Classifying the controls on differential movement of subduction interface blocks: a comparative study

General Background:

Deciphering how materials move and interact within subduction zones is crucial for understanding resultant processes, such as volcanoes and earthquakes, which we observe at Earth’s surface. As oceanic lithosphere subducts beneath a continent, it brings with it crustal material, which forms the interface between the subducting slab and the overlying mantle wedge. At this subduction interface, important devolitization reactions and processes occur that recycle volatiles into the mantle, including both carbon dioxide and water. The net flux of volatiles into the mantle wedge both generates magma and earthquakes, and sequesters CO₂ and H₂O in the deep mantle, playing a significant role in both water and carbon cycles.

Cartoon depiction of the upper ~100km of a subduction zone. The shaded region beneath the continental arc is the subduction interface.

            Exhumed subduction-interface terranes (mélange) typically consist of meter to kilometer-scale blocks of metamorphosed oceanic crust and mangle wedge surrounded by a fine-grained matrix dominated by chain and sheet silicates. Geodynamic modeling suggests that blocks can move at varying rates within the matrix, and has shown that this differential movement can result from several mechanisms: matrix viscosity, plate velocity, duration of subduction, and degree of hydration of the mantle wedge (Gerya et al, 2002).

Within the subduction interface, blocks are disaggregated from both the subducting slab and the overlying mantle wedge. Once these blocks are disaggregated, can they move within the fine-grained matrix? From Bebout and Penniston-Dorland (2016).

Modeling suggests that factors such as matrix viscosity, plate velocity, duration of subduction, and degree of mantle hydration all effect the size of the subduction interface and the degree of differential movement of material within the interface. From Gerya et al (2002).

            This goal of this study is to identify the dominant control on differential movement of material by comparing two exhumed terrains: the Kampos Mélange (Syros, Greece) and the Catalina Schist (Santa Catalina Island, California). These terranes appear to be similar in the degree of differential movement that they experienced; however, the Kampos Mélange experienced high-pressure/low-temperature metamorphism while the Catalina Schist experienced moderate-pressure/moderate-temperature metamorphism. 

Geologic map of Santa Catalina Island basement rock. Modified from Platt (1976).

Simplified geologic map of Syros, Greece showing the location of the Kampos Melange. From Kennedy et al (2003).

Methods

            In order to determine the peak metamorphic conditions for exotic blocks relative to non-exotic blocks in both exhumed terranes, a combination of field mapping, trace element thermometry (Tomkins et al, 2007; Hayden et al, 2008), inclusion barometry (Kohn, 2014b), and U/Pb geochronology (Zack et al, 2011) will be used. These methods have already been successfully applied to blocks from the Catalina Schist (both exotic and non-exotic) (Penniston-Dorland et al, in press; Harvey, unpublished data); therefore, this study relies on new samples and data to be collected for only the Kampos Mélange. The use of the same methods for each terrane allows their characteristics to be directly compared.

Sample data collected from the Catalina Schist showing comparison in peak P-T between exotic and non-exotic blocks of the same metamorphic grade. Temperature determined by Zirconium-in-rutile thermometry, and pressure determined by Raman Spectrometry. Data from Harvey et al (unpublished data) and Penniston-Dorland et al (in press). 

Funds Required

          This project will require funding for 1-2 graduate students, as well as funds for one field excursion to Syros, Greece, conference presentations, the creation of 40-50 thin (30μm) and corresponding thick (75μm) sections, and analysis time on the following instruments:

• Electron Microprobe Analyzer at the University of Maryland
• Laser Ablation Inductively Coupled Plasma Mass Spectrometer at the University of Maryland
• Raman Spectrometer at Boise State University (and associated travel costs)

Selected Literature Cited

Bebout, G.E., Penniston-Dorland, S.E. (20016) Fluid and mass transfer at subduction interfaces -the field metamorphic record. Lithos, 240, 228-258

Gerya, T.V., Stöchert, B., and Perchuk, A.L. (2002) Exhumation of high-pressure metamorphic rocks in a subduction channel: A numerical simulation. Tectonics, 21, doi:10.1029/2002TC001406

Hayden, L.A., Watson, E.B., and Wark, D.A. (2008) A thermobarometer for sphene (titanite). CMP 155, 529-540 (5) Kohn, M.J. (2014b) “Thermoba-Raman-try”: Calibrations of spectroscopic barometers and thermometers for mineral inclusions. EPSL, 388, 187-196 

Kennedy, A., Lagos, M., Ballhaus, C. (2003) Zirconz from Syros, Cyclades, Greece -Recrystallization and mobilization of zircon during high-pressure metamorphism. Journal of Petrology, 44(11), 1977-2002

Kohn, M.J., and Corrie, S.L. (2011) Preserved Zr-temperatures and U-Pb ages in high-grade metamorphic titanite: evidence for a static hot channel in the Himalayan orogen. EPSL, 311, 136-143 

Penniston-Dorland, S.P., Kohn, M.J., Piccoli, P.M., (in press) A mélange of subduction temperatures: Zr-in-rutile thermometry of the Catalina Schist, CA and its tectonic implications. 

Platt, J.P. (1975) Metamorphic and deformational processes in the Franciscan Complex, California: Some insights from the Catalina Schist terrain. GSA Bulletin, 6, 1337-1347. 

Tomkins, H.S., Powell, R., and Ellis, D.J. (2007) The pressure dependence of the zirconium-in-rutile thermometer. JMG, 25, 703-713