عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Intruduction: Red beds have been widely studied by palaeomagnetic methods for what they can tell us about the history of sedimentary basins and their subsequent deformation (Collinson, 1974, Turner, 1979a,b and 1981, McCabe and Elmore, 1989, Elmor, et al., 1993 and 2000). This study has investigated the palaeomagnetism of the Cambrian red sediments (Dezou and Dahou Formation) of the eastern margin of Central Iran. Davoudzadeh and Schmidt, (1984), have done some work on the region which includes these rocks and discuss the few rotations of the Central Iran micro plates. The aim of this study is to consider the AMS results to investigate the palaeo force field on the rock located at the northern side of the Persian Gulf.
Geological setting: The sampling sites of this study are situated in the Cambrian sediments of the eastern margin of Central Iran. These are the largest structural sedimentary units and are composed of complex geological structures. In this region, igneous, metamorphic and sedimentary rocks of the Precambrian to Quaternary Periods are preserved with a predominant number of outcrops of Mesozoic rocks. Most structural elements in different scales such as faults and folds are related to the tectonic activities of the Mesozoic and later eras. The most important structural element of the region is the Koohbanan fault with a NW-SE trend. Similar to the most other important faults of Central Iran, the existence of this fault is related to the late Precambrian Era.
The Dezou Formation is limited to the Rezou series (Late Precambrian) by a fault at the bottom and it can be separated by an unconformity from the Dahou Formation at the top in its area of study (Gazooie village, 30.8° N and 56.7° E, sites 13-14 & 16-20). It has a thickness of 380 m involving three major parts.
The Dahou Formation in the area of study (Gazooie village, 30.8° N and 56.7° E, site 20, figure. 4) rests with a basal breccia conformably on dolomites and limestone of the Dezou Formation. Middle to the end of early Cambrian Period was inferred from the stratigraphic position. It is covered by the Koohbanan Formation unconformably.
Rock magnetic studies: In the present study rock magnetic experiments including IRM acquisition, backfield IRM, thermomagnetic, and hysteresis and AMS studies have been performed on samples of the Dezou and Dahou Group sediments. Some of these experiments yield results that are grain size dependent and others are mineral type dependent; some are dependent on both properties.
Jelinek (1981) and Hrouda (1982) have proposed parameters T and Pj for the shape and corrected anisotropy degree (Tarling and Hrouda, 1993). A plot of T against Pj when Pj > 1 and –1 < T < 1 provides information about the shape and the degree of anisotropy on the same plot. T- Pj plot is shown for all sampling sites in the Dahou and Dezou Formations and also site 20 only. Most samples have less than 10% anisotropy but some of the samples tend to be more anisotropic..
Curie temperatures are mostly between 675-700?C and attributed to specularite and pigments of hematite. In a few examples magnetite with a Curie temperature of about 570?C can be seen as well. The clearest indication of magnetite here is RM ratio (RM ratio is the value of Ms at 100°C on the cooling curve to the value on the heating curve). RM ratio < 1 showing that magnetite has been oxidized to hematite with low Ms Values.
Rock magnetic studies show an agreement that both specular hematite and magnetite can occur in these sediments although the effect of hematite is usually dominant.
Site 13 that is sampled in red sandstone shows paramagnetic mineral content and hematite pigment with a Curie temperature of 680?C and a saturation field of 2000 mT. Low coercivity of 50 mT shows a low amount of magnetite.
Site 14 that is sampled in red sandstone shows a high amount of paramagnetic mineral content and finer grain size of hematite pigments (compare to site 13) with a Curie temperature of 680?C and a saturation field of 3000 mT. Lower coercivity than site 13 shows low amount of magnetite too.
Site 15 that is sampled in dolomite breccias shows very scattered results of IRM, hysteresis and Curie curves. And overall look suggest a predominantly diamagnetic mineral.
Site 16 that is sampled in red sandstone at its contact with dyke shows an overprinted magnetite mineral and it is obvious from the curie temperature of 580?C and very low saturation field in hysteresis and IRM curve.
Site 17 that is sampled in dolomite just above the dyke shows an overprinted magnetite. However, a predominant content of diamagnetic minerals is clear.
Site 18 and Site 19 that are sampled in red sandstone and red shale respectively show similar rock magnetic behaviour of paramagnetic mineral content and pigment and specular hematite with a high saturation field of 3000 mT and a back field of 500 mT.
Red sediments normally show a stable rock magnetic content. The presence of hematite in these samples that is evident from rock magnetic results also confirms the stability of magnetic properties and therefore we can rely on the AMS directional analysis.
Conclusion: Anisotropy of Magnetic Susceptibility (AMS) studies on Precambrian to early Cambrian red sediments, in the north side of the Kohbanan fault (Iran), show a predominant tectonic fabric with a low magnitude of oblate and prolate shape. The direction of the maximum axis of AMS lies parallel to the direction of the folded bed axis and therefore tectonic forces are probably matched up with the rotation of the Lut plate with respect to the Central Iran.