The Impact of Complex Volcanic Plumbing on the Nature of Seismicity in the Developing Magmatic Natron Rift, Tanzania

Miriam Christina Reiss,Cynthia J Ebinger,Frederik Link,Amani S Laizer,James D Muirhead,Emmanuel O Kazimoto,Georg Rümpker

doi:10.3389/feart.2020.609805

Miriam Christina Reiss, Cynthia J Ebinger + Show 5 more

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https://doi.org/10.3389/feart.2020.609805

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Abstract

Constraining the architecture of complex 3D volcanic plumbing systems within active rifts, and their impact on rift processes, is critical for examining the interplay between faulting, magmatism and magmatic fluids in developing rift segments. The Natron basin of the East African Rift System provides an ideal location to study these processes, owing to its recent magmatic-tectonic activity and ongoing active carbonatite volcanism at Oldoinyo Lengai. Here, we report seismicity and fault plane solutions from a 10 month-long temporary seismic network spanning Oldoinyo Lengai, Naibor Soito volcanic field and Gelai volcano. We locate 6,827 earthquakes with ML−0.85 to 3.6, which are related to previous and ongoing magmatic and volcanic activity in the region, as well as regional tectonic extension. We observe seismicity down to ∼17 km depth north and south of Oldoinyo Lengai and shallow seismicity (3–10 km) beneath Gelai, including two swarms. The deepest seismicity (∼down to 20 km) occurs above a previously imaged magma body below Naibor Soito. These seismicity patterns reveal a detailed image of a complex volcanic plumbing system, supporting potential lateral and vertical connections between shallow- and deep-seated magmas, where fluid and melt transport to the surface is facilitated by intrusion of dikes and sills. Focal mechanisms vary spatially. T-axis trends reveal dominantly WNW-ESE extension near Gelai, while strike-slip mechanisms and a radial trend in P-axes are observed in the vicinity of Oldoinyo Lengai. These data support local variations in the state of stress, resulting from a combination of volcanic edifice loading and magma-driven stress changes imposed on a regional extensional stress field. Our results indicate that the southern Natron basin is a segmented rift system, in which fluids preferentially percolate vertically and laterally in a region where strain transfers from a border fault to a developing magmatic rift segment.

Highlights

Continental rifting is a geodynamic process in plate tectonics
The magma-assisted rifting model by Buck (2004) describes how, if a steady supply of magma is available, extensional strain can be largely accommodated by dike intrusion at a fraction of the force required for tectonic faulting, which reduces activity on riftbounding border faults
We report seismicity and focal mechanisms from a ten-month deployment in 2019 in the southern Natron basin, which encapsulates recent volcanic activity at the only active natrocarbonatite volcano Oldoinyo Lengai worldwide and repeated seismic swarms consistent with volatile release and dike intrusion

Summary

Introduction

Continental rifting is a geodynamic process in plate tectonics. the forces enabling the onset of rifting of comparatively strong, cold, thick continental lithosphere and the relative importance of magmatic and extensional processes remain poorly understood (e.g., Bialas et al, 2010; Brune et al, 2017). Faulting and dike intrusion accommodate extension in the upper crust (Rubin and Pollard, 1988; Rowland et al, 2007; Baer et al, 2008; Trippanera et al, 2019), while released magmatic volatiles can weaken lithosphere through increased pore fluid pressures (Sibson, 2000; Reyners et al, 2007) and/or hydration mineral reactions (Moore and Rymer, 2007) All of these processes enhance the frequency of seismic activity and drive localization of fault-related strain (e.g., Muirhead et al, 2016; Chiodini et al, 2020)

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