Probing the nature of the Hikurangi margin hydrogeologic system


Evan A. Solomon (University of Washington), Marta Torres (Oregon State University), and Robert Harris (Oregon State University)

Fluid generation, migration, and pore fluid pressure at subduction zones are hypothesized to exert a primary control on the generation of seismicity, low-frequency earthquakes, and slow slip events (SSEs) (e.g. Ranero et al., 2008; Obana and Kodaira, 2009; Saffer and Tobin, 2011; Saffer and Wallace, 2015). The SAFFRONZ (Slow-slip and fluid flow response offshore New Zealand) project addresses the GeoPRISMS Subduction Cycles and Deformation Initiative Science Plan by testing interrelationships among fluid production, fluid flow, and slow slip at the Hikurangi Margin. The recognition of dramatic changes in the along-strike distributions of SSEs and their recurrence intervals, interseismic coupling, inferred pore pressure, and other subduction-related parameters (Fig. 1) have resulted in a concerted international effort to acquire seismological, geodetic, other geophysical, and geomechanical data both onshore and offshore the Hikurangi margin. This effort includes recent scientific ocean drilling, logging, and the deployment of two subseafloor observatories during IODP Expeditions 372 and 375 (see page 16 of this issue), as well as a 3-D seismic reflection survey on the northern margin. (p. 14).

SAFFRONZ will complement and extend these efforts by providing:

A continuous two-year record of fluid flow rates and composition over the timeframe of the next expected SSE,

Information on the present background state and past locations of fluid flow and how they relate to inferred pore fluid overpressure along the plate boundary, and

Comparative geochemical and hydrologic data between the northern and southern sections of the margin.

The SAFFRONZ field program is scheduled for January 10 to February 14, 2019 on the R/V Revelle. Our field strategy employs a nested approach to constrain the margin-wide fluid flow distribution tied to estimated pore pressure evolution along the plate boundary from modeling studies and seismic attributes.

We are specifically targeting fault zones and off-fault locations between the deformation front and the shelf-break (Fig. 2). Site locations will be guided by pre-existing multi- and single-beam sonar data (including seafloor backscatter and water column indicators of gas seepage), 2D/3D seismic reflection data, and real-time water column multi-beam sonar surveys during the research expedition. Violin-bow heat flow measurements and piston coring will guide ROV Jason hydroacoustic surveys, Jason heat flow probe measurements, and the collection of push cores to further identify sites of active fluid discharge. Finally, the ROV surveys will guide the deployment of benthic fluid flow meters (Fig. 3) to generate a record of fluid flow rates and composition over a two-year period – the approximate recurrence interval for SSEs in this region. We anticipate deploying about sixteen benthic fluid flow meters, some co-located with seafloor bottom pressure recorders managed by GNS Science, during the 2019 field program.Although our focus is on the northern margin, the location of shallow SSEs and most research activity, we will also conduct ship and ROV operations and deploy a subset of fluid flow meters in the southern region of the margin.

Comparison of fluid flow pathways, fluid composition, fluid flow rates, and flow transients between the northern and southern areas will provide information on the differences in the nature of dewatering between the accretionary southern portion of the margin hosting deep SSEs and the dominantly non-accretionary northern portion with shallow SSEs. The along-strike comparison will also provide a control (reference) transect to compare regional (i.e. flow in response to SSEs in the north) to other hydrologic phenomena.

From both scientific and societal perspectives, results from this project will contribute to our understanding of fault slip behavior offshore New Zealand that have global implications for the postulated interdependence of temperature, pore pressure, fluid flow, and SSEs at subduction zones. The ability to integrate the SAFFRONZ experiment with concurrent bottom pressure recorder deployments, onshore cGPS data, IODP borehole monitoring experiments, 2D and 3D seismic reflection data, and other data to be collected along the margin in the next few years greatly enhance the outcomes of this project. The synthesis of the results from all the concurrent experiments being conducted at Hikurangi promises to be very exciting and the integration of the hydrologic data produced from this project will result in an unprecedented view of deformation and hydrological responses to slow slip at subduction zones. ■

