Rates and style of Cenozoic deformation around the Gonghe Basin, northeastern Tibetan Plateau

Abstract

The northeastern Tibetan Plateau constitutes a transitional region between the lowrelief physiographic plateau to the south and the high-relief ranges of the Qilian Shan to the north. Cenozoic deformation across this margin of the plateau is associated with localized growth of fault-cored mountain ranges and associated basins. Herein, we combine detailed structural analysis of the geometry of range-bounding faults and deformation of foreland basin strata with geomorphic and exhumational records of erosion in hangingwall ranges in order to investigate the magnitude, timing, and style of deformation along the two primary fault systems, the Qinghai Nan Shan and the Gonghe Nan Shan. Structural mapping shows that both ranges have developed above imbricate fans of listric thrust faults, which sole into décollements in the middle crust. Restoration of shortening along balanced cross sections suggests a minimum of 0.8-2.2 km and 5.1-6.9 km of shortening, respectively. Growth strata in the associated foreland basin record the onset of deformation on the two fault systems at ca. 6-10 Ma and ca. 7-10 Ma, respectively, and thus our analysis suggests late Cenozoic shortening rates of 0.2 +0.2/-0.1 km/m.y. and 0.7 +0.3/-0.2 km/m.y. along the north and south sides of Gonghe Basin. Along the Qinghai Nan Shan, these rates are similar to late Pleistocene slip rates of ~0.10 ± 0.04 mm/yr, derived from restoration and dating of a deformed alluvial-fan surface. Collectively, our results imply that deformation along both fl anks of the doubly vergent Qilian Shan-Nan Shan initiated by ca. 10 Ma and that subsequent shortening has been relatively steady since that time.

Testing models of Tibetan Plateau formation with Cenozoic shortening estimates across the Qilian Shan–Nan Shan thrust belt

“…Specifically, our results suggest that the Qilian Shan frontal thrust system has absorbed a minimum of 53% northsouth Cenozoic shortening, which was accommodated by the north-directed thrust faults that place Mesozoic and Paleozoic rocks over Cenozoic sediments. Our estimated strain magnitudes are comparable to the results of a seismic reflection study ~450 km to the east (Gao et al, 2013) but higher than those obtained within the Qilian Shan-Nan Shan from surface geologic mapping alone (~5%-30% strain) (e.g., Gaudemer et al, 1995;Meyer et al, 1998;Lease et al, 2012;Craddock et al, 2014) (Fig. 3).…”

“…This disparity may be due to either a heterogeneous strain distribution in northern Tibet or an artifact of the limitations of strain estimates that are calculated from observations of the surface geology alone. Estimates derived from restoring only Cenozoic strata are significantly lower (<15% strain) (e.g., Lease et al, 2012;Craddock et al, 2014) than those that incorporate subsurface data (>40% strain) (J. Yang et al, 2007aYang et al, , 2007bYin et al, 2008b;Gao et al, 2013;this study).…”

“…Zheng et al, 2009W.J. Zheng et al, , 2013Champagnac et al, 2010;Yuan et al, 2011;Craddock et al, 2014). The variability and uncertainty of these rates arise because the magnitude of Cenozoic fault offset and total shortening remains poorly constrained throughout most of the Qilian Shan-Nan Shan.…”

“…4D). Craddock et al (2014) presented 10 line-length balanced north-south cross sections across the Qinghai and Gonghe Nan Shan, south of Qinghai Lake (Fig. 3), that are restored by bringing Neogene strata to horizontal ( Fig.…”

Initial Deformation of the Northern Tibetan Plateau: Insights From Deposition of the Lulehe Formation in the Qaidam Basin

“…Using Google Earth software, we measured the average width of the present‐day southern Qilian Shan to be ~105 km (Figure b). Based on previous crustal shortening estimates in the Qilian Shan (Craddock et al, ; Gaudemer et al, ; Lease et al, ; Meyer et al, ; Wei et al, ; Yin, Dang, Wang, et al, ; Zuza et al, ; Zuza et al, ), we assigned a ~30% post‐Lulehe Fm. crustal shortening in the southern Qilian Shan and thus assumed that the average width of the southern Qilian Shan during the deposition of Lulehe Fm.…”