language: en

The Neogene Xiyu Formation, a diachronous prograding gravel wedge at front of the Tianshan: Climatic and tectonic implications

Julien Charreau a, b, , Charles Gumiaux c, , Jean-Philippe Avouac a, 1, , Romain Augier c, 2 , Yan Chen c, 2, , Laurie Barrier d, 3, , Stuart Gilder e , Stéphane Dominguez f, 4, , Nicolas Charles c, 2 and Qingchen Wang g,
a Tectonics Observatory, California Institute of Technology,
Mail Code 100-23, Pasadena CA 91125, USA
b Centre de Recherche Pétrographique et Géochimique,
15 rue Notre Dame des Pauvres, 54501 Vandoeuvre lès Nancy, France
c Université d'Orléans, CNRS/INSU, Université François Rabelais ? Tours, Institut des Sciences de la Terre d'Orléans ? UMR 6113,
Campus Géosciences, 1A, rue de la Férollerie, 45071 Orléans Cedex 2, France
d Institut de Physique du Globe de Paris,
4 place Jussieu, 75252 Paris Cedex 05 France
e Ludwig Maximilians University, Department of Earth and Environmental Sciences, Geophysics Section,
Theresienstrasse 41, 80333 Munich, Germany
f Laboratoire Dynamique de la Lithosphère; UMR CNRS/UMII 5573,
Batiment 22 [cc. 60] Place Bataillon, 34095 Montpellier Cedex, France
g State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences,
Beijing 100029, China

Abstract The Tarim and Junggar basins in central Asia are capped by a thick pile of conglomerates, called the Xiyu Formation, that are commonly linked to a change in climate and/or accelerated uplift near the Plio-Pleistocene boundary. In order to better understand their origin and significance, we carried out a combined structural and magnetostratigraphic study in the Quilitage syncline (southern Tianshan), where the base of the Xiyu conglomerates is observed at both sides of the syncline. A balanced cross-section shows that, even at a local-scale, the base of the Xiyu conglomerates cannot be regarded as a single continuous stratigraphic layer. On the southern flank of the Quilitage syncline, we collected 172 samples collected for magnetostratigraphic dating identify 17 polarity chrons that date the new section from 5.2 to ~ 1.7 Ma and constrain the base of the Xiyu conglomerate here at ~ 1.7 Ma. This is 4.2 Ma younger than the age of the Xiyu previously found on the northern limb of the same syncline. Together with other magnetostratigraphic studies carried out around the Tianshan, our study unambiguously demonstrates that the onset of deposition of the Xiyu conglomerates is diachronous, and that the conglomerates are systematically younger toward the basin. Consequently, the Xiyu Formation should not be considered as a chronostratigraphic marker related to any particular tectonic or climatic event, but is instead a prograding gravel wedge that has prograded over the underthrusting forelands. A synthesis of chronologic and structural results yields progradation rates over the last 10 Ma on the order of ~ 2.0 mm/yr and ~ 3.9 mm/yr south and north of the Tianshan Mountains respectively. These rates are comparable to the shortening rate across the Tianshan range, suggesting that underthrusting is the main factor governing the progradation rate of the Xiyu Formation.

Keywords: Xiyu Formation; magnetostratigraphy; Tianshan; gravel wedge progradation; shortening rates

1. Introduction