References

Barnes, P.M., G. Lamarche, J. Bialas, I. Pecher, S. Henrys, G. Netzeband, J. Greinert, J. Mountjoy, K. Pedley, G. Crutchley, (2010), Tectonic and Geological framework for gas hydrates and cold seeps on the Hikurangi subduction margin, New Zealand, Marine Geology. doi: 10.1016/j.margeo.2009.03.012
Brown, K.M., M.D. Tryon, H.R. DeShon, L.M. Dorman, S.Y. Schwartz (2005), Correlated transient fluid pulsing and seismic tremor in the Costa Rica subduction zone, Earth Planet. Sci. Lett., 238, 189-203.
Obana, K., S. Kodaira,(2009), Low-frequency tremors associated with reverse faults in a shallow accretionary prism. Earth Planet. Sci. Lett. 287, 168–174.
Ranero, C. R., I. Grevemeyer, H. Sahling, U. Barckhausen, C. Hensen, K. Wallmann, W. Weinrebe, P. Vannucchi, R. von Huene, K. McIntosh (2008), Hydrogeological system of erosional convergent margins and its influence on tectonics and interplate seismogenesis, Geochemistry Geophysics Geosystems, 9(3), Q03S04.
Saffer, D.M., H.J. Tobin, (2011), Hydrogeology and mechanics of subduction zone forearcs: Fluid flow and pore pressure, Ann. Rev. Earth Planet. Sci., 39, 157-186.
Saffer, D.M., L.M. Wallace, (2015), The frictional, hydrologic, metamorphic and thermal habitat of shallow slow earthquakes. Nat. Geosci., 8(8), 594–600.
Saffer, D.M., L.M. Wallace, K. Petronotis, (2017), Expedition 375 Scientific Prospectus: Hikurangi margin coring and observatories. International Ocean Discovery Program. doi.org/10.14379/iodp.sp.375.2017
Solomon, E.A., M. Kastner, H. Jannasch, G. Robertson, Y. Weinstein, (2008), Dynamic fluid flow and chemical fluxes associated with a seafloor gas hydrate deposit on the northern Gulf of Mexico slope. Earth Planet. Sci. Lett. 270, 95-105.
Tryon, M.D., K.M. Brown, M.E. Torres, (2002), Fluid and chemical flux in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, Or, II: hydrological processes. Earth Planet. Sci. Let. 201, 541–557.
Wallace, L.M., J. Beavan, R. McCaffrey, D. Darby, (2004), Subduction zone coupling and tectonic block rotation in the North Island, New Zealand, J. Geophys. Res., 109, doi:10.1029/2004JB003241
Wallace, L.M., J. Beavan, (2010), Diverse slow slip behavior at the Hikurangi subduction margin, New Zealand, J. Geophys Res., 115(B12402), doi:10.1029/2010JB007717.

Reference information
Probing the nature of the Hikurangi margin hydrogeologic system
GeoPRISMS Newsletter, Issue No. 40, Spring 2018. Retrieved from http://geoprisms.org

Slow slip and future earthquake potential in New Zealand and Cascadia


Noel Bartlow (University of Missouri), Laura Wallace (UT Austin & GNS Science), Ryan Yohler (University of Missouri), and Charles Williams (GNS Science)

The New Zealand and Cascadia subduction zones are two GeoPRISMS primary sites that have captured the interest of geophysicists from around the world. Both subduction zones feature significant geological hazards, including the potential for large earthquake ruptures and tsunamis. In New Zealand, the capital city of Wellington sits directly atop a large patch of the main subduction plate interface which is frictionally locked, with the potential to rupture in future earthquakes. In Cascadia, there is also potential for a large earthquake on the main subduction interface, which may impact cities such as Portland, OR and Seattle, WA.

Both New Zealand and Cascadia also host slow slip events (SSEs). Slow slip events consist of slip on the subduction plate interface – just as would occur in an earthquake – except the slip takes place more slowly than it would during an earthquake. Now that slow slip events are widely recognized at many subduction zones, it is critical to better understand their role in the accommodation of plate motion. Many questions also exist regarding implications of slow slip for subduction zone mechanics, and the relationship between slow slip events and seismic slip events on the plate boundary.

In New Zealand, slow slip events have been shown to trigger regular earthquakes up to magnitude 6 (e.g. Wallace et al., 2017). Additionally, some evidence exists from the 2011 Tohoku-Oki Mw 9.0 earthquake (e.g. Ito et al., 2013) and the 2014 Mw 8.1 Iquique earthquake (Ruiz et al., 2014) that slow slip events may be able to trigger damaging megathrust events. Most slow slip events do not trigger earthquakes and we currently cannot differentiate slow slip events that might trigger large earthquakes from those that will not. It is possible, however, that further study will lead to methods that allow slow slip events to be used as reliable earthquake precursors.

PIs Noel Bartlow (Univ. of Missouri) and Laura Wallace (UTIG), along with collaborator Charles Williams (GNS Science New Zealand) and graduate student Ryan Yohler (Univ. of Missouri) are studying slow slip events and frictional locking in New Zealand and Cascadia.

One goal of the project is to create self-consistent catalogs of slow slip events in both subduction zones that capture the time varying behavior of slow slip, including how these events grow and decay and move along the subduction plate interface. The data used for these models consists of land-based geodetic GPS time series, and in New Zealand, we also use vertical deformation of the seafloor recorded for one slow slip event using absolute pressure sensors (Wallace et al., 2016).

