Reflection seismic imaging of the Lujiang–Zongyang volcanic basin, Yangtze Metallogenic Belt: An insight into the crustal structure and geodynamics of an ore district

Abstract

Abstract Five intersecting reflection seismic profiles were shot over the Lu–Zong volcanic basin, one of the seven ore districts in the middle and lower Yangtze River Metallogenic Belt in east China. The seismic data reveal diversified reflective patterns in the upper crust, which are interpreted as horsts and grabens in general. The middle crust presents both thrust related structures and boudinage and pinch-and-well structure on different profiles, suggesting a strong structural heterogeneity, which developed due to a change from compressional to extensional movement. Below the central part of a volcanic basin, the lower crust shows an obvious reflective anisotropy both at the crust–mantle and the brittle–ductile transition in the crust. This feature may be explained by magma system emplaced along a NE-trending fault or weak zone at different depths. The Tan-Lu fault zone is present as a northwest-dipping fault extending to the middle crust. The lower crust beneath the Tan-Lu fault and peripheries is possibly of Dabie affinities. The Lu–Zong volcanic basin is clearly defined as an asymmetric graben with master faults on both the east and north flanks. A contrasting reflection character suggests a division of the basin into northern and southern parts separated by a concealed fault. The relatively thin volcanic sequences, inferred to be 800–1000 m in thickness, cover Triassic and Paleozoic sequences at depth, that provide an exploration target in the northern part for deep porphyry and skarn deposits. The dominant deformation reflects NW–SE-oriented compression and extension formed during the Yanshanian orogenic episode. The contraction deformation is characterized by thrust faults with imbrication of crustal slices, whereas the extension deformation is featured by normal faults, regional detachments and basins. Underplating of mantle-derived magma and a multi-layer magma intrusion model are proposed to explain both seismic features and the NE-striking Lu–Zong volcanism and igneous intrusions in the region.