Haopeng Geng

Late Quaternary glacial chronology on the eastern slope of Gongga Mountain, eastern Tibetan Plateau, China

The Gongga Mountain is the largest area of modern glaciation in the Hengduan Mountains and, with a summit elevation of 7556 m, is the highest mountain on the eastern margin of the Tibetan Plateau. During the Quaternary glacial-interglacial cycles the Gongga Mountain was extensively and repeatedly glaciated, and glacial landforms and outwash deposits from multiple glaciations are well-preserved in valleys, in basins, and on piedmonts. To constrain the glacial chronology of the eastern slope of Gongga Mountain, sample sites were selected based on the distribution and weathering of glacial tills, relationships among glacial deposits, and soil development on moraines. Dating of the tills and glaciofluvial deposits was undertaken with electron spin resonance (ESR) and optically stimulated luminescence (OSL). The ages of the glacial deposits can be divided into four clusters: 2.2±0.5, 11.9±0.6, 35.9±2.7-58.0±6.3 and 119.2±15.9-194.2±32.8 ka. Five glacial advances in this region have been identified, which are equivalent in age to the Little Ice Age (LIA), Neoglaciation, marine oxygen isotope stage (MIS) 2, mid-MIS3, and MIS6. The largest local last glacial maximum (LGML) occurred on Gongga Mountain during mid-MIS3, characterized by a cold-humid climate, rather than the global Last Glacial Maximum (LGMG) of MIS2. The Gongga, Nanmenguangou (NMGG) and Yajiageng Glaciations occurred during the late part of the last glacial cycle, the middle of the last glacial cycle and the penultimate glacial cycle, respectively. On the basis of geomorphological, sedimentological, and compositional characteristics, landforms of the Moxi Platform and terraces can be grouped by facies and geochronology. In combination with the dating results, this analysis indicates the basal part of the Moxi Platform between Xinxin and the Moxi Hotel is correlative with the till of the Nanguanmen Glaciation (mid-MIS3). This basal unit has occasional lenses of glaciofluvial sandy gravel and lacustrine sediments. The remainder of the Moxi Platform and the terraces beside the platform are glaciofluvial deposits occasionally mixed with debris flow deposits and range in age from MIS3 to Holocene.

Rates and kinematics of active shortening along the eastern Qilian Shan, China, inferred from deformed fluvial terraces

In the eastern Qilian Shan, a flight of fluvial terraces developed along the Jinta River valley are deformed across the Nanying anticline. Four individual fluvial terraces are preserved at different elevations above the river, and higher terrace treads are draped by systematically thicker aeolian loess. Optically stimulated luminescence dating of deposits at the base of the loess provides constraints on the timing of surface abandonment; terraces were abandoned at 69 +/- 4 ka B.P. (T4), 57 +/- 4 ka B.P. (T3), and between 34 +/- 3 ka B.P. (T2), respectively. Differential GPS measurement of the terrace profile across the anticline allows reconstruction of subsurface fault geometry; we model terrace deformation above a listric thrust fault with a tip line at 2.2 +/- 0.1 km depth and whose dip shallows systematically to 23 +/- 3 degrees at depth of 5.8 +/- 1.1 km. Combining terrace ages with this model of fault geometry, we estimate a shortening rate of 0.8 +/- 0.2 mm/a across the Nanying fold and a shortening rate of similar to 0.1 mm/a across the mountain front fault since similar to 70 ka B.P. This rate suggests that the frontal fault system along the eastern Qilian Shan accomplishes crustal shortening at rates of approximately 0.9 +/- 0.3 mm/a during late Pleistocene time.

Folded fluvial terraces in a young, actively deforming intramontane basin between the Yumu Shan and the Qilian Shan mountains, NE Tibet

Intramontane basins in actively deforming regions contain significant information about the evolution of orogenic belts. We explored the tectonic characteristics and evolution of an intramontane basin between the Qilian Shan and Yumu Shan mountains on the NE Tibetan Plateau. We utilized the deformation of fluvial terraces along the Dahe River to constrain the rate and pattern of Quaternary deformation across the basin. Fluvial landforms include a widespread Mid-Pleistocene alluvial-fluvial fan surface and five terrace levels inset below this surface. We dated deposits associated with these landforms by optically stimulated luminescence (OSL) analyses on eolian loess and fluvial sediments. Our results yield ages of 142.8 ± 11.3 ka (Fs), 103–125 ka (T5), 96–115 ka (T4), 87–104 ka (T3), and 13–15 ka (T2), which we interpret to reflect the abandonment of fluvial terrace surfaces. Elevation surveys indicate that the terrace surfaces are folded along the Dahe anticline and are gently tilted northward across the basin. Analysis of terrace deformation suggests that the Dahe anticline grew by limb rotation and accommodated upper-crustal shortening at a rate of 0.14 +0.14/–0.03 mm/a. We determined the onset of deformation to have occurred between 0.3 and 0.9 Ma, based on the rotation rate of the southern limb of the anticline. This age is significantly younger than the onset ages of the thrust faults along the Qilian Shan (ca. 10 Ma) and the Yumu Shan (ca. 3.7 Ma). The character of the terrace deformation suggests that the Dahe anticline was growing from a south-dipping decollement, which also induced surface tilting across the entire basin. The Late Quaternary activation of the Dahe anticline folding and the detachment slipping provide evidence that the deformation of the intramontane basin occurred much later than that of the surrounding mountain ranges and accommodated part of the crustal shortening through basin narrowing.