Geodynamic Processes at Rifting and Subducting Margins
Search through the Google Scholar Bibliography to find a collection of publications issued from the E-FIRE Group. The bibliography is periodically updated by individuals from the E-FIRE group.
New research on ancient Alpine rocks may unveil clues to Earth’s evolution | October 25, 2016
Report from the Field – ExTerra Field Institute and Research Endeavor: Western Alps, Summer 2017 | GeoPRISMS newsletter issue 39, Fall 2017
EFIRE students, post-docs, and faculty joined ZIP faculty in the Western Alps and Syros, Greece for about 2 weeks. This trip allowed the ~30 participants to view different exposures of Alpine geology and also to view the subduction-related rocks of Syros, Greece as a comparison to those which we have studied in the Western Alps. We also spent time sharing the outcomes of our research with each other in workshop format during our time on Syros.
Panorama from the 2019 Field Institute (Pain de Sucre, Alpes, France)
EFIRE students, post-docs, and faculty joined ZIP faculty in the Western Alps for a tour of key field areas identified for student and post-doc research of rocks exhumed from fossil subduction zones. The group of almost 30 people spent about 2 weeks together learning from ZIP collaborators, seeing the rocks, and collecting samples for research projects. After this time together, the large group broke into smaller groups who traveled back to some of the localities to sample in greater detail. Altogether the group collected about 700 kg of rock samples!
Group photo in Monviso. Photo credit: Philippe Agard
Metamorphic map of Western Alps with stop locations from our 10-day group fieldwork overview. (1) Drive from Geneva with Pre-Alps overview, (2) Dent Blanche, (3) Lago Di Cignana, (4) Schistes Lustres, (5) Lanzo Massif, (6, 7) Monviso Ophiolite, (8) Relaxation day, (9, 10) Voltri Massif. Map modified after Beltrando et al. (2010).
| #Stop | Location | GPS Coordinates | More Info |
|---|---|---|---|
| #2 | Dent Blanche | 45.869N 7.328E | |
| #3 | Lago Di Cignana | 45.878N 7.592E | |
| #4 | Schistes Lustrés | 44.998N 6.883E | more info |
| #5 | Lanzo Massif | 45.177N 7.393E | |
| #6, #7 | Monviso Ophiolite | 44.666˚N 7.110˚E | more info |
| #9, #10 | Voltri Massif | 44.479˚N 8.601˚E | more info |
The Erro-Tobbio Unit in the Voltri Massif (Ligurian Alps, Italy) is mainly composed of high-pressure antigorite serpentinites and metaperidotites intruded by gabbroic and basaltic dikes. This unit has been interpreted as a slice of a variably serpentinized subcontinental mantle exposed on the Tethyan ocean floor that underwent partial dehydration during subduction in Cretaceous time. The peak metamorphic conditions of the Erro-Tobbio Unit were constrained to be 550–600 °C and 2.0–2.5 GPa based on the associated eclogitized metagabbros, metabasalts and metarodingites. The unit contains large and continuous outcrops of high-pressure antigorite serpentinites and metaperidotites with cross cutting veins containing secondary metamorphic olivine, magnetite and Ti-rich humite group minerals. The unique field relation and mineralogical assemblages in both the wall-rock and veins have been regarded as a snapshot of the early dehydration and subsequent fluid channelization that occurs in a subduction zone. Thus, studying the Erro-Tobbio Unit may provide key constraints in understanding those poorly understood deep Earth processes.
The E-FIRE group aims to understand fundamental questions on how and when water and other volatiles escape from a dehydrating rock, what the compositions of the liberated fluids are, and how such reactive fluids interact with and alter the wall-rock as it passes through. Those research questions will be addressed using a combination of phase petrology, whole-rock and mineral elemental and isotope geochemistry, and thermodynamic modeling of fluid-rock interactions. Samples collected mainly along the Gorzente River include deformed and undeformed partially dehydrated serpentinites/metaperidotites, cm-sized veins containing metamorphic olivine, magnetite and Ti-rich clinohumite assemblage, eclogitized metagabbro and metarodingite.
The Voltri massif is an exhumed meta-ophiolite that consist of portions of subcontinental lithospheric mantle, oceanic crust (predominantly metagabbroic dikes and rodingites), and minor sediments that formed part of the Ligurian Tethys Ocean that formed during the Jurassic. In different parts of the Voltri massif, these rocks variably record snapshots of their tectonic history from initial mantle processes and seafloor spreading, subduction up to eclogite facies conditions, and a greenschist facies overprint during exhumation. The entire massif has been interpreted as a tectonic mélange, with most structures forming during subduction and exhumation.
