Successive orthogonal and oblique extension episodes in a rift zone: Laboratory experiments with application to the Ethiopian Rift

Abstract

Abstract. Small-scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique (type I) and from oblique to orthogonal (type 2). In both the type 1 and type 2 models, faults produced during the first stage strongly control fault development during the second stage. In type 1 models, the oblique faults developed during the second oblique phase are confined within a first-phase graben, whereas in type 2 models the oblique faults, produced during the first phase, continue to develop during orthogonal extension and connect with each other to give sigmoidal fault blocks. Type 1 models are compared with the structural setting of the Ethiopian Rift; the evolution of the rift is related to a recent extensional event, whose principal direction of stretching trends at around 50 ø to preexisting major normal faults. Type 1 laboratory models are fairly comparable to the northem sector of the Ethiopian Rift, referred to here as MER. They account for both the development of the en echelon oblique faults of the Wonji Fault Belt and the sinistral shear gradient running parallel to the eastern border of the MER, which formed during an oblique rifting extension. The statistical analysis of the whole Ethiopian Rift fault pattern by reference to the experimental data allows the determination of a N100ø-N110 ø mean direction of stretching.

Strain accommodation in transitional rifts: extension by magma intrusion and faulting in Ethiopian rift magmatic segments

“…12). Bonini et al (1997) also come to this conclusion based on their model results, but these authors concluded that the transtension persists to the present day. In our interpretation, the duration of this phase of rifting is too short to accumulate measurable block rotations observable by palaeomagnetic investigations.…”

“…The obliquity of Miocene border faults and Quaternary faults in the Wonji belt led Bonini et al (1997) to propose a model of oblique extension along the border faults after a change in extension direction, which probably occurred at around 3.5 Ma to accommodate along-axis propagation in the Gulf of Aden (Wolfenden et al 2004). The Quaternary development, therefore, of the Wonji belt (our magmatic segments) has been attributed to oblique extension, with a left-lateral component of motion along the rift floor (e.g.…”

The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE): Probing the transition from continental rifting to incipient seafloor spreading

“…The distribution, kinematics, and estimated age of fault populations suggest that deformation in the Main Ethiopian Rift north of 8.5°N migrated to the center of the rift sometime in the interval 6.6-3 Ma, with an associated change from N130°E-directed extension to N105°E-directed extension (Bonini et al, 1997;Boccaletti et al, 1998;Wolfenden et al, 2004). Opening models of the East African Rift based on global positioning system (GPS) data and earthquake focal mechanisms constrain the current extension direction in the Main Ethiopian Rift to between N95°E and N110°E at rates of 4-7 mm/yr (Bilham et al, 1999;Fernandes et al, 2004;Calais et al, 2006;Stamps et al, 2008).…”

Formation and stability of magmatic segments in the Main Ethiopian and Afar rifts

“…The obliquity of Miocene border faults and Quaternary faults in the Wonji belt led Bonini et al (1997) to propose a model of oblique extension along the border faults after a change in extension direction, which probably occurred at around 3.5 Ma to accommodate along-axis propagation in the western Gulf of Aden (Wolfenden et al, 2004;Audin et al, 2004). The magmatic segments of the northern Main Ethiopian Rift have been attributed to oblique extension, with a left-lateral component of motion along the rift floor (e.g., Mohr, 1968;Boccaletti et al, 1998).…”