Variable magma flow in sills: Can a magma source be constrained?

It has been increasingly recognized that emplacement of mafic sills plays an important role in magma transport throughout the crust. However, further understanding of sill propagation and merging of flow within sill complexes in large igneous provinces is needed. This study focuses on outcrop along the KwaZulu-Natal North Coast (South Africa), where dolerite sills of the Karoo Large Igneous Province intruded shales, siltstones, and sandstones of the Permian-aged Vryheid Formation. These sills intruded at high crustal levels and are associated with significant vesiculation, host rock fluidization and brecciation. An integrated approach is used to examine sill morphology, intrusive relationships, and inherent structures in the sills in relation to the magma flow processes. Mesoscale magma flow indicators preserved in the sills include bridge structures, intrusive steps, deformed vesicles, and magma lobes. The predominantly prolate magnetic fabrics determined by anisotropy of magnetic susceptibility techniques indicate magma flow in the sills was oriented NW, NE-SW and SSE, and largely locally coaxial with field evidence. The mesoscale magma flow indicators developed from linkage between numerous sill segments and variation in the magma propagation direction within the plane of intrusion and show directional variation within (and between) sills. The local flow dynamics within each sill can be revealed in this way; however, the possible variation in flow in the intrusion must be considered when interpreting the gross magma flow direction of the entire sill network. The sills occur in close vertical proximity and are inferred to have intruded through lateral and upward magma flow within merging sill segments creating an interconnected sill network, fed from an eastward local magma source. Interpretation of magma flow in sills should be done with care, to avoid oversimplification regarding regional flow, and highly variable flow in sills is best reconciled at the field scale. (C) 2021 Elsevier B.V. All rights reserved.