F1b: Subduction in the past & today

During four years of the WP F1-2, we followed the proposed plan and worked in the two target regions: the Lesser Antilles and the Kamchatka subduction zones. We analyzed and interpreted of previously collected data and performed new field experiments. We also worked on the analysis of geophysical data from other subduction zones (Mexico, Indonesia) that contribute to understanding of active seismogenic and volcanic processes. This work resulted in several publications and presentations at international scientific meetings.
The support from Labex also helped us to obtain other funding are used to extend our studies of the subduction zone dynamics in the two target regions. The expertise developed by our team in the Lesser Antilles thanks to the support of the Labex program WP F1-2 allows for developing a new research program on the Ryukyu subduction zone in Japan in collaboration with colleagues of the Tokyo University and the Earthquake Research Institute. The program began in January 2016. It was funded by INSU in 2017 and BQR IPGP in 2018. Thanks to the expertise acquired during the Labex program on subaqueaous paleoseismology and volcanology, two young researchers (who get their PhD at IPGP) were selected to participate to Expedition IODP 372 in 2017 (Morgane Brunet) and IODP expedition 381 in 2018 (Gino de Gelder). In the framework of this project, they both obtained a 12-month CNRS (IODP-France) Post-Doc funding at IPGP for Gino de Gelder and Univ. Rennes for Morgane Brunet. Several PhD students have joined the program in 2018.

A – Lesser Antilles

To constrain a wide range of volcanic, tectonic and erosion processes in Lesser Antilles in order to characterize an integrated volcanic system, allowing evaluation of mass fluxes at the scale of the whole arc, we focused on deep offshore sediments as a continuous record of extreme volcanic events, earthquakes, masswasting events and erosion processes as drivers of the long-term evolution (construction and destruction) of a volcanic arc at convergent margins. This required the creation of a new multidisciplinary team (volcanology, tectonic, marine geophysics and sedimentology that must work towards a better understanding of volcanic arc evolution at a scale that fills the gap between conventional local onshore volcanological studies and plate-scale geodynamical investigations. The first step of the work in 2015 was to consolidate this new IPGP team with a new post-doc (Hervé Guyard who was hired between April 1 2015 and march 31 2017) and a PhD student (Chloé Siebert, 2015-2018) specialist in core sedimentology.

Thanks to the collaborations we have initiated through the Labex project, 6 others students, PhD students or post-docs have joined the group in 2016-2017
• Kelly Faukemberge is working at UMR Epoc (Bordeaux) and IPGP on processes of sediment transfers between the Lesser Antilles arc, deep basins of the accretionary prism and the trench.
• Arthur Bieber is a PhD student in Cotutelle between the Quebec University in Rimouski (UQUAR, CANADA) and IPGP. He will work and paleomagnetic signature of the sediments deposited during extreme events in the Lesser Antilles arc (earthquakes, volcanic eruptions and hurricanes) in the deep basins of the prism and in the trench.
• Pierre Morena is working at IFREMER (Brest), IPGP and GEOAZUR (Nice) on the sediments as proxies of large earthquakes and tsunami in the Lesser Antilles arc
• Morgane Brunet get her phD at IPGP on December 2015, she was working on a new history and understanding of landslides processes offshore Martinique (Lesser Antilles)
• Jaume Llopart was a post-doc (February 2017-july 2018) working on the numerical simulation of sediment deformation by volcanic activity in island-arc domain, using physical properties analyses and geotechnical tests.
In 2018, 5 other students have joined the program
• Gaëlle Benâtre is a PhD student a IPGP (2018-2021). She will work on active deformation of the Barbados accretionnary wedge by combining geophysical and sedimentological studies
• Giulia del Manzo is a PhD student at IPGP (2018-2021). She will work on volcanological history of the Lesser Antilles arc by studing tephra deposits in all cores we collected in 2012 during the IODP340 program and 2016 during the CASEIS program
• Louise Cordrie is a PHD student of IPGP/CEA (2018-2021). She will model the impact of tsunami induced by earthquakes along the coast of the Lesser Antilles island. In particular our objective is to develop news codes allowing for sediment transport.
• Pablo is a PhD student (2018-2021), IPGP and BRGM funding) working on numerical simulation of debris avalanche flow in oceanic context offshore Montserrat and Martinique. He will also deal with numerical simulation of tsunami generated by landslides.
• Maude Biguenet, is a PhD student (2018-2021 Université de la Rochelle (Dir : E. Chaumillon), She will work on tsunami deposits in coastal area of the Lesser Antilles arc islands.

