Eruptive history and volcano-tectonic evolution of Paka volcanic complex in the northern Kenya rift: Insights into the geothermal heat source

In this paper, we provide insights into the eruptive history and volcano-tectonic evolution of the Paka volcanic complex in a revised stratigraphic framework. We integrate all available 40Ar/39Ar radiometric dates along with field observations, remote sensing data, and the analysis of surface and cutting samples from a 2552 m deep borehole (PK-01). Chemical analyses distinguish basalt, hawaiite, mugearite, benmorite, and trachyte rock type in Paka. It also indicates that the eruptive products are genetically related by fractional crystallization. Major element mass balance and Rhyolite-MELTS models are consistent with the trachyte being generated by 74-83% fractional crystallization of Paka basalt. Before the growth of the volcano, volcanic activity was characterised by plateau fissure eruptions at 582-405 ka. Eruptions directly related to the Paka edifice are divided into four different volcanic sequences. Sequences 1-4 span the periods 390-278 ka, 247-205 ka, 160-36 ka, and 36-8 ka. The Pre-Paka plateau eruption products outcrop at the base of the volcano flanks and are overlain by edificeforming trachyte. Initial caldera-related subsidence is expressed by arcuate structures to the west and southeast parts of the volcano. These initial collapses are associated with tuff and pyroclastic eruptions estimated to have occurred at similar to 38 ka. Following these explosive eruptions, a similar to 1.5 by similar to 1.6 km main caldera developed. The lithological information from the drilled geothermal exploration borehole PK-01 indicates a minimum thickness of similar to 2200 m for the extrusive lava sequences, with the uppermost 1050 m of the strata being directly related to eruptions from the Paka volcano (Phases 1-4). We estimate a minimum total bulk volume of similar to 50 km(3) for the volcanic material erupted from Paka during the last -390 ka. This translates to an average eruption rate of similar to 1.2 x 10(-4) km(3)/yr. The associated advected magmatic heat supported an 'excess' heat flux ranging between 110 and 138 mW/m(2) or about twice present day average continental flux (57 mW/m(2)) and about 1.3 times the background values in geothermal areas (90 mW/m(2)). The apparent high heat flux implies a magma-driven geothermal system, where convection of the hydrothermal fluid is occurring above the heat source. Structural mapping revealed normal faults, strike-slip faults, lineaments, and an array of eruptive vents and domes as the main volcano-tectonic structures. We infer at least four faulting and fault reactivation events that have occurred during Paka geological history. The structural orientations in the area are dominated by NNE and NW with subordinate N-S and NNW striking structures. Evidence suggests that the intersection of NNE and NW trending structures may have had a major influence on the volcanism in the area.