Geometry and field relations disclose the emplacement dynamics of the SW Sinai Dyke Swarms (Egypt)

Abstract The first results on the structural attributes and emplacement mechanics of mafic and felsic dyke swarms exposed in the Precambrian basement in SW Sinai (Egypt) are presented. These results are based on data analysis of remote sensing, dyke geometries and relationships, and trace element chemistry available from literature. A total of 2376 dyke segments (2056 mafic and 320 felsic) formed parallel (N25°E-oriented mafic and N35°E-oriented felsic) and N35°W-oriented radial dyke swarm (eastern and western) sets. The dykes have a variety of attributes (length, thickness, and aspect ratio), inferred magma chamber overpressures, and depths, indicating different sources of origin. The average magma overpressures are estimated at 14, 29, 14, and 13 MPa, at depths of 20, 23, 17, and 5 km, for the parallel mafic swarm, parallel felsic swarm, the eastern, and western occurrences of the radial dyke swarms, respectively. The mafic dykes were fed by deep-seated basaltic magma reservoirs or chambers located at the crust-mantle boundary or in the lower crust, close to the Moho (20–25 km) and the Conrad (15–18 km) discontinuity in the area. The felsic dykes propagated from shallow-level, 5 km deep, rhyolitic chamber(s) in the upper crust. Three extensional episodes of dyke emplacement were recognized. The first episode is marked by a regional maximum tensile stress field (σ3) which is responsible for the formation of the parallel mafic swarm that strikes N25°E. Felsic dykes oriented N35°E were affected by the same stress field oriented N55°W flipped 10° clockwise from the preceding field emplaced coevally or shortly after the dykes of the first episode. Later, N35°W-trending mafic dykes were radially emplaced affected by local stress field evolved from volcanic edifices developed from two successive phases of local minimal stress fields σ3. The parallel set of dykes show calc-alkaline affinities with negative Nb Ta anomalies; typical of basaltic and rhyolitic magma for the mafic and felsic dykes. These were emplaced at active continental margins in a subduction-related setting mostly parallel to the plate boundaries at the time of their emplacement. The radial swarms have a transitional to mildly alkaline chemistry, without negative Nb Ta anomalies consistent with a continental tectonic setting of the source region. Insights concluded enabled comparisons to worldwide dyke swarms.