The 2006 March 25 Fin earthquakes (Iran) - insights into the vertical extents of faulting in the Zagros Simply Folded Belt

Roustaei, M. and Nissen, E. and Abbassi, M. and Gholamzadeh, A. and Ghorashi, M. and Tatar, M. and Yamini-Fard, F. and Bergman, E. and Jackson, J. and Parsons, B. (2010) The 2006 March 25 Fin earthquakes (Iran) - insights into the vertical extents of faulting in the Zagros Simply Folded Belt. Geophysical Journal International, 181 (3). pp. 1275-12991. DOI

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We investigate the depth of faulting and its connection with surface folding in the Zagros Simply Folded Belt of Iran. Our focus is a sequence of earthquakes (Mw 5.7, 5.5, 5.2, 5.0, 4.9) that struck the Fin region, in the south-eastern Simply Folded Belt, on 2006 March 25. Modelling ground displacements measured with radar interferometry, we find that either N- or S-dipping model reverse faults can reproduce the observed fringe patterns. Despite the uncertainty in fault orientation, we can constrain the vertical extents of rupture to between a top depth of 223C520136 km and a bottom depth of 223C9201310 km, consistent with the 223C8 km centroid depth of the largest earthquake. We suggest that the faulting ruptured the thick 'Competent Group' of Paleozoic and Mesozoic conglomerates and platform carbonates, which makes up the lower part of the sedimentary cover. The rupture probably terminated within the Precambrian Hormuz salt at its base, and the Cretaceous Gurpi marls at its top. These mechanically weak layers act as barriers to rupture, separating faulting within the Competent Group from deformation in the layers above and below. The pattern of coseismic surface uplift is centred on the common limb of the Fin syncline and Guniz anticline, but is oblique (by 20°) to the trend of these open, symmetric, 'whaleback' folds, and also overlaps a section of the Fin syncline axis. These observations suggest that locally, surface folding is decoupled from the underlying reverse faulting. Although the Fin syncline and Guniz anticline are symmetric structures, some other nearby folds show a strong asymmetry, with steep or overturned southern limbs, consistent with growth above N-dipping reverse faults. This suggests that the Simply Folded Belt contains a combination of forced folds and detachment folds. We also investigate the distribution of locally recorded aftershocks in the weeks following the main earthquakes. Most of these occurred at depths of 223C10201330 km, with a particularly high concentration of events at 223C20201325 km. These aftershocks therefore lie within the crystalline basement rather than the sedimentary cover, and are vertically separated from the main rupture. This study confirms earlier suggestions that earthquakes of Mw 520136 are capable of being generated within the thick 'Competent Group' of Paleozoic and Mesozoic sediments, as well as in the basement below the Hormuz Salt Formation.

Item Type: Article
Uncontrolled Keywords: 2010AREP; IA60;
Subjects: 02 - Geodynamics, Geophysics and Tectonics
Divisions: 02 - Geodynamics, Geophysics and Tectonics
Journal or Publication Title: Geophysical Journal International
Volume: 181
Page Range: pp. 1275-12991
Identification Number:
Depositing User: Sarah Humbert
Date Deposited: 04 May 2010 16:00
Last Modified: 23 Jul 2013 09:57

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