Palaeomagnetism and 40Ar/39Ar geochronology of upper Palaeogene volcanic rocks from Central Tibet: implications for the Central Asia inclination anomaly, the palaeolatitude of Tibet and post-50 Ma shortening within Asia

Abstract

SUMMARY Palaeomagnetic results from upper Palaeogene volcanics from three localities in the Qiangtang terrane of Central Tibet record a maximum likelihood palaeolatitude of 28.7 ± 3.7°N. These are the first volcanic-based palaeomagnetic results from the plateau interior that appear to average secular variation well enough to yield a reliable palaeolatitude estimate. This palaeolatitude estimate is significantly lower than the ∼36 ± 3°N predicted for Central Tibet by late Palaeogene Eurasia reference poles, consistent with the so-called Central Asia inclination anomaly, but it is consistent with values predicted from upper Palaeogene reference poles for Mongolia. The mean inclination of coeval sedimentary rocks from Central Tibet is ∼15 ± 5° shallower than our volcanic results, similar to the magnitude of sedimentary inclination shallowing estimated from Elongation/Inclination analysis of remanent directions in clastic rocks from the same region. We conclude that most of the inclination anomaly observed in volcanic rocks and that which remains after removal of sedimentary flattening in clastic rocks probably is due to the combined effect of reference poles that are not representative of stable Asia and by Cenozoic intracontinental shortening within Central Asia on the order of several hundred (not more than 1000) kilometers. Our compilation of palaeomagnetic data from the Qiangtang terrane further suggest that the southern margin of the Lhasa terrane (the southern margin of pre-India Asia) was located as far south as 20°N throughout Eocene time. These palaeolatitudes, in conjunction with the upper crustal shortening record for Asia, indicate that only approximately one third of the post-50 Ma India–Asia convergence is partitioned in Asian lithosphere. Low latitudes also imply that much of the proto-Tibetan plateau occupied southerly, tropical to subtropical latitudes, which has important implications for the weathering and erosion of the Tibetan plateau, palaeoaltimetry proxies and early Cenozoic climate dynamics.