NEW KINEMATIC CONSTRAINTS ON THE QUATERNARY TECTONIC EVOLUTION OF THE AFAR TRIPLE JUNCTION

The Afar region of east Africa comprises the triple junction of divergent plate boundaries between the Nubia, Somalia, and Arabia plates. Proposed kinematic models for the tectonic evolution of the region describe the triple junction as either a collection of growing / shrinking microplates, a set of fault blocks sliding past each other in response to a clockwise rotation, or as crust being stretched out by the counterclockwise rotating Danakil block. Distinguishing each model depends on several factors: time-dependent fault slip rates, the presence or lack of strike-slip / oblique motion, time-transgressive initiation of graben formation, and spatial variations in fault slip rates. We present new evidence, using late Quaternary fault slip rates, graben initiation ages and uplift rates from stream incision modeling, and remote sensing observations that provide improved insight on the evolution of the triple junction. Fault slip rates are determined using fault scarp degradation / mass diffusion modeling, and stream incision rates are determined using bedrock fluvial incision models. Model results are calibrated using ³⁶Cl terrestrial cosmogenic nuclide measurements. There appears to be no significant variation in late Quaternary fault slip rates compared to early-middle Quaternary uplift rates as determined with stream incision modeling. Stream incision modeling indicates that graben bounding faults propagated towards the northwest, showing decreased graben age to the northwest. Additionally, slip rates and uplift rates are significantly higher in the southeast compared to the northwest, which corresponds with the spatial variation in graben age. Furthermore, palinspastic reconstruction of field and remote sensing observations indicates that paleo-drainages dissected by normal faulting show minimal oblique offset. Combining this evidence suggests that the Afar triple junction likely follows a “Crank-arm” kinematic model, where the crust is stretched out in response to the rotating Danakil block. Future work can refine this model to better understand this key transition.