Several others Master students have worked on the project at IPGP and elsewhere in 2017 and 2018: Gaelle Benâtre (2017 and 2018, IPGP) , Olivia Bouchet (2017 PGP/MNHN) ,vv Harry Linang (2017 IPGP) ,Monteil C. (2017, ENSG Nancy/université de Bordeaux) ,Grébert B. (2017, Université de Bordeaux) ,Blet S. (2017 and 2018, Université de Bordeaux and IPGP), Pablo Poulain (2017, Université Paris 7, IPGP).

The group benefits of a considerable set of high-resolution geophysical marine data (bathymetry, seismic, imagery) and cores acquired during the last 15 years in the Lesser Antilles (AGUADOMAR-1999, CARAVAL- 2002, JCR123-2005, GWADASEIS-2009, JC45/46-2010 cruises) including an exceptional set of deep drilling data from the Integrated Ocean Drilling Program (IODP) Expedition 340 (March-April 2012 on the R/V JOIDES Resolution). We create a comprehensive database including all geophysical and coring data with ARCGIS.
The database has been enriched by 42 cores (33 giant piston cores and 9 box cores) for a total of 510 m of sediments and new geophysical data during the CASEIS cruise between may 27 and July 5 2016 on the French Research Vessel Pourquoi Pas?. The dataset available in the Lesser Antilles is now exceptional, covering time and space scales never attained in other volcanic arcs Worldwide. We will have the potential to access to the 4 million years history of extreme events having impacted the whole arc over distances of several hundred kilometers. Numerous papers are in preparation in the frameworks of the numerous PhD thesis. The results where presented in several international and national meeting.

A-1- Extreme events influencing the arc evolution at convergent margin:

To study the history of extreme events in the Lesser Antilles, we focused first on several cores obtained in 2009 during the GWADASEIS Cruise. High-resolution physical (SCOPIX imaging, grain size analyses) and geochemical (μXRF) analyses have been carried out at EPOC (U. Bordeaux 1) and ISTerre (U. Chambéry) laboratories on 8 marine sediment cores collected during the 2009 campaign. This multiproxy study combining large geophysical and high- resolution sedimentological datasets allowed the identification and characterization of several Holocene mass movements and turbidites from distinct basins in the Western part of the Lesser Antilles Arc. Some of these events were possibly triggered by regional seismicity and 12 sedimentary intervals were subsampled, treated and sent for foraminifera 14C AMS dating to test the synchronicity of these events and add new insights into sedimentary processes in the area over the last 30 ka (Aguilar et al., 2015 ; Guyard et al., 2015 ).
We completed this dataset by stratigraphical, magnetical and sedimentological analyses on 50 meters of sediments sampled with U-Channel at Gulf Coast Repository (IODP) to get information on older Late Pleistocene eruptions, earthquakes, and submarine landslide likely associated with volcanic flank collapses in the lesser Antilles arc (Guyard et al., in prep 1, Carlut et al., in prep). To extend our study in the whole arc and above the subduction interface in the prism and at the trench where we expect to retrieve the sedimentological signature of the largest earthquakes, tsunami and eruptions, we collected 42 new cores as well as geophysical data (bathymetry, imagery, seismic, chirp, magnetism, gravimetry) in the eastern part of the arc in deep basins of the fore-arc and the accretionary prism and in the trench during the CASEIS cruise (may 27-July 5 2016, R/V Pourquoi Pas?). Thanks to the Labex support WP F1-2 we have developed a collaboration with the Quebec University (UQAR) with Guillaume St Onge and we had access to his GEOTEK Multisensor Core Logger (MSCL) during the cruise. This equipment has been successfully used 24/7 during 38 days. It allowed us to made physical measurements and photographs on all cores. We measured 510 m of sediment. CASEIS Cores along the Lesser Antilles trench also permit the study of the sediment transfer between the Venezuela and the trench over several thousand kilometers and the recycling of the sediments at the trench. In 2017 and 2018 thanks to the Labex support, we performed CT-scan, SCOPIX imagery, micro-CATSCAN on U-channels, XRF measurements, granulometric analysis, magnetic measurement (Paleodirection of the of foraminifera) and 14C dating on several CASEIS cores at Ifremer Brest, EPOC Bordeaux, LSCE and ISTERRE Chambery and UQAR Canada, in the framework of the thesis of Chloé Seibert (IPGP), Pierre Moreno (IFREMER) and Arthur Bieber (UQAR). The Geophysical data of the CASEIS cruise (bathymetry, imagery, seismic profiles, chirp, magnetism) have been all processed and were coupled with older data in a the ArcGis database of the Lesser Antilles during the Master 2 internship of Gaelle Benâtre. All those new analysis will permit to identify and to establish the chronology of the extreme events (earthquakes, eruptions and tsunamis) having occurred in the Lesser Antilles arc.
In addition, thanks to the marine database we collected in the Lesser Antilles, we performed a study on interactions between tectonic, volcanic and sedimentological processes along the Lesser Antilles volcanic arc (Leclerc et al., 2016) and a study on the effect of the M6.3 2004 Saintes earthquake in Guadeloupe on the seafloor (coseismic ruptures and mass-wasting, Escartin et al., 2016). This study will serve as an example for a better understanding of sedimentological processes related to past earthquakes in the arc. A PhD thesis began at IPGP in 2017 (Mathilde Henry) on this subject using very high resolution ROV and AUV data we acquired during the SUBSAINTES cruise in 2017. To complement our offshore record of large earthquakes, we have also investigated the effect of large subduction earthquakes on the reefs of the Lesser Antilles (Weil Accardo et al., 2016, Philibosian et al., 2015).