Previous time varying slow slip event modeling studies usually assume a uniform, elastic half-space (e.g. Bartlow et al., 2014). These new models utilize spatially-varying elastic properties within the earth based on seismic velocity models in both New Zealand and Cascadia, calculated using the PyLith finite element code. This leads to more accurate models of slip during slow slip events, and therefore, more accurate estimates of the amount of slip taken up in slow slip as opposed to being available for release in future earthquakes. Preliminary models for both New Zealand (Williams et al., 2017) and Cascadia (Bartlow et al., 2017) were shown at the American Geophysical Union 2017 Fall meeting. Additionally, a time-dependent model incorporating both onshore GPS and offshore pressure measurements for the 2014 Gisborne, New Zealand slow slip events was shown at the meeting by graduate student Ryan Yohler (Yohler et al., 2017). This model is shown in Figure 1. This slow slip event occurred near the locations of two historical 1947 earthquakes that caused damaging tsunami waves. This is the first time that seafloor geodetic data have been used in a time-dependent deformation model.

As part of this project, a team led by Laura Wallace, including Bartlow and other collaborators, have recently reported the occurrence of a large, shallow (<15 km), two-week slow slip event at the Northern Hikurangi margin triggered dynamically by passing seismic waves from the November 2016 magnitude 7.8 Kaikōura earthquake, over 600 km away (Fig. 2; Wallace et al., 2017). Long-duration (>1 year), deep (>25 km) slow slip was also triggered at the southern Hikurangi margin (Kapiti region), and afterslip occurred on the subduction interface beneath the northern South Island of New Zealand (Fig. 1).

Triggered slow slip at southern Hikurangi is more likely due to large static stress changes induced by the Kaikōura earthquake, given that area’s closer proximity to the earthquake (Wallace et al., 2018). Prior studies had already identified cases of slow slip events triggering earthquakes, and nearby earthquakes prematurely stopping ongoing slow slip events, but these studies are the first to show that dynamic and/or static stress changes from passing seismic waves may also trigger large-scale, widespread slow slip events. We are still discovering the wealth of possible complex interactions between slow slip events and earthquakes, and what they might mean for hazards. ■

References

Bartlow, N.M., C.A. Williams, L.M. Wallace, (2017), Building a catalog of time-dependent inversions for Cascadia ETS events. In AGU Fall Meeting Abstracts.
Bartlow, N.M., L.M. Wallace, R.J. Beavan, S. Bannister, P. Segall, (2014), Time‐dependent modeling of slow slip events and associated seismicity and tremor at the Hikurangi subduction zone, New Zealand. J. Geophys. Res.: Solid Earth, 119(1), 734-753.
Ito, Y., R. Hino, M. Kido, H. Fujimoto, Y. Osada, D. Inazu, Y. Ohta, T. Linuma, M. Ohzono, S. Miura, M. Mishina, (2013), Episodic slow slip events in the Japan subduction zone before the 2011 Tohoku-Oki earthquake. Tectonophysics, 600, 14-26.
Ruiz, S., M. Metois, A. Fuenzalida, J. Ruiz, F. Leyton, R. Grandin, C. Vigny, R. Madariaga, J. Campos, (2014), Intense foreshocks and a slow slip event preceded the 2014 Iquique Mw 8.1 earthquake. Science, 345(6201), 1165-1169.
Wallace, L.M., S. Hreinsdottir, S. Ellis, I. Hamling, E. D’Anastasio, P. Denys, (2018), Triggered slow slip and afterslip on the southern Hikurangi subduction zone following the Kaikōura earthquake. Geophys. Res. Lett., 45, doi.org/10.1002/2018GL077385
Wallace, L.M., Y. Kaneko, S. Hreinsdóttir, I. Hamling, Z. Peng, N.M. Bartlow, E. D’Anastasio, B. Fry, (2017), Large-scale dynamic triggering of shallow slow slip enhanced by overlying sedimentary wedge. Nat. Geosci., 10(10), 765.
Wallace, L.M., S.C. Webb, Y. Ito, K. Mochizuki, R. Hino, S. Henrys, S. Schwartz, S., A.F. Sheehan, (2016), Slow slip near the trench at the Hikurangi subduction zone, New Zealand. Science, 352(6286), 701-704.
Williams, C.A., L.M. Wallace, N.M. Bartlow, (2017), Time-dependent inversions of slow slip at the Hikurangi subduction zone, New Zealand, using numerical Green’s functions. In AGU Fall Meeting Abstracts.
Yohler, R.M., N.M. Bartlow, L.M. Wallace, C.A. Williams, (2017), Constraining slip distributions and onset of shallow slow slip in New Zealand by joint inversions of onshore and offshore geodetic data. In AGU Fall Meeting Abstracts.

