The Day Nui Con Voi mylonitic belt in Southwestern China and Its implications for the early Cenozoic extrusion of Indochina

Abstract

The early Cenozoic India-Asia collision resulted in the extrusion of large crustal fragments southeast from the Eastern Himalayan syntaxis, with large shear zones at their boundaries that could have accommodated displacements of hundreds to perhaps a thousand kilometers. Along the northeastern edge of the Indochina extruded fragment, the belt of mylonitic metamorphic rocks generally referred to as the Ailao Shan/Red River shear zone forms the extrusion boundary. This shear zone actually consists of at least two belts, the Ailao Shan and the Day Nui Con Voi, which are separated by a narrow belt of unmetamorphosed Triassic sedimentary rocks. In the Chinese extension of the Day Nui Con Voi, the presence of sillimanite and garnet indicates the shear zone formed at amphibolite grade, and the mylonitic fabric defined by muscovite and biotite indicate left-lateral shearing. Ar/Ar cooling ages indicate the metamorphic rocks reached the cooling temperature of muscovite and biotite 26.07 ± 0.20 to 32.46 ± 0.25 Ma, ages that match those in the Day Nui Con Voi in north Vietnam. These data come from both the core orthogneiss of the shear zone as well as a narrow carapace of metasedimentary rocks of unknown age. Both rock units form an antiform in southern China that plunges below Triassic sedimentary rocks of South China. These relations show that: 1) the Day Nui Con Voi in China is the direct continuation of the same belt in north Vietnam, 2) the Day Nui Con Voi does not directly connect with the Ailao Shan shear zone, 3) the Day Nui Con Voi shear zone has a structural (?) cover of South China Mesozoic sedimentary rocks, 4) structural relations limit the amount of late stage left-lateral shear on the Indochina boundary, and 5) the structural relations require a more complex history for the shear zone along the NE boundary of the extruded Indochina crustal fragment than proposed by all earlier workers.