Evolution of lithospheric mantle in the north of Nain‐Baft oceanic crust (Neo‐Tethyan ophiolite of Ashin,Central Iran)

This study is focused on a plagioclase‐bearing spinel lherzolite from Chah Loqeh area in the Neo‐Tethyan Ashin ophiolite. It is exposed along the west of left‐lateral strike‐slip Dorouneh Fault in the northwest of Central‐East Iranian Microcontinent. Mineral chemistry (Mg#^olivine < ~ 90, Cr#^{clinopyroxene} < ~ 0.2, Cr#^spinel < ~ 0.5, Al₂O₃^{orthopyroxene} > ~ 2.5 wt%, Al₂O₃^{clinopyroxene} > ~ 4.5 wt%, Al₂O₃^spinel > ~ 41.5 wt%, Na₂O^{clinopyroxene} > ~ 0.11 wt%, and TiO₂^{clinopyroxene} > ~ 0.04 wt%) confirms Ashin lherzolite was originally a mid‐oceanic ridge peridotite with low degrees of partial melting at spinel‐peridotite facies in a lithospheric mantle level. However, some Ashin lherzolites record mantle upwelling and tectonic exhumation at plagioclase‐peridotite facies during oceanic extension and diapiric motion of mantle along Nain‐Baft suture zone. This mantle upwelling is evidenced by some modifications in the modal composition (i.e. subsolidus recrystallization of plagioclase and olivine between pyroxene and spinel) and mineral chemistry (e.g. increase in TiO₂ and Na₂O of clinopyroxene, and TiO₂ and Cr# of spinel and decrease in Mg# of olivine), as a consequence of decompression during a progressive upwelling of mantle. Previous geochronological and geochemical data and increasing the depth of subsolidus plagioclase formation at plagioclase‐peridotite facies from Nain ophiolite (~ 16 km) to Ashin ophiolite (~ 35 km) suggest a south to north closure for the Nain‐Baft oceanic crust in the northwest of Central‐East Iranian Microcontinent.     

Reduction of local scour at river conf luences using a collar

River confluences (RCs) are important features within river systems where the three dimensional (3D) flow structures and the downstream mixing of flows can cause deep scour holes. Despite this, few methods have been proposed to control scouring at RCs. In this study, application of a collar was experimentally examined for local scour control at the point where two rivers flow together. In parti-cular, experimental tests were done with and without collar application at three different locations. The results reveal that the scour depth is directly proportional to the discharge ratio, i.e. the ratio of lateral discharge to that in the channel downstream of the confluence, and the densimetric Froude number (Frg). In addition, installation of a collar at RCs can decrease the scour depth up to 100%, thus completely avoiding the scour process. The results also show that by increasing the Frg the optimal installation location for a collar changes and moves towards the river bed level. Using a collar can also reduce the height of the point bar formed downstream of the confluence. The outcomes of the study allow deri-vation of an equation for predicting scour depth when a collar is applied as a countermeasure. The analysis of this equation shows that the estimates are mostly affected by the Frg.     

Early Oligocene Continental Alkalibasalts of the Central Toveireh Area (Southwest of Jandaq,Isfahan Province,Iran)

Geotectonics - The Early Oligocene alkalibasalts exposed in the Central Toveireh area located in the southwest of Jandaq city in Isfahan Province (Iran) and northwest of the Central-East Iranian...