Permo‐Triassic Palaeomagnetism In the Emei Mountain Region, Southwest China

Abstract

A palaeomagnetic study has been made of samples collected from Permo‐Triassic type sections in the Emei Mountain region of southwest China. No results were obtained from the Lower Permian Qixia Formation limestones or the (Lower)‐Upper Permian Maokou Formation limestones. the Upper Permian (Longtanian) Emeishan Basalt Formation shows excellent stability to both alternating field and thermal demagnetization. Results from 24 flows (125 samples) are all of normal polarity, have a positive fold test (Cretaceous or Tertiary folding), and give a pole position at 54.5°N, 254.3°E with dp= 1.8°, dm =3.5°. the basalts were thus presumably extruded after the end of the Kiaman Reversed Superchron. the overlying Upper Permian Xuanwei Formation of red mudstones and sandstones (Changxingian in age) also shows a positive fold test from N = 12 samples after thermal treatment and has a positive reversals test at level C. the pole position is close to that of the Emeishan Basalt and lies at 57.0°N, 260.9°E with dp=4.3°, dm = 8.5°. the Lower Triassic (Griesbachian to mid‐Nammalian) Feixianguan Formation is primarily a red sandstone sequence at this locality. Thermal treatment of the collection of 114 samples reveals two main components. the A component is always observed below 600 °C and probably represents a DRM either from magnetite or specular haematite. the B component resides in haematite and is often of the opposite polarity superimposed on the A component. It is revealed only above 600 °C and is a CRM acquired during diagenesis. Although normal components dominate both, the A and B components also have reverse polarities. the normal A and normal B components are statistically indistinguishable and the normal B component shows a positive fold test. the reverse B component is, however, not quite antiparallel to the normal B component and may therefore be a second CRM acquired at a slightly different time to the normal B component. All these components are thought to have arisen during deposition and diagenesis over a few million years as has been observed in other studies of redbeds. the dominance of normal components is consistent with the observation that the earlier part of the Griesbachian Stage of the Lower Triassic is mainly a normal polarity chron. the overall mean of the N = 101 both normal and reverse A and B components is considered to be the best estimate of the palaeofield in the Early Triassic giving a pole position at 56.2°N, 227.6°E with dp = 1.7°, dm = 3.4°. This pole lies 15.2° away from the Emeishan Basalt pole suggesting a polar shift occurred near the Permo‐Triassic boundary. A review of Late Permian and Early Triassic poles from the South China Block shows a similar polar shift. Copyright © 1993, Wiley Blackwell. All rights reserved