Reference information
Slow slip and future earthquake potential in New Zealand and Cascadia
GeoPRISMS Newsletter, Issue No. 40, Spring 2018. Retrieved from http://geoprisms.org

Revealing the secrets of the New Zealand GeoPRISMS Primary Site


Slow slip and future earthquake potential in New Zealand and Cascadia

Slow slip and future earthquake potential in New Zealand and Cascadia

Noel Bartlow (University of Missouri), Laura Wallace (UT Austin & GNS Science), Ryan Yohler (University of Missouri), and Charles Williams (GNS Science) The New Zealand and Cascadia subduction zones are two GeoPRISMS primary sites that have...
Probing the nature of the Hikurangi margin hydrogeologic system

Probing the nature of the Hikurangi margin hydrogeologic system

Evan A. Solomon (University of Washington), Marta Torres (Oregon State University), and Robert Harris (Oregon State University) Fluid generation, migration, and pore fluid pressure at subduction zones are hypothesized to exert a primary control...
Volatile cycling through the Hikurangi forearc, New Zealand

Volatile cycling through the Hikurangi forearc, New Zealand

Jaime D. Barnes (UT Austin), Jeffrey Cullen (UT Austin), Shaun Barker (Univ. of Waikato), Samuele Agostini (Istituto di Geoscienze e Georisorse), Sarah Penniston-Dorland (Univ. of Maryland), John C. Lassiter (UT Austin), Andreas Klügel (Univ. of ...
The NZ3D Experiment – Adding a new dimension for understanding slow slip events

The NZ3D Experiment – Adding a new dimension for understanding slow slip events

MGL1801 Participants - Ryuta Aral (JAMSTEC), Stephen Ball (Univ. of Wisconsin, Madison), Nathan Bangs (UT Austin), Dan Barker (GNS Science), Joel Edwards (UC Santa Cruz), Melissa Gray (Imperial College London), Shuoshuo Han (UT Austin), Harold Leah...
IODP tackles the Hikurangi Margin of New Zealand with two drilling expeditions to unlock the secrets of slow-slip events

IODP tackles the Hikurangi Margin of New Zealand with two drilling expeditions to unlock the secrets of slow-slip events

MGL1801 Participants - Ryuta Aral (JAMSTEC), Stephen Ball (Univ. of Wisconsin, Madison), Nathan Bangs (UT Austin), Dan Barker (GNS Science), Joel Edwards (UC Santa Cruz), Melissa Gray (Imperial College London), Shuoshuo Han (UT Austin), Harold Leah...
Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) - Revealing the environment of shallow slow slip

Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) - Revealing the environment of shallow slow slip

Susan Schwartz (UC Santa Cruz), Anne Sheehan (University Colorado, Boulder), Rachel Abercrombie (Boston University) In the last fifteen years, it has become evident that slow slip events (SSEs) are a common and important part of the subduction process....
Sizing up the Taniwha: Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE)

Sizing up the Taniwha: Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE)

Jeff Marshall (Cal Poly Pomona) andJessica Pilarczyk (University of Southern Mississippi) “A Live Dragon” Beneath the Sea In Māori culture, the Taniwha is a dragon-like beast that lives beneath the water, sometimes protecting seafarers, whil...
Assessing changes in the state of a magma storage system over caldera-forming eruption cycles, a case study at Taupo Volcanic Zone, New Zealand

Assessing changes in the state of a magma storage system over caldera-forming eruption cycles, a case study at Taupo Volcanic Zone, New Zealand

Kari Cooper (UC Davis), Adam Kent (Oregon State University), Chad Deering (Michigan Tech), and collaborator Darren Gravley (University of Canterbury, New Zealand) The largest volcanic eruptions are rare events but when they occur can represent a...
SISIE: South Island Subduction Initiation Experiment

SISIE: South Island Subduction Initiation Experiment

Erin Hightower (Caltech) and Brandon Shuck (UT Austin) The South Island Subduction Initiation Experiment (SISIE) was an international collaborative active-source seismic survey of the Puysegur subduction margin conducted aboard the R/V Marcus G....

ExTerra Field Institute and Research Endeavor: Western Alps, Summer 2017


Besim Dragovic (Boise State University) Paul G. Starr (Boston College)

Subduction zone field geologists are a proud bunch. In 2011, the name ExTerra (Exhumed Terranes) was coined to describe those in the GeoPRISMS community who investigate rocks exhumed from fossil subduction zones, rocks whose evolution illuminates processes otherwise hidden beneath the surface of active subduction systems (kudos to Sarah Penniston-Dorland and Maureen Feineman for the name “ExTerra”).

Traditional field studies have often been conducted by individuals or only small groups of researchers. One fundamental aspect of this project, termed the ExTerra Field Institute and Research Endeavour (or E-FIRE for short) was to conduct collaborative fieldwork to collect materials held communally, foster broad interactions through workshops, and incorporate student exchanges among research laboratories. The E-FIRE group consisted of researchers (including seven PhD students and three postdocs) from nine U.S.-based universities and research institutions, each with different analytical expertise in metamorphic petrology and geochemistry (e.g. stable isotopes, geochronology, thermodynamic modeling).

In addition, ExTerra partnered with a sister European organization, the ZIP project (Zooming In between Plates). The ZIP project, coordinated by Philippe Agard, consists of researchers from twelve universities across Europe with support from a number of different industry partners. The project has been running since 2013, with many of the twelve PhD students being in the final stage of their projects when we arrived in the field this summer.

The overall big picture of this project was to trace the cycle of rocks and fluids through the subduction process. For this, we proposed to go to the Earth’s premier example of a fossil subduction zone – the Western Alps, Europe in the summer of 2017.

