Crustal Accommodation Space Generated during Rifting in Iceland
Knowing the volume of crustal accommodation space generated during crustal rifting is crucial to understanding both the mechanics of rift systems as well as their economic potential. Whereas subsidence mechanisms in continental rifts and rifted margins have been studied in detail, relatively little attention has been paid to volcanic rift systems. Rifting processes in Iceland are analogous to a wide range of volcanic rift and rifted margin systems and may provide insights into shallow to mid-crustal processes in these types of extensional environments. Icelandic rift zones are represented by an array of mechanically independent spreading segments, each a few tens of kilometers in length. Rift segment morphology, structure and subsidence patterns vary along strike, from segment centers to segment ends. Near complete exposure of extinct spreading segments in northern Iceland allows for along-strike variations in the volume of accommodation space generated during rifting to be calculated. Segment centers are sights of the most robust magmatic construction and intense hydrothermal alteration. In these areas, localized magma chambers at depths of a few kilometers feed shallow-level intrusions and lava flows. Inclined sheet swarms present at segment centers result in crustal thickening of at least 1-2 km via intrusion. Subdued topography at segment centers indicates that this crustal thickening is accommodated by localized mid-crustal subsidence rather than by building surface topography. Caldera-like, flexural basins on the order of several hundreds of meters deep indicate that equivalent amounts of subsidence out-paced the back-filling of these depressions by lavas and sedimentary breccias. Segment ends show evidence of substantially different processes. These areas have shallow flexures characterized by gently dipping lavas, dike swarms and more subdued subsidence. Along-strike variations in crustal thickness suggest that flow of middle crustal material is generally directed from segment centers towards segment ends. This redistribution of material away from segment centers creates the accommodation space required for flexural basins as well as thick intrusive sheet swarms to form. Similar processes are likely to occur beneath all magmatically active rifts, rifted margins and mid-ocean ridge spreading axes.