Strain partitioning between border faults and axial magmatic segments in the Afar Rift

Continental break-up is a key stage of the rifting process as it marks the transition to ocean seafloor spreading. Predictive models of the rift-to-seafloor spreading transition suggest that, during incipient continental break-up, the rift margins deactivate and extension focuses at the rift axis along a series of en-echelon magmatic segments which are offset by either transform or non-transform faults. However, how strain is partitioned between the rift margins and axial magmatic segments during the last phases of the rifting process remains poorly understood. Furthermore, how and when transform and non-transform offsets form during the rifting process is still unclear. Northern Afar, in the East African Rift System, is the ideal place to address these open questions as it shows these processes occurring during the final stages of continental rifting exposed at surface. In this thesis, I adopted a multi-disciplinary approach based on InSAR, seismicity and structural analyses to investigated the tectonic deformation and the fault kinematics at the offset between the two axial magmatic segments of Erta Ale and Tat’Ali (Afrera Plain) and also along the North-Western Afar Margin. The results show that the Afrera Plain is an active rift-linkage zone characterized by en-echelon, oblique, left-lateral faults striking in a ~NS direction. The structural architecture of the Afrera Plain is characterized by dominant East-dipping faults at the center and dominant West-dipping faults at the eastern tip, close to the Tat’ Ali segment. Such structural architecture is consistent with a kinematic model of rift-linkage where Erta Ale and Tat’ Ali segments interact trough a right-lateral transfer zone characterized by en-echelon oblique faults, striking in a ~NS-direction. Furthermore, InSAR time-series and models, combined with seismic data, have revealed great variability in the fault behavior. Faults at the center of the Afrera Plain are characterized by dominant stick-slip faulting with episodic slip events accompanied by ML ≥ 5 earthquakes. Conversely, a more complex fault behavior encompassing creep, micro-seismicity and episodic slip characterize the tips of the linkage zone. Such heterogenous fault behavior could be likely influenced by the high heat flows and the strong hydrothermal circulation at the Afrera Plain. Intense seismicity also characterizes the North-Western Afar Margin. Here ongoing tectonic extension along active border faults generate moderate seismicity with several Mw > 5 earthquakes occurred in the past decades. Recently, a ML 5.3 seismic sequence occurred in March-April 2018 rupturing the deep portion (15-30 km) of crustal border faults. The analysis of focal mechanism and relocated seismicity shows that slip occurred along major West-dipping faults and minor conjugate East-dipping faults. The fault kinematics has dominant normal component associated with both minor left- and right-later components. Deep seismicity in the area is focused below the Dergaha marginal graben where there is geophysical evidence of partial melt in the lower crust. Such observations suggest that deep seismicity along the North-Western Afar Margin could be triggered by fluids migration in the lower crust associated with magmatic processes. Conversely, seismicity outside the Dergaha graben is shallower (< 15 km) and likely associated to brittle faulting in the upper crust. The results of this thesis provide one the few direct observations of the tectonic processes occurring at the interaction between two magmatic segments in Afar. Furthermore, they provide new contributions towards the understanding of the kinematic of rift margins in Afar during incipient continental break-up suggesting that fluid migration in the crust may play a role in influencing the fault activity along the North-Western Afar Margin.