Asymmetric Architecture of an Active Rift Zone in the East Volcanic Zone, Iceland
Romain Plateaux , Françoise Bergerat and Bernard Mercier de Lépinay
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Introduction & purposes
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Determine the architecture of the volcanic
systems in active rift zone
Map large scale fracture length
Evaluate the distribution of throw and length of
fractures
Infer the fracturing mechanisms
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Institute of Oceanography, National Taiwan University, ISTeP, CNRS-UPMC, Géoazur Université de Nice Sophia-Antipolis
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3
Conclusion & perspective
Results
Building photogrammetric models and field checks
Objectives
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• An
example of a photogrammetric model in the Barðarbunga-Veiðivötn Volcanic System and the real objects in the field.
Conclusion
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Asymmetric geometry of the graben like-structures
within an active rift system
Tension fracture (mode I) and normal fault (shear failure)
Correlation between mean offset and fracture length
• Different development stage of grabens
• Fracture growth by coalesance of neighboring fractures
Slight deviation of the stress inferred from measured
fractures and the spreading direction
Introduction
How does the active rift look like ? : made of volcanic
systems constituted of central volcanoes, eruptive and non
eruptive fissures, and normal faults. Why the East Volcanic Zone (EVZ) in Iceland ? : (1) the most active rift
segment, (2) well exposed fractures. What objects ? The
Barðarbunga-Veiðivötn Volcanic System (BVVS) and the
Laki Volcanic System (LVS). How ? using photogrammetric technique because difficult access in the field and large
fracture scale.
Perspective
•A
comparison between the different fissure swarms in
Iceland
Figure 4: Detection of tie points (semiautomatic) between pair of photographs (a) and a block image showing all the tie points including ground points in a strip photographs (b). Resolution of the block image : 1 pixel = 0.5 m in
the field, focal length 153.15 mm, mean misfit (in pixel) 0.33±0.82, number of tie points : 4072, ground points : 18. c, d, e and f comparison of a normal fault between the photogrammetric model (c and d) and the field measurement.
Important Result
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Fracturing mechanisms are characterized by tension fractures (mode I) and normal faults (shear failure).
Fractures growth are controlled by coalescence of neighboring fractures
The orientation of ~σ3 inferred from fracture orientations slightly deviated from the regional spreading direction (more than 20◦)
Tectonic maps of the volcanic systems : BVVS & LVS
• Þingvellir fissure swarm ([5])
• Vogar fissure swarm ([6])
• This fissure swarm ([7])
• and others
• Geometrical
and statistical comparison
• Geomechanical modeling of the fault displacement using
angular dislocations (Poly3D)
References
[1] D. D. Pollard and P. Segall.
Theoretical displacements and stresses near fractures in rock : With applications
to faults, joints, veins, dikes, and solution surfaces.
Fracture Mechanics by B. K. Atkinson, Academic Press (San Diego, Calif.), 1987.
[2] C. H. Scholz.
The Mechanics of Earthquakes and Faulting.
Cambridge university press, New York, 1990.
[3] C. DeMets, R. G. Gordon, F. Argus, and S. Stein.
Current plate motions.
Geophysical Journal International, 101 :425–478, 1990.
[4] Águst Guðmundsson.
Rock Fractures in Geological Processes.
Cambridge university press, 2011.
Figure 1: Geological setting of Iceland and the studied area. The central volcanoes as
follow : ; B, Barðarbunga, E, Eyjafjallajökull ; G, Grímsvötn ; Ha, Hagöngur ; Ham, Hamarinn ;
He, Helka ; K, Katla ; Þ, Þeistareykir ; T, Torfajökull ; V, Vonarskarð, Open arrows show local
velocities of Eurasian and North American plates with respect to the hotspot.
[5] L. Sonnette, J. Angelier, T. Villemin, and F. Bergerat.
Faulting and fissuring in active oceanic rift : Surface expression, distribution and
tectonic–volcanic interaction in the Thingvellir Fissure Swarm, Iceland.
Journal of Structural Geology, 32(4) :407–422, April 2010.
Methods
• Photogrammetry
[6] T. Villemin and F. Bergerat.
From surface fault traces to a fault growth model : The Vogar Fissure Swarm of the
Reykjanes Peninsula, Southwest Iceland.
Journal of Structural Geology, 51 :38–51, June 2013.
technique : Aerotriangulation
(ERDAS software)
[7] R. Plateaux.
Architecture et mécanismes de fracturation du rift islandais dans la région de
Vatnajökull.
PhD thesis, Université de Nice Sophia-Antipolis, 2012.
Figure 5: A, The Barðarbunga-Veiðivötn Volcanic System ; B, The Laki Volcanic System ; C, a zoom on the Laki mountain.
Fracture length and throw
Fracturing mechanisms and stress
[8] F. Bergerat and R. Plateaux.
Architecture and development of (Pliocene to Holocene) faults and fissures in the East
Volcanic Zone of Iceland.
Comptes Rendus Geoscience, 344 :191–204, March 2012.
• Tension
fractures (mode I) and normal faults (shear failure, Fig. 7c).
• Orientation of ~
σ3 deviated from the spreading direction [3].
Figure 2: Principle : with one photography not possible to determine the depth (a),
unless a Digital Elevation Model is known (b) or two photographs are used (c) and the
rays cut each other and depth is then fixed (photogrammetry). Orientation : inner and
outer. Aerotriangulation : it combines inner and outer orientation calculations in a
geographic frame over a large project, using the coplanarity principle solved by a least
square method.
Contact Information
• Web
:
http ://homepageromainplateaux.com/
• Email : [email protected]
• Phone : +886 (0)988 157 640
• 3D
visualization and mapping (ERDAS & ArcGis
softwares)
Figure 6: Distribution of the number of fractures as function of the fracture length (a), the same distribution but
in log − log plot (b), superimposed offset profiles normalized showing 23 profiles with mean offset in red and ±
the standard deviation in dashed lines (c), correlative diagram between mean fault offset and fault length (d).
• Fig.
Figure 3: 3D Visualization to observe and map normal faults facing NE (red lines), SW
(blue lines) or fractures with no displacement observable (yellow lines) on a photomosaic
(made from aerotriangulation) , and then create a database in a GIS.
6a & b : classical distribution showing a logarithmic law.
• Fig. 6c mean profile showing an half ellipse with abrupt tips, consistent
with a first order dislocation model ; i.e. :[1, 2].
• Fig. 6d Such correlation traduces the fractures growth are controlled by
coalescence of neighboring fractures (largely studied elsewhere). This
distribution may also reflect the different stages of development of the
graben like-structures.
Figure 7: a and b, ~σ3 as denoted by green open arrows (parallel to ~σhmin) inferred from the fracturing mechanisms
and fracture orientation. c example of a fault system at earth surface in Iceland [4].