Abstract

Magnetic stripes parallel to mid-ocean ridges are one of the most significant consequences of seafloor spreading, and have played an essential role in the establishment of the plate tectonics theory and the determination of seafloor spreading rates. Similar magnetic anomaly patterns have not been well documented subaerially in continental rifts transitioning into seafloor spreading centers. Here, using high-resolution magnetic data that were collected across the Tendaho Graben in the Afar Depression, Ethiopia, we document one of the first examples of subaerial magnetic lineations similar in pattern and amplitude to those that characterize seafloor spreading centers. The ∼50-km-wide graben is the southernmost structural and geomorphological expression of the on-land continuation of the Red Sea propagator, which is taken to represent the Arabian-Nubian plate boundary within Afar. The graben is bounded by northwest-trending border faults, with the footwalls dominated by ca. 1.7 Ma basalts and the downthrown blocks constituting progressively younger basalts toward the center of the graben, reaching ca. 35 ka. The Tendaho magnetic field is characterized by an ∼10-km-wide linear negative magnetic anomaly that corresponds to a normal-polarity zone that is flanked by two parallel, ∼20-km-wide linear positive magnetic anomalies of reversed polarity. This work shows that magnetic stripes can be developed in transitional continental rifts before the development of oceanic spreading centers. The common assumption that magnetic stripes can be used to date the onset of seafloor spreading may need to be re-evaluated in light of the evidence provided here.

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