Cenozoic Exhumation of the Ailaoshan‐Red River Shear Zone: New Insights From Low‐Temperature Thermochronology

Abstract

Continental-scale shear zones play an important role in accommodating block extrusion and rotation as shown by deformation on the Ailaoshan-Red River shear zone (ASRRSZ) in the SE Tibetan Plateau. This study presents 13 apatite (U-Th)/He, 11 zircon (U-Th)/He, and three apatite fission track dates, together with thermal modeling in the Ailaoshan and Xuelongshan segments (ALSZ and XLSZ) of the shear zone to investigate its Cenozoic exhumation history and mechanism, which are critical for understanding its tectonic and landscape evolution. Our results, combined with published chronologic data, reveal that shear zone rocks along the ALSZ experienced prominent and rapid cooling from high temperature (>500°C) to 120-60°C at a rate of 75-100°C/Myr during 29-17 Ma with northwestward younging onset. A second, lower magnitude accelerated cooling occurred at 14-10 Ma along the ALSZ at a rate of 20-30°C/Myr, with a later initiation on the XLSZ at~5 Ma and continuing to present with a cooling rate of~20°C/Myr. Thermal modeling reveals a single rapid cooling phase with a rate of 17-14°C/Myr in the Eocene to early Oligocene for samples outside the shear zones. These three fast cooling episodes are directly related to deformation stages including crustal shortening across the SE plateau, sinistral ductile shearing along the ASRRSZ, and dextral faulting with a dip-slip component on the Red River and Weixi-Qiaohou faults along the shear zone flanks. Furthermore, the northward migration of kinematic reversal and associated cooling along strike since the mid-late Miocene likely reflects the northward advance of the eastern Himalayan syntaxis.

Diffuse Deformation in the SE Tibetan Plateau: New Insights From Geodetic Observations

“…These shear zones extend hundreds to over a thousand kilometers, separate crustal (sub‐) blocks, and accommodate hundreds of kilometers of displacement from Cenozoic (sub‐) block extrusion and rotation (Figure 1a; e.g., Akciz et al, 2008; Deng et al, 2014; Leloup et al, 1995, 2001; Liu et al, 2012; Zhang et al, 2010). High‐grade metamorphic rocks, including gneiss and migmatite, and highly deformed mylonite and schist are exhumed to the surface from a deep structural level, indicating significant exhumation as the result of oblique displacement (e.g., Leloup et al, 1995, 2001; Wang et al, 2020). However, these tectonites are barely observed in small‐scale faults, leading to the hypothesis that the strain may have mostly been localized on large‐scale tectonic boundaries during the late Oligocene to mid‐Miocene.…”

“…Therefore, dextral shear stresses likely dominate the whole SE Tibetan Plateau, leading to right‐lateral movements along these NW/NNW‐striking faults. Sinistral motion along the Xianshuihe‐Xiaojiang fault system and dextral motion along the Red River fault both initiated at ~14–10 Ma (e.g., Leloup et al, 2001; Li et al, 2015; Roger et al, 1995; Wang et al, 2016, 2020). Low‐temperature thermochronologic and paleoaltimetric studies suggest the Chuandian block may have experienced surface uplift since the mid‐Miocene (e.g., Clark et al, 2005; Li et al, 2015; Wang et al, 2016, and references therein).…”

Eocene–Oligocene Crustal Thickening-Collapse of the Eastern Tibetan Plateau: Evidence from the Potassic Granitoids in SW China

“…However, the potassic igneous rocks in these areas are not spatially restricted to these shear zones or adjacent areas. In addition, sinistral shearing along the ASRR shear zone occurred mainly during 32-20 Ma or even as late as 17 Ma, which is significantly younger than the timing of formation of the potassic igneous rocks (e.g., [49,97,[99][100][101][102][103]). This indicates that any block extrusion processes cannot be responsible for the formation of the potassic igneous rocks in this area.…”

“…Numerous geological and geophysical observations indicate that lithospheric shortening and crustal thickening in the eastern Tibetan Plateau are the result of continuous continental convergence since the Paleogene (e.g., [11,109]). Numerous Eocene to early Oligocene NW-NE-to NNW-SSE-trending anticline-syncline pairs are present in the Gaoligong, Diancangshan, Xuelongshan, Lanping-Simao, and Chuxiong areas (Figure 1(c); [103,110]). The NNW-SSE-trending Ludian-Zhonghejiang fold and thrust belt between Xuelong and Diancang Shan might also have formed at 50-39 Ma, as evidenced by the timing of deposition of the Baoxiangsi sedimentary sequence in the Jianchuan basin (e.g., [111]).…”

Existence of a continental-scale river system in eastern Tibet during the late Cretaceous–early Palaeogene

“…20,61 ). Blue circles correspond to AFT and AHe thermochronological ages of ~50-100 Ma from low-relief plateau areas 24,[31][32][33][34]36,67 . Hexagons indicate sites with late Cretaceous-early Palaeogene cooling histories (shown in Fig.…”