Subvolcanic subsidence and caldera formation during subaerial seafloor spreading in Iceland

Many caldera systems worldwide are interpreted to form as a result of catastrophic subsidence into space vacated by eruption. Some basaltic calderas, however, both associated with modern active volcanoes and in ancient, deeply eroded terranes, lack evidence for catastrophic formation and thus may have formed incrementally. The Vatnsdalur structural basin in northern Iceland is one such caldera-like volcanic depression. The Vatnsdalur structural basin is an elliptical depression with dimensions of ~6 km × ~3 km and a structural depth of ~1.5 km. It occurs within the gently westdipping, Tertiary basaltic lava sequences of the Skagi region, northern Iceland. Tilted basaltic to rhyolitic lava flows define a largescale, rim monocline surrounding the basin. Exposures that form the southern rim of the Vatnsdalur structural basin reveal details of the pre-, syn-, and postsubsidence volcanic and tectonic processes during basin evolution. In these exposures, the dips of lava flows steepen continuously from horizontal to as much as 60 W, toward the center of the basin. Basaltic lava flows and mass-flow breccia units were deposited unconformably over the dipping lava flows and thicken substantially downdip, toward the basin center. The dip of bedding and flow contacts in these overlying units decreases up section, gradually becoming conformable with the regional lava-flow stratigraphy. Brittle folding of the lava flows within the rim monocline occurred by a combination of slip on minor faults and syntectonic dike intrusion. Based on structural evidence and comparisons to worldwide caldera systems, we conclude that the Vatnsdalur structural basin formed as a result of either (1) loading of the subsurface by dense, intrusive material or (2) shallow crustal magma redistribution accommodating many small subsidence events. In the context of crustal construction in Iceland, the Vatnsdalur structural basin and adjacent areas demonstrate significant along-strike variability in the mechanism(s) of crustal construction that occurs during subaerial seafloor spreading.