We just published a new paper in Tectonics titled, Early onset of Franciscan subduction. Our study focused on the Franciscan complex of California, which is considered a type example of an ancient subduction zone. The timing of Franciscan subduction initiation, however, has long been debated, resulting in conflicting models for the tectonic history of western North America. The study was unique because we dated the mineral zircon preserved as inclusions within garnet formed during the early stages of subduction. We also dated zircon outside of garnet formed during a younger event within the subduction history. The zircon inclusions within garnet yielded an age of ~176 Ma, whereas the zircon outside the garnet had a younger age of ~160 Ma. These ages require that Franciscan subduction began by 180 Ma, significantly older than commonly believed and constrain models for the tectonic evolution of the western North American margin. Data used in the paper and supplemental material, as well as, python code to construct some plots within the paper are available on Github and Zenodo.
Zircon grains from Franciscan amphibolite. Circles show regions of zircon dated by SHRIMP-RG.
GSA in Seattle is drawing near and I’m excited to take part in a number of student authored paper this year. I’m also chairing a session on the tectonic evolution of convergent margins view through the lens of the Andean margin. I’m looking for students for the upcoming 2018-2019 academic year to work on the structural and metamorphic evolution of subduction zone and arc processes. If you’re interested please come and find me at any of the below presentations or send me an email to arrange a time to meet.
Student authored presentations
- Cordova et al. (Sun, 57-6), Initiation and early evolution of a subduction zone: T-t-D history of the Easton Metamorphic Suite, northwest Washington State.
- Webber et al. (Tues, 213-11), U/Pb of the Sierra de Maz: Implications for the timing of terrane accretion and translation along the western margin of Gondwana.
- Tholt et al. (Tues, 291-7), Metamorphic evolution of the Sierra de Maz: Implications for the timing of terrane accretion along the western margin of Gondwana.
- Houlihan et al. (Tues, 291-8), 40Ar/30Ar geochronology and kinematic analysis of the Sierra de Maz: Implications for the timing and orientation of deformation on the Paleozoic margin of western Gondwana.
Non-student authored presentations
- McClelland et al. (Tues, 291-6), Detrital zircon characterization of metasedimentary units of the Sierra de Maz, Western Sierras Pampeanas, Argentina.
- Roeske et al. (Tues, 213-12), The link between Ordovician-Early Devonian terrane accretion and translation and the sudden arc flare-up and cessation of the Famatina arc.
- Mulcahy et al. (Tues, 213 and 291), Understanding the interplay of structural, metamorphic, and magmatic processes in the evolution of convergent margins: Lessons from the Proterozoic to present-day Andean margin. Invited speakers include:
We just published a new paper in American Mineralogist, by lead author Marie Jackson. The paper, Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete, explored seawater interaction with zeolite minerals to produce Al-tobermoreite, a zeolite mineral that significantly increases the durability of ancient Roman cement compared to modern day materials. The paper received considerable press with articles by the Gaurdian, Nature News, and the Washington Post, among others. My contribution came from electron probe microanalysis (EPMA) of phillipsite and Al-tobermorite and an analysis of published compositions. The paper is open access and all of the data are available in the supplemental material. In addition, the EPMA data and code to generate related figures are available on Github and archived on Zenodo.
Next week is AGU and I’m excited to take part in two student presentations. Shaina Myers, an undergraduate student at Western, is presenting the results of her senior research on lawsonite stability and REE chemistry from the Franciscan subduction zone. Michael Kenney, a recent M.S. graduate at UC Davis, is presenting his thesis research using U-Pb dating of titanite to try and understand a protracted metamorphic history in a collisional orogen of northwest Argentina.
Franciscan lawsonite-blueschist assemblages, Myers & Mulcahy, AGU 2016, T31E-2962
Shaina Myers’ poster is on Wed from 08:00-12:20
T31E-2962: Retrograde lawsonite formation in the Franciscan subduction complex
Michael Kenney’s poster is on Monday from 13:40-18:00
V13D-2888: Differentiating metamorphic events in a polymetamorphic terrane using Zr-in-Ttn thermometry and titanite U-Pb geochronology
Andy Tholt is starting at Western this fall. He and I are headed to Argentina in August to test models for the metamorphic evolution of the Famatina margin. Andy earned his B.S. degree from U.C. Berkeley and then spent a year working at the Berkeley Geochronology Center. I’m really excited to have Andy join on as my first graduate student.
Andy trekking in Nepal, summer 2016.
Western graduate student Jeremy Cordova was awarded and EarthScope AGeS Program grant. Jeremy is working with Liz Schermer and I to date the initiation and duration of subduction metamorphism in the Northwest Cascades. Jeremy will be working with Laura Webb in UVM Noble Gas Lab to date high pressure mineral assemblages associated with subduction.
Large-scale faults (>1000 km) occur on every continent and many long-lived intracontinental fault systems record complex reactivation histories that localize convergent and strike-slip deformation over 100s of millions of years. The presence of these fault zones defies continuum deformation models of the continents, wherein faults are viewed as passive features responding to mantle flow. This research will test competing hypotheses for the origin of persistent large-scale faults by studying the tectonic history of exhumed middle- and lower-crustal sections of an ancient fault zone. End member models predict that such fault zones originate either as: 1) convergent-collisional boundaries between blocks of different strength, 2) transform boundaries along pre-existing zones of weakness, or 3) strike-slip boundaries within the arc-forearc region of oblique subduction settings.
The Valle Fértil fault zone of western Argentina is a ~1200 km major crustal lineament that records at least 400 m.y. of intermittent deformation and is an ideal location to test the above models because of excellent geophysical constraints on crustal strength contrasts, variable depths of exposure along strike, well-constrained tectonic evolution, and ideal mineral assemblages for dating the history of deformation within the fault. The results from the Valle Fértil fault can be applied to other intracontinental faults to address what factors determine the origin of major structures within orogenic belts and the processes by which they ultimately become large-scale faults with complex reactivation histories. The results will inform a broad group of scientists on deep crustal processes that control the location of seismicity, high heat flow, and hydrothermal systems. The collaborative research with colleagues Sarah Roeske (UC Davis), Bill McClelland (Univ. of Iowa), and Vinícius Meira (Univ. of Campinas, Brazil) will support undergraduate and graduate education and foster international collaboration between U.S. and South American scientists.
I’ll be in San Francisco next week for the annual AGU conference. I have a coauthored poster on Monday afternoon and I’m presenting an invited talk on Wednesday. I’m also a member of the VGP Student Awards Committee and excited to see what students are presenting this year. Speaking of students, I’m on the lookout for MS students to come and work with me at Western. Be sure and flag me down if you’re interested.
The poster by lead author Marie Jackson:
V13C-3150: Authigenic Mineral Cycling in Roman Seawater Concrete with Campi Flegrei Pumiceous Ash Pozzolan
The talk, with my coauthor Jeff Vervoort:
V34A-02: Complexities of Lu-Hf geochronology in convergent orogens