Paleomagnetic Results Of The Jurassic-Cretaceous Strata From Gyangze And Kangma Counties,Southern Tibet And Their Tectonic Implications

The amalgamation of the Tethyan Himalaya with the Lhasa block has been traditionally regarded as representing the India-Asia collision.Recently,however,a dual-collision model has been proposed,in which Tethyan Himalayais considered as a separate block that rifted from Greater India's northern margin in Late Cretaceous.The Tethyan Himalaya first collided with Lhasa block and then India craton collided with the Tethyan Himalaya.The dual-collision model hinges on the speculated rifting of Tethyan Himalaya from Greater India in Late Cretaceous,which,however,has not been paleomagnetically constrained because the Late Cretaceous paleomagnetic data from Tethyan Himalaya are lacking.Clearly,elucidating the Mesozoic;particularly the Late Cretaceous,tectonic evolution of Tethyan Himalaya is crucial to understanding the India-Asia collisional processes.To better constrain the paleogeographic position and drift history of Tethyan Himalaya during the Mesozoic,particularly in Late Cretaceous.A paleomagnetic investigation was conducted on the Chuangde section in Gyangze County and Tianba section in Kangma County of southern Tibet.In Chuangde areas,Paleomagnetic samples were collected from 39 sites,among which five were from grey andesite of the Xiare Fm,25 from siltstone and sandstones of the Weimei Fm,and 9 sites from limestones.In Tianba areas,paleomagnetic samples were collected from 6 sites,all of them were from shales and sandstones of the Jiabula Fm.Based on the paleomagnetic and rock magnetic results from the strata;The following conclusions can be drawn.(1)The demagnetization results show that samples from the Xiare Fm and the Weimei Fm of Chuangde section as well as samples from Jiabula Fm of Tianba section,did not yield characteristic remanent magnetization(ChRM).Specimens from the Chuangde Fm of Chuangde section yield stable ChRMs upon demagnetization that display both normal and reversed polarities,and passed a reversal test.The age of studied Chuangde Fm constrained by magnetostratigraphic sequence is consistent with that by biostratigraphy.(2)Taken together,the sedimentological,rock magnetic,and paleomagnetic results suggest that the Chuangde ChRMs likely represent primary remanence that was acquired during 86.3-74.0 Ma,which was constrained based on both magnetostratigraphy and foraminera data.The mean of the ChRMs of specimens from the Chuangde Fm is Ds=152.0°,Is=-52.9°,?=18.0,?95=5.9°after tilt correction.The results indicate that Tethyan Himalaya was located at 33.1°±5.6°S in Late Cretaceous(80.1±6.2 Ma).(3)The paleolatitude of Tethyan Himalaya is similar to the coeval paleolatitude of India(reference site:28.00 N,88.50 E),suggesting that Tethyan Himalaya and India were not separated by oceans,but likely formed a single crustal block in the Campanian.Therefore,the paleomagnetic data obtained from the Chuangde Formation do not support the dual-collision model.(4)The paleomagnetic data from Chuangde Fm provide new constrains on the collision between India craton and Asia continent.Together other geological constraints,the results of this study support the traditional model,that is,the collision between Tethyan Himalaya and Lhasa block represents the India-Asia collision.