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Title: The Iceland Plate Boundary Zone: Propagating Rifts, Migrating Transforms, and Rift-Parallel Strike-Slip Faults: ICELAND PLATE BOUNDARY ZONE
Author(s) / Creator(s):
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geochemistry, Geophysics, Geosystems
Page Range / eLocation ID:
4043 to 4054
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract

    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.

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  2. The structure of the lithosphere-asthenosphere boundary (LAB) beneath oceanic plates is key to understanding how plates interact with the underlying mantle. Prior contradictory geophysical observations have been used to argue for a thin, melt-rich boundary that decouples the plate from the rest of the mantle, or for a much broader anisotropic and thermally controlled boundary that indicates significant coupling with the rest of the mantle. The predictions of models based on these interpretations can be tested most easily in a subduction zone setting where the steady increase in pressure at the base of the subducting plate’s LAB will have differing effects on melt and anisotropy. Melt remains stable within the mantle to ~150-250 km (for carbonate melt) or to ~330 km (for silicate melt), while anisotropy induced by different processes should have no significant change until ~250 km to ~440 km depth. We calculate P-to-S receiver functions (PRFs) using varying frequency bands at broadband seismic stations with >4 years of data from the Servicio Geológico Colombiano’s Red Sismológica Nacional de Colombia to investigate the characteristics of the LAB of the subducting Nazca oceanic plate from the coast to the Andean foreland (corresponding to slab LAB depths of ~50 km to >400 km). The use of PRFs permits identification and analysis of anisotropy across the boundary while calculation at a range of frequency bands permits tuning of the PRFs to differing spatial scales to determine the size and abruptness of the boundary. We find that the P-to-S converted phase of the subducted Nazca plate’s LAB is detectable 4-5 seconds after the converted phase of the plate’s Moho to at least ~150 km depth. Assuming the slab has an average Vp/Vs of 1.75 to 1.78 and Vp of 8.2 km/s (+2.5% dVp), this corresponds to a plate thickness of ~50 km, matching the expected thickness given the Nazca plate’s age in the region (~10-20 Myrs). We find that the Nazca plate’s LAB is most consistently detectable in the <0.24 Hz band and largely undetectable in the <2.4 Hz band, indicating the LAB is gradational and between 10 and 30 km in thickness. Amplitude variations and complexities in the LAB converted phases further indicate that the boundary marks a change in anisotropy most consistent with the LAB representing a sheared zone between the plate and underlying mantle. 
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  3. Abstract

    Below the seismogenic zone, faults are expressed as zones of distributed ductile strain in which minerals deform chiefly by crystal plastic and diffusional processes. We present a case study from the Caledonian frontal thrust system in northwest Scotland to better constrain the geometry, internal structure, and rheology of a major zone of reverse-sense shear below the brittle-to-ductile transition (BDT). Rocks now exposed at the surface preserve a range of shear zone conditions reflecting progressive exhumation of the shear zone during deformation. Field-based measurements of structural distance normal to the Moine Thrust Zone, which marks the approximate base of the shear zone, together with microstructural observations of active slip systems and the mechanisms of deformation and recrystallization in quartz, are paired with quantitative estimates of differential stress, deformation temperature, and pressure. These are used to reconstruct the internal structure and geometry of the Scandian shear zone from ~10 to 20 km depth. We document a shear zone that localizes upwards from a thickness of >2.5 km to <200 m with temperature ranging from ~450–350°C and differential stress from 15–225 MPa. We use estimates of deformation conditions in conjunction with independently calculated strain rates to compare between experimentally derived constitutive relationships and conditions observed in naturally-deformed rocks. Lastly, pressure and converted shear stress are used to construct a crustal strength profile through this contractional orogen. We calculate a peak shear stress of ~130 MPa in the shallowest rocks which were deformed at the BDT, decreasing to <10 MPa at depths of ~20 km. Our results are broadly consistent with previous studies which find that the BDT is the strongest region of the crust.

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