50-6 Strain Migration and Localization During Active Crustal Necking: A Case Study of Plio-Pleistocene Deformation in the Magma-Rich Turkana Rift Zone, Kenya

Session: Latest Research Advances in Structural Geology and Tectonics

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Abstract:

Divergent plate boundaries form through continental rifting, where successive deformation phases progressively localize strain toward the eventual site of breakup. The transition from early-stage rifting, marked by distributed deformation and limited crustal thinning, to late-stage rifting, defined by localized deformation and pronounced crustal thinning, is governed by a key process: the necking of the continental crust. However, the processes driving necking and its impact on landscape evolution remain poorly understood. This knowledge gap arises because previous studies of the necking phase have been limited to continental rifted margins, where rifting has ceased and tectonic processes can only be inferred from the post-breakup crustal structure. The Turkana Rift Zone of the East African Rift System is actively undergoing crustal necking, offering a rare natural laboratory for direct investigation of the necking phase during magma-rich rifting. Here, we interpret a dense grid of onshore and offshore 2-D seismic reflection profiles tied to outcrops and boreholes in the northern Turkana Basin. We map several key Plio-Pleistocene stratigraphic horizons and basin-bounding fault structures to produce a series of basin-scale isopach and structure maps. These reveal progressive strain migration followed by partial abandonment along the basin-bounding fault system as the necking fault complex develops along the rift axis. Present-day seismicity along the rift axis suggests that the necking phase involves strain partitioning between faulting and magmatism. Necking is further accompanied by detached deformation of the Plio-Pleistocene sequence, driven by detachment faulting and mobilization of over-pressured shale. Together, these findings provide new insight into the necking phase of magma-rich rifting and highlight the development of detached structures during rapid burial of fluvial-lacustrine strata along the rift axis.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8351

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