Carboniferous palaeomagnetism of the Rocky Creek Block, northern Tamworth Belt, and the New England pole path

Abstract

Palaeomagnetic, rock magnetic and magnetic fabric results are presented for a Carboniferous (Visean to Westphalian) succession of felsic, mainly ignimbritic, volcanic and volcaniclastic rocks from the Rocky Creek Block of the northern Tamworth Belt, southern New England Orogen. Detailed thermal demagnetisation of 734 samples from 64 sites show three groups of magnetic components with low (<300°C), intermediate (300-600°C) and high (500-680°C) unblocking temperature ranges. Well-defined primary magnetisations have been determined for 28 sites with evidence of four overprint phases. The overprints arise from a mid-Tertiary weathering event (or possibly recent viscous origin), and from fluid movements associated with the Late Cretaceous opening of the Tasman Sea, thrusting during the Middle Triassic main phase of the Hunter-Bowen Orogeny, and latest Carboniferous - Early Permian formation of the Bowen-Gunnedah-Sydney Basin system. Rock magnetic tests establish that the primary magnetisation carriers in the volcanic rocks are mainly magnetite (predominantly single domain, or pseudo-single domain, and little or no multidomain) and hematite. Optimal magnetic cleaning is achieved at high to very high temperatures, with subtle, but systematic, directional and statistical differences between primary components derived from the mainly hematite fraction and pseudo-components derived from the mainly magnetite fraction. The 28 primary magnetisation results are presented as six mean-site results, summarised below and representing 25 sites, and three single-site results. Fold tests could be applied to five mean-site results. These are all positive, but one of these results may represent a secondary magnetisation. The primary magnetisation results define a Visean to Westphalian pole path. This long pole path indicates extensive latitudinal and rotational movement for the Rocky Creek Block, and potentially for the New England Orogen, as follows: (i) Yuendoo Rhyolite Member (Caroda Formation, Visean) pole 235.8°E, 27.7°S, ED95=9.0°, n=3; (ii) Peri Rhyolite Member/Boomi Rhyolite Member (Clifden Formation, Namurian, 318.0± 3.4 Ma) pole 177.4°E, 63.4°S, ED95=5.2°, n=3; (iii) tuffaceous beds above Boomi Rhyolite Member (Clifden Formation?, Namurian) pole 162.2°E, 59.1°S, ED95= 10.2°, n=3; (iv) upper Clifden Formation/lower Rocky Creek Conglomerate (Namurian/Westphalian) pole 95.3°E, 49.6°S, ED95=8.1°, n=3 (possible overprint)]; (v) Rocky Creek Conglomerate (Westphalian) pole 136.5°E, 57.6°S, ED95 =5.3°, n=5; (vi) Lark Hill Formation (Westphalian) pole 127.0°E, 50.4°S, ED95=4.8°, n=8. Show more