A-2-Arc-scale landslides as drivers for volcanic arc evolution

IODP Expedition 340 successfully drilled a series of sites offshore Montserrat, Martinique and Dominica in the Lesser Antilles from March to April 2012. These are among the few drill sites gathered around volcanic islands, and the first scientific drilling of large and likely tsunamigenic volcanic island-arc landslide deposits. Sites that penetrated landslide deposits recovered exclusively seafloor-sediment, comprising mainly turbidites and hemipelagic deposits, and lacked debris avalanche deposits. Le Friant et al., 2015 proposed a new model where i/ volcanic debris avalanches tend to stop at the slope break, and ii/ widespread and voluminous failures of pre-existing low-gradient seafloor sediment can be triggered by initial emplacement of material from the volcano. Offshore Martinique, the landslide deposits comprised blocks of parallel strata that were tilted or micro-faulted, sometimes separated by intervals of homogenized sediment (intense shearing), while Site offshore Montserrat penetrated a flat-lying block of intact strata. Brunet et al. 2016, proposed a new interpretation of the submarine landslide deposit, comprising up to 300 km3 of remobilized seafloor sediment, that extends for 70 km away from the coast of Martinique and covers an area of 2100 km2. Brunet et al. 2017 and Pablo, 2017 simulate the emplacement of the debris avalanche generated by the last flank collapse event of Montagne Pelée volcano (30-45 ka). They underline the importance of the submarine slope break during the flow and suggest that large collapses probably occurred in several times with successive volumes smaller than about 5 km3 entering the sea. These results have implications for the magnitude of tsunami generation. Under some conditions, volcanic island landslide deposits comprised of mainly seafloor sediment will tend to form smaller magnitude tsunamis than equivalent volumes of subaerial blockrich mass flows rapidly entering water. Expedition 340 also successfully drilled sites to access the undisturbed record of eruption fallout layers intercalated with marine sediment which provide an outstanding high-resolution dataset to analyze eruption and landslides cycles, improve understanding of magmatic evolution as well as offshore sedimentation processes (Emmanuel et al., 2017, Villemant et al., in prep, PhD Del Manzo).

More than 300 m of CT-scans from IODP sediment cores were also analyzed and processed using the Osirix software and revealed highly complex internal deformation and microstructures. These original data provide unique information on the landslide composition, dynamics and emplacements, and allow a better understanding of the deformation processes. The upper and lower limits of this thick submarine landslide deposit have also been better constrained by characterizing its internal architecture. By coupling thesev results with δ18O measurements and new calibrated AMS 14C ages, the top of the deformation is now dated around 130 ka BP which is consistent with the age of the first volcanic flank collapse of Mount Pelée (Guyard et al., 2015 ; Guyard et al., in prep2). The analyses of the sediments of the IODP cores also permit to have constrains on the travelling of pumiceous pyroclastic density currents over water (Jutzeler et al., 2016), to discover a large 2.4 Ma Plinian eruption of Basse-Terre (Guadeloupe) which was never documented before (Martin R. Palmer et al., 2016) and to better constrain the relationship between eruptive activity, flank collapse and sea-level in the Lesser Antilles (Coussens et al., 2016).

