip faulting produces rigid-body motion and internal deformation quantifiable by paleomagnetism when integrated with structural information. We wonder if paleomagnetic studies of the fault-bounded blocks in central Tibet would allow us to differentiate the competing models for the formation of V-shaped conjugate faults. We collected over 300 paleomagnetic samples (40 sites) from stratigraphic sections in or near to the Seling Co Basin of central Tibet. Sampled units include the Paleogene Linzizhong Volcanics, Cretaceous aged Jingzhushan Fm., Langshan Fm., and Duoni Fm., and late Jurassic Rila Limestone. We have characterized the stable components of natural remnant magnetization (NRM) of these samples through detailed thermal (mainly) and alternating field (AF) demagnetization. We have also conducted rock magnetic investigation to identify the magnetic carriers in these rocks. Results from the Linzizhong volcanics are of mixed polarities, but unfortunately do not average secular variation and shouldn't be used for structural studies at this point. Results from redbeds of the Jingzhushan Fm. predict a 28.2+/-10.4° counterclockwise rotation and 699+/-1199 km of N-S convergence relative to an early-mid Cretaceous reference pole (Torsvik et al., 2008). Results from limestones from the Langshan Fm. and siltstones from the Duoni Fm. have a similar direction and so are combined into one pole. This pole predicts 2.2 +/- 5.7° counterclockwise rotation and 544+/-577 km of N-S convergence. Results from the Rila Limestone predict 29.2+/-13.3° of counterclockwise rotation and -350+/-1200 km of N-S convergence. The Rila Limestone results, along with that from the Jingzhushan, both predict large CCW rotation of the Seling Co basin, and potentially bounding strike-slip faults. The results from the Langshan and Duoni formations, however, are of higher quality and pass local fold tests at greater than 95% confidence. These formations therefore provide the best representation of tectonic motion of the Seling Co basin.