Syn-rift margin architecture influenced by surficial loads

Passive margins, the transition zones between continents and oceans, document the history of continental extension and final breakup. Depending on abundant syn-rift mafic magmatism or not, two end members are categorized as either volcanic or volcanic-poor margins. Volcanic passive margins are filled with syn-extension thick piles of volcanic rock, visible in seismic images as seaward-dipping reflectors bounded by continentward-dipping faults. Here, we address the hypothesis that the cumulated weight of syn-rift deposition could shape, in addition to tectonics, the architecture of volcanic passive margins. To address this, we conducted thermomechanical numerical simulations. Our findings highlight that the dense solidified magma favors the development of continent-ward dipping normal faults from a single discontinuity. This dense material, such as lava flows or intrusive sills, leads to the formation of parallel fault arrays that dip toward the continent, bounding seaward-dipping wedges. Conversely, when the half-graben is filled with less dense material, the hanging wall breaks earlier, producing a more symmetrical pattern of conjugate faults. The surface load from basin infillings, whether heavy magma or lighter sediments, significantly impacts how strain is distributed and ultimately shapes the overall rift architecture of passive margins before complete breakup. These insights underscore the critical role of surface material properties in the geological evolution of these significant transition zones.