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

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.