Paleomagnetic and 40Ar/39Ar geochronological results from the Linzizong Group, Linzhou Basin, Lhasa Terrane, Tibet: Implications to Paleogene paleolatitude and onset of the India-Asia collision

The Linzizong Group (64-44 Ma) of the Lhasa Terrane in Tibet is critically positioned for establishing the paleoposition of the southern leading edge of the Asian continent during Paleogene times and constraining onset of the India-Asia collision. Here we report paleomagnetic results from a collection comprising 384 drill-core samples from 34 sites embracing all three formations of this group. Comprehensive demagnetization and field tests isolate characteristic remanent magnetizations (ChRM) summarized by overall tilt-corrected formation-mean directions of D = 183.6°, I = −12.4° (α95 = 8.1°) for the Dianzhong (64-60 Ma), D = 1.0°, I = 18.1° (α95 = 8.1°) for the Nianbo (60-50 Ma), and D = 12.4°, I = 23.2° (α95 = 7.3°) for the Pana (50-44 Ma). Fold tests are positive in each formation suggesting a pre-folding origin and we interpret the magnetizations as quasi-primary and acquired at, or slightly later than, formation of the Linzizong Group. Revised Paleogene paleopoles with Ar-Ar age constraints for the Lhasa Terrane indicate that onset of the India-Asia collision occurred no later than ∼60.5 ± 1.5 Ma at a low paleolatitude of ∼10°N. Analysis of 60 site-mean observations from a range of studies of the Pana Formation in the higher part of the succession highlight a large dispersion of ChRM directions; a number of possible causes are suggested but further study of this formation over a wider area is required to resolve this issue.