GEOLOGIC AND PALEOMAGNETIC EVIDENCE FOR SPREADING-RELATED MAGMATISM, FAULTING AND CRUSTAL FLEXURE IN THE BLÖNDOUS AREA, NORTHERN ICELAND

Tertiary-Recent lava flows in Iceland are structurally characterized by a series of “flexure zones” defined by regionally extensive lavas that dip inward toward synclinal axes. Previous authors have suggested that these flexure zones represent fossil spreading centers along the northern Mid-Atlantic Ridge abandoned by a series of eastward-directed ridge jumps that have left the active ridge crest at its present position in east-central Iceland. Although the geometry of ridge-inward dipping lavas observed in Icelandic flexures is counter to commonly assumed models of ridge crest structure, this overall geometry is similar to that recently observed along some well-exposed sections through Pacific Ocean crust. In our studies, we are testing the notion that lavas, cross-cutting diabase dikes and faults will provide a history of the magmatic and structural history of events related to Icelandic flexure zones and, ultimately, to structurally similar areas observed in the oceans. The eastern side of a regional synclinal flexure east of Blöndous in north-central Iceland is characterized by westward-dipping, ~6-8 Ma lavas cut by a myriad of east-dipping normal faults and diabase dikes. Paleomagnetic and structural data from 27 sites in lavas and cross-cutting dikes in this area demonstrate that (1) west-dipping lavas and east-dipping dikes result from structural rotations and (2) dikes were intruded both during and after normal fault-related tilting. Significantly, these data indicate that fault-related tilting within the eastern Blöndous flexure was initiated within the presumably narrow neovolcanic zone of the ridge and is not wholly related to off-axis thermal subsidence. These data are similar to our previous structural and paleomagnetic observations from several “tectonic windows” into intermediate to super-fast spread crust.