Planning and logistics

Weekly Google Hangouts offered the early stage researchers an opportunity to discuss papers on Western Alps geology and conduct webinars on analytical techniques, modeling, and field observation. In addition, an important component of the E-FIRE initiative was to have open, collaborative documentation and data sharing, with the end goal of opening the complete sample and data collection to any future researchers interested in subduction zone research. Hangout sessions before fieldwork included discussion with Frank Spear about the use of MetPetDB (a global database of various metamorphic petrology data) and with members of SESAR (System for Earth System Registration) about utilizing International Geo Sample Numbers (IGSNs – unique numbers and barcodes given to each sample).

One of the first major steps in the E-FIRE project was the first joint E-FIRE-ZIP workshop/retreat held in the Marin Headlands, close to San Francisco, in December 2016. This was the first time many of us had met in person. It provided a great opportunity for everyone to get to know each other. We also got to see some of our first subduction zone rocks together during a mini fieldtrip to nearby outcrops of eclogites and blueschists of the Franciscan Complex. It was also exciting to have such a large group of young researchers, from across North America and Europe!
Much of the credit for the fieldwork planning and organization must go to the E-FIRE PI triumvirate of Matt Kohn, Maureen Feineman, and Sarah Penniston-Dorland, as well as our main European collaborators Philippe Agard, Marco Scambelluri, Othmar Müntener, Samuel Angiboust, and their students.

E-FIRE Group Fieldwork overview – 7/26/17 – 8/6/17

This would turn out to be a different field experience for many of us. At any one time, there were 25-30 of us in the group, including our European collaborators. For a majority of the time, we stayed in Italian hostels and rifugios with beautiful mountain vistas (I know what you’re thinking…rough stuff). Thankfully, it just so happens that many of the world’s premier metamorphic rocks are associated with many of the Alp’s premier mountains: the Matterhorn, the Dent Blanche, and Monviso. Lunchtime in the field would often consist of grab bags of breads, cheeses, and cured meats from the local market (don’t worry, we ate some fruits and vegetables).

After a few close scares with delayed flights, everyone arrived safely in time for our first group E-FIRE dinner in Geneva. The next morning, we headed off for our first day in the field, consisting of an introduction to Alpine geology with rapid-fire stops along the way driving from Geneva to the Aosta Valley in Italy. This part of the trip was led by Alpine geology maestro, Philippe Agard, who demonstrated an incredible ability to explain complex Alpine features whilst hand-drawing cross-sections in front of some fantastic Alpine vistas.

No trip to the Alps would be complete without a hard slog up some steep mountainous terrain and our second day in the field delivered just that! Having hiked up some 1200 m of relief (most of us still suffering from jetlag), we were rewarded with spectacular views and some equally exciting geology in the Dent Blanche area. This area is interpreted to be a well-exposed example of an ancient subduction interface, where continental material of the colliding overlying plate is juxtaposed against the lower plate of European affinity. Recent work by some of our European collaborators, led by Samuel Angiboust, has suggested that this could be one of the best natural analogues for a subduction zone interface near the base of the upper plate crust.

Our third day started with an immaculate view of Monte Cervino (or it’s more well-known German name, Matterhorn). A beautiful mountain-side trail took us up to Lago Di Cignana, an artificial lake located in Valtournenche, in the Aosta Valley. This site is most well known as an ultra-high pressure (UHP) locality, consisting of various coesite (high-pressure polymorph of quartz)-bearing eclogites and schists. Recent work has found evidence for micro-diamonds within fluid inclusions in garnet, suggesting that these rocks were buried to depths greater tha 100 km and potentially provide a unique record of processes occurring deep within the subduction zone.

The next day consisted of a transect across the Schistes Lustres – one of the most complete sections across a fossil accretionary wedge complex consisting of blueschist and eclogite facies metasediments. The fieldwork was punctuated by brief outbursts of some stormy Alpine weather (thankfully one of the only rainy days in the whole trip!).

Day 5 featured a trip to the Lanzo Massif and a shift in focus from the processes operating during Alpine orogenesis to those occurring on the seafloor, prior to subduction. In contrast to many of the Alpine ophiolites seen on this fieldtrip, the Lanzo Massif has largely escaped metamorphic overprinting. Our leader for this day, Othmar Muntener, showed us well-preserved examples of seafloor serpentinisation and discussed the evidence for Lanzo, and some other Alpine ophiolites, belonging to an ultra-slow spreading ridge system. Of particular interest to the group was discussion of evidence for subducted sub-continental lithospheric mantle that would have been exhumed to the seafloor along large-scale detachment faults as part of a slow-spreading ridge system.

For many of us, one of the highlights on the trip was the Monviso area, in the Italian Alps. The two-day trip featured a tour around one of the best exposed fragments of subducted oceanic material, interpreted as a coherent slice of the oceanic crust/mantle interface. Whilst the hiking around the area featured some fantastic vistas of Monviso and the adjacent peaks, it was perhaps equally memorable for the dense clouds that would appear out of nowhere and reduce the visibility to just a few meters. Perhaps unsurprisingly, given a group of thirty geologists easily distracted by the rocks, we managed to lose some of the group in the fog! After about a thirty-minute search, and a lot of shouting and whistling, the lost E-FIRE folks were located – already on their way to the Rifugio for an early happy hour.