Low hydraulic conductivity of hemipelagic sediments were measured on IODP cores (Lafuerza et al., 2014). This is related to low rates of dewatering in turbidity current deposits and tephra layers, leading to excess pore fluid pressures development with depth. Combined downhole logging data acquired during IODP Expedition 340 with a rock physics models also concluded that unusually high excess pore pressures occur in multiple sand-rich zones (Hornbach et al., 2015). Lateral transfer of elevated pore pressures in the sedimentary column is key to initiate landslides. In the Grenada Basin, elevated pore fluid pressures can be the result of progressive long term mechanisms such as high sedimentation rates or fluid (gas or water) migration from deep sources, or short term mechanisms, as debris avalanche emplacement. The post-doctoral work of Jaume Llopart is addressing the excess pore fluid formation in the Grenada basin sedimentary sequence and is modelling the slope stability under subaerial debris avalanche loading. The results will be used to constrain the existing conceptual landslide formation models (Llopart et al., ready to submit).

B – Kamchatka

B-1 Geochemical sampling of water sources for C and Cl fluxes

In 2017, we have continued building a model for the injection of carbon from surface reservoirs into the mantle via subduction zones and for the return flux back to surface via arc magmatism. The first step has been to describe the nature and the amount of carbon entering the subduction zones in a manuscript entitled « The fate of carbon during alteration of oceanic crust through time” that is ready for submission to Geochimica Cosmochimica Acta. While doing that we faced a problem of unbalanced exchange of C between the mantle and the surface of the Earth. This has been the matter of a discussion between the Russian Collegues (Shilobreeva and Polyakov) and us (Martinez and Agrinier). This puzzle considerably delayed the submission of our work (Shilobreeva et al. 2017; Shilobreeva et al. ready to submission). We will continue next year by examining the Cl cycle via a comparison between the subduction injected Cl and that released by arc volcanoes.

B-2 Seismological studies of active volcanoes

We continued the analysis of the data from permanent seismic stations operated by Kamchatka Branch of the Russian Geophysical Survey (these data were obtained via a collaboration established during the labex project). First, we developed a new method for detecting and locating volcanic tremors (Shapiro et al., 2014, 2015; Droznin et al., 2015; Soubestre et al., 2018). Then, we performed a study of long-period volcanic seismicity and established its relation to the preparation of volcanic eruptions (Shapiro et al., 2017). We also worked on seismic imaging of the crust and upper mantle beneath the Klyuchevskoy Group of Volcanoes (Droznina et al., 2017; Koulakov et al., 2017). Another direction is applying the noise based seismic monitoring to Kamchatka volcanoes (Gómez-García, 2018). In 2016, we continued our field experiments on Kamchatka volcanoes (Jakovlev et al., 2016; Abramenkov et al., 2016). In particular, we finalized a new seismological field experiment on the Klyuchevskoy Group of Volcanoes (Shapiro et al., 2017, EOS) that was started during the summer 2015. 83 temporary seismic stations were installed in 2015 and we collected then in 2016. Main objectives of this experiment are studying the structure of the crust and the uppermost mantle in order to understand the origin of this very large volcanic group and, then, to study seismic signals caused by different types of volcanic activity. The experiment is organized in collaboration with Russian institutions: Institute of Volcanology and Seismology and Geophysical Service in Kamchatka and the University of Novosibirsk and with the GFZ, Potsdam fromv Germany and benefited from co-funding from the Russian Science Foundation and the Idex USPC. At this moment, we are working on consolidating the database and are starting its analysis.

B-3 Geodetic studies of the seismotectonic cycle in the Kamchatka subduction zone

Via our collaboration with the Kamchatka Branch of the Russian Geophysical Survey, we obtained the records of permanent GPS stations operating in the peninsular since 2005. Initial analysis of these time series revealed a few episodes of transient deformation preceding the 2013 deep Okhotsk Sea Earthquake (Walpersdorf et al., 2016). In continuation of this work, we plan to refine the position time series with improved corrections for seasonal atmospheric and hydrological perturbations and to compare the results with the catalogues of seismicity.

C – Other subduction zones

In addition to studies in Lesser Antilles and Kamchatka, we worked with the data from other subduction zones. The seismological dataset from Mexico has been used to study the role of slow earthquakes in the seismotectonic cycle (Frank et al., 2016). In Indonesia, we performed a set of seismic tomography studies of active volcanoes (Koulakov et al., 2016b,c; Jaxybulatov et al., 2016). We also developed a new research program on the Ryukyu subduction zone in Japan in collaboration with the Tokyo University and the Earthquake Research Institute. The program began in January 2016. It was funded in 2017 by the CNRS-INSU and in 2018 by the BQR IPGP and we already performed 5 fieldtrips (2 in 2018) to analyze coral microatolls in order to better constrain the seismic cycle of the Ryukyu trench. The preliminary results were presented in several international meetings and several papers are in preparation.