Bob Myhill
I am interested in using seismology and petrology to understand the geochemical and geodynamical evolution of the planets, from the very beginnings of the solar system to the present day.
In February 2016, I joined the Department of Earth Sciences at the University of Bristol to work on NASA's InSight Mission to Mars. I am combining existing geophysical and geochemical information to piece together Mars' history and its place in the solar system. Our intention is to use the seismic data returned from Mars to provide crucial data on the structure and composition of the planet; data for which we've been waiting since the Viking Landers in the 1970s!
Previously, I've worked on understanding the formation of Earth's core and mantle using experimental data and thermodynamics. I've played with squeezing tiny samples of rock to incredible pressures to simulate melting that we believe to be taking place 700 km beneath our feet.
As a student, I was a geologist and seismologist studying subduction through field and petrological studies of ophiolites and geophysical studies of deep focus earthquakes.
Address: University of Bristol
School of Earth Sciences
Wills Memorial Building
Queens Road
Bristol BS8 1RJ
In February 2016, I joined the Department of Earth Sciences at the University of Bristol to work on NASA's InSight Mission to Mars. I am combining existing geophysical and geochemical information to piece together Mars' history and its place in the solar system. Our intention is to use the seismic data returned from Mars to provide crucial data on the structure and composition of the planet; data for which we've been waiting since the Viking Landers in the 1970s!
Previously, I've worked on understanding the formation of Earth's core and mantle using experimental data and thermodynamics. I've played with squeezing tiny samples of rock to incredible pressures to simulate melting that we believe to be taking place 700 km beneath our feet.
As a student, I was a geologist and seismologist studying subduction through field and petrological studies of ophiolites and geophysical studies of deep focus earthquakes.
Address: University of Bristol
School of Earth Sciences
Wills Memorial Building
Queens Road
Bristol BS8 1RJ
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Papers by Bob Myhill
Bands or lineations of dense seismicity are associated with the hinge zones of identified folds. The focal mechanisms of earthquakes within these bands reveal that the mapped fold hinges are commonly perpendicular to the directions of maximum coseismic extension and compression. The hinges plunge at a variety of angles, resulting in systematic deviations from the downdip stress field expected within planar slabs. Slab synforms are typified by earthquake focal mechanisms indicating in-plane compression (e.g. Izu-Bonin, Tonga), while antiforms have earthquake focal mechanisms indicating in-plane extension (e.g. Solomons) or a mixture of in-plane compression and extension (e.g. Tonga).
Slab buckling explains both the clustering of earthquakes and the observed focal mechanism orientations within fold hinges. The localization of strain within buckle zones results in several of the peaks observed in regional earthquake depth distributions. During buckling, the directions of maximum shortening and extension are expected to be perpendicular to the fold hinges, in agreement with deep earthquake moment tensors. Displacement of the minimum-strain surface away from the centre of each seismogenic zone can explain the predominance of in-plane compression within synforms and in-plane extension within antiforms. More complex local variation in focal mechanism orientations in the Tonga slab can be explained by a superposition of in-plane compression and bending strain.
Buckling appears to be a common mechanism facilitating convergence between subducting slabs and the lower mantle. The consequent rotation and translation of fold limbs may explain the discrepancy between estimates of convergence based on subduction velocities and long-term coseismic strain.
Both conventional and pseudosection thermobarometry have been used to yield estimated peak pressure-temperature (P-T) conditions of these tectonic sheets. Toward the leading edge of the ophiolite, subophiolitic rocks of the Vourinos Complex record peak metamorphic temperatures of 770 ± 100 °C. Pressures of 4 ± 1 kbar beneath the Vourinos are estimated on the basis of hornblende composition and are similar to the expected pressures from ophiolitic overburden.
Beneath the exposed Dramala Complex, at the trailing edge of the ophiolitic body southwest of the Vourinos, estimated temperatures reached 800 ± 40 °C and 12.00 ± 1.27 kbar at the top of an apparent inverted metamorphic gradient imposed by discrete phases of accretion. High pressure assemblages beneath ophiolitic bodies imply exhumation relative to the overlying ophiolite. Estimated homologous temperatures in the upper plate are similar to those inferred for channeled exhumation during continental collision.
Mineral assemblages lower in the Dramala sole indicate reduced temperatures and peak pressures. Similar pressures obtained within lower temperature sole rocks beneath Vourinos and Pindos suggest that a shallowly dipping thrust may have been responsible for obduction. Peak temperatures and pressures are in agreement with those estimated for secondary thrust propagation beneath a proto-arc after subduction in an intra-oceanic setting.
Bands or lineations of dense seismicity are associated with the hinge zones of identified folds. The focal mechanisms of earthquakes within these bands reveal that the mapped fold hinges are commonly perpendicular to the directions of maximum coseismic extension and compression. The hinges plunge at a variety of angles, resulting in systematic deviations from the downdip stress field expected within planar slabs. Slab synforms are typified by earthquake focal mechanisms indicating in-plane compression (e.g. Izu-Bonin, Tonga), while antiforms have earthquake focal mechanisms indicating in-plane extension (e.g. Solomons) or a mixture of in-plane compression and extension (e.g. Tonga).
Slab buckling explains both the clustering of earthquakes and the observed focal mechanism orientations within fold hinges. The localization of strain within buckle zones results in several of the peaks observed in regional earthquake depth distributions. During buckling, the directions of maximum shortening and extension are expected to be perpendicular to the fold hinges, in agreement with deep earthquake moment tensors. Displacement of the minimum-strain surface away from the centre of each seismogenic zone can explain the predominance of in-plane compression within synforms and in-plane extension within antiforms. More complex local variation in focal mechanism orientations in the Tonga slab can be explained by a superposition of in-plane compression and bending strain.
Buckling appears to be a common mechanism facilitating convergence between subducting slabs and the lower mantle. The consequent rotation and translation of fold limbs may explain the discrepancy between estimates of convergence based on subduction velocities and long-term coseismic strain.
Both conventional and pseudosection thermobarometry have been used to yield estimated peak pressure-temperature (P-T) conditions of these tectonic sheets. Toward the leading edge of the ophiolite, subophiolitic rocks of the Vourinos Complex record peak metamorphic temperatures of 770 ± 100 °C. Pressures of 4 ± 1 kbar beneath the Vourinos are estimated on the basis of hornblende composition and are similar to the expected pressures from ophiolitic overburden.
Beneath the exposed Dramala Complex, at the trailing edge of the ophiolitic body southwest of the Vourinos, estimated temperatures reached 800 ± 40 °C and 12.00 ± 1.27 kbar at the top of an apparent inverted metamorphic gradient imposed by discrete phases of accretion. High pressure assemblages beneath ophiolitic bodies imply exhumation relative to the overlying ophiolite. Estimated homologous temperatures in the upper plate are similar to those inferred for channeled exhumation during continental collision.
Mineral assemblages lower in the Dramala sole indicate reduced temperatures and peak pressures. Similar pressures obtained within lower temperature sole rocks beneath Vourinos and Pindos suggest that a shallowly dipping thrust may have been responsible for obduction. Peak temperatures and pressures are in agreement with those estimated for secondary thrust propagation beneath a proto-arc after subduction in an intra-oceanic setting.