Paleomagnetism and geochronology of upper Eocene volcanic rocks from the western Qiangtang block: Constraints on the post-collisional shortening in western Tibet

摘要

Understanding the dynamics of the uplift of the Tibetan blocks requires constraints on the timing and magnitude of the crustal shortening accommodated by the different blocks. However, the estimates of the magnitude of post-collisional intra-Asian convergence range from >3000 km to a few hundred kilometers. Here we present new paleomagnetic and geochronological results from the Meisu Formation lavas on the southwestern margin of the western Qiangtang block, western Tibet. Zircon U/Pb data reveal that these volcanic rocks erupted during the late Eocene (-40 Ma). Following progressive thermal demagnetization, stable characteristic remanent magne-tizations (ChRMs) were successively isolated from 28 sites. These ChRMs passed the fold and reversal tests, consistent with a primary remanence. The paleopole at 50.2 degrees N, 163.2 degrees E with A95 = 5.9 degrees yields a paleolatitude of-29.5 +/- 5.9 degrees N at ca. 40 Ma for the southern margin of the western Qiangtang block (33.2 degrees N, 80.9 degrees E). A comparison of the Eocene latitudes between the western Qiangtang and Tarim block indicates a -700 km of post-40 Ma latitudinal crustal shortening between them at the longitude of-80 degrees E. Our review of published Cretaceous-Paleogene paleopoles from western Tibet suggests that the western Qiangtang block was positioned at a stable latitude during-116-30 Ma. Our paleomagnetic compilation also indicate a discrepancy in the latitude (-9.7 +/- 4.3 degrees) of western Lhasa and Qiangtang during the interval of 132 and 67 Ma, which vanished at ca. 30 Ma. We suggest a crustal shortening model to interpret this 1076 +/- 477 km discrepancy, which was accommodated by the interior of the Lhasa block, the Bangong-Nujiang suture zone, and/or an eastward extrusion of Indochina from an original position between the Lhasa and Qiangtang blocks during-67-30 Ma. We conclude that the crustal shortening caused by the ongoing India-Asia collision spread progressively northwards into Asia. We emphasize that reliable paleomagnetic data from Paleocene and lower Eocene units in the western Qiangtang and Lhasa blocks have the potential to further refine these crustal shortening estimates.