Exhumation of the ultrahigh‐pressure continental crust in east central China: Cretaceous and Cenozoic unroofing and the Tan‐Lu fault

Abstract

The orogenic architecture of the world's largest ultrahigh‐pressure exposure, the Hong'an‐Dabie Mountains of the Triassic Qinling‐Dabie orogenic belt, is dominated by Cretaceous and Cenozoic structures that contributed to its exhumation from ≤30 km depth. Cretaceous magmatic crustal recycling (≥50% for the entire Dabie) and heating (>250° to >700°C) were most prominent in Dabie, and exhumation, magmatism, and cooling were all controlled by Cretaceous transtension. Exhumation was accomplished principally by an asymmetric Cordilleran‐type extensional complex in the northern Dabie (Northern Orthogneiss unit) between 140 and 120 Ma, at rates as fast as 2 mm/yr and average horizontal stretching rates of up to 6 mm/yr. Cretaceous reactivation occurred within a regional transtensional strain field as a result of far‐field collisions and Pacific subduction. The onset of crustal extension was preceded and possibly facilitated by a reheating of the Hong'an‐Dabie crust (∼140 Ma) coeval with the onset of voluminous magmatism in eastern China (∼145 Ma), which resulted from a change in Pacific subduction from highly oblique to orthogonal. The Tan‐Lu continental‐scale fault was a normal fault zone in the mid‐Cretaceous (∼110‐90 Ma) and underwent ≥5.4 km dip slip and ≥4 km throw in the Cenozoic. During the India‐Asia collision the Qinling‐Dabie belt acted as the structural discontinuity between the strike‐slip‐dominated escape tectonics south of the Qilian‐Qinling‐Dabie belt and the rifting‐dominated tectonism north of it. The most prominent Cretaceous and Cenozoic structures of the Hong'an‐Dabie, the Xiaotian‐Mozitang and the Jinzhai fault zones, respectively, reactivated major lithospheric structures of the Triassic orogen, i.e., the Huwan detachment zone and the suture.