The Longmenshan Tectonic Complex and adjacent tectonic units in the eastern margin of the Tibetan Plateau: A review

摘要

The Longmenshan Tectonic Complex (LSTC), along the eastern margin of the Tibetan Plateau, the site of devastating earthquakes such as the magnitude 8.0 (Wenchuan) earthquake on 12 May 2008, preserves an exceptionally complete history of the tectonic evolution of the Yangtze block and its relations to adjacent tectonic units. Due to sequential tectonic superposition and tectonic reactivation, the tectonic nature of the LSTC, and in particularly the older history, has been profoundly debated and many different tectonic models have been proposed. Herein we summarize the current understandings of the major tectonic events that have shaped this important tectonic complex, highlighting problems left to be solved by future work, including:(1) The nature and constraints for at least 6 regional tectonic events, i.e., building of the metamorphic basement (Art), the Columbia/Nuna supercontinent (Pt1t), the Rodinia supercontinent (Pt3t), the Paleozoic passive continental margin (Pzt), the Paleotethys orogeny (Mzt) and the Neotethys orogeny (Kzt); (2) Metamorphic basement exposures and their tectonic implications, including rock types and geochronological constraints for the Archean, Paleoproterozoic and Neoproterozoic basements; (3) Nature of the present LSTC and its affinity with adjacent tectonic units; (4) Consideration of the NE-striking Longmenshan thrust belt and arcuate-shape Yanyuan-Muli thrust belt as parts of a single tectonic feature; (5) Mountain-basin coupled systems recording past tectonic eposides. We draw the following conclusions and tectonic models based on published research combined with our own recent studies: (1) The well preserved Archean Yudongzi gneiss group in the LSTC has a genetic affinity with the Kongling group, and thus belongs to the Yangtze block; (2) The Paleoproterozoic Hejiayan group, juxtaposed adjacent to the Archean Yudongzi group, may represent a 2000–1800 Ma orogenic belt, which corresponds to the supercontinent Nuna/Columbia amalgamation event; (3) A Neoproterozoic trench-arc-basin system, which is reconstructed based on identification of a Neoproterozoic ophiolite complex, arc-type magmatic rock assemblages and volcaniclastic basinal deposits along the western margin of the Yangtze block and the LSTC, may represent the record of eastward subduction of the Neoproterozoic Mozambique oceanic lithosphere beneath the Yangtze block during the assembly of the Rodinia supercontinent; (4) A complete bidirectional Wilson cycle was reconstructed by the formation of the late Permian to the middle-late Triassic back-arc Ganze-Litang rift and ocean following the early Paleozoic Mianlue continental rift and ocean, and subsequent closure of the ocean basin by simultaneous bidirectional northward and southwestward subduction and later collision. This relatively uncommon bidirectional Wilson cycle might be attributed to the formation of the three-armed rift system in the eastern Paleotethys associated with the late Permian Eemeishan Large Igneous Province in the LSTC; (5) A three-stage tectonic sequence of, in-sequence imbricate thrust in the LSTC during India-Eurasian collisional orogeny at 55–15 Ma, extrusion from 15 to 5 Ma and the plateau uplift since ∼5 Ma resulting from lower crustal channel flow, is proposed for the formation of the present LSTC.