Volcano deformation at active plate boundaries: Deep magma accumulation at Hekla volcano and plate boundary deformation in south Iceland

Most magmatic systems on Earth are located at actively deforming plate boundaries. In these systems, the magmatic and plate boundary deformation signals are intertwined and must be deconvolved to properly estimate magma flux and source characteristics of the magma plumbing system. We investigate the inter-rifting and inter-seismic deformation signals at the Eastern Volcanic Zone (EVZ)-South Iceland Seismic Zone (SISZ) ridge-transform intersection and estimate the location, depth, and volume rate for magmatic sources at Hekla and Torfajkull volcanoes, which are located at the intersection. We solve simultaneously for the source parameters of the tectonic and volcanic deformation signals using a new ten-year velocity field derived from a dense network of episodic and continuous GPS stations in south Iceland. We find the intersection of the axes of the EVZ and the SISZ is located within the Torfajkull caldera, which itself is subsiding. Deformation at Hekla is statistically best described in terms of a horizontal ellipsoidal magma chamber at 24_-2⁺⁴ km depth aligned with the volcanic system and increasing in volume by 0.017_-0.002^+0.007 km³ per year. A spherical magma chamber centered at 24_-2⁺⁵ km depth with a volume rate of 0.019_-0.002^+0.011 km ³ per year, or a vertical pipe-shaped magma chamber between 10 _-1⁺³ km and 21_-4⁺⁷ km with a volume rate of 0.008_-0.001^+0.003 km³ per year are also plausible models explaining the deformation at Hekla. All three models indicate magma accumulation in the lower crust or near the Moho under Hekla.