After the Monviso trip, we took a field break to decompress and to discuss the nascent project ideas the students and postdocs were thinking about in the context of the sites we had visited. It was exciting to hear students bouncing ideas off each other and contemplating how each of their individual projects goals will tie into each other’s. We smell collaboration (and field boots)!
After a well-needed day of rest and relaxation, we drove east to the Ligurian Alps, with the port city of Genoa as our base of operations over the next three days. Here, we drove north, as the serpentinite guru, Marco Scambelluri, led us into the Voltri Massif. In this portion of the Western Alps, it is suggested that convergence between Europe and Apulia occurred at an ocean-ocean plate interface (as opposed to northwest at Monviso, which is suggested to be ocean-continent). We got to contemplate whether changes in plate dynamics here would have resulted in differing metamorphic conditions and preservation of buried lithologies. Our first day was an introduction to the regional geology, and a quick introduction to fieldwork closer to the Mediterranean, with temperatures at times in excess of 100˚F. Much of the terrane is heavily vegetated, so the best field exposures were in riverbeds, like those of the Gorzente River, in the Erro Tobio Unit. The Erro Tobio Unit consists of variably serpentinized ultramafic rocks, with the most striking feature in these rocks being coalescing veins of metamorphic olivine resulting from dehydration during subduction.

The next day, Marco led us to several roadside locations in the Beguia Unit of the Voltri Massif, a region of large (tens to hundreds of meters) lenses of metamorphosed gabbro in a matrix of serpentinite. It has been hypothesized that these lenses may represent a tectonic mélange or simply an extension of what we all observed earlier in the trip. The temperature that day did not let up, and hammering dense Fe-Ti meta-gabbros did not provide a break either (pun intended), but we collected some spectacular samples and witnessed Marco’s expert wielding of a sledge hammer. At the end of days like this, a cool refreshment is always welcome, as well as a nice stroll along the rocky coast of Genoa.

Small Group Fieldwork | 8/8/17 – 8/28/17

Groups of researchers went back to Monviso to collect more stunning examples of slab/mantle fluid-rock interaction, to Dent Blanche for finer-scale sampling of the subduction interface, and out by ferry to explore an extension of the high-pressure Western Alpine rock in Corsica. The authors, with a group of others, went back to the Voltri Massif for more sampling of meta-gabbros and serpentinites, but also to the Apennines, where relics of unsubducted gabbro and serpentinite are preserved. There, we experienced two firsts: tripe (with mixed responses!), and having the carabinieri (the Italian military police) called on us for hammering rocks. This time with smaller groups going back to some of the same locations we visited earlier in the trip, the focus was on more detailed sampling for individual project goals, and expanded discussion of field observations and any tectonic interpretations.

After several days in smaller teams, the E-FIRE group was re-assembled back in Geneva, Switzerland for one more day, and what could possibly beat the adventures we experienced in the field? Why, it’s sample packing! In a parking lot at the Université de Lausanne, we showed each other some of the rocks we collected for our projects, and in some cases, samples we set out to collect for each other’s projects. Samples were packed onto a pallet and wrapped like no rocks had ever been wrapped before. That night, we had one final group dinner in Geneva before we all split up again, where many were back out into field and others were off to the Goldschmidt conference in Paris.

All told, roughly 700 kg of rock was collected by the E-FIRE group during the field excursion, and shipped to Penn State University where the samples will be held in a sample repository. This is of course until we crush, pick at, dissolve, and shoot lasers at them. In all seriousness, it is safe to state that the 2017 ExTerra Field Institute and Research Endeavour was the experience of a lifetime, especially for the early stage researchers; a unique opportunity to interact and initiate collaborations with each other and our new European colleagues, observe and contemplate subduction zone processes in a classic field location, and travel through one of the most beautiful terranes on Earth.

Acknowledgments

We are grateful to Philippe Agard, Marco Scambelluri, Othmar Müntener, and Samuel Angiboust (along with other European students, postdocs, and research scientists) for their guidance in field logistics and immense knowledge of Western Alps geology. We would like to thank the principal investigators Matt Kohn, Sarah Penniston-Dorland, and Maureen Feineman for the leadership and organizational skills necessary for such a large field-based research endeavor. We finally thank W.O. Bargone for field assistance. This field institute was supported by NSF EAR-1545903 and readers like you. ■

“Report from the Field” was designed to inform the community of real-time, exciting GeoPRISMS -related research. Through this report, the authors expose the excitement, trials, and opportunities to conduct fieldwork, as well as the challenges they may have experienced by deploying research activities in unique geological settings. If you would like to contribute to this series and share your experience on the field, please contact the GeoPRISMS Office at info@geoprisms.org. This opportunity is open to anyone engaged in GeoPRISMS research, from senior researchers to undergraduate students.
We hope to hear from you!

