Lost in Iceland? Fracture Zone Complications Along the mid-Atlantic Plate Boundary

Abstract

The mid-Atlantic plate boundary breaks up into a series of segments across Iceland. Two transform zones, the South Iceland Seismic Zone (SISZ) and the Tj\"{o}rnes Fracture Zone (TFZ) separate the on land rift zones from the Reykjanes Ridge (RR), and the Kolbeinsey Ridge (KR), offshore N-Iceland. Both are markedly different from fracture zones elsewhere along the plate boundary. The 80 km E-W and 10--15 km N-S SISZ is made up of more than 20 N-S aligned, right-lateral, strike-slip faults whereas the TFZ consists of a broad zone of deformation, roughly 150 km E-W and 75 km N-S. The over-all left-lateral transform motion within the SISZ is accommodated by bookshelf faulting whereas the right-lateral transform motion within the TFZ is incorporated within two WNW-trending seismic zones, spaced $\sim$40 km apart, the Gr\'{\i}msey Seismic Zone (GSZ) and the H\'{u}sav\'{\i}k-Flatey fault (HFF). Recently collected EM300 and RESON8101 multibeam bathymetric data along with CHIRP subbottom data has unveiled some tectonic details within the TFZ. The GSZ runs along the offshore extension of the Northern Volcanic Rift Zone (NVRZ) and is made up of four left-stepping, en-echelon, NS-striking rift segments akin to those on land. Large GSZ earthquakes seem to be associated with lateral strike-slip faulting along ESE-striking fault planes. Fissure swarms transecting the offshore volcanic systems have also been subjected to right-lateral transformation along the spreading direction. As the Reykjanes Peninsula, the on land extension of the RR, the GSZ bears the characteristics of an oblique rift zone. The plate boundary segments connecting to the RR and KR are thus symmetrical with respect to the plate separation vector (105$\deg$) and orientation of individual volcanic systems. The HFF has an overall strike of N65$\deg$W and can be traced continuously along its 75--80 km length, between the Theistareykir volcanic system within the NVRZ, across the central TFZ-graben, the Skj\'{a}lfandi bay, and into the largest and westernmost graben, Eyjafjardar\'{a}ll (EG). Four pull-apart basins occur along the fault, the largest at the intersection with the EG, the southward magma-starved, continuation of the KR. Dikes, parallel to the HFF bear witness to it being a leaky transtensional feature. RESON8101 maps expose the tectonic fabric along the tidally swept shoreline adjacent to the main fault. The southwesternmost margin of the fault is characterised by NE-striking lavas which, along the coast, dip steeply (30--50$\deg$) westwards, towards the EG. The lavas are dissected by en echelon arrays of minor strike-slip faults intersecting the main fault at angles of N20$\deg$--30$\deg$W and N20$\deg$E. Some can be traced onto land where they exhibit complicated flower patterns. Destructive earthquakes occurred on the HFF in 1755, 1867 during an eruption offshore Tj\"{o}rnes, i.e. north of the fault, and in 1872. The 1867 earthquakes where most likely associated with rift-transform interaction on the easternmost section of the fault. An intense earthquake sequence on April 17, 1872 culminated with two $\sim$M6.5 earthquakes at 10 a.m. and 11 a.m. the next day. Based on intensity and damage reports, the $\sim$M6.5 earthquakes originated at different segments of the HFF, near H\'{u}sav\'{\i}k and Flatey.

Department

Center for Coastal and Ocean Mapping

Publication Date

12-2003

Volume

84, Number 46

Journal Title

EOS Transactions, American Geophysical Union

Conference Date

Dec 8 - Dec 12, 2003

Publisher Place

San Francisco, CA, USA

Publisher

American Geophysical Union Publications

Document Type

Conference Proceeding

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