Tectonic Details of the Tjornes Fracture Zone, an Onshore-Offshore Ridge-transform in N-Iceland

Abstract

The Tjornes Fracture Zone (TFZ) links the northern rift zone (NVZ) in Iceland with the Kolbeinsey Ridge north of Iceland. The TFZ was initiated during the Miocene (about 7 Ma), following an eastward jump of the spreading axis in northern Iceland. A roughly 150 km long (EW) and 50 km wide (NS) deformation zone has since developed incorporating both right-lateral movement along WNW-trending strike-slip faults and oblique extension (105�) within three major N-S trending grabens (from west to east the Eyjafjardar�ll, Skj�lfandi and Oxarfjordur basins). Recently collected EM300 and RESON8101 multibeam bathymetric data, and CHIRP subbottom data combined with onshore mapping have enhanced our understanding of the rift-transform interactions within the TFZ. The transform motion is incorporated within two seismically active WNW trending zones, the Gr�msey Seismic Zone (GSZ) and the H�sav�k-Flatey fault (HFF), spaced ~40 km apart along the margins of the extensional basins. Being the propagating continuation of the NVZ offshore the GSZ has both the characteristics of an oblique rift zone and a transform whereas the HFF is more akin to oceanic transform faults. Four left-stepping, en-echelon, NS-striking rift segments (volcanic systems) exist along the GSZ. Large GSZ earthquakes, however, seem to be mainly associated with lateral strike-slip faulting along WNW-striking fault planes. Fissure swarms transecting the offshore volcanic systems also indicate right-lateral strike-slip motion parallel to the spreading direction. The HFF has an overall strike of N65�W and can be traced continuously onshore and offshore along its 75-80 km length, between the NVZ, across Skj�lfandi and into Eyjafjardar�ll. Four pull-apart basins occur along the fault, the largest at the intersection with Eyjafjardar�ll, the southward but magma-starved, continuation of the KR. Tertiary dikes, parallel to the HFF indicate it has been a leaky transtensional feature. The southwestern margin of the fault is characterized by NE-striking lavas which dip steeply (30-50�) towards Eyjafjardar�ll. The lavas are dissected by en echelon arrays of conjugate strike-slip faults intersecting the HFF fault at angles of N20�-30�W and N20�E. Some can be traced onto land where they exhibit complicated flower patterns. Destructive earthquakes occurred on the HFF in 1755, 1867 and 1872. The 1867 events were most likely associated with rift-transform interaction within the M�n�reyjar volcanic system, similar to the 1975-1989 Krafla rifting episode, when a lateral intrusion event triggered a M6.5 strike-slip earthquake at the junction of the Krafla fissure swarm and the GSZ. Although transform motion within the TFZ is currently taken up by two parallel fault systems the Tjornes microplate will merge with the North American plate as continued northward propagation of the divergent plate boundary gradually deactivates the extensional basins and HFF.

Department

Center for Coastal and Ocean Mapping

Publication Date

12-2004

Volume

85, Issue 47

Journal Title

EOS, Transactions American Geophysical Union, Fall Meeting, Supplement

Conference Date

Dec 13 - Dec 17, 2004

Publisher Place

San Francisco, CA, USA

Publisher

American Geophysical Union Publications

Document Type

Conference Proceeding

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