Internal Structure of the Extinct Skagi-Hunafloi Rift Zone and Implications for Magmatic Construction

Structural relief ~1.5 km of basaltic crust on the eastern flank of the now-extinct Skagi-Hunafloi rift zone (active between 8 and 4 Ma) is exposed in two mountain ranges bounding the glacially excavated Vatnsdalur in northwestern Iceland. This area reveals both the along- and across-strike variations within an abandoned Tertiary rift zone. Regionally the rift zone is characterized by a "flexure zone" with basaltic lavas that dip 5-10° westward, toward the rift axis. Superimposed on this structure is a bowl-shaped depression at least 800 m deep and ~5 km in diameter in which lava flows dip as much as 50° inward. The lowest package of lavas associated with the depression are basaltic and minor rhyolitic flows (7.62±0.32 Ma), which are at least 400 m thick and have been tectonically rotated during subsidence, as indicated by field relationships and preliminary paleomagnetic data. Overlying these tilted, pre-subsidence flows are syn-subsidence basaltic lavas, which are at least 150 m thick and breccias all of which thicken toward the center of the depression. Overlying the syn- subsidence flows is a ~250 m thick basaltic lens (6.98±0.32 Ma). Ages and thicknesses of units associated with the depression roughly constrain the local subsidence rates at ~1 km/my. Just to the west, gabbroic to granophyric intrusions and hydrothermally altered igneous rocks and breccias mark a dissected volcanic center. Dense (cone?) sheet swarms dip radially inward. This entire assemblage is overlain by gently dipping basaltic to rhyolitic lavas showing that magmatic construction did not result in generation of high relief. Crustal thickening was accommodated by subsidence and backfilling near the depression and by basaltic to minor rhyolitic sheet intrusions over a broader surrounding area. Folding of the pre-subsidence lavas was likely accommodated by slip on steeply dipping fractures and within flow top breccia units. Focused subaxial subsidence in rift zones may result from subsidence into accommodation space created by evacuation of shallow crustal magma chambers beneath central volcanoes. The along- and across-strike variations in Tertiary rift zones in Iceland may provide an analog for the internal structure of mid-ocean ridge spreading segments.