Our primary research goals in the Voltri massif are to constrain the PT conditions and timing of prograde subduction and the composition and movement of fluids during subduction. Current projects involve using thermodynamic and geochemical modeling, garnet geochronology, Li isotopes, and fluid inclusions to answer these questions.
A former accretionary wedge, the Schistes Lustrés high-pressure (HP) unit in the Western Alps is composed of calcschists and metapelites subducted to blueschist facies conditions. Other lithologies entrained locally in the metasediments include serpentinites and, less often, metabasalts. Pressure and temperature conditions within the Schistes Lustrés vary from E-W and can show subtle, but distinct evidence for pressure jumps between individual subunits, which are interpreted to have been juxtaposed during subduction. The northern part of the Schistes Lustrés is overlain by the Dent Blanche unit. On the western side of the Dent Blanche, temperatures in the Schistes Lustrés decrease to the west from structurally deeper units in the east. In the southern section of the Western Alps, where the Schistes Lustrés borders the Monviso meta-ophiolite to the west, individual subunits grade continuously in temperature (350 – 500ºC), with the deepest, and hottest, units exposed in the west. A distinct jump in pressure of ~5 kbar between the Lower Unit and Median Unit likely indicates a post-peak pressure pairing of these two subunits. The whole unit was exhumed at 35 – 40 Ma based on Rb/Sr and Ar/Ar dating and reached peak pressure between 55 – 45 Ma, although the HP event is difficult to discern due to a widespread greenschist-facies overprint.
The Schistes Lustrés also represents an important locale as a key to quantifying the subduction component of the global carbon cycle. Stable isotope (δ18Ο and δ13C) data suggest externally derived H2O-rich fluids percolated into the unit. However, on average, the Schistes Lustrés experienced only minor decarbonation.
The Monviso Ophiolite is made up of two intact sections of Tethyan oceanic crust that both reached eclogite facies conditions during subduction and collision of the European margin beneath the African plate. Peak metamorphic conditions of of 2.2-2.6 GPa and 480-550 °C were reached at ~50Ma. Both ophiolite sections are comprised of metasedimentary cover, metabasalt, metagabbro and serpentinized peridotite with varying amounts of deformation and retrogression (particularly in the metabasalts). The lower section is cut by several shear zones which contain blocks of the various lithologies in a serpentinite matrix and show evidence of pervasive fluid rock interaction.
E-FIRE research on this field location aims to understand the duration and distribution of fluid-rock interaction and the sources of these fluids, the age of important metamorphic events, and the preservation of ocean-floor isotopic stratigraphy to eclogite facies. Samples collected include undeformed and unaltered examples of the entire lithologic stratigraphy of the lower ophiolite section, various examples of fluid-rock interaction (veins and detailed transects of metasomatic rinds), rodingites, calcite-bearing metasediments, and examples of exceptional mineral textures (mylonitized to extremely coarse-grained eclogite, static to mylonitized serpentinite).
The ExTerra Field Institute and Research Endeavor (E-FIRE) united US scientists studying rocks exhumed from paleo-subduction zones (through ExTerra: Exhumed Terranes) with European colleagues also working on subduction systems (through ZIP: Zooming In between Plates, a Marie Curie training network). E-FIRE Field Institutes gathered ExTerra and ZIP scientists in the field to trace the cycle of rocks and fluids through the subduction process as recorded in Earth’s premier example of a fossil subduction zone: the Western Alps, Europe. Eleven early-stage researchers (ESRs), including PhD students and post-doctoral fellows based at nine different U.S. institutions, collected field data and rock samples to address three overarching research questions:
The projects adopted a variety of approaches to address these questions, including mineralogical and petrological analysis; textural characterization; geochemical analysis of major and trace elements (e.g., HFSE, REE, halogens), stable isotopes (e.g., δ7Li, δ11B, δ13C, δ18O, δ37Cl), and radiogenic isotopes (e.g., U-Th-Pb, Sm-Nd, Rb-Sr); and thermodynamic and geodynamic modeling.
The first ExTerra Field Institute was held from October 11-13, 2014 in the Santa Lucia Mountains of central coastal California. The institute featured a tilted view through the Late Cretaceous Salinian arc and its framework from shallow exposure levels to the deep crust, accretionary wedge sediments and various metamorphosed subduction assemblages of the Franciscan Complex, and possible intrusions of lithospheric upper mantle-derived melts in the Escondido ultramafic body. Convened by Mihai Ducea (University of Arizona), Alan Chapman (Missouri S&T), Maureen Feineman (Penn State), and Sarah Penniston-Dorland (University of Maryland), the institute was attended by 11 geoscientists. Of the participants, 6 were female and 5 male; 6 were graduate students or early-career faculty and 5 were mid-career or senior faculty. Fruitful group discussions took place throughout the trip, many of which were guided by the big questions identified in the ExTerra White Paper:
7) What are the mass fluxes into and out of the crust over geologic time?
