Kinematic analysis of fractures in the Great Rift, Idaho: Implications for subsurface dike geometry, crustal extension, and magma dynamics

Extension across the southern Great Rift of the Eastern Snake River Plain (ESRP), Idaho, was measured to calculate the dimensions of underlying dikes and interpret magmatic and extensional processes. Cumulative rift‐perpendicular extension ranges from 0.64 to 4.50 m along the 14 km long Kings Bowl segment, from 1.33 to 4.41 m along the 14 km long New Butte segment, and from 0.74 to 1.57 m along the 4 km long Minidoka segment. Along strike of each segment, extension increases toward coeval vents. Each rift segment is interpreted to be underlain by a subsurface dike, whose dimensions are calculated using buoyancy equilibrium and boundary element models. Dikes are calculated to have tops that are 950–530 m deep, bottoms that are 23–31 km deep, and widths that taper to zero from a maximum of 2–21 m. Modeling suggests that the Kings Bowl dike has a maximum probable width of ∼8 m and a volume of ∼2 km 3 , about 400 times the volume of its coeval lava flow. Dike widths and ages at the southern Great Rift provide evidence for a Holocene ESRP strain rate of about 1 to 3 × 10 −16 s −1 , which is as much as an order of magnitude slower than strain rates in the adjacent, seismically active Basin and Range province. Eruptive fissures are present where rift width is <1650 m. This corresponds to a depth to dike top of <700 m, which we propose was the depth where vesiculation initiated, thus increasing magma pressure and inducing eruption.