Reference information
Magnetotelluric and Seismic Investigation of Arc Melt Generation, Delivery, and Storage beneath Okmok Volcano. K. Key, N. Bennington
GeoPRISMS Newsletter, Issue No. 39, Fall 2017. Retrieved from http://geoprisms.org

Magnetotelluric and Seismic Investigation of Arc Melt Generation, Delivery, and Storage beneath Okmok Volcano


Kerry Key (Scripps Institution of Oceanography) and Ninfa Bennington (University Wisconsin-Madison)

It all sounded so easy when we were writing the proposal. Sure, we can deploy 54 seafloor magnetotelluric (MT) instruments around a remote Aleutian island, no problem, we have done lots of marine MT surveys before. Add on an array of onshore magnetotelluric and passive broadband seismic stations covering the flanks and caldera of a volcano that erupted without almost no warning back in 2008? Sure, that won’t be too hard either since we will have a helicopter transporting the field teams and science equipment, and we can base our camp at a remote cattle ranch used by previous field teams studying Okmok volcano. So we worked up a budget, wrote the proposal text and submitted it to the July 2014 target date for proposal submissions to the National Science Foundation’s GeoPRISMS program.

Fast forward to early January 2015 when we received an email from Bil Haq, then one of the two NSF Program Managers for GeoPRISMS, stating “Your proposal did well in the competition for GeoPRISMS funds and I plan to fund it at this time”.

Yes!!!!! Woohoo!!!!!! Seriously, this was good news.

Then comes the word that the field work will start in mid-June. We were supposed to get everything in place for two short cruises and three weeks of onshore field work in just a few months. Time to get moving!

The Logistics

We started a seemingly endless chain of emails and conference calls to work out the logistics for the onshore field work. We would be working out of a field camp at Bering Pacific Ranch on the abandoned WWII military base Fort Glenn, located on the eastern flank of Umnak island. Our tasks were to get a helicopter, about fifty barrels of helicopter fuel, seismometers, magnetotelluric instruments, cooking supplies and food for about 160 person-days delivered by the start of field operations around June 20th. The tiny city of Dutch Harbor, conveniently located about 100 km away on neighboring Unalaska Island, is the country’s largest fishing port by volume, so we planned to ship our stuff from the lower 48 states up to Dutch Harbor, where it would be consolidated and then shipped to Fort Glenn. Easy right?

Amazingly, this plan actually worked out. Once all the geophysical equipment, batteries, helicopter fuel, non-perishable food and cooking supplies arrived in Dutch Harbor, it was loaded onto the Island Packer, a small landing craft, and then ferried on a 60-km journey from Unalaska Island to a makeshift dock at the beach near Fort Glenn. Then two ranch hands transported it 5 km up to the field camp at the ranch.

By comparison, preparing for the marine part of the project was relatively straightforward since the Scripps lab does this routinely and all we had to do was get the marine MT equipment to the ship. By coincidence, our deployment cruise was scheduled on the RV Thompson, which happened to be passing through San Diego on its way north, so we lucked out and loaded the marine MT gear onboard for a free ride up to the Aleutians.

June 16-17, 2015 | Making it to Dutch Harbor

Dutch Harbor was also the port of departure for the marine MT deployment cruise so we flew into Anchorage and then boarded connecting flights to Dutch Harbor. While anywhere else in the US this would likely be an easy connection, flights to the Aleutian islands are unpredictable due to frequent low hanging clouds and fog. When the planes take off in Anchorage, they don’t know how the weather will be in Dutch Harbor so they load enough fuel on board to make the return trip if visibility is so bad they can’t see the Dutch Harbor runway. This was indeed the case for several of our connecting flights, and so it took a few attempts spread out over a few days for everyone to finally make it to Dutch Harbor. At the local grocery store Safeway, which was unexpectedly well stocked with a cornucopia of fresh produce, we gave the manager a lengthy shopping list of fresh produce, dairy, meat and seafood and he promised it would be delivered to the airport on the morning of June 22, where it would be loaded onto the charter flights taking us west to Fort Glenn.

June 18-21, 2015 | Deployment Cruise

We pushed off the dock around mid-day on the 18th and by 01:00 on the 19th the ship arrived at the first station, located on the northern end of the survey profile in the Bering Sea. By 10:00, we had already deployed seventeen seafloor MT receivers. The sky was filled with low hanging clouds so we couldn’t see Umnak Island, but the lack of view was made up for by the lack of wind and almost no swell – perfect conditions for the maiden marine flight of our consumer-grade drone, allowing us to capture some 4K high definition videos of ship and the science team deploying MT receivers. While waiting in port during the previous days, we had done a lot of prep work, including putting batteries in the 54 data loggers for the MT receivers, synchronizing their crystal oscillator clocks with GPS time and programming them to startup around the time we predicted they would be on the seafloor. So now for each receiver deployment, all we had to do is mount the magnetic field induction coil sensors on the receiver frame along with the two long electric dipole arms, attach the external electronic compass, plug in all the sensor cables, secure the concrete anchor strap, test the acoustic release system, test the stray-line buoy’s radio and finally attach the bright orange flag to the frame. There is a well-developed procedure for all these steps and checklist to make sure nothing is skipped, so it all goes like clockwork thanks to the careful efforts of the students, postdocs and technicians working either the noon-to-midnight or midnight-to-noon shifts (ship-time is not cheap so the vessel works 24 hours a day).

