Ordos Block (Basin) is a key to understand early continental evolution and tectonic regime of the North China Craton

Abstract

<p indent="0mm">The Ordos Block is one of the key elements of the North China Craton (NCC), and the Precambrian compositions and structures record the early-stage evolution of the NCC, such as cratonization of early continental crust, Paleoproterozoic mobile belts, Meso- and Neoproterozoic rifting, and platform development. Many basic issues on the early-stage tectonic evolution bear upon the Ordos Block. The following are some new achievements in fundamental geology of the Ordos Block. (1) Drill cores reveal that the Ordos Block possesses Neoarchean-Paleoproterozoic metamorphic basement. The Neoarchean granitic gneisses and Paleoproterozoic pelitic gneisses and granites of the Ordos Block are quite similar in geochemistry and metamorphism to the Paleoproterozoic metamorphic complexes and migmatite of the central and northern portions of the NCC. (2) The proto-Ordos Basin is an intra-cratonic basin with old crystalline basements. Precambrian sedimentary successions are mostly made with the Changcheng and Jixian systems, with the Qingbaikou and Nahua systems being largely absent. The Sinian or Ediacaran strata are partially preserved. The Paleozoic sequence of the Ordos Block is consistent with that of the rest parts of the NCC. (3) A Greater Ordos Basin existed in the Late Triassic, which extended eastward to the Bohai Gulf and Xuhuai belt. Precambrian basement rocks of the Lüliang and Taihang mountains were exhumed due to the NW-SE crustal shortening in the Mid-Late Jurassic, leading to the disruption of the Greater Ordos Basin. The Ordos Basin in a narrow sense was formed largely since the Late Jurassic. (4) The Ordos Basin proper is now surrounded by major active faults. Precambrian metamorphic rocks, which outcrop discontinuously beside the major faults, are compatible well with their counterparts in the other parts of the NCC in many respects like geochronology, geochemistry, and metamorphism. (5) The Ordos Block was in an extensional setting during Cretaceous and Cenozoic times, as manifested by rift basins, such as the Early Cretaceous rift zone in the western Ordos Basin and the Tertiary Shanxi, Weihe and Yinchuan grabens. The Lüliang and Taihang mountains were further uplifted as a result of crustal stretching. Recent studies confirm that the Ordos Block and the other parts of the NCC share the identical metamorphic basement and Mesoproterozoic-Paleozoic sedimentary sequences. Therefore, reconstructing tectonic history of the NCC should be based on careful analysis and logical explanation of geologic data as well as the reevaluation of some long-held conceptions. There have remained a number of disputes on how the North China continent originated and evolved. The Ordos Block can serve as an archive for solving the crux of the problems.