Fault-magma interactions during early continental rifting: Seismicity of the Magadi-Natron-Manyara basins, Africa
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Date
2017
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Geochemistry, Geophysics, Geosystems
Abstract
Although magmatism may occur during the earliest stages of continental rifting, its role in
strain accommodation remains weakly constrained by largely 2-D studies. We analyze seismicity data from a
13 month, 39-station broadband seismic array to determine the role of magma intrusion on state-of-stress
and strain localization, and their along-strike variations. Precise earthquake locations using cluster analyses
and a new 3-D velocity model reveal lower crustal earthquakes beneath the central basins and along projections
of steep border faults that degas CO2. Seismicity forms several disks interpreted as sills at 6–10 km
below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions
at Oldoinyo Lengai volcano. After determining a new ML scaling relation, we determine a b-value of
0.8760.03. Focal mechanisms for 65 earthquakes, and 13 from a catalogue prior to our array reveal an
along-axis stress rotation of 608 in the magmatically active zone. New and prior mechanisms show predominantly
normal slip along steep nodal planes, with extension directions N908E north and south of an
active volcanic chain consistent with geodetic data, and N1508E in the volcanic chain. The stress rotation
facilitates strain transfer from border fault systems, the locus of early-stage deformation, to the zone of
magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent
lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border
faults, and hydraulic fracture around the margins of magma bodies. Results indicate that earthquakes are
largely driven by stress state around inflating magma bodies.