Late Cretaceous-early Eocene Tectonic Evolution Of The Southern Tibetan Plateau: Key Geological Reords From Linzhou Basin

The tectonic evolution of the southern Lhasa terrane during the Late Cretaceous-Early Eocene is defined by three tectonic events: the transition from marine to terrestrial sedimentation,the strong deformation of pre-Mesozoic strata,and a lull between magmatic flare-ups.Accurate constraints of the ages of the tectonic events are crucial for recognizing how the southern Tibetan Plateau evolved from a marine environment to the present “roof of the world”.The Linzhou basin,located ~30 km north of Lhasa,is an ideal location to determine the timing of the three tectonic events.The marginal-marine Takena Formation is unconformably overlain by the strongly deformed Shexing Formation which is in turn overlain by Late Cretaceous-early Tertiary volcanic strata of the Linzizong Formation.We measured six volcanic-bearing sedimentary sections in the Linzhou basin,making detailed depositional descriptions and collecting samples of interlayered tuffaceous and volcanic-sedimentary strata.The upper Takena Formation was deposited in a marginal-marine environment,and the uppermost sandstone has a maximum depositional age of 95 Ma.The Shexing Formation is a suite of lacustrine-fluvial deposits which can be divided into two members: the meandering-lacustrine deposits in the lower member and braided river deposits in the upper member.Zircon U-Pb ages from tuffaceous layers in the lower and upper members constrain deposition between 83 Ma and 78 Ma.Thus we accurately constrain the transition from marine to terrestrial at 95 Ma and the unconformity between the Takena and Shexing Formations between 91 and 83 Ma.Undeformed,~72 Ma diorite porphyritic dikes cut the strongly deformed Shexing Formation.Late Cretaceous volcanic rocks with zircon U-Pb age of 70 Ma unconformably overly the Shexing Formation,further constraining the timing of deformation between ~78 Ma and ~72 Ma.The Linzizong volcanic rocks consist of~3500 m of volcanic-dominated strata that are divided into four units from the base to the top.The Paleocene Dianzhong Formation(E1d)is chiefly composed of basaltic and andesitic lava flows;the Eocene Nianbo Formation(E2n)consists of fluvial sandstones interbedded with minor lacustrine limestones and tuffs,cut by mafic dyke swarms;the lower Eocene Pana Formation(E2p1)is mostly composed of rhyolitic andesites and andesitic ignimbrites with columnar joints;and the upper Eocene Pana Formation(E2p2)is mostly composed of fluvial deposits and several interbedded andesitic ignimbrites.After the stratigraphic and petrological study,we conductedsystematic U-Pb zircon dating on 18 selected samples(one sandstone and the rest volcanic rocks)taken from the Dianzhong and Qiangga sections crossing the Linzizong volcanic rocks exposed in the Linzhou basin.The U-Pb dating yielded an age range of ca.66 to 47 Ma for the Linzizong volcanic rocks.The Dianzhong Formation(E1d),Nianbo Formation(E2n),lower Pana Formation(E2p1),and upper Pana Formation(E2p2)were formed during ca.66-59 Ma,ca.56-54 Ma,ca.54-50 Ma,and ca.49-47 Ma,respectively.There is a ca.3 Ma magmatic gap between the Dianzhong Formation and the Nianbo Formation and an uninterrupted volcanic-sedimentary succession from the Nianbo Formation to the upper Pana Formation.The Linzhou basin was a retro-arc foreland basin during the late Cretaceous,when continuous Gangdese arc uplift provided the main sediment source.As the subduction angle of the Neotethyan slab increased,the regional stress field transitioned from compressional to extensional,causing the Linzhou area to transform from a retro-arc foreland basin into an arc and intramontane basin.