How has the generation and recycling of continental crust changed over time?

 

Modern continental crust forms by arc volcanism at convergent margins, but there is uncertainty/disagreement as to how continents were generated and modified in the early Earth, and how much continental crust has been present at any given time in Earth history. My current research uses novel statistical techniques to uncover trends in the igneous and sedimentary geochemistry of continental rocks over time (Garber et al. in prep), case studies of exhumed lower crust to understand its formation and long-term thermal record (Cipar et al., submitted; Smye et al., in prep), as well as the rates and mechanisms of deep continental subduction (Garber et al., several in prep).

How do subduction zones initiate and thermally/chemically evolve?

 

Subduction zones are the main locus of chemical cycling between the surface and the deep Earth. There is therefore intense interest in i) their thermal structures, ii) the locus and magnitude of chemical exchange along subducting slabs, and iii) how each of (i) and (ii) vary within/along individual subduction zones, and throughout Earth history. My research involves using ophiolites to understand the sequence of metamorphic and tectonic events associated with subduction initiation (Searle et al. 2015 Geosphere; Rioux et al. 2016 EPSL; Garber et al. 202x Tectonics [in revision]), as well as exhumed subduction-zone rocks to understand slab major-element metasomatism (Garber et al. and Weinheimer et al., 2019 AGU) and the evolving thermal structure of subducted slabs (Rutte et al., in prep).

What are the mechanisms, pace, extent, and tectonic consequences of fluid-rock interaction in the deep crust?

 

When dry, continental lower crust may serve as a load-bearing structure in the lithosphere; when wet, it loses its strength and may undergo plastic flow. Understanding this dichotomy is critical for determining the long-term rheological behavior of continental crust. Exhumed lower crustal terranes show a range of fluid states, with localized and punctuated fluid-rock interaction controlling crustal rheology. Metasomatic events may also only be captured by certain phases, and in rocks that appear unaltered at outcrop or in hand sample. My research focuses on using key minerals sensitive to fluid alteration (especially titanite: Garber et al., 2017 J Pet; Y. Li, Penn State senior thesis) from well-characterized, exhumed lower-crustal terranes for which thermal and tectonic histories are broadly known, to track the timing, expression, origin, and consequences of fluid-rock interaction during orogenic events.

Petrochronology: how do trace elements and isotopes in dateable minerals record tectono-metamorphic processes?

 

Trace elements in metamorphic minerals – particularly those with appreciable U, Th, and Pb – can help reconstruct the petrological, geochemical, and tectonic evolution of metamorphic rocks. Analytical advances in LA-ICPMS permit rapid, precise, and high spatial resolution trace element measurements – allowing new ways to access these records. My research efforts combine cutting-edge analytical methods with innovative statistical methods, yielding groundbreaking insights into metamorphic records of titanite (Garber et al. 2017 J Pet) and zircon (Garber et al. 2020 CMP). Such work can further yield temperature-time histories from single crystals (Smye et al., 2018), providing novel opportunities to test tectonic hypotheses for the evolution of the deep crust and upper mantle.

© 2020 by JM Garber. All opinions expressed on this page are those of the author only, and do not reflect those of any institution to which I have been or may be affiliated with.