Growth of the Huoerguosi anticline (north Tianshan Mountains) by limb rotation since the late Miocene

Abstract

The Huoerguosi anticline, located in the north Tianshan Mountains piedmont fold-and-thrust belt, is a trending east-west fault-related fold. In the cross section along the Jingou River, its south limb is composed of the pre-growth strata of the Anjihaihe (E2-3 a), the Shawan ((E3-N 1)s), the Taxihe (N1 t) and the lower part of the Dushanzi (N2 d) Formations, trending east-west and dipping to south 55°, and the growth strata of the upper part of the Dushanzi (N2 d) and Xiyu ((N2-Q1)x) Formations, dips of which decrease from 55° at the base of the growth strata to 47° at the bottom of the Xiyu ((N2-Q1)x) Formation to ∼0° at the top of the Xiyu ((N2-Q1)x) Formation. The strata at the north limb of the anticline are vertical or over-turned, and are cut by the breakthrough thrusts to result in the drag fold. In the depth, the anticline is symmetric, and its core comprises the Cretaceous and the Jurassic coal-bearing beds. In the seismic profile, the seismic reflectors of pre-growth strata at the south limb of the anticline dip to south constantly, and ones of the growth strata fan southward, whose dips decrease upward. The geometry of the south limb growth strata outcropped along the Jingou River valley and the deep structure of the anticline shown in the seismic profile indicate that the Huoerguosi anticline is a detachment fold anticline growing by limb rotation. Based on the growth model and magnetostratigraphic age, during the growing process of the Huoerguosi anticline, the average shortening rate absorbed by the south limb is ∼0.46 mm/a, and the average uplifting rate of the anticline is ∼0.86 mm/a which exceeds the average deposition rate, which is in accordance with the fact that the top of the anticline is intensely eroded. Considering symmetric geometry of the Huoerguosi anticline and ignoring the breakthrough thrusts, the shortening of the whole anticline should be more than ∼0.92 mm/a, doubling the shortening rate determined from the growth at the south limb. © 2008 Science in China Press and Springer-Verlag GmbH.