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Imperial College London (1975)

Recent and contemporary crustal deformation in Easter Iran

Mohajer-Ashjai, Arsalan

Titre : Recent and contemporary crustal deformation in Easter Iran

Auteur : Mohajer-Ashjai, Arsalan

Université de soutenance : Imperial College London

Grade : Doctor of Philosophy (PhD) 1975

Résumé
Iran lies in a zone of active mountain building which forms a complex boundary between the Arabian and Eurasian crustal plates. In an attempt to gain a better understanding of current tectonic processes, and to investigate the applicability of the crustal microplate concept, geodetic, microearthquake and geological field studies have been made of certain major faults in the eastern part of the country, supplemented by studies of aerial photographs and ERTS 1 satellite imagery. On geological and seismological grounds, attention was concentrated on the Dasht-e-Bayaz and Great Kavir (Doruneh) faults, which form the northern boundary of the Lut Block, a proposed micro-plate. A small number of geodetic networks, consisting of lines a kilometer or so in length spanning these faults, were constructed and measured in October 1971, May 1972 and November 1974 with a Mekometer, a highly accurate electro-optic distance-measuring instrument. Siting of these networks was aided by a complementary study of microearthquake activity, using portable smoked drum seismographs. This revealed considerable activity, exceeding 15 events per day in places, with a tendency for the greatest activity to be associated with the ends of the strike slip faults, where a concentration of stress might be expected on theoretical grounds. Eight well recorded shocks were located with accuracy of better than 1 km. in N. Kashmar and seven similar shocks in the Dasht-e-Bayaz areas. These,plus other less accurately located shocks seem to be associated with a newly found fault in N. Kashmar and the Dasht-e-Bayaz fault respectively. Two composite fault plane solutions are also presented for these microearthquakes and low b values (about 0.5) were found from the recurrence curves obtained for different stations. To provide a basis for interpreting these results, a detailed map of recent and former fault traces has been prepared, based on aerial and satellite photographs. The angular relationship of the recent fault trends revealed by these photographs, together with available information on Neogene folding and the source mechanisms of the major earthquakes occuring in the area, suggest a consistent NE-SW compression of the crust and an almost uniform strain distribution. This is consistent with the results of the geodetic study, where measurements across the Dasht-e-Bayaz fault revealed left-lateral displacement combined with extension in the NW-SE direction, also with a composite fault plane solution showing a dominant left-lateral movement with a N46E axis of compression. Regional seismicity, internal deformation, dislocation lines cutting across the Lut Block,and low b values obtained from microearthquake studies, imply non-rigid behaviour of this proposed plate and, therefore, throw doubt on the micro-plate concept as an explanation of the observed recent and contemporary crustal deformation taking place in eastern Iran. Three regions near Birjand, Zahedan and Tabas are tentatively classified as the tectonic knots where major earthquakes might be expected in the future.

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Page publiée le 13 juin 2020