Prospective students who are interested in these or similar topics are encouraged to contact me (roholder [at] umich [dot] edu) to discuss potential research projects and apply to the University of Michigan graduate program (link). Students with complementary interests who are motivated to pursue their own research goals are also encouraged to apply; I am happy to help plan, support, and provide guidance for appropriately interesting ideas.
When did plate tectonics begin and how has it changed through time?
Earth’s mantle convection, which facilitates planetary heat loss, is manifested at the surface as plate tectonics. When plate tectonics began and how it has evolved through time are two of the most fundamental and challenging questions in Earth science. Many have inferred plate tectonics to have operated since at least ~2.5 Ga, based on observations such as change in the composition of sediments and igneous rocks or the transition from Archean dome-and-keel terranes to linear fold–thrust belts (e.g. Cawood et al. 2018 PTRS, and references therein). However, it is unclear whether these data are truly indicative of plate tectonics, or record another transitional tectonic mode (e.g. Lenardic et al. 2018 PTRS). In my work, I address these questions through examination of the metamorphic rock record. On modern Earth, plate tectonics is characterized by metamorphic rocks that show a bimodal distribution of apparent thermal gradients (temperature change with depth [pressure]: T/P) in the form of paired metamorphic belts at convergent margins (Miyashiro 1961 JPet). Paired metamorphism has occurred globally since the Paleoproterozoic (c. 2.2 Ga), but the global distribution of metamorphic T/P has broadened and become more distinctly bimodal since that time, suggesting an evolution in the styles of subduction and collisional orogenesis (Holder et al. 2019 Nature). However, a globally bimodal distribution of metamorphic T/P has not yet been demonstrated for the Archean, raising the question of whether plate tectonics (as we observe and define it today) was occurring. My ongoing work consists of targeted case studies to reconstruct the geodynamic histories of Archean terranes and whether plate tectonics was responsible for their formation.
Extreme pressure–temperature conditions in the crust and their geodynamic significance
Ultrahigh-temperature metamorphism (UHTM: >900°C at normal crustal depths; e.g. Kelsey and Hand, 2015 GF) represents the thermal limit of metamorphism within Earth’s crust. Due to the large volumes of granitic melt that can be generated during UHTM, understanding how UHT terranes form is fundamental to our understanding of how: 1) the crust differentiates into a more felsic upper crust and a more residual/mafic lower crust; and 2) the lithosphere is weakened during orogenesis and is permanently strengthened after orogenesis: processes fundamental to the evolution of the continental crust.
Ultrahigh-pressure metamorphism (UHPM: coesite-stable metamorphism, representing depths greater than ~80 km; e.g. Chopin, 1984 C.M.P.) represents the maximum depths to which continental crust has been subducted and exhumed. Petrological, chronological, and structural data from UHPM terranes provide fundamental constraints for geodynamic models of plate tectonics and lithospheric strength and define our understanding of both subduction and continental collision, particularly in the Phanerozoic.
Together, UHPM and UHTM reveal the radically different geodynamic processes acting on Earth’s crust. Understanding the mechanical and thermal processes that lead to these metamorphic extremes is fundamental to our understanding of secular changes in plate tectonics and the chemical and physical evolution of continents. Understanding the petrologic and structural development of such terranes provides us with analogs for interpreting and understanding the behavior of the middle and lower crust of modern orogenic systems, which we cannot directly observe.
Recent related work has focused on constraining the timing and duration of continental subduction during the Caledonian Orogeny in the UHP Western Gneiss Region, Norway (Holder et al. 2015 CG; Hacker et al. 2015 CG; Hacker et al. 2019 JMG); determining the tectonometamorphic evolution of the Bohemian Massif during the Variscan Orogeny (Štípská et al. 2015 JPet, 2016 JMG; Peřestý et al. 2016 JMG); and studying UHTM in southern Madagascar during the East African Orogeny as an analog for the middle to lower crust of the modern India–Asia collision (Horton et al. 2016 Tectonics; Holder et al. 2018 Geology; Holder et al. 2018 JMG; Holder et al. 2019 CMP; Holder and Hacker 2019 CG).
U–Pb geochronology—and the growing field of petrochronology: linking geochronology to rock-forming process through petrology and geochemistry—is one of the most fundamental types of data used to interpret the rock record. While broader questions of my research are concerned with tectonics and the evolution of Earth’s continents and mountain belts, another aspect of my research is developing new analytical approaches and perspectives on U–Pb dating for more rigorous interpretation of the rock record. Recent projects have focused on:
• The development and application of U–Pb carbonate dating.
• Evaluating the limits of zircon’s ability to record complex, high-temperature processes in the crust (Štípská et al. 2016 JMG; Holder et al. 2018 JMG).
• Characterizing the relationships between monazite composition and the conditions at which it grew (Holder et al. 2015 CG; Hacker et al. 2015 CG; Holder et al. 2018 JMG; Hacker et al. 2019 JMG).
• Calculating elemental diffusivities in titanite (Holder et al. 2019 CMP) and assessing mechanisms of titanite recrystallization (Holder and Hacker 2019 CG) to improve understanding of the geological significance of U–Pb titanite dates.