Continental rift systems form by propagation of isolated rift segments that interact, and eventually evolve into continuous zones of deformation. This process impacts many aspects of rifting including rift morphology at breakup, and eventual ocean-ridge segmentation. Yet, rift segment growth and interaction remain enigmatic. Here we present geological data from the poorly documented Ririba rift (South Ethiopia) that reveals how two major sectors of the East African rift, the Kenyan and Ethiopian rifts, interact. We show that the Ririba rift formed from the southward propagation of the Ethiopian rift during the Pliocene but this propagation was short-lived and aborted close to the Pliocene-Pleistocene boundary. Seismicity data support the abandonment of laterally offset, overlapping tips of the Ethiopian and Kenyan rifts. Integration with new numerical models indicates that rift abandonment resulted from progressive focusing of the tectonic and magmatic activity into an oblique, throughgoing rift zone of near pure extension directly connecting the rift sectors.
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<p>The Ririba rift represents the southern termination of the Main Ethiopian Rift and formed from the southward propagation of this latter during, or shortly after, the emplacement of subalkaline basalts that produced a widespread basaltic lava basement, at ~3.7 Ma.</p><p>The activity of the Ririba rift was short-lived and ceased between 2.8 and 2.3 Ma, when deformation migrated westward into an oblique, throughgoing rift zone directly connecting the Ethiopian and Kenyan rifts. Rifting was followed by the eruption of limited volumes of Late Pleistocene-Holocene alkaline basalts, associated to several, monogenetic volcanic centres, forming the Dilo-Dukana and Mega volcanic fields.</p><p>We provide new petrological, geochemical and isotopic data on the still poorly studied magmatic products emplaced by the volcanic activity of the Ririba rift with the aim of investigating the source of both the older Pliocene lava basement and the Late Pleistocene-Holocene alkaline basalts as well as their pathways to the surface.</p><p>Major and trace elements, besides discriminating the Pliocene lavas from the other younger alkaline products, reveal that the Dilo-Dukana and Mega samples always overlap in composition. On the whole they display variable major and trace element contents compared to a limited variation in silica (43-46wt.%) describing slightly defined trends. Regular and evident trends are observed comparing some incompatible trace elements (e.g., Rb, Ba, Zr, Nb) suggesting a prominent role of fractional crystallization for their differentiation.</p><p>Isotopes reveal that the products of the two volcanic fields have small but significantly different behaviour: the Dilo-Dukana products are isotopically homogeneous and clustered around <sup>87</sup>Sr/<sup>86</sup>Sr values of 0.70303 and <sup>143</sup>Nd/<sup>144</sup>Nd values of 0.51292, whereas the Mega lavas and pyroclastics display a small but wider variability partially overlapping the Dilo-Dukana samples, with <sup>87</sup>Sr/<sup>86</sup>Sr ranging from 0.70300 to 0.70334 and <sup>143</sup>Nd/<sup>144</sup>Nd from 0.51293 to 0.51290. Conversely, isotopes data corroborates the evidence that the younger Dilo-Dukana and Mega products are well distinct from the Pliocene basaltic lava basement (as well as with respect to all the older magmatic rocks of the area) and are characterised by a more prominent mantle signature.</p><p>Moreover, the Sr vs Nd isotopes variation among the younger Holocenic lavas of Mega describe a negative well-defined trend allowing to make inferences about the possible role of crustal contamination during the Ririba rift magmatic activity.</p><p>All these evidences are consistent with the interpretation that the two young volcanic fields of Dilo-Dukana and Mega are fed by deep structures directly transferring mantle melts up to the surface, as also suggested by the large abundance of mantle xenoliths in the different products. As a consequence, this strongly corroborates the interpretation that the two volcanic fields are not related to the major faults of the Ririba rift, but are associated to different, deep, NE-SW-trending inherited structures which cut the roughly N-S boundary faults of the rift.</p>
<p>The volcano-tectonic evolution of the Main Ethiopian Rift (MER) is punctuated with periods of intense silicic volcanism, characterized by large explosive caldera-forming eruptions and the production of several ignimbrite deposits. These volcanic paroxysms require large volume of evolved silicic magma accumulated in shallow chambers into the continental crust; however, the relations between magmatism and tectonics during rifting, and the influence of the distribution and timing of regional tectonics on the ascent of magma and its stalling in large magmatic reservoirs remain poorly defined.</p><p>We present new geochronological data (<sup>40</sup>Ar/<sup>39</sup>Ar dataset of 29 samples) providing new constraints on the timing, evolution and characteristics of volcanism in the Central sector of the MER, where large ignimbrite deposits and remnants of several calderas testify the recurrence of silicic flare-ups. Specifically, we investigate in detail the eastern margin of the rift, where a voluminous, widespread, crystal-rich ignimbrite (Munesa Crystal Tuff, MCT) has been described. This deposit has been correlated to a thick ignimbrite occurring at the bottom of geothermal wells in the rift, pointing to a giant eruptive event attributed to a huge caldera structure, presumably buried beneath the rift floor. At least other two widespread ignimbrite units are present along the same margin for several tens of kilometres, testifying the high volcanicity of this sector of the MER.</p><p>Our survey and analyses suggest that, at least in the eastern margin of the rift, activity was clustered in periods of large magma production and emission, resulting in the recurrence of intense volcanic phases interspersed with periods of rest of volcanism. Ignimbrites and other volcanic deposits occur in the investigated area, spanning an age interval from 3.5 Ma to as recent as 150 ka. The MCT emission, around 3.5 Ma, was followed, after a long quiescence, by an important phase with the emplacement of both mafic (lava flows and scoria cone) and evolved (ignimbrites) products between 1.9-1.6 Ma. After that, volcanism occurred more frequently, possibly with a lower amount of erupted magma and still alternating with quiescent periods, with volcanism clusters at ~ 1.3-1.2 Ma, ~ 0.8-0.7 Ma and ~ 0.3-0.2 Ma. This clustered volcanic activity will be compared with the episodic rifting of this sector of the Main Ethiopian Rift.</p>
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