Crustal accretion of thick mafic crust in Iceland: implications for volcanic rifted margins

Farrell

Chutas³

et al. 2018

Tectonics

Strongly lineated terrain outside of Iceland's active plate boundary zones is created by faults and dikes aligned with the rift zones where they formed, similar to the spreading fabric defined by abyssal hills generated at mid‐ocean ridge spreading centers. As expected, rift‐parallel normal faults and fissures dominate in the active rift zones, but in older crust to the east and west, faults with strike‐slip and oblique‐slip displacements dominate. Some areas have widespread, small‐scale, strike‐slip, and oblique‐slip faults, while others have more widely spaced, larger, strike‐slip fault zones. In most cases, the strike‐slip and oblique‐slip faults strike subparallel to nearby older dikes and normal faults assumed to indicate the orientation of the rift zones where they formed. Strike‐slip displacements overprinting normal faults and along dike margins suggest reactivation of spreading‐related zones of weakness. More complicated fault geometries and kinematics occur near the oblique rifts and the major transform fault zones. The sense of movement on the strike‐slip and oblique‐slip faults is broadly systematic with respect to the active Northern and Eastern Rift Zones supporting the interpretation that they are the result of crustal block rotations on either side of rift zones that propagate to the north and south away from the center of the Iceland hot spot. Similar fault kinematics may occur along mid‐ocean ridges and other magmatic rifts where rift propagation occurs on a range of scales.

Section: Discussionmentioning

confidence: 99%

Rift‐Parallel Strike‐Slip Faulting Near the Iceland Plate Boundary Zone: Implications for Propagating Rifts

Farrell

Chutas³

et al. 2018

Tectonics

“…The thermal structure (Fl ovenz & Saemundsson, 1993) and crustal-level magma bodies imaged seismically (Brandsd ottir & Menke, 2008;Einarsson, 1978) and even encountered by drilling (Elders et al, 2011), suggest that the gabbroic middle to lower crust is rheologically weak and may flow laterally along axis during spreading (Jones & Maclennan, 2005). Thus, as in other settings with hot, weak, mafic crust, the mobile lower crust may change thickness substantially relative to that of the rapidly cooled upper crust during spreading (Bell & Buck, 1992;Karson, 2016).…”

Section: Geochemistry Geophysics Geosystemsmentioning

confidence: 98%

The Iceland Plate Boundary Zone: Propagating Rifts, Migrating Transforms, and Rift‐Parallel Strike‐Slip Faults

Geochem Geophys Geosyst

2017

Unlike most of the Mid‐Atlantic Ridge, the North America/Eurasia plate boundary in Iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures. Accordingly, geologic processes in Iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid‐ocean ridge system. Combining existing and new data from across Iceland provides an integrated view of this active, mostly subaerial plate boundary. The broad Iceland plate boundary zone includes segmented rift zones linked by transform fault zones. Rift propagation and transform fault migration away from the Iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading‐related structures. Rift propagation results in block rotations that are accommodated by widespread, rift‐parallel, strike‐slip faulting. The geometry and kinematics of faulting in Iceland may have implications for spreading processes elsewhere on the mid‐ocean ridge system where rift propagation and transform migration occur.

“…The northern and eastern rift zones extend to the north and south from the hot spot and are linked to the offshore spreading centers by the Tjörnes Fracture Zone and South Iceland Seismic Zone transforms, respectively (Figure ). Both the northern and eastern rift zones are actively propagating away from the hot spot (Karson, ). Rift propagation has resulted in “ridge jumps” to the east, in order for the locus of plate spreading to remain above the hot spot as Iceland migrates west‐northwestward with respect to the hot spot.…”

Section: Spreading In Icelandmentioning

confidence: 99%

“…Rift propagation has resulted in “ridge jumps” to the east, in order for the locus of plate spreading to remain above the hot spot as Iceland migrates west‐northwestward with respect to the hot spot. Spreading on the eastern rift zone is replacing spreading on the western rift zone with which it overlaps (Einarsson, ; Karson, ; LaFemina et al, ). Similarly, spreading on the northern rift zone replaced spreading that occurred on a rift zone to the west in the Skagi area between ~7 and 12 Ma (Hardarson & Fitton, ; Jancin & Young, ; Kristjánsson, et al, ; Kristjánsson & Jónsson, ; Sæmundsson, ; Walters et al, ).…”

Section: Spreading In Icelandmentioning

confidence: 99%

Along‐Axis Structure and Crustal Construction Processes of Spreading Segments in Iceland: Implications for Magmatic Rifts

Siler

2017

Tectonics

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.