REGIONAL MAGMA FLOW IN SILLS OF THE KAROO LIP, SOUTH AFRICA

Determining magma flow direction is a first order question in igneous systems. This is particularly true for Large Igneous Provinces (LIPs), such as the Karoo province, where immense volumes of mafic magma have been transferred from the mantle towards the Earth’s surface over relatively short periods of time. The architecture of LIPs and their sill and dike feeder systems is becoming better understood through seismic reflection in sedimentary basins, combined with numerical modelling, field-based petrologic and structural studies, such as those in the Dry Valleys of Antarctica. In undeformed areas, the lower part of LIPs is characterized by interconnected vertical dikes and horizontal sills. The upper part of LIPs is typically represented by flood basalts, mafic volcaniclastic deposits, and hydrothermal vents. Sills are interpreted as staging chambers of volcanic edifices; dikes are thought to be feeders to both sills and volcanoes. Here we focus on magma dynamics in sills of the Karoo LIP.

Magnetic fabrics, such as the anisotropy of magnetic susceptibility (AMS), provide an effective and accurate mean to determine magma flow direction in mafic intrusive rocks. A previous study had shown that the AMS fabric is a reliable proxy for magma flow as long as samples are collected from the upper chilled margin of a sill. The central part of sills is more complex due to interference caused by thermal convection and inverse magnetic fabrics. Oriented core samples were collected from 30 sills and yielded 1598 specimens for AMS measurements. The low-field magnetic susceptibility Km ranges widely from about 100 to 20,000 microSI, while the degree of anisotropy P' ranges from 1.01 to 1.10. Thermomagnetic experiments reveal that the main magnetic carrier is titanomagnetite with variable ulvöspinel content and pseudo-single domain grain size.

Our results show that magma flow followed a main NW-SE direction in the Karoo Basin. The AMS axes are consistent with the nearly horizontal attitude of the sill in 23 out of 30 sills, with subvertical K3 axes. In 5 out of 30 sills, K3 axes are subhorizontal, with scattered directional data and are considered anomalous AMS fabrics. K1 axes are systematically subhorizontal and mark the magma flow direction. This regional scale flow pattern indicates that the Karoo LIP maximum magma flow was not located under the Drakensberg basalts, which are the thickest part of the Karoo volcanic pile. Instead the center of magmatic activity was most likely located to the NW of the Karoo Basin, somewhere in what is today Namibia.