Along‐Axis Structure and Crustal Construction Processes of Spreading Segments in Iceland: Implications for Magmatic Rifts. Issue 10 (17th October 2017)

• Record Type: Journal Article
• Authors: Siler, D. L.; Karson, J. A.
• Is Part Of: Tectonics. Volume 36:Issue 10(2017)
• Page Start: 2068
• Page End: 2084
• Publication Date: 2017-10-17
• Subjects: magmatic rifts -- Iceland -- subsidence -- segmentation -- seafloor spreading -- crustal structure; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1002/2017TC004629
• Languages: English
• ISSNs: 0278-7407

Abstract

Abstract: Magmatic rift systems are composed of discrete spreading segments defined by morphologic, structural, and volcanic features that vary systematically along strike. In Iceland, structural features mapped in the glaciated and exhumed Miocene age upper crust correlate with analogous features in the seismically and volcanically active neovolcanic zone. Integrating information from both the active rift zones and ancient crust provides a three‐dimensional perspective of crustal structure and the volcanic and tectonic processes that construct crust along spreading segments. Crustal exposures in the Skagi region of northern Iceland reveal significant along‐strike variations in geologic structure. The upper crust at exhumed magmatic centers (segment centers) is characterized by a variety of intrusive rocks, high‐temperature hydrothermal alteration, and geologic evidence for kilometer‐scale subsidence. In contrast, the upper crust along segment limbs, which extend along strike from magmatic centers, is characterized by thick sections of gently dipping lava flows, cut by varying proportions of subvertical dikes. This structure implies relatively minor upper crustal subsidence and lateral dike intrusion. The differing modes of subsidence beneath segment centers and segment limbs require along‐axis mass redistribution in the underlying upper, middle, and lower crust during crustal construction. This along‐axis material transport is accomplished through lateral dike intrusion in the upper crust and by along‐axis flow of magmatic to high‐temperature solid‐state gabbroic material in the middle and lower crust. These processes, inferred from outcrop evidence in Skagi, are consistent with processes inferred to be important during active rifting in Iceland and at analogous magmatic oceanic and continental rifts. Plain Language Summary: Extensional tectonic plate boundaries (or rifts) are locations where the Earth's crust is stretched and extended. Many such settings are volcanically active; that is, new crust is built by injection of magma into existing crust and by eruption of lava on to the Earth's surface. Our research is based on outcrop geology in the Skagi area, northern Iceland. The crust in Skagi was built ~2–4 km beneath Iceland's rift zone ~7–12 Myr ago. The Skagi crust has since been glacially eroded, exposing these rocks at the surface. These rocks, therefore, preserve evidence of the geologic and tectonic processes occurring beneath the rift during extension. Our work shows that extension of the crust occurs along with two important processes: subsidence (downward movement of the crust) and axial transport (movement of material along the axis of the rift). The spatial patterns of these two processes are uniform and consistent between Iceland and many rifts in diverse geologic settings. These results are, therefore, applicable to the characterization of rift tectonics worldwide. They are also applicable to the understanding of geologic hazards (e.g., earthquakes and volcanoes) and energy/mineral resources (e.g., geothermal energy, fossil fuel deposits, and ore deposits), all of which are abundant in rifts. Key Points: Crust in Iceland and other magmatic rifts is constructed along discrete spreading segments Magmatic construction and subsidence vary systematically along spreading segments Local subsidence occurs at segment centers; regional subsidence, lava pile thickening, and lateral dike intrusion occur at segment limbs