The clouds partially lifted in the late afternoon of the 19th as we entered Umnak Pass, a narrow channel that separates Umnak Island on the west from Unalaska Island on the east. The MT deployment carried on like clockwork and by the 20th we were making way into the Pacific Ocean, which was starting to kick up with strong winds and some moderate swell. We finished the last MT deployment about twelve hours ahead of schedule so we decided to use the extra time to collect high resolution multibeam bathymetry on the forearc slope before heading back to Dutch Harbor. Our journey back to port went about the northern shore of Unalaska, and with luck the clouds lifted partially to give some nice views of Mount Makushin volcano. As usual, soon after the ship tied up most of the crew and science party headed to the local bar to re-equilibrate after a few days on a dry ship.

June 21-July 8, 2015 | Onshore field work

The next phase of the project started with flights from Dutch Harbor to Fort Glenn that transported the science party, two ranch hands, and the perishable food that Safeway had just delivered. From the gravel landing strip (left over from WWII) at Fort Glenn, a ranch hand drove the science party and food up to the camp house where we would stay. The camp house was basically three trailer units arranged in a u-shape with an aluminum roof overtop and a giant garage door on the open side of the U. One unit was a cooking trailer with full kitchen and dinner area. Another was a bunk house and the third was a bathroom, shower and laundry facility. While we weren’t going to exactly be roughing it, nobody had stayed here in several years and everything was covered in mold and black volcanic dust, and the window sills were graveyards of giant fly corpses. We spent much of the first day cleaning up the place, stocking the kitchen and setting up workbenches for the geophysical equipment in the enclosed space in the middle of the three trailers. Sometime during the first day the helicopter arrived and everything was coming together for us to begin operations the next day.

Our seven-person science team would helicopter into and around Okmok volcano. Half of the crew carried out an onshore magnetotelluric survey collected in an array using a combination of long-period and wide-band MT systems, with nineteen stations within the caldera and ten stations outside. The remainder of our field team installed thirteen temporary broadband seismometers both in and around the volcano. The temporary seismic array recorded seismic data until its retrieval in summer 2016. In tandem with the Alaska Volcano Observatory’s twelve permanent seismic stations, there were twelve seismic instruments within/at the rim of the caldera and fourteen seismic instruments outside the caldera.

Both the seismic and MT teams operated in parallel so the helicopter went back and forth ferrying the teams around. That meant we always had to be prepared to be left overnight (or longer) at field stations if the fog came in and the helicopter couldn’t return to pick us up (luckily this never happened, despite a few close calls). We also had to be prepared for being chased by one or more of the ~7000 feral cattle that roam the outer flanks of the caldera. We quickly developed a protocol where after dropping off a team, the helicopter would fly in a 1-km circle around the station chasing away any nearby cattle. Despite this, there was an occasion where the seismic team had to make haste into a ravine to get away from an angry bull. While the MT systems only needed to record data for a few days and thus were all recovered by the end of the first field season, the broadband seismic systems were going to record seismic waves for the next year and would be picked up during the second field season.

July 9-14, 2015 | Recovery cruise

For the marine MT recovery cruise, we were on a different ship, the newly built RV Sikuliaq. Recovering the marine MT receivers meant driving up to them in the ship, sending an acoustic command that tells the instrument to let go of its anchor and then waiting for the instrument to rise to the sea surface. Once on the surface, the instrument’s stray line buoy radios the ship with its GPS position. The ship then drives up to the floating instrument from the downwind side and once its alongside the ship, we toss a grapnel around the stray line and use that to attach the instrument to the ship’s remotely operated crane, which then lifts it aboard. We successfully recovered all instruments except one that was deployed in a dicey location in Umnak Pass where there were strong currents that we suspect carried the instrument away after it released its anchor.

July 29 – Aug 6, 2016 | Recovery of seismic instruments

In summer 2016, we returned to Umnak Island to recover the seismic instruments. This time our operations were based on marine vessel Maritime Maid. Operations continued in a similar fashion to the previous field season with helicopter providing the team’s transportation to and from Okmok. However, this year there was an added level of excitement as take-offs and landings were carried out on the ship’s small helipad. Due to unusually cloudless blue skies and warm temperatures, we demobilized all thirteen seismic sites in a matter of several days. Amazingly, and quite happily, we found that the majority of stations were still up and running when returning to the sites for demobilization. After a rapid and successful field season, we departed from the wonderful Maritime Maid crew and made our way back home. ■