--> Fault Scaling in an Incipient Rift: Example From the Afar Triple Junction, Ethiopia and Djibouti

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Fault Scaling in an Incipient Rift: Example From the Afar Triple Junction, Ethiopia and Djibouti

Abstract

Rift basins and passive margins continue to be significant targets for hydrocarbon exploration. Modern, active rift basins allow us to closely examine the evolution of structural traps in such settings. The Afar region of east Africa is the only sub-aerial exposure of a rift-rift-rift triple junction, and thus provides a rare view of the final stages of continental break-up. In the Afar, the East African rift meets the Red Sea and Gulf of Aden mid-ocean ridge rift propagators. Here, the ridges do not link discretely; rather diffuse normal faulting with some minor components of strike slip faulting accommodates extension between the overlapping rift propagators. All Quaternary deformation dissects extensive flood basalts of the 1 – 4 Ma Afar Stratoid Basalt series, such that minimum estimates of fault displacement can be easily determined by examining vertical offsets of the relict stratoid surface. One question that we seek to address in the Afar rift is the contribution of small faults to the overall extensional budget of the region. The contribution of large faults has been studied in past decades, but as faulting is very diffuse in this region, the contribution of small faults cannot be ignored. Fault displacement and length scaling relations provide a means of assessing the bulk strain and fault density in a rift basin. To examine fault growth and fault scaling relationships, we analyzed a set of stereographic panchromatic and orthorectified multispectral images. The stereographic images were processed into a 5 m DEM, which was used with the panchromatic and multispectral images were used as basis for mapping in ArcGIS. Once mapped, the fault traces were analyzed using a script in ArcGIS, with fault throws being calculated along the length of each fault. This analysis shows that fault maximum throw in Dobe graben scales proportionally to fault length following a power law. Additionally, fault length appears to follow a power law distribution. Previous work has shown that fault length follows an exponential distribution in regions experiencing high amounts of strain (e.g. mid ocean ridges; ɛ > 8%), while following a power law distribution in regions of lesser strain. Further mapping and statistical analysis should help pinpoint the transition between largely exponentially distributed fault length in the hyperextended Asal rift with power law fault scaling in the central Afar rift and Main Ethiopian Rift.