Revealing the structure and dynamics of dyke swarms using 3D seismic reflection data

Abstract:

Swarms of dyke intrusions extending for 10’s to 1000’s of kilometres can rapidly transfer large volumes of magma through the crust. Due to their geometry and large scale, dyke swarm emplacement can drive crustal extension, thereby influencing plate tectonic processes on Earth and shaping other planetary bodies. Their unique morphology and short-lived emplacement history also means dyke swarms provide a record of syn-emplacement stress conditions and represent key spatial and temporal markers for palaeogeographic reconstruction. Understanding dyke swarm emplacement is therefore crucial to a wide range of Earth Science disciplines. Yet the 3D structure and large-scale dynamics of dyke swarms remains poorly understood. Here, we use an extensive suite of 2D and 3D seismic reflection data from the North Carnarvon Basin on the Gascoyne Margin, offshore NW Australia, to characterise the structure of a previously unidentified dyke swarm; we name this the Exmouth Dyke Swarm. We mapped 26, N-trending, up to 155 km long, regularly spaced dykes distributed across ~40,000 km². Seismic-stratigraphic correlations and borehole data indicate all dykes were emplaced within Triassic and older strata and have heights of >9 km. Overlying many dykes along their length are normal fault-bound graben and locally developed sub-vertical pipes, which extend up to and deform the Base Cretaceous unconformity (~148 Ma); we interpret these features as dyke-induced normal faults and pit craters, which implies dyking occurred in the latest Jurassic. Based on the presence of similar, potentially dyke-related normal faults and pits observed elsewhere in the North Carnarvon Basin, we suggest the Exmouth Dyke Swarm is >500 km long, ~200–300 km wide, and radially disposed along a 39° arc centred on the Cuvier Margin. Our work demonstrates seismic reflection data is a powerful tool for identifying dykes and dyke swarms in sedimentary basins and along continental margins. Such data therefore provide a unique opportunity to examine the 3D structure of natural dyke swarms, allowing us to test hypotheses concerning their emplacement and how dyking translates to surface deformation.