Abstract
The investigation of deformation rates on a mountain piedmont can provide key information for improving our understanding of the overall dynamics of a mountain range. Here, we estimate the shortening rate absorbed by a Quaternary emergent detachment fold on the southeastern piedmont of the Tianshan (China). Our work is primarily based on new 10Be cosmogenic exposure dating of deformed alluvial surfaces. The method we have developed combines depth profiling with sampling of surface cobbles, thereby allowing exposure time, erosion rate and inheritance to be simultaneously constrained. The exposure ages of the uppermost uplifted alluvial surfaces are around 140±17 ka, 130±9 ka and 47±9 ka, from west to east. A terrace lying below the 140 ka surface is dated at 65±5 ka. The ages of the uplifted and folded alluvial surfaces were then combined with estimates of shortening obtained using two distinct methods: (1) the excess area method, where sedimentation rates, extracted from magnetostratigraphic studies, are used to determine the amount of sedimentation after the abandonment of the river; and (2) a folding model derived from sandbox experiments. The late Pleistocene shortening rates are shown to be between 0.4±0.1 mm/yr and 0.8±0.5 mm/yr on the western part of the fold and 2.1±0.4 mm/yr along its central part. The central part of the frontal Yakeng anticline therefore accommodates up to 25% of the total shortening currently absorbed across the whole Eastern Tianshan range (8 mm/yr). However, this situation seems to have prevailed for only the last 150 ka, as the shortening rate absorbed by this nascent fold was previously ten times slower. While the initiation of folding of the Yakeng anticline can be traced back to 5.5 Ma ago, the basinward migration of the active deformation front onto the Yakeng fold is a relatively recent phenomenon and appears to be diachronous from west to east, probably in relation to the tectonic activity of the folds in the hinterland.
| Original language | English |
|---|---|
| Pages (from-to) | 333-348 |
| Number of pages | 16 |
| Journal | Earth and Planetary Science Letters |
| Volume | 434 |
| DOIs | |
| Publication status | Published (in print/issue) - 15 Jan 2016 |
Funding
We propose that the irregularities of the northern limit of the emerged fold are due not only to the differences in sedimentation rate ( Hubert-Ferrari et al., 2007 ), but also to the initiation of a back-thrust in the central part of the fold ( i.e. between the Yaha and the Erbatai River, see Fig. 1 ). This hypothesis is supported by the asymmetry of the limbs at this location. The topographic profile along the Yaha River displays a shorter and steeper northern limb ( Fig. 3c and 3d ), and the topographic map reveals a northward shift in the position of the crest line ( Fig. 1 ). This study was financed by the French INSU, CNRS SYSTER program. We are grateful to Robert Joussemet (STEVAL mineral processing pilot plant, LEM, Nancy) for his help in separating quartz and to the ASTER team for the cosmogenic nuclide concentration measurements (M. Arnold, G. Aumaître, K. Keddadouche, L. Léanni and F. Chauvet). The ASTER national AMS facility (CEREGE, Aix en Provence) is supported by the INSU, CNRS , the French Ministry of Research and Higher Education , IRD and CEA . We also thank two anonymous reviewers for their careful reading and contribution to the improvement of this manuscript. This is CRPG contribution n° 2415.
Keywords
- Active folding
- Foreland basin
- Shortening rates
- Terrestrial cosmogenic nuclide
- Tianshan