Magnetic fabric and paleomagnetism of the Middle Triassic siliciclastic rocks from the Nanpanjiang Basin, South China: Implications for sediment provenance and tectonic process

Abstract

A combined magnetic fabric and paleomagnetic study has been carried out on the siliciclastic rocks gathered from a stratigraphic cross-section through the Nanpanjiang Basin, South China, in an attempt to extract the paleoflow information preserved in and, thus, constrain the possible origins of these clastic rocks. The sediments used for this study were formed by sediment-gravity flows along the southern margin of the South China block in the Middle Triassic time (ca. 245-228 Ma). The results show a normal distribution of both low field magnetic susceptibility values and natural remanent magnetization intensities, which along with the monotonic detrital framework mode, mainly comprising quartz and lithic particles, may suggest a single provenance involved in deposition of these clastic deposits. Anisotropy of magnetic susceptibility (AMS) analysis acquires primarily the sedimentary magnetic fabrics, which, in this study, reveal paleoflow directions ranging from NNW to ENE with an overall mean orientation of NE. Demagnetization on a part of samples isolates a characteristic remanent component averaged at D = 44.8 degrees, I= 16.9 degrees, K = 9.7, alpha(95) = 6. 5 degrees, n = 55, corresponding to a paleolatitude N8.6 degrees and a clockwise rotation of ca. 45 degrees since the Middle Triassic for the studied cross-section. This mean direction passes fold tests and is consistent with the reference direction expected from the South China block at the 95% confidence level. Restoring this similar to 45 degrees declination renders an overall northward paleoflow, which, combined with other evidence, suggests a southern provenance for these sediments during deposition in the Middle Triassic time. In terms of the early Mesozoic plate framework of southeastern Asia, a tectonic scenario is proposed here, whereby the nearly N-S convergence of the Indochina and South China blocks and its related Indosinian orogeny in the Middle Triassic caused the formation of the Nanpanjiang foreland basin, which was filled by voluminous detritus shed from the uplifted orogenic belt on its southern side. (C) 2013 Elsevier Ltd. All rights reserved.Marginal Sea Opening Foundation [MSGL 12-06]; National Science Foundation for Young Scholars of China [41202149