Alan Chapman leads a morning discussion at the Whale Point Cabin at the Landels-Hill Big Creek Reserve.
Two significant ideas for future research in the Santa Lucia Mountains emerged from group discussions. The first relates to the processes by which metasedimentary rocks are emplaced into arc lower crust (>30 km paleodepth). Possibilities include downward flow of upper crustal sections, retroarc thrusting from the foreland, and tectonic underplating and/or diapirism from the trench side. Discriminating between these mechanisms will require focused structural, U- Pb zircon geochronologic, and zircon Hf isotopic analyses, as each model predicts different temporal and spatial patterns in pluton and supracrustal contaminant geochemistry. The group collected samples from several areas with the goal of performing such analyses. Sampled areas included Sur Series metamorphic framework rocks, deep plutonic rocks of the Coast Ridge belt, and trench deposits of the Franciscan Complex. The second effort will be to constrain the gabbroic (mantle-derived) input in a continental magmatic arc. Accordingly, samples exhibiting mafic compositions were collected from various locations within the Salinian arc and also from the Escondido ultramafic body that intrudes the arc. Other proposed ideas for study included building a comprehensive geochemical, structural, and geophysical cross-section of the Salinian arc crust, and following the devolatilization, chemical evolution, and development of preferred orientation in a sediment pile as it is subducted and underplated.
Eighteen samples were collected for research purposes and were registered with IGSN (International Geo Sample Number) through the System for Earth Sample Registration (SESAR).
Day 1: Salinian
Day 2: Franciscan
Day 3: Salinian
Conveners: Maureen Feineman, Sarah Penniston-Dorland, Brian Savage
6:00-6:30 | Greeting, reception, and introduction – Juli Morgan, Maureen Feineman
6:30-7:00 | Keynote: Interdisciplinary study of exhumed subduction zones – Bradley Hacker
7:00-7:30 | First Breakout: identify scientific objectives for ExTerra
7:30-7:45 | Focusing interdisciplinary study through sample and data management – Maureen Feineman
7:45-8:15 | Second Breakout: identify logistical and organizational needs
8:15-8:45 | Synthesis
This mini-workshop, to be held the evening of Wednesday, December 7, during the 2011 Fall Meeting of the American Geophysical Union in San Francisco, CA, aims to explore the interdisciplinary utility of studying exhumed terranes from extinct subduction zones. Exhumed terranes represent parts of the subduction zone that are not directly observable at currently active margins. By going to exhumed terranes, we are able to make direct measurements and observations that can be invaluable for ground-truthing seismic observations, thermal structure models, and interpretations of geochemical and petrological processes based on the eruptive products of active subduction zones. Due to the nature of the dynamic processes that bring exhumed terranes to the Earth’s surface, it is not possible to identify a single locality that represents the full range of subsurface regimes, including the subducted slab, the mantle wedge, the overlying arc crust, and exposed fault systems in the crust or accretionary prism. The goal of this workshop is twofold;
Identify what samples and data will be most useful to the diverse geophysical and geochemical subdisciplines involved in the study of active convergent margins, and
Determine how to best integrate studies of globally distributed exhumed terranes into a unified and directed body of research.
We will discuss in detail the benefits and burdens of sample archiving and data sharing. We encourage participation not only of those involved in the direct observation, sampling, and analysis of exhumed terranes, but also those who would make use of data gathered from these studies, including seismologists, modelers, and experimentalists.
The NSF GeoPRISMS Subduction Cycles and Deformation SCD) Science Plan identified the study of exhumed terranes as an important component of subduction zone research. It remains to be determined how to best integrate the study of exhumed terranes and high pressure rocks into GeoPRISMS SCD. GeoPRISMS largely follows the very effective model used previously by MARGINS of building a research program around a few select focus sites at active subduction zones. This focused research has been a clear strength of the MARGINS program. Work at focus sites, however, may not be the best way to approach exhumed terranes and HP-UHP rocks. During active subduction, these features are buried deep beneath the surface. Of necessity, exhumation most often occurs during or following the death of a subduction zone. The nature of exhumation processes is such that entire subduction zones are rarely if ever exposed in a single location, requiring field work to be conducted at multiple locations, and most likely by multiple research groups using different techniques and approaches, before a comprehensive range of pressure and temperature conditions can be represented. Currently, the study of exhumed terranes is included in the GeoPRISMS implementation plan as a thematic approach. The goal of this meeting is to explore how we can best organize research on exhumed terranes and HP-UHP rocks under the umbrella of GeoPRISMS SCD such that we might accomplish more as a group than we could as individuals working independently.
The GeoPRISMS SCD Implementation Plan identifies the following seven questions to be addressed within the initiative. These should be a good place to start in terms of focusing our research within the context of the SCD initiative.
