The Ailao Shan–Red River shear zone revisited: Timing and tectonic implications

Abstract

Continental strike-slip shear zones that may bear important information about the evolution of convergent tectonics often occur to accommodate plate convergence. When and how shearing along the shear zones responds to plate interactions, however, are often debated. In this study, we investigated the Oligocene–Miocene leucocratic dikes from the Ailao Shan–Red River shear zone, which was active during India-Eurasia plate convergence, to constrain the timing and mechanism of ductile shearing along the shear zone. The dikes are structurally grouped into pre-, syn-, and postkinematic types with respect to ductile shearing. Prekinematic dikes from ca. 41 to 30 Ma have low whole-rock 87Sr/86Sr(i) values (0.707–0.710), generally high εNd(t) values (–3.31∼–7.98), and variable εHf(t) values (–7.9∼+5.7). Their magma sources involved high thermal perturbation inducing partial melting of the lower crust, and contributions from the mantle that were possibly related to extensional collapse of the orogenic belt prior to tectonic extrusion of the Sundaland block. Syn- and postkinematic dikes from ca. 28 to 20 Ma dominantly have high whole-rock 87Sr/86Sr(i) (0.707–0.725) and low εNd(t) (–5.83 to –9.76) values, and either negative or positive zircon εHf(t) values (broadly in the range of –12 to + 7.6) for coeval but separate crustal magma sources. The results imply that major shearing accompanying retrograde metamorphism along the Ailao Shan–Red River shear zone was localized to crustal level. A synthesis of regional structural data suggests that Oligocene–Miocene shearing along the Ailao Shan–Red River shear zone and lateral tectonic extrusion of the Sundaland block proceeded in response to progressive India-Eurasia plate convergence. Distributed and inhomogeneous middle- to lower-crustal flow along the boundaries of and within the Sundaland block occurred during the tectonic extrusion.

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

“…Field observations support this hypothesis. Migmatites exposed along the ASRRSZ exhibit pervasive alternating layers of leucosomes and melanosomes, and gneisses show well‐defined mineralogical banding; various volumes of leucocratic dikes/sills/veins cut across or are concordant with foliation (e.g., Leloup et al, 1995, 2001; Liu et al, 2012, 2020; Searle et al, 2010; Zhang et al, 2014, 2017). All these indicate high‐temperature and substantial melt production at a deep structural level, which possibly occurred since the Eocene in response to crustal shortening (Figure 8b).…”

“…Ages of pre‐, syn‐, and post‐shearing dykes or leucogranite veins indicate that sinistral ductile shearing along the ASRRSZ may have continued from ~33 to ~20 Ma (e.g., Cao, Liu, et al, 2011; Deng et al, 2014; Liu et al, 2020; Searle et al, 2010; Zhang et al, 2014). Argon data suggest a later cessation at ~17 Ma (Leloup et al, 2001 and references therein).…”

“…It shows similar lithological and structural features with the DCSZ and XLSZ: Medium‐ to high‐grade metamorphic rocks are exposed along strike, superimposed upon by mylonite and schist; pervasive foliation, subhorizontal lineation, and other sinistral ductile shearing fabrics are well observed (e.g., Leloup et al, 1995; Liu et al, 2012; Zhang et al, 2006). Based on the ages of pre‐, syn‐, and post‐shearing dykes, the duration of left‐lateral shearing is constrained from ~30 Ma to ~20 Ma (e.g., Liu et al, 2020; Searle et al, 2010). 40 Ar/ 39 Ar dating of amphibole, biotite, and K‐feldspar indicates that the shear zone rocks were exhumed rapidly to various depth from 29 Ma to 17 Ma (e.g., Harrison et al, 1992, 1996; Leloup et al, 1995, 2001; Liu et al, 2013).…”

“…The ASRRSZ consists of, from NW to SE, the Xuelongshan, Diancangshan, Ailaoshan, and DayNuiConVoi metamorphic massifs, extending more than 1,000 km from SE Tibet to the South China Sea (Figures 1a and 1b; e.g., Deng et al, 2014; Leloup et al, 1995, 2001). Exhumed remnants of once partially molten mid‐lower crust are preserved within the ASRRSZ, revealed by extensively exposed leucogranite dike networks, granitoid, gneiss, and migmatite (e.g., Liu et al, 2020; Searle et al, 2010; Zhang et al, 2014, 2017). They are superimposed upon by mylonite and schist with steeply dipping foliation, subhorizontal lineation, and other deformation fabrics related to ductile strike‐slip shearing (e.g., Cao et al, 2011; Leloup et al, 1995, 2001; Liu et al, 2012; Zhang et al, 2006).…”

“…Extensive chronologic data, including U‐Pb and 40 Ar‐ 39 Ar analyses, have been produced over the years in order to constrain the timing of left‐lateral shearing (e.g., Cao et al, 2011; Cao, Neubauer, et al, 2011; Leloup et al, 2001, and references therein; Liu et al, 2012, 2020; Tapponnier et al, 1990; Zhang et al, 2006, 2014, 2017). 40 Ar‐ 39 Ar data, which record cooling through ~500°C to ~250°C, show a broad range of ages from the late Oligocene to early Pliocene along the ASRRSZ (e.g., Cao, Neubauer, et al, 2011; Chen et al, 2015; Harrison et al, 1992, 1996; Leloup et al, 1995, 2001; Li et al, 2001; Liu et al, 2013; Zhang et al, 2017).…”