by Maria V.S. Ask, Maria V.S. Ask, Ilmo Tapio Kukkonen, Odleiv Olesen, Bjorn Gabriel Lund, Ake Fagereng, Jonny Rutqvist

The project DAFNE aims at scientific drilling in the intraplate Pärvie fault system (PFS) in northern Sweden, which is the longest known postglacial fault (PGF) in the world (155 km long, at least 25 m surface offset). The PFS ruptured at the final stage of the Weichselian glaciation, about 10 ka BP, most probably in a great earthquake (Mw=8.0±0.4) triggered by the rapid uplift of Fennoscandia due to relaxation of accumulated stress under the disappearing ice sheet. The PFS is still seismically active; from 2003-2018, 1812 microseismic events were recorded down to ~35 km depth along the fault system. It is a key site for improving our understanding of intraplate earthquakes. The PGFs in general and PFS in particular represent a type of intraplate seismicity that is still poorly understood; It has great relevance to seismology in general, and may pose high risks for society. With the aid of scientific drilling, we aim at a comprehensive study of the geology, fault properties, tectonic history, stress field, seismicity, hydrogeology, subsurface life and geothermics of the PFS. In 2009, ICDP approved our workshop-proposal to develop research ideas for investigating characteristics of PGFs. The workshop was held in 2010, and the results were reported (Kukkonen et al. 2011). In 2013, the central PFS was identified as the best drilling target of the known large PGFs. Thereafter, significant new data sets have been collected including seismicity, reflection seismics, magnetotellurics, structural geology mapping, and cosmogenic dating. We have identified two drill sites located c. 30 km west of the city of Kiruna. This part of the PFS has been proposed to reflect a flower structure. To understand this complex structure, multiple boreholes, logging, in situ testing and monitoring are required. Site DAFNE-1, with Holes 1A and 1B aims at penetrating the east-dipping main fault. Site DAFNE-2, with Holes 2A and 2B) aims at penetrating the west-dipping subsidiary fault identified with a strong seismic reflector. Ongoing seismicity is related to both structures. The short arctic field season result in a three year long project, including: Site 1 preparations (Year 1), Site 1 drilling, and site 2 preparations (Year 2), and Site 1 drilling (Year 3). In Years 2-3, logging, in situ stress testing and installation of borehole observatories are achieved. Post-expedition Science Parties are organized after field seasons of Years 2 and 3.

• DAFNE has the following scientific objectives:
• 1.
• To improve insight in both how large intraplate earthquakes are triggered and why seismicity persists for very long time periods in areas of previous large ruptures;
• 2.
• To reach a better understanding of accumulation and release of strain along a postglacial intraplate fault zone, as well as revealing the consequences of seismicity, brittle deformation and stress on the thermal and hydrogeological regimes and the deep biosphere;
• 3.
• To improve the understanding of glacial isostatic adjustment (GIA). We anticipate that new data and results from drilling and numerical analyses will support prediction of new postglacial-type seismicity that is expected due to shrinking of ice sheets during current climatic warming at high latitudes (e.g. Greenland, Antarctica, Iceland, Alaska, and major mountain chains); and 4.
• To study the distal effect of the passive margin development along the western Fennoscandian Shield by extending the 3D modelling of temperature, stress and strain in the Nordland area eastwards to cover the PFS. Finite element modelling shows that the Pleistocene erosion along the coast and deposition along the shelf edge cause temperature and stress anomalies in the order of 25-70° and c. 10 MPa, respectively, in the underlying crust. Measurements of stress and temperature in the PFS borehole will help to constrain the contribution from this process to the generation of PGFs.

DAFNE, Deep Life, Earthquakes, Europe, Faults, Finland, Fluids, PFDP, Stress, Sweden

Latitude: 67.849148773, Longitude: 19.441595078