The following objectives and scientific questions were identified during the GeoPRISMS SCD Implementation Meeting breakout session on Exhumed Terranes, which took place in parallel with the Focus Site breakout groups. We realized that there were a number of individuals at the meeting working in various disciplines whose work was not easily tied to a focus site in an active subduction zone. Those present fell into one or more of the four sub;fields of research, given below.
We will split out twice into four breakout groups to facilitate inclusion of everyone in the discussion. Please be prepared to discuss the following issues. The ideas and feelings of the group with respect to these and any other issues/questions that arise will be assembled in a white paper to be submitted to GeoPRISMS.
ExTerra: Understanding Convergent Margin Processes Through Studies of Exhumed Terranes | AGU 2011
ExTerra: Understanding Convergent Margin Processes Through Studies of Exhumed Terranes
AGU Fall Meeting 2011, San Francisco
1Pennsylvania State University; 2University of Maryland; 3University of Rhode Island
On the evening of December 7, 2011, about 35 geoscientists convened in the ExTerra mini-workshop during the fall AGU Meeting to discuss how to integrate the study of exhumed rocks into the GeoPRISMS Subduction Cycles and Deformation (SCD) initiative (Figure 1). After introductory presentations by the convenors and keynote speaker Brad Hacker (University of California, Santa Barbara), workshop participants divided into four groups based on different types of exhumed terranes: subducted slab, mantle wedge, arc crust, and fault systems. The group discussion was divided into two areas: identification of scientific objectives and organizational strategies. Details of the outcomes from each discussion group are outlined at http://www.geoprisms.nineplanetsllc.com/scd/exterra.html. This is an ongoing discussion leading to a white paper contribution to the GeoPRISMS SCD Science Plan, and we invite all interested parties to participate!
The NSF GeoPRISMS Science Plan for the SCD Initiative identified the study of exhumed terranes as an important component of subduction zone research. It remains to be determined how to best integrate the study of exhumed terranes and high pressure rocks into GeoPRISMS SCD. GeoPRISMS largely follows the very effective model used previously by MARGINS of building a research program around a few locations, referred to as primary sites, atactive subduction, these features are buried deep beneath the surface. Of necessity, exhumation most often occurs during or following the death of a subduction zone. The nature of exhumation processes is such that entire subduction zones are rarely if ever exposed in a single location, requiring fieldwork to be conducted at multiple locations, and most often by multiple research groups using different techniques and approaches, before a comprehensive range of pressure and temperature conditions can be represented. Currently, the study of exhumed terranes is included in the GeoPRISMS Implementation Plan as a thematic study. The goal of this mini-workshop and the resulting white paper is to explore how we can best organize research on exhumed terranes under the umbrella of GeoPRISMS SCD such that we might accomplish more as a group than we could as individuals working independently.
Figure 2. SOTA fieldtrip to see Cycladic subduction zone rocks on the island of Syros, Greece.
The integration of studies of exhumed systems through GeoPRISMS can organize individual efforts towards major interdisciplinary objectives. Integration of data from multiple sites allows coverage of a broad range of conditions not observable at a single site. Studies of exhumed systems under the umbrella of GeoPRISMS have the potential to link experiments and seismic observation to physical reality, adding the components of space and time. Collaboration and communication between different communities represented within GeoPRISMS allow sample and data collection to be tuned to serve the needs of other groups (geochemists helping seismologists, petrologists helping modelers, etc.).
Four target areas have been identified as significant to improving our understanding active subduction processes by the study of exhumed terranes: 1) subducted slab, including HP and UHP rocks such as blueschists, eclogites, and metapelites; 2) mantle wedge, including serpentinites, ophiolites, and peridotites; 3) middle and lower arc crust, including granitoids, gabbros, migmatites, gneisses, amphibolites, granulites; and 4) exhumed fault systems, including accretionary prisms.
Several different ideas have been suggested in order to facilitate communication among different geoscientists. One idea is to hold focused, interdisciplinary field trips in order to provide the opportunity for non-field geologists to observe exhumed rocks and create an environment for exchange of ideas between field geologists and non-field geologists. Another idea is to create a sample repository and associated database that will allow sample collectors to connect with those who have use for rock samples. For example, experimental petrologists can make use of a sample repository to find materials for their experiments.
Figure 4. AGU Fieldtrip to see subduction zone rocks of the Franciscan Complex, CA.
We recognize that there are many challenges facing the integration of the study of exhumed terranes into GeoPRISMS. How do we open the dialog between petrologists, geophysicists, and modelers? How can studies of worldwide exhumed terranes be related to current GeoPRISMS focus sites? GeoPRISMS is a small program, and we will need to leverage with funds from outside sources.
